Two-time statistics of inertial particles dynamics in wind tunnel grid generated turbulence

The two-time statistics of finite-sized inertial particles dynamics in grid generated wind tunnel turbulence are discussed in this paper. For different sets of particle normalized sizes ranging from 12η → 26η (η is the Kolmogorov´s length scale of the flow); and densities normalized with that of air varying from 1 to 70, the Lagrangian velocity signals are recorded using Ultrasonic Doppler Velocimetry (UDV). These signals are then processed employing Maximum Likelihood Algorithm (MLA). The Lagrangian velocity increments PDF of all the data sets show that particle dynamics is intermittent. The signature of intermittency as determined through time evolution of flatness of Lagrangian Velocity is noticed to depend upon particle density. The Lagrangian integral time scale for same particle size depends upon the particle´s density, with heavier particles having longer correlation times. From acceleration autocorrelations it is found that the particle´s response time to the turbulent forcing τ_exp is always of the order of Kolmogorov´s time scale. Thus, an effective or experimental stokes number St_eff is defined which is based upon the actual response time of the particles τ_exp. For our data sets this effective stokes number varies from 0.1 → 0.2.