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Aerodynamic Effects of Shape, Camber, Pitch, and Ground Proximity on Idealized Ground-Vehicle Bodies
Abstract
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.
... A key milestone in the understanding of ground effect diffuser flows was the discovery, aided by flow visualization, of counterrotating vortices near the side edges of the diffuser [10]. It was shown that these vortices, shown in Fig. 3, not only help to prevent or delay flow separation at the sharp diffuser inlet edge [7,9,10,[12][13][14][15] but also directly contribute to downforce generation by inducing ...
... A key milestone in the understanding of ground effect diffuser flows was the discovery, aided by flow visualization, of counterrotating vortices near the side edges of the diffuser [10]. It was shown that these vortices, shown in Fig. 3, not only help to prevent or delay flow separation at the sharp diffuser inlet edge [7,9,10,[12][13][14][15] but also directly contribute to downforce generation by inducing ...
... George [10] found that the relationship between downforce and rake angle is approximately linear for angles between 630 deg, for a body in the freestream with a 20 deg diffuser and no end plates. Furthermore, significant changes in the flow behavior were reported, including the formation of counter-rotating vortices along the entire length of the body at rake angles of 10 deg and above. ...
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.
... However, most of the studies mentioned above made use of a stationary ground boundary condition [4,5,7,8]. In a series of wind tunnel experiments on idealized groundvehicle buff bodies, George [10] found that there is a significant difference in the lift and drag characteristics of the models between stationary and moving ground. The effect of the boundary condition was also investigated by Yang et al. [9], who investigated the use of three different boundary conditions for the ground plane. ...
... Although there is a discrepancy between the results obtained from the wind tunnel and the CFD model, the effect of the moving ground boundary condition can easily be modelled numerically. Since, according to the literature [10,11], a moving ground boundary condition is preferred, the wall boundary condition in the CFD model was adapted to the free-stream velocity. ...
... Furthermore, the addition of a moving ground plane and a track wall have been investigated for the AeroCity model. Based on reports available in the literature [10,11,22], the influence of the ground boundary condition on the aerodynamic performance of the WIG vehicle is significant. However, the current numerical results suggest that this may not be true in all cases. ...
The AeroCity is a new form of transportation concept that has been developed to provide high-speed ground transportation at a much lower cost than the existing high-speed railway. Utilizing the Wing-in-Ground (WIG) effect, the AeroCity vehicle does not require complex infrastructures like other contemporary concepts, such as the Hyperloop or Maglev trains. In the current work, the aerodynamic characteristics of the AeroCity vehicle are examined through a Computational Fluid Dynamics (CFD) analysis. The results from the CFD analysis qualitatively match with the findings of wind tunnel experiments. Surface streamlines and boundary layer measurements correspond well with the numerical data. However, the force measurements show a discrepancy. It is found that the separation bubble over the side plates is not captured by the CFD, and this is responsible for an under-prediction of the drag at higher free-stream velocities. The Transition SST model improved the matching between the experiments and numerical simulations. The influence of the moving ground is numerically investigated, and the effect of non-moving ground on the vehicle aerodynamics was found not to be significant. Finally, the inclusion of the track wall is examined. It is found that the merging of the wingtip vortices is responsible for a significant drag increase and, therefore, an alternative track geometry should be investigated.
... 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]. It was shown that these vortices, depicted in Fig ...
... Vitally for automotive applications, wind tunnel measurements showed that the diffuserassociated drag penalty was minimal [106]. This is explained partly by ground effect and partly by the diffuser pumping phenomenon, both of which induce low pressure mainly under the flat section of the underfloor, which is parallel to the freestream and therefore does not contribute to drag. ...
... George [106] found that the relationship between downforce and pitch angle is approximately linear for angles between ±30°, for a body in the freestream with a 20°diffuser and no end plates. Furthermore, significant changes to the flow patterns were reported, including the formation of counter-rotating vortices at pitch angles of 10°and above. ...
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.
... The aerodynamic performances of diffusers in ground effect have been investigated experimentally by force balance measurements (George, 1981;Ruhrmann and Zhang, 2003) as well as by surface pressure measurements (Senior, 2002;Ruhrmann and Zhang, 2003). Oil flow visualization and Laser Doppler Velocimetry (LDV) measurements have been proven useful to understand the diffuser flow topology (Senior, 2002;Cooper et al., 1998). ...
This investigation proposes a novel LPT facility featuring Helium-Filled Soap Bubbles flow tracers, LED illumination and two high-speed cameras to characterize the dominating flow patterns within automotive underbodies. A remote control (RC) car model, fitted with custom-made floor and diffusers, traverses a region of seeded air following the Ring of Fire methodology. Underground-placed cameras view the car through a transparent panel, providing unparalleled optical access to the underbody of the car. The on-site measurement setup and the interaction between car model and ground enhance the realism and fidelity of the experiments, while potentially reducing testing costs associated with wind tunnel operation. The setup is shown to be a valid alternative to conventional testing approaches to capture flow separation, 3D flow evolution and differences in the flow field between the four tested configurations, whereby the diffuser angle was varied in the range between 5° and 20°. The 15° diffuser led to the largest velocity and pressure peaks under the car, whereas the 10° diffuser produced the most downforce thanks to the diffuser “pumping” effect, leading to a large region of low pressure under the vehicle. Notably, the 20° diffuser featured the most prominent flow separation at the diffuser’s leading edge, heavily affecting its ability to sustain low pressures under the car. The results show that the wide tyres have a major impact on the underbody flow, because their large wakes induce mass flow leakage through the sides of the car, thus disrupting the mechanism of downforce generation and impairing the generation of streamwise vortices.
... This is followed by a drop in speed by the expanding flow region without causing flow separation. George [1981] studied the effect of ground clearance on downforce using a diffuser angle of 10ᵒ, and found that as the ground clearance is decreased, downforce increases until it hits the critical peak value. ...
The role of aerodynamics has always been an important factor in motorsports.
Since the front wing of a car faces the freestream first, the front wing plays the key
role in controlling the flow on the downstream components and generating downforce.
The front wing generates about thirty percent of the car’s total downforce.
The scope of this study is to investigate the performance of a front wing using
different arrangements. The effects of ground clearance, angle of attack and Reynolds
number on the front wing is studied using numerical methods. In addition to that, the
effectiveness of the vortex generators on the prevention of flow separation occurring
with the increased angle of attack is observed.
Providing insight about the aerodynamics of a front wing under ground effect is
the aim of this study. Generating higher downforce at higher ground clearances with
increased angle of attack and vortex generators can be valuable for the wing design
against the limiting regulations.
... Shadmani, et al. [18] experimentally demonstrated that drag force of Ahmed body with plasma actuator system situated in the center of the rear slant surface can be decreased by 3.65% and 2.44% in steady and un steady actuations respectively. Additionally, the impact of ground clearance had been studied a lot as in [19][20][21][22][23][24][25][26] The impact of car modification on reducing the drag had been investigated a lot in terms of changing the shape of the geometries utilizing different turbulence model. The baseline model in this study is Ahmed body, as the most famous geometry in aerodynamic of road vehicles. ...
In overall car performance and ride stability, external car aerodynamic study is of great importance, making it a key element in effective automobile design. In this study, the effect of the vehicle's length on the drag coefficient was numerically investigated. For this purpose, a CFD analysis based on RANS turbulence models was carried out for six configurations for the Ahmed body model with different length in addition to the baseline model. Tetrahedral cells were adopted throughout air enclosure except some prism cells around the vehicle's surfaces. Good agreement for the benchmark model was obtained by comparing current numerical results with experimental related data. The numerical results demonstrate that 1244mm is the optimal length of the Ahmed body. Increasing the overall length of the Ahmed body by about 19.15% leads to decreasing drag coefficient by 8.95%.
... SOFV is a very popular method for troubleshooting poor flow behavior and it has been used for more than 50 years in aerodynamic research. This visualization technique involves a paint mixture, usually consisted of oil and dye and it has been used by a number of authors in low [3,10] or high speed experiments [5,14,16] due to the advantages of easy implementation and inexpensiveness. Dye is constituted by very fine pigments of Titanium Dioxide (TiO2) or fluorescein sodium in various colors. ...
Flow visualization is one of many available tools in experimental fluid mechanics and is used from the primary stages of fluid mechanics research in order to identify the physical sizes and locations of the flow features under consideration. Most of the fluids used in engineering applications are transparent (water, air, etc) and flow visualization techniques are used in order to make their flow patterns visible. Simple flow visualization experiments are relatively inexpensive and they can be easily implemented providing with a first feeling of the characteristics of the flow domain. Subsequently, flow visualization techniques are of great applicability in complex flow fields and especially in turbomachinery applications where the flow is characterized by three dimensional and secondary flow patterns. In general, fluid motion can be visualized by surface flow visualization, by particle tracer methods or by optical methods. The former flow visualization technique reveals the streamlines of fluid flows around a solid surface. In this paper flow visualization techniques applied in two different cases of experimental testing (fans in crossflow and cascade experiment) are presented. In both cases, the mixture of paint was prepared using a highly volatile light mineral or heavy machine oil of viscosities of approximately 100cP and 200cP , respectively, together with very fine pigments of Titanium Dioxide (TiO 2 ) or fluorescein sodium in various colors. After the preparation of the mixture, a homogenous thin film was applied onto the whole plate surface by painting it with a soft brush. The air stream which flows over the surface of the plate, modifies the concentration and the homogeneity of the oil film, according to the flow conditions very close to the wall. The film was dried by the airflow and photographed for further consideration while the time taken for drying depended on the wind tunnel velocity as well as, on the pigmentation of the mixture. Successful and un-successful flow visualization tests are herein presented while each case is respectively commented as far as the mixtures, the proportions used and the application onto the rigs are concerned.
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.
The diffuser is a critical component in sports cars, enhancing aerodynamics by increasing downforce and reducing drag. Previous studies have focused on its dependence on diffuser incidence, height, and base pressure. The design of the car, particularly the rear end shape and the rear wing's presence, affect base pressure and the diffuser's performance. Previous studies have investigated the effects of diffuser geometry on aerodynamic performance, but the current study is the first to examine the relationship between the diffuser and the rear tires. It also provides specific and quantitative results on the impact of different diffuser design parameters on drag and downforce. The relationship between the rear tires and the double-element inverted wing diffuser using computational fluid dynamics (CFD) was investigated. This is an essential problem because the diffuser is a critical component of sports cars, and its design can significantly impact aerodynamic performance. CFD was used to simulate the flow of air around the car model. The CFD model was based on the Nissan Sunny (Versa) type Almera design, and the diffuser main element and flap wing angles were set at 4 and 15.5°, respectively. The flap gap, overlap distance, and wing ride height above the ground were varied to achieve an optimal aerodynamic design. The study found that the wing's ride height significantly influences the flow through the diffuser. The diffuser significantly impacts base pressure and downforce production. Increasing the ride height decreases base pressure, leading to an increase in downforce until a specific point near the car body, where downforce further increases. The study concluded that the best double-element diffuser design was selected based on lift-to-drag results and the allowable dimensions of the car, wing ride height, element gap, and overlap distances. Ultimately, the best diffuser wing design features a ride height of 154 mm, a gap distance of 10 mm, and an overlap of 5 mm. This design reduces drag by approximately 2.7 % and remarkably increases downforce ten times compared to the baseline car model.
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