Non-Hydrostatic Modelling of Kelvin-Helmholtz Billows Along the North Passage of Yangtze River Estuary, China

Shi, Jian; Zheng, Jinhai; Tong, Chaofeng

doi:10.9753/icce.v36.currents.81

Cited by 1 publication

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“…Tidal-scale processes in fresh-salt water mixing are well understood through in situ observations, laboratory experiments and numerical simulations, however, relative little is know about the hydrodynamic processes on intra-tidal time scales. These small scale flow structures, such as turbulence and instabilities, are the dominant mechanism for the transition from organized flow to turbulence, which is critical to the understanding of the hydrodynamics and mass transport in estuarine areas [4][5][6][7].…”

Section: Introductionmentioning

confidence: 99%

Kelvin-Helmholtz Billows Induced by Shear Instability along the North Passage of the Yangtze River Estuary, China

Shi

Tong

Zheng

et al. 2019

JMSE

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Kelvin-Helmholtz (K-H) instability plays a significant role in mixing. To investigate the existence of K-H instability along the North Passage of the Yangtze River Estuary, the non-hydrostatic model NHWAVE is utilized to simulate the fresh-salt water mixing process along the North Passage of the Yangtze River Estuary. Using high horizontal resolution, the structure of K-H billows have been successfully captured within the Lower Reach of the North Passage. The K-H instability occurs between the max flood and high-water slack. The duration and length scale of the K-H billows highly depends on the local interaction between fresh-water discharge and tide. The horizontal length scale of the instability is about 60 m, similar to the observations in other estuaries. In the vertical direction, the K-H billows exist within the pycnocline with length scale ranging from 6 to 7 m. The timescale of the billows is approximate 6 min. By analyzing the changes of potential energy during the mixing process, results show that the existence of K-H instability induces intense vertical mixing, which can greatly increase mixing efficiency in the North Passage of the Yangtze River Estuary.Miles [10] and Howard [11] showed that a necessary condition for K-H instability in a parallel, stratified, inviscid flow, is that the gradient Richardson number (Ri = N 2 /S 2 , where N = −(g/ρ)(∂ρ/∂z) is the Brunt-Väisälä frequency, S = du/dz is the velocity shear, g is the gravity acceleration, ρ is density and u is a representative flow speed) is less than 0.25 somewhere in the flow. However, it has been demonstrated this criterion is not sufficient [12], because it is possible to have a stable shear layer when Ri < 0.25 at the pycnocline. Fringer and Street [13] found the interfacial wave can be stable with Ri < 0.25, but unstable perturbations occurred when Ri < 0.13. Barad and Fringer [14] used an adaptive numerical method to evaluate the critical Ri for instability, and the result showed a similar value of Ri < 0.1 is required for instability. Based on laboratory experiments, Fructus et al. [15] proposed a new criterion for instability, which is L x /λ > 0.86, where L x is the length of the region with Ri < 0.25 and λ is the wave width. Another alternative criterion for instability is based on the linear stability analysis with the Taylor-Goldstein equation. Troy and Koseff [16] used the equation to derive the criterion for instability, which requiresσ i T w > 5, where T w is the time the fluid spends in region with Ri < 0.25 andσ i is the averaged growth rate of the instabilities in the region.

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Section: Introductionmentioning

confidence: 99%