  The artificial viscosity is reconsidered in smoothed particle hydrodynamics
to prevent inter-particle penetration, unwanted heating, and unphysical
solutions. The coefficients in the Monaghan's standard artificial viscosity are
considered as time variable, and a restriction on them is proposed such that
avoiding the undesired effects in the subsonic regions. The shock formation in
adiabatic and isothermal cases are used to study the ability of this modified
artificial viscosity recipe. The computer experiments show that the proposal
appears to work and the accuracy of this restriction is acceptable.
