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  • 1.
    Albernaz, Daniel
    et al.
    KTH.
    Do, Quang Minh
    KTH.
    Amberg, Gustav
    KTH.
    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, article id 043012Article in journal (Refereed)
    Abstract [en]

    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.
    Albernaz, Daniel L.
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Do-Quang, Minh
    KTH.
    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, p. 853-860Article in journal (Refereed)
    Abstract [en]

    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.
    Albernaz, Daniel L.
    et al.
    Royal Inst Technol, Dept Mech, Linne Flow Ctr, S-10044 Stockholm, Sweden..
    Do-Quang, M.
    Royal Inst Technol, Dept Mech, Linne Flow Ctr, S-10044 Stockholm, Sweden..
    Hermanson, J. C.
    University of Washington, Seattle, USA.
    Amberg, Gustav
    KTH.
    Droplet deformation and heat transfer in isotropic turbulence2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 820, p. 61-85Article in journal (Refereed)
    Abstract [en]

    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 single droplet undergoing phase change in isotropic turbulence using numerical simulations with a hybrid lattice Boltzmann scheme. Phase separation is controlled by a non-ideal equation of state and density contrast is taken into consideration. Droplet deformation is caused by pressure and shear stress at the droplet interface. The statistics of thermodynamic variables are quantified and averaged over both the liquid and vapour 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 analysed 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, whereas nonlinearities are produced with the increase of Re A, as intermediate frequencies are seen to overlap. As an outcome, a continuous spectrum is observed, which shows a decrease in the power spectrum that scales as similar to f(-3) Correlations between the droplet Weber number, deformation parameter, fluctuations of the droplet volume and thermodynamic variables are also developed.

  • 4.
    Albernaz, Daniel L.
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    Amberg, Gustav
    KTH.
    Lattice Boltzmann Method for the evaporation of a suspended droplet2013In: Interfacial phenomena and heat transfer, ISSN 2167-857X, Vol. 1, no 3, p. 245-258Article in journal (Refereed)
    Abstract [en]

    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.

  • 5.
    Albernaz, Daniel L.
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    Hermanson, James C.
    University of Washington, Seattle, USA.
    Amberg, Gustav
    KTH.
    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, article id 125105Article in journal (Refereed)
    Abstract [en]

    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.

  • 6.
    Aldaeus, Fredrik
    et al.
    KTH.
    Lin, Yuan
    KTH.
    Amberg, Gustav
    KTH.
    Roeraade, Johan
    KTH.
    Multi-step dielectrophoresis for separation of particles2006In: Journal of Chromatography A, ISSN 0021-9673, E-ISSN 1873-3778, Vol. 1131, no 1-2, p. 261-266Article in journal (Refereed)
    Abstract [en]

    A new concept for separation of particles based on repetitive dielectrophoretic trapping and release in a flow system is proposed. Calculations using the finite element method have been performed to envision the particle behavior and the separation effectiveness of the proposed method. As a model system, polystyrene beads in deionized water and a micro-flow channel with arrays of interdigited electrodes have been used. Results show that the resolution increases as a direct function of the number of trap-and-release steps, and that a difference in size will have a larger influence on the separation than a difference in other dielectrophoretic properties. About 200 trap-and-release steps would be required to separate particles with a size difference of 0.2%. The enhanced separation power of dielectrophoresis with multiple steps could be of great importance, not only for fractionation of particles with small differences in size, but also for measuring changes in surface conductivity, or for separations based on combinations of difference in size and dielectric properties.

  • 7.
    Aldaeus, Fredrik
    et al.
    KTH.
    Lin, Yuan
    KTH.
    Roeraade, Johan
    KTH.
    Amberg, Gustav
    KTH.
    Superpositioned dielectrophoresis for enhanced trapping efficiency2005In: Electrophoresis, ISSN 0173-0835, E-ISSN 1522-2683, Vol. 26, no 22, p. 4252-4259Article in journal (Refereed)
    Abstract [en]

    One of the major applications for dielectrophoresis is selective trapping and fractionation of particles. If the surrounding medium is of low conductivity, the trapping force is high, but if the conductivity increases, the attraction decreases and may even become negative. However, high-conductivity media are essential when working with biological material such as living cells. In this paper, some basic calculations have been performed, and a model has been developed which employs both positive and negative dielectrophoresis in a channel with interdigitated electrodes. The finite element method was utilized to predict the trajectories of Escherichia coli bacteria in the superpositioned electrical fields. It is shown that a drastic improvement of trapping efficiency can be obtained in this way, when a high conductivity medium is employed.

  • 8.
    Amberg, Gustav
    KTH.
    Semisharp phase field method for quantitative phase change simulations2003In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 91, no 26, article id 265505Article in journal (Refereed)
    Abstract [en]

    The standard phase field model for simulation of phase change requires an asymptotic analysis in a vanishing interface width, in order to connect the model parameters to the sharp interface parameters, which has hampered the quantitative usefulness of the method. In this Letter the method is simplified to the point that the relevant reduced problem can be solved analytically, allowing the sharp and phase field parameters to be identified, in principle, without restrictions on the model parameters. The scheme is tested for standard cases of two-dimensional solidification, showing excellent agreement with sharp interface kinetics.

  • 9.
    Amberg, Gustav
    KTH.
    Solidification microstructure, dendrites and convection2004In: Phase Change With Convection: Modelling And Validation / [ed] Kowalewski, TA., Gobin, D., Wien: Springer, 2004, p. 1-53Conference paper (Refereed)
  • 10.
    Amberg, Gustav
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    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, p. 378-386Article in journal (Refereed)
    Abstract [en]

    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.

  • 11.
    Barcena, L. T.
    et al.
    Universidad de Navarra, San Sebastián, Spain.
    Shiomi, J.
    Universidad de Navarra, San Sebastián, Spain / KTH.
    Amberg, Gustav
    KTH.
    Control of oscillatory thermocapillary convection with local heating2006In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 286, no 2, p. 502-511Article in journal (Refereed)
    Abstract [en]

    In this experimental work, a proportional feedback control was applied to attenuate an oscillatory thermocapillary flow in an open cylindrical container (annular configuration) filled with silicon oil with high Prandt1 number (Pr = 14 at 25 degrees C). The control was realized by locally heating the free surface with two point source heaters strategically positioned in different azimuthal positions. The heaters were actuated using the local temperature signals fed back from paired sensors. It is suggested that the shortcoming of the control performance accompanied with the amplification of the harmonic frequency components is due to the coupling of the fundamental and the harmonic modes caused by the local control. A remedy is demonstrated to validate the suggestion, where the coupling can be attenuated by increasing the azimuthal length of the actuation region.

  • 12.
    Brunet, P.
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Alfredsson, P. Henrik
    KTH.
    Control of thermocapillary instabilities far from threshold2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 10, article id 104109Article in journal (Refereed)
    Abstract [en]

    We report experiments on control of thermocapillary instabilities at high temperature differences, in an annular geometry. Previous studies [Phys. Fluids 14, 3039 (2002)] showed that a reasonable control of oscillatory instability could be achieved by optimizing a local heating feedback process. We conducted experiments with a basic flow converging from periphery to center. This constitutes a more unstable configuration than previously, and enables appearance of higher-order instabilities and chaos. Applying successfully local feedback control to the periodic state close to the threshold, we extend the process to higher temperature differences, where nonlinear as well as proportional/derivative control laws are necessary to obtain a significant decrease of the temperature fluctuations. Finally, proportional control allows us to synchronize a chaotic state, to a periodic one.

  • 13.
    Carlberg, T
    et al.
    Mid-Sweden University.
    Winkler, C
    KTH.
    Amberg, Gustav
    KTH.
    Weak convection influencing radial segregation2001In: First International Symposium On Microgravity Research & Applications In Physical Sciences And Biotechnology, Vols I And Ii, Proceedings, European Space Agency, 2001, p. 431-436Conference paper (Refereed)
    Abstract [en]

    The influence of weak convection, caused by surface tension forces, on radial segregation occuring in crystals grown under microgravity conditions is studied in a project involving both numerical simulations and space experiments. The geometry studied is a Bridgman configuration with a partially coated surface. Small slots in the coating give free surface in a controlled way, and the free surface area is the parameter in the investigation. The experiments will be performed in a GAS module, which will be flown in the beginning of year 2001. The paper presents the experimental technique in detail as well as some results obtained during the development phase. Numerical simulations have been performed and results of how the radial segregation varies with the convection level will be presented. The project is the first phase of a research program, which will continue with experiments in the ISS.

  • 14.
    Carlborg, Carl Fredrik
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    Stemme, Göran
    KTH.
    Amberg, Gustav
    KTH.
    van der Wijngaart, Wouter
    KTH.
    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), Institute of Electrical and Electronics Engineers (IEEE), 2008, p. 599-602Conference paper (Refereed)
    Abstract [en]

    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.

  • 15.
    Carlson, Andreas
    et al.
    KTH.
    Bellani, Gabriele
    KTH.
    Amberg, Gustav
    KTH.
    Contact line dissipation in short-time dynamic wetting2012In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 97, no 4, article id 44004Article in journal (Refereed)
    Abstract [en]

    Dynamic wetting of a solid surface is a process that is ubiquitous in Nature, and also of increasing technological importance. The underlying dissipative mechanisms are, however, still unclear. We present here short-time dynamic wetting experiments and numerical simulations, based on a phase field approach, of a droplet on a dry solid surface, where direct comparison of the two allows us to evaluate the different contributions from the numerics. We find that an important part of the dissipation may arise from a friction related to the motion of the contact line itself, and that this may be dominating both inertia and viscous friction in the flow adjacent to the contact line. A contact line friction factor appears in the theoretical formulation that can be distinguished and quantified, also in room temperature where other sources of dissipation are present. Water and glycerin-water mixtures on various surfaces have been investigated where we show the dependency of the friction factor on the nature of the surface, and the viscosity of the liquid.

  • 16.
    Carlson, Andreas
    et al.
    KTH.
    Bellani, Gabriele
    KTH.
    Amberg, Gustav
    KTH.
    Universality in dynamic wetting dominated by contact-line friction2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 85, no 4, article id 045302Article in journal (Refereed)
    Abstract [en]

    We report experiments on the rapid contact-line motion present in the early stages of capillary-driven spreading of drops on dry solid substrates. The spreading data fail to follow a conventional viscous or inertial scaling. By integrating experiments and simulations, we quantify a contact-line friction mu(f) which is seen to limit the speed of the rapid dynamic wetting. A scaling based on this contact-line friction is shown to yield a universal curve for the evolution of the contact-line radius as a function of time, for a range of fluid viscosities, drop sizes, and surface wettabilities.

  • 17.
    Carlson, Andreas
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    Amberg, Gustav
    KTH.
    Dissipation in rapid dynamic wetting2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 682, p. 213-240Article in journal (Refereed)
    Abstract [en]

    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.

  • 18.
    Carlson, Andreas
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    Amberg, Gustav
    KTH.
    Droplet dynamics in a bifurcating channel2010In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 36, no 5, p. 397-405Article in journal (Refereed)
    Abstract [en]

    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.

  • 19.
    Carlson, Andreas
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    Amberg, Gustav
    KTH.
    Modeling of dynamic wetting far from equilibrium2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 12, article id 121701Article in journal (Refereed)
    Abstract [en]

    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.

  • 20.
    Carlson, Andreas
    et al.
    KTH / Harvard University, Cambridge, United States / Princeton University, Princeton, United States.
    Kim, P.
    Korea Advanced Institute of Science and Technology, Daejeon, South Korea / Princeton University, Princeton, United States.
    Amberg, Gustav
    KTH.
    Stone, H. A.
    Princeton University, Princeton, United States.
    Short and long time drop dynamics on lubricated substrates2013In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 104, no 3, article id 34008Article in journal (Refereed)
    Abstract [en]

    Liquid infiltrated solids have been proposed as functional solvent-phobic surfaces for handling single and multiphase flows. Implementation of such surfaces alters the interfacial transport phenomenon as compared to a dry substrate. To better understand the interface characteristics in such systems we study experimentally the dynamics of a pendant water drop in air that contacts a substrate coated by thin oil films. At short times the water drop is deformed by the oil that spreads onto the water-air interface, and the dynamics are characterized by inertial and viscous regimes. At late times, the the oil film under the drop relaxes either to a stable thin film or ruptures. In the thin film rupture regime, we measure the waiting time for the rupture as a function of the drop equilibrium contact angle on a dry substrate and the initial film height. The waiting time is rationalized by lubrication theory, which indicates that long-range intermolecular forces destabilize the oil-water interface and is the primary mechanism for the film drainage.

  • 21.
    Do-Quang, Minh
    et al.
    KTH, Mekanik.
    Amberg, Gustav
    KTH, Mekanik.
    Modeling of Time-Dependent 3D Weld Pool Flow Due to a Moving Arc2003In: Modeling, Simulation and Optimization of Complex Processes: Proceedings of the International Conference on High Performance Scientific Computing, March 10-14, 2003, Hanoi, Vietnam / [ed] Hans Georg Bock, Ekaterina Kostina, Hoang Xuan Phu, Rolf Rannacher, Berlin: Springer, 2003, p. 127-138Conference paper (Other academic)
  • 22.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    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, New York: Springer, 2005, p. 127-138Conference paper (Refereed)
    Abstract [en]

    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.

  • 23.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Modelling of time-dependent 3D weld pool flow2003In: Mathematical modelling of weld Phenomena 7 / [ed] Cerjak, H; Bhadeshia, H. K. D. H.; Kozeschnik, E., Graz: Graz Techn. Univ. , 2003, p. 91-112Conference paper (Refereed)
    Abstract [en]

    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.

  • 24.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    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, p. 1915-1917Conference paper (Refereed)
    Abstract [en]

    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.

  • 25.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    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, p. 1664-1673Article in journal (Refereed)
    Abstract [en]

    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.

  • 26.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    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, p. 1772-1789Article in journal (Refereed)
    Abstract [en]

    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.

  • 27.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    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 2, article id 022102Article in journal (Refereed)
    Abstract [en]

    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.

  • 28.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Brethouwer, Gert
    KTH.
    Johansson, Arne V.
    KTH.
    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, article id 013006Article in journal (Refereed)
    Abstract [en]

    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.

  • 29.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Carlberg, Torbjörn
    Mid Sweden University.
    Three-dimensional modelling of radial segregation due to weak convection2004In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 269, no 2-4, p. 454-463Article in journal (Refereed)
    Abstract [en]

    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.

  • 30.
    Do-Quang, Minh
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Pettersson, Claes-Ove
    Sandvik Materials Technology.
    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 9, article id 092102Article in journal (Refereed)
    Abstract [en]

    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.

  • 31.
    Do-Quang, Minh
    et al.
    KTH.
    Carlson, A
    KTH.
    Amberg, Gustav
    KTH.
    The impact of ink-jet droplets on a paper-like structure2011In: Fluid Dynamics & Materials Processing, ISSN 1555-256X, E-ISSN 1555-2578, Vol. 7, no 4, p. 389-402Article in journal (Refereed)
    Abstract [en]

    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.

  • 32.
    Do-Quang, Minh
    et al.
    KTH.
    Geyl, Laurent
    KTH.
    Stemme, Göran
    KTH.
    van der Wijngaart, Wouter
    KTH.
    Amberg, Gustav
    KTH.
    Fluid dynamic behavior of dispensing small droplets through a thin liquid film2010In: Microfluidics and Nanofluidics, ISSN 1613-4982, E-ISSN 1613-4990, Vol. 9, no 2-3, p. 303-311Article in journal (Refereed)
    Abstract [en]

    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 ± 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.

  • 33.
    Do-Quang, Minh
    et al.
    KTH.
    Shiomi, Junichiro
    University of Tokyo, Tokyo, Japan / CREST, Japan Science and Technology Agency, Tokyo, Japan.
    Amberg, Gustav
    KTH.
    When and how surface structure determines the dynamics of partial wetting2015In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 110, no 4, article id 46002Article in journal (Refereed)
    Abstract [en]

    The motion of a three phase contact line, as in a droplet spreading over a dry surface, is ubiquitous in nature and common in technology, but is still not well understood. As has been recently shown, line friction may play an important role in rapid dynamic wetting. Recognizing this as a sometimes dominating factor, we identify the possible scenarios for dynamic wetting of a partially wetting fluid, given the fluid and substrate properties. In doing so, we also reconcile the seemingly different interpretations of dynamic wetting that have been put forward in the recent literature. Copyright (C) EPLA, 2015

  • 34.
    Do-Quang, Minh
    et al.
    KTH.
    Stemme, Göran
    KTH.
    van der Wijngaart, Wouter
    KTH.
    Amberg, Gustav
    KTH.
    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, p. 857-861Conference paper (Refereed)
    Abstract [en]

    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.

  • 35.
    Do-Quang, Minh
    et al.
    KTH.
    Villanueva, Walter
    KTH.
    Singer-Loginova, Irina
    KTH.
    Amberg, Gustav
    KTH.
    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, p. 229-237Article in journal (Refereed)
    Abstract [en]

    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.

  • 36.
    Engblom, S.
    et al.
    Uppsala University.
    Do-Quang, Minh
    KTH.
    Amberg, Gustav
    KTH.
    Tornberg, Anna-Karin
    KTH.
    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, p. 879-915Article in journal (Refereed)
    Abstract [en]

    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.

  • 37.
    Farzadi, A.
    et al.
    Sharif University of Technology, Tehran, Iran / KTH.
    Do-Quang, Minh
    KTH.
    Serajzadeh, S.
    Sharif University of Technology, Tehran, Iran.
    Kokabi, A. H.
    Sharif University of Technology, Tehran, Iran.
    Amberg, Gustav
    KTH.
    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 6, article id 065005Article in journal (Refereed)
    Abstract [en]

    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.

  • 38.
    Gey, Laurent
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    van der Wijngaart, Wouter
    KTH.
    Stemme, Göran
    KTH.
    Study Of The Flight Of Small Liquid Droplets Through A Thin Liquid Film For Picolitre Liquid Transfer2006In: 19th IEEE International Conference on Micro Electro Mechanical Systems (IEEE MEMS 2006), New York: Institute of Electrical and Electronics Engineers (IEEE), 2006, p. 24-27Conference paper (Refereed)
    Abstract [en]

    We introduce and successfully demonstrate a novel method and system for subsequent 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. In this study we tested and modelled the flight of liquid droplets, ejected from an inkjet print head, through a suspended liquid film. Model and experiment are in accordance.

  • 39.
    Gullman-Strand, Johan
    et al.
    KTH.
    Törnblom, Olle
    KTH.
    Lindgren, Björn
    KTH.
    Amberg, Gustav
    KTH.
    Johansson, Arne V.
    KTH.
    Numerical and experimental study of separated flow in a plane asymmetric diffuser2004In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 25, no 3, p. 451-460Article in journal (Refereed)
    Abstract [en]

    Computations of the turbulent flow through plane asymmetric diffusers for opening angles from 8degrees to 10degrees have been carried out with the explicit algebraic Reynolds stress model (EARSM) of Wallin and Johansson [J. Fluid Mech. 403 (2000) 89]. It is based on a two-equation platform in the form of a low-Re K - omega formulation, see e.g. Wilcox [Turbulence Modeling for CFD, DCW Industries Inc., 1993]. The flow has also been studied experimentally for the 8.5degrees opening angle using PIV and LDV. The models under-predict the size and magnitude of the recirculation zone. This is, at least partially, attributed to an over-estimation of the wall normal turbulence component in a region close to the diffuser inlet and to the use of damping functions in the near-wall region. By analyzing the balance between the production and dissipation of the turbulence kinetic energy we find that the predicted dissipation is too large. Hence, we can identify a need for improvement of the modeling the transport equation for the turbulence length-scale related quantity.

  • 40.
    Kekesi, Timea
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Wittberg, L. Prahl
    KTH.
    Drop deformation and breakup in flows with shear2016In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 140, p. 319-329Article in journal (Refereed)
    Abstract [en]

    A Volume of Fluid (VOF) method is applied to study the deformation and breakup of a single liquid drop in shear flows superimposed on uniform flow. The effect of shearing on the breakup mechanism is investigated as a function of the shear rate. Sequential images are compared for the parameter range studied; density ratios of liquid to gas of 20, 40, and 80, viscosity ratios in the range 0.5-50, Reynolds numbers between 20, a constant Weber number of 20, and the non-dimensional shear rate of the flow G = 0-2.1875. It is found that while shear breakup remains similar for all values of shear rate considered, other breakup modes observed for uniform flows are remarkably modified with increasing shear rate. The time required for breakup is significantly decreased in strong shear flows. A simple model predicting the breakup time as a function of the shear rate and the breakup time observed in uniform flows is suggested.

  • 41.
    Knaust, Stefan
    et al.
    Uppsala University.
    Andersson, Martin
    Uppsala University.
    Rogeman, Niklas
    Uppsala University.
    Hjort, Klas
    Uppsala University.
    Amberg, Gustav
    KTH.
    Klintberg, Lena
    Uppsala University.
    Influence of flow rate, temperature and pressure on multiphase flows of supercritical carbon dioxide and water using multivariate partial least square regression2015In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 25, no 10, article id 105001Article in journal (Refereed)
    Abstract [en]

    Supercritical carbon dioxide (scCO(2)) is often used to replace harmful solvents and can dissolve a wide range of organic compounds. With a favorable critical point at 31 degrees C and 7.4 MPa, reaching above the critical point for scCO(2) is fairly accessible. Because of the compressible nature of scCO(2) and the large changes of viscosity and density with temperature and pressure, there is a need to determine the behavior of scCO(2) in microfluidic systems. Here, the influence of how parameters such as flow rate, temperature, pressure, and flow ratio affects the length of parallel flow of water and scCO(2) and the length of the created CO2 segments are investigated and modeled using multivariate data analysis for a 10 mm long double-y channel. The parallel length and segment size were observed in the laminar regime around and above the critical point of CO2. The flow ratio between the two fluids together with the flow rate influenced both the parallel length and the segment sizes, and a higher pressure resulted in shorter parallel lengths. Regarding the segment length of CO2, longer segments were a result of a higher Weber number for H2O together with a higher temperature in the channel.

  • 42.
    Kudo, M.
    et al.
    Tokyo University of Science, Chiba, Japan.
    Shiomi, J.
    University of Tokyo, Tokyo, Japan.
    Ueno, I.
    Tokyo University of Science, Chiba, Japan.
    Amberg, Gustav
    KTH.
    Kawamura, H.
    Tokyo University of Science, Chiba, Japan.
    Experiment on multimode feedback control of non-linear thermocapillary convection in a half-zone liquid bridge2005In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 36, no 4, p. 57-63Article in journal (Refereed)
    Abstract [en]

    Feedback control on was carried out on non-linear thermocapillary convections in a half-zone liquid bridge of a high Prandtl number fluid. In the liquid bridge, the convection changes from a two-dimensional steady flow to a three-dimensional oscillatory one at a critical temperature difference. Feedback control was realized by locally modifying the free surface temperature using local temperature measured at different positions. The present study aims to develop a new control scheme by taking spatio-temporal azimuthal distribution of temperature fluctuation into account. The new control scheme achieved significant attenuation of the temperature oscillation compared with the previous one in a high Marangoni number range.

  • 43. Kudo, M.
    et al.
    Shiomi, J.
    Ueno, I.
    Amberg, Gustav
    KTH.
    Kawamura, H.
    [Feedback control of nonlinear thermocapillary convection in a half-zone liquid bridge]2005In: Transactions of the Japan Society of Mechanical Engineers, Part B, ISSN 0387-5016, Vol. 71, no 706, p. 1617-1624Article in journal (Refereed)
    Abstract [en]

    A feedback control upon nonlinear thermocapillary convections in a half-zone liquid bridge of a high Prandtl number fluid was conducted. In the liquid bridge, the convection changes from two-dimensional steady flow to three-dimensional oscillatory one at a critical temperature difference. The feedback control was realized by locally modifying the free surface temperature using local temperature measured at different positions. The present study aims to develop a new control scheme by taking spatiotemporal azimuthal distribution of temperature fluctuation into account. The performance of the control was evaluated with amplitude of the surface temperature with and without control. The behavior of the present controlled system was compared with that of our previous one on the spectral analysis. The new control scheme achieved significant attenuation of the temperature oscillation compared with the previous one for high Marangoni number region.

  • 44.
    Kékesi, Timea
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Prahl Wittberg, Lisa
    KTH.
    Drop deformation and breakup2014In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 66, p. 1-10Article in journal (Refereed)
    Abstract [en]

    A Volume of Fluid (VOF) method is applied to investigate the deformation and breakup of an initially spherical drop in the bag- and shear breakup regimes, induced by steady disturbances. The onset of breakup is sought by studying steady-shape deformations while increasing the Weber number until breakup occurs. A parameter study is carried out applying different material properties and a wide range of drop Reynolds numbers in the steady wake regime. Density ratios of liquid to gas of 20, 40, and 80, viscosity ratios in the range 0.5-50, and Reynolds numbers between 20 and 200 are investigated for a constant Weber number of 20. The critical Weber number is found to be 12, in agreement with observations of earlier studies. For Weber number of 20 varying density, viscosity ratios and Reynolds numbers, interesting mixed breakup modes are discovered. Moreover, a new regime map including all modes observed is presented. A criterion for the transition between bag-and shear breakup is defined relating the competing inertial and shear forces appearing in the flow. Furthermore, results on breakup times and the time history of the drag coefficient are presented; the latter is concluded to be a potential parameter to indicate the occurrence of breakup. (C) 2014 Elsevier Ltd. All rights reserved.

  • 45.
    Laurila, T.
    et al.
    Aalto University School of Science, Aalto, Finland.
    Carlson, Andreas
    KTH.
    Do-Quang, Minh
    KTH.
    Ala-Nissila, T.
    Aalto University School of Science, Aalto, Finland / own University, Providence, United States.
    Amberg, Gustav
    KTH.
    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, article id 026320Article in journal (Refereed)
    Abstract [en]

    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.

  • 46.
    Lavalley, R.
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Alfredsson, P. Henrik
    KTH.
    Experimental and numerical investigation of nonlinear thermocapillary oscillations in an annular geometry2001In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 20, no 6, p. 771-797Article in journal (Refereed)
    Abstract [en]

    The present work investigates thermocapillary flow in a cylindrical configuration using large Prandtl number liquids. The flow is studied using coordinated simulations and experimental optical methods such as PIV and flow visualization. In this way, properties of the oscillatory state can be obtained in great detail. Considerable attention is given to the search for the parameters influencing the onset of the instability. It is found that the onset of oscillations can be correlated using a thermocapillary Reynolds number. The oscillations take the form of a standing wave close to the onset, which is replaced by a travelling wave for stronger forcing. The selection of azimuthal wave number of the oscillatory mode is determined from geometrical parameters, and resembles the wave number selection in vortex ring instabilities. Throughout we obtain good agreement between experiments and simulations using a mathematical model with an undeformed adiabatic free surface.

  • 47.
    Levenstam, M.
    et al.
    Chalmers University of Technology.
    Amberg, Gustav
    KTH.
    Winkler, C.
    KTH.
    Instabilities of thermocapillary convection in a half-zone at intermediate Prandtl numbers2001In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 13, no 4, p. 807-816Article in journal (Refereed)
    Abstract [en]

    The stability of thermocapillary convection inside a cylindrical liquid bridge is studied using both a direct numerical simulation of the three-dimensional problem and linear stability analysis of the axisymmetric basic state. Previously this has been studied extensively for low and high Prandtl numbers. However, the intermediate range of Prandtl numbers between approximately 0.07 and 0.8 which joins the low and high ranges is quite complicated and has not been studied to the same extent. One striking feature is that the axisymmetric base state is much more stable in this intermediate range than at high or low Prandtl numbers. We identify four different oscillatory modes in this range, which have different qualitative features. Direct numerical simulations have been carried out for representative parameter values, and show that the bifurcations are supercritical.

  • 48.
    Lin, Yuan
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Aldaeus, Fredrik
    KTH.
    Roeraade, Johan
    KTH.
    Simulation of dielectrophoretic motion of microparticles using a molecular dynamics approach2006In: 4th International Conference on Nanochannels, Microchannels and Minichannels, ICNMM2006, 2006, p. 1-10Conference paper (Refereed)
    Abstract [en]

    We model and simulate dielectrophoresis of microscale particles using the finite element method. A soft sphere system molecular dynamics model is presented, which solves a set of equations for the motion of every particle. The model couples most of the significant forces, i.e. the dielectrophoresis (DEP) forces, the particle-particle electrostatic forces, particle-particle interfacial repulsive forces, particle-wall repulsive forces and the hydrodynamic forces in Stokes flow. Since the system of equations is stiff, an implicit scheme is used. To obtain the particle trajectories, a constant time-step is applied. We present some numerical tests computing hydrodynamic force, electrostatic force and DEP force using our model, including simulated trapping of particles in a micro channel by dielectrophoresis. The results are in agreement with the theories and the experimental observations.

  • 49.
    Lin, Yuan
    et al.
    SINTEF Materials and Chemistry, Trondheim, Norway.
    Shiomi, Junichiro
    University of Tokyo, Tokyo, Japan.
    Amberg, Gustav
    KTH.
    Numerical calculation of the dielectrophoretic force on a slender body2009In: Electrophoresis, ISSN 0173-0835, E-ISSN 1522-2683, Vol. 30, no 5, p. 831-838Article in journal (Refereed)
    Abstract [en]

    In this paper, a model is proposed to numerically calculate the dielectrophoretic (DEP) force acting on a straight slender body in a non-uniform electric field. The induced charges are assumed to be located along the centerline of the slender body. By enforcing the boundary conditions at the interfaces of the two dielectrics, an integral equations system is obtained with the induced charge densities as unknowns. Based on the calculated induced charge densities, expressions to calculate the DEP force and torque are obtained. The calculated induced charge density of a prolate ellipsoid under a uniform electric field is compared with the analytic solution and an excellent agreement is achieved. The smaller the slenderness (the ratio of maximum radius to length of the slender body), the smaller the error is. The DEP force that a prolate ellipsoid experiences in a general electric field is numerically calculated and compared with the results obtained by the commonly accepted effective dipole moment method. The current model is expected to possess higher accuracy than the effective dipole moment method and to demand less calculation work than the Maxwell stress tensor method.

  • 50.
    Lin, Yuan
    et al.
    SINTEF Materials and Chemistry, Trondheim, Norway.
    Shiomi, Junichiro
    University of Tokyo, Tokyo, Japan.
    Maruyama, Shigeo
    University of Tokyo, Tokyo, Japan.
    Amberg, Gustav
    KTH.
    Dielectric relaxation of water inside a single-walled carbon nanotube2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 4, article id 045419Article in journal (Refereed)
    Abstract [en]

    We report a molecular dynamics study of anisotropic dynamics and dielectric properties of water confined inside a single-walled carbon nanotube (SWNT) at room temperature. The model includes dynamics of an SWNT described by a realistic potential function. A comparison with simulations assuming a rigid nanotube demonstrates that the popular assumption severely overestimates the dielectric constant for small diameter SWNTs. Simulations of water inside flexible SWNTs with various diameters reveal strong directional dependence of the dynamic and dielectric properties due to the confinement effect. The obtained dielectric permittivity spectra (DPS) identify two different dipolar relaxation frequencies corresponding to the axial and the cross-sectional directions, which are significantly smaller and larger than the single relaxation frequency of bulk water, respectively. The frequency variation increases as the SWNT diameter decreases. The results suggest that DPS can be used as a fingerprint of water inside SWNTs to monitor the water intrusion into SWNTs.

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