IDENTIFYING THE ACTIVE FLOW REGIONS IN ACOUSTIC-HYDRODYNAMIC FEEDBACK MECHANISMS

Acoustic-hydrodynamic feedbacks are a common theme in jet noise. Strong sound emissions are supported by fluid instabilities, whose core is not necessarily localized in space. A common example is the feedback-loop insta- bility of cavity flows, impinging jets or the flow past air- foils. The feedback-loop is composed of a convective in- stability, which is usually an instability of the shear layer, and an acoustic pressure wave or a hydrodynamic non- local effect. Despite the fact that such a mechanism is widely accepted, a precise identification of the most sen- sitive spatial regions underpinning the instability is miss- ing. Herein, we propose a non-local decomposition of the structural sensitivity, which allows us to precisely identify the most sensitive regions of the flow responsible for the closure of the feedback-loop. The systematic use of these techniques could be applied in the design of passive flow control devices.