Hydrodynamic drag in cosmological simulations

We present a study of hydrodynamic drag forces in smoothed particle hydrodynamic (SPH) simulations. In particular, the deceleration of a resolution-limited cold clump of gas moving through a hot medium is examined. The drag at subsonic velocities exceeds that expected from simple dynamics-based arguments. The excess is shown to be a result of the hydrodynamic method. SPH encourages the accretion of particles from the hot medium on to a shell around the cold clump, effectively increasing the radius of the clump. For sonic and supersonic velocities, the effective area of the clump is shown to be set by the symmetrized smoothing length. The relationship of the effective area to the smoothing length breaks the expected dependency of drag on density since the symmetrized smoothing length increases with decreasing density. The consequences for cosmological simulations are discussed.