Design and Analysis of Turbulence Grids for Aeroacoustic Measurements

The effect of turbulence generating passive grids and their self noise has been studied. The geometric properties of the grids and the position within the tunnel contraction nozzle were varied to influence the turbulence generation. The resulting flow conditions and far field noise emission was analysed. A total of 12 passive grids were tested, all with square shaped bars of various bar diameter and mesh sizes, where each grids' flow properties were characterised and the far field noise was measured. It was found that grids closer to the contraction nozzle exit showed higher levels of turbulence intensity with the penalty of a sound spectra contaminated with the grids' self noise. Grids further upstream of the contraction nozzle exit offered lower self noise but the spectra was still contaminated with tones and broadband humps. However, the grids positioned furthest upstream from the contraction nozzle exit showed that it could generate similar levels of turbulence intensity to grids closest to the contraction nozzle exit without the self noise of the grid affecting the normal running background noise of the wind tunnel. In a case study using a NACA 0012 airfoil it was found that the interaction noise was sensitive to the geometry of the grids.