Aeroacoustics of flow duct with multiple cavities

The objective of this paper is to verify the concept of passive noise reduction in an in-duct device (cavity) through enhancement of noise interference. Design requirements and space limitations often cause the flow to change direction, thus leading to branching and the necessity of introducing in-duct devices, which will invariably affect the flow and acoustics inside the ductworks. Turbulence, and consequently noise, is also generated at these in-duct devices. A simple yet common form of in-duct device is a cavity. Aeroacoustic behavior of 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 in-duct devices. The flow-resonant behavior and noise radiation of a cavity is altered. Even though each in-duct device would generate its own noise, possibilities exist that the devices could be properly arranged so as to strengthen noise interference, thus leading to less overall noise radiation in the in-duct far field. This possibility of noise control in in-duct devices is investigated in the present study. A two dimensional approach is adopted and duct aeroacoustics is calculated by means of a one-step aeroacoustics simulation based on finite difference direct numerical simulation (DNS) technique. An additional cavity located on the opposite duct wall is used as the controlling device. The position of the additional cavity is varied. Its dimensions and relative locations are varied and the resultant aeroacoustics behavior is assessed. It was found that a 7.9 db reduction of noise power is possible with an offset of one-half cavity length.