Abstract:
This paper employs Direct Numerical Simulation to study the orientation effects of ellipsoid roughness elements on the statistics of rough-wall turbulent channel flows. Roughness elements are directly resolved using a sharp-interface immersed boundary method. The simulations consider roughness elements with four different orientations, namely random orientation, vertical placement, inclined 45° downstream, and inclined 45° upstream. For each orientation, three different roughness element spacings are considered. Numerical results show that the roughness elements’ orientation significantly affects turbulence statistics. For a small roughness element spacing (i.e.
l = 2.0
r, where
ris the minor semi-axis length of the ellipsoid), the randomly oriented roughness elements result in the largest sandgrain roughness length (
ks); for large roughness elements spacing (i.e.
l = 2.8
r and 3.5
r),
ks of the vertically placed roughness elements is the largest. The value of
ks and the Reynolds normal stresses in viscous units of the roughness elements inclined 45° downstream or upstream are nearly the same; the streamwise Reynolds normal stress is much higher than those of other rough walls. On the other hand, the streamwise dispersive stress of roughness elements inclined 45° downstream is higher than that of the opposite orientation. The vertical and spanwise dispersive stresses are much smaller than the streamwise one. Randomly oriented roughness elements yield the strongest vertical and spanwise dispersive stresses.