  The Richardson scaling law states that the mean square separation of a fluid
particle pair grows according to t3 within the inertial range and at
intermediate times. The theories predicting this scaling regime assume that the
pair separation is within the inertial range and that the dispersion is local,
meaning that only eddies at the scale of the separation contribute. These
assumptions ignore the structural organization of the turbulent flow into
large-scale shear layers, where the intense small-scale motions are bounded by
the large-scale energetic motions. Therefore, the large scales contribute to
the velocity difference across the small-scale structures. It is shown that,
indeed, the pair dispersion inside these layers is highly non-local and
approaches Taylor dispersion in a way that is fundamentally different from the
Richardson scaling law. Also, the layer's contribution to the overall mean
square separation remains significant as the Reynolds number increases. This
calls into question the validity of the theoretical assumptions. Moreover, a
literature survey reveals that, so far, t3 scaling is not observed for initial
separations within the inertial range. We propose that the intermediate pair
dispersion regime is a transition region that connects the initial Batchelor-
with the final Taylor-dispersion regime. Such a simple interpretation is shown
to be consistent with observations, and is able to explain why t3 scaling is
found only for one specific initial separation outside the inertial range.
Moreover, the model incorporates the observed non-local contribution to the
dispersion, because it requires only small-time-scale properties and
large-scale properties.
