Properties of particle-generated turbulence in the final-decay period

The properties of turbulence generated by uniform fluxes of polydisperse spherical particles moving through uniform flowing air are studied experimentally, emphasizing the properties of the turbulent interwake region surrounding the individual particle-wake disturbances. Mean and fluctuating velocities, as well as probability density functions, energy spectra and integral and Taylor length scales of velocity fluctuations, were measured within a counterflow particle/air wind tunnel using particle-wake discriminating laser velocimetry. Test conditions involved various binary mixtures of spherical glass particles having nominal diameters of 0.55, 1.1, and 2.2 mm and particle Reynolds numbers of 106, 373, and 990. When combined with earlier measurements limited to monodisperse spherical glass particles, the test conditions included mean particle spacings of 13-208 mm, particle volume fractions less than 0.003%, direct rates of dissipation of turbulence kinetic energy by particles less than 4%, and turbulence generation rates sufficient to yield streamwise and cross-stream gas velocity fluctuations, normalized by the streamwise mean velocity of the particles relative to the gas, in the range 0.2-1.5%. The turbulent interwake region for these conditions has properties that correspond to the final-decay period of grid-generated turbulence, including homogeneous and nearly isotropic turbulence having probability density functions that were well approximated by Gaussian functions with turbulence Reynolds numbers of 0.4-3.5. Mixing rules were developed, which successfully extended earlier results for the interwake turbulence properties of monodisperse particle phases to polydisperse particle phases, based on dissipation weighting of the properties of each particle size group. The flow in the final-decay period consisted of vortical regions that filled the turbulent interwake region but were sparse, which resulted in several unusual features of this region compared to conventional isotropic turbulence, as follows: enhanced rates of dissipation of turbulence kinetic energy, unusually large ratios of integral/Taylor length scales for conditions involving small turbulence Reynolds numbers, and decreasing ratios of integral/Taylor length scales with increasing turbulence Reynolds numbers, which is just opposite to the behavior of conventional grid-generated turbulence at large turbulence Reynolds numbers. The large range of scales where effects of viscosity were small in the final-decay region also yielded a Kolmogorov-like -5/3 power inertial decay region of one-dimensional energy spectra on dimensional grounds, similar to the inertial decay region of conventional turbulence at large turbulence Reynolds numbers.