Turbulent airflow, Reynolds stress, and sand transport response over a vegetated foredune
Date
2011-08-16
Authors
Chapman, Constance Alida
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Abstract
Recent research has revealed that quasi-instantaneous turbulent Reynolds stresses (RS, -u’w’) and decomposed ‘quadrant’ activity (e.g., ejections and sweeps) over dunes in fluvial and wind tunnel studies has shown that turbulent stresses at the toe of a dune often exceed time-averaged, streamwise shear stress (u*2) estimates. It is believed that semi-coherent turbulent structures are conveyed toward the bed along concave streamlines in this region, and these activities cause fluctuations in local surface stresses that assist in grain entrainment. This study focuses on event-based landform scale interactions between turbulent airflow and sediment transport over a vegetated foredune through the assessment of two different experiments that took place at Greenwich Dunes, Prince Edward Island National Park, P.E.I., Canada. Reynolds decomposition of quasi-instantaneous fluctuating u’ and w’ signals into quadrant (Q) activity (i.e., Q1 outward interactions: u’>0, w’>0; Q2 ejections: u’<0, w’>0; Q3 inward interactions: u’<0, w’<0; Q4 sweeps: u’>0, w’<0) is explored to identify patterns of Reynolds stress signal distributions over the dune. Over flat surfaces, Q2 ejections and Q4 sweeps often dominate RS signals, whereas Q1 outward and Q3 inward interactions are less frequent and contribute negatively to RS generation. Over dunes, however, topographically forced streamline curvature effects alter quadrant activity distributions and, hence, near-surface RS generation by enhancing (at the toe) or inhibiting (at the crest) turbulent motions. This results in Q2 ejection and Q4 sweep activity dominating stress generation on the beach, dune toe, and lower stoss slope, whereas, toward the crest, there is a shift toward Q1 outward and Q3 inward interactions. A flow 'exuberance effect' was identified that explains the contribution of positive to negative contributing activities that varies over the dune and helps explain the spatial pattern in RS. RS generation and sand transport depend on location over the dune (via topographic forcing effects on streamline curvature and flow stagnation/acceleration) and on incident flow direction via topographic steering effects that alter the apparent ‘steepness’ of the dune to flow streamlines. Transport on the lower portion of the dune was driven predominantly by ejection and sweep activity, while toward the crest it became dominated by outward and inward interactions, likely due to increased frequency of streamwise gusts (+u’) and vertical lift (+w’) in topographically compressed flow.
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Keywords
foredune, turbulence, quadrant activity, sand transport, Reynolds stress