مهندسی آب

Experimental study on ten modes of transverse waves due to vertical cylinders in open channels

 Abstract

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. In this paper attention is focused on the transverse waves generated in an open channel, where the flow passes through a cluster of rigid vertical cylinders. The wooden circular cylinders with different diameters were screwed into the bed of the flume at different setting up patterns and spacing. 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. A comprehensive set of experiments (760 in total) were conducted in flume. In some of the experiments (119 Exp.) the flow was submerged on the obstructions and for some of these experiments the waves still were formed. 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. The discharge of flow was varied from 2.2 l/s to 25 l/s. 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 a new formulation were obtained from dimensional analysis, to estimate the amplitude of the transverse wave generated by vortex shedding in open channels.

 Key Words: ransverse wave, vertical cylinders, wave amplitude, open channels.  

  Introduction

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. The results for this kind of vortex shedding can be found in published studies. In contrast, research on the vortex shedding and related phenomena for water as the transfer medium is very limited. In 1939, Crasse reported his findings on wave oscillations in open channels and this is thought to be the first time that such phenomena have been reported and analysed. In the study with Stone and Stone and Shen, they found that the average depth of water in a flume was difficult to determine during several experimental runs because of severe oscillations of the water surface. Stone was investigating the energy loss created by flow through an array of wooden dowels that simulated vegetation. Wavelike surface disturbances have also been observed in hydraulic model studies of channel flows through a dense array of vertical stems or cylinders. Tsujimoto’s studies were conducted in a laboratory flume, 12 m long and 0.4 m wide. Thirty percent of the 0.4 m width of the flume was covered with vertical, 1.2 mm diameter bamboo stems. Zima and Ackermann used two flumes, of widths 150 and 450 mm and used two rods of diameter 12.7 and 25.4 mm, respectively. They observed only two modes of vibration. They proposed a formula for simulating the maximum values of the dimensionless displacement (A/H) that may occur in a canal.