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Flow patterns, pressures, and forces on the underside of idealized ground effect vehicles.
Abstract
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.)
... 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 ...
... Further experiments and computational fluid dynamics simulations confirmed that the effect of reducing the ride height of a diffuserequipped bluff body in ground effect is an increase in downforce at a growing rate [3,6-9, 11,17,[19][20][21]. This is mainly the result of increased pressure recovery, which results in a stronger suction peak at the diffuser inlet, and stronger vortices [6-9, 11,12,20]. However, Cooper et al. and Jowsey and Passmore [3,20] also observed a sharp change in the streamwise pressure recovery rate in the vicinity of the leading edge of the underfloor. ...
... At even lower ride heights, downforce continues to increase despite the presence of the separation bubble, which gradually moves toward the diffuser inlet. At the critical ride height, the separation bubble is swept to one side, causing asymmetric vortex breakdown and flow separation at the inlet over a substantial part of the diffuser width, resulting in a large recirculation region and a significant loss of downforce [6-9, 11,12,17]. Ehirim et al. [8,9,18] also observed that the direction of the asymmetric stall depends on the relative strength of the vortices prior to breakdown, with the stronger vortex surviving the subsequent stall. Furthermore, Ruhrmann and Zhang,Zhang et al.,and Ehirim [7,11,22] observed that the process of downforce loss is subject to significant hysteresis at diffuser angles of 15 deg and above, as the vortices reform and reattach the flow at higher ride heights than when they break down. ...
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.
... George [106] added wheels to his model, and observed strengthened vortical flow and an increase of the critical diffuser angle up to about 20°, whereas adding roughness to the flat underfloor had the opposite effect. In a follow-up study, George and Donis [107] showed that as the underbody was sealed with side skirts, the vortices could not form, leading to flow separation inside the diffuser and a decrease in downforce. However, this only occurred at diffuser angles of 10°and above, as at lower angles the flow inside the diffuser remained attached despite the absence of vortices, due to the sufficiently weak adverse pressure gradient. ...
... . 2.16, not only help to prevent or delay flow separation at the sharp diffuser inlet edge[103,[105][106][107][108][109], but also directly contribute to downforce generation by inducing low-pressure regions on the sides of the diffuser surface[102,105,107,110]. The vortices were also shown to grow in size while moving inboard and upwards as they propagate downstream through the diffuser channel, and in some cases to detach from the diffuser surface[104,109,111,112]. ...
... . 2.16, not only help to prevent or delay flow separation at the sharp diffuser inlet edge[103,[105][106][107][108][109], but also directly contribute to downforce generation by inducing low-pressure regions on the sides of the diffuser surface[102,105,107,110]. The vortices were also shown to grow in size while moving inboard and upwards as they propagate downstream through the diffuser channel, and in some cases to detach from the diffuser surface[104,109,111,112]. ...
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.
... A different flow behavior of the longitudinal vortices was subsequently reported, however, by George and Donis [27] when they investigated the influence of side skirts on the vortices. As observed from the wind tunnel investigations, the "Venturi" bluff body model-consisting of a Venturi-like diffuser passage ( Fig. 11)-had an effect on the diffuser flow. ...
... Surface pressure, particle image velocimetry (PIV) and laser Doppler anemometry (LDA) measurements indicated that as the ride height of the diffuser was reduced toward its maximum-downforce height, downforce enhancement was accompanied by increasing suction at the diffuser inlet, enhancement of streamwise flow velocity and low pressure along the lengthwise sides of the diffuser due to the strengthening of the longitudinal vortices as reported in Refs. [26] Fig. 10 Bluff body geometries as studied by George [26] (dimensions in mm) Fig. 11 Bluff body geometry of the "Venturi" model as studied by George and Donis [27] 3 Throughout this paper, C L is þve upwards so ÀC L represents downforce. and [27]. ...
... [26] Fig. 10 Bluff body geometries as studied by George [26] (dimensions in mm) Fig. 11 Bluff body geometry of the "Venturi" model as studied by George and Donis [27] 3 Throughout this paper, C L is þve upwards so ÀC L represents downforce. and [27]. At the maximum downforce ride height, the strong vortices are attached at the early part of the diffuser ramp but toward the diffuser exit they become diffused and detached from the ramp. ...
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.
... The second objective of the investigation was to explore the influence of vortices on downforce gains and stall prevention. Previous studies [3,[5][6][8][9][10] have identified them as critical in underbody operation as well as closely related to underbody "stall" (or the loss of lift along the body as it approaches the ground). ...
... This over-prediction of C D is found to be a common trend and is discussed later on in this paper FRONT WING DESIGN -The same basic four element design was incorporated again in design of the front wing package. Another method to gain higher lift coefficient values is putting the front wing in ground effect, as discussed by Soso & Wilson [1,15] and other authors [2,3,5,[8][9][10][11][12]. However, the concern rose that the height of the wing assembly would change as the car maneuvers in a race. ...
... Some underlying conclusions are briefly presented below. These are developed more by various authors, who also offer suggestions to mitigate negative flow effects [2,3,9,10]. ...
... However, at a pitch angle of -10° with a 10° diffuser ramp, the longitudinal vortex flow was established by the upstream separating free shear layer. George and Donis (1983) investigated the aerodynamic effect of diffuser endplates and discovered that when the endplates for 10° and 15° diffusers are sealed to the ground, the diffuser inflow induced by the longitudinal vortex pair is weaker and there is less downforce, but the opposite effect is observed when the diffuser endplates are unsealed. Cooper et al. (1998Cooper et al. ( , 2000 investigated the relationship between diffuser length and area ratio under fixed ground and moving ground conditions using a bluff body with two diffuser lengths (25 % and 75 % of bluff body length). ...
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.
... ii. A venturi-or diffuser-type flow including vortices at low ride heights [George and Donis (1983)]. At low ride heights, the diffuser angle becomes relevant and separation effects dominate the flow structure. ...
This paper summarizes an experimental study of an isolated bluff body in ground effect and the same body with the addition of nearby non-rotating wheels. First, theoretical and experimental trends relating to ground proximity and diffuser mechanics are reviewed. Next, experimental forces and flow patterns for a body alone were found, resulting in a maximum lift coefficient of approximately 0.80. Subsequently, the addition of stationary wheels, not attached to the body, significantly diminished the downforce generation by as much as 65%. Quantitative trends as well as tuft and neutrally buoyant bubble flow observations were carried out to infer the appropriate flow physics. Specifically, it is concluded that the wheels decrease body downforce by impeding the creation of strong vortices in the diffuser, deflecting flow in a potential manner, and introducing energy dissipating wake turbulence into the diffuser. Comparison to computational work performed by Desai, et al. (2008) correlated well with the wind tunnel data and subsequent conclusions. Future work is proposed.
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.
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.
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.
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.
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