Leung R., So R.

Subsonic flow through ductwork has been a topic of interest among aeroacoustics researchers. Seldom can the fluid inside the ductworks flow through without encountering any obstructions or restrictions. Because of design requirements and space limitations, the flow needs to change direction, thus leading to branching and the necessity to introduce duct devices, which will invariably affect the flow and acoustics inside the ductworks. Turbulence, and consequently noise, is also generated at these duct devices. It is a common goal of the engineering designers that the occurrence of noise should be minimized since they are the major source of destructive unsteady loads and noise. Noise generation from flow past an open cavity is strongly dependent on the fluid-resonant behavior within the cavity. The shear layer resonates at Rossiter frequencies and drives the acoustic wave propagation towards the upstream far field. However, in such realistic applications as gas transport systems, the cavity does not exist alone but is enclosed by solid walls and/or surrounded by different kinds of structural discontinuities. The resonant behavior and noise radiation of a cavity altered due to the presence of duct walls and another cavity. If the two cavities are offset, the resultant flow interaction may result in a noise power reduction. This possibility of passive control of noise from duct devices is investigated in the present study. A two dimensional approach is adopted and the flow dynamics and aeroacoustics are calculated by means of a one-step aeroacoustics simulation using a finite difference DNS technique. Two fully open cavities on the opposite sides of a flow duct are studied. Their dimensions and the relative locations are varied and the resultant aeroacoustics behavior is assessed. It was found that a 30% reduction of noise power is possible with an offset of half cavity length.