Influence of wavy structured surfaces and large scale polymer structures on drag reduction

摘要

Drag reduction was studied for turbulent flow over a structured wall that contained 600 sinusoidal waves with a wavelength of 5 mm and an amplitude of 0.25 mm. A concentrated solution of a co-polymer of polyacrylamide and sodium acrylate was injected into the flow through wall slots. Laser Doppler velocimetry was used to measure turbulence. A fluorescence technique was developed that enabled us to demonstrate the existence, under certain circumstances, of large gelatinous structures in the injected polymer solution and in the flow channel.At maximum drag reduction, the Reynolds shear stress was zero and the velocity field was the same as found for a smooth surface. Larger drag reductions could be realized for a wavy wall because the initial drag was larger. The influences of polymers on the turbulent fields are similar for smooth and wavy boundaries. These results are of interest since the interaction with the wall can be quite different for water flow over smooth and wavy boundaries (which are characterized as being completely rough). An important effect of polymers is a decreasing relative importance of high frequency fluctuations with increasing drag reduction that is characterized by a cut-off frequency. This cut-off is the same for smooth and wavy walls at maximum drag reduction. The sensitivity of drag reduction to the method of preparing and delivering the polymer solution suggests that aggregation of polymers could be playing an important role for the system that was studied. For example, drag reduction was enhanced when large polymer structures are present.