Skin-friction drag modulation and riblet-like clusters in a semi-dilute particle-laden turbulent channel flow at $Re_\tau = 180$

In this study, we explore how coherent structures near the wall can be modulated by particles to help alter the skin-friction drag that is produced by them. This can help regulate the overall mass flow rate of the fluid by increasing or decreasing the overall drag caused by the skin-friction generated near the wall due to these coherent structures. We explore this modulation within Euler-Lagrange simulations of a four-way coupled turbulent particle-laden channel flow at $Re_\tau = 180$. Two separate cases are run at a lower ($St^+ = 6$) and higher inertia ($St^+ = 30$). The particles are present at a semi-dilute concentration, i.e., such that the average volume fraction is low ($2.4 10^-4 – 7.2 10^-4$), but mass loading is significant ($M = 0.2 – 0.6$). The results show that Particle inertia is vital to the type of modulation near the wall. Particles with a lower inertia ($St^+=6$) tend to enhance the drag, decreasing the overall mass flow rate. Due to their inertia, these particles move towards the wall, where they form small and fragmented clusters without significant coherence. Contrastingly, particles at $St^+ = 30$ cause a redaction of drag and an increase in the mass flow rate. These higher inertia particles accumulate near the walls and form very long longitudinal clusters akin to riblets that align with the flow low speed streaks. We measure the contribution of the particles to the total shear stress, and show how this directly relates to the change in the mass flow rate.