An experimental investigation of the fluid mechanics of an unsteady, three-dimensional separation.

摘要

low separation often has devastating consequences. Separation reduces heat transfer, stalls compressors, increases vehicle drag, and reduces aircraft lift. The conventional model for stationary, two-dimensional separation has repeatedly been found to be inadequate for three-dimensional and/or nonstationary flows, which currently are regimes of considerable interest.;The objective of the present experimental program has been to characterize the time development and decay of a pressure-driven, nonstationary, three-dimensional, laminar separation. To achieve this goal, a water tunnel and its flow control scheme were completely redesigned. The modified tunnel enables the study of complex, stationary and nonstationary, two- and three-dimensional flows.;A three-dimensional separation was generated on a flat plate by impulsively altering the freestream velocity field. A square-wave oscillation was chosen in order to distinguish between fast and slow responses of the separating boundary layer. Flow visualization and two-component LDA velocity measurements have been used to document the evolving flow. Known initial conditions and boundary conditions aid comparison with the direct numerical simulation of Pauley, et al. (1988).;The results reveal that an initially two-dimensional, laminar boundary layer evolves through several stages as a three-dimensional separation develops and decays. The developing three-dimensional separation passed through four distinct stages as it approached quasi-steady state. The quasi-steady separation embodied a complex, unsteady, vortical structure known as an Owl-face of the first kind. Discrete vortices shed from the quasi-steady, separated shear layer at a characteristic frequency f that satisfies