Cavity flows and wake behind an elliptic cylinder translating above the wall
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- Institutt for marin teknikk 
Vortex formation is one of the fundamental modes in fluid mechanics and it can develop in almost every realization of fluid motion. Studying its fundamental dynamics and interactions is of great interest for engineering applications; the vortex shedding behind slender cylindrical structures with various cross sections like pipelines, risers, bridges, buildings and wind turbine blades, can cause fluctuating drag and lift forces, which lead to the vortex-induced vibration (VIV), causing the material subjected to periodic bending stresses, which eventually can lead to fracture; the vortex shedding behind the offshore pipeline placed on/close to the seabed determines the gentle slope of the downstream scour hole. In order to understand the complex physics underpinning the vortex dynamics, some classical flow problems, i.e. oscillatory lid-driven cavity flows, cavity flows with an inserted cylinder and wake behind an elliptic cylinder translating above a wall, which are dominated by the vortex flow have been investigated numerically in the present thesis. Flow in a two-dimensional oscillatory lid-driven rectangular cavity with a depth-to-width ratio 1:2 is investigated, covering a wide range of Reynolds numbers (based on the velocity amplitude and the cavity depth) and Stokes numbers (based on the lid oscillation angular frequency and the cavity depth) where this flow is known to be in the two-dimensional regime. Effects of these two parameters on vortex dynamics, vertical and horizontal centerline velocities and the drag force on the lid are presented and discussed. Four different flow patterns are classified based on the vortex dynamics. Moreover, the corner singularity effect on the flow patterns is also presented and discussed. Effects of an inserted circular cylinder on a steady lid-driven cavity flow are investigated and discussed for different Reynolds numbers (based on the lid motion velocity and the cavity depth), depth-to-width ratios, cylinder radii and locations. An immersed boundary method is applied to treat the circular cylinder surface. Numerical results concerning the vortex structures and pressure distribution around the cylinder are presented and discussed. For the depth-to-width ratio of 1:2, seven flow patterns have been classified based on the vortex structures and their distributions are presented as a function of the Reynolds numbers and the cylinder radii for a given cylinder location. Wake behind an elliptic cylinder translating above a plane wall is investigated numerically for Reynolds numbers less than 150 and gap ratios from 0.1 to 5 (i.e., the ratio between the gap and semi-major axis length of the elliptic cylinder). Numerical results concerning the steady and unsteady wake structures Karman vortex street, the two-layered wake and the secondary vortex street), the hydrodynamic forces and the onset location of the two-layered wake are presented and discussed. Four flow patterns are classified based on the wake structure and their distributions are given in the space of the Reynolds number and the gap ratio. Numerical simulations of the oscillating boundary layer on a plane wall have been conducted for = 500 and 1120 (based on the boundary layer thickness and the amplitude of the oscillation velocity), and the present results for the wall shear stress and instantaneous vorticity contours are in a good agreement with previous numerical and experimental results.
Består avZhu, Jianxun; Holmedal, Lars Erik; Wang, Hong; Myrhaug, Dag. Vortex dynamics and flow patterns in a two-dimensional oscillatory lid-driven rectangular cavity. European journal of mechanics. B, Fluids 2020 ;Volum 79. s. 255-269 https://doi.org/10.1016/j.euromechflu.2019.09.013 This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Zhu, Jianxun; Holmedal, Lars Erik; Wang, Hong; Myrhaug, Dag. Flow patterns in a steady lid-driven rectangular cavity with an embedded circular cylinder. Fluid Dynamics Research 2020 CC BY-NC-ND 3.0
Zhu, Jianxun; Holmedal, Lars Erik; Myrhaug, Dag; Wang, Hong. Near-wall effect on flow around an elliptic cylinder translating above a plane wall. Physics of Fluids 2020 ;Volum 32.(9) s. 1-17 https://doi.org/10.1063/5.0020818
Zhu, Jianxun; Holmedal, Lars Erik. Numerical simulation of oscillatory boundary layer
Paper 1: Zhu, Jianxun; Holmedal, Lars Erik; Myrhaug, Dag; Wang, Hong. Fluid flow in steady and oscillatory lid-driven square cavities. IOP Conference Series: Materials Science and Engineering 2017 ;Volum 276.(1) s. 1-8 htps://doi.org/10.1088/1757-899X/276/1/012015 This is an open access article under the CC BY license
Paper 2: Zhu, Jianxun; Holmedal, Lars Erik; Myrhaug, Dag. Effects of an inserted circular cylinder on a steady lid-driven rectangular cavity flow. IOP Conference Series: Materials Science and Engineering 2019 ;Volum 700. s. 1-8 https://doi.org/10.1088/1757-899X/700/1/012008 This is an open access article under the CC BY license