Effect of Drag-Reducing Additives on Boundary-Layer Turbulence

Abstract

A preliminary investigation has been made concerning the effects of polyethylene oxide on the spectral density of turbulent fluctuations in a boundary layer. The Polyox solution was injected into a two-dimensional boundary layer along a flat plate which was towed at 9.5 fps in the Naval Academy's 85-ft tow tank. Measurements of the mean and fluctuating components of the velocity near the wall were made with conical hot film sensors. Polymer injection resulted in an increase in rms level of the fluctuations and in an increase in spectral density below 200 Hz. Above this frequency, the spectral density decreased, the effect being more pronounced at higher frequencies and at higher polymer concentrations. Measurements of the mean and fluctuating components of the wall shear stress were made with flush-mounted hotfilm sensors submerged in the viscous sublayer. Polymer injection resulted in a decrease in mean shear, in rms level, and in spectral density of the fluctuating wall shear stress. The latter effect increased with increasing frequency and all effects increased with higher molecular weight of the additives. All results suggest the polymer shifts the scale of turbulence away from the energy-dissipating small eddies.