Drag Reduction by Dimples on Surfaces in Plane–Plane Contact Lubrication

Abstract

An experimental system was designed to evaluate the drag reduction by dimples on surfaces in plane–plane contact lubrication. Because of the dimples on the surfaces, in some of the experiments, it was observed that stable gas bubbles existed in the lubricant film. The results of the experiments showed that the existence of gas was decided by the depth of dimples and the volume of lubricant supply; the deeper the dimples were and the less the lubricant supply, the higher the probability of gas existence. The reason for the existence of gas, based on a model analysis, was the meniscus force of the gas–liquid interface, which caused the gas to be enclosed in the dimples. When a small amount of lubricant was supplied, the friction force was reduced because of a mixed lubrication film of oil and gas. For different amounts of lubricant supply, an optimal depth of dimples existed in plane–plane contact lubrication, which could obviously reduce the friction force. An optimization model for full film lubrication was presented to determine the optimal depth of dimples. Based on the model, the optimal dimple depth is approximately proportional to the distance between the two surfaces. The theoretical analysis agreed with the experiments.