Direct numerical simulation of settling of a large solid particle during bioconvection

Settling of a large solid particle in bioconvection flow caused by gyrotactic microorganisms is investigated. The particle is released from the top of the bioconvection chamber; its settling pattern depends on whether it is released in the centre of the bioconvection plume or at its periphery. The Chimera method is utilized; a subgrid is generated around a moving particle. The method suggested by Liu and Wang (Comput. Fluid 2004; 33 :223–255) is further developed to account for the presence of a moving boundary in the streamfunction‐vorticity formulation using the finite‐difference method. A number of cases for different release positions of the particle are computed. It is demonstrated that bioconvection can either accelerate or decelerate settling of the particle depending on the initial position of the particle relative to the plume centre. It is also shown that the particle impacts bioconvection plume by changing its shape and location in the chamber. Copyright © 2005 John Wiley & Sons, Ltd.     

Application of Chimera grid to modelling cell motion and aggregation in a narrow tube

A computational scheme using the Chimera grid method is presented for simulation of three‐dimensional motion and aggregation of two red blood cells (RBCs) in a narrow tube. The cells are modelled as rigid ellipsoidal particles; the computational scheme is applicable to deformable fluid‐filled particles. Attractive energy between two RBCs is modelled by a depletion interaction theory and used for simulating aggregation of two cells. Through the simulation, we show that the Chimera grid method is applicable to the simulation of three‐dimensional motion and aggregation of multiple RBCs in a microvessel and microvascular network. Copyright © 2006 John Wiley & Sons, Ltd.     

A Conservative Interface Algorithm for Moving Chimera )Overlapped) Grids

A fully conservative zonal interface algorithm for overlapped (Chimera) grid has been extended to handle moving body flows. To ensure flow conservation, the motion of the computational grid needs to be taken into account. Several new issues arising from moving and deforming grids, e.g., vanishing and new-born cell problems, are addressed with a cell-merging-unmerging technique. The grid velocities are determined in a way which satisfies free-stream preserving (the Geometric Conservation Laws)The flow solver is based on a fully implicit, cell-centered finite volume discretization with MUSCL reconstruction and Roe's approximate Riemann solver. The interface algorithm is first validated by tackling the case of supersonic flow over a cylinder. Then several moving body flow problems are simulated using the current approach to demonstrate its capabilities.     

Application of hybrid Cartesian grid and gridless approach to moving boundary flow problems

The paper presents a hybrid Cartesian grid and gridless approach to solve unsteady moving boundary flow problems. Unlike the Chimera clouds of points approach, the hybrid approach uses a Cartesian grid to cover most of the computational domain and a gridless method to calculate a relatively small region adjacent to the body surface, making use of the flexibility of the gridless method in handling surface grid with complicated geometry and the computational efficiency of the Cartesian grid. Four cases were conducted to examine the applicability, accuracy and robustness of the hybrid approach. Steady flows over a single NACA0012 airfoil and dual NACA0012 airfoils at different Mach numbers and angles of attack were simulated. Moreover, by implementing a dynamic hole cutting, node identification and information communication between the Cartesian grid and the gridless regions, unsteady flows over a pitching NACA0012 airfoil (small displacement) and two‐dimensional airfoil/store separation (large displacement) were performed. The computational results were found to agree well with earlier experimental data as well as computational results. Shock waves were accurately captured. The computational results show that the hybrid approach is of potential to solve the moving boundary flow problems. Copyright © 2013 John Wiley & Sons, Ltd.     

A fully automated Chimera methodology for multiple moving body problems

A fully automated Chimera methodology has been developed in this study to provide geometric or stencil information required to facilitate inter‐grid data communications. Chimera holes are cut automatically in each grid of an overset grid system based on whether the grid overlaps with non‐penetrable surfaces (NPS) and/or blocked regions. The efficiency of the hole‐cutting algorithm is boosted with search algorithms based on the state‐of‐the‐art alternating digital tree (ADT) data structures. The automated nature of the hole‐cutting algorithm is ideally suited for handling multiple moving body problems. Several cases, both steady and unsteady, are used to demonstrate the effectiveness of the methodology. Copyright © 2000 John Wiley & Sons, Ltd.     

Fluid–structure interaction approaches on fixed grids based on two different domain decomposition ideas

This paper presents two domain decomposition techniques for fixed grid fluid–structure interaction simulations that can be applied to the interaction of general structures with incompressible flows. One approach is based on an overlapping domain decomposition idea while the other uses non-overlapping domains. The first technique combines a fixed grid Chimera approach with arbitrary Lagrangean Eulerian based methods, the second one is based on an eXtended Finite Element Method (XFEM) strategy. Both techniques are used in a partitioned and strong coupling fluid–structure framework. The usage of such fixed-grid methods considerably increases the range of possible applications. Several test examples demonstrate key features of both methods.     

Computation of viscous incompressible flow using pressure correction method on unstructured Chimera grid

In this paper, a pressure correction algorithm for computing incompressible flows is modified and implemented on unstructured Chimera grid. Schwarz method is used to couple the solutions of different sub-domains. A new interpolation to ensure consistency between primary variables and auxiliary variables is proposed. Other important issues such as global mass conservation and order of accuracy in the interpolations are also discussed. Two numerical simulations are successfully performed. They include one steady case, the lid-driven cavity and one unsteady case, the flow around a circular cylinder. The results demonstrate a very good performance of the proposed scheme on unstructured Chimera grids. It prevents the decoupling of pressure field in the overlapping region and requires only little modification to the existing unstructured Navier–Stokes (NS) solver. The numerical experiments show the reliability and potential of this method in applying to practical problems.     

A Chimera method for the incompressible Navier–Stokes equations

The Chimera method was developed three decades ago as a meshing simplification tool. Different components are meshed independently and then glued together using a domain decomposition technique to couple the equations solved on each component. This coupling is achieved via transmission conditions (in the finite element context) or by imposing the continuity of fluxes (in the finite volume context). Historically, the method has then been used extensively to treat moving objects, as the independent meshes are free to move with respect to the others. At each time step, the main task consists in recomputing the interpolation of the transmission conditions or fluxes. This paper presents a Chimera method applied to the Navier–Stokes equations. After an introduction on the Chimera method, we describe in two different sections the two independent steps of the method: the hole cutting to create the interfaces of the subdomains and the coupling of the subdomains. Then, we present the Navier–Stokes solver considered in this work. Implementation aspects are then detailed in order to apply efficiently the method to this specific parallel Navier–Stokes solver. We conclude with some examples to demonstrate the reliability and application of the proposed method. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamics of large solid particles in bioconvective sedimentation

Settling of one or two large solid particles in a bioconvection flow induced by gyrotactic motile microorganisms is investigated using a 2D numerical model. The results of varying the initial positions of large particles on the bioconvection flow pattern are investigated. The Chimera method is utilized to generate subgrids around the moving particles. It is demonstrated that the introduction of a single large particle displaces bioconvection plume and changes its shape. The introduction of two particles on the same side of the bioconvection plume further displaces the plume while the introduction of two particles on opposite sides reduces this displacement. The influence of the bioconvection plume on the particles' settling paths and particles' settling velocities is investigated. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical scheme for riser motion calculation during 3-D VIV simulation

This paper presents a numerical scheme for riser motion calculation and its application to riser VIV simulations. The discretisation of the governing differential equation is studied first. The top tensioned risers are simplified as tensioned beams. A centered space and forward time finite difference scheme is derived from the governing equations of motion. Then an implicit method is adopted for better numerical stability. The method meets von Neumann criteria and is shown to be unconditionally stable. The discretized linear algebraic equations are solved using a LU decomposition method. This approach is then applied to a series of benchmark cases with known solutions. The comparisons show good agreement. Finally the method is applied to practical riser VIV simulations. The studied cases cover a wide range of riser VIV problems, i.e. different riser outer diameter, length, tensioning conditions, and current profiles. Reasonable agreement is obtained between the numerical simulations and experimental data on riser motions and cross-flow VIV a/D. These validations and comparisons confirm that the present numerical scheme for riser motion calculation is valid and effective for long riser VIV simulation.