Instabilities of the undertow

Li, Li; Dalrymple, Robert A.

doi:10.1017/s0022112098001694

Cited by 30 publications

(5 citation statements)

References 9 publications

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“…This picture is reminiscent of descriptions based on measurements and simulations at the laboratory scale (Nadaoka et al, 1989;Watanabe et al, 2005;Lubin and Glockner, 2015) but, here, the instabilities are generated by the wave-mean shear flow rather than by direct breaking -breaking-induced turbulence is parametrized. It is therefore more consistent with the instability of the undertow profile described by Li and Dalrymple (1998).…”

Section: Structure and Production Of Vertical Shear Instabilitysupporting

confidence: 87%

“…In addition, these studies do not always clearly distinguish whether the instability is associated with the instantaneous plunging and rebounding jet produced by breakers or with the mean shear flow caused by momentum transfer. Yet, the two processes may be sorted by their timescale, i.e., smaller than the wave period for the rebounding jet (Watanabe et al, 2005) and longer for the mean shear turbulence (Li and Dalrymple, 1998). If confirmed, the latter could therefore be an intermediate phenomenon between breaker-induced turbulence and large-scale surf eddies.…”

Section: Introductionmentioning

confidence: 97%

“…They are mostly studied in laboratory experiment (Nadaoka et al, 1989) and laboratoryscale Large-Eddy Simulations (LES) using 2.5D CFD models applied to individual wave breaking (Lin and Liu, 1998;Li and Dalrymple, 1998;Watanabe and Saeki, 1999;Watanabe et al, 2005;Lubin and Glockner, 2015). These previous studies show that the spanwise (mostly alongshore) component of vorticity is an important aspect of the breaking process.…”

Section: Introductionmentioning

confidence: 99%

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Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

et al. 2021

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Flash rips and surf eddies are transient horizontal structures of the order of 10 to 100 m, which can be generated in the surfzone in the absence of bathymetric irregularities. They are traditionally evaluated in a depth averaged setting which involves intrinsic horizontal shear instabilities and the direct generation of vorticity by short-crested waves. In this article, we revisit the processes of surf eddy generation with a new three-dimensional wave resolution model (CROCO) and provide a plausible demonstration of new 3D non-hydrostatic instability and turbulent cascade. We first present a quick overview of a compressible free surface approach suitable for nearshore dynamics. Its ability to simulate the propagation of surface gravity waves and nearshore wave-driven circulation is validated by two laboratory experiments. Next, we present a real world application from Grand Popo Beach, Benin, forced by waves with frequency and directional spreading. The generation of surf eddies by the 3D model differs from depth-averaged models, due to the vertical shear associated with shallow breaking waves. In this case, the generation of eddies from both horizontal shear instability and the breaking of short-crested waves is hampered, the former by stretching the alongshore current and the latter by inhibiting the inverse energy cascade. Instead, the vertical shear flow is subjected to forced wave group variability and Kelvin-Helmholtz type instability at an inflection point. Primary and secondary instabilities generate spanwise and streamwise vorticity connecting small-scale eddies to larger horizontal surfzone structures. Streamwise 1 filaments, appearing as 5 m wide ribs or mini-rips, can extend beyond the surfzone but with moderate energy. These results appear consistent with the velocity spectra and observed patterns of tracers and suspended sediments at Grand Popo Beach. The timescale associated with the mean shear-induced turbulence is several times the wave period and suggests an intermediate range between breaker-induced turbulence and large-scale surf eddies.

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Section: Structure and Production Of Vertical Shear Instabilitysupporting

confidence: 87%

Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

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Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

et al. 2021

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“…The corresponding time-averaged velocity field is a shoreward mean flow above the trough level, and an undertow returning the water offshore. Li & Dalrymple (1998) demonstrated that, before an incoming wave breaks, a vortex surface in the mean parallel shear flow tends to be unstable, so that periodic undulations of the vortex surface are produced. In the mean shear surface, this process is analogous to Kelvin-Helmholz (K-H) instability.…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional vortex structures under breaking waves

2005

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The large-scale vortex structures under spilling and plunging breakers are investigated, using a fully three-dimensional large-eddy simulation (LES). When an overturning jet projecting from the crest in a breaking wave rebounds from the water surface ahead, the vorticity becomes unstable in a saddle region of strain between the rebounding jet and a primary spanwise vortex, resulting in spanwise undulations of the vorticity. The undulations are amplified on a braid in this saddle region, leading to a vortex loop with counter-rotating vorticity. This vortex loop consequently envelops adjacent primary vortices, to form a typical rib structure. This rib component (the stretched vortex loop) in the large-scale vortex structure, which intensifies in the strains associated with the multiple primary vortices generated throughout the splash-up cycle, appears to be the previously found obliquely descending eddy.

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“…(1988Matsunaga et al . ( , 1994 and Li & Dalrymple (1998) have shown that the undertow may be unstable and gives rise to seaward-migrating vortices with shorter length-scales and longer time-scales than the wavefield. However, this aspect of the phenomenon is not presently considered.…”

Section: Introductionmentioning

confidence: 99%

Sea waves and mass transport on a sloping beach

Blondeaux

Brocchini

Vittori

2002

Proc. R. Soc. Lond. A

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The steady streaming induced by a sea wave shoaling on a sloping beach and partly reflected at the coastline is determined in the region seaward of the breaker line. Shallow waters and waves of small amplitude are considered. Moreover, the Reynolds number is assumed to be large but still within the laminar regime and the flow domain is split into a bottom boundary layer and a core region. For an incoming wave which is fully absorbed at the coast the solution shows that close to the bottom the steady streaming is onshore directed even though the depth-averaged value represents an offshore directed flow. Moreover, the vertical velocity distribution depends on the ratio between the wave amplitude a * and the thickness δ * of the bottom boundary layer. For a fully reflected wave, steady recirculation cells are induced, the form and strength of which depend on the ratio a * /δ * . A complex flow is generated for reflection coefficients falling between 0 and 1.

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Instabilities of the undertow

Cited by 30 publications

References 9 publications

Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

Three-dimensional vortex structures under breaking waves

Sea waves and mass transport on a sloping beach

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