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• 1.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Multirelaxation-time lattice Boltzmann model for droplet heating and evaporation under forced convection2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 91, no 4, 043012Article in journal (Refereed)

We investigate the evaporation of a droplet surrounded by superheated vapor with relative motion between phases. The evaporating droplet is a challenging process, as one must take into account the transport of mass, momentum, and heat. Here a lattice Boltzmann method is employed where phase change is controlled by a nonideal equation of state. First, numerical simulations are compared to the D-2 law for a vaporizing static droplet and good agreement is observed. Results are then presented for a droplet in a Lagrangian frame under a superheated vapor flow. Evaporation is described in terms of the temperature difference between liquid-vapor and the inertial forces. The internal liquid circulation driven by surface-shear stresses due to convection enhances the evaporation rate. Numerical simulations demonstrate that for higher Reynolds numbers, the dynamics of vaporization flux can be significantly affected, which may cause an oscillatory behavior on the droplet evaporation. The droplet-wake interaction and local mass flux are discussed in detail.

• 2.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Simulation of a suspended droplet under evaporation with Marangoni effects2016In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 91, 853-860 p.Article in journal (Refereed)

We investigate the Marangoni effects in a hexane droplet under evaporation and close to its critical point. A lattice Boltzmann model is used to perform 3D numerical simulations. In a first case, the droplet is placed in its own vapor and a temperature gradient is imposed. The droplet locomotion through the domain is observed, where the temperature differences across the surface is proportional to the droplet velocity and the Marangoni effect is confirmed. The droplet is then set under a forced convection condition. The results show that the Marangoni stresses play a major role in maintaining the internal circulation when the superheated vapor temperature is increased. Surprisingly, surface tension variations along the interface due to temperature change may affect heat transfer and internal circulation even for low Weber number. Other results and considerations regarding the droplet surface are also discussed.

• 3.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Lattice Boltzmann Method for the evaporation of a suspended droplet2013In: Interfacial phenomena and heat transfer, ISSN 2167-857X, Vol. 1, 245-258 p.Article in journal (Refereed)

In this paper we consider a thermal multiphase lattice Boltzmann method (LBM) to investigate the heating and vaporization of a suspended droplet. An important benefit from the LBM is that phase separation is generated spontaneously and jump conditions for heat and mass transfer are not imposed. We use double distribution functions in order to solve for momentum and energy equations. The force is incorporated via the exact difference method (EDM) scheme where different equations of state (EOS) are used, including the Peng-Robinson EOS. The equilibrium and boundary conditions are carefully studied. Results are presented for a hexane droplet set to evaporate in a superheated gas, for static condition and under gravitational effects. For the static droplet, the numerical simulations show that capillary pressure and the cooling effect at the interface play a major role. When the droplet is convected due to the gravitational field, the relative motion between the droplet and surrounding gas enhances the heat transfer. Evolution of density and temperature fields are illustrated in details.

• 4.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Thermodynamics of a real fluid near the critical point in numerical simulations of isotropic turbulence2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 12, 125105Article in journal (Refereed)

We investigate the behavior of a fluid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent flow with an ideal gas. The primary focus of this work is to analyze fluctuations of thermodynamic variables (pressure, density, and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. Fluctuations in pressure, density, and temperature increase, followed by changes in their respective probability density functions. Due to the non-linearity of the EOS, it is seen that variances of density and temperature and their respective covariance are equally important close to the critical point. Unlike the ideal EOS case, significant differences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature, and kinetic energy.

• 5.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. University of Washington, USA. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Droplet deformation and heat transfer in isotropic turbulence2016Manuscript (preprint) (Other academic)

The heat and mass transfer of deformable droplets in turbulent flows is crucial to a wide range of applications, such as cloud dynamics and internal combustion engines. This study investigates a droplet undergoing phase change in isotropic turbulence using numerical simulations with a hybrid lattice Boltzmann scheme. We solve the momentum and energy transport equations, where phase separation is controlled by a non-ideal equation of state and density contrast is taken into consideration. Deformation is caused by pressure and shear stress at the droplet interface. The statistics of thermodynamic variables is quantified and averaged in terms of the liquid and vapor phases. The occurrence of evaporation and condensation is correlated to temperature fluctuations, surface tension variation and turbulence intensity. The temporal spectra of droplet deformations are analyzed and related to the droplet surface area.Different modes of oscillation are clearly identified from the deformation power spectrum for low Taylor Reynolds number $Re_\lambda$, whereas nonlinearities are produced with the increase of $Re_\lambda$, as intermediate frequencies are seen to overlap. As an outcome a continuous spectrum is observed, which shows a decrease that scales as $\sim f^{-3}$.Correlations between the droplet Weber number, deformation parameter, fluctuations of the droplet volume and thermodynamic variables are also examined.

• 6.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. University of Washington, USA. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Real fluids near the critical point in isotropic turbulenceIn: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666Article in journal (Refereed)

We investigate the behavior of a uid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent ow with an ideal gas. The primary focus of this work is to analyze uctuations of thermodynamic variables (pressure, density and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. These phenomena include increased uctuations in pressure, density and temperature, followed by changes in their respective probability density functions (PDFs). Unlike the ideal EOS case, signicant dierences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature and kinetic energy.

• 7.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Thermocapillary convection and phase change in welding2008In: International journal of numerical methods for heat & fluid flow, ISSN 0961-5539, E-ISSN 1758-6585, Vol. 18, no 3-4, 378-386 p.Article in journal (Refereed)

Purpose - In welding there is an intricate coupling between the composition of the material and the shape and depth of the weld pool. In certain materials, the weld pool may not penetrate the material easily, so that it is difficult or impossible to weld, while other seemingly quite similar materials may be well suited for welding. This is due to the convective heat transfer in the melt where the flow is driven primarily by surface tension gradients. This paper aims to study how surface active agents affect the flow and thus the welding properties by surveying some recent 3D simulations of weld pools. Design/methodology/approach - Some basic concepts in the modelling of flow in a weld pool are reviewed. The mathematical models for a convecting melt, with a detailed model for the surface tension and the Marangoni stress in the presence of surfactants, are presented. The effect of the sign of the Marangoni coefficient on the flow pattern, and thus, via melting and freezing, on the shape of the weld pool, is discussed. Findings - It is seen that it is beneficial to have surfactants present at the pool surface, in order to have good penetration. Results from a refined surface tension model that accounts for non-equilibrium redistribution of surfactants are presented. It is seen that the surfactant concentration is significantly modified by the fluid flow. Thereby, the effective surface tension and the Marangoni stresses are altered, and the redistribution of surfactants will affect the penetration depth of the weld pool. Originality/value - The importance of surfactants for weld pool shapes, and in particular the convective redistribution of surfactants, is clarified.

• 8. Boyanova, P.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Block-preconditioners for conforming and non-conforming FEM discretizations of the Cahn-Hilliard equation2012In: Large-Scale Scientific Computing, Springer Science+Business Media B.V., 2012, Vol. 7116 LNCS, 549-557 p.Conference paper (Refereed)

We consider preconditioned iterative solution methods to solve the algebraic systems of equations arising from finite element discretizations of multiphase flow problems, based on the phase-field model. The aim is to solve coupled physics problems, where both diffusive and convective processes take place simultaneously in time and space. To model the above, a coupled system of partial differential equations has to be solved, consisting of the Cahn-Hilliard equation to describe the diffusive interface and the time-dependent Navier-Stokes equation, to follow the evolution of the convection field in time. We focus on the construction and efficiency of preconditioned iterative solution methods for the linear systems, arising after conforming and non-conforming finite element discretizations of the Cahn-Hilliard equation in space and implicit discretization schemes in time. The non-linearity of the phase-separation process is treated by Newton's method. The resulting matrices admit a two-by-two block structure, utilized by the preconditioning techniques, proposed in the current work. We discuss approximation estimates of the preconditioners and include numerical experiments to illustrate their behaviour.

• 9.
Department of Information Technology, Uppsala University.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. Department of Information Technology, Uppsala University.
Cahn-Hilliard finite elements non-conforming Crouzeix-Raviart two-by-two block matrices preconditioning Schur complement2011Report (Other (popular science, discussion, etc.))

In this work we consider preconditioned iterative solution methods for numerical simulations of multiphase flow problems, modelled by the Cahn-Hilliard equation. We focus on diphasic flows and the construction and efficiency of a preconditioner for the algebraic systems arising from finite element discretizations in space and the theta-method in time. The preconditioner utilizes to a full extent the algebraic structure of the underlying matrices and exhibits optimal convergence and computational complexity properties. Large scale umerical experiments are included as well as performance comparisons with other solution methods.

• 10.
Department of Information Technology, Uppsala University.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Department of Information Technology, Uppsala University.
Efficient Preconditioners for Large Scale Binary Cahn-Hilliard Models2012In: Computational Methods in Applied Mathematics, ISSN 1609-4840, E-ISSN 1609-9389, Vol. 12, no 1, 1-22 p.Article in journal (Refereed)

In this work we consider preconditioned iterative solution methods for numerical simulations of multiphase flow problems, modelled by the Cahn-Hilliard equation. We focus on diphasic flows and the construction and efficiency of a preconditioner for the algebraic systems arising from finite element discretizations in space and the method in time. The preconditioner utilises to a full extent the algebraic structure of the underlying matrices and exhibits optimal convergence and computational complexity properties. Various numerical experiments, including large scale examples, are presented as well as performance compar- isons with other solution methods.

• 11.
Department of Information Technology, Uppsala University.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. Department of Information Technology, Uppsala University.
Solution Methods for the Cahn-Hilliard Equation Discretized by Conforming and Non-Conforming Finite Elements2011Report (Other (popular science, discussion, etc.))

In this work we consider preconditioned iterative solution methods for numerical simulations of multiphase flow problems, modelled by the Cahn-Hilliard equation. We focus on the construction and efficiency of various preconditioning techniques and the effect of two discretization methods - conforming and non-conforming finite elements spaces - on those techniques.

• 12.
KTH, School of Electrical Engineering (EES), Microsystem Technology (Changed name 20121201).
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Electrical Engineering (EES), Microsystem Technology (Changed name 20121201). KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Electrical Engineering (EES), Microsystem Technology (Changed name 20121201).
Continuous flow switching by pneumatic actuation of the air lubrication layer on superhydrophobic microchannel walls2008In: 21st IEEE International Conference on Micro Electro Mechanical Systems (IEEE MEMS 2008), IEEE conference proceedings, 2008, 599-602 p.Conference paper (Refereed)

This paper introduces and experimentally verifies a method for robust, active control of friction reduction in microchannels, enabling new flow control applications and overcoming previous limitations with regard to sustainable liquid pressure. The air pockets trapped at a

superhydrophobic micrograting during liquid priming are coupled to an actively controlled pressure source, allowing the pressure difference over the air/liquid interface to be dynamically adjusted. This allows for manipulating the friction reduction properties of the surface, enabling active control of liquid mass flow through the channel. It also permits for sustainable air lubrication at theoretically unlimited liquid pressures, without loss of superhydrophobic properties. With the non-optimized grating used in the experiment, a difference in liquid mass flow of 4.8 % is obtained by alternatively collapsing and recreating the air pockets using the coupled pressure source, which is in line with a FE analysis of the same geometry. A FE analysis of a more optimized geometry predicts a mass flow change of over 30%, which would make possible new microfluidic devices based on local friction control. It is also experimentally shown that our method allows for sustainable liquid pressure 3 times higher than the Laplace pressure of a passive device.

• 13.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Dissipation in rapid dynamic wetting2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 682, 213-240 p.Article in journal (Refereed)

In this article, we present a modelling approach for rapid dynamic wetting based on the phase field theory. We show that in order to model this accurately, it is important to allow for a non-equilibrium wetting boundary condition. Using a condition of this type, we obtain a direct match with experimental results reported in the literature for rapid spreading of liquid droplets on dry surfaces. By extracting the dissipation of energy and the rate of change of kinetic energy in the flow simulation, we identify a new wetting regime during the rapid phase of spreading. This is characterized by the main dissipation to be due to a re-organization of molecules at the contact line, in a diffusive or active process. This regime serves as an addition to the other wetting regimes that have previously been reported in the literature.

• 14.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Droplet dynamics in a bifurcating channel2010In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 36, no 5, 397-405 p.Article in journal (Refereed)

In the present paper we present a phenomenological description of droplet dynamics in a bifurcating channel that is based on three-dimensional numerical experiments using the Phase Field theory. Droplet dynamics is investigated in a junction, which has symmetric outflow conditions in its daughter branches. We identify two different flow regimes as the droplets interact with the tip of the bifurcation, splitting and non-splitting. A distinct criterion for the flow regime transition is found based on the initial droplet volume and the Capillary (Ca) number. The Rayleigh Plateau instability is identified as a driving mechanism for the droplet breakup close to the threshold between the splitting and non-splitting regime.

• 15.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Modeling of dynamic wetting far from equilibrium2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 12Article in journal (Refereed)

In this paper we present simulations of dynamic wetting far from equilibrium based on phase field theory. In direct simulations of recent experiments [J. C. Bird, S. Mandre, and H. A. Stone, Phys. Rev. Lett. 100, 234501 (2008)], we show that in order to correctly capture the dynamics of rapid wetting, it is crucial to account for nonequilibrium at the contact line, where the gas, liquid, and solid meet. A term in the boundary condition at the solid surface that naturally arises in the phase field theory is interpreted as allowing for the establishment of a local structure in the immediate vicinity of the contact line. A direct qualitative and quantitative match with experimental data of spontaneously wetting liquid droplets is shown.

• 16.
KTH, Superseded Departments, Mechanics.
Melt convection in welding and crystal growth2004Doctoral thesis, comprehensive summary (Other scientific)

A parallel finite element code with adaptive meshing was developed and used to study three dimensional, time-dependent fluid flows caused by thermocapillary convection as well as temperature and dopant distribution in fusion welding and floating zone crystal growth.

A comprehensive numerical model of the three dimensional time-dependent fluid flows in a weld pool had been developed. This model considered most of the physical mechanisms involved in gas tungsten arc welding. The model helped obtaining the actual chaotic time-dependent melt flow. It was found that the fluid flow in the weld pool was highly complex and influenced the weld pool’s depth and width. The physicochemical model had also been studied and applied numerically in order to simulate the surfactant adsorption onto the surface effect to the surface tension of the metal liquid in a weld pool.

Another model, a three dimensional time-dependent, with adaptive mesh refinement and coarsening was applied for simulating the effect of weak flow on the radial segregation in floating zone crystal growth. The phase change equation was also included in this model in order to simulate the real interface shape of floating zone.

In the new parallel code, a scheme that keeps the level of node and face instead of the complete history of refinements was utilized to facilitate derefinement. The information was now local and the exchange of information between each and every processor during the derefinement process was minimized. This scheme helped to improve the efficiency of the parallel adaptive solver.

• 17.
KTH, Superseded Departments, Mechanics.
Parallel computations on fusion welding and floating zones2003Licentiate thesis, comprehensive summary (Other scientific)
• 18.
KTH, Superseded Departments, Mechanics.
KTH, Superseded Departments, Mechanics.
Modeling of Time-Dependent 3D Weld Pool Flow Due to a Moving Arc2003In: Proceedings of High Performance Scientific Computing, 2003Conference paper (Other academic)
• 19.
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.
Modelling of time-dependent 3D weld pool due to a moving arc2005In: Modelling, Simulation And Optimization Of Complex Processes / [ed] Bock, HG; Kostina, E; Phu, HX; Rannacher, R, 2005, 127-138 p.Conference paper (Refereed)

It is well recognized that the fluid flow is an important factor in overall heat and mass transfer in molten pools during arc welding, affecting geometry of the pool and temperature distribution in the pool and in the HAZ. These in turn influence solification behavior, which determine the mechanical properties and quality of the weld fusion zone. Here, a comprehensive numerical model of the time dependent weld pool flow in GTA welding, with a moving heat source has been developed. This model included heat transfer, radiation, evaporation, electromagnetic forces and Marangoni stress in the free surface boundary. With this 3D, fully time dependent model, the true chaotic time dependent melt flow is obtained. The time dependent properties of flow velocities and temperature of numerical results are examined. It shows that the temperature fields axe strongly affected by convection at the weld pool surfaces. The fluid flow in the weld pool is highly complex and it influences the weld pool's depth and width. Moreover, the velocity field at the surface of the specimen determines the streamlines defining the traveling paths of inclusions such as slag particles.

• 20.
KTH, Superseded Departments, Mechanics.
KTH, Superseded Departments, Mechanics.
Modelling of time-dependent 3D weld pool flow2003In: Mathematical modelling of weld Phenomena 7 / [ed] Cerjak, H, 2003, 91-112 p.Conference paper (Refereed)

The fluid flows in molten pools during arc welding are important factors. These in turn influence in overall heat and mass transfer, which determine the mechanical properties and quality of the weld fusion zone. Here, modelling results are presented concerning the time dependent weld pool flow and temperature in gas tungsten arc welding (GTA) of the difference type of stainless steels. It is proved that the temperature fields are strongly affected by the convection at the weld pool’s surfaces. With the stainless steel type 304 (low sulfur content 0.0005 weight % and high sulfur content 0.0139 weight %), the actual chaotic time dependent melt flow is obtained with a fully time dependent model. In those cases, the fluid flow in the weld pool is highly complex and it influenced the weld pool`s depth and width. For the 645 SMO steel, which has an extremely low sulfur content and low conductivity, the chaotic fluid flows did not appear. The calculated geometry of the weld fusion zone and heat affected zone were in good agreement with the experimental results, both with or without chaotic fluid flows.

• 21.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Modelling the influence of wetting properties on the solid liquid impact2008In: PROCEEDINGS OF THE 6TH INTERNATIONAL CONFERENCE ON NANOCHANNELS, MICROCHANNELS, AND MINICHANNELS, PTS A AND B, NEW YORK: AMER SOC MECHANICAL ENGINEERS , 2008, 1915-1917 p.Conference paper (Refereed)

The impact of a solid object on a free liquid surface is quite complex. This problem has challenged researchers for centuries and remains of interest today. Recently Duez et al. [1] published experimental results on the splash when a solid sphere enters a liquid Surprisingly, a small change in the surface chemistry of the object can turn a big splash into an inconspicuous disappearance and vice versa. We study this problem by solving the Navier-Stokes together with the Cahn-Hilliard equations, [2, 3], which allows us to simulate the motion of a free air-water surface in detail, in the presence of surface tension and dynamic wetting. Quantitative computational modeling of dynamic wetting is difficult in itself, but here the use of this tool allows us to study in detail how the wetting properties determine whether a splash appears or not. Our simulated results are compared with the experiments of Duez et al.

• 22.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Numerical simulation of the coupling problems of a solid sphere impacting on a liquid free surface2010In: Mathematics and Computers in Simulation, ISSN 0378-4754, E-ISSN 1872-7166, Vol. 80, no 8, 1664-1673 p.Article in journal (Refereed)

This paper presents a model, using a phase-field method, that is able to simulate the motion of a solid sphere impacting on a liquid surface, including the effects of capillary and hydrodynamic forces. The basic phenomena that were the subject of our research effort are the small scale mechanism such as the wetting property of the solid surface which control the large scale phenomena of the interaction. The coupled problem during the impact will be formulated by the inclusion of the surface energies of the solid surface in the formulation, which gives a reliable prediction of the motion of solid objects in/on/out of a liquid surface and the hydrodynamic behaviours at small scales when the inertia of fluid is less important than its surface tension. Numerical results at different surface wettabilities and impact conditions will be presented and compared with the experiments of Duez el al. [C. Duez, C. Ybert, C. Clanet, L. Bocquet, Nat. Phys. 3 (2007) 180-183] and Lee and Kim [D. Lee. H. Kim, Langmuir 24 (1) (2008) 142]. (C) 2009 IMACS. Published by Elsevier B.V. All rights reserved.

• 23.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Simulation of free dendritic crystal growth in a gravity environment2008In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 227, no 3, 1772-1789 p.Article in journal (Refereed)

In this paper we simulate the evolution and free particle motion of an individual nucleus that grows into a dendritic crystal. The melt flow and the convective heat transfer around the crystal are simulated as they settle due to gravity. There is an intricate coupling between the settling and the evolution of the crystal. The relative flow induced by the settling enhances the growth at the downward facing parts, which in its turn affects the subsequent settling motion. Simulations have been done in two dimensions using a semi-sharp phase-field model. The flow was constrained to a rigid body motion by using Lagrange multipliers inside the solidified part. The model was formulated using two different meshes. One is a fixed background mesh, which covers the whole domain. The other is an adaptive mesh, where the node points are also translated and rotated with the movement of the solid particle. In the latter, the dendritic growth is simulated by the semi-sharp phase-field method.

• 24.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
The splash of a solid sphere impacting on a liquid surface: Numerical simulation of the influence of wetting2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 2Article in journal (Refereed)

The impact of a solid sphere on a liquid surface has challenged researchers for centuries and remains of interest today. Recently, Duez [Nat. Phys. 3, 180 (2007)] published experimental results of the splash generated when a solid sphere enters water. Interestingly, the microscopic properties of the solid surface control the nature of the macroscopic behavior of the splash. So by a change in the surface chemistry of the solid sphere, a big splash can be turned into an inconspicuous disappearance and vice versa. This problem was investigated by numerical simulations based on the Navier-Stokes equations coupled with the Cahn-Hilliard equations. This system allows us to simulate the motion of an air-water interface as a solid sphere impacts the liquid pond. The inclusion of the surface energies of the solid surface in the formulation gives a reasonably quantitative description of the dynamic wetting. Numerical results with different wetting properties and impact speed are presented and directly compared with the recent experimental results from Duez.

• 25.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Simulation of finite-size fibers in turbulent channel flows2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, no 1, 013006- p.Article in journal (Refereed)

The dynamical behavior of almost neutrally buoyant finite-size rigid fibers or rods in turbulent channel flow is studied by direct numerical simulations. The time evolution of the fiber orientation and translational and rotational motions in a statistically steady channel flow is obtained for three different fiber lengths. The turbulent flow is modeled by an entropy lattice Boltzmann method, and the interaction between fibers and carrier fluid is modeled through an external boundary force method. Direct contact and lubrication force models for fiber-fiber interactions and fiber-wall interaction are taken into account to allow for a full four-way interaction. The density ratio is chosen to mimic cellulose fibers in water. It is shown that the finite size leads to fiber-turbulence interactions that are significantly different from earlier reported results for point like particles (e.g., elongated ellipsoids smaller than the Kolmogorov scale). An effect that becomes increasingly accentuated with fiber length is an accumulation in high-speed regions near the wall, resulting in a mean fiber velocity that is higher than the mean fluid velocity. The simulation results indicate that the finite-size fibers tend to stay in the high-speed streaks due to collisions with the wall. In the central region of the channel, long fibers tend to align in the spanwise direction. Closer to the wall the long fibers instead tend to toward to a rotation in the shear plane, while very close to the wall they become predominantly aligned in the streamwise direction.

• 26.
KTH, Superseded Departments, Mechanics.
KTH, Superseded Departments, Mechanics.
Three-dimensional modelling of radial segregation due to weak convection2004In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 269, 454-463 p.Article in journal (Refereed)

A comprehensive three-dimensional, time-dependent model of heat, momentum and solute transfer during solidification is carried out to illustrate the influence of weak convection, caused by surface tension forces, on radial dopant segregation occurring in crystal growth under micro-gravity conditions. 3D adaptive finite element method is used in order to simulate the motion and deformation of the solidification interface. The geometry studied is a Bridgman configuration with a partly coated surface. The small slots in the coating gives a free surface in a controlled way, and is varied in order to alter the Marangoni flow. In this study, A comparison is made between the numerical results and the experimental results. A good agreement has been observed for the effective distribution coefficient keff and for the radial segregation [Delta]c’. The radial dopant segregation is affected by weak convection.

• 27.
KTH, Superseded Departments, Mechanics.
KTH, Superseded Departments, Mechanics.
Experimental and Numerical study of the influence of sulfur redistributation in welding of SAF-2507 stainless steel.2004In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936Article in journal (Other academic)
• 28.
KTH, Superseded Departments, Mechanics.
KTH, Superseded Departments, Mechanics.
Modellingof the adsorption kinetics and the convection of surfactants in GTA welding2004Article in journal (Other academic)
• 29.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Modeling of the adsorption kinetics and the convection of surfactants in a weld pool2008In: Journal of heat transfer, ISSN 0022-1481, E-ISSN 1528-8943, Vol. 130, no 9Article in journal (Refereed)

This paper presents a comprehensive three-dimensional, time-dependent model for simulating the adsorption kinetics and the redistribution of surfactants at the surface and in the bulk of a weld pool. A physicochemical approach that was included in this paper allows the surfactant concentration at the surface and in the bulk to depart from its thermodynamical equilibrium. The Langmuir equilibrium adsorption ratio was based on the k(seg) coefficient of Sahoo (1988, "Surface-Tension of Binary Metal-Surface-Active Solute Systems Under Conditions Relevant to Welding Metallurgy," Metall. Trans. B, 19B, pp. 483-491) and was finally used for calculating fluid flow and heat transfer in gas tungsten arc welding of a super duplex stainless steel, SAF 2507. In this study, the authors applied the multicomponent surfactant mass transfer model to investigate the effect of the influence of sulfur and oxygen redistribution in welding of a super duplex stainless steel.

• 30.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
The impact of ink-jet droplets on a paper-like structure2011In: Fluid Dynamics and Materials Processing, ISSN 1555-2578, Vol. 7, no 4, 389-402 p.Article in journal (Refereed)

Inkjet technology has been recognized as one of the most successful and promising micro-system technologies. The wide application areas of printer heads and the increasing demand of high quality prints are making ink consumption and print see-through important topics in the inkjet technology. In the present study we investigate numerically the impact of ink droplets onto a porous material that mimics the paper structure. The mathematical framework is based on a free energy formulation, coupling the Cahn-Hilliard and Navier Stokes equations, for the modelling of the two-phase flow. The case studied here consists of a multiphase flow of air-liquid along with the interaction between a solid structure and an interface. In order to characterize the multiphase flow characteristics, we investigate the effects of surface tension and surface wettability on the penetration depth and spreading into the paper-like structure.

• 31.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Electrical Engineering (EES), Microsystem Technology. KTH, School of Electrical Engineering (EES), Microsystem Technology. KTH, School of Electrical Engineering (EES), Microsystem Technology. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Fluid dynamic behavior of dispensing small droplets through a thin liquid film2010In: Microfluidics and Nanofluidics, ISSN 1613-4982, Vol. 9, no 2-3, 303-311 p.Article in journal (Refereed)

This paper presents a technology for dispensing droplets through thin liquid layers. The system consists of a free liquid film, which is suspended in a frame and positioned in front of a piezoelectric printhead. A droplet, generated by the printhead, merges with the film, but due to its momentum, passes through and forms a droplet that separates on the other side and continues its flight. The technology allows the dispensing, mixing and ejecting of picolitre liquid samples in a single step. This paper overviews the concept, potential applications, experiments, results and a numerical model. The experimental work includes studying the flight of ink droplets, which ejected from an inkjet print head, fly through a free ink film, suspended in a frame and positioned in front of the printhead. We experimentally observed that the minimum velocity required for the 80 pl droplets to fly through the 75 Â± 24 lm thick ink film was of 6.6 m s-1. We also present a numerical simulation of the passage of liquid droplets through a liquid film. The numerical results for different initial speeds of droplets and their shapes are taken into account. We observed that during the droplet-film interaction, the surface energy is partially converted to kinetic energy, and this, together with the impact time, helps the droplets penetrate the film. The model includes the Navier- Stokes equations with continuum-surface-tension force derived from the phase-field/Cahn-Hilliard equation. This system allows us to simulate the motion of a free surface in the presence of surface tension during merging, mixing and ejection of droplets. The influence of dispensing conditions was studied and it was found that the residual velocity of droplets after their passage through the thin liquid film well matches the measured velocity from the experiment.

• 32.
KTH, Superseded Departments, Mechanics.
KTH, Superseded Departments, Mechanics. KTH, Superseded Departments, Mechanics.
Problem Solving Environment for Parallel Adaptive Computation2004In: Mathematics and Computers in Simulation, ISSN 0378-4754, E-ISSN 1872-7166Article in journal (Other academic)
• 33.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Electrical Engineering (EES), Microsystem Technology. KTH, School of Electrical Engineering (EES), Microsystem Technology. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Numerical Simulation of the Passage of Small Liquid Droplets Through a Thin Liquid Film2008In: PROCEEDINGS OF THE 6TH INTERNATIONAL CONFERENCE ON NANOCHANNELS, MICROCHANNELS, AND MINICHANNELS, NEW YORK: AMER SOC MECHANICAL ENGINEERS , 2008, 857-861 p.Conference paper (Refereed)

We simulate numerically a novel method for dispensing, mixing and ejecting of picolitre liquid samples in a single step. The system consists of a free liquid film, suspended in a frame and positioned in front of a droplet dispenser. On impact, a picolitre droplet merges with the film, but due to its momentum, passes through and forms a droplet that separates on the other side and continues its flight. Through this process the liquid in the droplet and that in the film is mixed in a controlled way. We model the flow using the Navier Stokes together with the Cahn-Hilliard equations. This system allows us to simulate the motion of a free surface in the presence of surface tension during merging, mixing and ejection of droplets. The influence of dispensing conditions was studied and it was found that the residual velocity of droplets after passage through the thin liquid film matches the measured velocity from the experiment well.

• 34.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Parallel adaptive computation of some time-dependent materials-related microstructural problems2007In: Bulletin of the Polish Academy of Sciences: Technical Sciences, ISSN 0239-7528, Vol. 55, no 2, 229-237 p.Article in journal (Refereed)

Some materials-related microstructural problems calculated using the phase-field method are presented. It is well known that the phase field method requires mesh resolution of a diffuse interface. This makes the use of mesh adaptivity essential especially for fast evolving interfaces and other transient problems. Complex problems in 3D are also computationally challenging so that parallel computations are considered necessary. In this paper, a parallel adaptive finite element scheme is proposed. The scheme keeps the level of node and edge for 2D and level of node and face for 3D instead of the complete history of refinements to facilitate derefinement. The information is local and exchange of information is minimized and also less memory is used. The parallel adaptive algorithms that run on distributed memory machines are implemented in the numerical simulation of dendritic growth and capillary-driven flows.

• 35. Engblom, S.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
On diffuse interface modeling and simulation of surfactants in two-phase fluid flow2013In: Communications in Computational Physics, ISSN 1815-2406, E-ISSN 1991-7120, Vol. 14, no 4, 879-915 p.Article in journal (Refereed)

An existing phase-fieldmodel of two immiscible fluids with a single soluble surfactant present is discussed in detail. We analyze the well-posedness of the model and provide strong evidence that it is mathematically ill-posed for a large set of physically relevant parameters. As a consequence, critical modifications to the model are suggested that substantially increase the domain of validity. Carefully designed numerical simulations offer informative demonstrations as to the sharpness of our theoretical results and the qualities of the physical model. A fully coupled hydrodynamic test-case demonstrates the potential to capture also non-trivial effects on the overall flow.

KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Phase-field simulation of weld solidification microstructure in an Al-Cu alloy2008In: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 16, no 6Article in journal (Refereed)

Since the mechanical properties and the integrity of the weld metal depend on the solidification behaviour and the resulting microstructural characteristics, understanding weld pool solidification is of importance to engineers and scientists. Thermal and fluid flow conditions affect the weld pool geometry and solidification parameters. During solidification of the weld pool, a columnar grain structure develops in the weld metal. Prediction of the formation of the microstructure during welding may be an important and supporting factor for technology optimization. Nowadays, increasing computing power allows direct simulations of the dendritic and cell morphology of columnar grains in the molten zone for specific temperature conditions. In this study, the solidification microstructures of the weld pool at different locations along the fusion boundary are simulated during gas tungsten arc welding of Al-3wt% Cu alloy using the phase-field model for the directional solidification of dilute binary alloys. A macroscopic heat transfer and fluid flow model was developed to assess the solidification parameters, notably the temperature gradient and solidification growth rate. The effect of the welding speed is investigated. Computer simulations of the solidification conditions and the formation of a cellular morphology during the directional solidification in gas tungsten arc welding are described. Moreover, the simulation results are compared with existing theoretical models and experimental findings.

• 37. Laurila, T.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Thermohydrodynamics of boiling in a van der Waals fluid2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 85, no 2, 026320- p.Article in journal (Refereed)

We present a modeling approach that enables numerical simulations of a boiling Van der Waals fluid based on the diffuse interface description. A boundary condition is implemented that allows in and out flux of mass at constant external pressure. In addition, a boundary condition for controlled wetting properties of the boiling surface is also proposed. We present isothermal verification cases for each element of our modeling approach. By using these two boundary conditions we are able to numerically access a system that contains the essential physics of the boiling process at microscopic scales. Evolution of bubbles under film boiling and nucleate boiling conditions are observed by varying boiling surface wettability. We observe flow patters around the three-phase contact line where the phase change is greatest. For a hydrophilic boiling surface, a complex flow pattern consistent with vapor recoil theory is observed.

• 38.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Diffuse interface method for a compressible binary fluid2016In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 93, no 1, 013121Article in journal (Refereed)

Multicomponent, multiphase, compressible flows are very important in real life, as well as in scientific research, while their modeling is in an early stage. In this paper, we propose a diffuse interface model for compressible binary mixtures, based on the balance of mass, momentum, energy, and the second law of thermodynamics. We show both analytically and numerically that this model is able to describe the phase equilibrium for a real binary mixture (CO2 + ethanol is considered in this paper) very well by adjusting the parameter which measures the attraction force between molecules of the two components in the model. We also show that the calculated surface tension of the CO2 + ethanol mixture at different concentrations match measurements in the literature when the mixing capillary coefficient is taken to be the geometric mean of the capillary coefficient of each component. Three different cases of two droplets in a shear flow, with the same or different concentration, are simulated, showing that the higher concentration of CO2 the smaller the surface tension and the easier the drop deforms.

• 39.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Numerical simulation of particle formation in the rapid expansion of supercritical solution process2014In: Journal of Supercritical Fluids, ISSN 0896-8446, E-ISSN 1872-8162, Vol. 95, 572-587 p.Article in journal (Refereed)

In this paper, we numerically study particle formation in the rapid expansion of supercritical solution (RESS) process in a two dimensional, axisymmetric geometry, for a benzoic acid + CO2 system. The fluid is described by the classical Navier-Stokes equation, with the thermodynamic pressure being replaced by a generalized pressure tensor. Homogenous particle nucleation, transport, condensation and coagulation are described by a general dynamic equation, which is solved using the method of moments. The results show that the maximal nucleation rate and number density occurs near the nozzle exit, and particle precipitation inside the nozzle might not be ignored. Particles grow mainly across the shocks. Fluid in the shear layer of the jet shows a relatively low temperature, high nucleation rate, and carries particles with small sizes. On the plate, particles within the jet have smaller average size and higher geometric mean, while particles outside the jet shows a larger average size and a lower geometric mean. Increasing the preexpansion temperature will increase both the average particle size and standard deviation. The preexpansion pressure does not show a monotonic dependency with the average particle size. Increasing the distance between the plate and the nozzle exit might decrease the particle size. For all the cases in this paper, the average particle size on the plate is on the order of tens of nanometers.

• 40.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Numerical Simulation of Rapid Expansion of Supercritical Carbon Dioxide2015In: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 61, no 1, 317-332 p.Article in journal (Refereed)

Axisymmetric rapid expansion of supercritical carbon dioxide is investigated in this article. The extended generalized Bender equation of state is used to give a good description of the fluids over a wide range of pressure and temperature conditions. The locations of Mach disks are analyzed and compared with an experimental correlation for the case where there is no plate positioned in front of the nozzle exit. It is found that the disagreement between our numerical results and the experimental formula is very small when the pressure ratio is small, and increases as the pressure ratio increases. It is also found that with different equations of state, the predicted positions of Mach disks do not differ a lot, but the temperature profiles in the chamber differ a lot. The case where there is a plate positioned in front of the nozzle exit is also studied in this article. A universal similarity solution is obtained.

• 41.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Thermohydrodynamics of boiling in binary compressible fluids2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 92, no 4, 043017Article in journal (Refereed)

We numerically study the thermohydrodynamics of boiling for a CO2 + ethanol mixture on lyophilic and lyophobic surfaces in both closed and open systems, based on a diffuse interface model for a two-component system. The corresponding wetting boundary conditions for an isothermal system are proposed and verified in this paper. New phenomena due to the addition of another component, mainly the preferential evaporation of the more volatile component, are observed. In the open system and the closed system, the physical process shows very different characteristics. In the open system, except for the movement of the contact line, the qualitative features are rather similar for lyophobic and lyophilic surfaces. In the closed system, the vortices that are observed on a lyophobic surface are not seen on a lyophilic surface. More sophisticated wetting boundary conditions for nonisothermal, two-component systems might need to be further developed, taking into account the variations of density, temperature, and surface tension near the wall, while numerical results show that the boundary conditions proposed here also work well even in boiling, where the temperature is nonuniform.

• 42.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Kinematics and dynamics of suspended gasifying particle2017In: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 228, no 3, 1135-1151 p.Article in journal (Refereed)

The effect of gasification on the dynamics and kinematics of immersed spherical and non-spherical solid particles have been investigated using the three-dimensional lattice Boltzmann method. The gasification was performed by applying mass injection on particle surface for three cases: flow passing by a fixed sphere, rotating ellipsoid in simple shear flow, and a settling single sphere in a rectangular domain. In addition, we have compared the accuracy of employing two different fluid-solid interaction methods for the particle boundary. The validity of the gasification model was studied by comparing computed the mass flux from the simulation and the calculated value on the surface of the particle. The result was used to select a suitable boundary method in the simulations combined with gasification. Moreover, the reduction effect of the ejected mass flux on the drag coefficient of the fixed sphere have been validated against previous studies. In the case of rotating ellipsoid in simple shear flow with mass injection, a decrease on the rate of rotation was observed. The terminal (maximum) velocity of the settling sphere was increased by increasing the ratio of radial flux from the particle boundary.

• 43. Neytcheva, M.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Element-by-element Schur complement approximations for general nonsymmetric matrices of two-by-two block form2010In: Large-Scale Scientific Computing, Springer Berlin/Heidelberg, 2010, 108-115 p.Conference paper (Refereed)

We consider element-by-element Schur complement approximations for indefinite and general nonsymmetric matrices of two-by-two block form, as arising in finite element discretized systems of PDEs. The paper provides some analysis of the so-obtained approximation and attempts to quantify the quality of the underlying two-by-two matrix splitting in a way similar to that used for symmetric positive definite matrices. The quality of the approximation is illustrated numerically.

• 44. Nita, Satoshi
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics.
Electrostatic cloaking of surface structure for dynamic wetting2017In: SCIENCE ADVANCES, ISSN 2375-2548, Vol. 3, no 2, e1602202Article in journal (Refereed)

Dynamic wetting problems are fundamental to understanding the interaction between liquids and solids. Even in a superficially simple experimental situation, such as a droplet spreading over a dry surface, the result may depend not only on the liquid properties but also strongly on the substrate-surface properties; even for macroscopically smooth surfaces, the microscopic geometrical roughness can be important. In addition, because surfaces may often be naturally charged or electric fields are used to manipulate fluids, electric effects are crucial components that influence wetting phenomena. We investigate the interplay between electric forces and surface structures in dynamic wetting. Although surfacemicrostructures can significantly hinder spreading, we find that electrostatics can " cloak" themicrostructures, that is, deactivate the hindering. We identify the physics in terms of reduction in contact-line friction, which makes the dynamic wetting inertial force dominant and insensitive to the substrate properties.

• 45. Ogden, S.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
Fluid behavior of supercritical carbon dioxide with water in a double-Y-channel microfluidic chip2014In: Microfluidics and Nanofluidics, ISSN 1613-4982, E-ISSN 1613-4990, Vol. 17, no 6, 1105-1112 p.Article in journal (Refereed)

The use of supercritical carbon dioxide (scCO(2)) as an apolar solvent has been known for decades. It offers a greener approach than, e.g., hexane or chloroform, when such solvents are needed. The use of scCO(2) in microsystems, however, has only recently started to attract attention. In microfluidics, the flow characteristics need to be known to be able to successfully design such components and systems. As supercritical fluids exhibit the exciting combination of low viscosity, high density, and high diffusion rates, the fluidic behavior is not directly transferrable from aqueous systems. In this paper, three flow regimes in the scCO(2)-liquid water two-phase microfluidic system have been mapped. The effect of both total flow rate and relative flow rate on the flow regime is evaluated. Furthermore, the droplet dynamics at the bifurcating exit channel are analyzed at different flow rates. Due to the low viscosity of scCO(2), segmented flows were observed even at fairly high flow rates. Furthermore, the carbon dioxide droplet behavior exhibited a clear dependence on both flow rate and droplet length.

• 46.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
Effect of fluid and particle inertia on the rotation of an oblate spheroidal particle suspended in linear shear flow2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 91, no 5, 053017Article in journal (Refereed)

This work describes the inertial effects on the rotational behavior of an oblate spheroidal particle confined between two parallel opposite moving walls, which generate a linear shear flow. Numerical results are obtained using the lattice Boltzmann method with an external boundary force. The rotation of the particle depends on the particle Reynolds number, Rep = Gd-2 nu(-1) (G is the shear rate, d is the particle diameter,. is the kinematic viscosity), and the Stokes number, St = alpha Re-p (a is the solid-to-fluid density ratio), which are dimensionless quantities connected to fluid and particle inertia, respectively. The results show that two inertial effects give rise to different stable rotational states. For a neutrally buoyant particle (St = Re-p) at low Re-p, particle inertia was found to dominate, eventually leading to a rotation about the particle's symmetry axis. The symmetry axis is in this case parallel to the vorticity direction; a rotational state called log-rolling. At high Re-p, fluid inertia will dominate and the particle will remain in a steady state, where the particle symmetry axis is perpendicular to the vorticity direction and has a constant angle phi(c) to the flow direction. The sequence of transitions between these dynamical states were found to be dependent on density ratio alpha, particle aspect ratio r(p), and domain size. More specifically, the present study reveals that an inclined rolling state (particle rotates around its symmetry axis, which is not aligned in the vorticity direction) appears through a pitchfork bifurcation due to the influence of periodic boundary conditions when simulated in a small domain. Furthermore, it is also found that a tumbling motion, where the particle symmetry axis rotates in the flow-gradient plane, can be a stable motion for particles with high r(p) and low alpha.

• 47.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
The dynamical states of a prolate spheroidal particle suspended in shear flow as a consequence of particle and fluid inertia2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 771, 115-158 p.Article in journal (Refereed)

The rotational motion of a prolate spheroidal particle suspended in shear flow is studied by a lattice Boltzmann method with external boundary forcing (LB-EBF). It has previously been shown that the case of a single neutrally buoyant particle is a surprisingly rich dynamical system that exhibits several bifurcations between rotational states due to inertial effects. It was observed that the rotational states were associated with either fluid inertia effects or particle inertia effects, which are always in competition. The effects of fluid inertia are characterized by the particle Reynolds number Rep=4Ga2/ν, where G is the shear rate, a is the length of the particle major semi-axis and ν is the kinematic viscosity. Particle inertia is associated with the Stokes number St=α· Rep, where alpha is the solid-to-fluid density ratio. Previously, the neutrally buoyant case (St=Rep) was studied extensively. However, little is known about how these results are affected when St≢Rep, and how the aspect ratio rp (major axis/minor axis) influences the competition between fluid and particle inertia in the absence of gravity. This work gives a full description of how prolate spheroidal particles in the range 2≤ rp≤ 6 behave depending on the chosen St and Rep. Furthermore, consequences for the rheology of a dilute suspension containing such particles are discussed. Finally, grid resolution close to the particle is shown to affect the quantitative results considerably. It is suggested that this resolution is a major cause of quantitative discrepancies between different studies. Thus, the results of this work and previous direct numerical simulations of this problem should be regarded as qualitative descriptions of the physics involved, and more refined methods must be used to quantitatively pinpoint the transitions between rotational states.

• 48.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Orientational dynamics of a triaxial ellipsoid in simple shear flow: Influence of inertia2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 1, 013109Article in journal (Refereed)

The motion of a single ellipsoidal particle in simple shear flow can provide valuable insights toward understanding suspension flows with nonspherical particles. Previously, extensive studies have been performed on the ellipsoidal particle with rotational symmetry, a so-called spheroid. The nearly prolate ellipsoid (one major and two minor axes of almost equal size) is known to perform quasiperiodic or even chaotic orbits in the absence of inertia. With small particle inertia, the particle is also known to drift toward this irregular motion. However, it is not previously understood what effects from fluid inertia could be, which is of highest importance for particles close to neutral buoyancy. Here, we find that fluid inertia is acting strongly to suppress the chaotic motion and only very weak fluid inertia is sufficient to stabilize a rotation around themiddle axis. Themechanism responsible for this transition is believed to be centrifugal forces acting on fluid, which is dragged along with the rotational motion of the particle. With moderate fluid inertia, it is found that nearly prolate triaxial particles behave similarly to the perfectly spheroidal particles. Finally, we also are able to provide predictions about the stable rotational states for the general triaxial ellipsoid in simple shear with weak inertia.

• 49.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
Orientational dynamics of a tri-axial ellipsoid in simple shear flow: influence of inertiaManuscript (preprint) (Other academic)
• 50.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Mechanics. KTH Royal Institute of Technology.
KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Mechanics.
Quantitative analysis of the angular dynamics of a single spheroid in simple shear flow at moderate Reynolds numbers2016In: Physical Review Fluids, ISSN 2469-990X, Vol. 1, no 4, 044201-1-044201-21 p.Article in journal (Refereed)

A spheroidal particle in simple shear flow shows surprisingly complicated angular dynamics; caused by effects of fluid inertia (characterized by the particle Reynolds number Rep) and particle inertia (characterized by the Stokes number St). Understanding this behavior can provide important fundamental knowledge of suspension flows with spheroidal particles. Up to now only qualitative analysis has been available at moderate Rep. Rigorous analytical methods apply only to very small Rep and numerical results lack accuracy due to the difficulty in treating the moving boundary of the particle. Here we show that the dynamics of the rotational motion of a prolate spheroidal particle in a linear shear flow can be quantitatively analyzed through the eigenvalues of the log-rolling particle (particle aligned with vorticity). This analysis provides an accurate description of stable rotational states in terms of Rep,St, and particle aspect ratio (rp). Furthermore we find that the effect on the orientational dynamics from fluid inertia can be modeled with a Duffing-Van der Pol oscillator. This opens up the possibility of developing a reduced-order model that takes into account effects from both fluid and particle inertia.

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