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  • 1.
    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.

  • 2.
    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.

  • 3.
    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.

  • 4.
    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.

  • 5.
    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.

  • 6.
    Shiomi, J.
    et al.
    KTH.
    Amberg, Gustav
    KTH.
    Active control of a global thermocapillary instability2002In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 14, no 9, p. 3039-3045Article in journal (Refereed)
    Abstract [en]

    Active control was applied to oscillatory thermocapillary flow in an open cylindrical container filled with Silicone oil. Thermocapillary convection was driven by imposing a radial temperature gradient on a flat free surface. This is an extension of the previous work by Shiomi who applied proportional feedback control by locally heating the surface at a single position using the local temperature signal at a different position fed back through a simple algorithm. Although significant attenuation of the oscillation was detected, an uncertainty remained if global stabilization was achieved. In the present paper, two sensor/heater pairs were installed to achieve the global suppression of the oscillation. Successful global stabilization of the oscillation was achieved in a range of Marangoni number, with the best performance in the weakly nonlinear regime. Using a reliable temperature measurement method, quantitative analysis is carried out to quantify the performance of the control. The optimal values of gain and relative position of sensor/heater pairs were identified.

  • 7.
    Shiomi, J.
    et al.
    University of Tokyo, Tokyo, Japan.
    Amberg, Gustav
    KTH.
    Numerical investigation of feedback control of thermocapillary instability2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 5, article id 054107Article in journal (Refereed)
    Abstract [en]

    Control of oscillatory thermocapillary convection in an annular geometry with a horizontal free surface is investigated by means of a numerical simulation. The objective is to suppress oscillations using a feedback opposition control. The temperature is measured at certain positions on the interface, this signal is amplified and used to apply local heating on the free surface. Many features of the controlled system observed in previous experiments could be reproduced by the simulation. The numerical simulation allows us to clarify the picture of the spatial structures of the controlled oscillation, which was not accessible in the experiments. In addition to what was found in the previous experiments, the present simulations also permit us to investigate the importance of the positioning of sensors and heaters, and the influence of the properties of the heaters.

  • 8.
    Wang, Yuli
    et al.
    KTH.
    Do-Quang, Minh
    KTH.
    Amberg, Gustav
    KTH.
    Viscoelastic droplet dynamics in a y-shaped capillary channel2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 3, article id 033103Article in journal (Refereed)
    Abstract [en]

    Non-Newtonian droplet dynamics commonly exist in microfluidic systems. We report simulations of viscoelastic (VE) droplets traveling in a two dimensional capillary bifurcation channel. A numerical system combining phase field method, VE rheology, and Stokes flow equations is built. As a generic microfluidic two-phase problem, we study how a non-Newtonian droplet that approaches a channel bifurcation will behave. We identify conditions for when a droplet will either split into two or be directed into one of the branches. In particular, we study the importance of the non-Newtonian properties. Our results reveal two different non-Newtonian mechanisms that can enhance splitting and non-splitting of droplets with respect to Newtonian droplets, depending on the size of droplet and capillary number.

1 - 8 of 8
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