Effects of Surface-Roughness Geometry on Separation-Bubble Transition

[+] Author and Article Information
Stephen K. Roberts

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

Metin I. Yaras

Department of Mechanical and Aerospace Engineering,  Carleton University, Ottawa K1S 5B6, Canadametin-yaras@carleton.ca

J. Turbomach 128(2), 349-356 (Feb 01, 2005) (8 pages) doi:10.1115/1.2101852 History: Received October 01, 2004; Revised February 01, 2005

This paper presents measurements of separation-bubble transition over a range of surfaces with randomly distributed roughness elements. The tested roughness patterns represent the typical range of roughness conditions encountered on in-service turbine blades. Through these measurements, the effects of size and spacing of the roughness elements, and the tendency of the roughness pattern toward protrusions or depressions (skewness), on the inception location and rate of transition are evaluated. Increased roughness height, increased spacing of the roughness elements, and a tendency of the roughness pattern toward depressions (negative skewness) are observed to promote earlier transition inception. The observed effects of roughness spacing and skewness are found to be small in comparison to that of the roughness height. Variation in the dominant mode of instability in the separated shear layer is achieved through adjustment of the streamwise pressure distribution. The results provide examples for the extent of interaction between viscous and inviscid stability mechanisms.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Surface roughness at the leading edge of a HP turbine nozzle (a), and near the suction peak of a LP turbine blade (b). Note: magnification is higher in (b).

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Figure 2

Schematic of the wind tunnel test section

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Figure 3

Streamwise distribution of the acceleration parameter

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Figure 4

Time-averaged streamwise velocity field—the separation-bubble contour is shown with the dividing streamline. ReL=35,000, krms=0.7μm

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Figure 5

Measured and predicted locations of transition inception

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Figure 6

Streamwise intermittency distributions for selected test cases

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Figure 7

Fourier power spectrum of u′∕Uref for ReL=350,000, CP1 pressure distribution-(a) krms=0.7μm; (b) krms=107μm

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Figure 8

Growth rate of disturbance energy in the u′∕Uref spectrum at fMA (ReL=350,000, CP1 pressure distribution)

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Figure 9

Sample hot-wire signals in the separation bubble for the CP1 (a) and CP4 (b), (c) pressure distributions



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