Extensive strain along gradient trajectories in the turbulent kinetic energy field

Based on direct numerical simulations of forced turbulence, shear turbulence, decaying turbulence, a turbulent channel flow as well as a Kolmogorov flow with Taylor-based Reynolds numbers Re-lambda between 69 and 295, the normalized probability density function of the length distribution (P) over tilde((l) over tilde) of dissipation elements, the conditional mean scalar difference at the extreme points as well as the scaling of the two-point velocity difference along gradient trajectories are studied. Using the field of the instantaneous turbulent kinetic energy k as a scalar, we find good agreement between the model equation for (P) over tilde ((l) over tilde) as proposed by Wang and Peters (2008 J. Fluid Mech. 608 113-38) and the results obtained in the different direct numerical simulation cases. This confirms the independence of the model solution from both the Reynolds number and the type of turbulent flow, so that it can be considered universally valid. In addition, we show a 2/3 scaling for the mean conditional scalar difference. In the second part of the paper, we examine the scaling of the conditional two-point velocity difference along gradient trajectories. In particular, we compare the linear s/tau scaling, where tau denotes an integral time scale and s the separation arclength along a gradient trajectory in the inertial range as derived by Wang (2009 Phys. Rev. E 79 046325) with the s . a(infinity) scaling, where a(infinity) denotes the asymptotic value of the conditional mean strain rate of large dissipation elements.