Papers by Jens Honore Walther
The Nordic Seminar on Computational Mechanics, 2010
We study the effect of piston motion on the in-cylinder swirling flow in a low speed, large two-s... more We study the effect of piston motion on the in-cylinder swirling flow in a low speed, large two-stroke marine diesel engine. The work involves experimental, and numerical simulation using OpenFOAM platform, Large Eddy Simulation was used with three different models, One equation Eddy, Dynamic One equation Eddy, and Ta Phouc Loc model, to study the transient phenomena of the flow. The results are conducted at six cross sectional planes along the axis of the cylinder and with the piston displaced at four fixed piston positions covering the air intake ports by 0%,25%, 50%, and 75% respectively, for the fully opened case LES model with 8/12 million mesh points were used. We find that the flow inside the cylinder changes as the ports are closing, from a Rankine/Burger vortex profile to a solid body rotation while the axial velocity profiles change from a wake-like to a jet-like profile.
International Journal of Numerical Methods for Heat & Fluid Flow, Feb 25, 2014
Purpose-the purpose of this paper is to study the effect of piston position on the in-cylinder sw... more Purpose-the purpose of this paper is to study the effect of piston position on the in-cylinder swirling flow in a simplified model of a large two-stroke marine diesel engine. Design/Methodology/Approach-Large Eddy Simulations with four different models for the turbulent flow are used: a one-equation model, a dynamic one-equation model, a localized dynamic one-equation model and a mixed-scale model. Simulations are carried out for two different geometries corresponding to 100 % and 50 % open scavenge ports. Findings-It is found that the mean tangential profile inside the cylinder changes qualitatively with port closure from a Lamb-Oseen vortex profile to a solid body rotation while the axial velocity changes from a wake-like profile to a jet-like profile. The numerical results are compared with particle image velocimetry measurements (?) and in general we find a good agreement. Limitations/implications-Considering the complexity of the real engine, we designed the engine model using the simplest configuration possible. The setup contains no moving parts, the combustion is neglected and the exhaust valve is discarded. Originality/value-Studying the flow in a simplified engine model, the setup allows studies of fundamental aspects of swirling flow in a uniform scavenged engine. Comparing the four turbulence models, the local dynamic one-equation model is found to give the best agreement with the experimental results.
Journal of Fluid Mechanics, Dec 28, 2016
We present a combined numerical (particle vortex method) and experimental (soap film tunnel) stud... more We present a combined numerical (particle vortex method) and experimental (soap film tunnel) study of a symmetric foil undergoing prescribed oscillations in a two-dimensional free stream. We explore pure pitching and pure heaving and contrast these two generic types of kinematics. We compare measurements and simulations when the foil is forced with pitching oscillations, and we find a close correspondence between flow visualisations using thickness variations in the soap film and the numerically determined vortex structures. Numerically we determine wake maps spanned by oscillation frequency and amplitude, and we find qualitatively similar maps for pitching and heaving. We determine the drag-thrust transition for both pitching and heaving numerically, and we discuss it in relation to changes in wake structure. For heaving with low oscillation frequency and high amplitude we find that the drag-thrust transition occurs in a parameter region with wakes in which two vortex pairs are formed per oscillation period in contrast to the common transition scenario in regions with inverted von Kármán wakes.
Experiments in Fluids, Mar 1, 2013
The turbulent and swirling flow of a uniflowscavenged two-stroke engine cylinder is investigated ... more The turbulent and swirling flow of a uniflowscavenged two-stroke engine cylinder is investigated using a scale model with a static geometry and a transparent cylinder. The swirl is generated by 30 equally spaced ports with angles of 0°, 10°, 20°, and 30°. A detailed characterization of the flow field is performed using stereoscopic particle image velocimetry. Mean fields are calculated using both a fixed coordinate system and a coordinate system based on the instantaneous flow topology. Timeresolved measurements of axial velocity are performed with laser Doppler anemometry, and power spectra are calculated in order to determine vortex core precession frequencies. The results show a very different flow dynamics for cases with weak and strong swirl. In the strongly swirling cases, a vortex breakdown is observed. Downstream of the breakdown, the vortex becomes highly concentrated and the vortex core precesses around the exhaust valve, resulting in an axial suction effect at the vortex center. Mean fields based on the instantaneous flow topology are shown to be more representative than mean fields based on a fixed coordinate system in cases with significant variations in the swirl center location.
Journal of Computational Physics, May 1, 2017
We present a Brinkman penalization method for three-dimensional (3D) flows using particle vortex ... more We present a Brinkman penalization method for three-dimensional (3D) flows using particle vortex methods, improving the existing technique by means of an iterative process. We perform simulations to study the impulsively started flow past a sphere at Re = 1000 and normal to a circular disc at Re = 500. The simulation results obtained for the flow past a sphere are found in qualitative good agreement with previously published results obtained using respectively a 3D vortex penalization method and a 3D vortex method combined with an accurate boundary element method. From the results obtained for the flow normal to a circular disc it is found that the iterative method enables the use of a time step that is one order of magnitude larger than required by the standard non-iterative Brinkman penalization method.
Physical Review Fluids
The important role of unicellular flagellated micro-organisms in aquatic food webs is mediated by... more The important role of unicellular flagellated micro-organisms in aquatic food webs is mediated by their flagella, which enable them to swim and generate feeding currents. The flagellum in many predatory flagellates is equipped with hairs (mastigonemes) that reverse the direction of thrust compared to the thrust due to a smooth flagellum. Conventionally, the mechanism of such reversal has been attributed to drag-based thrust of individual hairs, neglecting their hydrodynamic interactions. However, at natural densities of hairs, hydrodynamic interactions are important. In fact, using fully resolved three-dimensional computational fluid dynamics, we show here that hydrodynamic interactions are key to thrust-generation and reversal in hairy flagellates, making their hydrodynamics fundamentally different from the slender-body theory governing smooth flagella. We reveal the significant role of the curvature of the flagellum, and using model analysis we demonstrate that strongly curved flagellar waveforms are optimal for thrust-generation. Our results form a basis for understanding the diverse flagellar architectures and feeding modes of predatory flagellates.
The important role of flagellates in aquatic microbial food webs is mediated by their flagella th... more The important role of flagellates in aquatic microbial food webs is mediated by their flagella that enable them to swim and generate a feeding current. The flagellum in most predatory flagellates is equipped with rigid hairs that reverse the direction of thrust compared to the thrust due to a smooth flagellum. Conventionally, such reversal has been attributed to drag anisotropy of individual hairs, neglecting their hydrodynamic interactions. Here, we show that hydrodynamic interactions are key to thrust-generation and reversal in hairy flagellates, making their hydrodynamics fundamentally different from the slender-body theory governing microswimmers with smooth flagella. Using computational fluid dynamics and model analysis, we demonstrate that long and not too closely spaced hairs and strongly curved flagellar waveforms are optimal for thrust-generation. Our results form a theoretical basis for understanding the diverse flagellar architectures and feeding modes found in predatory ...
Journal of Computational Physics, 2019
We present an implicit boundary particle method with background mesh adaptation. We use a Brinkma... more We present an implicit boundary particle method with background mesh adaptation. We use a Brinkman penalisation to represent the boundary of the domain and a remeshed particle method to simulate viscous flow with high Reynolds numbers. A penalty term is added to the Navier-Stokes equations to impose the boundary conditions. The boundary conditions are enforced to a specific precision with no need to modify the numerical method or change the grid, achieving an implicit approach for flow around complex boundaries/geometries simulation. The main idea of the Brinkman penalisation method is to model the solid obstacle as a porous medium. The governing equations for the compressible fluid
Physical review. E, Statistical, nonlinear, and soft matter physics, 2010
We study shear-induced instabilities of lipid bilayers immersed in water using coarse-grained mol... more We study shear-induced instabilities of lipid bilayers immersed in water using coarse-grained molecular dynamics simulations. The shear imposed by the flow of the water induces initially microscopic structural changes of the membrane, starting with tilting of the molecules in the direction of the shear. The tilting propagates in the spanwise direction when the shear rate exceeds a critical value and the membrane undergoes a bucklinglike deformation in the direction perpendicular to the shear. The bucklinglike undulation continues until a localized Kelvin-Helmholtz-like instability leads to membrane rupture. We study the different modes of membrane undulation using membranes of different geometries and quantify the relative importance of the bucklinglike bending and the Kelvin-Helmholtz-like instability of the membrane.
Applied Energy, 2022
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Combustion and Flame, 2018
This paper aims to simulate diesel spray flames across a wide range of engine-like conditions usi... more This paper aims to simulate diesel spray flames across a wide range of engine-like conditions using the Eulerian Stochastic Field probability density function (ESF-PDF) model. The ESF model is coupled with the Chemistry Coordinate Mapping approach to expedite the calculation. A convergence study is carried out for a number of stochastic fields at five different conditions, covering both conventional diesel combustion and low-temperature combustion regimes. Ignition delay time, flame lift-off length as well as distributions of temperature and various combustion products are used to evaluate the performance of the model. The peak values of these properties generated using thirty-two stochastic fields are found to converge, with a maximum relative difference of 27% as compared to those from a greater number of stochastic fields. The ESF-PDF model with thirty-two stochastic fields performs reasonably well in reproducing the experimental *Manuscript Click here to view linked References 2 flame development, ignition delay times and lift-off lengths. The ESF-PDF model also predicts a broader hydroxyl radical distribution which resembles the experimental observation, indicating that the turbulence-chemistry interaction is captured by the ESF-PDF model. The validated model is subsequently used to investigate the flame structures under different conditions. Analyses based on flame index and formaldehyde distribution suggest that a triple flame, which consists of a rich premixed flame, a diffusion flame and a lean premixed flame, is established in the earlier stage of the combustion. As the combustion progresses, the lean premixed flame weakens and diminishes with time. Eventually, only a double-flame structure, made up of the diffusion flame and the rich premixed flame, is observed. The analyses for various ambient temperatures show that the tripleflame structure remains for a longer period of time in cases with lower ambient temperatures. The present study shows that the ESF-PDF method is a valuable alternative to Lagrangian particle PDF methods.
Journal of The Royal Society Interface, 2019
Leuconoid sponges are filter-feeders with a complex system of branching inhalant and exhalant can... more Leuconoid sponges are filter-feeders with a complex system of branching inhalant and exhalant canals leading to and from the close-packed choanocyte chambers. Each of these choanocyte chambers holds many choanocytes that act as pumping units delivering the relatively high pressure rise needed to overcome the system pressure losses in canals and constrictions. Here, we test the hypothesis that, in order to deliver the high pressures observed, each choanocyte operates as a leaky, positive displacement-type pump owing to the interaction between its beating flagellar vane and the collar, open at the base for inflow but sealed above. The leaking backflow is caused by small gaps between the vaned flagellum and the collar. The choanocyte pumps act in parallel, each delivering the same high pressure, because low-pressure and high-pressure zones in the choanocyte chamber are separated by a seal (secondary reticulum). A simple analytical model is derived for the pump characteristic, and by im...
Proceedings of the National Academy of Sciences, 2017
SignificanceMicrobes compose the majority of life in aquatic ecosystems and are crucial to the tr... more SignificanceMicrobes compose the majority of life in aquatic ecosystems and are crucial to the transfer of energy to higher trophic levels and to global biogeochemical cycles. They have evolved different foraging mechanisms of which our understanding is poor. Here, we demonstrate for filter-feeding choanoflagellates—the closest relatives to multicellular life—how the observed feeding flow is inconsistent with hydrodynamic theory based on the current understanding of the morphology. Instead, we argue for the widespread presence of flagellar vanes and suggest an alternative pumping mechanism. We also demonstrate a trade-off in filter spacing that allows us to predict choanoflagellate prey sizes. These mechanistic insights are important to correctly understand and model microbial heterotrophs in marine food webs.
Advanced Functional Materials, 2015
IUTAM Symposium on Advances in Micro- and Nanofluidics, 2009
We propose control algorithms to enhance the efficiency of a hybrid model coupling continuum and ... more We propose control algorithms to enhance the efficiency of a hybrid model coupling continuum and atomistic descriptions of dense liquids. Time and length scales are decoupled by using an iterative Schwarz domain decomposition algorithm. In this algorithm, the lack of periodic boundary conditions in the MD simulations leads to spurious density fluctuations at the continuum-atomistic interface. We remedy this problem by using an external boundary force determined by a simple control algorithm that acts to cancel the density fluctuations. The conceptual and algorithmic simplicity of the method makes it suitable for any type of coupling between atomistic, mesoscopic and continuum descriptions of dense liquids.
Scientific Reports, 2015
Rough surfaces immersed under water remain practically dry if the liquid-solid contact is on roug... more Rough surfaces immersed under water remain practically dry if the liquid-solid contact is on roughness peaks, while the roughness valleys are filled with gas. Mechanisms that prevent water from invading the valleys are well studied. However, to remain practically dry under water, additional mechanisms need consideration. This is because trapped gas (e.g. air) in the roughness valleys can dissolve into the water pool, leading to invasion. Additionally, water vapor can also occupy the roughness valleys of immersed surfaces. If water vapor condenses, that too leads to invasion. These effects have not been investigated and are critically important to maintain surfaces dry under water. In this work, we identify the critical roughness scale, below which it is possible to sustain the vapor phase of water and/or trapped gases in roughness valleys – thus keeping the immersed surface dry. Theoretical predictions are consistent with molecular dynamics simulations and experiments.
The European Physical Journal Special Topics, 2015
We present continuum simulations of water flow past fullerene molecules. The governing Navier-Sto... more We present continuum simulations of water flow past fullerene molecules. The governing Navier-Stokes equations are complemented with the Navier slip boundary condition with a slip length that is extracted from related molecular dynamics simulations. We find that several quantities of interest as computed by the present model are in good agreement with results from atomistic and atomistic-continuum simulations at a fraction of the cost. We simulate the flow past a single fullerene and an array of fullerenes and demonstrate that such nanoscale flows can be computed efficiently by continuum flow solvers, allowing for investigations into spatiotemporal scales inaccessible to atomistic simulations.
New Journal of Physics, 2014
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Journal of Computational Physics, 2012
Computers & Mathematics with Applications, 2010
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit:
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Papers by Jens Honore Walther