Modeling Turbulence Effects
Turbulence in the lubricant film flow can occur due to various reasons. In high-speed rotors, inertial effects of the flow become significant. Thus, flow no longer remains laminar and also gives rise to turbulent shear stresses. These effects, in turn, significantly change the behavior of the bearing as compared to a laminar approximation. Surface roughness of the journal and bushing surfaces can also induce turbulence in the flow even at moderate speeds. A detailed modeling of the flow pattern in the film, including turbulence effects, makes the problem very complicated. However, with the help of homogenization techniques, the problem can still be solved in a simplified manner by slightly modifying the Reynolds equation by introducing flow factors and shear stress factors that account for turbulence effects in the flow.
Consider two rough surfaces pressing against each other and supported by a lubricant film between them. The surfaces are in a relative sliding motion with each other with velocity vJ and vB, respectively. When the lubricant film between them is thick enough, asperities in the surfaces do not come in contact with each other. This is usually the case for when the contact load is small. Such a lubrication is referred to as full film lubrication. When the contact load is very high, the surfaces can come close to each other by squeezing the film. In such a scenario, the gap between the surfaces can be of the same order as the height of the asperities. Therefore, asperities can come in contact at various locations. As a result, the contact load is not only supported by the pressure in the lubricant film but also by the contact pressure between asperities. Such a lubrication is referred to as mixed lubrication.
Figure 7-7: Rough surfaces in contact.