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THE DECAY OF HOMOGENEOUS TURBULENCE GENERATED BY MULTI-SCALE GRIDS

P. C. Valente
Turbulence, Mixing and Flow Control Group, Department of Aeronautics Imperial College London, London SW7 2AZ, United Kingdom

John Christos Vassilicos
Turbulence, Mixing and Flow Control Group, Department of Aeronautics Imperial College London, London SW7 2AZ, United Kingdom

Аннотация

An experimental investigation of freely decaying homogeneous, quasi-isotropic turbulence generated by a low-blockage space-filling fractal square grid is presented. We find good agreement with previous work by Mazellier & Vassilicos ["Turbulence without the Richardson-Kolmogorov cascade", Phys. Fluids 22, 075101 (2010)] but also extend the length of the assessed decay region and consolidate the results by repeating the experiments with a second probe of increased spatial resolution. It is confirmed that this moderately high Reynolds number turbulence (150 < Reλ < 400) does not follow the classical high Reynolds number scaling of the dissipation rate ε ~ u'3/L which is in fact equivalent to a proportionality between the Taylor-based Reynolds number Reλ and the ratio of integral scale L to Taylor micro-scale λ. Instead we observe a constant ratio L/λ whilst Reλ decays during free turbulence decay. Alternative reasons for this non-classical behaviour are discussed, including various ways in which the turbulence may fall into a self-preserving, single-length-scale state. It is also shown that the measured 3D energy spectra can be reasonably collapsed using a single length-scale over the entire decay region even though the Reynolds number is high enough for conventional decaying turbulence to display spectra with two-scale (inner and outer) Kolmogorov scaling.