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Item type:Publication, Access status: with files , Numerical modeling of granular bulk cargoTo study the behavior of unsaturated granular materials on vessels, a monolithic multi-physics approach is implemented within a Finite Volume framework. This includes constitutive models representing various cargo failure mechanisms which are applied to 2D parameter studies that provide comprehensive insights into cargo behavior under diverse conditions. The models include a rigid-perfectly plastic soil model, a porous media approach for rigid materials, an incompressible elastic model, and a coupling of the rigid-perfectly plastic model with the porous media. Thorough validation and verification of all implemented models are presented, along with a proof-of-concept study demonstrating a complete three-dimensional simulation of a loaded bulk carrier in waves.Publicationtype: Doctoral Thesis53 57 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adjoint shape optimization of blood flow applications under uncertainties(2025); This thesis aims to advance traditional gradient-based shape optimization techniques by incorporating blood-specific features. The work can be broadly classified into four main categories, (a) computational blood flow modeling, (b) parameter-free shape optimization, (c) the continuous adjoint method and (d) robust shape optimization. As regards (a), this work presents key aspects related to the computational fluid dynamics (CFD) or fluid-structure interaction (FSI) simulations employed for the solution of the blood flow problem (primal problem). The underlying primal problem is enhanced to account for blood damage (hemolysis) by the numerical solution of an additional partial differential equation (PDE). The minimization of hemolysis is one of the main objectives of interest in this work. In order to account for the shear-thinning behavior of blood, several non-Newtonian viscosity models are also employed. The numerical solution of the primal problem is verified against analytical solutions and literature-reported results when deemed necessary. The shape optimization problem is formulated based on a parameter-free approach. To this end, the identification of the shape gradient or at least, a suitable descent direction, based on the shape sensitivity is addressed. Several methods that compute the descent direction based on the solution of one (or two) additional PDE(s) are presented. Auxiliary aspects related to the optimization problem are also discussed in the context of (b). The thesis presents the novel derivation of continuous adjoint equation systems dual to the blood-specific problems. Specifically, contributions to (c) relate to the development of the continuous adjoint to the steady-state Navier-Stokes equations for an incompressible fluid with hemolysis modeling and with non-Newtonian viscosity properties. The accuracy of the novel systems in estimating the shape sensitivity is assessed against second-order accurate finite difference (FD) studies. On the topic of (d), this work focuses on the necessary uncertainty quantification techniques able to drive a robust shape optimization procedure. The problem is considered uncertain by means of the employed parameters or the boundary conditions thus rendering the objective functional a statistical quantity. The optimization targets the minimization of the statistical moments of the objective functional through the use of methods based on sampling or the method of moments. The biomedical applications studied in this thesis refer to an idealized medical device and an idealized bypass-graft.Publicationtype: Doctoral Thesis120 142 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , An all-resolution cumulant-based lattice Boltzmann approach for simulating high Reynolds number engineering shear flows(2024); The thesis addresses the development of a lattice Boltzmann fluid solver for the accurate and efficient simulation of turbulent flows at high Reynolds numbers. The central goal is a combined treatment of turbulence and collision models. Building on a comprehensive study of frequently used collision operators, which are fundamental for modeling the flow physics, the advancement of a particular model based on a cumulant approach and its validation is performed. To comply with the flexible resolution requirements associated with simulating technical flows, an optional wall model is supplemented to the final formulation.Publicationtype: Doctoral Thesis109 465 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , CAD-free adjoint shape optimization of floating vessels(2024); This thesis aims to improve fluid-dynamic shape optimization methods for non-parameterized (CAD-free) shapes. The focus is computing shape transformations for shapes with Lipschitz boundary and approximating the direction of the steepest descent in Banach spaces with the help of shape sensitivity information, which is calculated using an adjoint-based method. Further, considering geometric constraints and their algorithmic treatment is a central aspect of the thesis. The work investigates a p-Laplace relaxation to approximate Lipschitz transformations with the help of W-1-p functions. Further attempts are made to improve the approximation of the steepest descent direction in W-1-∞ using an approach based on the ADMM algorithm. The suggested approaches are applied in multiple numerical experiments, which successively deform a domain by point-wise movement of the computational grids.Publicationtype: Doctoral Thesis135 242 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Datenbasierte Modellierung der aerodynamischen Lasten auf Überwasserschiffe(2023); The aerodynamic loads on the superstrucure of ships are influenced by different environmental and operational parameters. In order to determine the loads depending on the system parameters on board in near-real time, data-based surrogate models are developed. High-resolution pressure fields based on hybrid RANS/LES simulations are used as data basis. For the data basis a dimension reduction is performed. To create a fully parametrized model, the functional relationship between the system parameters and the model coefficients is represented by a regression. The surrogate modelling allows an efficient and accurate prediction of the pressure fields and resulting loads for arbitrary values of the system parameters.Publicationtype: Doctoral Thesis116 121 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adjoint-based shape optimization constraint by turbulent two-phase Navier-Stokes systems(2021-10); The thesis aims to advance gradient-based optimization methods for non-parametrized shapes exposed to immiscible two-phase flows using an adjoint Reynolds-Averaged Navier-Stokes (RANS) approach. Attention is given to the trade-off between adjoint consistency and industrial process capability. The efforts can be structured into four building blocks (I-IV): Compatible fully turbulent (I) primal (physical) and (II) dual (mathematical, adjoint) relationships together with appropriate objective functional formulations (III) are analysed with curiosity and scepticism at the same time, in order to attain the required accuracy, robustness and efficiency. The geometry engine (IV), which translates the computed sensitivities into shape deformations and grid adjustments, is crucial for the efficiency of the process, the technical usability of the result, and the HPC capability. Different aspects of the simulation-driven shape optimization process are addressed. This research’s common ground is to analyse potential issues in greater depth rather than to formulate ad-hoc measures. The guiding principle frequently follows the concept of ”Learning from the Adjoints”. It is seen that potential weaknesses displayed by an adjoint approach –e.g. when developing (continuous) analytical solutions, in conjunction with (discrete) convergence problems– are often attributable to weaknesses of the primal formulation and a twist of the research question can lead to fruitful insights. Contributions to (I) refer to the development of an efficient engineering Cahn-Hilliard (CH) Volume-of-Fluid (VoF) branch. In line with analytical considerations for a model problem, a nonlinear Equation of State (EoS) is derived to relate an indicator function with the fluid properties. Building block (II) covers the derivation of discrete adjoint VoF formulations and the implementation of an adjoint VoF sub-cycling strategy. The suggestion of a discretely differentiable EoS, together with a novel combination of an inconsistent adjoint VoF method and the CH-VoF approach, allows for a robust and (flexible-) consistent adjoint two-phase formulation. Another major part is concerned with a continuous adjoint complement to 2D, incompressible, first-order boundary-layer equations. The findings support the heuristic neglect of the adjoint transposed convection term and offer analytical expressions for adjoint laminar boundary-layer parameters. The thesis is also concerned with improving adjoint investigations of turbulent flows using mixing-length arguments for the frozen turbulence strategy and a Law of the Wall consistent approach. An algebraic expression provides a consistent closure of the adjoint momentum equation in the logarithmic layer. Spatial decoupling of the control from the objective (III) affects the formulation of boundary conditions and reduces iterative efforts when the design surface does not cover the entire wetted surface. Additionally, an implicit surface metric approach is presented to extract the inherently smooth gradient (IV) out of the possible rough sensitivity derivative. Attention is devoted to compliance with geometrical constraints, e.g. constant volume or maximum outer dimensions. Finally, an adaptive floatation module is added to the gradient-based optimization procedure, which is not differentiated and considered frozen during the adjoint simulation. Examples included underline the capability of the frozen floatation approach and provide partially drastically improved ship hull shapes. It is demonstrated that fixed floatation can lead to optimization losses when the final shape is released. Almost without exception, applications relate to maritime two-phase flows at the industrial level. Some of them are conducted even in full-scale. They refer to a Kriso container ship, a generic submarine, a double-ended ferry, and an offshore-supply vessel.Publicationtype: Doctoral Thesis565 2429 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Numerical modelling of multi-body hydrodynamics in multi-phase simulationsThis thesis is concerned with the simulation of mechanically coupled bodies in a multi-phase environment. While the applications of such cases are very wide, they are of particular interest for offshore wind parks, where numerical investigations can support the safety margins of an operation. The computational focus of the present study is to supplement a viscous three-phase fluid-soil solver featuring the overset grids technique by appropriate mechanical models to analyse the hydrodynamics of complex systems composed of multiple mechanically coupled bodies. A quaternion-based motion modeller featuring several basic joint elements is used to model their influence in a multi-body arrangement or the mechanical interaction between different moving parts of a structure, which can be seen as a multi-body system. Implicit seaway boundary conditions allow the use of compact domain sizes and variable headings of incident waves. Validation examples included refer to rigid links, ropes, fenders or guide frames restricting the motion in experiments, which aim to illustrate the predictive capabilities of the procedure. Finally, investigations on the simulation of a floating tug in waves, boatlanding for safe transfer, grounding of gravity foundations for wind turbines and the installation process of jack-up rigs in seaway involving structure-seabed interactions has been considered for application purposes while utilizing all features of the computational framework.Publicationtype: Doctoral Thesis393 1274 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A consistent smoothed particle hydrodynamics simulation approach with variable resolutionPublicationtype: Doctoral Thesis171 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Analysis of coupling techniques for overset-grid finite-volume methods(2019-09); Coupling of unstructured, overlapping 3D grids in finite-volume methods (FVM) is usually based on a local interpolation of field values, which (a) is insufficient to guarantee mass conservation and (b) requires a sufficient overlap of the wetted domain. Effects of the non-conservative coupling are analysed and the mass-dependent pressure in incompressible FVM is stabilised by global mass conservation practices. Areas with an insufficient overlap are numerically manipulated to allow the simulation of dynamic body contact. This enhanced approach is validated and applied, e.g. within the simulation of a gear pump and for contact simulations of a cylinder/sphere with a wall. The latter reveal a significant increase and a coupling of additional resistance and inertia forces.Publicationtype: Doctoral Thesis513 690 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Development of a scalable method for the efficient simulation of flows using dynamic goal-oriented local grid-adaptation(2019); A dynamic local grid adaptation method has been implemented in an existing framework of a parallel general-purpose pressure-correction based RANS code for unstructured grids, to efficiently simulate flows in marine applications. Various refinement indicators were implemented, ranging from flow feature based criteria to goal oriented error estimators, derived from the adjoint solution of the primal flow problem. The efficiency of free surface flow simulations has been further enhanced by an explicit interface sharpening (EIS) algorithm, which can be used as an add-on algorithm for existing VoF methods, and a dedicated sub-cycling technique. All developed methods were extensively verified and validated before being applied to complex three-dimensional problems.Publicationtype: Doctoral Thesis337 1100