Sound From Flow Over a Rough Surface

Abstract

The mechanism for sound production from flow over a rough surface is not well understood. Measurements of radiated noise and low-wavenumber unsteady surface pressures were carried out in order to better understand the sound production mechanism. The initial results of an ongoing experimental investigation of the sound produced by flow over a rough surface are presented. In order to investigate scaling relationships, the flow speed, roughness height, and roughness element distribution were varied. Previous investigations have reported roughness noise levels that scale on flow velocity, roughness height, and fetch area and have indicated that the sound production may be dipole or quadrupole in nature. Prevailing analytical models assume that both types of sources are present. The scaling of roughness noise for large roughness height (k+ = uτk/ν of order 1000) has not been investigated previously and is part of the current study. The scaling behavior of low-wavenumber surface pressures is discussed, in addition to the comparison of radiated noise spectra obtained by phased microphone array measurements.

Boundary-layer noise induced by arrays of roughness elements

“…However, there were significant disagreements over suggested scaling laws, controlling parameters and the physical mechanisms behind the theories. The lack of understanding and predictive tools for roughness noise prompted a number of investigations in recent years, including the study of Liu, Dowling & Shin (2006 aimed at evaluating and modelling the contribution of roughness noise to airframe noise, and a concerted effort (Blake et al 2008) consisting of theoretical Glegg & Devenport 2009), experimental (Anderson et al 2007;Smith et al 2008;Alexander et al 2009Alexander et al , 2010, 2013Devenport et al 2010Devenport et al , 2011 and numerical (Yang & Wang 2008 investigations. Liu & Dowling (2007) evaluated Howe's semi-empirical model (Howe 1998) using a number of different smooth wall pressure-spectrum theories with enhanced friction velocities and boundary-layer thicknesses to adjust for the presence of roughness, and 284 Q.…”

Excitation of Airborne Acoustic Surface Modes Driven by a Turbulent Flow

“…Experimental studies have examined the impact of roughness [1][2][3] and discontinuous surface features [4] using wall-jet measurements with a turbulent flow. Various surface topologies have been developed to mitigate the effects of these surfaces on the flow: for instance, the study of flow-generated sound and pressure fluctuations produced by a range of surface treatments, including ordered roughness fetches [5][6][7][8], bioinspired canopy structures [9], and porous surfaces [10].…”