The overset grid method, applied to the solution of the incompressible Navier-Stokes equations in two and three spatial dimensions

• Main Author: Skillen, Alex
• Other Authors: Iacovides, Hector
• Published: University of Manchester 2012
• Subjects: 512.12
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.553380

The generation of structured grids around complex geometries is generally a difficult task. Thistask is typically a major bottleneck in the overall solution procedure; however, the overset gridmethod can be used to relieve much of this burden. An overset grid consists of a set of simplecomponent grids, which can overlap arbitrarily (provided there is sufficient overlap to interpolatefrom). The union of all simple grids should then delineate the global domain. This allows complex domains to meshed using a series of simple meshes. Interpolation boundary conditions are enforced at internal boundaries to ensure a continuous solution. Standard tri-linear interpolation is typically used for this purpose, although there are alternative methods that attempt to enforce global conservation. A new CFD code has been developed that incorporates the overset grid method in three spatialdimensions. This code uses the steady state, finite volume discretisation method. SIMPLE pressure velocity coupling has been used on a colocated grid with Rhie-Chow interpolation for face velocities. Different interpolation methods have been compared for the information transfer at internal boundaries from one grid to the next. It has been shown that for a variety of test cases, continuous and accurate solutions are obtained from one grid to another, which are comparable to those of the single-block or block-structured solutions, or to experimental data (where available). A new hole cutting algorithm and bulk correction outlet condition are presented. Improvements to existing digital tree data structures are also described. Lid driven cavity flow, the flow around rotating cylinders, and flow impingement onto a concavesurface are considered in order to demonstrate the method. The flow over a backward facing step, over a multi-element airfoil, through a bifurcating artery and over a wing-body junction are then considered (with experimental comparison). This demonstrates the range of applicability of the method. In all cases, the overset method offers significant advantages over block-structured solutions that are available in the literature. It is shown that greater numerical efficiency is generally achievable via the use of an overset simulation: Since the gridding is flexible, high aspect ratio cells need not propagate into the domain (as is often the case for a block-structured arrangement). Also, much of the domain away from localised regions of geometrical complexity can be meshed with efficientCartesian grids.