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Cfd study of formula 1 shark fins effect on the aerodynamic performance and yaw stability

A small research carried out as a part of one of my MSc modules, focused on analysing the impact of these "polemic"
devices over the aerodynamic performance of the rear wing and general stability of a F1 racing car.

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Cfd study of formula 1 shark fins effect on the aerodynamic performance and yaw stability

  1. 1. CFD study of Formula 1 Shark fins: Effect on the aerodynamic performance and yaw stability ABSTRACT This year, Formula 1 has brought back to life an aerodynamic device which disappeared a couple of years ago: The shark fin over the engine cover. It is clear that this element has a big impact on the aesthetics of the car. In this research, a CFD study of the effect of different shark fins has been undertaken in order to establish a quantitative analysis of the effects on car stability and rear wing performance that the device produces. A baseline Formula 1 car was designed and imported to CFD software Star CCM+. Two different fins were added to a baseline car without the element. All three geometries were tested in three different positions related to the air flow: 0º, 4º and 8º of yaw angle. Forces and coefficients of the rear wing were extracted in order to make a comparison of each car at a certain yaw condition. Pressure distribution and shear stress on the wing surfaces, as well as wake vorticity were analyzed with the aim of build a clear picture of how the introduction of shark fin devices impact on aerodynamic performance at the rear end of the car. Lateral forces and yaw moment coefficients generated by the shark fin were extracted from the simulations and results of the three cars were again compared at each yaw scenario. Results shown a loss of downforce generated by the rear wing on straight line performance when the element is introduced. However, as yaw angle is increased, losses are reduced and flow tends to be uniform towards the rear end. Moreover, the stability is highly improved as a consequence of shark fin introduction. INTRODUCTION A racing car changes continuously as it is moving along the track. How would aerodynamics change around it? Yawed flows tend to change the balance of the car and can lead to a loss in lateral stability and poor handling. Many researchers studied the performance changes produced when the car is cornering. (Milliken, Dell'Amico and Rice, 1976) carried out a steady-state analysis of car lateral stability. Nevertheless, all of them focused their work on passenger cars. (Howell, 2015) performed a wing tunnel test in order to determine how drag coefficient varies with changes in yaw. Other effects of cornering, such as side forces, are taken into account by Okada et al. (2016) to determine Drag variations using CFD analysis. But cornering is not the only reason why the stability of a racing car can be compromised. (Theissen et al., 2011) pointed out that a deep knowledge of car’s behaviour under crosswinds is essential in order to evaluate car performance. (Broadley and Garry, 2000) studied the instability derived from the braking manoeuvre. This year, Formula 1 cars have introduced a new element close to the rear end, with the aim of improving yaw stability: An aerodynamic element called “Shark fin”. (Dahlberg, 1999) found that the front end of the car experiences a higher lateral force than the rear one under yawed flows. The main aim of this research is to assess through CFD simulation the real effect over the aerodynamic forces generated as a consequence of the introduction of different shaped shark fins, as well as to define the role that the device plays in the lateral stability of the vehicle. The introduction of the shark fin should improve significantly car stability under yaw conditions. Will the “Shark fins” stay in formula 1 longer? With the purpose of answer this question, this research was undertaken. CFD MODEL GEOMETRY A Formula 1 car body and wing were designed using Solidworks according to the dimensions within the 2017 Formula 1 technical regulations. Two different shark fin models were added to the baseline geometry for a total of three cars to test (Figure 1). Figure 1: No fin, Spine fin and Square fin geometries
  2. 2. CAD models were imported into Star CCM+. All the geometries were combined into different groups according to its relevance in the final results (e.g., Wing was divided into upper and lower surfaces, whereas the body of the car was one single part). MESH VALUES Since the geometry had to be tested in different positions, surface wrapper technique was applied to the model. Another chosen model was Trimmer. The combination of both models was translated into a reduction of computational time spent as expecting regarding results obtained by Pachpund et al. (2012). In terms of mesh size, three different volumes were added to the main geometry with the aim of gently reduce mesh size towards the rear end of the car. Wing and fin mesh sizes were refined since they are goal parts of the research. An example of the refined mesh can be seen in figure 2. Moreover, a contact prevention set was applied to wing planes, in order to avoid the gap to be closed. Figure 2: Example of mesh refined at rear wing BOUNDARY AND PHYSICS CONDITIONS Inlet boundary was given a velocity of 60m/s and outlet was defined as a flow-split boundary type. The flow is considered turbulent. The k-epsilon model was chosen as a turbulence model for all simulations. (Shetty and Patil, 2013) implemented this model in a CFD analysis of the external aerodynamics of a car with positive results. CONVERGENCE Results had been considered as full converged when all residuals were below 1E-03. Results converged after 4000 iterations. RESULTS AND DISCUSSION 0º YAW CASE Forces in the rear wing Forces and force coefficients were extracted from the simulation and can be found in Tables 1 and 2. Results revealed downforce loss of 3.7% in the spine fin case, and about 7.3% whit the use of the Square fin. Furthermore, drag force slightly decreased in both cases. Since downforce reduction is considerable, analysis of the flow was undertaken in order to understand the reason. No fin Spine fin Square fin Yaw CL CD CL CD CL CD 0º 4.610 0.509 4.441 0.471 4.297 0.472 4º 3.675 0.482 3.416 0.465 3.585 0.451 8º 2.736 0.522 2.673 0.482 3.040 0.458 Table 1: Rear wing force coefficients No fin Spine fin Square fin Yaw DF Drag DF Drag DF Drag 0º 1392.2 153.7 1340.7 169.7 1297.3 142.4 4º 1341.3 175.8 1246.4 169.7 1308.2 164.7 8º 1167.1 222.5 1140.2 205.5 1295.6 195.2 Table 2: Rear wing aerodynamic forces (Newtons) Flow structure With the aim of understand the phenomena of force reduction, wake vorticity was measured close to the rear wing. Results can be seen in Figure 3. No fin and Spine fin cases presented similar results, though slightly higher vorticity values can be noticed in the region below the main plane. However, a weak but perceivable vortex structure appeared in the center of the wing, above the flap plane.
  3. 3. Figure 3: Vortex structures at the wake for 0º yaw case (No fin, Spine fin, Square fin.) Studying this vortex in depth was observed that is generated in the square fin and continues towards the rear wing. Vortex structure impacts against the main plane, generating the previously mentioned downforce and drag loss. Evolution of this vortex as it moves through the wing can be seen in Figure 4. Figure 4: Vortex evolution through wing main plane Wing pressure distribution Static pressure distribution along the wingspan was extracted from a surrounding area of the leading edge of the main plane (Figure 5) and the flap (Figure 6). Pressure distributions are symmetric as expected. In the main plane, no fin and Spine fin cases show similar distributions except from the two light curves on the upper surface in the Spine fin case. Vortices coming from the fin diverge producing a decrease in pressure, thus decreasing downforce. Nevertheless, those vortices are weak. Figure 5: Static pressure distribution on the main plane The main difference appears in the Square fin case. The vortex created mentioned in the previous sub-section hit the main plane, reducing the pressure in the upper face
  4. 4. and increasing it in the lower face. Thus, being the cause of the decrease in downforce found when shark fins were introduced. Figure 6: Static pressure distribution over the flap (0º yaw) Pressure distribution in the flap showed the same results. The fluctuant pressure that appeared on the main plane tips became low pressure zones on the flap as a consequence of vorticity. The inverted peak created by the square fin vortex was also noticed in the upper face of the Shark fin case. 4º YAW CASE Forces in the rear wing All results are shown in Tables 1 and 2. As a result of the yawed flow, a diminution of 3.65% in downforce was noticed with no shark fin in the car compared to the straight line case. Drag force suffered an increase of 14.43%. The introduction of the spine fin did not reported any benefit in terms of drag or downforce at a yaw angle of 4 degrees, showing an important reduction in downforce compared to the car with no fin incorporated drag forces are similar. As expected, the “Square fin” did not show a massive difference over both previous scenarios. However, a lower value of the drag force allows the wing to be more efficient. Flow structure Vorticity in the wake (Figure 7) revealed some of the reasons governing the wing performance. Windward vortices were found to be weaker and leeward ones stronger in all three cases as expected. The structure generated by introducing the spine fin is indicative of the downforce loss in the wing, since vortices are weaker. Introducing the squared fin, vortices tend to be similar to those found in absence of fin and a rise in the central vortex generated by the fin was noted. Figure 7: Wake vorticity for No fin, Spine fin and Square fin. Since this last vortex seemed to be stronger that in the 0º of yaw case, vorticity in the trailing edge location of the shark fin was measured in order to find out its effects over the rear wing. Results in figure 8 show that the structure moved upwards before hitting the rear wing, and as a consequence of that downforce will be higher.
  5. 5. Figure 8: Square fin tip vortex Wing pressure distribution The vortex produced by the squared fin has migrated upwards and the inverted peak of the lower main plane surface disappeared. However, small effects of this vortex can still be perceived in the upper surface, with a small pressure decrease towards the center of the wing. Hence, downforce loss between no fin and square fin cases is less. The spine fin case showed a decreased pressure gradient close to the right end plate. That agrees with the weak vortex structured depicted in the previous section, being the reason of performance loss in this case. Peak pressure gradients were displaced right as a consequence of the yawed flow. As expected, the squared fin led the flow slightly straighter towards the rear wing and therefore, the minimum value is closer to the middle plane. In terms of the flap, the flow followed the same pattern showing the decreased pressure gradient in the right zone of the spine fin case. In the windward side of the square fin case a bigger pressure gradient was noticed, due to better flow alignment. Figure 9: Pressure distributions in the rear wing at 4º of yaw 8 DEGREES OF YAW Forces in the rear wing As expected, the biggest difference was seen with a yaw angle of 8 degrees. In the car with no fin the flow detaches from the lower surface of the main plane, causing a notable loss of downforce. Since the “Spine fin” is not long enough to drive the flow straight towards the rear wing, flow detachment was seen, although the drag force is reduced in 7.6%. The “Square fin” had the biggest impact over wing performance. A more symmetric pressure distribution is translated into a less flow detachment over the wing. Hence a substantial increase in downforce and a slightly lower drag force generated enhances the efficiency of the wing. Flow structure Windward tip vortices were stronger than the other 2 cases. At high yaw conditions, the fin aligns the flow with the rear wing and generates more suction, enhancing downforce.
  6. 6. Figure 10: Wake vorticity for No fin, Spine fin and Square fin at 8º yaw In this case, wall shear stress (Figure 11) helped to understand the effect of the fin. The flow in the lower surface of the windward side detached and generated a vortex close to the endplate. The effect disappeared with the introduction of the spine fin and square fin increased the stress on that zone, revealing that flow alignment is better. Separation and downforce loss in the no fin case was expected regarding the results obtained in (Gogel and Sakurai, 2006). Figure 11: Wall shear stress. Left to right: No fin, Spine fin and Square fin This separation and downforce loss in the no fin case was expected regarding the results obtained in ( Wing pressure distribution Figure 12 shows static pressure in the wing. In the main plane a decrease in pressure in the square fin case was perceived. That leads to an increment in rear downforce. The minimum pressure peak was also displaced to the center of the main plane, since flow hit the rear wing straighter. Figure 12: Wing pressure distribution at 8º yaw In terms of the flap, the windward side showed higher pressure in the no fin case as a consequence of the flow separation produced. Improvements can be seen with the introduction of the fins, being the square fin case the most favorable. EFFECT ON PERFORMANCE AND STABILITY From a cornering point of view, the introduction of a massive plate at the rear end of the car would surely produce an important handling change. With the purpose of measure the impact of the shark fin, yaw moment coefficients were extracted from the model.
  7. 7. Figure 13: Yaw moment coefficient variation Figure 13 displays the yaw moment coefficient for each car at the different tested yaw angles. As can be perceived straightforward, both square and spine fins increase the yaw moment in the opposite direction of the steering angle. However, curves corresponding to the no fin and spine fin cases followed a similar tendency when yaw angle becomes more extreme, whereas the shark fin still increases until a value of -0.7076, which almost doubles the value found in the no fin case. Static pressure distribution over both sides of the shark fin showed a notable differential pressure between left and right surfaces producing a net side force. This lateral force improves the stability of the car in potential oversteering situations. Lateral forces generated also allow the car to drive faster through corners. In addition to these effects, downforce increase helps making the car stable. (Howell and Le Good, 1999) stated the effect of lift in yaw stability. CONCLUSION A CFD analysis of how the introduction of shark fins affects the performance and stability of a F1 car has been carried out. In terms of performance, downforce loss is experienced in straight line with the presence of the square shark fin. The longer the element, the greater the decrease in downforce experienced. However, the curve of performance loss is less pronounced when yaw angle is increased. Regarding stability, results confirm that for big side areas in the rear, stability is largely improved. The square fin improves stability in cornering and crosswind conditions. Results show that the benefits obtained in yawed conditions using the square fin are larger than the loss of wing performance experienced in straight line. It is also clear as well that the spine fin does not report the benefits of the square fin at yaw, stating that intermediate solutions are not convenient. REFERENCES 1. Milliken, W., Dell'Amico, F. and Rice, R. (1976). The Static Directional Stability and Control of the Automobile. SAE Technical Paper Series. 2. Howell, J. (2015). Aerodynamic Drag of Passenger Cars at Yaw. SAE International Journal of Passenger Cars - Mechanical Systems, 8(1).. 3. Okada, Y., Nakashima, T., Tsubokura, M., Morikawa, Y., Kouno, R., Okamoto, S., Matsuhiro, T. and Nouzawa, T. (2016). Aerodynamics Evaluation of Road Vehicles in Dynamic Maneuvering. SAE Technical Paper Series. 4. Theissen, P., Wojciak, J., Heuler, K., Demuth, R., Indinger, T. and Adams, N. (2011). Experimental Investigation of Unsteady Vehicle Aerodynamics under Time-Dependent Flow Conditions - Part 1. SAE Technical Paper Series. 5. Dahlberg, E. (1999). Yaw Instability Due to Longitudinal Load Transfer During Braking in a Curve. SAE Technical Paper Series. 6. Broadley, I. and Garry, K. (2000). Improving the Aerodynamic Stability of a Practical, Low Drag, Aero-Stable Vehicle. SAE Technical Paper Series. 7. Shetty, S. and patil, V. (2013). Recent Advances in External Aerodynamics Simulation of a Hatchback Production Car. SAE Technical Paper Series. 8. Pachpund, S., Madhavan, J., Pandit, G. and Chimner, T. (2012). Development of CFD Methodology for Drag Force Prediction on Passenger Car with Rear Mounted Spoiler. SAE Technical Paper Series. 9. Gogel, D. and Sakurai, H. (2006). The Effects of End Plates on Downforce in Yaw. SAE Technical Paper Series. 10. Howell, J. and Le Good, G. (1999). The Influence of Aerodynamic Lift on High Speed Stability. SAE Technical Paper Series.