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Transport equations for the normalized nth-order moments of velocity derivatives in grid turbulence
Published online by Cambridge University Press: 16 November 2021
Abstract
Transport equations for the normalized moments of the longitudinal velocity derivative ${F_{n + 1}}$ (here,
$n$ is
$1, 2, 3\ldots$) are derived from the Navier–Stokes (N–S) equations for shearless grid turbulence. The effect of the (large-scale) streamwise advection of
${F_{n + 1}}$ by the mean velocity on the normalized moments of the velocity derivatives can be expressed as
$C_1 {F_{n + 1}}/Re_\lambda$, where
$C_1$ is a constant and
$Re_\lambda$ is the Taylor microscale Reynolds number. Transport equations for the normalized odd moments of the transverse velocity derivatives
${F_{y,n + 1}}$ (here,
$n$ is 2, 4, 6), which should be zero if local isotropy is satisfied, are also derived and discussed in sheared and shearless grid turbulence. The effect of the (large-scale) streamwise advection term on the normalized moments of the velocity derivatives can also be expressed in the form
$C_2 {F_{y,n + 1}}/Re_\lambda$, where
$C_2$ is a constant. Finally, the contribution of the mean shear in the transport equation for
${F_{n + 1}}$ can be modelled as
$15 B/Re_\lambda$, where
$B$ (
$=S^*{S_{s,n + 1}}$) is the product of the non-dimensional shear parameter
$S^*$ and the normalized mixed longitudinal-transverse velocity derivatives
${{S_{s,n + 1}}}$; if local isotropy is satisfied,
$S_{s,n + 1}$ should be zero. These results indicate that if
${F_{n + 1}}$,
${F_{y,n + 1}}$ and
$B$ do not increase as rapidly as
$Re_\lambda$, then the effect of the large-scale structures on small-scale turbulence will disappear when
$Re_\lambda$ becomes sufficiently large.
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- © The Author(s), 2021. Published by Cambridge University Press
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