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 fully coupled system in which a vehicle s aerodynamic and handling responses can be simulated has been designed and evaluated using a severe crosswind test. Simulations of this type provide vehicle manufacturers with a useful alternative to on road tests, which are usually performed at a late stage in the development process with a proto- type vehicle. The proposed simulations could be performed much earlier and help to identify and resolve any aerodynamic sensitivities and safety concerns before significant resources are place in the design. It was shown that for the simulation of an artificial, on-track crosswind event, the use of the fully coupled system was unnecessary. A simplified, one-way coupled system, in which there is no feedback from the vehicle s dynamics to the aerodynamic simulation was sufficient in order to capture the vehicle s path deviation. The realistic properties of the vehicle and accurately calibrated driver model prevented any large attitude changes whilst immersed in the gust, from which variations to the aerodynamics could arise. It was suggested that this system may be more suited to other vehicle geometries more sensitive to yaw motions or applications where a high positional accuracy of the vehicle is required.

Overset, or Chimera, meshes are used to discretize the governing equations within a computational domain using multiple meshes that overlap in an arbitrary manner. The overset meshing technique is most applicable to problems dealing with multiple or moving bodies. Deep water simulations were carried out using both single and overset grid techniques for the evaluation of the overset grid application. These simulations were carried out using the commercial CFD code STAR-CCM+ by CD-adapco. The geometry simulated is that of a SES model (T-Craft) tested at the Naval Surface Warfare Center Carderock Division. The craft is simulated with two degrees of freedom, allowing movement in heave and pitch in response to displacement of the free surface. Agreement between the single and overset grid techniques was deemed reasonable to extend to future shallow water cases. However, due to longer run times of the overset mesh, the traditional or single mesh technique should be employed whenever applicable. In order to extend existing full craft CFD simulations of a surface effect ship (SES) into shallow water and maneuvering cases, an overset mesh is needed. Simulations of the SES were performed and monitored at various depth Froude numbers resulting in subcritical, critical, and supercritical flow regimes. Resistance, pitch response, and free surface response of the SES were compared between the shallow water simulations. The SES produced wider wakes, perpendicular to the craft, at simulations closer to the critical flow regime. Critical flow occurs at a depth Froude number between 0.9 and 0.95. Progression of shallow water effects through the three flow regimes agrees well with shallow water theory. / Master of Science

Das Ziel dieser Forschungsarbeit ist die Bestimmung charakteristischer aerodynamischer Koeffizienten im Schwebeflug mit Hilfe von Strömungssimulationen. Diese Koeffizienten werden für eine modellbasierte Regelung eines vollaktuierten Multikopters benötigt. Für die Simulationen wird aufbauend auf vorangegangenen Arbeiten ein ‚Sliding-mesh‘-Modell optimiert und einem ‚Overset-mesh‘-Modell gegenübergestellt. Die Verfahren werden anhand von Mess- sowie Referenzdaten hinsichtlich ihrer Rechenzeit und Genauigkeit verglichen. Die Messdaten werden im Rahmen dieser Arbeit ausgewertet. Die Referenzdaten stammen aus älteren Untersuchungen. Das ‚Overset-mesh‘-Verfahren liefert bei viermal höherer Rechenzeit gleichwertige adäquate Ergebnisse. Aus diesen Gründen werden anknüpfende Untersuchungen des Zeitverhaltens der Rotorschubkraft bei Drehzahländerungen mit dem ‚Sliding-mesh‘-Modell durchgeführt. Es wird beobachtet, dass die Schubkraft innerhalb der Simulation den Messdaten bei Drehzahlsprüngen vorauseilt.:1 Einleitung 1.1 Motivation 1.2 Zielsetzung und Aufbau der Arbeit 2 Theoretische Grundlagen 2.1 Strömungsmechanik 2.1.1 Grundgleichungen 2.1.2 Zusätzliche Gleichungen 2.1.3 Navier-Stokes-Gleichungen 2.1.4 Laminare und turbulente Strömungen 2.1.5 Grenzschicht 2.2 Numerische Strömungsmechanik 2.2.1 Numerische Diskretisierung 2.2.2 Numerische Lösungsmodelle 2.2.3 Turbulenzmodelle 2.3 Rotorströmungen 2.3.1 Strahltheorie 2.3.2 Numerische Rotorsimulation 3 Stand der Technik 3.1 Motivation 3.2 Rotorströmung 3.3 Aeroelastizität 3.4 Nachlaufströmung 4 Umsetzung 4.1 Modellbildung 4.1.1 ‚Sliding-mesh‘-Verfahren 4.1.2 ‚Overset-mesh‘-Verfahren 4.2 Validierung 4.2.1 ‚Sliding-mesh‘-Verfahren 4.2.2 ‚Overset-mesh‘-Verfahren 4.3 Modifikationen 4.3.1 ‚Sliding-mesh‘-Verfahren 4.3.2 ‚Overset-mesh‘-Verfahren 4.4 Auswertung 4.4.1 ‚Sliding-mesh‘-Verfahren 4.4.2 ‚Overset-mesh‘-Verfahren 5 Messaufbau und Messauswertung 5.1 Messaufbau 5.2 Messauswertung 5.2.1 Schubkraftverlauf 5.2.2 Drehmomentenverlauf 6 Zeitverhalten 6.1 Modellbildung 6.2 Auswertung Schubkraftverlauf 6.2.1 Drehzahlsprung 1 6.2.2 Drehzahlsprung 2 7 Zusammenfassung und Ausblick 7.1 Zusammenfassung 7.2 Ausblick

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.

Simulations of fluid flows over complex geometries are typically solved using a solution technique known as the overset meshing method. The geometry is meshed using grid types appropriate to the local geometry in a patchwork fashion, rather than meshing the entire geometry with one type of mesh. The strand-Cartesian approach is a simplification of this process. While high-order accurate solvers on Cartesian grids are simple to implement, strand grids are usually restricted to second-order accuracy, resulting in poor quality solutions. Flux correction is a high-order accurate solution method, specifically designed for use on strand grids. The flux correction method on strand grids is evaluated in conjunction with an overset Cartesian grid. Fundamental studies are considered which demonstrate the effectiveness of high-order methods in solving practical flows of interest.

Recent increases in computing power and memory have created renewed interest in alternative grid schemes such as unstructured grids, which facilitate rapid grid generation by relaxing restrictions on grid structure. Three rotor models have been incorporated into a popular fixed-wing unstructured computational fluid dynamics (CFD) solver to increase its capability and facilitate availability to the rotorcraft community. The benefit of unstructured grid methods is demonstrated through rapid generation of high fidelity configuration models. The simplest rotor model is the steady state actuator disk approximation. By transforming the unsteady rotor problem into a steady state one, the actuator disk can provide rapid predictions of performance parameters such as lift and drag. The actuator blade and overset blade models provide a depiction of the unsteady rotor wake, but incur a larger computational cost than the actuator disk. The actuator blade model is convenient when the unsteady aerodynamic behavior needs to be investigated, but the computational cost of the overset approach is too large. The overset or chimera method allows the blades loads to be computed from first-principles and therefore provides the most accurate prediction of the rotor wake for the models investigated. The physics of the flow fields of these models for rotor/fuselage interaction are explored, along with efficiencies and limitations of each methodology.

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.