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Grid Turbulence: How Important are Initial Conditions?

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The importance of initial conditions in turbulent flows is a contentious issue, which could have important ramifications relative to our understanding of turbulence and its modelling. This is particularly true for the idealised case of homogeneous isotropic turbulence (HIT). An important theoretical result for HIT is that the turbulence decays following a power-law, with some support for the universality of the power-law exponent. However, recent theoretical, numerical and experimental studies have argued against such a universality. The present study attempts to address these issues and resolve a number of ambiguities found in the literature for decaying grid turbulence and its relation to the ideal case of decaying HIT . Extensive measurements of the turbulence decay downstream of various grids were undertaken with and without a secondary contraction to investigate systematically the effect of initial conditions for this flow. It will be shown that the turbulence only decays following a self-similar manner approximately. It is demonstrated that grid turbulence decays following a non-universal power-law. However, we argue that the decay would be universal should the turbulence be truly homogeneous and isotropic, although a dependence on R-lambda is expected. This dependence should disappear asymptotically as the Reynolds number goes to infinity.

This talk is part of the Engineering Department Energy, Fluids and Thermo seminars series.

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