arXiv2026
This paper is concerned with the large-time behavior of inviscid and viscous shocks for scalar conservation laws on the half-line $(-\infty,0)$ under an incoming \emph{time-periodic} boundary condition. This problem arises naturally from classical piston problems in fluid mechanics. Even for arbitrarily small periodic oscillations of the boundary data, the effect of the boundary manifests itself through a persistent time-periodic component near the boundary, which behaves as a \emph{boundary wave}. For inviscid shocks moving away from the boundary ($s<0$), this boundary wave repeatedly interacts with the shock over infinitely many periods. We show that the repeated interactions do not generate secular growth, but instead produce a finite \emph{cumulative} correction to the asymptotic shock location. For the inviscid stationary case $s=0$, the time-periodic boundary \emph{modulates} the background stationary shock into a genuinely time-periodic shock configuration, to which the solution converges. In the viscous case with $s<0$, the boundary wave couples with the viscous shock through a transitional region induced by diffusion, and the solution converges asymptotically to their superposition. Our results show that for scalar conservation laws, the persistent boundary wave modifies the shock without producing resonance.