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Byline: Yi Wang, Bo Yu, Jin-jia Wei, Feng-chen Li, Yasuo Kawaguchi A discrete-element model (spring-dumbbell model) is used to investigate the mechanism of drag-reduction by adding polymer in a turbulent channel flow of Newtonian fluid by Direct Numerical Simulations (DNSs). By using this model, the spatial concentration of the polymer can be obtained numerically. In this paper, the effect of the spring coefficient on the drag-reduction is studied in detail. The quantitative relations between the drag-reduction rate and viscoelastic and Reynolds shear stress, etc., are obtained. Discussions are performed to clarify the relation between the turbulent structure of the polymer drag-reducing flow and the spring coefficient of the discrete-element model.

Byline: Jin-Jia Wei, Bo Yu, Wen-Quan Tao, Yasuo Kawaguchi Analysis of a discretised scalar equation shows that high-resolution composite schemes containing a characteristic line of ˆφf = 1 on the Normalised Variable Diagram (NVD), although following the Convective Boundedness Criterion (CBC) proposed by Gaskell and Lau, may result in numerical results dependent on initial field, and thus may predict overshoot/undershoot behaviour. The finding is confirmed by testing a case of pure convection of a double-step profile in an oblique uniform velocity field. A slightly revised CBC is proposed by excluding the line of ˆφf = 1 from the CBC region, and can predict bounded numerical results without losing accuracy.

Abstract: The quantitative characteristics of vortex structures and turbulent events in turbulent channel flows with and without drag reduction were investigated experimentally. Drag reducing aqueous solutions of CTAC (CTAC – cetyltrimethyl ammonium chloride) with concentration of 25ppm and 75ppm at 30°C were tested. Particle image velocimetry (PIV) was used to measure the instantaneous velocity field in the streamwise–wall-normal plane of the flow. Through visualizing the instantaneous vector fields, contour maps of the swirling strength and instantaneous value of uv (u and v are the streamwise and wall-normal velocity fluctuations, respectively), the characteristic angle of vortex packets was quantified. It was shown that the drag-reducing CTAC additive reduced both the strength and frequency of turbulent bursts near the wall, and the characteristics of vortex structures and bursts were not only dependent on drag-reduction level but also on concentration of additive. From the quantified parameters characterizing turbulent events in a wall-bounded turbulent flow, it was obtained that f T was linearly proportional to the product of frequency and strength of turbulent events. [Copyright &y& Elsevier]