Jet flow over foredunes

Abstract

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

Flow dynamics over a foredune scarp

“…Another jet is formed centered on 0.45 m above the surface at 2D‐6 (~2.2 m downwind of the scarp crest) as the flow continues to accelerate up the foredune slope. Such jets as seen just seaward of the scarp crest (2D‐4) and on the foredune stoss slope (2D‐6) are common near, and at the crest of steep cliffs and scarps and also over high foredunes (Hsu, ; Hesp et al, ; Yasin and Al‐Harbi, ; Pires et al, ; Elkhoury, ; Hesp and Smyth, ).…”

“…A jet may form at the scarp crest (Hsu, ; Arens, ; Tsoar et al, ; Hesp et al, , ; Jarmalavicius et al, ; Yassin and Al‐Harbi, ; Pires et al, ; Hesp and Smyth, ), and the degree of turbulence at the crest is greatest for vertical scarps, and declines as the scarp or escarpment gradient lowers (Yasin and Al‐Harbi, ; SPires et al, ). If the morphology of the dune (or other topography) is flat to relatively flat downwind of the scarp crest, flow separation usually occurs and a region of turbulent reversing flow develops (Wilhelm et al, ; Largeau and Moriniere, ; Pires et al, ; Rowcroft et al, ; Elkhoury, ).…”

Topographic change and numerically modelled near surface wind flow in a bowl blowout

“…These local changes in the pressure field cause the wind to deviate in both speed and direction, producing complex streamline behaviour Hesp and Smyth, 2016). As wind flow approaches an obstacle or a topographic change, such as a blowout, secondary flow patterns are created as the boundary layer is altered by topographically generated changes in fluid pressure.…”

“…As wind flow approaches an obstacle or a topographic change, such as a blowout, secondary flow patterns are created as the boundary layer is altered by topographically generated changes in fluid pressure. These local changes in the pressure field cause the wind to deviate in both speed and direction, producing complex streamline behaviour Hesp and Smyth, 2016). In blowouts, complex patterns of near surface wind flow occur most notably at marked topographic breaks, along and above steep erosional walls that adjoin the deflation basin, and up deflation basins during oblique incident flows (Gares and Nordstrom, 1995;Hesp and Hyde, 1996;Hesp and Pringle, 2001;Hugenholtz and Wolfe, 2009;Hesp and Walker, 2012;Smyth et al, 2013).…”

Barrier response to sea level rise and storms

“…They also argue that the apparent development (or recovery) of a dune towards a maximum (or equilibrium) height (see Houser et al ., ) is not based on a reduction in sediment transport due to stagnation, but a lack of available sediment (from the nearshore) and the need for a greater volume of sediment to yield a unit increase in height. Results of this study highlight the need for continued investment in field studies, and the need to reconcile numerical models with field data that may include localized flows that are difficult to capture in a model (Hesp and Smyth, , this issue).…”