Thomas Rung

Adjoint complement to viscous finite-volume pressure-correction methods

Abstract A hybrid-adjoint Navier–Stokes method for the pressure-based computation of hydrodynamic objective functional derivatives with respect to the shape is systematically derived in three steps: The underlying adjoint partial differential equations and boundary conditions for the frozen-turbulence Reynolds-averaged Navier–Stokes equations are considered in the first step. In step two, the adjoint discretisation is developed from the primal, unstructured finite-volume discretisation, such that adjoint-consistent approximations to the adjoint partial differential equations are obtained following a so-called hybrid-adjoint approach. A unified, discrete boundary description is outlined that supports high- and low-Reynolds number turbulent wall-boundary treatments for both the adjoint boundary condition and the boundary-based gradient formula. The third component focused in the development of the industrial adjoint CFD method is the adjoint counterpart to the primal pressure-correction algorithm. The approach is verified against the direct-differentiation method and an application to internal flow problems is presented.

Adjoint-based Hull Design for Wake Optimisation

Abstract The hull of a generic container vessel was redesigned with the aid of RANS-based adjoint sensitivity analysis in order to improve the quality of the wake field. The redesign was carried out manually at model scale using the sensitivity derivatives of two wake objective functions introduced into the adjoint method. Both flow and adjoint solvers are based on an unstructured finite volume discretisation of incompressible flow equations. The distribution of the sensitivity derivatives with respect to design parameters yields valuable insight into the design problem from the objective point of view. The method can identify key influences and lead to an improved wake field after two design cycles.

Viscous-Inviscid Coupling Methods for Advanced Marine Propeller Applications

The paper reports the development of coupling strategies between an inviscid direct panel method and a viscous RANS method and their application to complex propeller ows. The work is motivated by the prohibitive computational cost associated to unsteady viscous flow simulations using geometrically resolved propellers to analyse the dynamics of ships in seaways. The present effort aims to combine the advantages of the two baseline methods in order to reduce the numerical effort without compromising the predictive accuracy. Accordingly, the viscous method is used to calculate the global flow field, while the inviscid method predicts the forces acting on the propeller. The corresponding reaction forces are employed as body forces to mimic the propeller influence on the viscous flow field. Examples included refer to simple verification cases for an isolated propeller blade, open-water validation simulations for a complete propeller, and more challenging investigations of a manoeuvring vessel in seaways. Reported results reveal a fair predictive agreement between the coupled approach and fully viscous simulations and display the efficiency of the coupled approach.

Validation of the GPU-Accelerated CFD Solver ELBE for Free Surface Flow Problems in Civil and Environmental Engineering

This contribution is dedicated to demonstrating the high potential and manifold applications of state-of-the-art computational fluid dynamics (CFD) tools for free-surface flows in civil and environmental engineering. All simulations were performed with the academic research code ELBE (efficient lattice boltzmann environment, http://www.tuhh.de/elbe). The ELBE code follows the supercomputing-on-the-desktop paradigm and is especially designed for local supercomputing, without tedious accesses to supercomputers. ELBE uses graphics processing units (GPU) to accelerate the computations and can be used in a single GPU-equipped workstation of, e.g., a design engineer. The code has been successfully validated in very different fields, mostly related to naval architecture and mechanical engineering. In this contribution, we give an overview of past and present applications with practical relevance for civil engineers. The presented applications are grouped into three major categories: (i) tsunami simulations, considering wave propagation, wave runup, inundation and debris flows; (ii) dam break simulations; and (iii) numerical wave tanks for the calculation of hydrodynamic loads on fixed and moving bodies. This broad range of applications in combination with accurate numerical results and very competitive times to solution demonstrates that modern CFD tools in general, and the ELBE code in particular, can be a helpful design tool for civil and environmental engineers.

Towards Online Visualization and Interactive Monitoring of Real-Time CFD Simulations on Commodity Hardware

Real-time rendering in the realm of computational fluid dynamics (CFD) in particular and scientific high performance computing (HPC) in general is a comparably young field of research, as the complexity of most problems with practical relevance is too high for a real-time numerical simulation. However, recent advances in HPC and the development of very efficient numerical techniques allow running first optimized numerical simulations in or near real-time, which in return requires integrated and optimized visualization techniques that do not affect performance. In this contribution, we present concepts, implementation details and several application examples of a minimally-invasive, efficient visualization tool for the interactive monitoring of 2D and 3D turbulent flow simulations on commodity hardware. The numerical simulations are conducted with ELBE, an efficient lattice Boltzmann environment based on NVIDIA CUDA (Compute Unified Device Architecture), which provides optimized numerical kernels for 2D and 3D computational fluid dynamics with fluid-structure interactions and turbulence.

Advanced Lagrangian Approaches to Cavitation Modelling in Marine Applications

The paper scrutinizes different approaches to cavitation modelling in the framework of volume of fluid based marine engineering Navier-Stokes simulations. Traditional Eulerian cavitation models compute the vapor content based on computationally efficient, semi-empirical mass-transfer models for cavitation. In conjunction with Lagrangian cavitation models, separate equations for the bubble size and momentum are solved for each individual bubble/nuclei of a dispersed vapor phase, and a subsequent mapping procedure provides the vapor-volume fraction of the Eulerian mixture. The paper aims to advocate the benefits of a combined approach, which reduces the computational effort of the Lagrangian approach whilst maintaining its enhanced predictive realm in critical flow regimes. Validation examples refer to 2D hydrofoils and outline the strong parameter dependency for the Eulerian cavitation models as well as its insensitivity to water-quality aspects. On the contrary, Lagrangian cavitation models return an improved accuracy and capture the influence of water quality. Results of an open-water propeller flow investigation confirm these findings and display a fair predictive agreement in conjunction with a combined modelling approach which allows to perform accurate cavitation predictions at reasonable cost.

Filtered Gradients for Adjoint-based Shape Optimisation

Smooth gradient distributions are obtained through an explicit filtering technique that complements an adjoint Navier-Stokes method in the framework of CAD-free shape optimisation. The gradients of the objective functional are obtained through a continuous adjoint method that uses consistent discretisation schemes devised based on the primal discretisation. After a verification study based on the direct-differentiation method, the approach is used to optimise the shape of ducts for incompressible flow. The suggested filtering approach is shown to be first-order equivalent to the well-established smoothing based on so-called “Sobolev gradients”.

SPH Simulations of Ship Propeller Induced Harbour Bed Erosion

The paper reports on the predictive prospects of Smoothed-Particle-Hydrodynamics (SPH) for simulations of ship propeller induced scours in harbours. Such erosions represent unpleasant phenomena, especially if they occur close to quay walls, and generate cost intensive counter measures. These measures are usually based on a rather weak background knowledge. SPH simulations can help to analyse the erosional processes and to understand the interaction between ship, water, soil and structure. In the present research, a body-force propulsor model based on the open water characteristics is used to represent the ship’s propeller. The evolution of the liquid and granular phase particles is obtained from an SPH-integration of the continuity and momentum equations. The fluid is considered to be Newtonian and the viscosity of the soil-phase is modelled in line with the Mohr-Coulomb yield stress criterion. Water and soil particles interacting in a suspension layer are assigned to a viscosity that is derived from a Chezy-relation between the shear stress and the local flow velocity. A variable particle resolution strategy is applied to handle large domains, in which the areas around the ship hull demand a fine resolution. A complex full-scale application example included refers to the starting sequence of a container ship propeller.Copyright

Prediction of Roll Damping Using Viscous Flow Solvers

This article illustrates the use of a RANSE solver coupled to a motion solver to predict the free roll decay and the associated damping coefficients of floating bodies. The necessary building blocks to perform such a prediction are described briefly. A sensitivity study for the convergence criterion, the time step, the domain size and the grid resolution is performed for a simple 2-dimensional barge. The results are compared to results from experiments. Furthermore a simulation for a free roll decay of a Navy Combatant is performed, considering the results of the parameter study for the Barge. Overall results indicate that the natural roll frequency can be well predicted, while the prediction of the roll damping coefficients is afflicted with some uncertainties.Copyright

Computation of mechanically coupled bodies in a seaway

ABSTRACT The paper is concerned with the simulation of mechanically coupled bodies in seaway. While the applications of such cases are very wide, they are of particular interest for offshore operations, e.g. towing and boat landing. The focus of the present study is to supplement a viscous flow solver by appropriate mechanical models to analyse the hydrodynamics of coupled bodies due to mechanical joints. A quaternion-based motion modeller has been implemented using several basic joint elements to model their influence in a multi-body system. Examples included refer to rigid links, ropes, fenders or guide frames to restrict the motion in experiments, and aim to illustrate the predictive accuracy of the procedure and generic applications utilising all features of the computational framework including an overset grid technique.