The average patterns of the velocity and scalar fields near turbulent/non-turbulent interfaces (TNTI), obtained from direct numerical simulations (DNS) of planar turbulent jets and shear free turbulence, are assessed in the strain eigenframe. These flow patterns help to clarify many aspects of the flow dynamics, including a passive scalar, near a TNTI layer, that are otherwise not easily and clearly assessed. The averaged flow field near the TNTI layer exhibits a saddle-node flow topology associated with a vortex in one half of the interface, while the other half of the interface consists of a shear layer. This observed flow pattern is thus very different from the shear-layer structure consisting of two aligned vortical motions bounded by two large-scale regions of uniform flow, that typically characterizes the average strain field in the fully developed turbulent regions. Moreover, strain dominates over vorticity near the TNTI layer, in contrast to internal turbulence. Consequently, the most compressive principal straining direction is perpendicular to the TNTI layer, and the characteristic 45-degree angle displayed in internal shear layers is not observed at the TNTI layer. The particular flow pattern observed near the TNTI layer has important consequences for the dynamics of a passive scalar field, and explains why regions of particularly high scalar gradient (magnitude) are typically found at TNTIs separating fluid with different levels of scalar concentration. Finally, it is demonstrated that, within the fully developed internal turbulent region, the scalar gradient exhibits an angle with the most compressive straining direction with a peak probability at around 20. The scalar gradient and the most compressive strain are not preferentially aligned, as has been considered for many years. The misconception originated from an ambiguous definition of the positive directions of the strain eigenvectors.@en

The scaling of turbulent motions is investigated by considering the flow in the eigenframe of the local strain-rate tensor. The flow patterns in this frame of reference are evaluated using existing direct numerical simulations of homogeneous isotropic turbulence over a Reynolds number range from up to 1131, and also with reference to data for inhomogeneous, anisotropic wall turbulence. The average flow in the eigenframe reveals a shear layer structure containing tube-like vortices and a dissipation sheet, whose dimensions scale with the Kolmogorov length scale, . The vorticity stretching motions scale with the Taylor length scale, , while the flow outside the shear layer scales with the integral length scale, . Furthermore, the spatial organization of the vortices and the dissipation sheet defines a characteristic small-scale structure. The overall size of this characteristic small-scale structure is in all directions based on the coherence length of the vorticity. This is considerably larger than the typical size of individual vortices, and reflects the importance of spatial organization at the small scales. Comparing the overall size of the characteristic small-scale structure with the largest flow scales and the vorticity stretching motions on the scale of shows that transitions in flow structure occur where and 250. Below these respective transitional Reynolds numbers, the small-scale motions and the vorticity stretching motions are progressively less well developed. Scale interactions are examined by decomposing the average shear layer into a local flow, which is induced by the shear layer vorticity, and a non-local flow, which represents the environment of the characteristic small-scale structure. The non-local strain is in width and height, which is consistent with observations in high Reynolds number flow of a wide instantaneous shear layer with many -scale vortical structures inside (Ishihara et al., Flow Turbul. Combust., vol. 91, 2013, pp. 895–929). In the average shear layer, vorticity aligns with the intermediate principal strain at small scales, while it aligns with the most stretching principal strain at larger scales, consistent with instantaneous turbulence. The length scale at which the alignment changes depends on the Reynolds number. When conditioning the flow in the eigenframe on extreme dissipation, the velocity is strongly affected over large distances. Moreover, the associated peak velocity remains Reynolds number dependent when normalized by the Kolmogorov velocity scale. It signifies that extreme dissipation is not simply a small-scale property, but is associated with large scales at the same time.@en

Direct numerical simulations (DNS) of forced (statistically stationary) isotropic turbulence are used to assess the characteristics of the intense vorticity structures (IVS) or "worms"at higher Reynolds numbers than previously available. The simulations cover a range of Taylor-based Reynolds numbers in the range 90≤Reλ≤399, for a resolution of kmaxη≈2.0, where kmax is the maximum resolved wave number and η is the Kolmogorov microscale. Most of the IVS characteristics are confirmed at the higher Reynolds numbers analyzed in this work; e.g., the results confirm that the mean radius of the IVS (Rivs) is approximately equal to (Rivs)/η≈4-5 and that the mean radius is equal to the radius of the stationary Burgers vortex (Rivs/RB)≈1.0, with RB=2(ν/α)1/2, where ν is the kinematic viscosity and α is the (locally) imposed rate of strain. Moreover, the tangential velocity of the IVS scales with the root-mean-square velocity of the flow Uivs∼u′. These IVS characteristics seem to be robust and relatively independent of the Reynolds number; however, there are other quantities for which the classical results do not hold. A notable example is the mean length of the IVS (Livs), which previous works, carried out at smaller Reynolds numbers, claimed to scale with either the Taylor microscale λ or the integral scale of turbulence L. It turns out that the correct scaling for this quantity can be observed only at Reλ200, and the results consistently show that Livs scales with the Kolmogorov microscale Livs∼η, with a mean value equal to (Livs)≈60η. The present findings provide further evidence that Reλ200 is required for the small scales to be fully developed.@en