Aeroacoustic response of a T-junction with bias flow

This paper reports a numerical study on the aeroacoustic response of a rectangular T-junction with bias flow in the side-branch. The Mach number of the grazing flow in the main duct is fixed at 0.1. The primary motivation of the present work is to study and explain the in recent experiments observed high sound amplification at small bias flows. The study is conducted by performing numerical simulation, which solves the 2D compressible linearized Navier-Stokes equations (LNSEs) in the frequency domain. The time averaged flow is first solved by using RANS along with a k-? turbulence model, and the turbulent viscosity is then added to the kinematic viscosity in the equations. The overall agreement with the experimental acoustic 3-port scattering data is very good. Remaining discrepancies are likely due to that the present simulations are performed in 2D neglecting the 3D effects that exist in the rectangular test channel used for the experiments. The numerical results confirm that the rectangular T-junction is very sensitive to small bias flows. Compared to the case with no bias flow, a strong shear layer is created along the downstream main duct by the mixed grazing-bias flow. For small bias flows this shear layers extends far downstream of the actual junction. This creates a region of vortex-sound interaction which is much larger than for the no bias flow case. Based on the Howe formula for vortex sound power the dominating sound generation is then moved downstream of the actual junction. This is different from the no bias flow case where all the sound generation occurs across the junction. The amplified sound power level is very sensitive to the Mach number of the bias flow. A high level sound amplification is observed when the Mach number of the bias flow is below 0.03.