  The flow properties of a continuum model for an active nematic is studied and
compared with recent experiments on suspensions of microtubule bundles and
molecular motors. The velocity correlation length is found to be independent of
the strength of the activity while the characteristic velocity scale increases
monotonically as the activity is increased, both in agreement with the
experimental observations. We interpret our results in terms of the creation
and annihilation dynamics of a gas of topological defects.
