  We investigate stationary gas flows in a fixed, rotating barred potential.
The gas is assumed to be isothermal with an effective sound speed c_s, and the
equations of motion are solved with smoothed particle hydrodynamics (SPH).
Since the thermal energy in cloud random motions is negligible compared to the
orbital kinetic energy, no dependence of the flow on c_s is expected. However,
this is not the case when shocks are involved.
  For low values of c_s an open, off-axis shock flow forms that is
characteristic for potentials with an inner Lindblad resonance (ILR). Through
this shock the gas streams inwards from x_1 to x_2-orbits. At high sound speeds
the gas arranges itself in a different, on-axis shock flow pattern. In this
case, there is no gas on x_2-orbits, demonstrating that the gas can behave as
if there were no ILR. The critical effective sound speed dividing the two
regimes is in the range of values observed in the Milky Way.
  We give a heuristic explanation for this effect. A possible consequence is
that star formation may change the structure of the flow by which it was
initiated. Low-mass galaxies should predominantly be in the on-axis regime.
  A brief comparison of our SPH results with those from a grid-based
hydrodynamic code is also given.
