Assessment of fluctuating drag model for a single particle larger than the Kolmogorov length scale Available to Purchase

When a solid particle in a fully developed turbulence is larger than the Kolmogorov length scale, the turbulent momentum, transmitted to the particle by random advection past particle of dissipative structures by eddies of order of the particle diameter, tends to reduce the relative velocity between the particle and the fluid. In this work, a corresponding model of the effective drag is discussed and numerically assessed with the experimental study. The velocity gradient in the fluid around the particle is the key-variable of this model, and consequently, the model highlights the role of the flow structure on the particle dynamics. In the simulation of the particle motion in the periodic box turbulence (the latter is resolved by Direct Numerical Simulation—DNS), the model reproduces fairly well the statistical properties known from measurements. First, in accordance with measurements, the simulation shows a universality of normalized distributions of the particle velocity and its fluctuation rate of the particle acceleration and its autocorrelation function—these distributions along the particle trajectory are almost insensitive to parameters of the particle inertia. Thereby, the typical correlation time of the particle acceleration is of the order of the Kolmogorov timescale, i.e., this correlation time is much less than the viscous relaxation time to the low-frequency solicitations in turbulence. In turn, the particle acceleration variance does depend on parameters of the particle inertia, and this dependency is predicted in the simulation consistently with the experiment. Second, the computed distributions of the particle acceleration as well as those of the particle velocity increments at small time lags expose the stretched tails. This is the way in which the intermittency of the flow structure is manifested: the intense velocity gradients in the fluid induce the strong fluctuations of the particle drag. Probability density functions of normalized Voronoï volumes indicate that increasing the particle density and, to a lesser extent, the particle size favors the preferential accumulation of particles above the Kolmogorov size.