Effects of a boundary layer trip in the prediction of noise from flow past tandem cylinders

Abstract

The use of boundary layer trips in wind tunnel experiments forces transition to fully turbulent flow, which affects the resulting acoustic signature. Boundary layer trips are often too small to be modeled geometrically in computational simulations, making it difficult to consider their influence. The goal of this study was to determine the best method for representing the effects of a boundary layer trip in the benchmark aeroacoustic problem of flow over tandem cylinders in a wind tunnel. This case is pertinent to the study of aircraft landing gear noise because periodic vortex shedding results in pronounced acoustic tones in addition to broadband noise from turbulent wake interactions. A hybrid Lattice-Boltzmann Method/Ffowcs-Williams Hawkings technique was used to predict the transient flow field and the resulting noise. This study compared four methods for simulating the trip's effects: 1) applying a surface roughness on the upstream cylinder; 2) reducing the viscosity of the fluid; 3) applying a geometric trip on the upstream cylinder that is one volume element thick; and 4) forcing velocity perturbations in the incoming free stream. Of these methods, reducing the fluid's viscosity was the most effective way to reproduce the highly turbulent flow patterns and the acoustic signature.