Study on Langevin model parameters of velocity in turbulent shear flows

This paper deals with the stochastic equation used to predict the fluctuating velocity of a fluid particle in a nonhomogeneous turbulent flow, in the frame of probability density function (PDF) approaches. It is shown that a Langevin-type equation is appropriate provided its parameters (drift and diffusion matrices) are suitably specified. By following the approach proposed in the literature for homogeneous turbulent shear flows, these parameters have been identified using data from direct numerical simulations (DNS) of both channel and pipe flows. Using statistics extracted from the computation of the channel flow, it is shown that the drift matrix of the stochastic differential equation can reasonably be assumed to be diagonal but not spherical. This behavior of the drift coefficients is confirmed by the available results for a turbulent pipe flow at low Reynolds number. Concerning the diffusion matrix, it is found that this matrix is anisotropic for low Reynolds number flows, a property which has been observed earlier for a homogeneous turbulent shear flow. The pertinence of the present estimation of the drift and diffusion tensors is assessed through different kinds of tests including the incorporation of these parameters in a purely Lagrangian, or stand-alone, PDF computation.