STUDY ON TURBULENCE STRUCTURE AND ACTIVE CONTROL OF A THREEDIMENSIONAL SEPARATED FLOW
Project/Area Number  05452148 
Research Category 
GrantinAid for General Scientific Research (B)

Allocation Type  Singleyear Grants 
Research Field 
Fluid engineering

Research Institution  HOKKAIDO UNIVERSITY 
Principal Investigator 
KIYA Masaru Hokkaido University, Fac.of Eng., Professor, 工学部, 教授 (50001160)

CoInvestigator(Kenkyūbuntansha) 
IDO Yasusi Hokkaido University Fac.of Eng., Instructor, 工学部, 助手 (40221775)
MOCHIZUKI Osamu Hokkaido University, Fac.of Eng., Associate Professor, 工学部, 助教授 (50157830)

Project Period (FY) 
1993 – 1994

Project Status 
Completed(Fiscal Year 1994)

Budget Amount *help 
¥6,200,000 (Direct Cost : ¥6,200,000)
Fiscal Year 1994 : ¥2,700,000 (Direct Cost : ¥2,700,000)
Fiscal Year 1993 : ¥3,500,000 (Direct Cost : ¥3,500,000)

Keywords  Turbulent shear flow / Flow separation / Flow reattachment / Flow control / Active control / Sinusoidal disturbance / Separation bubble / Separated shear layr / はく離流れ / 乱流はく離泡 / 周期的攪乱 / 乱流 / 鈍頭物体 
Research Abstract 
A turbulent separation bubble formed by the boundarylayr separation from the leading edge of a blunt circular cylinder of semiinfinite length was forced by a singlefrequency and twofrequency 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 singlefrequency 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 mainflow velocity. A flow model was proposed to successfully interpret this frequency (referred to as the mosteffective 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 mainflow velocity. This
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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 selfexcited flow maintained by a feedback loop : The disturbance produced by a largescale 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 rollingup of the separated shear layr. (4) The structure of largescale 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 twofrequency 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 twofrequency forcing yields approximately the same reduction of the reattachment length as the singlefrequency forcing if the forcing amplitude is of the order of 10% of the mainflow velocity. A discretevortex 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

Report
(3results)
Research Output
(20results)