Very recently we have published a new view on the way dilute polymers affect turbulent flows. Taking it far from the boundaries we also put it in a special context of NO MEAN SHEAR turbulent flow (shearless mixing). In such flow, the typical ‘cascade model’ is not applicable and one has to deal with the fact that dilute polymers still work – therefore they do not ‘cut-off’ energy that comes from large scales, but deal with turbulent energy as is, changing e.g. intermittency, entrainment rate and dissipation.
A. Liberzon, M. Holzner, B. Luthi, M. Guala and W. Kinzelbach, On turbulent entrainment and dissipation in dilute polymer solutions, Physics of Fluids, 21, 035107 (2009). DOI: 10.1063/1.3097006
We present a comparative experimental study of a turbulent flow developing in clear water and dilute polymer solutions (25 and 50 wppm polyethylene oxide). The flow is forced by a planar grid that oscillates vertically with stroke S and frequency f in a square container of initially still fluid. Two-component velocity fields are measured in a vertical plane passing through the center of the tank by using time resolved particle image velocimetry. After the forcing is initiated, a turbulent layer develops that is separated from the initially irrotational fluid by a sharp interface, the so-called turbulent/nonturbulent interface (TNTI). The turbulent region grows in time through entrainment of surrounding fluid until the fluid in the whole container is in turbulent motion. From the comparison of the experiments in clear water and polymer solutions we conclude: (i) Polymer additives modify the large scale shape of the TNTI. (ii) Both, in water and in the polymer solution the mean depth of the turbulent layer, H(t), follows the theoretical prediction for Newtonian fluids H(t), where KS2f is the “grid action.” (iii) We find a larger grid action for dilute polymer solutions than for water. As a consequence, the turbulent kinetic energy of the flow increases and the rate of energy input becomes higher. (iv) The entrainment rate =ve/vrms (where ve=dH/dt is the interface propagation velocity and vrms is the root mean square of the vertical velocity) is lower for polymers (p0.7) than for water (w0.8). The measured values for are in good agreement with similarity arguments, from which we estimate that in our experiment about 28% of the input energy is dissipated by polymers. ©2009 American Institute of Physics