(518a) Turbulence Collapse in a Particle-Gas Suspension

The flow of turbulent particle-gas suspensions is considered at high flow Reynolds number, moderate particle Reynolds number (ratio of fluid inertia and viscosity at the particle scale) and high Stokes number (ratio of particle inertia and fluid viscosity). At moderate particle Reynolds number, the interaction between the particles and fluid is described by a Reynolds number dependent extension of the Stokes drag law valid for Reynolds number up to 100. At high Stokes number, the particles cross streamlines due to high inertia. The volume fraction is in the range 10^-3, but the mass fraction is O(1) due to the high ratio of the particle and gas densities. There is two-way coupling, where the fluid turbulent fluctuations exert a force on the particles due to drag, and the particles exert a reverse force on the fluid. The gas turbulence is simulated using DNS (Direct Numerical Simulations) where all the turbulence scales are resolved, while the particles are explicitly simulated using the point particle approximation.

It is observed that at very low particle volume fraction in the range 10^-3, there is a critical volume fraction at which there is turbulence collapse. There is a discontinuous decrease in the turbulence intensities by 1-2 orders of magnitude in all the directions and in the Reynolds stress when the volume fraction is decreased by less than 10^-4. It is shown that this decrease is due to a disruption of the rate of turbulent energy production by the particles, and this disruption occurs at the largest turbulence scale which is the flow scale. This is contrary to the conventional wisdom, that the turbulence attenuation is a gradual process which is due to the increase in the dissipation of energy due to particle drag.