Experimental Study On The Characteristics Of Two Oscillating Grid Turbulence With Viscoelastic Fluid

Turbulent drag-reducing flow is one of the hot topics in Non-Newtonian fluid dynamical field.Since Tom's effect,many authors have carried out lots of studies on turbulent drag-reducing technology from theoretical,experimental and numerical aspects.Some remarkable results have been obtained,such as,turbulent dragreducing mechanism,the maximum drag-reducing asymptotic line and the application of drag-reducing technology,etc.But now,there still exists many problems to be solved,such as,more comprehensive turbulent drag-reducing mechanism,large eddy simulation for drag-reducing flows at high Reynolds number,etc.Now,more scholars mainly focus on the turbulent drag-reducing flows for viscoelastic fluids with wall-bounded effect,such as,pipe flow,channel flow and boundary-layer flow,etc.The existence of wall can induce the inhomogeneous flow characteristics,so it is diffcult to research the interaction between molecular microstructures of viscoelastic fluids and vortex structures.However,in isotropic turbulence without wall effect,the existence of viscoelastic fluids cannot change the momentum flux,but it can influence the energy transportation process among different scales structures.Therefore,it is imporatant to investigate the isotropic turbulence with viscoelastic fluids,which can promote the characteristics and mechanism of turbulent drag-reducing flows.Firstly,it adopted the idea of synchronously moving towards each other in opposite directions,and design the experimental system of two-oscillating grids.Two classical drag-reducing solutions with viscoelastic characteristics were chosen: high polymer and surfactant solutions.Rotational rheometer with Kinexus Pro Series were used to measure the rheological property.The results show that there exists shear-thinning characteristics of shear viscosity for polyacrylamide(PAM)and polyphenylene oxide(PEO)solutions with different concentrations;however,it shows the process of shearthinning,shear-thicking and shear-thinning again for cetyltrimethylammonium chloride(CTAC)and natrium salicylicum solution(Na Sal).Then,particle image velocimetry(PIV)were used to experimently investigate twooscillating grid turbulence with viscoelastic fluids so as to analyze the characteristics of flow field and turbulent important parameters,such as intergral scale,Taylor micro-scale,turbulent kinetic energy,etc.The results show that the existence of viscoelastic fluids remarkably influence the above flow parameters.It proposed the defination for drag-reducing rate based on turbulent kinetic energy,and it is found that in two-oscillating grids without wall effect,it also shows the remarkable dragreducing effect and there exists a critical drag-reducing concentration in CTAC/Na Sal solution cases.From turbulent kinetic energy spectrum,it elaborates the effect of viscoelastic fluids on energy transportation process.Based on the high-order correlation function of velocity partial derivative,it analyzes the small-scale intermittency of flow and is found that the small-scale intermittency is inhibited by viscoelastic fluids.Based on PIV data,proper orthogonal decomposition(POD)and wavelet transformation were used to investigate the multi-scale characteristics of vortex structures in two-oscillating grid turbulence.From POD results,it is clearly seen that for viscoelastic fluid cases,it only needs few POD eigenfunctions to capture the vortex structures of flows.It is suggested that the vortex structures of fows are inhibited so as to show the remarkable drag-reducing effect.According to the results of Daubechies wavelet transformation for multi-scale vortex dynamics,it is shown that at the same decomposition level(smaller scale),the existence of viscoelastic fluid remarkably changes the small-scale structures.Besides,at all decomposition scales,the contribution of viscoelastic fluids on intermittency is weaker.Finally,it studies the classical Kolmogorov p/3 scaling law,extended self-similar scaling law and hierarchical scaling law in the classical drag-reducing solution flows from four aspects,such as grid oscillating frequency,solution concentration,kinds of drag-reducing additives and grid fractal characteristics.It is found that the above four parameters can influence the scaling relationship and the scaling exponent deviates Kolmogorov p/3 scaling law for all cases,and it shows the abnormal characteristics.Besides,it analyzes the intermittent parameters of hierarchical scaling law,and is found that the existence of viscoelastic fluids inhibits the intermittent characteristics.