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INFLUENCE OF UPSTREAM TURBULENCE ON SELF-SUSTAINED OSCILLATIONS IN AN OPEN CAVITY

Sang Bong Lee
Deparment of Mechanical Engineering, Korea Advanced Institute of Science and Technology 373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701 Korea

Hyung Jin Sung
Department of Mechanical Engineering, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea

Abstract

Direct numerical and large eddy simulations of incompressible turbulent flows over deep and shallow cavities were performed in the range of 600 ≤ ReD ≤ 12000 to investigate the influence of the incoming turbulent boundary layer on self-sustained oscillations of the shear layer. When the turbulent boundary layer of Reθ = 300 approached the open cavity with ReD = 3000, the energy spectra of the pressure fluctuations showed energetic frequencies in the range of 0.15 ≤ ωθ ≤ 0.3. Conditionally averaged flow fields disclosed that the energetic frequencies arise from the separation of high speed streaky structures rather than from a geometric peculiarity of the cavity. The same energetic frequencies were observed in a backward-facing step flow as well as in deep and shallow cavity flows, despite the different geometries of these systems. In the turbulent cavity flow of ReD = 12000, however, the peak frequencies of the energy spectra at cavity lengths of L/D = 1 and 2 were found to correspond to the N th modes with N = 2 and 3 respectively. These N th modes were very similar to the frequency characteristics of self-sustained oscillations reported for laminar cavity flows. Inspection of instantaneous pressure fluctuations as well as spanwise-averaged pressure fluctuations revealed that regular shedding of quasi two-dimensional vortical structures was responsible for the peak frequency in the energy spectra.