Spectral/hp iLES-SVV simulation methodology study on an Ahmed Body squared back 2018-36-0320

The Ahmed Body is one of the most widely studied bluff bodies used for automotive conceptual studies and Computational Fluid Dynamics - CFD software validation. With the advances of the computational processing capacity and improvement in cluster costs, high-fidelity turbulence models, such as Detached Eddies Simulation – DES and Large Eddies Simulation – LES, are becoming a reality for industrial cases, as studied by BUSCARIOLO et al. (2016) [4], evaluating DES models to automotive applications.

This work presents a correlation study between a computational and physical model of an Ahmed Body with slant angle of 0 degree, also known as a squared back. Physical results are from a wind tunnel test, performed by STRACHAN et al. (2007) [11] considering moving ground and Reynolds number of 1.7M, based on the length of the body.

CFD simulations were performed by the code Nektar++, which is an open source spectral/hp element high-order solver, which methodology combine both mesh refinement (h), with higher polynomial order (p) for lower error propagation and better convergence. It employs a high-fidelity turbulence model known as Spectral Vanish Viscosity – iLES-SVV model, which works as a filter for high frequencies. Same physical test conditions and tunnel test section were considered, with a total time of 4 convective lengths.

The 4 cases studies consider high-order mesh of 6^th order, divided in two polynomial orders: 5^th and 6^th for two different mesh setups: one base mesh setup with around 95,000 elements corresponding to 6.3Million of DOFs and a second mesh considering a refinement (h) with around 310,000 elements, corresponding to 19.8 Million of DOFs. Meshes were generated by NekMesh, which works with Nektar++ for high-order mesh generation. In order to improve the computational costs, only half of the model is simulated, considering symmetric condition.

Considering the converged drag coefficient values for cases of 5^th and 6^th polynomial order and refined mesh, the maximum difference found, compared with experimental results with moving ground configuration, there is a maximum difference of 5%. For the lift coefficient the maximum difference between the simulation results compared to experimental data is 25%, but the closest result is fully correlated. There is also a good agreement between the LDA measurements on the end of the body with the results from the simulation. The methodology shows promising results against the open literature once an appropriate validation study has been undertaken. Despite the relatively course resolution adopted the results are encouraging. Having identified an appropriate resolution, we will next consider other slant angles, to see how well these correlate with the experimental studies.