This paper presents a new method to reduce turbulent frictional drag by injecting high-viscosity fluid into the boundary layer. When the turbulent region of the boundary layer is filled with high-viscosity fluid, and the viscosity of the viscous sublayer is kept low, the Reynolds stress in the turbulent region is reduced and therefore requires a greater velocity gradient to transfer the momentum. The greater velocity gradient in the turbulent region results in a reduction of the velocity gradient at the viscous sublayer, which causes a drop in shear stress at the wall. Such a boundary-layer structure could be created by injecting two different fluids from double slits on a wall. Sugar syrup and water were used as the high-viscosity fluid and the low-viscosity fluid, respectively. The shear stress was directly measured by shear stress pick-ups mounted flush on the wall. The shearing stress was reduced by more than 50 percent at the optimum injection condition. A water/water injection experiment was also performed to show the effect of injection itself.

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