Waterjet Propulsion and Negative Thrust Deduction

The thrust deduction fraction of waterjet-propelled hulls is often reported to be negative in the speed range close to the operating speeds. In this paper, employing a numerical method, the bare hull and the self-propelled hull flows are studied. The changes between the bare hull and self- propelled hull resistances are investigated for understanding whether it is the waterjet hull resistance decrease which contributes to the negative thrust deduction fraction or there are some other effects rather than the resistance increment

The problem of verification and validation processes of CFD simulations of planing hulls

In the context of Computational Fluid Dynamic (CFD) application in ship hydrodynamic field it is well known that the numerical simulations of planing crafts are significantly less reliable rather displacement hulls. For this reason it is important to perform a comprehensive approach to the verification and validation (V&V) methodologies and procedures in order to obtain high-quality results of CFD simulations of planing hulls. In the first stage of this work, an assessment of the accuracy and effectiveness of different simulations setups and techniques for planing craft is performed, paying particular attention to the different techniques of moving mesh, such as the single moving grid, overset/chimera grid, and morphing mesh, and to the problems related to the air-water interface models, such as the numerical ventilation of the hull bottom. In the second stage the results of the V&V study for four different hull models are reported at four Froude numbers (Fr). The Unsteady Reynolds Average Navier Stokes (URANS) code results are validated using benchmark experimental data obtained for three warped hulls, characterized by systematic variation of the slenderness ratio (L/B) and for one monohedral hull with comparable L/B. Grid independence, iteration, time-step, and statistical convergence analysis for response variables (resistance coefficients, wetted surfaces, and dynamic trim angles) are performed using the main uncertainty estimation methods available in the literature. The same procedures are repeated for the wave profiles analysis. The results of this work show that is possible to improve the reliability of the numerical simulation of the planing craft reducing the errors and uncertainties related to the predictions of resistance, running attitude and wave pattern. It should be note that the error has a significant hull geometry dependency. Moreover the results of the V&V study highlight that the sources of errors investigated have different importance on the numerical error and uncertainty and the modelling of the physics of the planing craft is a critical point to improve the reliability of the numerical simulation

Verifikationsmethodik für die rechnerische Windtechnik Vorhersage von Windlasten an Tragwerken

In this thesis, a new credibility assessment framework is developed for computational wind engineering (CWE) simulations. The framework is mainly developed for testing code implementation correctness and estimation of the discretization uncertainty for eddy-resolving, and unsteady simulations. The framework is composed of two main milestones. First, a modular and flexible procedure for code verification is developed with the ability of testing black box codes. The code verification procedure focuses on the consistency of the code implementation and convergence of field variables. The procedure for code verification consists of analytical benchmarks, either exact or manufactured, with increasing complexity to test the implementation of each term in the Navier-Stokes equation. Second, the credibility assessment framework has a guideline for the quantification of discretization error/uncertainty. More precisely, guidelines are defined for solution verification. The discretization error/uncertainty estimation is based on Richardson Extrapolation approach. A solution biased uncertainty estimator is used to account for using unstructured grids, non-uniform refinement, and non-asymptotic solutions. The newly developed framework has a new definition for the measurement of grid size, handling simulation data with anomalous behavior, and for the safety factor definition in the uncertainty quantification of the discretization error. The assessment methodology is suited to both well- and ill-behaved sequences of simulations. The performance of the assessment methodology is checked with a glimpse on validation with experimental data. Finally, it can be concluded that the developed verification methodology is highly qualified to judge the quality of CWE simulations. Moreover, the generality and modularity of the framework makes it applicable to any software environment regardless of the discretization scheme. Consequently, the methodology encourages further research on the identification of the reliability of CWE simulations.In dieser Arbeit wird ein neues Rahmenwerk zur Glaubwürdigkeitsbewertung für rechnergestützte Windsimulationen (CWE) entwickelt. Der Rahmen wird hauptsächlich für die Prüfung der Korrektheit der Code-Implementierung und die Abschätzung der Diskretisierungsunsicherheit für wirbelauflösende und instationäre Simulationen entwickelt. Das Framework besteht aus zwei Hauptmeilensteinen. Erstens wird ein modulares und flexibles Verfahren zur Code-Verifikation entwickelt, das die Möglichkeit bietet, Black-Box-Codes zu testen. Das Code-Verifikationsverfahren konzentriert sich auf die Konsistenz der Code-Implementierung und die Konvergenz der Feldvariablen. Das Verfahren zur Codeverifizierung besteht aus analytischen Benchmarks, entweder exakt oder hergestellt, mit zunehmender Komplexität, um die Implementierung jedes Terms in der Navier-Stokes-Gleichung zu testen. Zweitens verfügt das Rahmenwerk zur Glaubwürdigkeitsbewertung über einen Leitfaden zur Quantifizierung von Diskretisierungsfehlern/Unsicherheiten. Genauer gesagt, werden Richtlinien für die Verifizierung der Lösung definiert. Die Schätzung des Diskretisierungsfehlers/der Unsicherheit basiert auf dem Richardson-Extrapolationsansatz. Ein lösungsverzerrter Unsicherheitsschätzer wird verwendet, um die Verwendung unstrukturierter Gitter, ungleichmäßiger Verfeinerung und nicht asymptotischer Lösungen zu berücksichtigen. Der neu entwickelte Rahmen hat eine neue Definition für die Messung der Gittergröße, die Behandlung von Simulationsdaten mit anomalem Verhalten und für die Definition des Sicherheitsfaktors bei der Unsicherheitsquantifizierung des Diskretisierungsfehlers. Die Bewertungsmethodik eignet sich sowohl für gut als auch für schlecht verhaltene Simulationsfolgen. Die Leistungsfähigkeit der Bewertungsmethodik wird mit einem Blick auf die Validierung mit experimentellen Daten überprüft. Abschließend kann festgestellt werden, dass die entwickelte Verifikationsmethodik hoch qualifiziert ist, um die Qualität von CWE-Simulationen zu beurteilen. Darüber hinaus macht die Allgemeingültigkeit und Modularität des Rahmens es für jede Softwareumgebung unabhängig vom Diskretisierungsschema anwendbar. Folglich fördert die Methodik weitere Forschungen zur Identifizierung der Zuverlässigkeit von CWE-Simulationen

Development and Characterization of a Plate Fuel Hotspot Model in COMSOL

Aluminum clad plate fuel is common to many high performance water cooled research reactors, including the High Flux Isotope Reactor (HFIR) at ORNL. Fuel manufacturing defects associated with fuel segregation and incomplete bonding of the cladding to the fuel material currently limit the performance of HFIR. A high resolution multi-physics (HRMP) simulation of concurrent fuel segregation and incomplete bonding of fuel cladding is developed in this dissertation. The simulation development begins with a review of legacy modeling of the fuel segregation and cladding non-bond, and then proceeds to identify improvements possible in the HRMP framework. A contact conductance model is selected for the incomplete bonding of cladding to fuel, advancing previous models. A verification of the COMSOL simulation platform used to construct the evaluation model is performed using the method of manufactured solutions, including assessments of the solid conduction modeling domain, the fluid coolant channel domain, and coupled fuel to coolant channel domains. Solution verification is performed with the least squares, grid convergence index approach, and indirect validation of the evaluation model is performed using data generated to establish thermal performance limits for the Advanced Neutron Source reactor. The verification and validation efforts are also extensions to previous work using COMSOL for HRMP modeling of HFIR, and establish numerical and modeling uncertainties. The evaluation model is then employed in a sensitivity assessment of 18 parameters in the evaluation model using Latin Hypercube sampling methods to establish a ranking of parameter importance in predicting four quantities of interest