Towards a Standardized Assessment of Automotive Aerodynamic CFD Prediction Capability - AutoCFD 2: Windsor Body Test Case Summary

To improve the state of the art in automotive aerodynamic prediction using CFD, it is important to compare different CFD methods, software and modelling for standardized test cases. This paper reports on the 2^nd Automotive CFD Prediction Workshop for the Windsor body squareback test case. The Windsor model has high quality experimental data available and a simple geometry that allows it to be simulated with limited computational resources. The model is 1 metre long and operates at a Reynolds number of 2.7 million. The original Windsor model did not include wheels, but a second variant was added here with non-rotating wheels. Experimental data is available for integrated forces, surface pressure and wake PIV surveys. Eight standard meshes were provided, covering the two geometry variants, two near wall mesh spacings (relating to wall resolved and wall modelled) and two mesh densities in the wake (relating to RANS and eddy resolving). The mesh sizes ranged from 3.3 to 50 million cells. 73 solutions were submitted from 17 organizations. The data was integrated into a web-based dashboard to allow any participant to interrogate and compare their results. There was significant variability in predicted lift, drag and moment coefficients, even when using the same turbulence model on the workshop provided grid. Most simulations underpredicted drag, but in general the eddy resolving simulations were closer to experiment. For centerline pressure, all the simulations gave a higher pressure than the experiment on the forward facing upper surface. All the RANS predictions showed incorrect, almost axisymmetric, base pressure distributions. The eddy resolving pressure distribution is much improved, reflecting the flatter distribution found in the experiment. Some simulations reported a left-right asymmetry in base pressure which is consistent with the experimentalists reporting a bi-stability of the wake for the no wheels case.