Aeroacoustics of Rounded Forward-Facing Steps: Far-Field Behavior

Abstract

Measurements and predictions of far-field sound radiated by rounded forward-facing steps immersed in an incompressible turbulent boundary were performed. Four different corner rounding radii equal to 0%, 6.25%, 12.5%, and 25% of the step height were considered. The step height was 26.2% of the incoming boundary-layer thickness and the Reynolds number based on step height ranged between 35,000 and 104,000. Measurements of the spanwise averaged unsteady pressure show that the pressure on the downstream surface is a function of rounding due to change in the size and strength of the separation bubble, whereas on vertical face it remains independent of corner rounding. The pressure on either surface scales on the step height and free-stream velocity. Rounding the step corner leads to a consistent reduction in noise without changing the overall shape of the sound spectrum significantly. The sound from each rounded step exhibits the effects of step noncompactness, which prevents spectral scaling. The predictions of sound from an unrounded forward-facing step were made by combining a theoretical formulation with surface pressure measurements. The predictions describe the overall shape of the sound spectrum including the effects of step noncompactness accurately.