Clustering and turbulence modulation in particle laden shear flows

Abstract

Turbulent fluctuations induce the commonplace phenomenology on the transport of small inertial particles known as clustering. Particles spread disuniformly and form aggregates where their local concentration is much higher than it is in nearby rarefaction regions, the voids, where in extreme cases not even a single particle can be found. The underlying physics has been exhaustively analyzed in statistically homogeneous and isotropic flows under the so called oneway coupling regime, i.e. in conditions where the momentum exchange between the carrier fluid and the disperse phase is negligible. Recently it has been shown that the addition of a mean flow might have dramatic effects on the disperse phase, i.e. the mean flow, through its large scale anisotropy, induces a preferential orientation of the clusters. Due to inertial effects, their directionality can even increase in the smallest scales, contrary to the expectation based on the isotropy recovery behavior of velocity fluctuations. This finding opens new issues in presence of large mass loads, when the momentum exchange between the two phases becomes significant and the back-reaction of the particles on the carrier flow cannot be neglected. These aspects are discussed here by addressing direct numerical simulation data of particle laden homogeneous shear flow in the two-way coupling regime. Consistently with previous findings we observe an overall depletion of turbulent fluctuations. In particular, particles with order Kolmogorov scale relaxation time induce the energy depletion of the classical inertial scales and the amplitude increase of the smallest ones where the particle back-reaction pumps energy into the turbulent eddies increasing their energy content. We find that increased mass loads result in the substantial broadening of the energy co-spectrum thereby extending the range of scales driven by anisotropic production mechanisms. This is due to the clusters which form the spatial support of the back-reaction field and give rise to a highly anisotropic forcing active down to the smallest scales.