Experimental Study On Two-grid Oscillating Turbulence With Polymer Additive

Turbulent drag-reducing effect with additives has been a hot topic in theNonnewtonian fluid dyanmic field. Many researchers have been carried out thewall-bounded channel flow from theory, experiment and numerical simulation, andhave obtained many remarkable results, which can be deeply used to explain theturbulent drag-reducing mechanism and characteristics in wall-bounded flow. Butthere exists some difficult problem to be solved, such as, the turbulent drag-reducingmechanism and characteristics in isotropic turbulence without wall effect at highReynolds number, large eddy simulation for turbulent drag-reducing flow at highReynolds number, etc.In this paper, it elaborates the research background of this topic, andsummarizes the research actuality of turbulent drag-reducing flow and isotropicturbulence. So the study of grid turbulence in viscoelastic fluid has importanttheoretic meaning and scientific value. It designs the two-grid oscillating turbulenceexperiment setup, which satisfies the requirement for isotropic turbulence. Theadvaced measurement means in fluid flow-Particle Image Velocimetry (PIV) is usedto experimental study for two-grid oscillating turbulence. The high molecularpolymer-polyacrylamide (PAM) is used as additive, which has viscoelastic anddrag-reducing characteristics. The rheology characteristics of aqueous PAM solutionwith mass concentration2×10-4. The results show that the solution has viscoelasticcharacteristics. The two-grid oscillating turbulence with distilled water and aqueousPAM solution with mass concentration2×10-4are measured experimentally atdifferent grid oscillating frequency with10,15,20and25Hz.Then, the turbulent important parameters-turbulent kinetic energy, viscousdissipation rate, and vortex vector are analyzed. It is found that the existence ofPAM restrains the physical value of these three parameters in flow, especially at gridoscillating frequency with10Hz. In order to analyze the effect of viscoelastic on theflow coherent structures, proper orthogonal decomposition (POD) is used to studythe velocity vector field and the studies are mainly focused on the characteristics ofPOD eigenfunction and energy distribution ect, and the coherent structures areexamined. It is clearly seen that the coherent structures in aqueous PAM solutionhave been restrained, so the flow has become regular.Finally, The multi-scale characteristics of two-grid oscillating turbulence areanalyzed based on wavelet transform. Through the analysis of multi-scalecharacteristics of fluctuation velocity u, it is found that the existence of PAMrestrains the high frequency component of fluctuation velocity u. The flatness and local intermittence measure of fluctuation velocity u at different scales are computed,the results show that the existence of PAM restrains the intermittence at all scalesand the local intermittence at space, especially for the effect on small scales. Thenthe study on scaling law characteristics of fluctuation velocity u, it is clearly seenthat the scaling exponents for all experiments are far from Kolmogorov p/3and SLscaling law, and the existence of PAM also changes the scaling characteristics.