The Measurement of Velocity and Velocity Gradients in a Turbulent Channel by Using Different Particle Tracers

Nowadays the statistics of velocity and velocity gradients in a turbulent channel flow at low Reynolds numbers are well established following several numerical simulations and experiments. These measurements are usually referred to the flow phase, while the effect of particle tracers is disregarded: the velocity of tracers is just what is measured in experiments. In recent years, investigations on the role of particles in the near-wall region of turbulent channel flows have been performed. A useful parameter to be considered is the particle response time: τp=ρpd2/18ρfv, where ρp,f are particle and fluid density, d is the particle diameter and v the kinematic viscosity. This time scale must be compared with Kolmogorov time scale (τη). For values of the ratio τp/τη less than 100, particles are expected to enhance dissipation. Hereafter the attention is focused on velocity gradients.