High-Reynolds-number turbulent boundary layer friction drag reduction from wall-injected polymer solutions

Abstract

A set of controlled high-Reynolds-number experiments has been conducted at the William B. Morgan Large Cavitation Channel (LCC) in Memphis, Tennessee to investigate the friction drag reduction achieved by injecting aqueous poly(ethylene oxide) (PEO) solutions at three different mean molecular weights into the nearzero-pressure-gradient turbulent boundary layer that forms on a smooth flat test surface having a length of nearly 11 m. The test model spanned the 3.05 m width of the LCC test section and had an overall length of 12.9 m. Skin-friction drag was measured with six floating-plate force balances at downstream-distance-based Reynolds numbers as high as 220 million and free stream speeds up to 20 m s −1 . For a given polymer type, the level of drag reduction was measured for a range of free stream speeds, polymer injection rates and concentrations of the injected solution. Polymer concentration fields in the near-wall region (0 < y + < ∼ 10 3 ) were examined at three locations downstream of the injector using near-wall planar laserinduced-fluorescence imaging. The development and extent of drag reduction and polymer mixing are compared to previously reported results using the traditional K-factor scaling. Unlike smaller scale and lower speed experiments, speed dependence is observed in the K-scaled results for the higher molecular weight polymers and it is postulated that this dependence is caused by molecular aggregation and/or flowinduced polymer degradation (chain scission). The evolution of near-wall polymer concentration is divided into three regimes: (i) the development region near the injector where drag reduction increases with downstream distance and the polymer is highly inhomogeneous forming filaments near the wall, (ii) the transitional mixing region where drag reduction starts to decrease as the polymer mixes across the boundary layer and where filaments are less pronounced and (iii) the final region where the polymer mixing and dilution is set by the rate of boundary layer growth. Unlike pipe-flow friction-drag reduction, the asymptotic maximum drag reduction (MDR) either was not reached or did not persist in these experiments. Instead, the nearest approach to MDR was transitory and occurred between the development and transitional regions. The length of the development region was observed to increase monotonically with increasing polymer molecular weight, injection rate, concentration and decreasing free stream speed. And finally, the nearwall polymer concentrationis correlated to the measured drag reduction for the † Email address for correspondence: ceccio@umich.edu 260 E. S. Winkel and others three polymer molecular weights in the form of a proposed empirical drag-reduction curve.

A Short Review on Drag-Reduced Turbulent Flow of Inhomogeneous Polymer Solutions

“…Recently, this drag-reduced flow has received increasing attention for explaining the nonequilibrium of polymer solutions and its effect on turbulence. Some systematic studies for the drag reduction caused by inhomogeneous polymer solutions via slot-injection and wall-blowing methods have been conducted [44][45][46][47][48][49]. They strive to ascertain the development of the drag reduction, the modification of turbulent statistics and coherent structures, and the process of polymer diffusion.…”

“…Some studies have concentrated on the polymer drag reduction in TBL and channel flow with injection of polymer solutions [42,[45][46][47][48][49][51][52][53]. Drag reduction is found to begin downstream of the location where drag-reducing polymer has been injected into turbulent bulk flow even though the solution is not mixed uniformly in the flow system.…”

“…The concentrations of the injected polymer solutions used were relatively high and Reynolds numbers were set over a large range in the tests. Table 1 gives the experimental conditions and drag reduction obtained by polymer injection in typical researches [48][49][50]. The polyethylene oxide (PEO) with a mean molecular weight of about 2-8 million was used to be the polymer additive.…”

“…The polyethylene oxide (PEO) with a mean molecular weight of about 2-8 million was used to be the polymer additive. Previous researches indicated that the local concentration of the inhomogeneous polymer solution in near-wall region may determine the level of drag reduction [49,57]. Three regions of drag reduction with different variations along the streamwise direction can be typically categorized corresponding to the evolution of polymer concentration downstream of the slot.…”

An Experimental Study on the Effect of IS700 Coating on the Cavitation Inception and Development, and Noise Reduction of a Marine Propeller

“…A number of possible propeller coating types were introduced in order to use for different purposes. The several types were investigated by Coldron and Condé [4], Brasseur et al [5], Li et al [6], Winkel et al [7], Somandepalli et al [8], Elbing et al [9], Demirel et al [10] and Song et al [11].…”