STUDY ON TURBULENCE STRUCTURE AND ACTIVE CONTROL OF A THREE-DIMENSIONAL SEPARATED FLOW

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A turbulent separation bubble formed by the boundary-layr separation from the leading edge of a blunt circular cylinder of semi-infinite length was forced by a single-frequency and two-frequency sinusoidal disturbances introduced uniformly along the separation edge. The disturbance was generated by a woofer inside the cylinder and introduced into the separated shear layr through a thin slot. Main results for the single-frequency forcing may be summarized as follows :(1) The reattachment length attains a definite minimum at a particular forcing frequency if the r.m.s.forcing amplitude is less than 10% of the main-flow velocity. A flow model was proposed to successfully interpret this frequency (referred to as the most-effective frequency F) and the minimum reattachment length, which is a logarithmic function of the forcing amplitude.(2) The separation bubble can be eliminated in a range of the forcing frequency for the forcing amplitudes greater than 14% of the main-flow velocity. This … More range of the forcing frequency tends to increase with increasing forcing amplitude. This result is useful in active control of flow separation on aerofoils and cascades of turbomachinery.(3) The separation bubble is a self-excited flow maintained by a feedback loop : The disturbance produced by a large-scale vortex impinging on the surface in the reattachment region of the separation bubble propagates upstream as a pressure wave to be accepted at the sharp separation edge. The disturbance enhances the rolling-up of the separated shear layr.(4) The structure of large-scale vortices in the separation bubble forced at the most effective frequency was obtained in terms of cross correlations of velocity fluctuations at two points separated in the circumferential direction and flow visualization by smoke wires and tuft probes.Main results for the two-frequency forcing may be summarized as follows :(5) The combination of the most effective frequency F and its higher harmonic 2F yields a minimum reattachment length at a phase difference of 0 and a maximum reattachment at another phase difference of pi. This result was successfully interpreted in terms of the subharmonic instability of the classical plane mixing layr.(6) The two-frequency forcing yields approximately the same reduction of the reattachment length as the single-frequency forcing if the forcing amplitude is of the order of 10% of the main-flow velocity.A discrete-vortex numerical simulation of the forced separation bubble reproduced the relation between the reattachment length and the forcing frequency, and more importantly useful pieces of information on a feedback control of the separation bubble. Less