velocity at which the fluid is flowing this boundary layer is subject to viscous shear by the movement of the rotating inner cylinder 3 this shear may induce either laminar or turbulent flow which reflects a balance between viscous and inertial forces and hence relates to flow instability it is desirable to maintain flow in the laminar regime with minimal mixing between layers of oil in order to enhance lubrication the onset of turbulence causes adjacent layers of oil to mix and may result in the formation of circular vortexes and fluctuations in flow velocity thereby reducing lubrication efficiency the structure of the exterior surface of the inner cylinder 3 and the interior surface of the outer cylinder 5 can be defined as having both a degree of roughness and a directional contour to this roughness referred to as a surface u201c lay u201d roughness is a measurement of the small scale variations in the height of the physical structure of the surface e g peaks and valleys while a surface lay refers to the directional line in which the peaks and valleys lie roughness is sometimes an undesirable property as it may cause excessive friction wear drag and fatigue between two surfaces that are in contact with each other however in the presence of lubrication oil roughness may sometimes be beneficial as its texture can allow the surfaces to trap lubricants and prevent the surfaces from welding together roughness.