Three-dimensional Computational Fluid Dynamic experiments over a complex dune topography

Jet flows, which are localized flows exhibiting a high speed maxima, are relatively common in nature, and in many devices. They have only been occasionally observed on dunes, and their dynamics are poorly known. This paper examines computational fluid dynamic (CFD) two-dimensional (2D) modelling of jet flow over a foredune topography. Flow was simulated in 10° increments\ud from onshore (0°) to highly oblique alongshore (70°) incident wind approach angles. CFD modelling reveals that the formation of a jet is not dependent on a critical wind speed, and an increase in incident wind velocity does not affect the magnitude of jet flow. A jet is first formed at ~1.0m seawards of the foredune crest on the Prince Edward Island foredune morphology example examined here. A jet is not developed when the incident wind is from an oblique approach angle greater than ~50° because there is significantly less flow acceleration across a much lower slope at this incident angle. The presence of a scarp does influence the structure of the crest jet, in that the jet is more pronounced where a scarp is present. Surface roughness affects the magnitude of jet expansion and jets are better developed on bare surfaces compared to vegetated ones

Section: Methodsmentioning

confidence: 99%

Jet flow over foredunes

Hesp

Smyth

2016

“…Scarping is common on coastal dunes and can be defined as basal erosion and undercutting of the stoss slope of a dune due to wave attack or possibly stream and river erosion (Carter et al, ; Aagaard et al, ; Castelle et al, ). Scarping occurs on a variety of dune types in a number of different environments (Vellinga, ; Carter, ; Pye and Neal, ; Cooper et al, ; van Gent et al, ; van Thiel de Vries et al, ; Roelvink et al, ; Ruz et al, ; Pattanapol et al, ; Wakes, ). Factors such as the maximum wave height, maximum surge level, tide fluctuation, flood height in the case of a river, the time interval between significant wave events, and the previous morphology of the dune play a fundamental role in determining the degree of erosion and scarping (Carter and Stone, ; Christiansen and Davidson‐Arnott, ; Cooper et al, ; Fiore et al, ; van Rijn, ; Lopes et al, ; Li et al, ; Bonte and Levoy, ).…”

Section: Introductionmentioning

confidence: 99%

Flow dynamics over a foredune scarp

Piscioneri

Smyth

Hesp

2019

The nature of wind flow over a small, 0.6 m high foredune scarp is investigated on the Sir Richard Peninsula, South Australia during a variety of incident wind directions and speeds. The study provides additional supporting evidence that the presence of the scarp and the dune exerts a strong influence on a landwards trending reduction in wind velocity and an increase in turbulence, with the greatest area of turbulence occurring near and at the foredune scarp base. For an incident oblique wind, an alongshore helicoidal flow is formed within a separation region along the scarp basal region. In this region, the coefficient of variation (CV) of wind speed is high and displays significant fluctuations. The flow at the scarp crest is compressed, streamlined and accelerated, turbulence is suppressed, and local jets may occur depending on the incident wind approach angle. Jets are more likely where the incident flow is perpendicular or nearly so. A flow separation region does not develop downwind of the scarp crest where the morphology of the foredune stoss slope downwind of the scarp is more convex (as in this case) rather than relatively flat, and possibly due to the presence of vegetation at the scarp crest. A tentative model of the flow regions developed across a backshore–scarp–foredune region during oblique incident flow is provided. © 2018 John Wiley & Sons, Ltd.

“…Others have used a value of 0.05 m (e.g. Jackson et al, 2020;Wakes, 2013;Wakes et al, 2016Wakes et al, , 2021 as did we in some trial runs, and this leads to much larger values of wall shear stress, more extreme changes in velocity and TKE profiles, and slightly altered recirculation dynamics in front of the scarp and in the lee of the crest. A new generation of CFD simulations is required that uses field measurements of actual surface roughness characteristics to define a realistic modelling surface, with smooth conditions on the beach and rough conditions on the stoss slope due to vegetation cover.…”

Section: Discussionmentioning

confidence: 99%

CFD simulations of wind flow across scarped foredunes: Implications for sediment pathways and beach–dune recovery after storms

Bauer

Wakes

2022

Beach–dune systems are dynamic geomorphic environments subject to erosion during storms and reconstruction during swell‐dominated inter‐storm periods. Wave‐cut scarps are frequently carved into the beach–dune profile, occasionally eroding the stoss slope of the foredune. Although the rapid phases of erosion have been researched and modelled extensively, relatively little is known about beach–dune recovery, especially scarp healing by aeolian processes. Computational fluid dynamic (CFD) modelling was deployed in this study to yield insights into different flow patterns across beach–dune profiles with three model foredunes (unscarped, 1 m scarp, 2 m scarp) for varying wind approach angles (α = 0–45°). The results show that the presence of a scarp substantially modifies the flow dynamics on the backbeach in front of the foredune. Vertical profiles of turbulence kinetic energy indicate that the influence of this flow‐modified zone extends out to about −7h from the scarp base (where h is the scarp height). The reduction in onshore wind speed that is characteristic of this flow‐modified zone favours sand deposition on the backbeach and sand ramp development at the base of the scarp. Shore‐normal wind approach angles yield discontinuous roller‐like vortices with reversing (offshore) flow components at the sand surface, accompanied by the evolution of echo dunes. Oblique wind approach angles yield corkscrew (helicoidal) vortices that lead to the formation of continuous sand ramps alongshore. A conceptual model of scarp healing based on ideas previously discussed in the literature and enhanced by the CFD simulation results highlights the importance of sand ramps for reconnecting the transport system on the beach to the foredune stoss slope, thereby facilitating foredune growth and maintenance.