Effects Of Control Rods On Turbulent Wake Characteristics Of Flow Past A Circular Cylinder

The flow past bluff bodies has been studied extensively due to its relevance in connection with nature and industrial applications in industry.The bluff body sustains the fluctuating fluid forces arising from vortex shedding when the fluid flows past,which may cause considerable bending or even structural failure.However,ocean current energy can be harvested from the vibration process.Circular cylinder has been one of the most representative protype of the bluff bodies.Flow past circular cylinder is widely investigated in the interest of the control and motivation in fluid dynamics.Control rods have been proposed to modify the wake when placing at an appropriate location in the near wake.We can see that most previous numerical studies concerning wake control behind a cylinder focused on the laminar flow regime at low Reynolds number(Re),and the influence of control rods on the turbulent wake of the circular cylinder at a subcritical Reynolds number has not been investigated so far.In this thesis,the flow around a main circular cylinder with two small control rods is numerically investigated at a subcritical Reynolds number of 3900.Two small control cylinders with diameter ratio of d/D = 0.04(d and D are the diameters of the control rods and the main cylinder,respectively)are symmetrically placed inside the separated shear layers emanating from the main cylinder.The computation is performed with highly resolved direct numerical simulations employing a high-order spectral/hp element method.For the validation purpose,the results of the flow over a single cylinder are presented at first,and the current simulation is in good agreement with the existing experiments and large eddy simulations.For the main cylinder with the two control rods,the dynamics of shear layers is found to be modified strongly owing to the disturbance from the small control rods.As a consequence,the recirculation length is decreased by 89% as compared to that of a single cylinder.The presence of the small control rods further leads to a significant increase in the fluctuating lift and the mean drag exerting on the main cylinder.Furthermore,the statistical analysis and turbulent wake visualization imply that the turbulent intensity is amplified in the near wake region and becomes weaker further downstream.This study then suggests that the small control rods located in the near wake region of the main cylinder may work differently in the turbulent wake against that in the laminar wake.