Decay of almost periodic solutions of conservation laws

We consider the asymptotic behavior of solutions of systems of inviscid or viscous conservation laws in one or several space variables, which are almost periodic in the space variables in a generalized sense introduced by Stepanoff and Wiener, which extends the original one of H. Bohr. We prove that if u (x, t) is such a solution whose inclusion intervals at time t, with respect to E > 0, satisfy l(epsilon)(t)/t --> 0 as t --> infinity, and such that the scaling sequence u(T)(x, t) = u(Tx, Tt) is pre-compact as T --> infinity in L-1oc(1) (R-+(d+1)), then u(x, t) decays to its mean value (u) over bar, which is independent of t, as t --> infinity. The decay considered here is in L-1oc(1) of the variable xi = x/t, which implies, as we show, that M-x(\u(x,t) - (u) over bar \) --> ) 0 as t --> infinity,where M-x denotes taking the mean value with respect to x. In many cases we show that, if the initial data are almost periodic in the generalized sense, then so also are the solutions. We also show, in these cases, how to reduce the condition on the growth of the inclusion intervals 1(epsilon)(t) with t, as t --> infinity, for fixed epsilon > 0, to a condition on the growth of l(epsilon)(0) with epsilon, as epsilon --> 0, which amounts to imposing restrictions only on the initial data. We show with a simple example the existence of almost periodic (non-periodic) functions whose inclusion intervals satisfy any prescribed growth condition as epsilon --> 0. The applications given here include inviscid and viscous scalar conservation laws in several space variables, some inviscid systems in chromatography and isentropic gas dynamics, as well as many viscous 2 x 2 systems such as those of nonlinear elasticity and Eulerian isentropic gas dynamics, with artificial viscosity, among others. In the case of the inviscid scalar equations and chromatography systems, the class of initial data for which decay results are proved includes, in particular, the L-infinity generalized limit periodic functions. Our procedures can be easily adapted to provide similar results for semilinear and kinetic relaxations of systems of conservation laws.