Reynolds number effect on the concentration and preferential orientation of inertial ellipsoids

Using direct numerical simulation, the dynamics of ellipsoidal particles in a turbulent channel flow is examined at three values of the friction Reynolds number, Re-tau = 180, 400, and 550. Once the flow has reached a statistically steady state, the Lagrangian tracking of 300 000 prolate ellipsoids, modeled as point particles, is conducted. A global parameter based on entropy is used to follow the evolution of the particle concentration and reveals that, when expressed in wall units, the duration of the transient period increases with the Reynolds number. Once the particle distribution has reached a statistically stationary state, the distribution is closer to the uniform distribution for high values of Re-tau, with a small influence of the aspect ratio, but a noticeable effect of particle inertia. In the viscous sublayer, where the mean velocity gradient causes Jeffery-like orbiting of the ellipsoids, preferential orientation is affected by Re-tau. The stronger fluctuations of the fluid vorticity occurring at higher Re-tau are responsible for a destabilization of the weakly stable rotation orbits of the spheroids. Spinning particles are more likely to align with the fluid seen vorticity and spend less time aligned with the mean flow, while tumbling particles are more likely to be found out of the mean velocity-gradient plane.