In contradiction with common assumptions of a laminar lubricant flow, it is shown that high Reynolds number lubricant flows may occur in usual metal forming processes even at scales as small as the surface roughness asperities. This enhances the sensitivity of the flow to perturbations arising e.g. from the surface roughness and brings about the possibility of a roughness-induced transition toward an unsteady flow/turbulence. Via lattice Boltzmann simulations, it is shown that, under these circumstances, a qualitative change in flow properties may indeed be triggered by a variation of the wall roughness alone. We focus on the impact of various roughness parameters on the transition showing that it is not the roughness height alone which determines the onset of flow instability. Rather, it is the combined effect of the roughness height- and wave length which is essential. In particular, by an increase of the roughness wave length, it is possible to trigger flow instability even if the roughness slope is reduced. These findings are in line with experimental results on rough wall turbulence which indicate that a local Reynolds number based on the roughness height alone does not capture the roughness effects on the flow characteristics.

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