Drag-reducing characteristics of cationic surfactant solution flow in copper pipe

The drag-reducing characteristics of a cationic surfactant solution flow in copper pipe have been investigated experimentally.The tested drag-reducing fluid was an aqueous solution of the cationic surfactant cetyltrimethyl ammonium chloride(CTAC).The experimental results show that the maximum drag reduction percentage reduces with the increase of fluid temperature at low concentration of CTAC,such as 100×10-6 or 150×10-6.Furthermore,the concentration and temperature changes of CTAC solution have significant influences on the drag-reducing ability.The drag-reducing effect of CTAC additives shows great potentials in the application in a district heating/cooling(DHC)system,especially for the radiant floor heating(RFH)system.

TRPIV measurement of turbulent coherent structures in a drag-reducing flow by polymers

Fully developed turbulent flow fields with and without polymer solution at the same Reynolds number were measured by time-resolved particle image velocimetry （TRPIV） in a water channel to investigate the mechanism of drag-reducing solution from the view of coherent structures manipulation. The streamwise mean velocity and Reynolds stress profiles in the solution were compared with those in water. After adding the polymer solution, the Reynolds stress in the near-wall area decreases significantly. The result relates tightly to the decease of the coherent structures＇ bursting. The spatial topology of coherent structures during bursts has been extracted by the new mu-level criterion based on locally averaged velocity structure function. The effect of polymers on turbulent coherent structures mainly reflects in the intensity, not in the shape. In the solution, it is by suppressing the coherent structures that the wall friction is reduced.

Experimental study on the characteristics of ventilated cavitation around an underwater navigating body influenced by turbulent drag-reducing additives

In this study, a new control strategy for turbulent drag reduction involving ventilated cavitation is proposed. The configurational and hydrodynamic characteristics of ventilated cavities influenced by turbulent drag-reducing additives were experimentally studied in water tunnel. The test model was fixed in the water tunnel by a strut in the aft-part. Aqueous solutions of CTAC/Na Sal(cetyltrimethyl ammonium chloride/sodium salicylate) with weight concentrations of 100, 200, 400 and 600 ppm(part per million), respectively, were injected into the ventilated air cavity from the edge of the cavitator with accurate control by an injection pump. The cavity configurations were recorded by a high-speed CCD camera. The hydrodynamic characteristics of the test model were measured by a six-component balance. Experimental results show that, within the presently tested cases, the lengths of cavity influenced by drag-reducing solution are smaller than normal condition(ventilated cavity) in water, but the asymmetry of the cavity is improved. The drag resisted by the test model is reduced dramatically(the maximum drag reduction can reach to 80%) and the re-entrant jet is more complex after the CTAC solution is injected into the cavity. Turbulent drag-reducing additives have the potential in enhancement of supercavitating asymmetry and further drag reduction.

Direct numerical simulation study of the interaction between the polymer effect and velocity gradient tensor in decaying homogeneous isotropic turbulence

Direct numerical simulation of decaying homogeneous isotropic turbulence （DHIT） of a polymer solution is performed. In order to understand the polymer effect on turbulence or additive-turbulence interaction, we directly investigate the influence of polymers on velocity gradient tensor including vorticity and strain. By visualizing vortex tubes and sheets, we observe a remarkable inhibition of vortex structures in an intermediate-scale field and a small-scale field but not for a large scale field in DHIT with polymers. The geometric study indicates a strong relevance among the vorticity vector, rate-of-strain tensor, and polymer conformation tensor. Joint probability density functions show that the polymer effect can increase ＂strain generation resistance＂ and ＂vorticity generation resistance＂, i.e., inhibit the generation of vortex sheets and tubes, ultimately leading to turbulence inhibition effects.