| Under certain conditions, flow over a bluff body can resul to periodic vortex shedding, whose mechanism is complex, nonlinear, not entirely known to us. Furthermore, vortex shedding can exert periodic force on the bluff body, resulting in possible mechanical oscillation and destructive industrial consequences. Thus, to acquire an understanding of the mechanisms of flow over a bluff body have paramount academic value and practical significance to the enhancement of the stability of industrial apparatus.By taking a2-D circular cylinder between parallel walls and3-D circular cylinder within a rectangular channel, this research studies numerically the effects of the blockage ratio, in a2-D scenario, on cylinder wake flow transition and evolution, length and separation angle of the twin vortex, stress distribution around the cylinder and characteristic parameters that pertains to vortex shedding. Also, this paper studies numerically the effects of blockage ratio and the cylinder aspect ratio, in a3-D scenario, on cylinder wake flow evolution, characteristic parameters that pertains to vortex shedding, separation angle and surface stress of the cylinder. The achievements of this thesis are:(1) A model that can replicate the flow over a circular cylinder is established. In accordance to the wake character, a multi-grid O-type mesh is used. The discretization method of the momentum equation, pressure item are QUICK and Standard, respectively. Pressure-velocity coupling is accomplished via the SIMPLEC algorithm. Using these conditions,2-D and3-D flows over a circular cylinder were simulated. The computational results fit well with the results of previous achievements.(2) Flow over a circular cylinder for blockage ratio0.025-0.588is simulated. For various blockage ratios, the transition and evolution of the wake, length of twin vortex and etc. are obtained and analyzed. The changing pattern of wake character with varying blockage ratio is found. Between blockage ratio0.025-0.588, the critical Reynolds number of wake transition increases with blockage ratio. Length of twin vortex decreases with blockage ratio. Strouhal number St increases with blockage ratio and reaches a maximum when blockage ratio is0.5, then it decreases. Averaged drag coefficient increases with blockage ratio. Root mean square of lift coefficient initially increases with blockage ratio, reaching maximum when blockage ratio is0.2, it then decrease between0.2and0.5of blockage ratio. After blockage ratio0.5, it increases again.(3) Flow over a circular cylinder within a rectangular channel is simulated with aspect ratio10-35and blockage ratio0.025-0.1. For varying aspect ratio and blockage ratio, the wake patterns, cylinder surface stress, and flow characteristic parameter are obtained and analyzed. The results show:between aspect ratio10-35, St first decreases. When the aspect ratio reaches20, St is at its minimum, and start to increase afterward. Averaged drag coefficient decreases with aspect ratio. When aspect ratio reaches15, the averaged drag coefficient is almost constant. Root mean squared lift coefficient decreases with aspect ratio until20, when it reaches a minimum, then it increases. Between blockage ratio0.025-0.1, the averaged pressure coefficient of both the front and rear stagnation points increase with blockage ratio. St first increase with blockage ratio, until blockage ratio0.067. Then it decreases. Drag coefficient increases with blockage ratio, so does the lift coefficient. |
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