On the Diffusion of Fluid Particles by the Combined Model of Random Fourier Modes and Random Flight.

Abstract

To investigate the motion of fluid particles round a cylinder in turbulent flows, we have developed a new model simulating the trajectory of particles: the combined model of Kinematic Simulation (KS) and Random Flight (RF). The large-scale turbulence is simulated by a sum of random Fourier modes varying in space and time, and the small-scale turbulent fluctuation is simply modelled by ITO type of stochastic differential equation with a memory time comparable to a Lagrangian integral time scale TLS of small-scale motion. The Lagrangian properties of fluid particles in homogeneous isotropic turbulence with uniform mean flow has been examined. Some new results have been obtained using this new model: (a) the cross-correlation of the displacements by the large and small scale motions is negligibly small; (b) the random small-scale motion causes the decorrelation of the large-scale velocity; (c) the value of TLS used in RF is estimated from pure KS.