A parallel advancing front grid generation scheme

A parallel advancing front scheme has been developed. The domain to be gridded is first subdivided spatially using a relatively coarse octree. Boxes are then identified and gridded in parallel. A scheme that resembles closely the advancing front technique on scalar machines is recovered by only considering the boxes of the active front that generate small elements. The procedure has been implemented on the SGI origin class of machines using the shared memory paradigm. Timings for a variety of cases show speedups similar to those obtained for flow codes. The procedure has been used to generate grids with tens of millions of elements.&nbsp

Multistage explicit advective prediction for projection-type incompressible flow solvers

A multistep advective predictor has been developed within the context of projection schemes for incompressible flows. The key idea is to integrate with schemes of different order the different regions of the domain. In regions where advection dominates, multistepping yields a considerable benefit. In those regions where viscosity dominates, the scheme reverts naturally to the original one-step scheme. Several examples show savings of the order of 1:3–1:10 as compared with standard projection schemes, even for transient problems. Given that these benefits can be achieved with a very modest change in existing codes, the proposed multistage advective predictor should be widely applicable

Cache‐efficient renumbering for vectorization

A renumbering strategy for field solvers based on unstructured grids that avoids memory contention and minimizes cache‐misses is described. Compared with usual colouring techniques, the new renumbering strategy reduces the spread in point‐data access for edge‐based solvers by more than an order of magnitude. The technique is particularly suited for multicore, cache‐based machines that allow for vectorization or pipelining

Regridding Surface Triangulations

    An advancing front surface gridding technique that operates on discretely defined surfaces (i.e. triangulations) is presented. Different aspects that are required to make the procedure reliable for complex geometries are discussed. Notable among these are (a) the recovery of surface features and discrete surface patches from the discrete data, (b) filtering based on point and side-normals to remove undesirable data close to cusps and corners, (c) the proper choice of host faces for ridges, and (d) fast interpolation procedures suitable for complex geometries. Post-generation surface recovery or repositioning techniques are discussed. Several examples ranging from academic to industrial demonstrate the utility of the proposed procedure forab initiosurface meshing from discrete data, such as those encountered when the surface description is already given as discrete, the improvement of existing surface triangulations, as well as remeshing applications during runs exhibiting significant change of domain. &nbsp

Mesh adaptation in fluid mechanics

The development, application and impact of mesh adaptation procedures in the field of computational fluid mechanics (CFD) are reviewed. The discussion is restricted to unstructured (i.e. unordered) grids, such as those commonly encountered in finite element applications

On the modeling of pedestrian motion

A model for the simulation of pedestrian flows and crowd dynamics has been developed. The model is based on a series of forces, such as: will forces (the desire to reach a place at a certain time), pedestrian collision avoidance forces, obstacle/wall avoidance forces; pedestrian contact forces, and obstacle/wall contact forces. Except for the will force, it is assumed that for any given pedestrian these forces are the result of only local (nearest neighbour) situations. The near-neighbour search problem is solved by an efficient incremental Delaunay triangulation that is updated at every timestep. In order to allow for general geometries a so-called background triangulation is used to carry all geographic information. At any given time the location of any given pedestrian is updated on this mesh. The results obtained to date show that the model performs well for standard benchmarks, and allows for typical crowd dynamics, such as lane forming, overtaking, avoidance of obstacles and panic behaviour

Automatic unstructured grid generators

A review of automatic unstructured grid generators is given. These types of grids have found widespread use in computational fluid dynamics, computational structural dynamics, computational electro-magnetics and computational thermodynamics. The following topics are treated: the methods most commonly used, the specification of desired element size/shape and surface definition/meshing. Finally, the use of automatic grid generators as an enabling technology for moving body simulations and adaptive remeshing techniques is discussed

Extensions and improvements of the advancing front grid generation technique

We describe extensions and improvements to the advancing front grid generation technique that have proven useful over the years. The following areas are treated in detail: situations with thin or crossing surfaces, meshing of surfaces defined by triangulations, and ease of user input to define the desired element size in space. The first extension is important if one considers the generation of volumetric grids around shells, membranes, fabrics, or CAD-data that exhibit cusps. Traditional advancing front generators are likely to fail in these situations. We propose the introduction of a crossing environment variable attached to faces and points in order to filter out undesired or incorrect information during the grid generation process. The second extension is required for situations where the surfaces to be gridded are not defined analytically, but via a triangulation. Typical cases where such triangulations are used to define the domain are geophysical problems, climate modelling and medical problems. The third topic deals with the reduction of manual labour to specify element size in space. Sources, element size attached directly to CAD-data, and adaptive background grids are discussed. Adaptive background grids, in combination with surface deviation tolerances, are used to obtain surface triangulations that represent the geometry faithfully, and at the same time enable a smooth transition to volumetric meshes

Renumbering strategies for unstructured-grid solvers operating on shared-memory, cache-based parallel machines

Two renumbering strategies for field solvers based on unstructured grids that operate on shared-memory, cache-based parallel machines are described. Special attention is paid to the avoidance of cache-line overwrite, which can lead to drastic performance degradation on this type of machines. Both renumbering techniques avoid cache-misses and cache-line overwrite while allowing pipelining, leading to optimal coding for this type of hardwar

Projective prediction of pressure increments

A simple projective predictor of pressure increments has been developed. The procedure requires the storage of previous pressure increments and right-hand sides, i.e. a modest amount of storage. Based on this information, the known right-hand sides are projected onto the right-hand side at the new timestep. The projection coefficients are then used to predict the pressure increment at the new timestep. Numerical tests indicate that the number of iterations required is reduced considerably. Furthermore, the main gains are achieved with a very modest number of basis vectors. Typically, no more than 2 previous results have to be stored. The procedure is easy to implement and should be applicable to a large number of codes.&nbsp