In our study we determine the influence of wall roughness on friction for pipe and channel flows by numerically calculating the entropy production in the flow. It turns out, that there is an appreciable influence of wall roughness in laminar flows though this effect often is neglected completely. In addition to the friction factor results we gain an understanding of the physics since we have access to the dissipation distribution in the flow field close to the roughness elements. For a concise description of flows over rough walls there should be a reasonable choice of the wall location as well as the roughness parameter. Various options are discussed and assessed.

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