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Article

High Frequency Surface Heat Flux Imaging of Bypass Transition

[+] Author and Article Information
Richard J. Anthony

AFRL Propulsion Directorate Wright-Patterson, AFB, OH 45433 richard.anthony@wpafb.af.mil

Terry V. Jones

Department of Engineering Science,  University of Oxford, Oxford, UK terry.jones@eng.ox.ac.uk

John E. LaGraff

Department of Mechanical, Aerospace, and Manufacturing Engineering,  Syracuse University, Syracuse NY 13201 jlagraff@syr.edu

J. Turbomach 127(2), 241-250 (Mar 01, 2004) (10 pages) doi:10.1115/1.1860379 History: Received October 01, 2003; Revised March 01, 2004

A high-frequency surface heat flux imaging technique was used to investigate bypass transition induced by freestream turbulence. Fundamental experiments were carried out at the University of Oxford using high-density thin film arrays on a flat plate wind tunnel model. Bypass transition was induced by grid-generated turbulence with varying intensities of 2.3%, 4.2%, and 17% with a fixed integral length scale of approximately 12mm. Unique high resolution temporal heat flux images are shown which detail significant differences between unsteady surface heat flux events induced by freestream turbulence and the classical Emmons-type spots which many turbomachinery transition models are based on. The temporal imaging technique presented allows study of unsteady surface heat transfer in detail, and helps elucidate the complex nature of transition in the high-disturbance environment of turbomachinery.

Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

2D disturbances affecting turbine blade boundary layers. (Sketch by Coupland (28).)

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Figure 2

How do turbulent spot characteristics in a high disturbance environment differ from the classical Emmon’s type spot? (Sketch by Schubauer and Klebanoff 29.)

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Figure 3

Platinum gauge layout for flat plate model having 15 different thin film gauge arrays with 233 total thin film sensors. (Instrumented surface is 332mm×150mm.)

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Figure 4

The platinum arrays are sputtered onto a thin flexible sheet that is laid over the model surface

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Figure 5

Completed perspex flat plate model with platinum sensor surface and gold-plated pin-out connections

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Figure 6

Magnified image of two high density platinum thin film arrays aligned in the spanwise direction (perpendicular to the flow). Each array consists of 27 sensors. Note the platinum is shown dark and the substrate is shown light in this figure.

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Figure 7

Turbulent spot heat flux traces measured along streamwise arrays #5 and #6. Intermittent turbulent spots can be tracked as they convect downstream.

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Figure 8

Surface heat flux image in the x–t plane showing turbulent spots appearing and growing as they convect down the model surface

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Figure 9

Example surface heat flux image in the z–t plane recorded from a wide, low resolution array at the rear of the plate

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Figure 10

Test section and inlet of the Oxford 150mm×150mm suction tunnel with transient flow bypass flap

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Figure 11

Bar grid arrangement for three different turbulence intensities at constant integral length scale

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Figure 12

Surface heat flux footprints of large, mature turbulent spots in a low disturbance environment (top two images) compared with turbulent spot heat flux under moderate freestream turbulence (bottom three images). Turbulence-induced bypass spots appear at much lower Reynolds number and are very streaky in nature.

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Figure 13

Increasing local Reynolds number within the transition region under fixed freestream turbulence intensity Tu=2.3% (grid A). As Reynolds number increases, streaky turbulent patches merge and coalesce into a fully turbulent boundary layer.

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Figure 14

Unusual examples of spots with an apparent arrowhead shape under 2% turbulence intensity. Most spots seen under moderate freestream turbulence have a rather irregular asymmetrical shape.

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Figure 15

The numerical simulation by Jacobs and Durbin (12) shows the streaky nature of bypass transition under freestream turbulence. Note when comparing; this is a spatial image in the x–z plane, with flow from left to right, as opposed to a temporal heat flux image in the z–t plane where spots are pointed right to left.

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Figure 16

Irregular “nonarrowhead” spot shapes usually appear in the bypass transitional boundary layer. The swept-back spot leading edge angle, however, is fairly consistent and scales approximately with freestream velocity.

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Figure 17

Turbulence-induced spots in the x–t plane. Faint tracks of enhanced heat flux are seen upstream of the jump to turbulent heat flux.

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Figure 18

x–t image of turbulence-induced bypass transition at higher Reynolds number. Initial spot structures can be tracked through the turbulent boundary layer downstream.

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Figure 19

Effect of increasing freestream turbulence intensity at a set local Reynolds number of 1.6×105 and fixed integral length scale of approximately 12mm

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Figure 20

Wavy transitional heat flux structures under strong freestream turbulence intensity (Tu=17.5%, grid C). The color scale has been adjusted to bring out the detail of the turbulent heat flux structure.

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