| The properties of turbulence generated by uniform fluxes of polydisperse particles moving through gaseous flows were studied theoretically and experimentally, emphasizing the properties of the turbulent inter-wake region surrounding the individual particle wake disturbances. Mean and rms fluctuating velocities as well as PDF, energy spectra, and the integral and Taylor length scales of velocity fluctuations, were measured within a particle/air counterflow wind tunnel using particle-wake discriminating laser velocimetry. Present test conditions involving various binary mixtures of spherical glass particles were combined with earlier measurements limited to monodisperse spherical particles to yield conditions that were typical of practical applications in nature and technology. The present turbulent inter-wake regions had properties that correspond to the final-decay period of isotropic turbulence, e.g., turbulence Reynolds numbers of 0.4--3.5. Mixing rules were developed that successfully extended earlier methods of predicting the inter-wake turbulence properties of monodisperse particle phases to more practical polydisperse particle phases, based on dissipation weighting of the properties of each particle size group. Isotropic turbulence in the final-decay period within the turbulent inter-wake region had several unusual features compared to conventional isotropic turbulence in the initial-decay period, as follows: rates of dissipation of turbulence kinetic energy were enhanced (by factors of 100--10,000 for present conditions), ratios of integral/Taylor length scales were unusually large (ratios of 10--1,000 for present conditions), and ratios of integral/Taylor length scales decreased with increasing turbulence Reynolds numbers, which is just opposite to the behavior of conventional isotropic turbulence in the initial-decay period. In spite of the these differences between the properties of isotropic turbulence in the initial- and final decay periods, however, the one-dimensional energy spectra in both cases exhibited a -5/3 power decay region at large wave numbers; this behavior is observed because both flows satisfy the Kolmogorov requirements for the presence of an inertial decay region of the energy spectra and as a result yield this behavior on dimensional grounds. Finally, the new measurements provide an improved picture of the physical structure of isotropic turbulence in the final-decay period that helps explain the differences in behavior between isotropic turbulence in the initial- and final-decay period. |
