Force and torque acting on particles in a transitionally rough open-channel flow

Abstract

AbstractDirect numerical simulation of open channel flow over a geometrically rough wall has been performed at a bulk Reynolds number of ${\mathit{Re}}_{b} \approx 2900$. The wall consisted of a layer of spheres in a square arrangement. Two cases have been considered. In the first case the spheres are small (with diameter equivalent to $10. 7$ wall units) and the limit of the hydraulically smooth flow regime is approached. In the second case the spheres are more than three times larger ($49. 3$ wall units) and the flow is in the transitionally rough flow regime. Special emphasis is given to the characterisation of the force and torque acting on a particle due to the turbulent flow. It is found that in both cases the mean drag, lift and spanwise torque are to a large extent produced at the top region of the particle surface. The intensity of the particle force fluctuations is significantly larger in the large-sphere case, while the trend differs for the fluctuations of the individual components of the torque. A simplified model is used to show that the torque fluctuations might be explained by the spheres acting as a filter with respect to the size of the flow scales which can effectively generate torque fluctuations. Fluctuations of both force and torque are found to exhibit strongly non-Gaussian probability density functions with particularly long tails, an effect which is more pronounced in the small-sphere case. Some implications of the present results for sediment erosion are briefly discussed.