Lagrangian microscales in turbulence

Though difficult to obtain experimentally, lagrangian quantities are readily extracted from direct numerical simulations (DNS) of turbulence. Results from recent DNS studies on the temporal nature of lagrangian acceleration and strain rate are reviewed and analysed. Contrary to the long-accepted paradigm, it is found that turbulent straining is not persistent. Both for acceleration and strain rate, directional information is lost in a matter of one or two Kolmogorov timescales; whereas the amplitudes of acceleration and strain rate have longer timescales, that increase with Reynolds number (relative to the Kolmogorov timescale). It is shown that the lagrangian time series of dissipation (i.e. straining amplitude) can be reasonably approximated as the product to two independent random functions; the first is universal and scales with the Kolmogorov scales; the second has a longer timescale and accounts for internal intermittency.