Interaction of perturbed free shear layer with flat plate

Merati, Parviz; Adrian, Ronald J.

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https://hdl.handle.net/2142/112218 Copy

Description

Title

Interaction of perturbed free shear layer with flat plate

Author(s)

• Merati, Parviz
• Adrian, Ronald J.

Issue Date

1986-01

Keyword(s)

• Perturbed Free Shear Layer
• Flate Plate

Abstract

The interaction of a plane, two-dimensional turbulent shear layer with a thin, flat plate located downstream from the origin of the shear layer has been studied experimentally and theoretically. The shear layer was created by two parallel air streams with velocity U1 = 7.31 ms-1 and U2 = 2.74 ms-1 and Reynolds number of 620 based on the velocity difference and the initial momentum thickness. Flow motion was visualized using a smoke wire, and measured using hot wire probes. The spatial growth of the initial instablity of the non-impinging shear layer was determined from measurements of the power spectrum of the streamwise velocity component as a function of distance downstream. The initial spectrum consisted of background wind tunnel noise and period components associated with the passage of the blower blades. The shear layer exhibited a region of exponential growth of the disturbances (linear instability) up to 6 cm downstream, followed by regions non-linear saturation and growth whose location depended upon the frequency and implicitly upon the amplitude. Amplification rates in the linear region were in fair agreement with the linear theory of Monkewist and Huerre 1982. Development of harmonics caused by non-linearity was documented. Mean vortex passage rates decreased smoothly with downstream distance, but abrupt increases in the momentum thickness occurred at the mean location of vortex pairings. Studies with a thin flat plate whose leadin edge is located at a distance XL downstream on the centerline of the shear layer show that the shear layer vortices impinging upon the leading edge create acoustic disturbance that propogate upstream and influence the initial growth of the shear layer. This feedback effect is analyzed by a simple model in which the feedback is taken to be proportional to the disturbance at the leading edge, and the shear layer growth is described by linear stability theory. The theoretical closed-loop system response describes frequency dependent gain of the shear layer that is observed in the experiments as a function of XL. Closed loop gains more than six times larger than the open loop gain have been observed. The flow of the large scale shear layer vortices around the flat plate has been studied. Vortices are initial cut by the leading edge, where a small separation bubble may form. At the trailing edge, a wake appears when the plate is long enough, and secondary set of shear layer vortices is observed.

Publisher

Department of Theoretical and Applied Mechanics. College of Engineering. University of Illinois at Urbana-Champaign

Series/Report Name or Number

• TAM R 477
• 1986-6002

ISSN

0073-5264

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/112218

Sponsor(s)/Grant Number(s)

Tsi Inc St Paul Mn 86/01

Copyright and License Information

Copyright 1986 Board of Trustees of the University of Illinois

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