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Coherence measurements in synthetic turbulent boundary layers

Savaş, Ömer and Coles, Donald (1985) Coherence measurements in synthetic turbulent boundary layers. Journal of Fluid Mechanics, 160 . pp. 421-446. ISSN 0022-1120. doi:10.1017/S0022112085003548.

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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 × 10^6. 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.

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Additional Information:© 1985 Cambridge University Press. Received 11 September 1984 and in revised form 15 May 1985. Published online: 20 April 2006. The research described in this paper was supported by the National Science Foundation under Grants ENG 75-03694 and ENG 77-23541. Ö. Savaş gratefully acknowledges the Fellowship provided by the Scientific and Technical Research Council of Turkey (TUBITAK) during his stay at GALCIT.
Funding AgencyGrant Number
NSFENG 75-03694
NSFENG 77-23541
Scientific and Technical Research Council of Turkey (TUBITAK)UNSPECIFIED
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ID Code:38357
Deposited By: Tony Diaz
Deposited On:09 May 2013 20:07
Last Modified:09 Nov 2021 23:37

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