Frictional Drag Reduction by Injecting High-Viscosity Fluid

Abstract

This paper presents a new concept to reduce turbulent frictional drag by injecting high-viscosity fluid into boundary layer. When the turbulent region of boundary layer is filled with high-viscosity fluid and the viscosity of the viscous sublayer is kept low, the velocity profile in the boundary layer should change substantially. The Reynolds stress in turbulent region becomes less which requests more velocity gradient there to keep the momentum transfer same. It results in a reduction of velocity gradient at the viscous sublayer which gives the reduction of shear stress at the wall. Such a boundary layer structure could be realised by injecting two different fluids from double slits on a wall. Sugar syrup was used as the high-viscosity fluid at the experiment. The sugar syrup and water were injected into turbulent boundary layer on a flat plate through up-stream and down-stream slits respectively. A steady injection was realised by pushing out the fluid in a tank using compressed air. The shear stress was directly measured by shear stress pick-ups mounted flush on the wall. The experimental result showed a substantial reduction of shear stress by injecting sugar syrup and water from the double slits. The maximum reduction rate was more than 50%. A series of water/water injection experiment was also made to know the effect of injection itself.