Control of a boundary layer separation

Uruba, Václav; Knob, Martin; Popelka, Lukáš

doi:10.1002/pamm.200700945

Cited by 4 publications

(4 citation statements)

References 2 publications

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“…() show that the extent of the flow separation region varies according to incident wind velocity so it may be that the change in either incident velocity or direction may have produced a change in the flow structure over the scarp. Separated flows above forward‐facing steps or scarps are also characterised by a flapping motion particularly in the high shear layer along the surface of the separation envelope or region, so it is possible to have fluctuating velocities along that zone (Largeau and Moriniere, ; Uruba and Knob, ; Pearson et al, ). As the wind direction veered towards almost perpendicular onshore after ~12.30 pm, the highest placed anemometer at the scarp crest tended to display the highest velocities (Figure (b)).…”

Section: Three Dimensional (3d) Flowmentioning

confidence: 99%

“…Where the slope is lower than ~55° to 60°, basal flow separation does not occur, and speedup is common as the flow moves upslope (Jensen and Petersen, 1968; Bowen and Lindley, , ; Tsoar, ; Emeis et al, ). Above those slope gradients, the development of a turbulent reversing vortex within a flow separation region or envelope at the base of the scarp is common (Uruba and Knob, ; Pires et al, , ; Qian et al, ). The point at which flow separation occurs upstream is around –0.8 h to –1.2 h where h is the scarp height according to Pearson et al .…”

Section: Introductionmentioning

confidence: 99%

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Flow dynamics over a foredune scarp

Piscioneri

Smyth

Hesp

2019

Earth Surf Processes Landf

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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.

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Section: Three Dimensional (3d) Flowmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Flow dynamics over a foredune scarp

Piscioneri

Smyth

Hesp

2019

Earth Surf Processes Landf

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“…The flow separation region upwind of the scarp/slope is inherently turbulent and unsteady (Uruba and Knob, 2009). The degree of upwind turbulence and pressure increases with increasing slope (Xianwan et al, 1999), and the extent of flow separation depends on the Reynolds number (Hattori and Nagano, 2010).…”

Section: Introductionmentioning

confidence: 99%

“…The turbulence intensity of both streamwise and transverse velocity fluctuations increases as step/scarp height increases, and the downwind length of the separation region increases with step height (Abu-Mulaweh, 2005). The average re-attachment length of the separation region on the scarp or step plateau or terrace depends on the incident velocity (Largeau and Moriniere, 2007), but is variable due to the flapping behaviour (low frequency fluctuations) or unsteady motion of the shear layer above the separation region (Largeau and Moriniere, 2007;Uruba and Knob 2009;Pearson et al, 2013). This flapping motion is related to the ejection of flow within the separated region (Sherry et al, 2010).…”

Section: Introductionmentioning

confidence: 99%

CFD flow dynamics over model scarps and slopes

Hesp

Smyth

2019

Physical Geography

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As sea level rises, and during storm and surge events, coastal dunes may become cliffed or scarped by wave action. Knowledge of wind flow over dune scarps, and as scarps fill, their subsequent various slopes, is an essential first step to understanding sediment transport pathways from the beach to the dunes. In this study, flow over scarps (also termed forward facing steps) is reviewed, and the flow over a vertical scarp (90°) and three slopes of 45°, 24° and 14°, all 2 m in height, is examined via CFD modelling. The flow over three 90° scarps with heights of 1 m, 2 m and 4 m, and over a 2 m high vertical (90°) scarp for three increasingly oblique incident winds is also studied. The extent of wind flow deceleration, separation and recirculation becomes smaller with decreased slope, with maximum flow separation and reverse vortex development occurring in the front of the vertical scarp. The extent of crest wind flow separation and recirculation is greatest for the scarp (7.8 m in length), and is considerably less for the 45° slope (2.4 m in length). As scarp height increases, so too does the spatial extent of turbulent wind flow, wind speed, and extent of the flow separation region. For cases where the scarp slope varied but height remained constant, the extent of the flow separation region was greatest when the scarp was vertical. Wind flow separation was dramatically reduced below a scarp slope of 45°. As incident wind direction became more oblique over a vertical scarp, wind speed undergoes significantly less deceleration, and helicoidal vortices replace roller vortices. Our results demonstrate how scarp morphology and wind direction are likely to influence transport pathways.

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