Unstructured mesh generation and adaptivity

An overview of current unstructured mesh generation and adaptivity techniques is given. Basic building blocks taken from the field of computational geometry are first described. Various practical mesh generation techniques based on these algorithms are then constructed and illustrated with examples. Issues of adaptive meshing and stretched mesh generation for anisotropic problems are treated in subsequent sections. The presentation is organized in an education manner, for readers familiar with computational fluid dynamics, wishing to learn more about current unstructured mesh techniques

Unstructured Grid Adaptation: Status, Potential Impacts, and Recommended Investments Towards CFD 2030

International audienceUnstructured grid adaptation is a powerful tool to control Computational Fluid Dynamics (CFD) discretization error. It has enabled key increases in the accuracy, automation, and capacity of some fluid simulation applications. Slotnick et al. provide a number of case studies in the CFD Vision 2030 Study: A Path to Revolutionary Computational Aerosciences to illustrate the current state of CFD capability and capacity. The study authors forecast the potential impact of emerging High Performance Computing (HPC) environments forecast in the year 2030 and identify that mesh generation and adaptivity will continue to be significant bottlenecks in the CFD workflow. These bottlenecks may persist because very little government investment has been targeted in these areas. To motivate investment, the impacts of improved grid adaptation technologies are identified. The CFD Vision 2030 Study roadmap and anticipated capabilities in complementary disciplines are quoted to provide context for the progress made in grid adaptation in the past fifteen years, current status, and a forecast for the next fifteen years with recommended investments. These investments are specific to mesh adaptation and impact other aspects of the CFD process. Finally, a strategy is identified to di↵use grid adaptation technology into production CFD work flows

Anisotropic output-based adaptation with tetrahedral cut cells for compressible flows

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections."September 2008."Includes bibliographical references (leaves 153-164).Anisotropic, adaptive meshing for flows around complex, three-dimensional bodies remains a barrier to increased automation in computational fluid dynamics. Two specific advances are introduced in this thesis. First, a finite-volume discretization for tetrahedral cut-cells is developed that makes possible robust, anisotropic adaptation on complex bodies. Through grid refinement studies on inviscid flows, this cut-cell discretization is shown to produce similar accuracy as boundary-conforming meshes with a small increase in the degrees of freedom. The cut-cell discretization is then combined with output-based error estimation and anisotropic adaptation such that the mesh size and shape are controlled by the output error estimate and the Hessian (i.e. second derivatives) of the Mach number, respectively. Using a parallel implementation, this output-based adaptive method is applied to a series of sonic boom test cases and the automated ability to correctly estimate pressure signatures at several body lengths is demonstrated starting with initial meshes of a few thousand control volumes. Second, a new framework for adaptation is introduced in which error estimates are directly controlled by removing the common intermediate step of specifying a desired mesh size and shape. As a result, output error control can be achieved without the adhoc selection of a specific field (such as Mach number) to control anisotropy, rather anisotropy in the mesh naturally results from both the primal and dual solutions. Furthermore, the direct error control extends naturally to higher-order discretizations for which the use of a Hessian is no longer appropriate to determine mesh shape. The direct error control adaptive method is demonstrated on a series of simple test cases to control interpolation error and discontinuous Galerkin finite element output error. This new direct method produces grids with less elements but the same accuracy as existing metric-based approaches.by Michael Andrew Park.Ph.D

Computational and Experimental Investigation of the Aerodynamics of a W-shaped leading edge reversed delta plan-form wing

A feasibility analysis for an unconventional W-shaped leading edge, reversed delta plan-form wing has been carried out. The wing is believed to aid the Vertical/Short Take-off and Landing (V/STOL) capabilities of small aircraft. The main focus of the research was to carry out computational investigations of the flow phenomena associated with this unique shaped wing at cruise, take-off, and landing configurations. An interactive numerical and experimental method was used to baseline the important flow-field structures associated with this wing, and to identify the necessary areas for further comprehensive full-scale numerical investigations carried out herein. Numerical simulations solved the explicit quasi-steady compressible Navier-Stokes equations for the cruise conditions (run at a Reynolds Number of 3x107), while segregated quasi-steady incompressible Navier-Stokes equations were solved for the ground-effect analyses and low-speed wind tunnel simulations on a 5% scale of the wing (run at Reynolds Number of 3x105 and 3.6x105).Numerically, the ground was accounted for with the image method, and the static ground board method. The fuselage was not modelled in the numerical or experimental investigations. Hence, it needs to be noted that the additional lift-dependant drag caused by the modification of the span loading due to fuselage has not been accounted for. Also, the there are limitations on the ground height limited by the inclusion of the fuselage. In general, the wing was found to have a highly three-dimensional flow field. Both low-speed and high-speed free-flight results revealed that the wing exhibits soft stall and a good lift-to-drag ratio, as well as statically stable pitching moment response up to stall conditions. Maximum lift was reached at 14°< α < 16°, giving a lift-to-drag ratio of 18. On-surface streamline observations showed that the effect of the forward sweep assists in terminating the propagation of the flow separation along the entire part of the wing. High-speed numerical investigations showed regions of local supersonic flow, but with no detrimental effects on the performance of the wing. Near-wake results by both means of study revealed inboard vortex phenomena at higher angle of attack. The ground-effect results showed a great increase of the lift coefficient and lift-to-drag ratio for the W-wing in ground effect. Values of L/D=30 were achieved for h/b = 0.09, a 90 % increase as compared to the free-flight case. Regions of very low velocity and high pressure underneath the wing were resolved, suggesting a very strong “air cushion” effect being induced by the wing. Modification of the wing design suggested that in absence of the forward-sweep, the un-swept wing struggles to maintain attached flow, or indeed prevent further separation on the rest of the wing