The first step in the assembly of an overset grid system is to cut holes or to mark points that are inside a solid body and outside the domain of interest. Most existing approaches have been developed for use only with structured grids. A fast and robust approach that can be applied to structured, unstructured, or generalized grid topologies, with a minimum of user inputs, is desired. A new hole cutting process is presented that utilizes a Cartesian Binary tree representation of the geometry to provide a fast and efficient algorithm applicable to generalized grids. An algorithm has also been developed to mark the fringe points and find its donors. The effectiveness of the algorithm is demonstrated by testing it on generalized and structured grids.

<p>A National Aeronautics and Space Administration computational fluid dynamics code, OVERFLOW 2, was modified to utilize a harmonic balance solution method. This modification allows for the direct calculation of the nonlinear frequency-domain solution of a periodic, unsteady flow while avoiding the time consuming calculation of long physical transients that arise in aeroelastic applications.</p><p>With the usual implementation of this harmonic balance method, converting an implicit flow solver from a time marching solution method to a harmonic balance solution method results in an unstable numerical scheme. However, a relatively simple and computationally inexpensive stabilization technique has been developed and is utilized. With this stabilization technique, it is possible to convert an existing implicit time-domain solver to a nonlinear frequency-domain method with minimal modifications to the existing code.</p><p>This new frequency-domain version of OVERFLOW 2 utilizes the many features of the original code, such as various discretization methods and several turbulence models. The use of Chimera overset grids in OVERFLOW 2 requires care when implemented in the frequency-domain. This research presents a harmonic balance version of OVERFLOW 2 that is capable of solving on overset grids for sufficiently small unsteady amplitudes.</p> / Dissertation

A high fidelity computational fluid dynamics approach to perform direct simulations of ship maneuvers is presented in this thesis. The approach uses dynamic overset grids with a hierarchy of bodies to enable arbitrary motions between objects, and overcome the difficulties in simulation of the moving rudder and rotating propeller. To better resolve propeller/rudder interaction a Delayed Detached Eddy Simulation turbulence model based on Menter’s SST is used. The methodology was implemented in the general purpose RANS/DES/DDES research code REX, and is applied to the KRISO Container Ship (KCS) with moving rudder and rotating propeller in deep and shallow water. For the first time, a grid study is conducted for the self-propulsion condition for the propeller RPM, thrust, torque and lateral force, and for the roll and pitch motions, using grids of 8.7 (coarse), 24.6 (medium) and 71.3 (fine) million points. A grid study is also performed for the zigzag maneuver evaluating the maximum and minimum values of propeller thrust, torque and lateral force roll, pitch, yaw, roll rate, yaw rate and drift throughout the maneuver. An extensive comparison between predicted motions and forces of the direct simulations and the experimental data collected by Schiffbau-Versuchsanstalt Potsdam GmbH (SVA) and Flanders Hydraulics Research (FHR) are presented. While the results and comparisons with experimental data show that using direct CFD to compute modified and standard maneuvers with moving rudder and rotating discretized propeller is feasible, computational cost remains an impediment for many practical applications. Coupling a dynamic overset CFD solver with a potential propeller code can dramatically reduce the computational time to perform maneuvering simulations by using one order of magnitude larger time step than direct simulation. This thesis investigates the ability of a coupled CFD/potential propeller code approach to simulate maneuvers in ships, where the rudder is located downstream of the propeller. While the approach has been successfully applied to submarine maneuvers, in which the propeller wake is free of interference, the concept had not been evaluated before for cases where an object (the rudder) is immersed in the wake. The study is performed using the CFD code REX and the propeller code PUF-14. Performance of the coupled REX/PUF-14 approach is first tested studying propeller/rudder interaction, evaluating influence of the propeller/rudder gap size and rudder deflection on propeller performance curves and rudder forces, comparing against DDES simulations with a discretized rotating propeller. A grid study was performed for advance coefficient J=0.6 and a rudder angle δ=20 degrees for a propeller rudder gap of 0.2 times the rudder radius, with the resulting grid uncertainties for propeller thrust and torque coefficients suggesting that the effects of the grid changes are small for the present range of grid sizes. A 15/1 zigzag maneuver for the KCS container ship, in which case the rudder is very close downstream of the propeller, is then analyzed, and compared against discretized propeller simulations and experimental data. Self-propulsion coupled REX/PUF-14 results agree very well with experiments and discretized propeller simulations. Prediction of motions, forces and moments, and mean flow field with the coupled REX/PUF-14 approach are comparable to results obtained with discretized propeller simulations and agree with experiments well, though as implemented the coupled approach is unable to resolve tip vortices and other flow structures that interact with the rudder, potentially affecting prediction of flow separation. It can be concluded that coupled CFD/potential flow propeller approaches are an effective and economical way to perform direct simulation of surface ship maneuvers with CFD.

No description available.

Modern computational science permits the accurate solution of nonlinear partial differential equations (PDEs) on overlapping computational domains, known as an overset approach. The complex grid interconnectivity inherent in the overset method can introduce errors in the solution through “orphan” points, i.e., grid points for which reliable solution donor points cannot be located. For this reason, a variety of data transfer strategies based on scattered data interpolation techniques have been assessed with application to both overset and hybrid methodologies. Scattered data approaches are attractive because they are decoupled from solver type and topology, and may be readily applied within existing methodologies. In addition to standard radial basis function (RBF) interpolation, a novel steered radial basis function (SRBF) interpolation technique has been developed to introduce data adaptivity into the data transfer algorithm. All techniques were assessed by interpolating both continuous and discontinuous analytical test functions. For discontinuous functions, SRBF interpolation was able to maintain solution gradients with the steering technique being the scattered-data analog of a slope limiter. In comparison with linear mappings, the higher-order approaches were able to more accurately preserve flow physics for arbitrary grid configurations. Overset validation test cases included an inviscid convecting vortex, a shock tube, and a turbulent ship airwake. These were studied within unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations to determine quantitative and qualitative improvements when applying RBF interpolation over current methods. The convecting vortex was also analyzed on a grid configuration which contained orphan points under the state-of-the-art overset paradigm. This was successfully solved by the RBF-based algorithm, which effectively eliminated orphans by enabling high-order extrapolation. Order-of-magnitude reductions in error compared to the exact vortex solution were observed. In addition, transient conservation errors that persisted in the original overset methodology were eliminated by the RBF approach. To assess the effect of advanced mapping techniques on the fidelity of a moving grid simulation, RBF interpolation was applied to a hybrid simulation of an isolated wind turbine rotor. The resulting blade pressure distributions were comparable to a rotor simulation with refined near-body grids.

The flapping path of a single airfoil and dual airfoils in a biplane configuration is optimized for maximum thrust and/or propulsive efficiency. Unsteady, low speed viscous flows are computed using a Navier-Stokes solver in a parallel computing environment. A gradient based algorithm and Response Surface Methodology (RSM) are employed for optimization. The evaluation of gradient vector components and the design of experiments for RSM, which require unsteady solutions, are also carried out in parallel. Parallel computations are performed using Parallel Virtual Machine (PVM) library. First, a single airfoil undergoing a combined sinusoidal or non-sinusoidal pitching and plunging motion is studied. The non-sinusoidal flapping motion is described using an elliptic curve or Non-Uniform Rational B-Splines (NURBS). It is shown that the thrust generation may significantly be increased in comparison to the sinusoidal flapping motion. For a high thrust, the airfoil stays at high effective angle of attack values during the upstroke and the downstroke, and the effective pitching occurs at minimum and maximum plunge positions. Secondly, the optimization of sinusoidal and non-sinusoidal flapping paths of dual airfoils is considered. Moving and deforming overset grids are used for computations. The deforming overset grids remove the restrictions on the flapping motion, and improve the optimization results obtained earlier. At low flapping frequencies, an airfoil in a biplane configuration produces more thrust than a single airfoil. Yet, at high frequencies the airfoil in biplane configuration produces less thrust at a significantly lower efficiency than the single airfoil.

Airfoils &deg / apping in pitch and plunge are studied, and the &deg / apping motion parameters are op- timized to maximize thrust generation and the e&plusmn / ciency of the thrust generation. Unsteady viscous &deg / ow&macr / elds over &deg / apping airfoils are computed on overset grids using a Navier-Stokes solver. Computations are performed in parallel using Parallel Virtual Machine library routines in a computer cluster. A single &deg / apping airfoil and dual airfoils &deg / apping in a biplane con- &macr / guration are considered. A gradient based optimization algorithm is employed. The thrust production and the e&plusmn / ciency of the thrust production are optimized with respect to &deg / apping parameters / the plunging and pitching amplitudes, the &deg / apping frequency, and the phase shift between the pitch and plunge motions. It is observed that thrust generation of &deg / apping airfoils strongly depends on the phase shift and high thrust values may be obtained at the expense of reduced e&plusmn / ciency. For a high e&plusmn / ciency in thrust generation, the e&reg / ective angle of attack of the airfoil is reduced and large scale vortex formations at the leading edge are prevented. At a &macr / xed reduced &deg / apping frequency of 1, a single &deg / apping airfoil in pitch and plunge motion produces the maximum average thrust coe&plusmn / cient of 1:41 at the plunge amplitude of 1:60, the pitch amplitude of 23:5o, and the phase shift of 103:4o whereas the maximum e&plusmn / ciency of 67:5% is obtained at the plunge amplitude of 0:83, the pitch amplitude of 35:5o and the phase shift of 86:5o.

In this thesis, store separation from two different configurations are solved using computational methods. Two different commercially available CFD codes / CFD-FASTRAN, an implicit Euler solver, and an unsteady panel method solver USAERO, coupled with integral boundary layer solution procedure are used for the present computations. The computational trajectory results are validated against the available experimental data of a generic wing-pylon-store configuration at Mach 0.95. Major trends of the separation are captured. Same configuration is used for the comparison of unsteady panel method with Euler solution at Mach 0.3 and 0.6. Major trends are similar to each other while some differences in lateral and longitudinal displacements are observed. Trajectories of a fueltank separated from an F-16 fighter aircraft wing and full aircraft configurations are found at Mach 0.3 using only the unsteady panel code. The results indicate that the effect of fuselage is to decrease the drag and to increase the side forces acting on the separating fueltank from the aircraft. It is also observed that the yawing and rolling directions of the separating fueltank are reversed when it is separated from the full aircraft configuration when compared to the separation from the wing alone configuration.

This work extends a previously developed finite-volume overset-grid fluid flow solver to enable the characterisation of rigid-body-fluid interaction problems. To this end, several essential components have been developed and blended together. The inherent time-dependent nature of fluid-solid interaction problems is captured through the laminar transient incompressible Navier-Stokes equations for the fluid, and the Euler-Newton equations for rigid-body motion. First and second order accurate time discretisation schemes have been implemented for the former, whereas second and third order accurate time discretisation schemes have been made available for the latter. Without doubt the main advantage the overset-grid method offers regarding moving entities is the avoidance of the time consuming grid regeneration step, and the resulting grid distortion that can often cause numerical stability problems in the solution of the flow equations. Instead, body movement is achieved by the relative motion of a body fitted grid over a suitable background mesh. In this case, the governing equations of fluid flow are formulated using a Lagrangian, Eulerian, or hybrid flow description via the Arbitrary Lagrangian-Eulerian method. This entails the need to guarantee that mesh motion shall not disturb the flow field. With this in mind, the space conservation law has been hard-coded. The compliance of the space conservation law has the added benefit of preventing spurious mass sources from appearing due to mesh deformation. In this work, two-way fluid-solid interaction problems are solved via a partitioned approach. Coupling is achieved by implementing a Picard iteration algorithm. This allows for flexible degree of coupling specificationby the user. Furthermore, if strong coupling is desired, three variants of interface under-relaxation can be chosen to mitigate stability issues and to accelerate convergence. These include fixed, or two variants of Aitken’s adaptive under-relaxation factors. The software also allows to solve for one-way fluid-solid interaction problems in which the motion of the solid is prescribed. Verification of the core individual components of the software is carried out through the powerful method of manufactured solutions (MMS). This purely mathematically based exercise provides a picture of the order of accuracy of the implementation, and serves as a filter for coding errors which can be virtually impossible to detect by other means. Three instances of one-way fluid-solid interaction cases are compared with simulation results either from the literature, or from the OpenFOAM package. These include: flow within a piston cylinder assembly, flow induced by two oscillating cylinders, and flow induced by two rectangular plates exhibiting general planar motion. Three cases pertaining to the class of two-way fluid-interaction problems are presented. The flow generated by the free fall of a cylinder under the action of gravity is computed with the aid of an intermediate ‘motion tracking’ grid. The solution is compared with the one obtained using a vorticity based particle solver for validation purposes. Transverse vortex induced vibrations (VIV) of a circular cylinder immersed in a fluid, and subject to a stream are compared with experimental data. Finally, the fluttering motion of a rectangular plate under different scenarios is analysed.

La simulation numérique des écoulements (CFD) est largement utilisée aujourd'hui dans l'industrie aéronautique, de l'avant-projet à la conception des appareils. En parallèle, la puissance des calculateurs s'est accrue, permettant d'effectuer des simulations résolvant les équations de Navier-Stokes moyennées (RANS) dans un délai de restitution acceptable du point de vue industriel. Cependant, les configurations simulées sont de plus en plus complexes géométriquement, rendant la réalisation du maillage très coûteuse en temps humain. Notre objectif est de proposer une méthode permettant de simplifier la génération de maillages autour de géométries complexes, en exploitant les avantages de la méthode Chimère, tout en levant les difficultés principales rencontrées par cette méthode dans le calcul des connectivités. Dans notre approche, le domaine de calcul est découpé en régions proches et en régions éloignées des corps. Des grilles curvilignes de faible extension décrivent les régions autour des corps. Le maillage de fond est défini par un ensemble de grilles cartésiennes superposées aux grilles de corps, qui sont engendrées et adaptées automatiquement selon les caractéristiques de l'écoulement. Afin de traiter des maillages recouvrants autour de géométries complexes sans surcoût humain, les différentes grilles sont regroupées par composant Chimère. Des relations d'assemblage sont alors définies entre composants, en s'inspirant de la Géométrie de Construction des Solides (CSG), où un solide peut être construit par opérations booléennes successives entre solides primitifs. Le calcul des connectivités Chimère est alors réalisé de manière simplifiée. Des simulations RANS sont effectuées autour d'un fuselage d'hélicoptère avec mât de soufflerie et autour d'une aile NACA0015 en incidence, afin de mettre en oeuvre la méthode. / Computational fluid dynamics (CFD) is widely used today in aeronautics, while the computing power has increased, enabling to perform simulations solving Reynolds-averaged Navier-Stokes equations (RANS) within an acceptable time frame from the industrial point of view. However, the configurations are more and more geometrically complex, making the mesh generation step prohibitive. Our aim is here to propose a method enabling a simplification of the mesh generation over complex geometries, taking advantage of the Chimera method and overcoming the major difficulties arising when performing overset grid connectivity. In our approach, the computational domain is partitioned into near-body regions and off-body regions. Near-body regions are meshed by curvilinear grids of short extension describing the obstacles involved in the simulation. Off-body mesh is defined by a set of adaptive Cartesian grids, overlapping near-body grids. In order to consider overset grids over complex geometries with no additional cost, grids are gathered by Chimera component, and assembly relations are defined between them, inspired by Constructive Solid Geometry, where a solid can result from boolean operations between primitive solids. The overset grid connectivity is thus simplified. RANS simulations are performed over a helicopter fuselage with a strut, and over a NACA0015 wing.