Turbulence modulation with controlled particle injection and their migration*

Characterizing turbulence modulation in particle-laden turbulent flows is an active area of research. It has been observed that there is a discontinuous collapse of turbulence at a critical volume fraction [1]. At this critical volume fraction, there is a reduction of second moments of all the components of fluid velocity fluctuations by 1–2 orders of magnitude. The study revealed that this transition is a robust one, and the state beyond the critical volume fraction is a laminar state with fluid velocity fluctuation generated by the force exerted by the particle phase. An analysis of the fluid momentum equation revealed a sharp decrease in the divergence of Reynolds stress at the critical volume fraction. Similarly, in the energy equation, there is a dramatic decrease in the turbulent energy production rate at the transition volume fraction. However, there is a small decrease in the energy dissipation rate due to the mean flow and a small increase in the dissipation rate due to the particle drag. The results indicated that a decrease in the rate of turbulence production is the main reason behind the turbulence collapse. Such an observation motivated us to investigate whether turbulence modulation is possible by controlled particle injection at the zone of the channel where turbulence production is maximum. A series of DNS has been conducted by injecting particles in a controlled manner at particular zones of the channel as well as controlling their cross-stream migration. It is observed that when the particles are injected only at the zone of maximum turbulence production, there is a maximum 20% decrease in turbulent fluctuation, and the extent of turbulence attenuation is maximum when the particle migrates up to a maximum cross-stream distance, even at a lower solid volume fraction.

[1] Muramulla, P., Tyagi, A., Goswami, P.S. & Kumaran, V. Disruption of turbulence due to particle loading in a dilute gas–particle suspension. J. Fluid Mech. 889, A28, 2020