Relaxation enhancement by time-periodic flows Alexander KiselevRoman ShterenbergAndrej Zlatos 35K1535K5535K90parabolic equationstime-periodic flowsdiffusion enhancement We study enhancement of diffusive mixing by fast incompressible time-periodic flows. The class of \textit{relaxation-enhancing} flows that are especially efficient in speeding up mixing has been introduced in [P. Constantin, A. Kiselev, L. Ryzhik, and A. Zlatos, \emph{Diffusion and mixing in fluid flow}, Ann. of Math. (2) \textbf{168} (2008), 643--674]. The relaxation-enhancing property of a flow has been shown to be intimately related to the properties of the dynamical system it generates. In particular, time-independent flows $u$ such that the operator $u cdot abla$ has sufficiently smooth eigenfunctions are not relaxation-enhancing. Here we extend results of [P. Constantin, A. Kiselev, L. Ryzhik, and A. Zlatos, \emph{Diffusion and mixing in fluid flow}, Ann. of Math. (2) \textbf{168} (2008), 643--674] to time-periodic flows $u(x,t)$ and, in particular, show that there exist flows such that for each fixed time the flow is Hamiltonian, but the resulting time-dependent flow is relaxation-enhancing. Thus we confirm the physical intuition that time dependence of a flow may aid mixing. We also provide an extension of our results to the case of a nonlinear diffusion model. The proofs are based on a general criterion for the decay of a semigroup generated by an operator of the form $\Gamma + iAL(t)$ with a negative unbounded self-adjoint operator $\Gamma$, a time-periodic self-adjoint operator-valued function $L(t)$, and a parameter $A \gg 1$. Indiana University Mathematics Journal 2008 text pdf 10.1512/iumj.2008.57.3349 10.1512/iumj.2008.57.3349 en Indiana Univ. Math. J. 57 (2008) 2137 - 2152 state-of-the-art mathematics http://iumj.org/access/