Experimental Study on Two-Oscillating Grid Turbulence With Polymer Additives

In order to investigate the polymer effect on grid turbulence, the experiments study on grid turbulence has been built based on Particle Image Velocimetry. The Newtonian fluid flow and 200ppm polymer solution flow in grid turbulence were carried out at different grid oscillating frequency, such as 5Hz, 7.5Hz, 10Hz and 12.5Hz. The experimental results show that the viscous dissipation rate and vortex vector ω_z is smaller and more regular in space distribution in polymer solution case at grid oscillating frequency with 5Hz. It indicates that the existence of polymer additives inhibits enormously the viscous dissipation rate and vortex vector, but this phenomenon can be attenuated with the increase of grid oscillating frequency. From this result, it shows that there exists a critical Reynolds number for the inhibition of polymer effect, which is the same as that in turbulent channel flows with polymers. Then, proper orthogonal decomposition (POD) has been used to extract coherent structures in grid turbulence. It is found that it needs 24 and 4 POD eigenfunctions to examine coherent structure in the Newtonian fluid and the polymer solution cases respectively at grid oscillating frequency with 10Hz. It suggests that the coherent structures can be inhibited due to the existence of polymers so as to the flow field to be more regular. But, with the increase of grid oscillating frequency, the number of POD eigenfunctions for the Newtonian fluid case and the polymer solution case respectively are approaching the same. Through this analysis, it can be also seen that the inhibition effect of polymers is close relation with the grid oscillating frequency.