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The Influence of Porosity on the Structure and Behavior of Gravity Currents Propagating into an Aquatic Canopy

Author(s): Ayse Yuksel Ozan; George Constantinescu

Linked Author(s): George Constantinescu

Keywords: Gravity currents; Vegetated flows; Large Eddy Simulation; Coherent structures

Abstract: In the present study we consider gravity currents forming in relatively shallow environments that propagate in a region containing an array of fairly uniformly distributed plant stems. Experiment and shallow flow theory have showed that if the additional drag force induced by the obstacles is sufficiently high the current can transition to a drag-dominated regime where the front velocity decays with time even before the reflected bore catches the front of the current. In the present work we focus on the case of a fully vegetated channel in which a temperature driven current develops between two vegetated water regions containing emerged and respectively submerged vegetation penetrating close to the free surface. The temperature of the region containing emerged vegetation is smaller due to shading, which reduces the solar radiation compared to the region containing vegetation only beneath the free surface. We perform high resolution Large Eddy Simulation to investigate the effects of the volume fraction of solids (the range considered is 1. 25% to 12% ) on the structure of high-Reynolds number gravity currents, their front velocity and the total drag force acting on the gravity current. Simulations are performed at a channel Reynolds number defined with the buoyancy velocity and channel depth of 15, 000. The ratio between the diameter of the cylinders and the channel depth is kept constant (0. 032) in the simulations. It is found that for sufficiently large volume fraction of solids (larger than 3% ) the decay of the front velocity with the time of release, t, is proportional to t-0. 25. The coefficient is slightly different than the value predicted by analytical models based on shallow flow theory. The paper provides a detailed discussion of the structure of the gravity current during the drag dominated phase and discusses how the eddy content of the dissipative and tail regions changes with the volume fraction of solids and how mixing changes with the increase in the volume fraction of solids within the vegetated channel.


Year: 2014

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