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Register a new userLagrangian coherent structures and entrainment near the turbulent/non-turbulent interface of a gravity current
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Added by
Marius Mihai Neamtu Halic Mr.
Publication date
2019
fields
Physics
and research's language is
English
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In this paper, we employ Lagrangian coherent structures (LCSs) theory for the three dimensional vortex eduction and investigate the effect of large-scale vortical structures on the turbulent/non-turbulent interface (TNTI) and entrainment of a gravity current. The gravity current is realized experimentally and different levels of stratification are examined. For flow measurements, we use a multivolume three-dimensional particle tracking velocimetry technique. To identify vortical LCSs (VLCSs), a fully automated 3D extraction algorithm for multiple flow structures based on the so-called Lagrangian-Averaged Vorticity Deviation method is implemented. The size, the orientation and the shape of the VLCSs are analyzed and the results show that these characteristics depend only weakly on the strength of the stratification. Through conditional analysis, we provide evidence that VLCSs modulate the average TNTI height, affecting consequently the entrainment process. Furthermore, VLCSs influence the local entrainment velocity and organize the flow field on both the turbulent and non-turbulent sides of the gravity current boundary.
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We use DNS to study inter-scale and inter-space energy exchanges in the near-field of a turbulent wake of a square prism in terms of the KHMH equation written for a triple decomposition of the velocity field accounting for the quasi-periodic vortex shedding. Orientation-averaged terms of the KHMH are computed on the plane of the mean flow and on the geometric centreline. We consider locations between $2$ and $8$ times the width $d$ of the prism. The mean flow produces kinetic energy which feeds the vortex shedding coherent structures. In turn, these structures transfer energy to the stochastic fluctuations over all length-scales $r$ from the Taylor length $lambda$ to $d$ and dominate spatial turbulent transport of two-point stochastic turbulent fluctuations. The orientation-averaged non-linear inter-scale transfer rate $Pi^{a}$ which was found to be approximately independent of $r$ by Alves Portela et. al. (2017) in the range $lambdale r le 0.3d$ at a distance $x_{1}=2d$ from the square prism requires an inter-scale transfer contribution of coherent structures for this approximate constancy. However, the near-constancy of $Pi^a$ at $x_1=8d$ which was also found by Alves Portela et. al. (2017) is mostly due to stochastic fluctuations. Even so, the proximity of $-Pi^a$ to the turbulence dissipation rate $varepsilon$ in the range $lambdale rle d$ at $x_1=8d$ requires contributions of the coherent structures. Spatial inhomogeneity also makes a direct and distinct contribution to $Pi^a$, and the constancy of $-Pi^a/varepsilon$ close to 1 would not have been possible without it either in this near-field flow. Finally, the pressure-velocity term is also an important contributor to the KHMH, particularly at scales r larger than about $0.4d$, and appears to correlate with the purely stochastic non-linear inter-scale transfer rate when the orientation average is lifted.
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