The influence of compressibility effects in correlation issues for aerodynamic development of racing cars

In motorsport, Computational Fluid Dynamic (CFD) simulations of new designs are routinely compared to wind tunnel and track data in cross-validation exercises. Often, relatively poor correlation is found between these three methods. A Formula 1 car can reach speeds in excess of Mach 0.25 and, in addition, parts of the car operate in extreme ground effect thus accelerating the local flow to double or more the freestream Mach number. A 3-dimensonal CFD study was conducted into the significance of compressibility effects occurring over an entire open-wheeled racing car for a typical design-phase scenario whereby a 60% wind tunnel model is matched to Reynolds-scaled full-scale CFD (i.e. lower velocity). A 36% model was also investigated for the same Reynolds number. The overall lift and drag coefficients for the whole car were very similar, which could give a misleading impression of negligible compressibility effects. However, the results demonstrated significant differences (up to several percent) in both lift and drag over the major individual components of the wheels, front wing, diffuser and rear wing due to compressibility effects, when compared with incompressible benchmarks. The behaviour of vortices and separation points would be affected by the density changes, which would in turn have significant consequences for bodywork fine-tuned in incompressible simulations.