Jens Honore Walther

Technical University of Denmark | DTU · Department of Mechanical Engineering

This paper presents the numerical results of combustion and soot recession in the Engine Combustion Network (ECN) Spray A and D flames using large eddy simulations (LES). The nominal injector nozzle diameters for the ECN Spray A and D are 90μm and 186μm. A two-equation soot model is implemented to model the soot formation and oxidation processes. T...

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. Convention...

Using molecular dynamic simulations, the performance of carbon nitride (C2N) was studied for the separation of Cu2+ and Hg2+ as heavy metal ions from water. The high porosity of the single layer C2N nanosheet in comparison with other similar carbon-based membranes and the existence of nitrogen atoms at the edge of the pores increase the efficiency...

The aim of this numerical study is to investigate the soot formation processes in a ducted fuel injection (DFI) spray under engine-like conditions. Furthermore, the effect of the pre-injection dwell period on the spray, combustion, and soot characteristics are also investigated. The DFI configuration considered here is D3L14G2. Large eddy simulatio...

In this study, large eddy simulations, coupled a two-equation soot model, are performed to investigate the effects of ambient carbon dioxide (CO2) and water (H2O) additions on the soot formation and oxidation processes in an n-dodecane spray flame. In the soot model, acetylene (C2H2) is soot precursor and surface growth species, while hydroxyl radi...

Stricter regulations imposed on emissions are motivating the scientific community to consider studying alternative fuels to achieve low emission, high efficient dual-fuel (DF) marine engines. In this context, three dimensional computational fluid dynamic (CFD) simulations are performed to study the combustion and emission formation under two-stroke...

Accurate estimation of the actual nominal wake behind the ship in waves is important for propeller designers when improving efficiency and minimizing cavitation. The present CFD study investigates the nominal wake fields of the Kriso Container Ship (KCS) in regular waves with wavelength equal to the ship length in five different headings. The ship...

This paper presents the numerical study of soot formation and oxidation processes across different ambient temperatures (900K, 1000K, and 1100K) and oxygen levels (15% and 21% O2) using large eddy simulation coupled with a two-equation soot model. The predicted ignition delay time, lift-off length and soot distribution show good agreements with the...

The formation and evaporation of nanodroplets in steam ejectors is neglected in many numerical simulations. We analyse the influence of a primary nozzle on steam ejector performances considering phase change processes. The numerical model is validated in detail against experimental data of supersonic nozzles and steam ejectors available in the lite...

Numerical simulations using large eddy simulation (LES) and Unsteady Reynolds Averaged Navier–Stokes (URANS) are carried out to identify the underlying mechanisms that govern the early soot evolution process in an n-dodecane spray flame at 21% O2 by molar concentration. A two-equation phenomenological soot model is used here to simulate soot format...

In the present study, conjugate heat transfer (CHT) calculations are applied in a computational fluid dynamics (CFD) simulation to simultaneously solve the in-cylinder gas phase dynamics and the temperature field within the liner of the engine. The effects of different initial temperatures with linear profiles across the liner are investigated on t...

Numerical simulations are performed to investigate the effects of ambient density (ρam) and nozzle diameter (Dnoz) on the ignition characteristic of diesel spray combustion under engine-like conditions. A total of nine cases which consist of different ρam of 14.8, 30.0, and 58.5 kg/m³ and different Dnoz of 100, 180, and 363 μm are considered. The r...

Turbulent reactive flows are ubiquitous in industrial processes. Decoupling transport effects from intrinsic chemical reactions requires an in-depth understanding of fluid flow physics; computational fluid dynamics (CFD) methods have been widely used for this purpose. Most CFD simulations of reactive liquid-phase flows, where the Schmidt numbers (S...

Railway ballast affected by heavy cyclic loading degrades and spreads resulting in an uncomfortable transportation caused by undesirable vibrations. Restoring a well sorted track ballast can be expensive. This paper analyzes track ballast deformation using the Discrete Element Method (DEM). The simulations are performed using the STAR-CCM+ software...

Choanoflagellates are unicellular microscopic organisms that are believed to be the closest living relatives of animals. They prey on bacteria through the act of the continuous beating of their flagellum, which generates a current through a crown-like filter. Subsequently, the filter retains bacterial particles from the suspension. The mechanism by...

Turbulence modelling plays an important role in the numerical prediction of nonequilibrium condensations in transonic flows. The present study evaluates the effect of four different turbulence models, namely, k - ε standard, RNG, realizable, and k - ω SST, on the condensation behaviour in transonic flows considering shock waves. The numerical simul...

A prediction method, known as the Coupled Time Scale (CTS) method, is proposed in the current work to estimate the ignition delay time (IDT) of liquid spray combustion by only performing an inert spray simulation and a zero-dimensional (0-D) homogeneous reactor (HR) simulation. The method is built upon the assumption that if the majority of the vap...

Numerical simulations are carried out to evaluate the performance of different reduced n-heptane mechanisms in dual-fuel combustion. Three reduced n-heptane mechanisms with a size <70 species are tested in the present study. The ignition characteristics of methane and n-heptane for these mechanisms are first evaluated by performing zero-dimensional...

Sponges are suspension feeders that filter vast amounts of water. Pumping is carried out by flagellated chambers that are connected to an inhalant and exhalant canal system. In ‘leucon’ sponges with relatively high-pressure resistance due to a complex and narrow canal system, pumping and filtering are only possible owing to the presence of a gasket...

In the present study, large-eddy simulations (LES) are used to identify the underlying mechanism that governs the ignition phenomena of spray flames from different nozzle diameters when the ambient temperature (Tam) varies. Two nozzle sizes of 90µm and 186µm are chosen. They correspond to the nozzle sizes used by Spray A and Spray D, respectively,...

Water flow enhancement in amorphous silica nanochannels coated with monolayer graphene - Enrique Wagemann, Diego Becerra, Jens H. Walther, Harvey A. Zambrano

This paper presents a comparison of full-scale computational fluid dynamics (CFD) simulations with speed trial measurements for a ro-ro vessel and a general cargo vessel. Significant work has been done on validating CFD simulation in model scale. However, in full-scale very few publicly available studies have been conducted due to limited access of...

Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying and quantifying impact characteristics that are difficult t...

Devices integrated by nanoconduits hold great potential for clinical and biochemical analysis due to amplified sensibility, faster response and increased portability. In nanoconduits, wherein the electrical double layer may occupy a considerable part of the channel, the hydrodynamics of multivalent electrolytes is highly influenced by interfacial e...

The present study investigated a new microchannel profile design encompassing condensate drainage slits for improved moisture removal with use of triangular shaped plain fins. Heat transfer and pressure drop correlations were developed using computational fluid dynamics (CFD) and defined in terms of Colburn j-factor and Fanning f-factor. The microc...

The pressure-gradient driven flow in a three-dimensional cross-corrugated channel is investigated based on large eddy simulations. The channel geometry is highly tortuous so that the flow unsteadiness can be triggered at a moderate Reynolds number. The objective of this paper is to provide a better understanding of the laminar-to-chaotic transition...

Nearly frictionless water transport makes carbon nanotubes promising materials for use as conduits in nanofluidic applications. Here, we conduct Molecular Dynamics simulations of water flow within amorphous silica nanopores coated by a (39,39) Single Walled Carbon Nanotube (SWCNT). Our atomistic models describe the interaction between water and por...

The directed transport of liquids at the nanoscale is of great importance for nanotechnology applications ranging from water filtration to the cooling of electronics and precision medicine. Here we demonstrate such unidirectional, pumpless transport of water nanodroplets on graphene sheets patterned with hydrophilic/phobic areas inspired by natural...

The mitigation of CO2 emissions is an effective measure to solve the climate change issue. In the present study, we propose an alternative approach for CO2 capture by employing supersonic flows. For this purpose, we first develop a computational fluid dynamics (CFD) model to predict the CO2 condensing flow in a supersonic nozzle. Adding two transpo...

This paper presents a comparison of towing tank testing, ship scale computational fluid dynamics (CFD) simulations, sea trial measurements and in-service performance. The study includes extensive convergence tests and validation of both resistance, open-water and self-propulsion CFD simulations in both model and ship scale. The self-propulsion CFD...

Near-well flow analysis is an important tool for gaining detailed insight of the flow behaviour and for improving well design and production optimization of real reservoirs. One challenge of accurate numerical modelling of the flow field in the vicinity of the well is related to the scale disparity factor in space and time. The numerical scale gap...

This work reports on numerical investigation of effects of ambient pressure (Pam) on spray combustion under engine-like conditions. Three cases with different Pam of 42, 85 and 170 bar at a fixed ambient temperature of 1000 K are considered. Zero-dimensional calculations are first performed for autoignition of stagnant adiabatic homogenous mixtures...

Choanoflagellates are unicellular eukaryotes that are ubiquitous in aquatic habitats. They have a single flagellum that creates a flow toward a collar filter composed of filter strands that extend from the cell. In one common group, the loricate choanoflagellates, the cell is suspended in an elaborate basket-like structure, the lorica, the function...

Leuconoid sponges are filter-feeders with a complex system of branching inhalant and exhalant canals leading to and from the close-packed choano- cyte 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 c...

This poster presents a droplet of water after impacting on a metallic mesh. The model is using an enhanced VOF-based numerical code where a smoothing approach that filters out spurious current velocities as well as Kistler's dynamic contact angle treatment to accurately account for surface wettability effects have been implemented within the genera...

Near-well flow analysis is an important tool for gaining detailed insight of the flow behaviour and for improving well design and production optimization of real reservoirs. One challenge of accurate numerical modelling of the flow field in the vicinity of the well is related to the scale disparity factor in space and time. The numerical scale gap...

In this paper, we derive the non-singular Green's functions for the unbounded Poisson equation in one, two and three dimensions using a spectral cut-off function approach to impose a minimum length scale in the homogeneous solution. The resulting non-singular Green's functions are relevant to applications which are restricted to a minimum resolved...

A prescribed velocity‐vorticity boundary layer model for the vorticity transport equation is proposed, which corrects the unphysical upward deflection of the wake seen in a simpler prescribed velocity shear approach. A Lagrangian implementation of the boundary layer model has been investigated using our in‐house vortex solver MIRAS. The MIRAS code...

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 f...

Development of functional nanofluidic devices requires understanding the fundamentals of capillary driven flow in nanochannels. In this context, we conduct molecular dynamics simulations of water capillary imbibition in silica nanoslits under externally applied electric (E) fields with strengths between 0 to 1 V/nm. For increasing E-fields, we obse...

In the present study, three-dimensional (3D) computational fluid dynamics simulations are performed to examine the process of sulfuric acid (H2SO4) and water (H2O) condensation in a large two-stroke marine diesel engine. A skeletal n-heptane chemical mechanism is coupled with a sulfur (S) subset to simulate the combustion process as well as the for...

Thermophoresis is the phenomenon wherein particles experience a net drift induced by a thermal gradient. In this work, molecular dynamics simulations are conducted to study with atomistic detail the thermophoresis of water nanodroplets inside carbon nanotubes (CNTs) and its interplay with the retarding liquid-solid friction. Different applied tempe...

DNA molecules need to wait for entry into the nanopores of sequencers if the molecular length is larger than the pore width. Brownian motion explores diverse configurations randomly and some of them let the molecule pass through the nanopore. On the other hand, ink does not change its color when flowing through a pen where a constriction exists bec...

Regularized Green's functions for mixed unbounded-periodic domains are derived. The regularization of the Green's function removes its singularity by introducing a regularization radius which is related to the discretization length and hence imposes a minimum resolved scale. In this way the regularized unbounded-periodic Green's functions can be im...

The present numerical study aims to assess the performance of an Eulerian Stochastic Field (ESF) model in simulating spray flames produced by three fuel injectors with different nozzle diameters of 100 µm, 180 µm and 363 µm. A comparison to the measurements shows that although the simulated ignition delay times are consistently overestimated, the r...

Transport of water in hydrophilic nanopores is of significant technological and scientific interest. Water flow through hydrophilic nanochannels is known to experience enormous hydraulic resistance. Therefore, drag reduction is essential for the development of highly efficient nanofluidic devices. In this work, we propose the use of graphitic mater...

The development of efficient nanofluidic devices requires driving mechanisms that provide controlled transport of fluids through nanoconduits. Temperature gradients have been proposed as a mechanism to drive particles, fullerenes and nanodroplets inside carbon nanotubes (CNTs). In this work, molecular dynamics (MD) simulations are conducted to stud...

This paper reports the implementation and evaluation of a Lagrangian soot tracking (LST) method for the modeling of soot in diesel engines. The LST model employed here has the tracking capability of a Lagrangian method and the ability to predict primary soot particle sizing. The Moss-Brookes soot model is used here as the Eulerian method to simulat...

The oil production by horizontal wells is a complex phenomenon that involves flow through the porous reservoir, completion interface and the well itself. Conventional reservoir simulators can hardly resolve the flow through the completion into the wellbore. On the contrary, Computational Fluid Dynamics (CFD) is capable of modeling the complex inter...

Thermal Brownian Motors (TBMs) are nanoscale machines that exploit thermal fluctuations to provide useful work. We introduce a TBM-based nanopump which enables continuous water flow through a Carbon Nanotube (CNT) by imposing an axial thermal gradient along its surface. We impose spatial asymmetry along the CNT by immobilizing certain points on its...

It is well known that translocation events of a polymer or rod through pores or narrower parts of micro- and nanochannels have a stochastic nature due to the Brownian motion. However, it is not clear whether the objects of interest need to have a larger size than the entrance to exhibit the deviation from the dynamics of the surrounding fluid. We s...

Significance Microbes 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 multic...

The sand shot in the DISAMATIC process is simulated by the discrete element method (DEM) taking into account the influence and coupling of the airflow with computational fluid dynamics (CFD). The DEM model is calibrated by a ring shear test, a sand pile experiment and a slump test. Subsequently, the DEM model is used to model the propagation of the...

A hybrid filament-mesh vortex method is proposed and validated to predict the aerodynamic performance of wind turbine rotors and to simulate the resulting wake. Its novelty consists of using a hybrid method to accurately simulate the wake downstream of the wind turbine while reducing the computational time used by the method. The proposed method us...

In this work, three-dimensional computational fluid dynamics (CFD) studies of sulphur oxides (SOx) and sulphuric acid (H2SO4) formation processes in a large, low speed two-stroke marine diesel engine are carried out. The current numerical study aims to investigate the conversion of sulphuric dioxide (SO2) to sulphuric trioxide (SO3) and the possibi...

In the petroleum industry, water-and-gas breakthrough in hydrocarbon reservoirs is a common issue that eventually leads to uneconomic production. To extend the economic production lifetime, inflow-control devices (ICDs) are designed to delay the water-and-gas breakthrough. Because the lifetime of a hydrocarbon reservoir commonly exceeds 20 years an...

A hydrodynamic journal bearing has been investigated using both the traditional two-dimensional (2D) Reynolds equation, and the full solution being the three-dimensional (3D) Navier-Stokes equations. The two approaches are compared by performing an investigation of two inlet groove designs: the axial and the circumferential groove, respectively, on...

In this paper, we derive the non-singular Green's functions for the unbounded Poisson equation in two and three dimensions using a spectral approach to regularize the homogeneous equation. The resulting Green's functions are relevant to applications which are restricted to a minimum resolved length scale (e.g. a mesh size h) and thus cannot handle...

Graphene has attracted considerable attention due to its characteristics as a 2D material and its fascinating properties, providing a potential building block for nanofabrication. In nanochannels the solid-liquid interface plays a non-negligible role in determining the fluid dynamics. Therefore, for an optimal design of nanofluidic devices, a compr...

We present a stochastic turbulence generator based on a vorticity formulation where the generated turbulent field implicitly fulfills the kinematic constraints of an incompressible flow. The generator allows direct access to the turbulent velocity and vorticity field. Enforcing additional constraints such as a divergence-free vorticity field and a...

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 wit...

The condensation phenomenon of vapor plays an important role in various industries, such as the steam flow in turbines and refrigeration system. A mathematical model is developed to predict the spontaneous condensing phenomenon in the supersonic flows using the nucleation and droplet growth theories. The numerical approach is validated with the exp...

The present work presents numerical simulations of the complex particle motion in a supersonic separator with a delta wing located in the supersonic flow. The effects of the delta wing on the strong swirling flow is analysed using the Discrete Particle Method. The results show that the delta wings re-compress the upstream flow and the gas Mach numb...

In capillary imbibition, the classical Lucas-Washburn equation predicts a singularity as the fluid enters the channel consisting in an anomalous infinite velocity of the capillary meniscus. The Bosanquet’s equation overcomes this problem by taking into account fluid inertia predicting an initial imbibition regime with constant velocity. Nevertheles...