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Is grid turbulence Saffman turbulence?

Published online by Cambridge University Press:  11 December 2009

P.-Å. KROGSTAD  and
P. A. DAVIDSON
P.-Å. KROGSTAD*
Affiliation:
Norwegian University of Science and Technology, N-7491 Trondheim, Norway
P. A. DAVIDSON
Affiliation:
University of Cambridge, Cambridge CB2 1PZ, UK
*
†Email address for correspondence: per.a.krogstad@ntnu.no
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Abstract

There has been a longstanding debate as to whether the large scales in grid turbulence should be classified as of the Batchelor or Saffman type. In the former, the integral scales, u and ℓ, satisfy u2ℓ5 ≈ constant, while in Saffman turbulence we have u2ℓ3 = constant. For strictly homogeneous turbulence the energy decay rates in these two types of turbulence differ, with u2 ~ t−10/7 in Batchelor turbulence and u2 ~ t−6/5 in Saffman turbulence. We present high-resolution measurements of grid turbulence taken in a large wind tunnel. The particularly large test section allows us to measure energy decay exponents with high accuracy. We find that the turbulence behind the grid is almost certainly of the Saffman type, with u2ℓ3 = constant. The measured energy decay exponent, however, is found to lie slightly below the theoretical prediction of u2 ~ t−1.2. Rather we find u2 ~ t−n, with n = 1.13±0.02. This discrepancy is shown to arise from a weak temporal decay of the dimensionless energy dissipation coefficient, εℓ/u3, which is normally taken to be constant in strictly homogeneous turbulence, but which varies very slowly in grid turbulence.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 642 , 10 January 2010 , pp. 373 - 394
Copyright
Copyright © Cambridge University Press 2009

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References

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