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Effects of submergence and test startup conditions on local scour by plane

Author(s): N. Deshpande; R. Balachandar; K.A. Mazurek

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Keywords: Soil erosion; scour; water jets; tailwater; flow measurement

Abstract: Experiments were carried out to study the scour of cohesionless soils by submerged plane turbulent wall jets. The Reynolds number based on the jet exit conditions was 32,500 and the densimetric Froude number was 6.2. Submergence, defined as the ratio of the tailwater depth (yt) to nozzle exit thickness (bo) was varied from 4 to 20. Detailed velocity measurements were conducted using a laser Doppler anemometer. The results confirm the presence of two distinct types of flow fields, one that occurs at lower submergences and the other at higher submergences (yt/bo = 8). At low submergences, velocity histograms indicate that the jet is initially close to the bed and then flicks towards the water surface, whereas at the higher submergences, no such flicking movement was observed. At higher submergences, the jet impingement point on the sand bed was highly unsteady. Low pass filtering of the velocity data give further details of these processes. For tailwater conditions corresponding to the high submergence range, the frequency of motion of the impingement point tends to increase as the tailwater depth is reduced. Furthermore, variations in scour and velocity profiles were noted in the mound region across the flume cross-section. The extent of these variations was found to be dependent on the level of submergence. In an effort to clarify some of the differences in the scour characteristics noted in earlier studies, three different test startup conditions were adopted. ”Startup“ is defined as the commencement of flow through the nozzle on to the sand bed. The conditions include an instantaneous attainment of final velocity, a gradual increase and a stepwise increase to final flow conditions. The velocity measurements indicate that the flow gradually evolves to a state that is independent of the startup condition. However, the scour profiles appear to be dependent on the startup conditions for a longer duration.


Year: 2007

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