Using Wall Roughness

As detailed in the section describing the formulation of the wall laws, roughness is incorporated by a reduction of the turbulent wall function coefficient . This reproduces the effect of the erosion of the inner layer by moving the logarithmic region of the wall law downward.

As shown in the accompanying figure, for very large values of roughness parameter , the logarithmic profile might no longer intersect with the assumed linear profile in the sublayer. Should this occur in the fully-rough regime , there is no problem since the sublayer is irrelevant. In the event that these curves do not intersect in the transitional roughness regime , the logarithmic profile is used by STAR-CCM+, with the proviso that is never allowed to be less than zero.

It should be noted that it is not physically meaningful for , so the user should take care that the distance from each wall-adjacent cell centroid to the wall is larger than the wall roughness height. Should this condition be violated, STAR-CCM+ will locally limit the roughness height such that .