Date

2022-8-23

Author

KARIMIZINDASHTI, Golnesa

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A Discrete-Vortex Method (DVM)-based in-house code is developed and implemented to simulate two-dimensional low to medium-Reynolds number flows around circular and square cylinders undergoing either prescribed or free motions. To simulate the vorticity gradient, three different diffusion schemes, Random Walk, Particle Strength Exchange, and Vorticity Redistribution methods, are utilized. The performance of these diffusion schemes is examined by conducting simulations of flow passing over a stationary square cylinder for 𝑅𝑒 = 200, 500, 1000. The results obtained from these cases are compared against each other and the comprehensive data collected from previous numerical and experimental studies. These results are presented in terms of RMS lift and mean drag coefficients, Strouhal number, and base pressure coefficient. Also presented are the wake characteristics, as well as the mean and fluctuations of pressure distribution around the body surface. Based on its reasonable agreement with the literature and affordable computational cost, PSE is selected as the diffusion scheme for the subsequent studies.. The second part of the study examines the performance of the present algorithm on the moving boundary case; circular and square cylinders rotating with constant velocity. Subjecting a bluff body to rotary about its axis effectively reduces drag forces and suppresses the fluctuating lift forces. Both cylinders are exposed to a uniform flow of 𝑅𝑒 = 200 and an imposed rotation rate range of 0 ≤ 𝛼 ≤ 5.5 for the circular, and 0 ≤ 𝛼 ≤ 5 for the square cylinder. The present numerical tool is able to predict the vortex shedding suppression as a result of forced rotation. For both cases, a systematic increase in the time-averaged lift coefficient and a general descending trend in the time-averaged drag coefficient are detected. The second mode of vortex shedding for the circular cylinder is captured within a narrow range of rotation rates. A noticeable feature of the flow over rotating square cylinders is the emergence of local wakes in the near-body region, which develop independently from the wake behind the body. In the last part of the study, the fluid-structure interaction phenomenon is explored by studying the flow-induced vibration (FIV) of a rotating circular cylinder exposed to a uniform flow of 𝑅𝑒 = 200 and free to move in the direction parallel to the flow. The current study is the first to report the results of such a case numerically based on the Discrete Vortex Method. Two types of vibration are observed in the response of the cylinder. The first type (VIV-type vibration), characterized by low-amplitude, high-frequency vibrations, is detected in lower rotation rates. The second type (galloping-like vibration) is linked with high-amplitude and low-frequency oscillations, characteristic of the higher rotation rate ranges. Each vibration region is characterized by specific wake patterns.

Subject Keywords

Discrete Vortex Method, Two-Dimensional Flow, Circular & Square Cylinders, Flow-Induced Vibrations, Fluid-Structure Interaction

URI

https://hdl.handle.net/11511/99390

Collections

Graduate School of Natural and Applied Sciences, Thesis