On the Turbulent Dynamics of Polymer Solutions

We study properties of dilute polymer solutions which are known to depend strongly on polymer elongation. The probability density function (PDF) of polymer end-to-end extensions $R$ in turbulent flows is examined. We demonstrate that if the value of the Lyapunov exponent $\lambda$ is smaller than the inverse molecular relaxation time $1/\tau$ then the PDF has a strong peak at the equilibrium size $R_0$ and a power tail at $R\gg R_0$. This confirms and extends the results of \cite{Lumley72}. There is no essential influence of polymers on the flow in the regime $\lambda\tau1/\tau$ the majority of molecules is stretched to the linear size $R_{\rm op}\gg R_0$. The value of $R_{\rm op}$ can be much smaller than the maximal length of the molecules because of back reaction of the polymers on the flow, which suppresses velocity gradients thus preventing the polymers from maximal possible stretching.