Three-dimensional predictions of the rod wake-airfoil interaction noise by hybrid methods

Sound generated by an airfoil in the wake of a rod is predicted numerically by two different hybrid CFD/CAA approaches (Ma = 0.2). The configuration is a symmetric airfoil one chord downstream of a rod, whose wake contains both periodic and broadband vortical fluctuations. In particular, a significant broadening of the main Strouhal peak has been observed at subcritical vortex shedding conditions. This study addresses the overall ability of both CFD/CAA hybrid approaches to model broadband noise sources. The first approach computes the aerodynamic noise by solving the linearized perturbed compressible equations (LPCE) for the noise propagation, with the acoustic sources and hydrodynamic flow variables computed from the incompressible Large Eddy Simulation (iLES) using a computional grid of approximately 3 million grid cells and high-order compact finite difference schemes. The second approach uses the unsteady aerodynamic field of a compressible Detached Eddy Simulation (DES) and a Ffowcs Williams & Hawkings (FW-H) acoustic analogy formulation for the farfield noise calculations. The non-zonal DES approach solves either the unsteady Reynolds-averaged or spatially filtered Navier-Stokes equations by using a novel cubic explicit algebraic stress turbulence model based on a two-equation k-e model by Lien and Lechziner. An implicit formulation is used with second order accuracy and a grid of approximately 2.3 million cells. The results of these hybrid approaches are compared and subsequently validated with the measurements of Jacob et al. in the nearfield (HWA) and in the farfield (noise).