Turbulence collapses at a threshold particle loading in a dilute particle-gas suspension

Two mechanisms are considered responsible for the turbulence modification due to suspended particles in a turbulent gas-particle suspension. Turbulence augmentation is due to the enhancement of fluctuations by wakes behind particles, whereas turbulence attenuation is considered to result from the increased dissipation due to the particle drag. In order to examine the turbulence attenuation mechanism, Direct Numerical Simulations (DNS) of a particle-gas suspension are carried out at a Reynolds number of about 3333 based on the average gas velocity $\bar{u}$, channel width $h$, and the gas kinematic viscosity. The particle Reynolds number based on the particle diameter $d_p$, gas kinematic viscosity and the flow velocity $\bar{u}$ is about 42 and the Stokes number is in the range $7-450$. The particle volume fraction is in the range $0-2 \times 10^{-3}$, and the particle mass loading is in the range $0-9$. As the volume fraction is increased, a discontinuous decrease in the turbulent velocity fluctuations is observed at a critical volume fraction. from $9 \times 10^{-4}$ to $1 \times 10^{-3}$. There is a reduction, by one order of magnitude, in the mean square fluctuating velocities in all directions and in the Reynolds stress. Though there is a modest increase in the energy dissipation due to particle drag, this increase is smaller than the decrease in the turbulent energy production; moreover, there is a decrease in the total energy dissipation rate when there is turbulence collapse. Thus, turbulence attenuation appears to be due to a disruption of the turbulence production mechanism, and not due to the increased dissipation due to the particles. There is a discontinuous collapse in the turbulence intensities at a critical particle loading, instead of the continuous decrease as the particle loading is increased.