  Direct numerical simulation of open channel flow over a geometrically rough
wall has been performed at a bulk Reynolds number of approximately 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 on 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.
