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Freely decaying two-dimensional turbulence

Published online by Cambridge University Press:  12 July 2010

S. FOX*
Affiliation:
Department of Engineering, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, UK
P. A. DAVIDSON
Affiliation:
Department of Engineering, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, UK
*
Email address for correspondence: [email protected]

Abstract

High-resolution direct numerical simulations are used to investigate freely decaying two-dimensional turbulence. We focus on the interplay between coherent vortices and vortex filaments, the second of which give rise to an inertial range. We find that Batchelor's prediction for the inertial-range enstrophy spectrum Eω(k, t) ~ β2/3k−1, where β is the enstrophy dissipation rate, is reasonably well satisfied once the turbulence is fully developed, but that the assumptions which underpin the usual interpretation of his theory are not valid. For example, the lack of a quasi-equilibrium cascade means the enstrophy flux Πω(k) is highly non-uniform throughout the inertial range, thus the common assumption that β can act as a surrogate for Πω(k) becomes questionable. We present a variant of Batchelor's theory which accounts for the wavenumber-dependence of Πω; in particular we propose Eω(k, t) ~ Πω(k1)2/3k−1, where k1 is the wavenumber marking the start of the observed k−1 region of the enstrophy spectrum. This provides a better collapse of the data and, unlike Batchelor's original theory, can be justified on theoretical grounds. The basis for our proposal is the observation that the straining of the vortex filaments, which fuels the enstrophy flux through the inertial range, comes almost exclusively from the strain field of the coherent vortices, and this can be characterized by Πω(k1)1/3. Thus Eω(k) is a function of only k and Πω(k1) in the inertial range, and dimensional analysis then yields Eω ~ Πω(k1)2/3k−1. We also confirm the prediction by Davidson (Phys. Fluids, vol. 20, 2008, 025106) that in the inertial range Πω varies as Πω(k)/Πω(k1) = 1 − a−1 ln(k/k1), where a is a constant of order 1. This corresponds to ∂Eω/∂t ~ k−1. Surprisingly, the measured enstrophy fluxes imply that the dynamics of the inertial range as defined by the behaviour of Πω extend to wavenumbers much smaller than k1, but this is masked in Eω(k, t) by the presence of coherent vortices which also contribute to Eω in this region. In fact, we find that kEω(k, t) ≈ H(k) + A(t), or ∂Eω/∂t ~ k−1 in this extended low-k region, where H(k) is almost independent of time and represents the signature of the coherent vortices. In short, the inertial range defined by ∂Eω/∂t ~ k−1 or Πω(k) ~ ln(k) is much broader than the observed Eω ~ k−1 region.

Type
Papers
Copyright
Copyright © Cambridge University Press 2010

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References

REFERENCES

Batchelor, G. K. 1953 The Theory of Homogeneous Turbulence. Cambridge University Press.Google Scholar
Batchelor, G. K. 1969 Computation of the energy spectrum in homogeneous two-dimensional turbulence. Phys. Fluids 12, 233239.CrossRefGoogle Scholar
Brachet, M. E., Meneguzzi, M., Politano, H. & Sulem, P. L. 1988 The dynamics of freely decaying two-dimensional turbulence. J. Fluid Mech. 194, 333349.CrossRefGoogle Scholar
Davidson, P. A. 2008 Cascades and fluxes in two-dimensional turbulence. Phys. Fluids 20, 025106.CrossRefGoogle Scholar
Dritschel, D. G., Scott, R. K., Macaskill, C. & Tran, C. V. 2008 Unifying scaling theory for vortex dynamics in two-dimensional turbulence. Phys. Rev. Lett. 101 (9), 094501.CrossRefGoogle ScholarPubMed
Dritschel, D. G., Tran, C. V. & Scott, R. K. 2007 Revisiting Batchelor's theory of two-dimensional turbulence. J. Fluid Mech. 591, 379391.CrossRefGoogle Scholar
Fox, S. & Davidson, P. A. 2008 Integral invariants of two-dimensional and quasigeostrophic shallow-water turbulence. Phys. Fluids 20, 075111.CrossRefGoogle Scholar
Kolmogorov, A. N. 1941 Local structure of turbulence in an incompressible viscous fluid at very large Reynolds number. Dokl. Akad. Nauk SSSR 30 (4), 299303.Google Scholar
Kraichnan, R. H. 1967 Inertial ranges in two-dimensional turbulence. Phys. Fluids 10, 1417.CrossRefGoogle Scholar
Kraichnan, R. H. 1971 Inertial-range transfer in two- and three-dimensional turbulence. J. Fluid Mech. 47, 525535.CrossRefGoogle Scholar
Lindborg, E. & Alvelius, K. 2000 The kinetic energy spectrum of the two-dimensional enstrophy turbulence cascade. Phys. Fluids 12 (5), 945947.CrossRefGoogle Scholar
Lowe, A. & Davidson, P. A. 2005 The evolution of freely-decaying, isotropic, two-dimensional turbulence. Eur. J. Mech. B 24, 314.CrossRefGoogle Scholar
Maltrud, M. E. & Vallis, G. K. 1991 Energy-spectra and coherent structures in forced 2-dimensional and beta-plane turbulence. J. Fluid Mech. 228, 321.Google Scholar
Oetzel, K. G. & Vallis, G. K. 1997 Strain, vortices, and the enstrophy inertial range in two-dimensional turbulence. Phys. Fluids 9, 29913004.CrossRefGoogle Scholar
Tran, C. V. & Dritschel, D. G. 2006 Vanishing enstrophy dissipation in two-dimensional Navier–Stokes turbulence in the inviscid limit. J. Fluid Mech. 559, 107116.CrossRefGoogle Scholar