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Research Papers

Instability and Transition in a Separation Bubble Under a Three-Dimensional Freestream Pressure Distribution

[+] Author and Article Information
M. I. Yaras

Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, ON K1S 5B6, Canadametin_yaras@carleton.ca

J. Turbomach 134(3), 031019 (Jul 15, 2011) (11 pages) doi:10.1115/1.4000533 History: Received December 24, 2008; Revised September 01, 2009; Published July 15, 2011; Online July 15, 2011

This paper presents measurements of the instability and transition processes in separation bubbles under a three-dimensional freestream pressure distribution. The measurements are performed on a flat plate on which a pressure distribution is imposed by a contoured surface facing the flat test-surface. The three-dimensional pressure distribution that is established on the test-surface approximates the pressure distributions encountered on swept blades. This type of pressure field produces crossflows in the laminar boundary layer upstream of the separation and within the separation bubble. The effects of these crossflows on the instability of the upstream boundary layer and on the instability, transition onset, and transition rate within the separated shear-layer are examined. The measurements are performed at two flow-Reynolds numbers and relatively low level of freestream turbulence. The results of this experimental study show that the three-dimensional freestream pressure field and the corresponding redistribution of the freestream flow can cause significant spanwise variation in the separation-bubble structure. It is demonstrated that the instability and transition processes in the modified separation bubble develop on the basis of the same fundamentals as in two-dimensional separation bubbles and can be predicted with the same level of accuracy using models that have been developed for two-dimensional separation bubbles.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the wind-tunnel test-section with 3D ceiling configuration

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Figure 2

Test-section ceiling geometry: (a) 2D and (b) 3D

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Figure 3

Freestream turbulence intensity distribution: (a) ReL=340,000 and (b) ReL=470,000

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Figure 4

Power spectrum of freestream axial velocity fluctuations Vx′

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Figure 5

Streamwise distribution of freestream velocity for the 2D and 3D configurations near midspan

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Figure 6

Streamwise distribution of freestream velocity for the 3D configuration

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Figure 7

Streamwise distribution of freestream flow direction for the 3D configuration

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Figure 8

Tangential and crossflow velocity profiles

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Figure 9

Streamwise distribution of boundary-layer displacement thickness near midspan for 2D and 3D configurations

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Figure 10

Streamwise distribution of boundary-layer displacement thickness for 3D configuration (a) ReL=340,000 and (b) ReL=470,000

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Figure 11

Spanwise vorticity-magnitude and axial velocity distributions in the separation bubble

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Figure 12

Tangential and crossflow velocity profiles in the separation bubble

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Figure 13

Streamwise variation in the intermittency (a) ReL=340,000 and (b) ReL=470,000

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