Grid Generated Turbulence for Aeroacoustic Facility

Abstract

A preliminary version of this paper was presented at the AIAA Aviation 2020 forum (Paper 2020-2525).This paper provides an insight into the grid generated turbulence for aeroacoustic studies. Several passive grids of square bars were tested in the open-jet aeroacoustic facility at the University of Bristol. The geometric properties of the grids and their position within the tunnel contraction nozzle were varied to quantify their influence on the average and statistical flow properties as well as the generated self-noise. Moreover, case studies involving turbulence interaction noise generation with a NACA0012 airfoil and cylinder were conducted. Turbulence intensity, integral length scale, and the anisotropy of the flow generated by each grid were characterized by hot-wire measurements, and the associated far-field noise was measured by a far-field microphone array. The grid turbulence results show that the downstream evolution of the turbulence intensity and integral length scale was comparable to results of closed test-section wind-tunnel studies for the first hydraulic diameter downstream of the contraction nozzle exit. However, beyond the first hydraulic diameter, the turbulence intensity plateaus and the integral length scales show rapid growth. Moreover, the results show that grids positioned closest to the contraction nozzle exit produced turbulence closest to isotropy with high levels of turbulence intensity, but the measured noise spectra suffered from the contamination from the grids self-noise. The grids located farthest from the contraction nozzle exit performed best in terms of noise contamination and could generate almost the same level of turbulence properties as the grids closest to the contraction nozzle exit.