Lagrangian coherent structures and entrainment near the turbulent/non-turbulent interface of a gravity current

Neamtu-Halic, Marius M.; Krug, Dominik; Haller, George; Holzner, Markus

doi:10.1017/jfm.2019.635

Cited by 25 publications

(23 citation statements)

References 41 publications

(110 reference statements)

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“…Lagrangian methods are, however, not necessarily constrained by such requirements (Schlueter-Kuck and Dabiri 2017). This last feature drives the development of new post-processing strategies for the detection of coherent structures with data obtained from large-scale applications (Neamtu-Halic et al 2019;Wei et al 2019). Lagrangian (trajectory-based) analysis allows for improved understanding of various processes that take place in the ocean, atmosphere and even in the lab, among other problems, which often suffer from poor spatial resolution (Mathur et al 2019).…”

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confidence: 99%

Detection of vortical structures in sparse Lagrangian data using coherent-structure colouring

2021

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In this study, vortical structures are detected on sparse Shake-The-Box data sets using the Coherent-Structure Colouring (CSC) algorithm. The performance of this Lagrangian approach is assessed by comparing the CSC-coloured tracks with the baseline vorticity field. The ability to extract vortical structures from sparse data is accessed on two Lagrangian particle tracking data sets: the flow past an Ahmed body and a swirling jet flow. The effects of two normalized parameters on the identification of vortical structures were defined and studied: the mean track length and the mean inter-particle distance. The accuracy of the vortical-structure detection problem through CSC is shown to improve with decreasing inter-particle distance values, whereas little dependence on the mean track length is observed at all. Overall, the CSC algorithm showed to yield accurate detection of coherent structures for inter-particle distances smaller than 15% of the characteristic dimension of the structure. However, the results quickly deteriorate for sparser Lagrangian data.

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Section: List Of Symbols R Ijmentioning

confidence: 99%

Detection of vortical structures in sparse Lagrangian data using coherent-structure colouring

2021

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“…Moreover, others only considered a small volume limited to the current/ambient interface ( 4 × 4 × 2 cm from a domain 200 × 50 × 50 cm) (Krug et al 2015). In the same experimental domain, Neamtu-Halic et al(2019) identified vortical Lagrangian coherent structures in the flow body capable of affecting the height of the turbulent/non-turbulent interface, though measurements were again limited to the current-ambient interface (this work considering 4 regions of 9 × 9 × 4 cm) precluding identification of structures lower in the flow. As a result of the limitations of these works, significant questions regarding the structure of the gravity current body remain.…”

Section: Introductionmentioning

confidence: 99%

Observations of large-scale coherent structures in gravity currents: implications for flow dynamics

et al. 2021

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Density driven flows, also known as gravity currents, comprise a head, body, and tail. Yet whilst the body typically forms the largest part of such flows, its structure remains poorly understood. In this work, experimental data gathered using particle image velocimetry enables the instantaneous, whole-field dynamics of constant-influx solute-based gravity currents to be resolved. While averaged turbulent kinetic energy profiles are comparable to previous work, the instantaneous data sets reveal significant temporal variation, with velocity measurements indicating large-scale wave-like motions within the body. Spectral analysis and dynamic mode decomposition, of streamwise and vertical velocity, are used to identify the frequencies and structures of the dominant motions within the flow. By considering an idealised theoretical density profile, it is suggested that these structures may be internal gravity waves that form a critical layer within the flow located at the height of the maximum internal velocity. Irreversible internal wave breaking that has been postulated to occur at this critical layer suggests formation of internal eddy transport barriers, demonstrating that new dynamic models of turbulent mixing in gravity currents are needed. Graphic abstract

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“…The goal is to understand the impact of vortical structures as extracted from the flow on the dynamics of enstrophy near the TNTI. To identify the OECSs that are in proximity of the TNTI and to investigate their contribution to enstrophy and scalar transport, we use a recently developed conditional analysis by Neamtu-Halic et al [36].…”

Section: Introductionmentioning

confidence: 99%

“…Nowadays, it is widely accepted that entrainment rate reduces with increasing stratification as a consequence of the reduction of both the local entrainment velocity and the area of the TNTI [31,46,47]. Recently, Neamtu-Halic et al [36] used experimental data of a gravity current to show that OECSs modulate the area of the TNTI and that their modulating capacity diminish with increasing stratification. Moreover, OECSs have been observed to organize the flow field on the TNTI proximity thereby imposing the local entrainment velocity [36] and setting the mechanism that produces/destroys the surface area of the nearby TNTI [37].…”

Section: Introductionmentioning

confidence: 99%

“…Recently, Neamtu-Halic et al [36] used experimental data of a gravity current to show that OECSs modulate the area of the TNTI and that their modulating capacity diminish with increasing stratification. Moreover, OECSs have been observed to organize the flow field on the TNTI proximity thereby imposing the local entrainment velocity [36] and setting the mechanism that produces/destroys the surface area of the nearby TNTI [37]. It remains to be understood how OECSs affect the process of the entrainment and how do they adapt at different levels of stratification.…”

Section: Introductionmentioning

confidence: 99%

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Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification

Neamtu-Halic,

Mollicone,

van Reeuwijk

et al. 2020

Preprint

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In this paper, we investigate the enstrophy dynamics in relation to objective Eulerian coherent structures (OECSs) and their impact on the enstrophy and scalar transport near the turbulent/non-turbulent interface (TNTI) in flows with and without stable stratification. We confirm that vortex-stretching produces enstrophy inside the boundaries of the OECSs, while viscous diffusion transfers the enstrophy across the boundaries of the structures. Although often overlooked in the literature, viscous dissipation of enstrophy within the boundaries of vortical structures is significant. Conversely, for the weakly stratified flows also investigated here, the effect of the baroclinic torque is negligible. We provide evidence that the OECSs advect the passive/active scalar and redistribute it via molecular diffusion. Finally, we use conditional analysis to show that the typical profiles of the enstrophy and scalar transport equation terms across the TNTI are compatible with the presence of OECSs positioned at the edge between the turbulent sublayer and the turbulent core region. We show that when these profiles are further conditioned to the presence of OECSs, their magnitude is considerably higher.

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Lagrangian coherent structures and entrainment near the turbulent/non-turbulent interface of a gravity current

Cited by 25 publications

References 41 publications

Detection of vortical structures in sparse Lagrangian data using coherent-structure colouring

Detection of vortical structures in sparse Lagrangian data using coherent-structure colouring

Observations of large-scale coherent structures in gravity currents: implications for flow dynamics

Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification

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