A two-component laser-Doppler velocimeter, with high spatial and temporal resolution,
was used to study how the introduction of a drag-reducing surfactant to water
changes the fully-developed velocity field in an enclosed rectangular channel.
Measurements were made for four different Reynolds numbers, Re = 13300; 19100; 32000,
and 49100 (based on the bulk viscosity, the half-height of the channel, and the
viscosity of water). For a fixed volumetric flow the pressure drop was reduced by 62 to
76% when compared to a Newtonian flow with an equal wall viscosity. Measurements
were made of the mean streamwise velocity, the root mean square of two components
of the fluctuating velocity, the Reynolds shear stress and the spectral density function
of the fluctuating velocity in the streamwise direction. The Reynolds shear stress is
found to be zero over the whole channel and the spectra of the streamwise velocity
fluctuations show a sharp cutoff at a critical frequency,
fc. The ratio of the cutoff
frequency to the root mean square of the streamwise velocity fluctuations is found
to be approximately equal to 1 mm−1. The observation of a zero Reynolds shear
stress indicates the existence 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 mechanism other than that
usually found for a fully developed flow. Possible explanations for this behaviour
are presented.
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