Influence of the geometry on the two- and three-dimensional dynamics of the flow in a 180 degree sharp bend

  • Lintao Zhang

    Student thesis: Doctoral ThesisDoctor of Philosophy

    Abstract

    In this thesis, I numerically simulate the flow past a 180◦ sharp bend. The main objective of this research is to characterize the influences of the bend geometry on the flow regimes when Reynolds number (Re) is varied. Reynolds number and the opening ratio (β) (the definition of β is defined in section 4.1.1) of the bend are the control parameters which are varied to perform the parametric study. For the first part of our research, I focus on the flow past a twodimensional 180◦ sharp bend. An extensive parametric study on the transitions of the flow regimes by varying the values of Re and β is performed. The values of Re and β are in the ranges of (0...2500] and [0.1...10], respectively. As Reynolds number is increased, I find a laminar flow, then a flow with a first recirculation attached the inside boundary, then a flow with a second recirculation attached to the top boundary. The onset of the unsteadiness occurs through instability of the main stream and the vortex shedding starts from the inside boundary. For β < 0.3, the flow is characterised by the dynamics of the jet flow near the very narrow turning part. For β ≥ 0.3, the flow exhibits strong similarities with the flow behind an obstacle placed in a channel. For the second part of our work, I focus on the flow past a threedimensional 180◦ sharp bend. Simulations with periodic conditions are performed for selected Re and β in order to determine the validity of the 2D assumptions. The results show that two-dimensional dynamics can predict the features of three-dimensional flow in steady flow regimes, even for unsteady flow regimes to some extent. Simulations on the more realistic situations of a three-dimensional bend with walls are carried out as well. The results show that flow is symmetric to the center of the bend at low Re along spanwise direction. As Re increases, the influences of Dean flow on the mainstream flow are obvious. Near-wall quasi-symmetric shedding is presented in unsteady flow regimes as well. The unsteadiness is first originated from the shear layer between the first recirculation and the Dean flow region.
    Date of AwardApr 2013
    Original languageEnglish
    Awarding Institution
    • Coventry University
    SupervisorAlban Potherat (Supervisor)

    Keywords

    • Fluid mechanics
    • Bend geometry
    • Flow

    Cite this

    '