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Roughness effects on lubricant flows are investigated via 2D lattice Boltzmann simulations. At a Reynolds numbers of order 1000 a transition from laminar to unsteady flow is observed by an increase of the roughness height from about 10% to about 25% of the channel width. At lower Reynolds numbers ( where the flow is laminar in both channels), the transition is observed when increasing the wall roughness further. In other words, the critical Reynolds number for the transition from laminar toward unsteady flow decreases at higher wall roughness. Wall roughness may, therefore, qualitatively change the flow properties in confined geometry. Due to the ubiquitous presence of the wall roughness, the phenomenon is relevant in all cases where relatively high Reynolds number flow occur in strongly confined channels such as lubricant flow during the deformation of solid surfaces. For a fixed Reynolds number and channel geometry, time and spatial dependence of the velocity field and fluctuating quantities obey the scaling behavior as expected from the structure of the Navier - Stokes (NS) equations. This underlines the physical significance of the observed transition. As a possible application, wall roughness may be used in order to enhance mixing efficiency at a given Reynolds number.