Turbulence modulation and rotational dynamics of large nearly neutrally buoyant particles in homogeneous isotropic turbulence

This paper is an experimental investigation of turbulence modulation effects by Taylor-scale size particles in the dilute regime. Experiments are performed on a turbulence tank able to provide Homogeneous Isotropic Turbulence at Reλ ≈ 270. A novel experimental technique capable of simultaneously measuring rotational rates of arbitrarily shaped particles and fluid velocity using standard Stereoscopic Particle Image Velocimetry (Stereo-PIV) and Index-of-Refracion matching is presented here. Particles of the same IoR of water with embedded tracers allowed the measurement of the velocity of the portion of particles in the measurement plane. A novel algorithm based on the assumption of solid body rotation, is then used to extract particle rotation rates. We compare the results from two particle shapes to the single phase measurements: spherical and ellipsoidal particles with aspect ratio 2. It is found that spherical particles provide a 15% turbulence reduction, about five times more than what is provided by ellipsoidal particles at the same volume fraction (φv ≈ 0.1%), and with less particle surface area available. These result suggest that there might be an turbulence production mechanism for ellipsoidal particles that is not present for spheres. This hypothesis is supported by spectral analysis. Pivoting effect is observed for both spherical and ellipsoidal particles, but for the latter, the reduction in the small wavenumber region is less evident. Preliminary results of statistics of rotational rates shows that ellipsoidal particles tend to have an enhanced rotational velocity as compared to spheres.QC 20110610