Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel

Abstract

Topographic interactions generate multidirectional and unsteady airflow that limits the application of velocity profile approaches for estimating sediment transport over dunes. Results are presented from a series of wind tunnel simulations using Irwin-type surface-mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp-crested model dunes. Findings complement existing theories on secondary airflow effects on stoss transport dynamics and provide new information on the influence of lee-side airflow patterns on dune morphodynamics.For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a significant drop in time-averaged measurements of streamwise shear stress at this location. This effect is believed sufficient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and flow acceleration cause an increase in flow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60-70 per cent greater than at flow reattachment on the lower stoss of closely spaced dunes. Streamwise flow accelerations, rather than turbulence, have greater influence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed flow within the separation cell generates significant surface shear (30-40 per cent of maximum values) for both spacings. This supports field studies that suggest reversed flow is competent enough to return sediment to the dune directly or in a deflected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time-averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and flow steadiness increase within 6 h (h = dune height) of reattachment and approach upwind values by 25 h. A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to fluvial estimates.The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost-effective design shows promise for validating these findings in natural dune settings.

Source-to-Sink

“…Within these complex-system interactions, the ''primary'' flow (i.e., the flow before the bedforms) becomes slaved to the emergent bedform topography (and not the other way around) to produce a characteristic ''secondary'' flow over the bedforms. This characteristic secondary flow differs in speed, direction, shear stress, and level of turbulence from the primary flow as a function of position on the bedform (for eolian dunes, see Kocurek [1996a, 1996b] and Walker and Nickling [2003]). The most basic flow field over dunes consists of flow acceleration and erosion on the upstream or stoss slope and flow deceleration and deposition on the downstream or lee slope.…”

Visualization and laser measurements on the flow field and sand movement on sand dunes with porous fences

“…When an air-flow velocity becomes higher than the critical friction velocity, sand particles start to move. As for sand-movement dynamics on sand dunes, Walker (1999) and Walker and Nickling (2003) measured secondary airflows behind a dune, focusing on their influences on both stoss-side and lee-side transport dynamics in isolated and closely spaced dune systems. Andreotti et al (2002a, b) investigated surface waves on barchans, which were generated by dune collisions and changes in wind direction.…”

Foredune accretion under offshore winds

“…Further field research into airflow patterns in dune systems (e.g. Arens et al, 1995;Frank and Kocurek, 1996;Hesp, 2002;Kocurek et al, 1992;Tsoar, 1983;Walker, 1999;Walker and Nickling, 2002) as well as wind tunnel studies and numerical modelling (Parsons et al, 2004a,b;Schatz and Herrmann, 2006 ;van Boxel et al, 1999;Walker and Nickling, 2003) has yielded a more detailed understanding of lee-side air flow. The three broad categories--attached and undeflected flow, attached and deflected flow, and separated flow--determined by Sweet and Kocurek (1990) and modified by Walker and Nickling (2002) are dependent primarily on dune topography and the approach angle of the wind.…”