Diffusion layer thickness in turbulent flow

Burluka, Alexey (2020) Diffusion layer thickness in turbulent flow. International Journal of Heat and Fluid Flow, 81. p. 108530. ISSN 0142-727X

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Abstract

Average thickness of diffusive layers in a turbulent flow is described usingan idea of Lagrangian meso-scale element convected by mean flow and largescale turbulence. This idea enables a formulation of a simple model for thediffusive layer thickness assuming that its evolution is determined by thediffusive growth and two components, compressive normal and tangential, ofthe turbulent strain rate tensor. Analysis of the possible effects of the foldingaction of the turbulence leads to the conclusion that the folding becomessignificant only at the scales far superior to the considered dimensions of themeso-scale elements, thus it may be neglected in the present formulation. Theevolution equation for the meso-scale element thickness is derived and putto test against experiments conducted in plane and round jets. The modelproved capable of producing, using the same values of two model constants,values of the diffusive layer thickness in good qualitative agreement with themeasurements.While the present numerical simulations of the turbulent jets are madeusing very simple, perhaps simplistic, flow and turbulence description, theynonetheless allow a fairly accurate estimation of turbulence microscales atdifferent locations in a jet. It turns out that neither Kolmogorov nor Taylorscale provides a good universal reference scale for the diffusive layer thicknessand it is local turbulence conditions and history of the meso-scale elementdetermining the latter.

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