Development of an incompressible Navier-Stokes solver for moving body problems using an overset meshing approach

Mackenzie, Jessica Melanie (2017) Development of an incompressible Navier-Stokes solver for moving body problems using an overset meshing approach. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

An overset meshing approach is an effective method of simulating fluid flow involving multiple moving bodies. It consists of minor meshes representing solid objects, which overlap a Cartesian background grid, allowing bodies to move arbitrarily whilst retaining communication between grids. However, a hole beneath each overlapping mesh must be cut from the background grid, leaving a small overlap. Current hole-cutting methods tend to be complex with some requiring extensive user knowledge and input. Since the hole must be re-cut regularly for moving body problems, it can become very time-consuming. An original approach for performing a hole-cut has been implemented by employing the Cartesian cut-cell method. This method would ordinarily be used to cut the boundary of a solid object from a single Cartesian grid, as an alternative to the overset grids approach. Thus, the treatment of cut-cells has been modified for its new purpose of hole-cutting. The cut-cell method is already a well-established technique for cutting a Cartesian grid, and is fully automated. It has not been used for hole-cutting previously within the literature and offers a very different hole-cut to existing techniques; It cuts though cells rather than around them, simplifying the cutting process and providing a smooth cut. This approach has been applied to an incompressible Navier-Stokes solver for viscous single fluid flow. Unstructured, triangular minor meshes are used due to their ability to represent complex geometries accurately. An explicit time integration method is used on these minor meshes, but an implicit integration method is implemented on the Cartesian background mesh. This new hybrid of integration methods was found to significantly reduce the CPU time in comparison to using a fully explicit method. The solver has been validated using benchmark tests, including a lid driven cavity and flow past a stationary/oscillating cylinder. The results obtained were found to be in good quantitative agreement with published numerical results. The solver was developed for 2-phase flow problems. However, during the initial validation test, convergence issues were encountered, which meant a sufficient solution could not be obtained.