The effect of gravity on turbulence modulation in particle-laden horizontal open channel flow

Two-way coupled direct numerical simulation of turbulence in a horizontal open channel with Lagrangian tracking of heavy particles is performed to study the joint influence of particle inertia and wall-normal gravity on turbulence modulation. The Stokes numbers based on the wall units considered in the study are S t + = 1 (low inertia), 31 (moderate inertia), and 202 (high inertia) and the gravitational accelerations are g + = 0, 0.0068, 0.068, and 0.2221, varying from case to case. It is found that heavy particles (gravity involved in simulation) with low inertia subtly increase or have little significant influence on turbulence modulation, regardless of gravitational acceleration. Particles with high inertia and large gravitational acceleration significantly reduce the mean streamwise fluid velocity and the scales of turbulence structure near the wall, and vice versa in the outer layer. In this case, turbulence fluctuations are notably suppressed and the turbulence structures tend to be two-dimensional and laminar due to the strong rebound and trajectory-crossing effect. Moderate-inertia particles probably cause anisotropic turbulence modulation due to the competition between particle inertia and gravity settling. In the parameter range studied in this work, particles have indistinctive modification on turbulence when τ p + g + ( = S t + g + ) < 0.2 but suppress turbulence when τ p + g + > 0.2.