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Coherence measurements in synthetic turbulent boundary layers

Published online by Cambridge University Press:  20 April 2006

Ömer Savas  and
Donald Coles
Ömer Savas
Affiliation:
School of Aerospace. Mechanical, and Nuclear Engineering, University of Oklahoma, Norman, Oklahoma 73019
Donald Coles
Affiliation:
Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, California 91125
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Abstract

Synthetic turbulent boundary layers were constructed on a flat plate by generating systematic moving patterns of turbulent spots in a laminar flow. The experiments were carried out in a wind tunnel at a Reynolds number based on plate length of 1.7 × 106. Spots were generated periodically in space and time near the leading edge to form a regular hexagonal pattern. The disturbance mechanism was a camshaft that displaced small pins momentarily into the laminar flow at frequencies up to 80 Hz. The main instrumentation was a rake of 24 single hot wires placed across the flow in a line parallel to the surface. The main measured variable was local intermittency; i.e. the probability of observing turbulent flow at a particular point in space and time. The results are reported in numerous (x, z, t)-diagrams showing the evolution of various synthetic flows along the plate. The dimensionless celerity or phase velocity of the large eddies was found to be very nearly 0.88, independent of eddy scale. All patterns with sufficiently small scales eventually showed loss of coherence as they moved downstream. A novel phenomenon called eddy transposition was observed in several flows that contained appreciable laminar regions. The original large eddies were replaced by new eddies at new positions, intermediate to the original ones, while preserving the hexagonal pattern. The present results, together with some empirical properties of a turbulent spot, were used to estimate the best choice of scales for constructing a synthetic boundary layer suitable for detailed study as a model for a natural flow. The values recommended are: spanwise period/thickness ≈ 2.5, streamwise period/thickness ≈ 8.0.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 160 , November 1985 , pp. 421 - 446
Copyright
© 1985 Cambridge University Press

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