Experimental and Computational Studies on Flow structure of Turbulent Separated Regions and Their Manipulation

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1. Separated regions formed at the leading edge of a blunt plate and a blunt circular cylinder were experimentally studied to obtain (1) time-mean properties, Reynolds stresses, turbulence-energy spectra, correlations, etc. of fluctuating velocities and surface pressure; (2) unsteady-flow properties such as the reverse-flow intermittency, the frequency of switching of the local-flow direction, the convection velocity of coherent structures; (3) properties of the coherent structures in the reattaching regin of the separated shear layer in terms of the phase-averaged space-time distributions of the velocity, vorticity and surface; (4) similarity of the nominally two-dimensional and axisymmetric separated regions.2. Manipulation of the two-dimensional separated region can be effectively achieved by periodically segmenting the separation edge into isosceles triangle. The free-stream turbulence can serve as an effective means to reduce the longitudinal length of the separated region for the … More two-demensional and axisymmetric cases.3. A two-dimensional discrete-vortex method was obtained to simulate separated unsteady flows in centrifugal impellers, which appear when the flow eate is off the value at the design point. The unsteady flow due to the separated region was found to have a dominant effect on the total pressure rise through the impeller.4. A three-dimensional discrete-vortex model was employed to simulate interaction vetween vortex filaments of various shapes. These simulations gave us a clue to understand the spanwise or circumferential deformation of the separated shear layer and the eventual formation of the three-dimensional coherent structures in the reattaching region.5. The structure and motion of the coherent structures obtained in the present study successfully modelled the turbulent properteis and unsteady-flow properties in the separated regions. The pseudo periodic shedding of the coherent structures from the separated regions was suggested to be prodeced by a feedback mechanism in which disturbances produced by the impinging coherent structures propagate upstream to excite the shear layer near the separation edge. Less