Universal alignment in turbulent pair-dispersion
| When |
Feb 14, 2023
from 11:00 to 12:00 |
|---|---|
| Where | Salle des thèses |
| Attendees |
Ron Schnapp |
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The Richardson-Obukhov law of turbulent pair dispersion predicts that the mean squared distance between pairs of Lagrangian particles in turbulence grows algebraically with time with an exponent of 3. However, despite its discovery was made nearly a century ago, this hallmark of turbulence theory has evaded unequivocal experimental confirmation ever since. Among the issues that make its verification difficult are having too small Reynolds numbers, which limit the separation of scales, an intrinsic intermittency of the process, and an innate dependence on initial distances and velocities. To overcome these issues, we present a new way of identifying and verifying Richardson's law in empirical work using a new observable - the angle formed between the separation and the relative velocity vectors. Our theory predicts that the average value of this angle is a universal constant in the inertial range of turbulence, being independent of the scale, Reynolds number, or dissipation rate. We confirm these predictions through experimental and DNS results. Moreover, this constancy is a unique feature that enables identifying the super-diffusive regime in empirical work. Thus, our work allows identifying whether the super-diffusive regime exists and at which scales in future experiments, independently of the flow parameters.