A study on aerodynamics of Ultra-efficient cars

Ultra-efficient cars (i.e. Formula One racing cars) are loaded with many different aerodynamic components. They interact to produce highly nonlinear flows, which have a very complex effect on the airflow around the racing car. Clearing up fluid phenomena makes it possible to optimize aerodynamic components effectively. This paper reviews the aerodynamic performance of currently used shapes, as well as the definition of the design constraints for the vehicle. The designs are inspired by formula one cars, especially by Honda F1 Team, but they are adjusted according to the limit conditions of CFD simulation software Ansys CFX, and parameters are scaled accordingly based on the space requirements of this test. A level of velocity at 40 km/h is tested, giving ideas of the full model performance. Results are then compared and discussed to obtain a comprehensive and valid conclusion about the potential improvement in the aerodynamics of road cars, which can be extracted from those ultra-efficient ones.


Introduction
It is well known that aerodynamics is the study of the airflow around a car.The smoother the airflow, the lower the resistance, and the less fuel will be burnt at a specific speed.That is why car manufacturers are so keen on improving aerodynamic efficiency, because it means they can get better fuel consumption data and sell more cars.For manufacturers, aerodynamic efficiency improvements cost less money than expensive weight reduction or expensive engine technology.However, as for the ultra-efficient cars, their aerodynamic design partially shares a similarity with the road cars [1] in reducing drag and increasing stability, while the other parts are unique and far more complicated, which are still unfamiliar to the public [2].Since resistance increases significantly with speed, aerodynamics account for a much higher proportion of energy consumption (electricity or fossil fuel) on motorways than urban use.According to the Auto Research Center, aerodynamics accounts for 50% of highway energy use and only 20% of urban energy use.This means that a 10% reduction in air resistance on motorways will increase the fuel economy by about 5% and the city economy by 2%.With more research on aerodynamics, vehicle design can be more economic and practical.Therefore, it is critical to use high-performance vehicles as a reference for aerodynamics studies.In this paper, the author analyzes the aerodynamic performance of different component designs of ultra-efficient cars.The main purpose is to provide references to the impact factors of aerodynamic efficiency [3].

Front wing
The front wing (as shown in Fig. 1) generates about 20%~25% downforce for the entire vehicle.In addition, since the downforce generated by the front wing can easily affect the aerodynamic balance (Center of Pressure/CoP) of a racing car, this paper adjusts the CoP of the entire vehicle by adjusting the angle of attack of the front flaps.Because the front wing is upstream of the airflow of the vehicle, the upwash airflow and wing tip vortices generated by the front wing not only have a significant influence on the flow field in the rear of the vehicle, but also are very sensitive to changes in the vehicle direction caused by steer, pitch, and other factors.Because of these characteristics and the important role that the front wing plays in determining the CoP adjustable range of a racing car, the front wing has become the most time-consuming component in wind tunnel testing [2].

Effect of lower front wing pressure and upwash air flow
The ground effect of the front wing is not only beneficial to increase the lift-drag ratio, but also to control the airflow upstream of the front wing within a certain range.However, if the front wing generates excessive downforce, components such as the rear suspension arm can generate lift beyond the limit.Furthermore, the upwash airflow from the front wing reduces the actual angle of attack of the rear components to such an extent that the total downforce of the vehicle is reduced.In addition, turbulence from the front wing and the suspension arm reduces the dynamic pressure near the tail and directly reduces the downforce of the tail (particularly in the FSAE).For these reasons, the author points out the method of controlling the downforce generated by the front wing at a fixed value, or even less, without blindly increasing it.Fortunately, these optimizations are done within the regulation box of the front wing, and in addition to simply reducing the amount of upwash, there are other benefits, such as reducing steering sensitivity in racing cars (the effect picture is shown in Fig. 2).Strictly speaking, this paper optimizes the upwash distribution and steering sensitivity of the front wing by adjusting the height and shape of the front wing in the Y-axis (left and right direction of the racing car).At the same time, it is also necessary to optimize the tip geometry of the front wing according to the shape of the front wing end plate (FWEP) so that it can work together (example in Fig. 3) [6].

Interaction between the front wing tip vortex and tire
Another problem in the research and development of the front wing is how to deal with the wingtip vortices peeled off from the front wing end plate.In the lower part of the inner side of the tire, a separation vortex will peel off from the leading edge (here refers to the shoulder position), resulting in serious total pressure loss (Fig. 4).A feasible solution is to mix the separation vortex of the tire with the vortex of the front wing end plate.In the process of moving, the mixing vortex in the same direction will maintain the same position in the YZ plane (on the front cross section of the car), which can prevent the vortex from gradually shifting to the inside of the car and avoid the total pressure loss under the bottom plate (Fig. 5).In order to realize this scheme, it is necessary to create a stable front wing end plate vortex, which will peel off at a proper position, and it needs to be strong enough not to dissipate halfway.Interaction between the front end-plate and tire vortices (static pressure distribution, velocity vector) [5] In addition, the steering of the car has a significant impact on the design of the front wing.Steering creates a yaw angle with respect to the airflow in the racing car, so the front wing needs to be optimized so that there is no significant difference in performance between steering and non-steering conditions, especially when dealing with the position relationship between the separation vortices in the lower half of the windward tire and the front wing vortices.it is reasonable to control eddy currents at the tip of the front wing by installing an aileron on the front wing, or by installing a foot along the lower edge of the front wing, in conjunction with a semi-cone plate ("cone") at the rear of the end plate.Another method is to adjust the strength and position of the wing tip vortexes by the longitudinal vortexes generated by the "strips".The strake is a flat plate mounted parallel to the end plate under the middle section of the front wing.

Components above the chassis
The upper part of the chassis refers to a series of areas from the nose cone to the upper half and both sides of the single shell.The main function of the components above the chassis is to produce downwash airflow at the cost of a small part of the lift, and increase the downforce generated by the components below the car floor and tail (mainly the tail wing) [8].
The front cantilever plays an important role in deflecting the front wing upward wash direction downward (Fig. 6).Without the presence of the front cantilever, the control of the barge board vortex would be totally impossible.However, since the suspension is not allowed as an aerodynamic component, the FIA limits the ratio of section chord length to thickness, the chord length, and the angle of attack of the suspension arm.Therefore, air stall may occur on the upper surface of the suspension arm due to the effect of the wash airflow on the front wing.Accordingly, the usual response is to optimize the section of the front cantilever and the overall suspension geometry to maximize its aerodynamic effects.

Components under chassis
The lower part of the chassis refers to the area from the front axle to the front of the side box.This area is restricted by many rules, such as the minimum area and maximum width of components.The "shadow rule" is almost effective for the entire region.In addition, there are a series of other rules to limit the specific shape of components.Generally, in addition to producing downwash flow at the cost of lift through components above the chassis, downwash can also be produced through longitudinal vortices traveling along the vehicle body.The enhancement of downwash flow not only means that the airflow under the bottom plate is improved, but also means that it can generate down-pressure through the components in front of the bottom plate.Therefore, the interaction of all components in the area must be considered, and the front wing and diffuser must be covered.The performance of the components under the chassis accounts for a large proportion of the car's performance, and is also important for the aerodynamic performance [10].
The function of the barge board is to create downwash flow in front of the bottom plate by generating and controlling a series of longitudinal eddies (Fig. 7).Different teams have different shapes of barge boards, but their roles in their cars are basically the same.
A curved barge board produces upper and lower tip vortices (Fig. 8).Behind the windbreak, the airflow is directed by eddies to the outside of the racing car.During the downward journey, the eddies on the barge board flow outward and downward through the eddies, wings, cantilevers, side boxes, and a range of other components.The lower eddy gradually increases as it progresses downward.In view of the above reasons and the existence of the ground effect, the lower eddy will always remain in the same position on the YZ plane after it leaves the barge board.Because the upper eddy flows outwards and downwards, the position of the lower eddy remains unchanged, and the relative lateral displacement of the two eddies in the Y-axis direction will occur, while the airflow between the two eddies will be guided downwards.As a result, the barge board creates a downwash flow by creating a longitudinal eddy current.The downwash flow can improve the actual angle of attack of the bottom plate, thereby enhancing the suction effect of the front edge of the bottom plate.In addition, the lower eddy of the barge board can generate a little downforce by the suction action of the eddy current itself after it has flowed into the base plate.However, if the suction action of the front edge of the bottom plate is too strong, combined with the total pressure loss of the lower vortex center of the barge board, the boundary layer of the bottom plate will become thicker, damaging the performance of the diffuser (more prone to air stripping).Therefore, it is important to prioritize which components to optimize and which aspects to optimize for a single component.None of the other effects the barge board produces are as strong as the two eddies it creates.But it has some other minor effects.Fig. 9 shows the difference in streamlines when installing and removing the barge board.As it is shown that the outwash flow from the barge board pushes the separation flow from the inner front edge of the lower half of the front tire towards the outside of the racing car, avoiding loss of diffuser performance.Although the presence or absence of a barge board has little effect on the chaotic characteristics of the rear of the tire, the interaction of the front wing tip vortices and the tire chaotic currents mentioned above is sufficient to illustrate the considerable collaboration between the barge board and the front and rear aerodynamic kits.

Diffuser
The diffuser is similar to half a Venturi tube, but unlike the Venturi tube compressed fluid, it uses a diffuser section to slow down the flow (Fig. 10).Because the diffuser instantaneously expands the flow pipe and the fluid is still incompressible at this time, more air will be drawn into the diffuser from or around the chassis, causing the flow rate of the chassis to increase and the pressure to decrease.This pressure difference is used to generate a lot of downforce.The static pressure at the diffuser outlet is affected by the suspension assembly and lower rear wings.By changing the position and shape of these components and the position with the strongest suction effect, one proper approach is to reduce the static pressure at the diffuser outlet and increase the diffuser flow rate [13].

Effect of the body on the diffuser in the front half
The diffuser is affected by the airflow in the front half of the racing car because it is installed at the rear of the car.For example, if the suction effect of the bottom plate is increased or the lower eddy of the barge board is increased, the boundary layer under the bottom plate becomes thicker, which means that the diffuser flow is more easily separated.Therefore, choosing which components to develop further and when they will work (yaw, pitch, roll) will have a significant impact on the overall aerodynamic characteristics of the racing car.

Aerodynamic characteristics resulting from changing chassis height
The area ratio of the diffuser inlet to outlet varies with the height of the racing chassis.The relative position of the diffuser to the suspension assembly also changes.As a result of these changes, the downforce generated by the racing car also changes.Variations in chassis height can affect the generation of downforce in a number of ways, including the CoP characteristics at high-, low-speed corners and brakes.In most cases, in order to maintain stability when braking, the CoP of the racing car should move back when braking.This means that racing cars need to increase the dimensionless rear downforce (Cl*A) at the rear by increasing the height of the rear chassis when braking (dotted line in Fig. 12).To achieve this feature, it is essential to ensure that when the chassis is low, part of the diffuser flow is separated and the downforce generated by the diffuser is reduced.However, it must be noted that if the air separation is too severe, hysteresis will prevent the downforce of the racing car braking from returning to its previous level (solid line in Fig. 12).
Figure 12.Relationship between the downforce generated by a racing car and the chassis height [14].

Rear wing
Most of the drag generated by the rear wing is induced drag, and an increase in drag must be accompanied by an increase in downforce.Because the rear wing has a low chord, the tip vortices cause varying degrees of upwash in the direction of the incoming flow (Fig. 13).In addition, the downforce and resistance generated by the rear wing also account for a considerable proportion of the total downforce and total resistance of the car, so the priority is weighing the speed and cornering performance of the car by adjusting the rear wing.Because of this, the first thing is to use several different rear wing designs to generate different levels of downforce to cover all the speeds required on the track.The purpose of the rear wing research and development is to improve the overall L/D of the car through the following considerations: more efficient airfoil, pressure distribution along the Y-axis direction, end plate design optimized according to different downforce levels, etc. Fig. 14 shows the distribution of airflow angle of attack after removing the upper rear wing under CFD simulation.This result shows that there is a significant upwash at the wingtips on both sides of the rear wing.Fig. 15 shows the total pressure distribution near the tip of the tail wing on the YZ plane.In view of this (Fig. 16), the curvature of the rear tip will be reduced, thereby reducing the load and differential pressure resistance here and increasing L/D.In addition, optimizing the rear to change the induced drag is considerable (by elliptically distributing the pressure below the tail in different angles of attack and curvatures along the Y-axis to minimize the induced drag).Slots above the tail end plate can also act similarly by controlling the intensity of the tip vortices to influence the upstream characteristics along the Y-axis).Figure 16.The pressure distribution along the wing span and chordal direction of the rear wing, and the 450mm black curve at the lower left corner means that wash-up at this point results in higher pressure [5].

Conclusion
In conclusion, the strong non-linearity of the airflow around multiple components of the F1 chassis makes it difficult to fully understand the details behind the aerodynamic components in practice.However, by using CFD to qualitatively analyze key aerodynamic phenomena, in conjunction with quantitative data obtained in wind tunnel testing, it has been able to effectively develop the aerodynamic components of racing cars in reality.These methods also make the assessment of the aerodynamic performance of the racing car more accurate on the track.The airflow structure of the model F1 used for simulation in this paper is different, the processing level is different, and the accuracy of CFD is also very different.Some academic knowledge may not be observed in this simulation.In order to improve such a situation (CFD simulation), the following steps need to be taken into consideration.The physical conditions in this experiment are fixed, but it should be recognized that changes in external physical conditions, such as temperature and pressure, should not be ignored.For example, whether cavitation needs to be considered if there is a large negative pressure area when calculating a complex geometric model, whether compressibility and viscous heat need to be considered when calculating high-pressure gas, and, in some cases, whether there is a need to consider evaporation, condensation, and other phase transitions.Sometimes, these physical phenomena will lead to non-convergence and even calculation errors, but if not taken into account, the calculated values may become inaccurate.Mesh quality is always an important part of CFD.Good grid quality can enhance convergence, improve calculation accuracy, and reduce calculation time.Therefore, the grid quality should be improved as much as possible in the case of sufficient time.At the same time, the grid is densified for regions with complex flow conditions.After the calculation results meet the requirements, grid independence verification is also required.

Figure 3 .
Figure 3.The front wing of MP4-27 has different angles of attack, curvature and chord lengths along the wing span to optimize the upwash flow and pressure distribution generated by the front wing [7].

Figure 4 .
Figure 4. Turbulence with flaking front edge of the front tire [5].

Figure 5 .
Figure 5. Interaction between the front end-plate and tire vortices (static pressure distribution, velocity vector)[5]

Figure 8 .
Figure 8.The eddy current produced by the barge board, mainly the upper and lower eddies, creates the downwash through the relative displacement of the two eddies [11].

Figure 9 .
Figure 9. Barge board to guide the random flow of the front wheels, comparison between mounting and removing the barge board [5].

Figure 13 .
Figure 13.Upwash by wing tip vortices in the direction of incoming flow [5].