Pattern Observation during Bed-Form Development Due to Turbulence around Obstacles in river

English Abstract:

The objective of this study is to better understanding the mechanism of formation of different kinds of bedform structures due to obstacles immersed on the plane sediment beds in unidirectional fluid flow. Experiments have shown how local scour develops around obstacles of different shapes in non-cohesive sediments. The study shows that obstacle is the dominant cause of the initiations of scour pattern. Obstacle and the stagnation eddy together onset the scour hole at the bottom of the sediment bed, when the critical pressure gradient associated with the initiation of scour is reached. The pressure drop between the stagnation pressure upstream and wake pressure downstream of the obstacle induces this hydraulic gradient. This study explores the temporal development of scour around obstacles resting on flat bed composed of uniform, cohesionless sediment.

This study will show the statistical modeling of various turbulent parameters like velocities and Reynolds stresses; and predict the natural phenomena, like trends of coherent structure over equilibrium scour holes induced by obstacles of different shapes on the river bottom. The turbulent flow field within the equilibrium scour mark was measured in a laboratory flume using 3-D Micro acoustic Doppler velocimeter (ADV). The scours marks may be named as current-crescent preserved in geological record are traditionally used as indicators of palaeo-current direction. Using quadrant threshold technique for coherent motion, it has been shown that over the smooth surface ejections and sweeps are the largest contribution near bed region and outward and inward interactions are the largest contribution near the water surface, but within the scouring region that is not true. Also note that the qualitative and quantitative behaviors of bursting events fit well with experimental data on the smooth surface. But within the scour-hole qualitative behavior of bursting events fits well with experimental data. It is interesting to note that the yz and xy- planes are much more important in the scouring regions, but reverse are true for the smooth surface.

Moreover, this is important for interpretation of turbulence and flow structures across the scour marks available at the ancient sedimentary structures in the light of modern analog- present is the key to the past.