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

Abstract

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.

Turbulent Heat Transfer in Drag-Reducing Channel Flow of Viscoelastic Fluid

“…In case 4, no apparent log-law (i.e., constant ζ) region was found between the elastic layer and the core region. Similar behavior was seen in experimental results (Warholic et al, 1999;, where turbulent motions were suppressed and a nearly maximal level of DR was achieved. Hence, the y e in case 4 could not be given explicitly from the above procedure.…”

Electrokinetic Microactuator Arrays for Control of Vehicles

“…These polymers, however, can only be used in liquid flows, and their effect is largely degraded after the fluid has been subjected to very high shear rates, as occurs in pumps. More recent work (e.g., Chara et al 1993;Miska et al 1995;Warholic et al 1999;Zakin 2001) has used very low concentrations of surfactant ions which self-assemble into long-chain micelle structures, and in so doing produce a similar effect as long-chain polymer molecules. Unlike polymers, however, after passing through regions of high shear rates these micelles rapidly reassemble to recover their original effectiveness.…”

Heat transfer enhancement in turbulent drag reducing surfactant solutions by agitated heat exchangers

“…It is known, however, that the flow of DR solutions lacks some of the features of turbulent flow. Studies have shown that radial turbulence intensities are greatly reduced in DR flow [6], and it is accepted that this suppression of radial turbulence results in the reduced pressure loss [7].…”