Immediate blowup of entropy-bounded classical solutions to the vacuum free boundary problem of non-isentropic compressible Navier--Stokes equations

TL;DR

This paper demonstrates that classical solutions with bounded entropy to the vacuum free boundary problem of non-isentropic compressible Navier-Stokes equations blow up immediately under certain conditions, highlighting the importance of boundary degeneracy conditions.

Contribution

It provides new non-existence results for entropy-bounded classical solutions in vacuum boundary problems, emphasizing the role of boundary decay rates and physical degeneracies.

Findings

Classical solutions do not exist immediately for expanding initial velocities near the boundary.

Entropy blows up instantly if the initial density decay rate is not specific.

Non-existence results depend on the presence of heat conductivity and boundary decay rates.

Abstract

This paper considers the immediate blowup of entropy-bounded classical solutions to the vacuum free boundary problem of non-isentropic compressible Navier-Stokes equations. The viscosities and the heat conductivity could be constants, or more physically, the degenerate, temperature-dependent functions which vanish on the vacuum boundary (i.e., $\mu=\bar{\mu} \theta^{\alpha}, ~ \lambda=\bar{\lambda} \theta^{\alpha},~ \kappa=\bar{\kappa} \theta^{\alpha}$, for constants $0\leq \alpha\leq 1/(\gamma-1)$, $\bar{\mu}>0,~2\bar{\mu}+n\bar{\lambda}\geq0,~\bar{\kappa}\geq 0$, and adiabatic exponent $\gamma>1$). With prescribed decaying rate of the initial density across the vacuum boundary, we prove that: (1) for three-dimensional spherically symmetric flows with non-vanishing bulk viscosity and zero heat conductivity, entropy-bounded classical solutions do not exist for any small time, provided the initial velocity is expanding near the boundary; (2) for three-dimensional spherically symmetric flows with non-vanishing heat conductivity, the normal derivative of the temperature of the classical solution across the free boundary does not degenerate, and therefore the entropy immediately blowups if the decaying rate of the initial density is not of $1/(\gamma-1)$ power of the distance function to the boundary; (3) for one-dimensional flow with zero heat conductivity, the non-existence result is similar but need more restrictions on the decaying rate. Together with our previous results on local or global entropy-bounded classical solutions (Liu and Yuan, SIAM J. Math. Anal. (2) 51, 2019; Liu and Yuan, Math. Models Methods Appl. Sci. (9) 12, 2019), this paper shows the necessity of proper degenerate conditions on the density and temperature across the boundary for the well-posedness of the entropy-bounded classical solutions to the vacuum boundary problem of the viscous gas.