Particulate Impact and Erosion in a Turbulent Serpentine Channel

Abstract

Surface roughness often arises in consequence of erosion by particulate impact. We have studied particle impact in turbulent channel flow by direct numerical simulation (DNS). The channel is S-shaped, with periodicity between inlet and exit; hence, it is an infinite serpentine. The bend is fairly tight: its diameter is equal to the channel width. There is a separation bubble downstream of the bend. A range of particle masses was simulated. The particle Stokes numbers ranged from 0.5 to 6.0. In a straight, turbulent channel flow, eddies are responsible for particulate impact. On a Reynolds averaged level, turbophoresis has been given credit in the literature. In a curved channel, particle inertia may be the dominant cause of impact. However, turbulent convection is still important. This depends on the relative magnitude of particle and turbulent time-scales. The role of turbulent eddies is more complex than turbophoresis. In this paper we are able only to report our empirical findings. Transport in the serpentine geometry depends on the particle Stokes number. Heavier particles create a plume that leaves the inner bend and flows toward the outer wall. Turbulence then spreads particles away from the wall, forming a region of high concentration next to the surface. We have used an erosion formula to compare the distribution of particle impacts to erosion. Erosion depends on impact velocity and angle. The region of maximum erosion is not the region of maximum number of impacts. Higher Stokes number particles form more intense plumes and thicker layers next to the wall. Lighter particles form a plume, but the high concentration layer remains thin, reminiscent of plane channel flow.