A criterion for identifying strong flow regions in turbulence

We propose a new criterion for identifying strong flow regions in turbulence. This criterion may also be used as a first approximation to characterise where and when dissolved polymer molecules may become deformed by a turbulent flow, which is, for example, important for drag reduction. In arriving at the new criterion, we first review several existing criteria for identifying strong flow regions. They have the disadvantage of being either not objective or hard to compute with good accuracy. The new criterion, which depends on the second and third invariant of the rate-of-strain tensor, does not suffer from these disadvantages. After an analysis of the criterion and a comparison with an existing criterion using a simulated turbulent pipe flow, we conclude that it is indeed suitable for identifying strong flows. Finally, we show that the criterion is related to enstrophy production, and thus may also have some dynamical relevance.