Spatial and temporal scales of force and torque acting on wall-mounted spherical particles in open channel flow

TL;DR

This study analyzes the spatial and temporal scales of force and torque on wall-mounted spheres in open channel flow using direct numerical simulation data, revealing differences in scales and correlations with flow structures.

Contribution

It provides new insights into the spatial and temporal structure of forces and torques on particles in rough wall flows, including their correlation with flow features.

Findings

Spanwise torque statistics are similar to shear stress at smooth walls.

Drag and lift forces have smaller time and length scales than torque.

Flow-particle correlations reveal buffer-layer streaks and pressure effects.

Abstract

Data from direct numerical simulation of open channel flow over a geometrically rough wall at a bulk Reynolds number of 2900, generated by Chan-Braun et al. ["Force and torque acting on particles in a transitionally rough open-channel flow", J. Fluid Mech. 684, 441--474 (2011), 10.1017/jfm.2011.311] are further analysed with respect to the time and length scales of force and torque acting on the wall-mounted spheres. For the two sizes of spheres in a square arrangement (11 and 49 wall units in diameter, yielding hydraulically smooth and transitionally rough flow, respectively), the spatial structure of drag, lift and spanwise torque is investigated. The auto-correlation and spectra in time as well as the space-time correlation and convection velocities are presented and discussed. It is found that the statistics of spanwise particle torque are similar to those of shear stress at a smooth wall. Particle drag and lift are shown to differ from spanwise particle torque, exhibiting considerably smaller time and length scales; the convection velocities of drag and lift are somewhat larger than those of spanwise torque. Furthermore, correlations between the flow field and particle-related quantities are presented. The spatial structure of the correlation between streamwise velocity and drag/spanwise torque features elongated shapes reminiscent of buffer-layer streaks. The correlation between the pressure field and the particle drag exhibits two opposite-signed bulges on the upstream and downstream sides of a particle.