Numerical modeling and analysis of flow around stationary and oscillating circular cylinder / Niaz Bahadur Khan

Niaz, Bahadur Khan (2018) Numerical modeling and analysis of flow around stationary and oscillating circular cylinder / Niaz Bahadur Khan. PhD thesis, University of Malaya.

	PDF (The Candidate's Agreement) Restricted to Repository staff only Download (1293Kb)
Preview	PDF (Thesis PhD) Download (5Mb) | Preview

Abstract

The fluid–structure interaction problems associated between cylinder and fluid has gained considerable attention because of its importance in a wide range of applications including marine equipment, nuclear reactors, skyscrapers, bridges, wind turbines and chimneys. When a flow passes through a circular cylinder, vortex shedding would occur behind the cylinder alternately at the top and bottom sides resulting in an unwanted structural vibration, especially when the frequency of vortex shedding is equal to or near to the natural frequency of the structure. This phenomenon is known as a vortex-induced vibration (VIV). Despite the wide range of research on flow around cylinder at Reynolds number 3900, the study of mesh density in spanwise direction, use of cost effective turbulent model and impact of mesh pattern on flow around cylinder problems is limited. Flow around fixed cylinder provide basis for the VIV problem which is associated with oscillating cylinder case. The main objective of this study is to numerically investigate the unsteady nature of the flow around cylinder using computational fluid dynamics. This study is divided into two parts: fixed cylinder case and oscillating cylinder case. The first section analysed the effects of the spanwise domain, the spanwise mesh resolution and mesh resolution near-field grid on the recirculation length, the angle of separation and hydrodynamic coefficients. The effects of non-dimensional timestep and time statistic average on the accuracy of the statistical quantities are also investigated. In the first section, extensive numerical simulations have been performed using large eddy simulation (LES) code and Smagorinksy–Lilly SGS models to investigate the unsteady nature of the flow around a fixed cylinder at a Reynolds number (Re)=3900. Meshing and analysis are performed using ICEM-CFD and an ANSYS-fluent tool, respectively. The second section mainly focused on the VIV phenomenon for elastically mounted rigid cylinders. The objective of this section is to test the capability and accuracy of 2D and 3D RANS models to compute the maximum amplitude, the mode of vortex, and other hydrodynamic coefficients, and compare the performance of these models to that of computationally expensive models. In addition, the performance and capability of SST-kω is compared with realizable-kε (RKE) equations. In this study, a user-defined function code written in C language is used to facilitate the oscillation of the cylinder and record the fluid forces with the amplitude of the cylinder through dynamic mesh update method. Several important results are obtained in this study. For the fixed cylinder, the mesh density in the spanwise domain and near-field grids significantly affect the calculation of the recirculation length, the angle of separation, the hydrodynamic coefficients and the statistic in the wake region behind the cylinder. In addition, the recirculation length is observed to be a key parameter for assessing the accuracy of the numerical method. For the oscillating cylinder, the 2D RANS SST k-ω turbulent model, which is relatively less expensive, can predict the hydrodynamic forces, the maximum amplitude and all modes of the vortex at Re=104. In the RKE model, a delayed transition is observed between the upper and lower branches, resulting in the broad range of the ‘lock-in’ region. With very small mass-damping ratio, the 2D RANS SST k-ω turbulent model successfully predicted the maximum amplitude during the VIV analysis. The findings of this study significantly reduced the computational cost for the flow around fixed and oscillating cylinders.

Actions (For repository staff only : Login required)