Johan Roenby

• PhD
• Professor (Associate) at Roskilde University

Rivulets and droplets are naturally appearing shapes when small amounts of liquid are deposited on a partially wettable substrate. Here, we study, by means of numerical simulations, the dewetting dynamics of a ring-rivulet on substrates with various contact angles and wettability patterns. In particular, we consider, beyond the homogeneous case, an...

The offshore wind sector relies on floating foundations for deeper waters but faces challenges from harsh conditions, nonlinear dynamics, and low-frequency resonant motions caused by second-order hydrodynamic loads. We analyze these dynamics and extract higher harmonic motions for a semisubmersible floating foundation under extreme wave conditions...

Rivulets and droplets are naturally appearing shapes when small amounts of liquid are deposited on a partially wettable substrate. Here we study, by means of numerical simulations, the dewetting dynamics of a ring-rivulet on substrates with various contact angles and wettability patterns. In particular, we consider, beyond the homogeous case, an an...

In ocean environments, floating wind turbines are subject to both breaking waves and directional waves. While the response is often modelled through radiation-diffraction theory, the effects of higher-order wave loads are hereby limited to second order, and the damping often needs calibration against model tests. Better accuracy and full coverage o...

We present a non-iterative algorithm, FloatStepper, for coupling the motion of a rigid body and an incompressible fluid in computational fluid dynamics (CFD) simulations. The purpose of the algorithm is to remove the so-called added mass instability problem, which may arise when a light, floating body interacts with a heavy fluid. The idea underlyi...

We present a new OpenFOAM based open-source framework, TwoPhaseFlow, enabling fast implementation and testing of new phase change and surface tension force models for two-phase flows including interfacial heat and mass transfer. Capitalizing on the runtime-selection mechanism in OpenFOAM, the new models can easily be selected and benchmarked agains...

We present a non-iterative algorithm, FloatStepper, for coupling the motion of a rigid body and an incompressible fluid in computational fluid dynamics (CFD) simulations. The purpose of the algorithm is to remove the so-called added mass instability problem, which may arise when a light floating body interacts with a heavy fluid. The idea underlyin...

We consider the interfacial flow in and around porous structures in coastal and marine engineering.⋆ During recent years, interfacial flow through porous media has been repeatedly simulated with Computational Fluid Dynamics (CFD) based on algebraic Volume Of Fluid (VOF) methods [1, 2]. Here, we present an implementation of a porous medium interfaci...

We present a systematic study of geometric volume of fluid (VOF) methods provided in the gVOF and TwoPhaseFlow packages, which include algorithms that are among the most accurate proposed in recent years. In addition to contributing to their further validation, the main purpose is to evaluate, in terms of accuracy and efficiency, the relative advan...

We numerically study the coalescence dynamics of two sessile droplets with radii $R_0$. The droplets are placed on top of a rigid substrate with a contact angle of $\theta_{eq.} = \pi/9$. Having a highly wettable substrate ($\theta_{eq} \ll \pi/2$) theory predicts that the bridge height ($h_0$) scales according to $h_0(t) \sim t^{2/3}.$ This behavi...

We consider the interfacial flow in and around porous structures in coastal and marine engineering. During recent years, interfacial flow through porous media has been repeatedly simulated with Computational Fluid Dynamics (CFD) based on algebraic Volume Of Fluid (VOF) methods (Jensen et al., 2014; Higuera et al., 2014). Here, we present an impleme...

One of the prevailing challenges in Computational Fluid Dynamics is accurate simulation of two-phase flows involving heat and mass transfer across the fluid interface. This is currently an active field of research, which is to some extend impaired by a lack of a common programming framework for implementing and testing new models. Here we present a...

Free surfaces and fluid interfaces are encountered in a wide variety of gas-liquid configurations. Although many numerical approaches exist to solve such flows, there is still a need for improved simulation methods. Recently, a new efficient geometric VoF method for general meshes, called isoAdvector, was implemented in OpenFOAM®. More recently, th...

The performance of interFoam (a widely-used solver within the popular open source CFD package OpenFOAM) in simulating the propagation of a nonlinear (stream function solution) regular wave is investigated in this work, with the aim of systematically documenting its accuracy. It is demonstrated that over time there is a tendency for surface elevatio...

Simulations involving free surfaces and fluid interfaces are important in many areas of engineering. There is, however, still a need for improved simulation methods. Recently, a new efficient geometric VOF method called isoAdvector for general polyhedral meshes was published. We investigate the interface reconstruction step of isoAdvector, and demo...

In a recent publication, we presented a novel geometric VOF interface advection algorithm, denoted isoAdvector (Roenby et al. in R Soc Open Sci 3:160405 2016, [1]). The OpenFOAM\(^{\textregistered }\) implementation of the method was publicly released to allow for more accurate and efficient two-phase flow simulations in OpenFOAM\(^{\textregistered...

This paper presents the results of a numerical investigation of breaking waves over an artificial surf reef (ASR). In the surfing industry, it has become common to establish artificial surf reefs to enhance the surfability at popular surf locations, or to attract surfers to new locations. Besides enhancing the surfing quality, an ASR can also be se...

A computational fluid dynamics study is carried out on the inner nozzle flow and onset of liquid sheet instability in a large-scale pressure-swirl atomizer with asymmetric inflow configuration for high viscosity fluids. Large-eddy simulations (LES) of the two-phase flow indicate the unsteady flow character inside the nozzle and its influence on liq...

This paper presents a sharp free surface method for fully nonlinear flow of two immiscible phases for wave propagation problems in the Finite Volume framework. The method resolves a sharp interface between two phases by combining the geometric reconstruction Volume-of-Fluid scheme isoAdvector for accurate advection of the free surface with the Ghos...

The performance of interFoam (a widely-used solver within the popular open source CFD package OpenFOAM) in simulating the propagation of a nonlinear (stream function solution) regular wave is investigated in this work, with the aim of systematically documenting its accuracy. It is demonstrated that over time there is a tendency for surface elevatio...

Simulations involving free surfaces and fluid interfaces are highly important for many engineering subjects. There is, however, still a need for improved simulation methods. In this paper we introduce a novel computational interface reconstruction scheme based on calculation of a reconstructed distance function (RDF). By iterating over the RDF calc...

In a recent publication [1], we presented a novel geometric VOF algorithm , denoted isoAdvector. The OpenFOAM implementation of the method was publicly released[2] to allow more accurate and efficient multiphase flow simulations in OpenFOAM. In the present paper, we give a brief outline of the isoAd-vector method and test it with two pure advection...

Presentation of my work with isoAdvector, an accurate and efficient geometric VOF method for general meshes implemented in OpenFOAM. The slides show both practical applications of the method as well as some further developments. The original presentation contained a lot of videos. These are converted to links to YouTube videos in this version of th...

In this paper we present a new numerical modelling approach for coastal and marine applications where a porous media conceptual model was combined with a free surface volume-of-fluid (VOF) model and an immersed boundary method (IBM). The immersed boundary model covers the method of describing a solid object in a simple computational mesh without re...

To realise the full potential of Computational Fluid Dynamics (CFD) within marine science and engineering, there is a need for continuous maturing as well as verification and validation of the numerical methods used for free surface and interfacial flows. One of the distinguishing features here is the existence of a water surface undergoing large d...

We devise a numerical method for passive advection of a surface, such as the interface between two incompressible fluids, across a computational mesh. The method is called isoAdvector, and is developed for general meshes consisting of arbitrary polyhedral cells. The algorithm is based on the volume of fluid (VOF) idea of calculating the volume of o...

The open source finite volume based C++ code library OpenFOAM(R) can solve a variety of problems within computational continuum mechanics due to its tensorial approach and object-oriented nature. In this paper, we demonstrate how OpenFOAM(R) can be used as a platform for multi-physics simulations involving wave-seabed-structure interaction. Our pri...

Two new classes of analytical solutions for hollow vortex equilibria are presented. One class involves a central hollow vortex, comprising a constant pressure region having non-zero circulation, surrounded by an n-polygonal array of point vortices with . The solutions generalize the non-rotating polygonal point vortex configurations of Morikawa and...

The stability of offshore structures, such as wind turbine foundations, breakwaters, and immersed tunnels can be strongly affected by the liquefaction and cyclic mobility phenomena in the seabed. Our goal is to develop a numerical code for analysis of these situations. For this purpose, we start by formulating the strong interactions between soil s...

We consider interaction of a rigid body with a surrounding ideal fluid containing a number of point vortices. The fluid is assumed to be planar and unbounded and the body is assumed to be free to move in response to the fluid forces. Except that the body should be simply connected and rigid, no assumptions are made on the body shape or on its inter...

We study an idealized model of body–vortex interaction in two dimensions. The fluid is incompressible and inviscid and assumed to occupy the entire unbounded plane except for a simply connected region representing a rigid body. There may be a constant circulation around the body. The fluid also contains a finite number of point vortices of constant...

A number of problems related to particle trajectories in ideal 2D flows are discussed. Both regular particle paths, corresponding to integrable dynamics, and irregular or chaotic paths may arise. Examples of both types are shown. Sometimes, in the same flow, certain particles will follow regular paths while others follow irregular paths. Even in th...

We explore whether a rigid body moving freely with no circulation around it in a two-dimensional ideal fluid can carry a fluid ``atmosphere'' with it in its motion. Somewhat surprisingly, the answer appears to be ``yes.'' When the body is elongated and the motion is dominated by rotation, we demonstrate numerically that, indeed, regions of fluid fo...

The model of body—vortex interactions, where the fluid flow is planar, ideal and unbounded, and the vortex is a point vortex, is studied. The body may have a constant circulation around it. The governing equations for the general case of a freely moving body of arbitrary shape and mass density and an arbitrary number of point vortices are presented...

We explore the class of dynamical systems consisting of a rigid body and N point vortices in an ideal, unbounded, 2D fluid. The body is represented by a closed curve and is free to move in response to the fluid motion. It may have a prescribed circulation about it, which is conserved. The vortices have fixed strengths and are intended to model vort...

Auxiliary heating such as neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) will accelerate ions in ITER up to energies in the MeV range, i.e. energies which are also typical for alpha particles. Fast ions of any of these populations will elevate the collective Thomson scattering (CTS) signal for the proposed CTS diagnostic in...