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Title: Numerical Investigation of Two-Dimensional Low Reynolds Numbers Fluid Flow in Confined and Unconfined Channels using Lattice Boltzmann Method
Authors: , Naqibullah
Keywords: Physical Sciences
Issue Date: 2021
Publisher: COMSATS University, Islamabad.
Abstract: Numerical Investigation of Two-Dimensional Low Reynolds Numbers Fluid Flow in Confined and Unconfined Channels using Lattice Boltzmann Method The main motivation of this research thesis is to document and interlink the flow patterns and variation of physical parameters for different separation ratios and Reynolds numbers for some important engineering applications related problems. The systematic analyses at low Reynolds numbers for multiple bluff structures was limited and in some cases are far from complete. There are some applications of flow at low Reynolds number, for example in micro-devices, such as cooling of fibers and micro electro-mechanical-systems devices. Lattice Boltzmann method is used for systematic numerical investigations. The developed code is tested and the obtained results are in good agreement with the available results for flow past isolated cylinder. In case of flow past four side-by-side cylinders seven different flow regimes are identified for different range of separation ratios and Reynolds numbers. In case of channelized flow problem the hydrodynamic forces of the cylinder are strongly depends on the expansion ratio of the channel. The primary vortex shedding frequency role becomes dominant at higher spacings and the secondary cylinder interaction frequencies are dominant at low spacings. The gap flows effect the developing process of shed vortices at low spacings just near downstream of the cylinders. In case of blockage effect the irregular variation of wake structure observed throughout the computational domain. In case of nine cylinders the flip-flopping nature was observed at low separation ratios. Overall four different flow regimes were observed in case of nine cylinders array. As a result the secondary interaction frequencies dominate the flow. The shedding frequencies roles are dominant at the critical spacing between the cylinders. The drag and lift forces are completely irregular at low separation ratio and almost periodic at large separation ratios. In case of nine cylinders the physical parameters showing abrupt changes once the flow regime transformation occurs. The shear layers reattachment and wake and gap flow interactions considerably influenced the forces on the cylinders. The lessajesious representation of forces clearly states about the inphase and antiphase nature of the flow behind the individual cylinder. The obstruction at the channel entrance completely converts the laminar behavior of flow to unstable flow without any regular movement of shed vortices behind the cylinders. A classification map, demarcated by the separation ratios and Reynolds numbers, is derived for observed flow xi regimes in case of side-by-side cylinders and nine cylinders array. Physical features of each flow regime such as wake structure evolution, streamlines visualization, vortex dynamics, gap flow, time analysis of forces, shedding frequencies and hydrodynamic forces are thoroughly discussed for each problem. Keywords: Flow regimes; Fluid-structure interaction; Hydrodynamic forces; Lattice Boltzmann method; Reynolds number; Separation ratio; Strouhal number
Gov't Doc #: 23662
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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