Role of Plate Length on Noise from Wall Jets

Abstract

The present study experimentally investigates the effect of the growth of inner layer on noise emission characteristics of wall jets. The plate length L considered for the current study vary in the range of L/h = 2.5 to 30, where h is the nozzle height. The jet is issued from a nozzle having the exit dimensions of 20 cm in width and 2 cm in height h. The jet Reynolds number, based on the nozzle height and jet exit velocity Uj, is varied up to 7.0 · 104. Acoustic measurements revealed the distinct variations in the noise levels with different plate lengths. The L/h = 2.5 wall jet has an increase in noise levels by around 10 dB compared to that of a free jet (background noise). Wall jets in the range of L/h = 5 to 20 radiate higher noise levels compared to other plates, while the least noise emissions are observed from fully developed wall jets (L/h > 20). The significant sources identified for noise emissions are the trailing edge and the secondary shear layer in the wall jets. The low frequency noise corresponding to the Strouhal number (based on h) below 0.2 is characterized as the trailing edge noise. The spectra of the wall jets collapse in the Strouhal number range (based on the inner layer thickness of wall jets) of ∼0.2 to 1.0 indicating the secondary shear layer noise of wall jets.