| مستخلص: |
In this paper, the classical triple-deck formalism is employed to investigate two instability problems in which acoustic feedback loop plays an essential role. The first concerns a boundary layer over a flat plate, on which two well separated roughness elements are present. A spatially amplifying Tollmien-Schlichting (T-S) wave between the roughness elements is scattered by the downstream roughness to emit a sound wave, which propagates upstream and impinges on the upstream roughness to regenerate the T-S wave thereby forming a closed feedback loop in the streamwise direction. Numerical calculations suggest that at high Reynolds numbers and for moderate roughness heights the long-range acoustic coupling may lead to global instability, which is characterized by self-sustained oscillations at discrete frequencies. The dominant peak frequency may jump from one value to another as the Reynolds number, or the distance between the roughness elements, is varied gradually. The second problem concerns supersonic 'twin boundary layers', which develop along the two well-separated parallel flat plates. The two boundary layers are in mutual interaction through the impinging and reflected acoustic waves. It is found that the interaction leads to a new instability that is absent in the usual unconfined boundary layer. [ABSTRACT FROM AUTHOR] |
