After the re-balance and the re-drag processes, the data analysis in wind tunnel (WT) tests are more graphical. It is possible to plot several types of graphs to evaluate aerodynamic coefficients together with the ride heights (RH). The main ones are Stability, Sensitivity and Pressure Coefficient C_P.

Stability

The stability, in this case, is not related with the behavior of the car, which is a quite different story. Actually, it is in terms of performance of the car, which means the variation of the aerodynamic coefficients, balance over front and rear ride heights, FRH and RRH, respectively. This is a strong constraint since for each aerodynamic project it improves or not the drivability of the car. If the balance changes, this is something that the driver feels. Actually, this is the only aerodynamic parameter that the driver can feel. Not only in terms of averaged values, but also how much the balance varies when the car changes the ride height (RH). This something that induces or removes confidence of the driver on the car, thus it is clear that a car which the balance constantly changes with RH, is a car that induces less confidence for the driver. The reason is that the car will be driven at different conditions that motivates different RH, thus the car with low stability exhibits constant variations of balance. It is not possible to block the ride heights since the active suspensions were banned on the racing series. In corners, if the car changes the velocity, RH and F_bal, this could not be beneficial.

Different race cars face RH variation differently, because some of them are more sensible to RH. GT like race cars that has a big underfloor in terms of surface, the floor is close to the ground and the front underwing is not connected to the suction areas on the sides of the car. This creates a great dependency of downforce and balance on RH, because these aero devices operate with ground effect. This sensitivity to RH is reduced in prototypes, because it is more open to the air flow. In the open-wheels cars the sensitivity is even lower, because the under-floor area is smaller, it is open to the free stream and the wings are bigger.

The stability means the sensitivity of F_bal over FRH and RRH, as illustrated in Figure 2. However, it is possible to analyze stability in terms of C_ZFS, C_ZRS and C_ZS. The performance evaluation is made by checking the gradients, the balance variation relative to FRH and RRH between the baseline and the option.

Hence, it is possible to plot these two, the delta between them and the direction of he maximum that can be identified, thus the opposite of the isoline if it is changing between the baseline and the option.

This comparison should be placed in between the more stable option, because the isolines are less concentrated and the maximum variation means that maximum aero balance is smaller, while in the gradient there is a similar behavior, but more stable. Hence, it is possible to build aero maps for the front wing, stability maps for C_ZFS and, if the diffuser is being evaluated, to build aero maps for C_ZRS.

Sensitivity

The sensitivity is a similar evaluation than stability, but since the coefficients under variation can be plotted relative to just one cornering parameter, these must be yaw, steer or roll. The output is a 2D graph as illustrated by Figure 5. This example has three curves, which are three car configurations. The criteria to analyze this delta, which is the amount of variation of the aerodynamic coefficients that is given in degrees, is promoting the option that the variation is smaller and the regularity is higher.

For example, option 1 with respect to the baseline has linear behavior and a smaller range of C_ZFS variation overall, thus it is preferable to the baseline. Another example is the option 2 that exhibits a less predictable behavior with the same range. From the regularity point of view, option 1 is better than option 2 and overall. The same evaluation can be done for yaw and steer, but for each of these sensitivities, it must be identified C_ZFS, C_ZRS and C_ZS. Each of them is critical for the car. This is not a general rule, but it must be checked C_ZFS×Yaw and C_ZS×Roll to verify if the car has oversteer. It must be known the status of the baseline configuration of the car to be aware which is the characteristic that is less performing. For instance, in a car it is possible to identify if one target is improving C_ZRS×Yaw, because this is a stronger loss with yaw. This also has a clear aerodynamic meaning in evaluating the effect of the rear diffuser. Hence, this identifies the most critical option and if it promotes a regular behavior, less variation. It is possible to choose how the output from each wind tunnel data will have the possibility to select which aerodynamic coefficient will be evaluated. These will be used together with Yaw, Roll and Steer.

Cooling

The cooling analysis focuses mainly on two different radiators, its objective is to keep the powertrain system functional. The mass flow through the radiator is translated into the crossing velocity. Similarly, the brake cooling is defined by the mass flow through the ducts. The crossing velocity is obtained from the delta between the front and the rear surface. In addition, it is possible to trace the pressure at the front and rear surfaces. Using the calibration curve it is obtained the crossing velocity.

As averaged values or aero maps between the baseline and option configurations is plotted in Figure 8, it is possible to evaluate the distribution of the radiator crossing velocity or, most likely, the average. Hence, there is a sensible delta that allows to go in more detail where in the surface has a gain or loss.

The brakes are more difficult to measure from the wind tunnel point of view, because a static total pressure is used to measure the velocity. Hence, the use of a pitot tube for few static pressure measurements on sections as the one at Figure 9 is critical for the evalution. The CFD simulation usually is preferable for brake cooling evaluation, not only the entire car, but as a model of the brake system that has more detailed channels to solve the Navier Stokes Equations (NSE) and find the temperatures. These are useful, because it is impossible to install a pitot tube in a section like that in Figure 9 whitout ant interference on the flow in a such small duct.

Pressure coefficient at bodywork and underwing

The most common example of pressure distribution is the pressure coefficient C_P over a surface. In wind tunnel models WTM tents of pressure taps are used to measure C_P on the underfloor. These are very useful, because in the WT this information helps to understand the flow behavior and not rely only on the overall values of the car balance.

Hence from this plot (Figure 11) and comparing to plots of two configurations it is possible to detect if there is a stall on the rear diffuser or on the front wing, because also this can exhibit stall. It is possible to visualize if there is a gain or loss on the rear downforce and if this is located all over the floor or just in a specific region. In addition, it is possible to perform correlations. CFD allows to extract C_P on the same point as in WTM and compare if there are non-correlating results. C_P is used to calculate downforce generated on the area that is instrumented. This is something common in high level projects. The instruments can be applied in a full scale car. Figure 11 indicates the baseline, the option and the delta iso-surfaces for C_P. It is possible to visualize the location of a C_P loss. The difference, in this case, induces a cornering condition with a lower capability to develop higher speeds, because there is less suction on the zone highlighted. The averages of the aerodynamic coefficients, C_P, stability and cooling thresholds, these are aerodynamic results that should be evaluated in order to decide the best configuration. It is important to understand that this one does not account for parameters at the best or the worst condition. Actually, the objective is a balanced solution. For instance, to improve cooling, sometimes it is required to reduce downforce, improve stability and reduce the average coefficients. Therefore, it is important to know where to bring the car in terms of the targets and which of them are the stronger ones. This is a decision taken test by test or in specific moments of the run list.

References

• This article is based in lecture notes written by the author during the Industrial Aerodynamic lectures at Dallara Academy.