عنوان مقاله [English]
نویسندگان [English]چکیده [English]
There are many conditions in open channel flows where the steady flow of water passes through a cluster of vertical cylinders. These vertical cylinders may represent piers of bridge, jetty or vegetation on the floodplain of rivers. Research on vortex shedding has been mainly focused on the acoustic resonant noise generated from heat exchangers when a gas flows through bank of tubes. In contrast, research on the vortex shedding and related phenomena for water as the transfer medium is very limited. Consider a free stream of velocity U that flows past a cylinder of diameter D, Periodic vortex shedding occurs downstream from the cylinder when the Reynolds Number, defined in terms of the cylinder diameter is greater than approximately 40. When each vortex is shed, the cylinder and the fluid experience equal and opposite forces. These forces have components perpendicular to the direction of the mean flow and are the exciting forces capable to create transverse surface waves.
In this study attention is focused on the transverse waves generated in an open channel, where the flow passes through a cluster of rigid vertical cylinders. The experiments were conducted in a laboratory flume with 6m in length and 0.72m in width and 0.6m in height. The rigid rods were screwed into the bed of the flume at different distances and spacing (i.e. different T and P) and for pattern distributions (i.e. staggered and in-line). In this study, the height of the outlet control weir was changed from the highest to the lowest levels, with ten modes of oscillation occurring across the laboratory flume for each sequence and the discharge of flow was varied. A comprehensive set of experiments (3304 in total) were conducted in flume. In some of the experiments (398 Exp.) the flow was submerged on the obstructions and for some of these experiments the waves still were formed. The waves were produced from the periodic forces created by the vortex shedding from the cylinders. In specific conditions these forces may create transverse waves with maximum amplitude in the channel. The maximum relative amplitudes in each mode of the waves (in each type of the experiments) are selected and used for the further analysis. The results provided an explanation for different wave formations that might occur due to vortex shedding for free and submerged flow on the obstructions. Finally new formulations were obtained from dimensional analysis, to estimate the amplitude of the transverse wave generated by vortex shedding in open channels.