احسان مقدسی

Obstacles such as piers of a bridge inflow confuse and disturb its natural form. With passing the flow through these obstacle disturbances create downstream, called the Von Karman Street vortex shedding. When the frequency of the microwaves is equal to the natural frequency of the current approach, the resonance will form. If the flow walls are constant such as a flume, this phenomenon may see as a transversal wave. Actual examples of this phenomenon created near the bridge piers in a canal called the Delta Mendota, as well as in the New York canal around the I-84 Bridge (Schuster, 1967).Numerous people in this field have done research that mostly with variables such as channel width, shape, and placement of obstacles in the channel to study different wave characteristics, including its amplitude and frequency. Zima, L., and Ackermann (2002), Ghomeshi et al. (2007), Jafari et al. (2010), Meile et al. (2011), and Sarkar (2012) tried to find the maximum amplitude of the created wave by using obstacles with different diameters. Zhao et a. (2014) performed their experiments to measure the coefficient and tensile force created in the vicinity of obstacles while transmitting transverse waves. This study aimed to find a relationship between the percentage of obstacle immersion and wave characteristics.

A fundamental solution to save fossil fuels and to avoid air pollution is using small water turbines. In this study, an example of a hydrodynamic turbine that can generate electricity in low-flow and low-velocity flows is proposed. After extensive research on the problem's theory, the 3D shape of the turbine blades was modelled in SOLIDWORKS software and introduced as an input in the FLOW-3D model. For each turbine with different blade angles and for different depths of flow, the interaction between the fluid and blades were evaluated and analyzed to determine the turbine that has the highest torque and angular velocity. The basic premise of this study is that the turbine, which has the maximum angular velocity and torque speed, without oscillation and mutation and without the changes of torque direction, certainly has more power and has the optimal blade angle and flow conditions. All turbines had the highest torque and angular velocity at a discharge of 20  and at the depth of 5 cm and therefore had more power. Accordingly, it was concluded that the most suitable location for greater efficiency and greater power of the turbine is its location near the flow surface. Also a turbine with a blade angle of 70 degrees had the best efficiency among the turbine blades with respect to the created torque and angular velocity.

Vortex is shed by flow collision with obstacles in its path. If the frequency of vortex shedding equals the frequency of natural oscillations of flow, resonance will be created and transverse oscillation perpendicular to flow with greatest wave amplitude will occur. In this study, in order to investigate the characteristics of the transverse wave caused by the vortex shedding of the obstacles, 135 cylinder barriers with a diameter of 20 mm in 5 different configurations were arranged in the laboratory flume. In total, 900 tests were carried out which variables were flow discharge, average flow depth, channel slope, longitudinal and transverse distance between obstacles. In each test, after the formation of transverse oscillations, their characteristics including amplitude and frequency of wave were recorded. Then, the effective variables on transverse wave characteristics and their effects on the involved dimensionless numbers were investigated. The results indicated whatever the flow discharge is increased, the maximum wave amplitude due to resonance occurs in larger average flow depth, which has more amount. Also by changing the longitudinal distance of obstacles, Roshko's changes relative to increasing of Ursell were ascending at the beginning and they were reversed after reaching a certain range of Ursell number; moreover, by increasing the flow discharge, the rate of Ursell changes relative to Roshko decreased. Finally, by using dimensional analysis and statistical software, the equations between Roshko with Ursell and Froude numbers were proposed for each of the modes I and II and the validation of equations were approved (R2= 0.92).

By passing and strike flow with vegetation, pressure area in upstream of plant and low pressure area in its downstream is formed. The pressure difference created in this way triggers separation of the boundary layer and consequently formation of vortices downstream of vegetation. When the frequency of vortex rooted from vegetation becomes equal to the natural frequency of the channel, transverse stationary surface waves are generated. In natural channels in areas where the flow rate is on the wane, the probability of growing plants is on the rise, incontrovertibly there is a free flow beside a flow passing through the plants.
In this study, the characteristics of the surface transverse oscillations caused by vegetation, introduced as rigid barriers, were investigated under different configurations of barriers and over a range of width percentage covered by obstacles (WPO). Totally, 378 experimental tests were conducted in a rectangular channel of 9 m length and 50 cm width. The variables studied included flow discharge, flow velocity and WPO. Moreover, flow discharges were ranged from 5 to 15 liters per second. It is shown that, as long as WPO increases, transverse oscillation are formed at larger depths, which also have a larger oscillation amplitude. For a certain flow depth, increasing WPO results in growth of oscillation amplitudes. Based on dimensional analysis, regression relationships were extracted to predict the relative amplitude of transverse oscillations as a function of WPO, among other affecting parameters. Overall, the empirical equations proposed in this study were found to reproduce experimental results with acceptable accuracy.