Experimental Study On Oscillating Grid Turbulence With Viscoelastic Fluid By PIV

Since the discovery of turbulent drag reduction phenomenon by long-chainpolymer additives, a large amount of researchers have been engaged in studying theinteraction between turbulence and drag-reducing agents. Up to now, studies onturbulent drag-reducing flow of viscoelastic fluid have been focused on thewall-bounded turbulent flows, such as channel, pipe and boundary layer flows,where the wall plays an important role. However, the wall effect of all thewall-bounded turbulent flows induces additional difficulties in understanding theinteraction in viscoelastic drag-reducing solutions. Therefore it is helpful toinvestigate the effect of drag reducing agents on the characteristics of turbulence inisotropic turbulent flow without wall effects.First of all, the experimental device with a pair of vertically oscillating grids isdesigned to produce a region of isotropic turbulence in the homogeneous fluidcontained in a water tank. Experiments are carried out for different concentrations ofpolymer and surfactant solutions. Particle image velocimetry (PIV) is used tomeasure the fluctuating velocity of turbulence. The database of velocities in gridturbulence with viscoelastic fluid is built, which lays the foundation to researchdrag-reducing effect with viscoelastic fluid.Next, to study the drag-reducing with viscoelastic fluid, we compare thevelocity between Newtonian fluid and viscoelastic fluid, meanwhile calculate andcontrast the fluctuation velocity. The viscoelastic fluid has the same drag-reducingeffect in isotropic turbulence. With a larger concentration of viscoelastic fluid, abetter drag-reducing is found, while surfactant solutions have better drag-reducingthan polymer solutions. Furthermore, the ratio of the longitudinal to the transverseenergy components is calculated and verifies that the grid turbulence has a region ofisotropic turbulence.At last, the viscosity and the relaxation time are measured to study whether theproperties will influence the drag-reducing effect. The viscosity of viscoelastic fluiddoesn’t have an obvious effect on the drag-reducing effect. The viscoelastic fluidwith a long relaxation time has a better drag-reducing effect. Using the method ofthe proper orthogonal decomposition, the coherent structures are got. Comparedwith Newtonian fluid, viscoelastic fluid uses less numbers of eigenfunctions to getthe coherent structures. It is because the turbulence is inhibited in viscoelastic fluid.From this point the drag-reducing effect in isotropic turbulence is pointed out again.