Tomo-PIV measurement of small-scale structures in Newtonian and polymeric turbulence

The effects of polymer additives on turbulent fluid flows have attracted massive attention since the discovery of significant drag reduction by polymers in wall-bounded flows. Here we present an experimental study on the polymer–turbulence interaction at the center of the turbulent von Kármán swirling (VKS) flow system where the flow is far away from the boundary (bulk turbulence) with tomographic particle image velocimetry (Tomo-PIV). We used water–glycerol mixture to tune the viscosity of the working fluids, which facilitates us to resolve the dissipative scales and thus obtain all the nine components of the velocity gradient tensor directly. Our experiments demonstrate that at the center of the VKS flow, anisotropic properties extend from large scale to small scale, but gradually weaken with increasing Reynolds number. In polymeric turbulence, it is found that with increasing polymer concentration both the spatial averaged root mean square velocity and the average energy dissipation rate first decrease and then tend to stay at a constant value when concentration exceeds a critical value, implying that the effect of polymers saturates at high polymer concentration. We also find that the small scales become more anisotropic with the increasing concentration. The axisymmetry of small scales, however, is always retained, which can be employed to estimate the average energy dissipation rate from the planar PIV data. Moreover, we reveal that the number of the tube-like structures, the elementary structure in Newtonian turbulence, is strongly inhibited by the polymer additives, whereas the size of the tube-like structures is greatly enlarged.