Turbulence Structure and Polymer Drag Reduction in Adverse Pressure GradientBoundary Layers

摘要

The ability of some solutions of long chain polymers to greatly reduce thepressure drop in pipes is well documented and commercially applied. In addition, a number of experiments indicate that large reductions of wall friction are possible by injecting polymer into a boundary layer. Therefore, it is reasonable to expect that drag reducing polymers could significantly improve the performance of submersibles when introduced into the boundary layers on the fore and sides of the vessel. However, once polymer is injected at these locations it is unlikely to leave the boundary layer because these polymers have very low mass diffusivities in water and boundary layers continually entrain fluid rather than eject it. As a result, one can expect that drag reducing polymers will be present in the adverse pressure gradient boundary layers on the aft of a vessel whenever the frictional drag on the fore and sides is reduced with polymers. Direct measurements of adverse pressure gradient turbulent boundary layers in which drag reducing polymers are present are not available. However, at fixed speeds, the net drag on a propeller hydrofoil increases while lift decreases when submerged in polymer solution (Kowalski, 1971; Wu, 1969). This apparent increase in form drag suggests a sudden increase in the displacement thickness of the boundary layers which may indicate separation. Therefore, it is necessary to establish whether conditions exist under which turbulent boundary layers of polymer solutions will not separate when they encounter adverse pressure gradients. Assuming that such conditions exist, it is important from an engineering standpoint to model the flow.