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The Force and Pressure of a Diffuser-Equipped Bluff Body in Ground Effect
Abstract
The force and pressure behavior of a generic diffuser in ground effect were investigated. The diffuser model is a bluff body with a rear diffuser at 17 deg to the horizontal, and side-plates. Measurements were conducted in a low speed wind tunnel equipped with a moving ground facility. Techniques employed were force balance, pressure taps, and surface flow visualization. The diffuser flow in ground effect was characterized by vortex flow and three-dimensional flow separation. Four types of force behavior were observed: (a) down-force enhancement at high ride heights characterized by an attached symmetric diffuser flow, (b) maximum down-force at moderate ride heights characterized by a symmetric diffuser flow and separation on the diffuser ramp surface, (c) down-force reduction at low ride heights characterized by an asymmetric diffuser flow and flow separation, and (d) low down-force at very low ride heights, also characterized by an asymmetric diffuser flow and flow separation. The down-force reduction near the ground is attributed to flow separation at the diffuser inlet and subsequent loss of suction in the first half of the diffuser.
... Senior and Zhang [44] , Senior [45] , Zhang et al [46] and Ruhrmann and Zhang [47] identified four distinct flow regions as they lowered the ride height: force enhancement (a), force plateau (b), force reduction (c) and loss of downforce (d). ...
... Senior and Zhang [44] and Senior [45] studied diffuser equipped with an aft ramp at 17°. ...
... They observed symmetric counter-rotating vortices onset from the leading-edge corners. These vortices Zhang et al [46] extended the investigation conducted by Senior and Zhang [44] and Senior [45] . They found that in the force enhancement region (a), vortices are stable, highly concentrated and less turbulent at the core. ...
This dissertation aimed at investigating the nature of interaction between the new 2022 Formula One Front Wing and the Underbody. Understanding their interaction as two major downforce producing components is very important for design. Previous research studies have investigated the aerodynamics of Front Wing and Underbody in isolation. The subject of how Front Wing is affected by the proximity of the Front Tyres has also been touched a number of times. However, the mutual aerodynamic interaction between Front Wing and Underbody was left largely ignored and as a result its details weren’t well understood. As new design regulations have been introduced in Formula One in 2022, Front Wing featuring winglet and Underbody with fore- and aft-ramp were used in this study to make it more relevant to the design of new cars.
At first, the aerodynamics of the new Front Wing and Underbody were investigated in isolation at different ride heights. Then the combined simulations were run using an overset mesh. Underbody was fixed at its maximum downforce height in “clean air” while the upstream Front Wing ride height was varied. k-ε Realizable turbulence model was used as it is known to be effective for racing car and ground effect applications. The incoming velocity was 0.19 Mach. Compressibility effects were also considered.
Simulations of isolated Front Wing showed that upwash decreases and pressure losses increase in the wake with decreasing ride height. Through isolated simulations of the Underbody, its maximum downforce height was identified in “clean air”. It was found that both the Front Wing and Underbody lose significant amounts of downforce when operating together. Underbody suffers loss due to upwash and pressure losses in Front Wing wake. Loss in Underbody downforce increases with decreasing Front Wing ride height. On the other hand, Front Wing loses downforce because the underbody slows down the flow few metres ahead of itself resulting in a lower effective incoming velocity for the Front Wing. The effect of downstream Underbody on upstream Front Wing has been identified and explained for the first time in open literature.
Key words: Ground Effect, Inverted Wing, Underbody Diffuser, Aerodynamic Interaction, Overset Mesh
... • Diffuser pumping, where the increasing cross-sectional area results in slowing the airflow down and increasing its static pressure. Crucially, in the case of a bluff body or a car with a fixed base pressure at the diffuser exit, the pressure recovery manifests itself as a suction peak at the diffuser inlet, which propagates upstream, towards the front of the body [99,[101][102][103][104][105]. This phenomenon is illustrated in A key milestone in the understanding of ground effect diffuser flows was the discovery, aided by flow visualisation, of counter-rotating vortices near the side edges of the diffuser [106]. ...
... Cooper et al. [99] initially attributed this downforce loss to boundary layer merging between the underfloor and the ground, assuming that the phenomenon would not occur in inviscid flow, and therefore must be the result of viscous effects. This theory was later disproven, as it was shown that Reynolds number has no effect on downforce variations with ride height or on the critical ride height [102,116]. However, Senior and ...
... Zhang [102], Ruhrmann and Zhang [103] and Zhang et al. [110] later suggested that the boundary layer merging phenomenon is indeed present, but at ground separations even lower than where the downforce loss occurs. Both conclusions were later confirmed by laser Doppler anemometry (LDA) measurements of velocity profiles between the underfloor and the ground, which revealed a region of constant velocity between the boundary layers, even at ride heights where asymmetric vortex breakdown and downforce loss have already occurred, followed by apparent boundary layer merging at even lower ground separation [104,105]. ...
This research project was focused on two related topics—hardware-in-the-loop aero- dynamic optimisation, and aerodynamics of automotive underbody diffusers in the presence of rake, defined as an inclination of the underfloor with respect to the ground. Two experimental systems were used for automatic, closed-loop optimisation trials, and for mapping of aerodynamic performance. Each consisted of an Ahmed-type body with a diffuser, with three controlled degrees of freedom, i.e. the model’s height above the ground, and inclinations of the underfloor and diffuser plates. The systems were equipped with force acquisition for optimisation and performance quantification purposes, and with surface pressure measurements to inspect the underlying flow patterns.
The high-speed system was used for real-time optimisation runs using a range of algorithms in order to determine their suitability to problems of this type. Population-based algorithms, and genetic algorithms in particular, were found to provide the most reliable convergence in spite of the noise and hysteresis in the measurements. Reductions in pre-sampling delay and sampling time decreased the average function evaluation time without negatively impacting convergence performance, whereas combinatorial optimisation was used to minimise actuation overheads. Subsequently, both methods were shown to improve overall optimisation efficiency during experimental trials.
Finally, the impact of rake on diffuser aerodynamics was investigated through quasi- static variations of the three degrees of freedom. Introducing rake was found to induce significant pressure recovery beneath the underfloor, causing strong suction under the front of the body and increased downforce. Furthermore, two counter-rotating vortices were observed along the edges of the underfloor, whose formation and strength depended on the configuration of the model, and which significantly affected the stall characteristics of the diffuser.
... As the diffuser ride height is gradually lowered, the flow behavior within the diffuser changes from attached to separated and this influences the aerodynamic performance of the diffuser. Senior et al. [29][30][31] and Zhang et al. [32] investigated, over varying ride heights, the downforce performance of a bluff body equipped with a 17 deg diffuser (Fig. 14). ...
... Unlike the studies in Refs. [29][30][31], the tests in Ref. [33] included a parallel investigation where ride height was gradually reduced and increased. As a result, the flow hysteresis regions and a further downforcereduction region (Region E) were identified, as plotted in Fig. 16. ...
... Although at the diffuser inlet x=L B ¼ 0:409 ð Þ the simulations underpredicted the suction level with a predicted C p of À2.09 against an experimental value of À2.37, the shape of the distribution, including the extended suction along the lengthwise ends of the diffuser, attributed in Refs. [29][30][31] to the strong counter-rotating vortices along the ends of diffuser length, was well captured. As shown in Fig. 24, IDDES also captured at the maximum downforce ride height h=d ¼ 0:217 ð Þ , the axisymmetric separation bubble formed between the counter-rotating vortex pair and close to the diffuser inlet, as seen on the oil flow pathlines of the corresponding wind tunnel investigation. ...
The ground-effect diffuser has become a major aerodynamic device on open-wheel racing and sports cars. Accordingly, it is widely considered to be indispensable to their aerodynamic performance, largely due to its significant downforce contribution. However, the physics and characteristics that determine how it generates downforce and its application in the auto racing industry require an in-depth analysis to develop an understanding. Furthermore, research that could generate further performance improvement of the diffuser has not been defined and presented. For these reasons, this review attempts to create a systematic understanding of the physics that influence the performance of the ground-effect diffuser. As a means of doing this, the review introduces research data and observations from various relevant studies on this subject. It then investigates advanced diffuser concepts mainly drawn from the race car industry and also proposes a further research direction that would advance the aerodynamic performance of the diffuser. It is concluded that although the diffuser will continue to be paramount in the aerodynamic performance of racing cars, research is needed to identify means to further enhance its performance.
... Senior et al. [9][10][11] found that flow behavior and diffuser downforce are dependent on diffuser ride height, which regulates the constrained underbody airflow entering the diffuser. ...
... This implies that the surviving vortex is weaker than the same vortex at the maximum-force ride height (as confirmed by pressure measurements). The asymmetry of the diffuser flow at the force-reduction ride height was previously reported to be random, and the vortex bursting phenomenon switched from one vortex to the other [9][10][11]. ...
... Under the force-enhancement flow regime, the longitudinal vortex pair is formed when peak suction is established at the diffuser inlet fed by the diffuser pumping effect at the inlet [6][7][8][9][10][11][12][13][14]. ...
A ground-effect diffuser is an upward-sloping section of the underbody of a racing car that enhances aerodynamic performance by increasing the downforce, thus improving tire grip. The downforce generated by a diffuser can be increased by geometric modifications that facilitate passive flow control. Here, we modified a bluff body equipped with a 17 deg diffuser ramp surface (the baseline/plane diffuser) to introduce a convex bump near the end of the ramp surface. The flow features, force, and surface pressure measurements determined in wind-tunnel experiments agreed with previous studies but the bump favorably altered the overall diffuser pressure recovery curve by increasing the flow velocity near the diffuser exit. This resulted in a static pressure drop near the diffuser exit followed by an increase to freestream static pressure, thus increasing the downforce across most of the ride heights we tested. We observed a maximum 4:9% increase in downforce when the modified diffuser was compared to the plane diffuser. The downforce increment declined as the ride height was gradually reduced to the low-downforce diffuser flow regime.
... [30][31][32] This is followed by downforce reduction beyond the critical ride height and diffuser angle, as an excessive adverse pressure gradient in the diffuser causes a combination of gradual vortex breakdown and flow separation, followed by a sudden loss of downforce when one of the vortices collapses completely. [32][33][34][35] The process of downforce reduction is subject to significant hysteresis at diffuser angles of 15°and higher. 32,35 It is known that a positive rake angle causes a significant downforce increase and a forward shift of the centre of pressure, as well as significantly reduces the critical ride height, boosting the performance of the diffuser. ...
... In the case of automotive diffusers, a moving ground results in a stronger downforce peak at the diffuser inlet and has an effect on the downforce reduction phenomenon. 30,31,33 This investigation was carried out using a fixed-ground wind tunnel, as the primary purpose was to demonstrate the system and to perform the optimisation algorithm study. However, the system was also able to generate a large amount of experimental aerodynamic data. ...
... This crest was previously observed with respect to ride height at very similar angles and ride heights. 32,33 Crucially, the maximum-downforce diffuser angle decreases with decreasing ride height, from almost 30°at h r /d = 1, down to 10°at h r /d = 0.05. This is as increasing diffuser angle and decreasing ride height both have the effect of increasing the area ratio of the diffuser (the exit-to-inlet cross-sectional area ratio), and the downforce produced and the streamwise adverse pressure gradient are both functions of the area ratio. ...
A wind tunnel-based morphing system was devised and utilised for aerodynamic data collection and real-time optimisation of an Ahmed body equipped with a diffuser. Three degrees of freedom were controlled, i.e. ride height, rake angle of the underfloor, and angle of the diffuser plane. Their impact on performance was investigated with a fixed ground. Real-time optimisation was carried out with the aim of determining the most suitable optimisation method for this problem. Tests were carried out using simulated annealing, particle swarm optimisation, pattern search and two genetic algorithms. The results showed that the algorithms demonstrated significantly different performance. However, they were all able to converge on a solution in spite of the hysteresis, which is a characteristic of diffuser flows, and the noise inherent in the system. Pattern search provided the most efficient convergence to the global maximum, despite several discrete aerodynamic changes within the search space, such as sudden flow separation or vortex breakdown. However, it was found to be sensitive to the initial position and noise in the data. The genetic algorithms were found to provide the most reliable convergence, although they were hindered by their inability to make small adjustments in the final stage of convergence. Population sorting was demonstrated as a way to improve the performance of population-based algorithms. Several new trends in diffuser performance were also observed, most notably that rake, even at small angles, not only generates downforce, but also significantly decreases the critical ride height and energises the diffuser, allowing it to work at higher angles. Up to 1000 different configurations per hour could be tested, making the system attractive for multi-dimensional aerodynamic optimisation, which would be very costly using computational fluid dynamics or conventional wind tunnel testing.
... The study was performed using commercial CFD, Fluent © 6.3.26. CFD data is compared to experimental work done with similar bodies by Cooper et al. [1,2], George et al. [3,4], Zhang et al. [5,6], and others [7,8,9]. Emphasis is made on the study of vortex structures in bluff body flow. ...
... VORTEX STUDY WITH MULTIPLE BODIES -Previous research performed by George, Zhang, and others [3][4][5][6][7][8][9] highlighted the significant contribution edge leeward vortices make to downforce and stall prevention in a practical underbody. This investigation further examined the contribution of vortices to lift and stall prevention of underbodies, as well as factors that influence vortex development. ...
... This is described as high AR diffuser stall with vortex deterioration. This type of reasoning is supported by Zerihan & Zhang [5,16] and generally concurs with the rest of the research [3][4][5][6][7][8][9]. ...
Results are presented from a study on the use of Computational Fluid Dynamics (CFD) for automotive underbody design. A diffuser-equipped bluff body with endplates was examined in ground effect at varying ride heights in configurations with and without wheels. The study was performed using commercial CFD, Fluent© 6.3.26. CFD data is compared to experimental work done with similar bodies by Cooper et al. [1, 2], George et al. [3, 4], Zhang et al. [5, 6], and others [7, 8, 9]. Emphasis is made on the study of vortex structures in bluff body flow. Various mesh geometries and solvers were explored with computational models designed to operate on single-processor workstations or small networks. Steady-state solutions were modeled for all cases; boundary layers were approximated with wall functions. CFD results for lift coefficient measured within 15-25% of experimental cases, dependent on solver. Qualitative results matched well with experimentally measured flow structures. Downforce reduction due to stall was found at ride heights similar to those established experimentally, attributed to trailing-edge separation of both the underbody and ground boundary layers at the diffuser, as well as to vortex deterioration or breakdown. The effects of underbody vortices on downforce generation and stall prevention at low ride heights are discussed in detail from insights derived using the CFD models. After validating computational models against experimental baselines, CFD was explored for multi-body flow applications. Effects of wheel-shaped objects along side of, ahead of, and behind the bluff body in ground effect were examined. A decrease in force generation from the bluff body was found in certain configurations. This was found to relate to the disruption of vortex formation along the diffuser and underbody region, as well as from blockage. Similar experiments conducted by Breslouer and George [10] extend upon these findings. From the study, opportunities for practical automotive development using commercial CFD are discussed while indicating likely obstacles and limitations. Recommendations are made relating to solver choice and mesh generation practice with the aim to optimize model creation and run time while still realizing useful qualitative and quantitative results.
... This airfoil stems from modifications to the NASA-LANGLEY LS(1)-0413MOD profile. The complete listing of the profile at α = 3.6 • can be found in Ref. 16 . Note that for an inverted airfoil the angle of attack is defined positive for a rotation that brings the leading edge closer to the ground. ...
... In literature this phenomenon is referred to as diffuser pumping. 10,15,16 On a race car the diffuser can contribute up to one third of the total downforce. 18 According to Ref. 18 the most extensive work on the subject of diffuser aerodynamics is due to Cooper et al. 4,5 Their experiments were conducted on a 3D bluff body equipped with a diffuser in ground effect. ...
... A more recent work was performed by Zhang et al. 13,15,16 In Ref. 16 force and pressure behaviour of a bluff body equipped with a 17 • diffuser operating in ground effect were investigated by Senior and Zhang. With respect to the ride height, four distinct regions of downforce behavior were distinguished: downforce enhancement region, maximum downforce region, downforce reduction region and low downforce region. ...
... Senior and Zhang [8] and Ruhrmann and Zhang [9] investigated the flow field of bluff body configurations that were more applicable to open-wheeled racing car designs. The bluff body incorporated a generic diffuser of variable ramp angle (5,10,15,17, and 20u to the horizontal), which was tested above a moving ground plane with an upstream boundary layer suction. ...
... This experimental set-up was different from that of Senior and Zhang [8] in that their experimental set-up had the diffuser fitted in the same wind tunnel, i.e. above a moving ground plane with upstream boundary layer suction, and was suspended by an overhead support strut. In fact, Senior and Zhang [8] showed that with a fixed ground board under the diffuser, vortices were still produced. ...
... This experimental set-up was different from that of Senior and Zhang [8] in that their experimental set-up had the diffuser fitted in the same wind tunnel, i.e. above a moving ground plane with upstream boundary layer suction, and was suspended by an overhead support strut. In fact, Senior and Zhang [8] showed that with a fixed ground board under the diffuser, vortices were still produced. The subject of this paper therefore has a different boundary layer flow into the following wing. ...
In a further effort to provide additional insight into the aerodynamic factors that may influence the creation of overtaking opportunities in open-wheeled racing categories such as Formula 1, a series of wind tunnel experiments were initiated at the University of Southampton to provide information on the effect that an upstream diffuser in ground effect may have on a downstream single-element wing in ground effect. Investigations focused on force measurements and on the extraction of wake profiles from the downstream wing and its flow field. The upstream model configuration was altered by changing the angle of the diffuser ramp and by varying the height of the diffuser above the ground. It was found that a lower diffuser angle increased the efficiency of the downstream wing and that changes to the vortex wake generated by the diffuser induced changes to the wake characteristics of the wing. It was also found that decreasing the ride height of the bluff body produced an increase in the thickness of the wake of the downstream wing.
... Senior and Zhang [8] and Ruhrmann and Zhang [9] investigated the flow field of bluff body configurations that were more applicable to open-wheeled racing car designs. The bluff body incorporated a generic diffuser of variable ramp angle (5,10,15,17, and 20u to the horizontal), which was tested above a moving ground plane with an upstream boundary layer suction. ...
... This experimental set-up was different from that of Senior and Zhang [8] in that their experimental set-up had the diffuser fitted in the same wind tunnel, i.e. above a moving ground plane with upstream boundary layer suction, and was suspended by an overhead support strut. In fact, Senior and Zhang [8] showed that with a fixed ground board under the diffuser, vortices were still produced. ...
... This experimental set-up was different from that of Senior and Zhang [8] in that their experimental set-up had the diffuser fitted in the same wind tunnel, i.e. above a moving ground plane with upstream boundary layer suction, and was suspended by an overhead support strut. In fact, Senior and Zhang [8] showed that with a fixed ground board under the diffuser, vortices were still produced. The subject of this paper therefore has a different boundary layer flow into the following wing. ...
In an effort to provide more detailed insight into the aerodynamic factors that may influence the creation of overtaking opportunities in modern open-wheeled racing series, a set of wind tunnel experiments was initiated in the moving ground facilities at the University of Southampton. To generate data typical of one car following another, a single-element wing in ground effect was tested downstream of a bluff body that incorporated a diffuser and rear wing. The tests included variations in the height and angle of attack of the wing, while data collection was achieved via force and pressure measurements, flow visualization and flowfield surveys. The results were then compared with baseline data that were obtained without the presence of the bluff body. It was found that, while behind the upstream body, the wing experienced a decrease in its downforce values, with the amount of downforce lost depending on its height above the ground. It was also shown that more downforce was lost from sections closer to the mid-span of the wing than was the case from sections closer to the tips of the wing.
... This configuration has been utilized primarily on high performance vehicles to increase downforce, i.e. negative lift, therefore enhancing the overall mechanical grip. An important feature of the flow is that the pressure at the base of the bluff body remains relatively constant as the model height is varied (Senior and Zhang, 2001). Hence as the model height is reduced, pressure underneath of model (nearest to the ground plane) must be Ôpumped down' (Sovran, 1994), which leads to an increase in downforce. ...
... Apart from engineering and fundamental interests, correctly conducted tests and offsurface measurements, including turbulence data, would benefit numerical modelling efforts. In a recent study (Senior and Zhang, 2001), the overall force and on-surface features of a bluff body equipped with an aft upswept section were investigated in ground effect in wind tunnel model tests that employs a moving ground. As the bluff body is lowered towards the ground from a height in freestream, the overall downforce on the body first experiences an increase, the force enhancement phenomenon. ...
... The model is symmetric about the span-wise central plane. Detailed dimensions of the model can be found in Senior and Zhang (2001) and are also provided inFig. 1. ...
The vortices behind a bluff body equipped with an upswept aft section are studied in a model test. The bluff body operates in close proximity to ground. The principle measurement technique is laser Doppler anemometry (LDA), which is supported by surface flow, pressure and force measurements. The upswept surface has an angle of 17° to horizontal. With the presence of end-plates on the aft section, discontinuities in slope of the force curve exist at several heights in proximity to the ground. The characteristics of these changes are linked with edge vortices. The position and strength of the vortex are identified. Three main types of trailing vortices exist: (a) concentrated, symmetric with a high axial speed core, (b) diffused, symmetric with a low axial speed core, and (c) diffused and asymmetric. The study provides further clarification of major physics and a database for validating predictive methods.
... The latter, along with a proper diffuser angle, can provide a significant 32.5% increase in the downforce coefficient when compared to a baseline model. Another investigation on an Ahmed bluff body at a 0 • upper slant angle that had a diffuser with side plates was performed to elaborate the different types of flow regions for the configuration [16]. ...
... However, when testing the body close to the ground, the airflow under the body experiences an increase in its velocity caused by the ground constraint, a phenomena called ground interaction. This phenomenon has been studied rigorously in vehicle aerodynamics [16]. Descriptions of the major flow structures for different ride heights have been obtained, along with insights on the flow's physics. ...
The aerodynamic complexity of the underbody surfaces of conventional road vehicles is a matter of fact. Currently available literature is focused mainly on very simple Ahmed-body geometries as opposed to realistic car shapes, due to their complexity and computational cost. We attempted to understand the flow behaviour around different realistic conventional road car geometries, and we provide an extensive evaluation of the aerodynamic loads generated. The key findings of this article could potentially set a precedent and be useful within the automotive industry’s investigations on drag-reduction mechanisms or sources of downforce generation. The novelty of the work resides in the realistic approach employed for the geometries and in the investigation of barely researched aerodynamic elements, such as front diffusers, which might pave the way for further research studies. A baseline flat-underfloor design, a 7∘ venturi diffuser-equipped setup, a venturi diffuser with diagonal skirts, and the same venturi diffuser with frontal slot-diffusers are the main configurations we studied. The numerical predictions evaluated using RANS computational fluid dynamics (CFD) simulations deal with the aerodynamic coefficients. The configuration that produced the highest downforce coefficient was the one composed of the 7∘ venturi diffuser equipped with diagonal sealing skirts, achieving a CL value of −0.887, which represents an increase of around 1780% with regard to the baseline model. That achievement and the gains in higher vertical loads also entail a compromise with an increase in the overall air resistance. The performance achieved with diffusers in the generation of downforce is, as opposed to the one obtained with conventional wings, a cleaner alternative, by avoiding wake disturbances and downwash phenomena.
... Complementing the work of [4], [16] investigated a new bluff body equipped with a diffuser which extended over 41% of the body length and with inclination angle of 17 • and endplates in different ground heights. The result was the identification of four distinct regions of diffuser performance, all related to the model ground height. ...
... With substantial literature on the diffusers, such as the works from [4] and [16], we notice that each study has employed different bluff body geometries, that have not been previously studied without a diffuser. The work of [8] was the first to propose a computational study of diffusers on an Ahmed body with slant angle of 35 • with similar conditions of [18] study. ...
The Ahmed body is one of the most studied 3D bluff bodies used for automotive research and was first proposed by Ahmed in 1984. The variation of the slant angle of the rear upper surface on this body generates different flow behaviours, similar to a standard road vehicles. In this study we extend the geometrical variation to evaluate the influence of a rear underbody diffuser which are commonly applied in high performance and race cars to improve downforce. We perform parametric studies on the rear diffuser angle of two baseline configurations of the Ahmed body: the first with a 0 degree upper slant angle and the second with a 25 degrees slant angle. We employ a high-fidelity CFD simulation based on the spectral/hp element discretisation that combines classical mesh refinement with polynomial expansions in order to achieve both geometrical refinement and better accuracy. The diffuser length was fixed to the same length of 222 mm similar to the top slant angle that have previously been studies The diffuser angle was changed from 0 to 50 degrees in increments of 10 degrees and an additional case considering the angle of 5 degrees. For the case of a 0 degree slant angle on the upper surface the peak values for drag and negative lift (downforce) coefficient were achieved with a 30 degrees diffuser angle, where the flow is fully attached with two streamwise vortical structures, analogous to results obtained from Ahmed but with the body flipped upside down. For diffuser angles above 30 degrees, flow is fully separated from the diffuser. The Ahmed body with 25 degrees slant angle and a diffuser achieves a peak value for downforce at a 20 degrees diffuser angle, where the flow on the diffuser has two streamwise vortices combined with some flow separation. The peak drag value for this case is at 30 degrees diffuser angle, where the flow becomes fully separated.
... Force measurements ( Figure 1) on a diffuserequipped bluff body under moving ground wind tunnel conditions have revealed the presence of four distinct diffuser flow regimes (Senior and Zhang, 2001;Senior, 2002). In the force-enhancement flow regime (region A), downforce ( , ) increased as the bluff body is gradually lowered from an elevated ride height. ...
... When the ride height is reduced further, downforce reduction occurred in the regions C and D flow regimes. (Senior and Zhang, 2001;Senior, 2002). ...
A ground-effect diffuser is an upwardly-inclined section of an automobile’s underbody which increases aerodynamic performance by generating downforce. To understand the diffuser flow physics (force behaviour, surface and offsurface flow features), we established the near-wake (within one vehicle width of the base) velocity profiles and flow structures of an automotive ground-effect diffuser using a bluff body with a 17 degree slanted section forming the plane diffuser ramp surface (baseline geometry), and endplates extending along both sides of the ramp. Wind tunnel experiments were conducted at a Reynolds number of 1.8 million based on the bluff body length, and laser Doppler velocimetry was used to measure two-dimensional velocity components on three planes of the diffuser near-wake. We also measured the velocity field in the near-wake of diffusers with modified geometry (with an inverted wing or a convex bump) as passive flow control devices. The near-wake velocity profiles indicated that the passive flow control methods increased the diffuser flow velocity and that the longitudinal vortices along the diffuser determined the shape of the flow structures in the near-wake of the diffuser bluff body.
... Another limitation of the design here is that all the simulations were done in a partial half-model. A paper by Senior [31] illustrated the importance of symmetric flows in high-performance undertrays, and the analyses in this project have naturally enforced a symmetrical flow in the simulation. Hence, in future projects, it is also important that the undertray simulations are done using the whole car geometry. ...
Research into undertray devices in the high-speed racing industry is still privileged information — this is indicated by the lack of research papers which are forthcoming in the field. In this project, the downforce and drag of an undertray were investigated in two and three dimensions by employing Computational Fluid Dynamics (CFD). Three sets of analyses were undertaken, consisting of 2D enclosed flow, 2D open flow, and 3D open flow, and these were followed by calculations on prototype undertrays to gain a fundamental understanding of the effect of an undertray’s flow features on its performance in diverse geometrical setups. Two dimensional simulation was determined not to be optimal for undertray analysis, as it does not properly capture important flow features, which turn out to require three dimensional calculations. The three dimensional analysis has also shown the importance of strategic management of effective flow control using generation of vortices to maintain flow attachment on the undertray’s wall and to generate stable vortices to exploit the flow, improving the downforce and reducing drag. The final undertray design geometry was based on these analyses. The final result was found to give a 184.1 N increase in downforce and 10.5 N reduction in drag at 40 km/h. Further research into key flow features and vortex generation and their effect on the undertray overall performance is required to utilise the potential performance of an undertray fully.
... Specifically, it was observed a much larger change than drag using different ground clearance. In the more pragmatic racing car field, aerodynamic solutions and ads on, such as wings and diffusers, were developed to take advantage of the ground effect (Wright, 1982;Garry, 1996;Senior and Zhang, 2001;Zhang et al., 2004Zhang et al., , 2006. ...
The influence of ground clearance on the flow around a simplified high-speed train is investigated in this paper. Four clearance heights are studied using IDDES. After a grid independence study, the results of the simulations are validated against experimental data present in the literature. It is found that the drag decreases when reducing the clearance gap from the baseline height to a possibly critical height, while drag remains constant when the clearance is lower than this critical height. The negative lift (downforce) increases with the decreasing of the clearance gap. The flow is particularly influenced by the gap height at the underbody and wake regions, where a lower underbody velocity and a higher wake velocity are observed with lower clearance down to case h2. Therefore, the different topologies of the wake are presented and described. Particular attention is paid to the description of the wake flow and to the position and the formation of the flow mixing region. Specifically, with decreasing clearance, the mix of the tail downwash and underbody flows happen earlier, and the core of the counter- rotating vortices in the wake tends to develop with an increasing height trend. Overall, aerodynamic performance and flow structure descriptions show positive and negative effects when decreasing gap clearances, which should be taken into account for new design strategies.
... Puglisevich 21 has used the same approach with LES turbulence model. Senior 20 varied the ride height and studied the effect of separation in the diffuser with resulting losses in downforce. Ruhrman 22 has extended this work to investigate the ride height and ramp angle using a matrix approach. ...
Diffusers are typically used in motorsport to generate negative lift (downforce). They also reduce aerodynamic drag and so significantly enhance aerodynamic efficiency. The amount of downforce generated is dependent on ride height, diffuser ramp angle and its relative length to that of the vehicle length. This paper details a numerical investigation of the effects of ride height and diffuser ramp angle in order to find an optimum downforce and efficiency for the inverted Ahmed model. A short and long diffuser with ratios of 10% and 35%, respectively, to that of vehicle length are studied. The short diffuser produced lower maximum downforce and efficiency at a lower ride height and lower angle when compared to the longer diffuser. The long diffuser produced highest downforce and the best efficiency with a ramp angle of 25° at ride height ratio of 3.8% when compared to vehicle length. Different ride heights were found to correspond to different diffuser ramp angles to achieve optimum downforce and efficiencies.
... The ground effect, however, does not only apply to the front wing but also to the underbody of race cars, where the same observation regarding the correlation between ground clearance and downforce is made [12] while the aerodynamic load can be further increased by means of vortex generators [13]. However, in the wake of another flow body, i. e., during slipstream driving, the benefit of the ground effect is significantly marginalized as shown in [14,15]. ...
A realistic open-wheel race car model is investigated experimentally by means of surface flow visualization with UV active tufts, wall pressure as well as force measurements. The head restraint size is varied and crosswind conditions are reproduced inside a closed test section with a non-moving ground. An assessment of the aerodynamic components determines the contributions of front wing, rear wing and side wings in terms of the overall aerodynamic performance of the race car. Maximum downforce corresponding to c_L ≈ -0.35 (front wing only), c_L ≈ -1.4 (front wing and rear wing combined) and c_L ≈ -1.5 (complete aero package) was found. By means of flow visualization and image subtraction techniques, regions of highly turbulent flow on the rear wing are identified. They are shown to grow in size when the largest head restraint is installed. As a result, the pressure distribution for this configuration exhibits a considerable decrease in magnitude. As for the smaller investigated head restraints, marginal deviations are detected. However, we show that the second smallest head restraint produces a lap time reduction by a couple of split seconds when compared to the smallest device that is currently employed.
... The influence of the position and orientation of the underbody channel and the wing on the aerodynamic characteristics of a car body have also been clarified. Although the road-toroad aerodynamics are complicated by the existence of the wind tunnel's boundary layer [24], it can be avoided, as it does not exist on an actual road surface in wind tunnel experiments using actual car shapes [4]. Thanks to a new multi-set level method, boundary layer meshes can be built over an immersed complex geometry that is applied in a metric tensor field using gradation [2]. ...
A rear wing designed to improve motoring performance and enhance stability during cornering needs to generate a large downforce at a relatively low speed. If the angle of attack of the rear wing is large, then air resistance is increased during high-speed driving, and thus, fuel consumption is increased due to the large drag values. On the other hand, the performance on high-speed cornering will improve overall lap time with an increased angle of attack. To mitigate this disadvantage, we aimed to reduce the angle of attack during high-speed driving to reduce downforce and drag and thus to reduce fuel consumption. Meanwhile, during low-speed driving, for example in cornering, the angle of attack was increased and a large downforce generated to improve driving stability. In order to achieve both goals, we developed a passive-type variable rear wing. This rear wing was designed to have a three-step shape where the second step in the center was designed to swing. We first confirmed the behavior through both computer-aided engineering analysis and wind tunnel experiments, and then we constructed a full-size rear wing and measured the downforce on a student Formula SAE vehicle. The results showed that it is possible to generate a downforce of 80 N at a low speed of 30 km/h (8.3 m/s) and a downforce of 145 N at a high speed of 50 km/h (13.9 m/s).
... Vortices are used to augment the downforce on the vehicle, which allows greater tire grip and thus improved performance. For example, vortices created at end-plates=side-plates have been shown to have a significant contribution to the force on wings [4][5][6] and diffusers [7][8][9][10] in ground effect, as summarized by Zhang et al. [11]. Not only do engineers take advantage of these vortices inevitably created by features of the car, but they also use specific VGs. ...
Methods of modeling vortex generation in computational fluid dynamics calculations without meshing the vortex generating device are investigated. In this way, the effect of adding vortices to existing flows can be assessed without the need to modify the computational grid; this can represent a significant saving. Previous work in this area has focused on boundary layer control. This study looks at larger scale applications, such as using vortices for force augmentation or directing flow. Two different approaches are used: modeling the vortex generator and modeling just the vortex alone. For the former, an existing method, which acts to align the flow with the vortex generator by adding a forcing term to the governing equations, is tested, but found to be unsuitable for use on this scale. The other approach is to add specified vortex velocity profiles, allowing the introduction of arbitrary vortices. A new version is developed to add continuous 3D velocity distributions in regions where desired vortices are to be created. It is implemented using several different forms of forcing. After basic testing, all methods are applied in a practical engineering case using a commercial solver. [DOI: 10.1115/1.4005314]
... Similar works in this field were also reported by George (1981), Senior and Zhang (2001) and Zhang et al. (2004). All these studies emphasised the influence of underbody shape in the mechanism of generation and increasing of downforce. ...
This paper presents new results concerning the flow around the Ahmed body fitted with a rear underbody diffuser without endplates, to reveal the influence of the underbody geometry, shaped as a Venturi nozzle, on the main aerodynamic characteristics. The study is performed for different geometrical configurations of the underbody, radius of the front section, length and the angle of the diffuser being the parameters, which are varied. Later, based on a theoretical approach, the coefficients of the equivalent aerodynamic resistances of the front section of underbody and diffuser are computed, which help to evaluate the drag due to underbody geometry.
... Kumarasamy and Barlow [7] studied the ow over a half-cylinder close to a moving wall. An experimental study for three-dimensional ow with free-stream turbulence past an aerodynamic body placed near a moving ground was conducted by Senior and Zhang [8]. They found that the ow remains symmetric about the centre plane of the di user and the suction level at the di user inlet continues to rise as the ride height is reduced. ...
Flow past a cylinder of square cross-section placed near a plane sliding wall has been investigated. This work aims to address
questions regarding the characteristics of the vortex shedding regime and its modifications with the variation of wall to
cylinder gap height and Reynolds number. The governing unsteady Navier-Stokes equations are discretised through the finite
volume method. A SIMPLER algorithm has been used to compute the discretised equations iteratively. A uniform velocity profile
equal in speed of the wall speed impinges on the cylinder. An alternate vortex shedding is found in the cylinder near wake
for Reynolds number (based on cylinder height) greater than 80. The boundary layer along the moving wall separates and the
secondary vortex forms in the region downstream of the cylinder. Unlike the stationary ground case where the vortex shedding
suppression occurs beyond a critical value of gap length, here the vortex shedding takes place even at low gap length ratio
0.1. We found that the cylinder experiences a upward force at higher values of Reynolds number. The gap flow is strong and
the velocity profiles overshoots its free stream value within this region.
... In a more recent work, Senior & Zhang (2001) tried to generate additional information on the basic fluid dynamics of the diffuser flow. While testing a variety of diffuser angles, they identified two vortices forming at the side edges of the diffuser and concluded that the Reynolds number effects are not significant. ...
Race car performance depends on elements such as the engine, tires, suspension, road, aerodynamics, and of course the driver. In recent years, however, vehicle aero-dynamics gained increased attention, mainly due to the utilization of the negative lift (downforce) principle, yielding several important performance improvements. This review briefly explains the significance of the aerodynamic downforce and how it improves race car performance. After this short introduction various methods to generate downforce such as inverted wings, diffusers, and vortex generators are dis-cussed. Due to the complex geometry of these vehicles, the aerodynamic interaction between the various body components is significant, resulting in vortex flows and lifting surface shapes unlike traditional airplane wings. Typical design tools such as wind tunnel testing, computational fluid dynamics, and track testing, and their rel-evance to race car development, are discussed as well. In spite of the tremendous progress of these design tools (due to better instrumentation, communication, and computational power), the fluid dynamic phenomenon is still highly nonlinear, and predicting the effect of a particular modification is not always trouble free. Several examples covering a wide range of vehicle shapes (e.g., from stock cars to open-wheel race cars) are presented to demonstrate this nonlinear nature of the flow field.
An automotive diffuser is an open channel within the underbody of a vehicle that features a diverging ramp in the aft section. The performance of a diffuser is sensitive to ground effect where decreases in ride height result in increases in downforce. However, below a critical value, any further reduction in ride height results in a significant loss of downforce. Previous experimental investigations demonstrated that the dominant flow feature within underbody diffuser flows is a pair of counter-rotating longitudinal vortices, and the resulting downforce behavior is directly linked to the structure of the longitudinal vortex pair. This study investigates the effect of ride height on the behavior of the longitudinal vortex pair within an underbody diffuser flow in ground effect. The unsteady flow past a diffuser-equipped bluff body with a 17-degree diffuser ramp angle is simulated using large eddy simulation with wall-stress modeling, commonly referred to as wall modeled large eddy simulation (WMLES). The flow Reynolds number based on body length is 1.75 million. Numerical simulations are performed with OpenFOAM and WMLES is implemented with libWallModelledLES, a third-party WMLES library for OpenFOAM. Results show that the mean center-line surface pressure distributions along the underbody match well with experiments. Visualization of the vortices with isosurfaces of the Q-criterion demonstrates that the longitudinal vortices experience a spiral-type vortex breakdown which propagates upstream with decreasing ride height.
This article presents the results of an experimental investigation into the impact of rake, or inclination of the underfloor, on the aerodynamics of a bluff body equipped with an underbody diffuser. An extensive wind tunnel campaign, utilising a remotely-actuated model for faster data acquisition, showed that introducing rake results in a downforce increase at all ride heights and diffuser angles, with the strongest effect occurring at low ride heights. Surface pressure measurements on the underbody revealed this to be caused by three main effects. Firstly, a large increase in loading at the front of the floor, due to the inclination of the floor with rake angle and subsequently an increase in the pressure pumping effect. Secondly, a reduction in the suction peak at the throat of the diffuser, which leads to reduced pressure recovery in the diffuser, and less likely separation at high diffuser angles or low ride heights. Thirdly, stronger streamwise vortices along the edges of the underfloor and diffuser, which generate downforce directly due to their low-pressure cores, but also introduce upwash under the model, further inhibiting separation in the diffuser. As the related drag penalty is minimal, aerodynamic efficiency is also improved with increasing rake angle.
div class="section abstract"> Racing and high-performance vehicles utilize their underbody floor and diffuser as efficient mechanisms to generate the majority of their downforce. Previous work has primarily been focused on simplified bluff bodies with plane diffusers. The little published work on more complex multichannel diffusers has shown improved downforce generation over plane diffuser, but with limited understanding of the flow features and their response to ride height. This study analyses the performance and complex flow features of a high-performance vehicle equipped with a multichannel diffuser at various ride heights. A comparative assessment between RANS and DDES simulations is performed, which shows that both models adequately predict downforce and underbody flow features at high to medium ride heights, but only the DDES model is able to capture the unsteady flow behavior, which dominates the diffuser at low ride heights. Subsequently, an in-depth aerodynamic analysis of the vehicle’s ride height dependency is conducted using the DDES simulations. The analysis shows the crucial role of the diffuser’s side plate vortices on the flow entertainment and formation of vortices at the separators. Moreover, the analysis describes the influence of ride height on the strength and stability of the vortex flow in the diffuser. Findings at very low ride heights demonstrate that strong interaction between the vortices and flow along the ground plane cause the formation of relatively unstable vortices, which provide less flow entertainment and thereby cause a reduction in downforce.
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The Ahmed body is one of the most studied 3D automotive bluff bodies and the variation of its slant angle of the rear upper surface generates different flow behaviours, similar to a standard road vehicles. In this study we extend the geometrical variation to evaluate the influence of a rear underbody diffuser which are commonly applied in high performance and race cars to improve downforce. Parametric studies are performed on the rear diffuser angle of two baseline configurations of the Ahmed body: the first with a 0° upper slant angle and the second with a 25° slant angle. We employ a high-fidelity CFD simulation based on the spectral/hp element discretisation that combines classical mesh refinement with polynomial expansions in order to achieve both geometrical refinement and better accuracy. The diffuser length was fixed to the same length of 222 mm similar to the top slant angle that have previously been studies. The diffuser angle was changed from 0° to 50° in increments of 10° with an additional case considering the angle of 5°. The proposed methodology was validated on the classical Ahmed body considering 25° slant angle, found a difference for drag and lift coefficients of 13% and 1%, respectively. For the case of an 0° slant angle on the upper surface the peak values for drag and negative lift (downforce) coefficient were achieved with a 30° diffuser angle, where the flow is fully attached with two streamwise vortical structures, analogous to results obtained from [1] but with the body flipped upside down. For diffuser angles above 30°, flow is fully separated from the diffuser. The Ahmed body with 25° slant angle and a diffuser achieves a peak value for downforce at a 20° diffuser angle, where the flow on the diffuser has two streamwise vortices combined with some flow separation. The peak drag value for this case is at 30° diffuser angle, where the flow becomes fully separated.
To extract the maximum performance out of the engine and transmission unit the bodywork of the vehicle should be as streamed lined as possible. But at the same time down force must also be accounted for since many accidents take place at high speeds due to driver losing control. While drag reduction is important, the generation of downforce is also important as it plays a major role in determining the handling characteristics of the vehicle. The project is aimed at developing the bodywork of a high performance vehicle to induce the lowest drag possible along with high downforce by creating a diffuser and smoothening the underbody of the vehicle. The project will discuss the influence of shape of the bodywork in mainly underbody area (when using a diffuser) as it will increase the downforce of the vehicle without creating a lot of drag force as vehicles with underside whose engine and exhaust pipes and suspension arm exposed create a lot of flow separation which in turn increases drag and decreases downforce. The diffuser utilizes the ground effect produced due to Bernoulli’s principle in a vehicle to produce negative lift or downforce while the smooth underbody ensures lesser flow separations as they contribute towards vehicles instability at high speeds and lower efficiency of power unit. The smooth underbody along with teardrop driver and engine canopy reduce flow separation in areas of vehicle where it is common to experience it, helps in reaching Carbon emission to be reduced. The method of CFD simulation in ANSYS-FLUENT will be used in to conduct tests and obtain the results since accessibility to make physical model of vehicle and wind tunnel was not present and the other reason being the negligible error between the simulation and physical tests.
Motor racing, like other popular forms of competitive sports, requires physical fitness, concentration, and vigorous preparation and training. Although progress in technology may dominate the race, governing bodies are continuously updating the rulebooks to keep the human factor dominant in winning races. On the other hand, vehicle performance depends on elements such as the engine, tires, suspension, road, and aerodynamics. In recent years, however, vehicle aerodynamics has gained increased attention, mainly due to the utilization of the negative lift (downforce) principle, yielding several significant performance improvements. The importance of drag reduction and improved fuel efficiency are easily understood by the novice observer and are still at the center of racing vehicle design. Interestingly, however, generating downforce by the vehicle usually increases its drag but improves average speed in closed circuits. Consequently, various methods to generate downforce such as inverted wings, diffusers, and vortex generators will be discussed. Also, generic trends connecting a vehicle’s shape to its aerodynamics are presented, followed by more specific race-car examples. Due to the complex geometry of these vehicles, the aerodynamic interaction between the various body components is significant, resulting in vortex flows and wing shapes which may be different than those used on airplanes.
This article highlights the advances in the aerodynamic development and test process of both road and motorsports cars over the last two decades, including explaining the main driving forces behind this evolution. The relentless and continuous drive to improve efficiency and correlation between computational fluid dynamics, real-time simulation, wind tunnel testing, and the track is explained. Key enabling technologies are described, such as: continuous motion systems; high-speed data acquisition systems; steel moving belt ground planes with under floor load cells; on-demand robotic particle image velocimetry; pneumatic model tyres with integral sidewall and contact patch deflection systems; driver simulators and rapid prototyped rake systems for track cars. Finally, as aerodynamicists attempt to simulate and test within ever more complex and realistic environments potential future directions and emerging trends are outlined, including gusts, aeroelasticity, adaptive cooling, and cornering.
Being a continuous subject of research, this study presents new aspects regarding the relevance of underbody diffusers in road vehicle aerodynamics. Using a generic car model on wheels as a reference, the effect of the wheels on the body fitted with an underbody diffuser was studied, where the diffuser length and angle were varied within ranges which are applicable for hatchback passenger cars. The results show that the vortices which originate from the rear wheelhouses have a major impact on the aerodynamics of the underbody diffuser, which results in increasing of drag and lift of the body. For cases studied, the average drag and lift increment due to the addition of wheels were (ΔcD)mean = 0.058, respectively (ΔcL)mean = 0.243. The lift of the body on wheels decreases with both diffuser length and diffuser angle, and there are situations when it may become negative as for a body without wheels. The results show also the possibility to reach a minimum drag according with normalised diffuser length.
Being a continuous subject of research, this study presents new results concerning devices used to generate downforce and their influence in road vehicle aerodynamics. Thus, using a mass production car model, the study examines the effects of a rear wing and a short underbody diffuser on the main aerodynamic characteristics, lift and drag. The results show that the lift of the studied car decreases due to both diffuser and rear wing, more significantly due to the later one. Concerning the drag, it decreases only in the case of underbody diffuser.
Die Kategorie der Hochleistungsfahrzeuge umfasst eine Reihe ganz unterschiedlicher Automobile, nämlich:
Sportwagen; das sind für den Straßenverkehr zugelassene Fahrzeuge, die hohe Fahrleistungen bieten ohne dem Fahrer wesentliche Einschränkungen bezüglich ihrer Alltagstauglichkeit ab zu verlangen.
Rennwagen, deren ausschließlicher Zweck darin besteht, auf der Rennstrecke zur Austragung von Wettbewerben eingesetzt zu werden. Dazu zählen auch solche Autos, die aus Serienfahrzeugen abgeleitet sind.
Rekordfahrzeuge mit unterschiedlicher Zielsetzung wie etwa
höchste Geschwindigkeit;
niedrigster Verbrauch;
größte Reichweite;
Erprobung von Sonderantrieben.
Entsprechend differenziert sind auch die Anforderungen an die Aerodynamik. Gemeinsam ist all diesen Fahrzeugen, dass sie mit einem niedrigen Luftwiderstand auszurüsten sind. Sind sie für hohe Geschwindigkeiten auf kurvenreichen Kursen ausgelegt, tritt die Forderung nach niedrigem Auftrieb hinzu. Beide Forderungen stehen bezüglich der Maßnahmen, mit denen sie zu erfüllen sind, nicht selten in einem Widerspruch zueinander. Wie dieser auszutarieren ist, bildet einen Schwerpunkt der Darstellung.
Darüber dürfen die übrigen Aufgaben, die mit Hilfe der Aerodynamik zu lösen sind, nicht übersehen werden, als da sind:
Gewährleistung der Richtungsstabilität;
Verhalten bei Fahren im Windschatten;
Kühlung sämtlicher Aggregate;
Herstellung annehmbaren Komforts für den Fahrer.
Being a continuous subject of research, the authors present in this paper new aspects and results concerning the relevance of diffusers in vehicle aerodynamics. In the first stage of investigation, a generic car model with wheels was considered, starting from the Ahmed body, and the effect of adding wheels to the body was studied. Latter, the influence of diffuser length and angle, within ranges relevant to hatchback passenger cars, on drag and lift was studied for the Ahmed body with wheels. The results show that the addition of wheels and wheelhouses results in increasing of drag and lift due to the vortices which originate from the wheels. Concerning the underbody diffuser, the results show that both drag and lift of the bluff bodies with wheels are reduced with increasing diffuser length for moderate values of angle, up to six degrees for the cases studied.
In this paper, using the facilities offered by the ANSYS CFX, CFD code, the authors investigate numerically the flow around the Ahmed body for the rear slanted upper surface of 35°, fitted with a simple underbody diffuser, without endplates, in order to find the influence of the later one on the main aerodynamic characteristics, drag and lift. Relative motion between body and ground is simulated. The study is performed for different geometrically configurations, length and the angle of the diffuser being the parameters which are varied in ranges which are relevant for a hatchback passenger cars. Later, based on a theoretical approach, a coefficient of the equivalent hydraulic resistance of the diffuser is computed, which help to evaluate the drag due to underbody diffuser.
The fundamental aerodynamic influence of downstream wheels on a front wing flow field and vice versa has been investigated using generic wind tunnel models. The research has been conducted using a wing with a fixed configuration, whereas the wing ride height with respect to the ground has been varied as the primary variable. The overlap and gap between the wing and wheels have been kept constant within the context of the current paper. At higher ride heights the wheels reduce wing downforce and increase wing drag, whereas the drag of the wheels themselves also rises. At low ride heights, however, the opposite happens and the wing performance improves, while the wheels produce less drag. The ride height range has been subdivided into force regions with consistent characteristics throughout each of them. Force and pressure measurements, particle image velocimetry results, and oil flow images have been used to explain the differences between the force regions and to derive the governing flow mechanisms. The trajectories and interaction of vortices play a dominant role in the observed force behavior, both as force enhancing and reducing mechanisms. The effect of wheel circulation, flow separation, and flow channeling by the ground and by the wheels are among the other main contributors that have been discussed within this paper.
Methods of modeling vortex generation in computational fluid dynamics without mesh-ing the vortex generating device have been investigated. This is done by adding source terms to the governing equations to create vortices. Previous work in this area has focused on boundary layer control. This study looks at larger scale applications, such as using vortices for force enhancement. Two different approaches are tested. One is to model vortex generators directly, for which an existing method that replaces the force exerted on the fluid by a vortex generator with a source term is used. Also of this type, a simple im-mersed boundary method is used for comparison. The other approach uses source terms to create specified vortex velocity profiles. A method to add a continuous three-dimensional velocity is formulated and implemented in three ways; explicit calculation of the required forces from the Navier-Stokes equations, direct forcing (setting the velocity as boundary conditions), and penalty-type feedback forcing. After basic testing, all methods are applied in a practical engineering case using a commercial solver. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Unsteady Large Eddy Simulation (LES) is carried out for the flow around a bluff body equipped with an underbody rear diffuser in close proximity to the ground, representing an automotive diffuser. The goal is to demonstrate the ability of LES to model underbody vortical flow features at experimental Reynolds numbers (1.01 × 106 based on model height and incoming velocity). The scope of the time-dependent simulations is not to improve on Reynolds-Averaged Navier Stokes (RANS), but to give further insight into vortex formation and progression, allowing better understanding of the flow, hence allowing more control. Vortical flow structures in the diffuser region, along the sides and top surface of the bluff body are successfully modelled. Differences between instantaneous and time-averaged flow structures are presented and explained. Comparisons to pressure measurements from wind tunnel experiments on an identical bluff body model shows a good level of agreement.
We review the progress made during the last 30 years on ground effect aerodynamics associated with race cars, in particular open wheel race cars. Ground effect aerodynamics of race cars is concerned with generating downforce, principally via low pressure on the surfaces nearest to the ground. The "ground effect" parts of an open wheeled car's aerodynamics are the most aerodynamically efficient and contribute less drag than that associated with, for example, an upper rear wing. While drag reduction is an important part of the research, downforce generation plays a greater role in lap time reduction. Aerodynamics plays a vital role in determining speed and acceleration (including longitudinal acceleration but principally cornering acceleration), and thus performance. Attention is paid to wings and diffusers in ground effect and wheel aerodynamics. For the wings and diffusers in ground effect, major physical features are identified and force regimes classified, including the phenomena of downforce enhancement, maximum downforce, and downforce reduction. In particular the role played by force enhancement edge vortices is demonstrated. Apart from model tests, advances and problems in numerical modeling of ground effect aerodynamics are also reviewed and discussed.
Flow separation control using vortex generators on an inverted wing in ground effect is experimentally investigated, and its performance is characterized in terms of forces and pressure distributions over a range of incidence and ride height.Counter-rotating and co-rotating rectangular-vane type vortex generators are tested on the suction surface of the wing.The effect of device height and spacing is investigated.The counter-rotating sub-boundary layer vortex generators and counter-rotating large-scale vortex generators on the wing deliver 23% and 10% improvements in the maximum downforce, respectively, compared with the clean wing, at an incidence of one degree, and delay the onset of the downforce reduction phenomenon.The counter-rotating sub-boundary layer vortex generators exhibit up to 26% improvement in downforce and 10% improvement in aerodynamic efficiency at low ride heights.Chordwise pressure measurement confirms that both counter-rotating vortex generator configurations suppress flow separation, while the corotating vortex generators exhibit negligible effectiveness.This work shows that a use of vortex generators, notably of the counter-rotating sub-boundary layer vortex generator type, can be effective at controlling flow separation, with a resultant improvement in downforce for relatively low drag penalty.
This paper experimentally investigates the use of vortex generators for separation control on an inverted wing in ground effect using off-surface flow measurements and surface flow visualization.A typical racing car wing geometry is tested in a rolling road wind tunnel over a wide range of incidences and ride heights.Rectangular vane type of sub-boundary layer and large-scale vortex generators are attached to the suction surface, comprising counter-rotating and corotating configurations.The effects of both device height and spacing are examined.The counter-rotating sub-boundary layer vortex generators and counter-rotating large-scale vortex generators suppress the flow separation at the center of each device pair, while the counter-rotating large-scale vortex generators induce horseshoe vortices between each device where the flow is separated.The corotating sub-boundary layer vortex generators tested here show little evidence of separation control.Increasing the spacing of the counter-rotating sublayer vortex generator induces significant horseshoe vortices, comparable to those seen in the counter-rotating largescale vortex generator case.Wake surveys show significant spanwise variance behind the wing equipped with the counter-rotating large-scale vortex generators, while the counterrotating sub-boundary layer vortex generator configuration shows a relatively small variance in the spanwise direction.The flow characteristics revealed here suggest that counter-rotating sub-boundary layer vortex generators can provide effective separation control for race car wings in ground effect.
The forces and pressures on a generic bluff body in ground effect were investigated. The bluff-body model was equipped with interchangeable underbody diffuser ramps and side plates. Five different diffuser angles were tested: 5, 10, 15, 17, and 20 deg to the horizontal. The experiments were undertaken in a low-speed wind tunnel equipped with a moving ground. Load cells, pressure taps, and surface flow visualization were the techniques used to evaluate the flow field. The flow field is characterized by vortex flow and three-dimensional flow separation. A region of hysteresis was found for the 15, 17, and 20 deg diffusers. As the ride height is varied, five different flow types can be identified with three subtypes within the region of hysteresis. The force reduction phenomenon was found to be caused by both vortex breakdown and flow separation.
A numerical study on the flow past a square cylinder placed parallel to a wall, which is moving at the speed of the far field has been made. Flow has been investigated in the laminar Reynolds number (based on the cylinder length) range. We have studied the flow field for different values of the cylinder to wall separation length. The governing unsteady Navier–Stokes equations are discretized through the finite volume method on a staggered grid system. A SIMPLE type of algorithm has been used to compute the discretized equations iteratively. A shear layer of negative vortex generates along the surface of the wall, which influences the vortex shedding behind the cylinder. The flow-field is distinct from the flow in presence of a stationary wall. An alternate vortex shedding occurs for all values of gap height in the unsteady regime of the flow. The strong positive vortex pushes the negative vortex upwards in the wake. The gap flow in the undersurface of the cylinder is strong and the velocity profile overshoots. The cylinder experiences a downward force for certain values of the Reynolds number and gap height. The drag and lift are higher at lower values of the Reynolds number. Copyright © 2005 John Wiley & Sons, Ltd.
Underbody diffusers are used widely in race car applications because they can significantly improve the cornering capacity of the vehicle through the generation of a downforce. They are also likely to have a wider role in reducing the drag in road vehicles as it becomes increasingly important to reduce emissions of carbon dioxide.
This paper reports on a wind tunnel investigation, using a simplified bluff body model, into the effect of splitting a simple plane diffuser into multiple channels. Tests are reported for a range of diffuser geometries suitable for road and race car applications. The results for the lift, the drag, and the incremental changes to the lift-to-drag ratio are reported and discussed in terms of the underbody pressures.
While broadly similar trends to the single-channel plane diffuser are seen in the multiple-channel diffuser configurations, it was found that the effect of increasing the number of channels depended on the flow regimes present in the plane diffuser. At angles just above the plane diffuser optimum, where the flow is partially separated, the multiple-channel configurations give large improvements in the downforce with minimal increase in the drag, significantly extending the performance envelope. The pressure maps indicate that the gains occur through improved diffuser pumping and pressure recovery in both the inner and the outer channels.
Studies the flow underneath two types of 'ground effect' vehicles, plenum and venturi, noting application to racing car design. Discusses the differences between plenum and venturi flow noting the longitudinal vortex pair characteristics of the latter. A low speed wind tunnel was used to test the plenum and venturi models, and for some tests a moving belt ground plane was used. Presents results of pressure profile for the plenum model, noting importance of the inside pressure as a determinant of lift and drag. Shows how plenum pressure (nearly uniform) depends on relative gap sizes in high or low external pressure regions. Discusses static pressure distribution, flow visualization and lift/drag coefficient results for the venturi model. Examines how the lift and pressure distribution changed with modifications to the venturi model. (C.J.U.)
Results are presented from an experimental study of the lift, drag, pitching moment, and flow field of a series of rounded edge simple bluff bodies of various cambers and tapers. The bodies were proportioned to be similar to those of idealized ground vehicles such as automobiles, vans, and trucks. The models were tested with and without simulated wheels, underbody roughness, and proximity to a stationary and moving ground plane. The pitch angle was varied at zero yaw angle. The force and moment coefficients and flow visualization studies indicated the existence and importance of flow regimes characterized by a pair of trailing vortices on the leeward side of the body similar to those found over an inclined body of revolution and over slender delta wings. These flows can suppress bubble-type separation. The effects of a rough underbody are generally detrimental although less so if the rough surface is on the windward side. A moving ground plane was found to give significantly different lift and drag for small ground clearances characteristic of actual road vehicles.
The paper describes an experimental investigation concerning the effects of slanting the blunt base of three-dimensional bodies having either an axisymmetric or a rectangular cross section. It was found that base slant can have a very dramatic effect on body drag, particularly in a relatively narrow range of slant angles where the drag coefficient exhibits a large local maximum (overshoot). Detailed study of the flow showed that the drag maximum is related to the existence of two very different separation patterns at the rear of either body. One pattern is similar to that found behind axisymmetric bodies with no base slant, and its main feature is the presence of a closed separation region adjacent to the base. The other pattern is highly three-dimensional with two streamwise vortices approximately parallel to the slanted surface, one at each side of the body. The drag coefficient maximum occurs in the slant-angle range where a changeover from one flow pattern to the other takes place. The observed phenomenon may be thought of as being associated with a broader category of “critical geometries,” which is tentatively defined and discussed.
Performance data and flow characteristics for subsonic, two-dimensional, straight center line diffusers are presented. The four primary flow regimes which can occur are described and presented as functions of overall diffuser geometry. The performance of both stalled and unstalled diffusers is mapped for a wide range of geometries and inlet boundary layer thicknesses. An understanding of the relationships between flow regime and performance leads to a rational basis for diffuser design. The important maxima of performance and their location on the performance maps are presented. Both the range of data and correlations of optima of performance are extended beyond previous results.
A moving belt simulation of ground effect is necessary for tests in wind tunnels of road vehicle models. This need has been convincingly demonstrated using scale racing car models, scale saloon and sports car models and small scale truck models in a 2.1 × 1.7 m wind tunnel over the last twelve years. The development of new techniques enabled measurement of forces to be made on models with wheels rotating on a moving ground. The requirement for large models to increase Reynolds number and improve the model detail led to the construction of a large 2.4 × 5.3 m moving belt rig in a new 3.5 × 2.6 m wind tunnel. In order to measure forces and pressures on road vehicle models in crosswinds, a technique using yawed models with rotating wheels on a yawed moving ground was also developed.
Secondary flows in non-circular ducts are accompanied by a longitudinal component of vorticity. The equation of motion defining this component in a turbulent flow is composed of three terms giving the rates of production, diffusion and convection. Since the expression for production is the second derivative of Reynolds strees components, longitudinal vorticity cannot exist in laminar flow. For turbulent flow in a square duct the Reynolds stress tensor is examined in detail. Symmetry requirements alone provide relationships showing that the production is zero along all lines of symmetry. General characteristics of flow in circular pipes are sufficient to indicate where the production must be greatest. Experimental measurements verify this result and define the point density of production, diffusion and convection of vorticity. Data also indicate that the basic pattern of secondary flow is independent of Reynolds number, but that with increasing values of Reynolds number the flows penetrate the corners and approach the walls. A similar experimental investigation of a rectangular duct shows that the corner bisectors separate independent secondary flow circulation zones. Production of vorticity is again associated with the region near the bisector. However, there is some evidence that the secondary flow pattern is not so complex as inferred from the distortion of the main longitudinal flow.
It is no longer acceptable, in most circles, to present experimental results without describing the uncertainties involved. Besides its obvious role in publishing, uncertainty analysis provides the experimenter a rational way of evaluating the significance of the scatter on repeated trials. This can be a powerful tool in locating the source of trouble in a misbehaving experiment. To the user of the data, a statement (by the experimenter) of the range within which the results of the present experiment might have fallen by chance alone is of great help in deciding whether the present data agree with past results or differ from them. These benefits can be realized only if both the experimenter and the reader understand what an uncertainty analysis is, what it can do (and cannot do), and how to interpret its results.This paper begins with a general description of the sources of errors in engineering measurements and the relationship between error and uncertainty. Then the path of an uncertainty analysis is traced from its first step, identifying the intended true value of a measurement, through the quantitative estimation of the individual errors, to the end objective—the interpretation and reporting of the results. The basic mathematics of both single-sample and multiple-sample analysis are presented, as well as a technique for numerically executing uncertainty analyses when computerized data interpretation is involved.The material presented in this paper covers the method of describing the uncertainties in an engineering experiment and the necessary background material.
The kinematic and fluid-mechanic boundary conditions in underbody flow simulation
- G Sovran