Effect of particle shape on bedload sediment transport in case of small particle loading

The paper presents four direct numerical simulations of an open channel flow with geometrically resolved particles heavier than the fluid. In each simulation the same number of mono-disperse particles is considered, with the shape of the particles varied between the simulations, while conserving the volume. Three simulations are reported with non-spherical particles, here prolate ellipsoid, oblate ellipsoid, and Zingg-ellipsoid, and compared to a fourth one with spherical particles. The influence of the particle shape on the average fluid motion and on the disperse particle phase is investigated in the regime of low particle loading. It is found that the Reynolds stresses with spherical particles are much smaller compared to those with non-spherical particles. Of these, they are maximum in the case of oblate particles. Furthermore, the oblate spheroids are prominently found in the near-wall region and hardly roll. This behavior differs markedly from the behavior of the spherical particles, which preferably bounce and roll. The other two cases, with prolate ellipsoids and Zingg-ellipsoid behave between the two extreme cases of spherical and oblate particles. The oblate spheroids orient such that their symmetry axis is parallel to the wall-normal direction, whereas the prolate spheroids orient with their symmetry axis parallel to the streamwise direction. The Zingg-ellipsoids orient with their longest axis parallel to the streamwise direction and their smallest axis parallel to the wall-normal direction. This preferred orientation is more dominant in the near wall region. All effects mentioned are supported by quantitative evaluation of statistics and can serve as reference for future validations.