Vortex generators are passive devices that are used to reduce the flow separation on the suction side of the wing. This work summarises the use of vortex generators on a front wing of a Formula Student race car. As the front wing is very sensitive in terms of ground effects, a minute difference in the height-to-chord ratio of the wing can influence the downforce and drag significantly. When a car dives (pitch in the negative ordinate), there is a flow separation on the front wing, which can be overcome by the use of vortex generators. A team from the Vellore Institute of Technology, Vellore attempts to improve the aerodynamic efficiency of a multi-element front wing, which in turn, improves the cornering performance of the car.
For the last five decades, aerodynamics has been a major area of research in the automotive industry. The effort has been to reduce drag and increase the normal load on tyres, with the aim of increased grip without the corresponding addition of mass in race cars. The amount of grip available at the tyres along with aerodynamic drag and engine power set the theoretical limits for the vehicle's velocity around the track, especially while cornering. It is thus of particular interest when designing a vehicle to increase this grip, while keeping weight to a minimum.
Over the years, rules and regulations have been imposed by SAE to keep these aerodynamic advantages on a reasonable scale as technology has improved. In the design of an open wheel formula race car, one is faced with numerous instances of complex geometry, for example, rotating wheels, A-arms and the cockpit with a driver. This nature of the vehicle makes it difficult, if not impossible, to approach the problem of aerodynamic optimisation of the entire car analytically. It is due to these difficulties that methods of simulation such as computational fluid dynamics (CFD) commercial codes have been developed to aid research.
A number of experimentally-investigated studies for the inverted wing in ground effects have been previously carried out [5-11]. Typical features of a single-element wing in ground effect are described by Zerihan and Zhang using a moving ground [5, 6]. Their study showed not only an increase of downforce as the ride height is reduced but also captured the downforce reduction phenomenon, when the wing is mounted below the height where the maximum downforce is generated .
Flow separation is induced by high adverse pressure gradients, where the momentum of the flow along the chord is reduced, and the flow separates from the surface of the wing [3, 4]. At a moderate ride height, flow separation was observed near the trailing edge of the wing, and as the wing is brought close to the ground, the region of the flow separation increases, resulting in the loss of the downforce.
Characteristics of the edge vortices with respect to the ride height were also studied . It was shown that the edge vortices break down when the wing reaches maximum downforce height. The investigations suggest that separation control on the suction surface of a wing and a development of end plates could improve the aerodynamic performance of the wing, controlling the flow and edge vortices, leading to higher downforce or more efficient downforce-to-drag ratio. Flow separation, in general, is generated downstream of maximum suction point, a thicker turbulent boundary layer, yielded by the separation, induces a significant increase in drag, thus, adversely affects the performance of the wing.
A number of separation control methods have also been investigated. Lin suggested that effective devices for separation control are those that generate vortices along the direction of the flow such as those produced by vortex generators [12, 13].
Vortex generators commonly used for aerospace applications are attached upstream of the separation line and generate vortices, which accelerate mixing of momentum of the free stream and the flow in a boundary layer in order to overcome the large adverse pressure gradient. Sub-boundary layer vortex generators have a height between 10-50 % of the boundary layer thickness, and are more advantageous in terms of effectiveness and give less drag compared with large-scale vortex generators, which have device height of the same order as the boundary layer thickness .
This paper investigates the contribution of vortex generators on aerodynamic performance of an inverted front wing in ground effect by using CFD. The primary aim was to develop a multi-element wing system that provides better performance in pitching conditions. Counter rotating and co-rotating vortex generators with different heights, shapes and spacing are simulated on the suction surface of the wing.
1. Vehicle Pitch data
Kept at a height of 50 mm from the ground, the wing generated the maximum downforce. If the height is reduced beyond this, it results in a drop in the downforce value, which is in accordance with results obtained by Zhang and Zerihan .
The front and rear vibrational frequencies were iterated based on front wing deflection and within the range of human resonance frequency (2-4 Hz), according to ergonomics. First, the rear frequency was assumed at 3.2Hz and the front frequency was iterated in the range of 2-4 Hz by plotting it against front wing deflection (1). The value of 3.4 Hz was selected based on the human frequency range and vertical deflection of the front wing. Then keeping front frequency fixed (3.4 Hz) and iterating rear frequency in the range of 2-4 Hz by plotting it against pitch angle, the angle was calculated by taking moment of the car about the ground, and the value of 3.12 Hz was fixed. At last, the frequencies were finalised according to the availability of springs in market. Selected rear ride frequency was 3.1 Hz. Hence, the value of pitch is 0.02377 radians or 1.362 degree, (2).
2. Computer Aided Design
The car model shown in (3) is used to carry out the CFD simulations. Half-body model is used as the car is symmetric. Unnecessary body parts such as the rear wheel, rear wing, engine, etc. are omitted to reduce the mesh size and hence computational power required. The VG on the wings bottom surface is shown in (4).
3. Simulation Domain
All simulations were run on the Star-CCM+ software. Quarter-body model is used to reduce the simulation time; the body is symmetrical about the longitudinal axis while the components towards the rear of the car were not relevant to the study. Creating an appropriate domain is necessary to avoid any interaction of its walls and the vehicle, which can result in inaccurate results.
Blockage ratio should be maintained below 5 %. The domain size is five times the length of the car at front, 10 times the length of the car at rear and five times the length of the car from the side, (5).
Generating a good quality mesh is the answer to accuracy and stability of numerical computations. For volume mesh model, trimmer mesh is used. (5) shows the trimmer mesh formed on the car. The mesh has 8,136,706 cells, 24,151,961 faces and 8,854,852 vertices. The next most important part of setting-up the simulation is assigning boundary conditions. The first step is to set-up the physics model for simulation. The physics continuum used for doing computational analysis and defining the boundary conditions for the same is detailed in (6).
(7) describes the VG geometry parameters that were changed to optimise the performance. Additional parameters that were varied are include number of VG, position of VG on the airfoil, shape of the VG, and co-rotating & counter rotating arrangement. Upon varying the thickness of the VGs, it was observed that the downforce drops with increasing thickness (8). The optimum thickness was 2 mm, as it wasn’t possible to evaluate below that due to manufacturing constraints.
Upon varying the length of the VGs, it was observed that the downforce drops with increasing length. Peak downforce was observed at 35 mm length, while the drag variance with changes in length were negligible. Upon varying the angle of the VGs, peak downforce was observed at 22-degree angle to the longitudinal centreline of the car, beyond which a rapid drop in the downforce was observed. Drag variation remained negligible with no clearly observable pattern. Here, counter and co-rotating arrangements were simulated, which resulted in the co-rotating VG energising the Vortex in a much better manner than counter rotating.
The taper of the thickness of the VGs was varied, and it was observed that the peak downforce and minimum flow separation occur when the thickness is halved towards the rear of the VG. The height at the front of the VG was 2 mm, resulting in the rear height to be 4 mm. The position of the VGs was also moved along the chord of the aerofoil to determine the optimal position for placing the VG along the chord. It was observed that maximum downforce and minimum flow separation occur when the VGs are placed at the maximum camber position. The VGs were placed at 27.85 mm, which was the point of separation of air from the wing behind the max camber. The results, however, did not show any improvement. After post processing, it was found that the VG required maximum velocity of air to utilise that energy to swirl.
The aero package had two types of element systems. The number of VG was fixed at 12 and it was observed that the 3 element system in the aero package had enough pressure recovery that it didn’t require a VG to remove separation. Hence, all these VGs were placed in under the 4 element system.
Meanwhile, the Pressure Difference gives a quantitative measure of the strength of the vortex. More the pressure difference, more is the swirl of the air around the geometry. Hence, the best performing VG was the symmetric airfoil. (9) shows all the different geometries that were used to iterate.
The final result of the study found that a wing achieved a steeper pressure gradient during pitch and uniformly brought it back up to ensure no separation occurs. It is observed from the study that vortex generators can have a positive effect on the overall performance of an inverted multi-element wing operating in ground effects. The co-rotating vortex generator with symmetric airfoil gave the best performance of all the iterated shapes, heights and spacing.
The wake formation had significant drop due to implementation of VG, (10). It is also to be noted that the investigation only included straight line performance and no yawed situations were simulated, which obviously limits the conclusions that can be drawn. However, it is important to realise that CFD is an approximated representation of reality and on-track validation is necessary to investigate the actual performance of the wings.
Wind tunnel validation shall be done to validate the findings. Actual car data for yaw, roll and pitch from on-board data acquisition systems like IMU sensors and accelerometer are being collected to get a better understanding of the vehicle performance. Further, on-track methods like flow visualisation would be done.
 Gad-el-Hak, M., 1990, “Control of Low-Speed Airfoil Aerodynamics,” AIAA J., 28, pp. 1537–1552.
 Gad-el-Hak, M., and Bushnell, D. M., 1991, “Separation Control: Review,” ASME J. Fluids Eng., 113, pp. 5–30.
 Zerihan, J., and Zhang, X., 2000, “Aerodynamics of a Single Element Wing in Ground Effect,” J. Aircr., 37[6[, pp. 1058–1064.
 Zhang, X., and Zerihan, J., 2003, “Off-Surface Aerodynamic Measurements of a Wing in Ground Effect,” J. Aircr., 40[4[, pp. 716–725.
 Zhang, X., and Zerihan, J., 2003, “Aerodynamics of a Double-Element Wing in Ground Effect,” AIAA J., 41, pp. 1007–1016.
 Zhang, X., and Zerihan, J., 2004, “Edge Vortices of a Double-Element Wing in Ground Effect,” J. Aircr., 41, pp. 1127–1137.
 Zerihan, J., and Zhang, X., 2001, “Aerodynamics of Gurney Flaps on a Wing in Ground Effect,” AIAA J., 39[5[, pp. 772–780.
 Soso, M. D., and Wilson, P. A., 2006, “Aerodynamics of a Wing in Ground Effect in Generic Racing Car Wake Flows,” Proc. Inst. Mech. Eng., Part D [J. Automob. Eng.[, 220, pp. 1–13.
 Coe, D., Chipperfield, A., and Williams, C., 2006, “Transient Wing in Ground Effect Aerodynamics: Comparisons of Static and Dynamic Testing,” Sixth MIRA International Vehicle Aerodynamics Conference, pp. 404–410.According to information in personal communication with Charles F. Gidcumb, McDonnell Douglas, Long Beach, CA, 1996
 Lin, J. C., 2002, “Review of Research on Low-Profile Vortex Generators to Control Boundary-Layer Separation,” Prog. Aerosp. Sci., 38_4–5_, pp. 389–420.
 Lin, J. C., 1999, “Control of Turbulent Boundary-Layer Separation Using Micro-Vortex Generators,” AIAA Paper No. 1999-3404.
 Lin, J. C., Howard, F. G., and Selby, G. V., 1990, “Small Submerged Vortex Generators for Turbulent Flow Separation Control,” J. Spacecr. Rockets, 27_5_, pp. 503–507.
ANIRUDH GANESH SRIRAAM is a Student at VIT Vellore
DINESH MOHITE is a Student at VIT Vellore