Experimental study on the physical mechanism of the dragreduction of hydrophobic materials in laminar flow

摘要

In order to study the physical mechanism of the drag reduction of hydrophobic materials inlaminar flow, flat plat models, which are made from the combination of smooth /rough andhydrophilic/hydrophobic surfaces, were tested and studied experimentally to compare thevariation of the drag in laminar flow in a water tunnel. The rough hydrophobic model showedapparent reduction of the drag and the other three models were equivalent within error bars.Micro air bubbles existed on the surface of rough hydrophobic model. Such air bubbles areobserved in detail in a duct. They behaved a dynamic equilibrium under flow shear: randomlyemerged, grew, and finally were washed down by flow shear. From the results of drag test andair bubble observation, the mechanism of the drag reduction of hydrophobic materials wasdeduced as: the micro air bubbles existed on the surface results in apparent velocity slip on thewall in macroscopic field and gained drag reduction.