Flow-induced noise generated by sub-boundary layer steps

• The paper presents the flow-noise results for sub-boundary layer step-pairs of rectangular and triangular shapes.
• The paper presents detailed boundary layer data in the wake of the steps.
• Noise data shows the existence of both streamwise and cross-stream dipoles and the role of step shape on noise level.
• A new noise scaling law based on the perturbed boundary layer height is proposed and shown to be accurate.

The sound generated by a turbulent boundary layer with a smaller two-dimensional step was investigated and related to the perturbed flow field via a new scaling law. Experimental results were obtained using two types of sub-boundary layer steps (rectangular and triangular). Flow visualisation revealed an upstream separation zone and a downstream reattachment point. The sub-boundary layer steps, in general, resulted in a rapid thickening of the shear flow near the wall. The integral boundary layer results show the effect of the steps on the boundary layer can be likened to a disturbance followed by a boundary layer re-establishment region. The noise data revealed the nature of the noise created by the steps. The steps can be considered as acoustically compact (height much less than acoustic wavelength) and radiate noise in both streamwise and cross stream directions, suggesting the existence of compact superimposed dipole sources. The shape of the step has a large effect on the noise level, suggesting the front surface of the step has a dominant role in noise production. A new scaling law based on the perturbed boundary layer height was able to successfully collapse the noise data.