Dynamics of particle-laden turbulent Couette flow: Turbulence modulation by inertial particles

Abstract

In particle-laden turbulent flows, it is established that the turbulence in the carrier fluid phase gets affected by the dispersed particle phase for volume fractions above 10 - 4. Hence, reverse coupling or two-way coupling becomes relevant in that volume fraction regime. Due to their greater inertia, larger particles change either the mean flow or the intensity of fluid-phase fluctuations. In a recent study [Muramulla et al., "Disruption of turbulence due to particle loading in a dilute gas-particle suspension,"J. Fluid Mech. 889, A28 (2020)], a discontinuous decrease of turbulence intensity is observed in a vertical particle-laden turbulent channel flow for a critical volume fraction O(10 - 3) for particles with varying Stokes numbers (St) in the range of 1 - 420 based on the fluid-integral time scales. The collapse of turbulent intensity is found out to be the result of a "catastrophic reduction of turbulent energy production rate."Mechanistically, a turbulent Couette flow differs from a pressure-driven channel flow in many ways, such as fluid-phase mean-velocity profile and turbulent coherent structures. In the particle-laden Couette flow, particles are treated as neutrally buoyant. Therefore, it is worth investigating the mechanism of turbulence modulation by inertial particles in the particle-laden turbulent Couette flow. In this article, the turbulence modulation in the fluid phase in the presence of inertial particles is investigated using two-way coupled direct numerical simulations of a particle-laden sheared turbulent suspension. The particle volume fraction (φ) is varied from 1.75 × 10 - 4 to 1.05 × 10 - 3 and the Reynolds number based on the half-channel width (δ) and the wall velocity (U) (R e δ) is 750. The particles are of high inertia with S t ∼ 367 based on a fluid integral timescale represented by δ / U. A discontinuous decrease in turbulence intensity and Reynolds stress is observed beyond a critical volume fraction φ c r ∼ 7.875 × 10 - 4. The drastic reduction of shear production of turbulence leads to the collapse of fluid-phase turbulence. The stepwise particle injection and stepwise removal study confirm the role of critical volume loading in the discontinuous transition. Additionally, the effect of the nature of particle-particle and particle-wall collisions has been investigated. It is observed that the inelastic collisions increase the φ c r marginally although the nature of turbulence modulation remains similar. The explicit role of the inter-particle collisions has also been investigated by switching off the particle-particle collisions. In this case, φ c r increases more than in the case of an inelastic collision. The turbulence modulation carries the signatures of transition from sheared turbulence to particle-driven fluid fluctuation at higher volume loading.