On the estimation of unsteady aerodynamic forces and wall spectral content with immersed boundary conditions

Abstract

Immersed boundary conditions (IBC) have become a practical tool to simplify the meshing process for the simulation of complex geometries in CFD. This approach has reached a sufficient level of maturity to allow the simulation of compressible high Reynolds number flows. However, the access of physical quantities at the immersed wall is far from being straightforward. This paper provides two methods for the reconstruction of fluctuating wall quantities relying on the creation of explicit watertight surface meshes of the immersed boundary. These surface meshes are used for the investigation of highly unsteady compressible flows of two generic space launcher afterbody configurations using Zonal Detached Eddy Simulation (ZDES). Since the flows are massively separated, the side load arising from the pressure is mainly responsible for the global load unsteadiness. Therefore, in the present study the focus is put on the accuracy of the wall pressure reconstructed on immersed boundaries and compared to validated numerical simulations using a classical body-fitted approach and experimental data. The numerical results demonstrate the ability of the present approaches to accurately capture the global load fluctuation around both afterbody configurations. Moreover, the IBC surface meshes simplify the overall post-processing operations and allow the extraction of wall quantities for unsteady simulation at low computational cost. This last feature has been used for the spectral analysis on IBC surfaces which reproduced successfully the location and the intensity of the pressure fluctuation.