The influence of a drag-reducing surfactant on a turbulent velocity field

A two-component laser-Doppler velocimeter, with high spatial and temporal r esolution, was used to study how the introduction of a drag-reducing surfac tant to water changes the fully-developed velocity field in an enclosed rec tangular channel. Measurements were made for four different Reynolds number s, Re = 13 300, 19 100, 32 000, and 49 100 (based on the bulk viscosity, th e half-height of the channel, and the viscosity of water). For a fixed volu metric flow the pressure drop was reduced by 62 to 76% when compared to a N ewtonian flow with an equal wall viscosity. Measurements were made of the m ean streamwise velocity, the root mean square of two components of the fluc tuating velocity, the Reynolds shear stress and the spectral density functi on of the fluctuating velocity in the streamwise direction. The Reynolds sh ear stress is found to be zero over the whole channel and the spectra of th e streamwise velocity fluctuations show a sharp cutoff at a critical freque ncy, f(c). The ratio of the cutoff frequency to the root mean square of the streamwise velocity fluctuations is found to be approximately equal to 1 m m(-1). The observation of a zero Reynolds shear stress indicates the existe nce of additional mean shear stresses (or mean transfers of momentum) that are not seen with a Newtonian fluid. Furthermore, the presence of a random fluctuating velocity field suggests a production of turbulence by a mechani sm other than that usually found for a fully developed flow. Possible expla nations for this behaviour are presented.