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  • 1.
    af Klinteberg, Ludvig
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindbo, Dag
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tornberg, Anna-Karin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    An explicit Eulerian method for multiphase flow with contact line dynamics and insoluble surfactant2014In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 101, p. 50-63Article in journal (Refereed)
    Abstract [en]

    The flow behavior of many multiphase flow applications is greatly influenced by wetting properties and the presence of surfactants. We present a numerical method for two-phase flow with insoluble surfactants and contact line dynamics in two dimensions. The method is based on decomposing the interface between two fluids into segments, which are explicitly represented on a local Eulerian grid. It provides a natural framework for treating the surfactant concentration equation, which is solved locally on each segment. An accurate numerical method for the coupled interface/surfactant system is given. The system is coupled to the Navier-Stokes equations through the immersed boundary method, and we discuss the issue of force regularization in wetting problems, when the interface touches the boundary of the domain. We use the method to illustrate how the presence of surfactants influences the behavior of free and wetting drops.

  • 2.
    Alenius, Emma
    Department of Energy Sciences, Lund University, Sweden.
    Mode switching in a thick orifice jet, an LES and dynamic mode decomposition approach2014In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 90, p. 101-112Article in journal (Refereed)
    Abstract [en]

    The dynamics of a confined thick orifice plate jet, at Mach 0.4, are studied with dynamic mode decomposition (DMD), of the velocity from a large eddy simulation (LES). The jet exhibits strong periodic structures, due to an initially laminar shear layer, and a non-deterministic switching is observed between an axisymmetric and an azimuthal jet mode. The DMD captures the shape of these structures as different dynamic modes, but (by definition) not their true time-evolution. In order to study the time-evolution of semi-periodic structures in the flow, such as the jet modes that come and go in time, it is suggested to use DMD for identifying the shape of the structures and then calculate time-coefficients for them, by expressing the velocity field as a linear combination of the most important dynamic modes. These time-coefficients are then shown to capture the physics of the flow; they oscillate at the frequency of the corresponding mode, within an envelope with a non-deterministically varying period, representing the mode switching. Additionally, a time variation of the strength of the jet, represented by mode zero, is found to be related to this switching.

  • 3. Arlov, D.
    et al.
    Revstedt, J.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Numerical simulation of a gas-liquid Rushton stirred reactor - LES and LPT2008In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 37, no 7, p. 793-801Conference paper (Refereed)
    Abstract [en]

    Simulations of aerated stirred reactor is performed using a combination of large eddy simulation (LES) and Lagrangian particle tracking (LPT). A single impeller Rushton turbine is positioned at the center of the reactor and air is introduced at the bottom through a circular sparger. Effects of the gas volume flow, stirrer speed and sparger dimension are investigated. The results show that the time averaged liquid velocities in radial and tangential directions decrease with increasing gas volume fraction. In the axial direction, the gas redirects the radial jet upwards, breaking the symmetry of the ring vortices. Especially, for a narrower sparger, a more concentrated tilt upwards is observed with a larger region of negative axial velocity. Although, low aeration number is used, the periodicity from the impeller is decreasing and interfering with the creation of the trailing vortex pair. The gas dispersion increases with decreasing the sparger diameter.

  • 4. Bartuschat, D.
    et al.
    Fischermeier, E.
    Gustavsson, Katarina
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA.
    Rüde, U.
    Two computational models for simulating the tumbling motion of elongated particles in fluids2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 127, p. 17-35Article in journal (Refereed)
    Abstract [en]

    Suspensions with fiber-like particles in the low Reynolds number regime are modeled by two different approaches that both use a Lagrangian representation of individual particles. The first method is the well-established formulation based on Stokes flow that is formulated as integral equations. It uses a slender body approximation for the fibers to represent the interaction between them directly without explicitly computing the flow field. The second is a new technique using the 3D lattice Boltzmann method on parallel supercomputers. Here the flow computation is coupled to a computational model of the dynamics of rigid bodies using fluid-structure interaction techniques. Both methods can be applied to simulate fibers in fluid flow. They are carefully validated and compared against each other, exposing systematically their strengths and weaknesses regarding their accuracy, the computational cost, and possible model extensions.

  • 5. Brändle de Motta, J. C.
    et al.
    Costa, Pedro
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Derksen, J. J.
    Peng, C.
    Wang, L. -P
    Breugem, W. -P
    Estivalezes, J. L.
    Vincent, S.
    Climent, E.
    Fede, P.
    Barbaresco, P.
    Renon, N.
    Assessment of numerical methods for fully resolved simulations of particle-laden turbulent flows2019In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 179, p. 1-14Article in journal (Refereed)
    Abstract [en]

    During the last decade, many approaches for resolved-particle simulation (RPS) have been developed for numerical studies of finite-size particle-laden turbulent flows. In this paper, three RPS approaches are compared for a particle-laden decaying turbulence case. These methods are, the Volume-of-Fluid Lagrangian method, based on the viscosity penalty method (VoF-Lag); a direct forcing Immersed Boundary Method, based on a regularized delta function approach for the fluid/solid coupling (IBM); and the Bounce Back scheme developed for Lattice Boltzmann method (LBM-BB). The physics and the numerical performances of the methods are analyzed. Modulation of turbulence is observed for all the methods, with a faster decay of turbulent kinetic energy compared to the single-phase case. Lagrangian particle statistics, such as the velocity probability density function and the velocity autocorrelation function, show minor differences among the three methods. However, major differences between the codes are observed in the evolution of the particle kinetic energy. These differences are related to the treatment of the initial condition when the particles are inserted in an initially single-phase turbulence. The averaged particle/fluid slip velocity is also analyzed, showing similar behavior as compared to the results referred in the literature. The computational performances of the different methods differ significantly. The VoF-Lag method appears to be computationally most expensive. Indeed, this method is not adapted to turbulent cases. The IBM and LBM-BB implementations show very good scaling.

  • 6. Caraeni, D.
    et al.
    Fuchs, Laszlo
    Compact third-order multidimensional upwind discretization for steady and unsteady flow simulations2005In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 34, no 05-apr, p. 419-441Article in journal (Refereed)
    Abstract [en]

    We propose a new third-order multidimensional upwind algorithm for the solution of the flow equations on tetrahedral cells unstructured grids. This algorithm has a compact stencil (cell-based computations) and uses a finite element idea when computing the residual over the cell to achieve its third-order (spatial) accuracy. The construction of the new scheme is presented. The asymptotic accuracy for steady or unsteady, inviscid or viscous flow situations is proved using numerical experiments. The new high-order discretization proves to have excellent parallel scalability. Our studies show the advantages of the new compact third-order scheme when compared with the classical second-order multidimensional upwind schemes.

  • 7.
    Du, Lin
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Sun, X.
    Effect of flapping frequency on aerodynamics of wing in freely hovering flight2015In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 117, p. 79-87Article in journal (Refereed)
    Abstract [en]

    The two-dimensional incompressible Navier-Stokes equations are solved using the immersed boundary method. The wing is driven to translate in the horizontal direction and rotate periodically to emulate the wing motion of a fruit fly in normal hovering flight, while the motion in the vertical direction responds passively to the action of the wing aerodynamic lift and weight of the insect body. The insect body is modeled by a point mass. It is shown that flapping wing cannot produce required lift to maintain stable hovering flight in specified range with low flapping frequencies, if the insect weight is equivalent to the averaged wing lift in one cycle on the assumption of zero vertical velocity. The vertical velocity influences the instantaneous angle of attack of the hovering wing, which results in the variation in aerodynamics of the wing. The insect may experience fluctuating hovering flight with a reduced weight when the flapping frequency is low. The fluctuating amplitude decreases with increasing flapping frequency. The efficiency of hovering flight is also a problem of concern.

  • 8. Evegren, Philip
    et al.
    Revstedt, Johan
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Pulsating flow and mass transfer in an asymmetric system of bifurcations2011In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 49, no 1, p. 46-61Article in journal (Refereed)
    Abstract [en]

    Pulsating flow through bifurcations are of general interest. In the human body such flows are also very common; for example in blood vessels and the respiratory tract. The characteristics of the flow in arteries have been related to the process of atherogenesis, based on the observation that the initial manifestation of the process is observed at certain common locations, i.e., near bifurcations in vessels of certain size. Inspite of these observations there is no direct understanding between the flow itself and the pathological process. In fact, the flow itself is rather complex since it is unsteady and transitional. The paper considers both unsteady- and steady-flow through a three generation system of (non-symmetric) bifurcations. The geometry consists of a 90 degrees. bifurcation followed by two sets of consecutive symmetric bifurcations. The aim of the paper is to investigate the effects of the bifurcations on the flow and mass transport in such a geometrical configuration that is often found in physiological situations. Additionally, the effects of different inlet velocity conditions have been considered. The different inlet conditions are aimed at studying the sensitivity to variations of inflow conditions; variations found under normal physiological conditions. The results show that the geometrical asymmetry affects the velocity distribution even after a second bifurcation downstream. Two generations down this asymmetry does not have a significant effect any-more. The different inlet conditions affect the flow to the next generation of branches during parts of the cycle. At peak flow and further downstream in the system the effects are negligible. It is also found that over a cycle the mass flow distribution through the outlets can be affected by the inlet velocity conditions. The distribution of a passive scalar is not uniform but depends on the inlet conditions and the Schmidt number (i.e., molecular diffusion).

  • 9. Grosshans, H.
    et al.
    Movaghar, A.
    Cao, L.
    Oevermann, M.
    Szasz, R. -Z
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Sensitivity of VOF simulations of the liquid jet breakup to physical and numerical parameters2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 136, p. 312-323Article in journal (Refereed)
    Abstract [en]

    In this paper the characteristics of the primary breakup of a liquid jet is analyzed numerically. We applied the Volumes of Fluids (VOF) approach utilizing the Direction Averaged Curvature (DAC) model, to estimate the interface curvature, and the Direction Averaged Normal (DAN) model, to propagate the interface. While being used for the first time to predict liquid atomization, this methodology showed a high accuracy. The influence of varying the fluid properties, namely liquid-gas density and viscosity ratio, and injection conditions is discussed related to the required grid resolution. Resulting droplet sizes are compared to distributions obtained through the One-Dimensional Turbulence (ODT) model.

  • 10. Grosshans, H.
    et al.
    Szász, R. -Z
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Enhanced liquid-gas mixing due to pulsating injection2015In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 107, p. 196-204Article in journal (Refereed)
    Abstract [en]

    This paper considers the effects of intermittent injection of a liquid jet or spray on the initial break-up and mixing of one fluid with the surrounding ambient fluid. The aim of the analysis is to describe the physical process and indicate the mechanisms that control the mixing under different flow conditions (time-dependent injection and its frequency relative to the time scales of the flow) and fluid properties (density ratio), Schmidt number for a single phase case which is studied for comparison, or the Weber number for the two-phase cases. The computations use Large Eddy Simulation (LES) to account for turbulence, and either Volume Of Fluid (VOF) for the initial break-up or Lagrangian Particle Tracking (LPT) with droplet break-up model in the case of liquid droplets injected into the ambient gas. The results show that, depending on the physical properties of the liquid and ambient gas, the initial break-up and turbulent mixing can be enhanced substantially with intermittent injection. The numerical modeling is validated by recovering key results of experimental and analytical works. It can be observed that a main effect during the mixing is the suction of ambient fluid at the tail of the injected liquid, which depends on the fluid properties. Increased injection frequency shows to increase the mixing significantly during the initial transient phase.

  • 11.
    Hoffman, Johan
    et al.
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Jansson, Johan
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    de Abreu, Rodrigo Vilela
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Degirmenci, Niyazi Cem
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Jansson, Niclas
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Müller, Kaspar
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Nazarov, Murtazo
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Spühler, Jeannette Hiromi
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Unicorn: Parallel adaptive finite element simulation of turbulent flow and fluid-structure interaction for deforming domains and complex geometry2013In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 80, no SI, p. 310-319Article in journal (Refereed)
    Abstract [en]

    We present a framework for adaptive finite element computation of turbulent flow and fluid structure interaction, with focus on general algorithms that allow for complex geometry and deforming domains. We give basic models and finite element discretization methods, adaptive algorithms and strategies for efficient parallel implementation. To illustrate the capabilities of the computational framework, we show a number of application examples from aerodynamics, aero-acoustics, biomedicine and geophysics. The computational tools are free to download open source as Unicorn, and as a high performance branch of the finite element problem solving environment DOLFIN, both part of the FEniCS project.

  • 12. Khatri, Shilpa
    et al.
    Tornberg, Anna-Karin
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis, NA.
    A numerical method for two phase flows with insoluble surfactants2011In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 49, no 1, p. 150-165Article in journal (Refereed)
    Abstract [en]

    In many practical multiphase flow problems, i.e. treatment of gas emboli and various microfluidic applications, the effect of interfacial surfactants, or surface reacting agents, on the surface tension between the fluids is important. The surfactant concentration on an interface separating the fluids can be modeled with a time dependent differential equation defined on the moving and deforming interface. The equations for the location of the interface and the surfactant concentration on the interface are coupled with the Navier-Stokes equations. These equations include the singular surface tension forces from the interface on the fluid, which depend on the interfacial surfactant concentration. A new accurate and inexpensive numerical method for simulating the evolution of insoluble surfactants is presented in this paper. It is based on an explicit yet Eulerian discretization of the interface, which for two dimensional flows allows for the use of uniform one dimensional grids to discretize the equation for the interfacial surfactant concentration. A finite difference method is used to solve the Navier-Stokes equations on a regular grid with the forces from the interface spread to this grid using a regularized delta function. The timestepping is based on a Strang splitting approach. Drop deformation in shear flows in two dimensions is considered. Specifically, the effect of surfactant concentration on the deformation of the drops is studied for different sets of flow parameters.

  • 13.
    Kierkegaard, Axel
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Efraimsson, Gunilla
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Flow field eigenmode decompositions in aeroacoustics2010In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 39, no 2, p. 338-344Article in journal (Refereed)
    Abstract [en]

    In this paper an efficient method to study sound generation processes in low Mach number flows is presented. We apply the methodology on a two-dimensional flow over a cavity with smoothed corners. Instead of the full flow field obtained from, for example a Direct Numerical Simulation (DNS), we use a reduced model based on global modes to obtain the aeroacoustic sources. Global modes are eigenmodes to the Navier-Stokes equations, linearized about a steady base flow. In a reduced model the perturbations from a steady state are approximated by a linear combination of the eigenmodes. The time dependence is determined by the corresponding eigenvalues. Curie's equation is used to calculate the acoustic field, and by studying the source terms in Curie's equation, mechanisms for sources of sound are identified. Results of acoustic pressure in the far-field and source strengths for different superpositions of eigenmodes are presented.

  • 14.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Finite-volume scheme for the solution of integral boundary layer equations2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 132, p. 62-71Article in journal (Refereed)
    Abstract [en]

    An unstructured-mesh finite-volume formulation for the solution of systems of steady conservation laws on embedded surfaces is presented. The formulation is invariant to the choice of local tangential coordinate systems and is stabilized by a novel up-winding scheme applicable also to mixed-hyperbolic systems. The formulation results in a system of non-linear equations which is solved by a quasi-Newton method. While the finite volume scheme is applicable to a range of conservation laws, it is here implemented for the solution of the integral boundary layer equations, as a first step in developing a fully coupled viscous-inviscid interaction method. For validation purposes, integral boundary layer quantities computed using a minimal set of three-dimensional turbulent integral boundary layer equations are compared to experimental data and an established computer code for two-dimensional problems. The validation shows that the proposed formulation is stable, yields a well-conditioned global Jacobian, is conservative on curved surfaces and invariant to rotation as well as convergent with regard to mesh refinement.

  • 15.
    Lokatt, Mikaela
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Eller, David
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Robust viscous-inviscid interaction scheme for application on unstructured meshes2017In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 145, p. 37-51Article in journal (Refereed)
    Abstract [en]

    A coupled viscous-inviscid interaction scheme combining the continuity equation for potential flow with the three-dimensional integral boundary layer equations is presented. The inviscid problem is discretized by a finite-element approach whereas an upwind-biased finite-volume scheme is employed for the boundary layer equations. The discretization is applicable to unstructured tetrahedral-triangular meshes and results in a sparse system of non-linear equations which is solved by a Newton-type method. The mathematical reasons for the singularities commonly associated with the integral boundary layer equations in separated flow regions are analyzed and the connection between the mathematical singularities and the numerical ill-conditioning is discussed. It is shown that, by a suitable choice of closure relations, it is possible to obtain a boundary layer model free from numerical ill-conditioning in separated flow regions. The accuracy of the coupled viscous-inviscid model is investigated in a number of test cases including transitional and mildly separated flow over two different natural laminar flow airfoils and three-dimensional flow over a swept wing. It is concluded that the coupled method is able to provide reasonably accurate predictions of viscous and inviscid flow field quantities for the investigated cases.

  • 16.
    Marin, Oana
    et al.
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis, NA (closed 2012-06-30). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gustavsson, Katarina
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis, NA (closed 2012-06-30). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tornberg, Anna-Karin
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis, NA (closed 2012-06-30). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A highly accurate boundary treatment for confined Stokes flow2012In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 66, p. 215-230Article in journal (Refereed)
    Abstract [en]

    Fluid flow phenomena in the Stokesian regime abounds in nature as well as in microfluidic applications. Discretizations based on boundary integral formulations for such flow problems allow for a reduction in dimensionality but have to deal with dense matrices and the numerical evaluation of integrals with singular kernels. The focus of this paper is the discretization of wall confinements, and specifically the numerical treatment of flat solid boundaries (walls), for which a set of high-order quadrature rules that accurately integrate the singular kernel of the Stokes equations are developed. Discretizing by Nystrom's method, the accuracy of the numerical integration determines the accuracy of the solution of the boundary integral equations, and a higher order quadrature method yields a large gain in accuracy at negligible cost. The structure of the resulting submatrix associated with each wall is exploited in order to substantially reduce the memory usage. The expected convergence of the quadrature rules is validated through numerical tests, and this boundary treatment is further applied to the classical problem of a sedimenting sphere in the vicinity of solid walls.

  • 17. Mattsson, Ken
    et al.
    Svärd, Magnus
    Carpenter, Mark
    Nordstrom, Jan
    High-order accurate computations for unsteady aerodynamics2007In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 36, no 3, p. 636-649Article in journal (Refereed)
    Abstract [en]

    A high-order accurate finite difference scheme is used to perform numerical studies on the benefit of high-order methods. The main advantage of the present technique is the possibility to prove stability for the linearized Euler equations on a multi-block domain, including the boundary conditions. The result is a robust high-order scheme for realistic applications. Convergence studies are presented, verifying design order of accuracy and the superior efficiency of high-order methods for applications dominated by wave propagation. Furthermore, numerical computations of a more complex problem, a vortex-airfoil interaction, show that high-order methods are necessary to capture the significant flow features for transient problems and realistic grid resolutions. This methodology is easy to parallelize due to the multi-block capability. Indeed, we show that the speedup of our numerical method scales almost linearly with the number of processors.

  • 18.
    Muld, Tomas W.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Efraimsson, Gunilla
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aeroacoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Flow structures around a high-speed train extracted using Proper Orthogonal Decomposition and Dynamic Mode Decomposition2012In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 57, p. 87-97Article in journal (Refereed)
    Abstract [en]

    In this paper, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are used to extract the most dominant flow structures of a simulated flow in the wake of a high-speed train model, the Aerodynamic Train Model (ATM). The use of decomposition methods to successfully identify dominant flow structures for an engineering geometry is achieved by using a flow field simulated with the Detached Eddy Simulation model (DES), which is a turbulence model enabling time accurate solutions of the flows around engineering geometries. This paper also examines the convergence of the POD and DMD modes for this case. It is found that the most dominant DMD mode needs a longer sample time to converge than the most dominant POD mode. A comparison between the modes from the two different decomposition methods shows that the second and third POD modes correspond to the same flow structure as the second DMD mode. This is confirmed both by investigating the spectral content of the POD mode coefficients, and by comparing the spatial modes. The flow structure associated with these modes is identified as being vortex shedding. The identification is performed by reconstructing the flow field using the mean flow and the second DMD mode. A second flow structure, a bending of the counter-rotating vortices, is also identified. Identifying this flow structure is achieved by reconstructing the flow field with the mean flow and the fourth and fifth POD modes.

  • 19.
    Nair, Vineeth
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alenius, Emma
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Efraimsson, Gunilla
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Inspecting sound sources in an orifice-jet flow using Lagrangian coherent structures2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 140, p. 397-405Article in journal (Refereed)
    Abstract [en]

    A novel method is proposed to identify flow structures responsible for sound generation in confined flow past an inhibitor. Velocity fields obtained using Large Eddy Simulations (LES) are post-processed to compute the Finite Time Lyapunov Exponent (FTLE) field, the ridges of which in backward time represent an approximation to Lagrangian Coherent Structures (LCS), the structures that organize transport in the flow field. The flow-field is first decomposed using dynamic mode decomposition (DMD), and the organizing centers or vortices at the significant DMD frequencies are extracted. The results are then compared with the lambda(2) criterion. Features such as shear layer roll-up and development of secondary instabilities are more clearly visible in the FTLE field than with the lambda(2) criterion.

  • 20. Nordström, Jan
    et al.
    Ham, Frank
    Shoeybi, Mohammad
    van der Weide, Edwin
    Svärd, Magnus
    Mattsson, Ken
    Laccarino, Gianluca
    Gong, Jing
    Uppsala Univ., IT dept..
    A hybrid method for unsteady inviscid fluid flow2009In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 38, no 4, p. 875-882Article in journal (Refereed)
    Abstract [en]

    We show how a stable and accurate hybrid procedure for fluid flow can be constructed. Two separate solvers, one using high order finite difference methods and another using the node-centered unstructured finite volume method are coupled in a truly stable way. The two flow solvers run independently and receive and send information from each other by using a third coupling code. Exact solutions to the Euler equations are used to verify the accuracy and stability of the new computational procedure. We also demonstrate the capability of the new procedure in a calculation of the flow in and around a model of a coral.

  • 21.
    Pignier, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    O'Reilly, Ciarán
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Aerodynamic and aeroacoustic analyses of a submerged air inlet in a low-Mach-number flow2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 133, p. 15-31Article in journal (Refereed)
    Abstract [en]

    Computational aerodynamic and aeroacoustic analyses of a submerged air inlet are performed at a low Mach number. A hybrid method is used, in which the flow in the vicinity of the inlet is solved through detached eddy simulation (DES) and the acoustic pressure in the far-field is computed through the use of a Ffowcs Williams and Hawkings integral. Several surfaces of integration are used, both solid and permeable. The inlet design is based on an experimental inlet developed by the National Advisory Committee for Aeronautics (NACA). The flow is solved first through steady-state RANS simulation, then time-dependent DES is run from the converged results. The results from both RANS simulations and DES show good agreement with experimental data from NACA, both in terms of integral quantities and surface pressure coefficients. Pressure fluctuations are observed on both sides of the lip of the inlet, and are greater at low velocity ratios, with the velocity ratio defined as the ratio between the flow velocity at the duct entrance and in the free stream. A transition is observed between a quasi-laminar flow at a velocity ratio of 0.8 and a turbulent flow at velocity ratios of 0.6 and 0.4. This turbulent behaviour at low velocity ratios is associated with much higher acoustic levels in the far-field. At low velocity ratios, the acoustic spectra in the far-field exhibit a broadband character with maximum levels distributed around a characteristic frequency given by the ratio between the flow velocity at the duct entrance and the duct entrance depth. At high velocity ratios, the spectra show tonal characteristics with peaks at around 90 percent of this characteristic frequency and at the corresponding harmonics. A comparison between the spectra from solid and permeable surfaces reveals that volume sound sources are negligible at this low Mach number. A visualization of the integrands in the Ffowcs Williams and Hawkings integral show that sound sources are located on both sides of the lip of the inlet, at the position of impact of the vortices, and along the vortex wakes. Some observations regarding the use of solid and permeable surfaces of integration are made.

  • 22.
    Sakowitz, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Effects of velocity ratio and inflow pulsations on the flow in a T-junction by using Large Eddy Simulation2013In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 88, p. 374-385Article in journal (Refereed)
    Abstract [en]

    Large Eddy Simulations (LES) of the flow in a T-junction are performed and analyzed in terms of mixing quality, secondary structures and flow modes. Different mass flow ratios between the two inlet branches are applied and strong pulsating inflow conditions are considered in order to simulate engine-like conditions. The mixing quality is assessed by using several mixing parameters. For steady inflow conditions, it is found that the mixing quality depends on the mass flow ratio. For cases where the branch jet impinges on the opposite pipe wall, the largest turbulence intensities and mixing qualities are observed. Furthermore, the distribution of the mean concentration is dependent on the evolution of secondary structures, which also depend on the mass flow ratio. Flow pulsations are found to affect the mixing quality depending on the pulsation frequency. The spatial mixing quality is not necessarily enhanced by the pulsations. The flow structures are attributed to a Kelvin-Helmholtz instability due to the strong shear layer and vortex shedding past the branch jet. The normalized frequency of the vortex shedding is dependent on the mass flow ratio. Dynamical Mode Decomposition (DMD) is applied in order to investigate the spatial shapes of the flow modes. It is found that DMD is able to capture the vortex shedding mechanism even for the pulsating cases.

  • 23.
    Vilela de Abreu, Rodrigo
    et al.
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Jansson, Niclas
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Hoffman, Johan
    KTH, School of Computer Science and Communication (CSC), High Performance Computing and Visualization (HPCViz).
    Computation of aeroacoustic sources for a Gulfstream G550 nose landing gear model using adaptive FEM2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 124, p. 136-146Article in journal (Refereed)
    Abstract [en]

    This work presents a direct comparison of unsteady, turbulent flow simulations with measurements performed using a Gulfstream G550 nose landing gear model. The experimental campaign, which was carried out by researchers from the NASA Langley Research Center, provided a series of detailed, well documented wind-tunnel measurements for comparison and validation of computational fluid dynamics (CFD) and computational aeroacoustics (CAA) methodologies. Several computational efforts were collected and presented at the Benchmark for Airframe Noise Computation workshops, BANC-I and II. For our simulations, we used a General Galerkin finite element method (G2), where no explicit subgrid model is used, and where the computational mesh is adaptively refined with respect to a posteriori estimates of the error in a quantity of interest, here the source term in Lighthill's equation. The mesh is fully unstructured and the solution is time-resolved, which are key ingredients for solving problems of industrial relevance in the field of aeroacoustics. Moreover, we choose to model the boundary layers on the landing gear geometry with a free-slip condition for the velocity, which we previously observed to produce good results for external flows at high Reynolds numbers, and which considerably reduces the amount of cells required in the mesh. The comparisons presented here are an attempt to quantify the accuracy of our models, methods and assumptions; to that end, several results containing both time-averaged and unsteady flow quantities, always side by side with corresponding experimental values, are reported. The main finding is that we are able to simulate a complex, unsteady flow problem using a parameter-free methodology developed for high Reynolds numbers, external aerodynamics and aeroacoustics applications.

  • 24. Vuorinen, V.
    et al.
    Larmi, M.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Boersma, B. J.
    A low-dissipative, scale-selective discretization scheme for the Navier-Stokes equations2012In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 70, p. 195-205Article in journal (Refereed)
    Abstract [en]

    A new Scale-Selective Discretization (SSD) procedure for the Navier-Stokes equations is proposed. The aim is to reduce the numerical dissipation of already existing numerical schemes to make the SSD scheme easily implementable to the existing CFD codes. In particular, the new procedure is designed to decrease the dissipation errors arising from the discretization of the convection term using upwind-biased convection schemes. Such dissipative errors reduce the quality of high-fidelity simulation approaches in fluid dynamics such as Large-Eddy Simulations (LES). The new discretization procedure is based on separating small and large scales of the flow using a high-pass filter. As a first pre-processing step the convecting velocity field u i is decomposed into a rapidly fluctuating part ui' using the high-pass filter and a smooth part ui-ui'. After this the derivatives involving ui-ui' may be discretized with a centered scheme whereas the derivatives involving ui' can be discretized using an upwind method. The new procedure is tested in Navier-Stokes simulations by implementing the method into a second order accurate incompressible finite volume code based on the fractional step method. The numerical tests on the 2D lid-driven cavity at laminar conditions Re=2500 imply that the new method clearly improves the quality of the simulations. At Re=10,000 the SSD scheme captures the post-critical state of the cavity flow. The advantages of the new method are quantitatively assessed by studying a 2D temporally evolving shear layer. The results imply that the SSD scheme significantly reduces the numerical diffusion in contrast to the conventional upwind-biased schemes. Results from marginally resolved turbulent channel flow at Re τ=590 imply that the new scheme can be used for 3D simulations.

  • 25.
    Winkler, Niklas
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Drugge, Lars
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Stensson Trigell, Annika
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Efraimsson, Gunilla
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, VinnExcellence Center for ECO2 Vehicle design.
    Coupling aerodynamics to vehicle dynamics in transient crosswinds including a driver model2016In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 138, p. 26-34Article in journal (Refereed)
    Abstract [en]

    In this paper we assess the order of model complexity needed to capture a vehicle behaviour during a transient crosswind event, regarding the interaction of the aerodynamic loads and the vehicle dynamic response. The necessity to perform a full dynamic coupling, including feedback in real-time, instead of a static coupling to capture the vehicle performance both with respect to aerodynamics and the vehicle dynamics is evaluated. The computations are performed for a simplified bus model that is exposed to a transient crosswind. The aerodynamic loads are obtained using Detached Eddy Simulation (DES) with the overset mesh technique coupled to a single-track model for the vehicle dynamics including a driver model with three sets of controller parameters to obtain a realistic scenario. Two degrees of freedom are handled by the vehicle dynamics model; lateral translation and yaw motion. The results show that the full dynamic coupling is needed for large yaw angles of the vehicle, where the static coupling over-predicts the aerodynamic loads and in turn the vehicle motion. © 2016 Elsevier Ltd

  • 26. Yee, H. C.
    et al.
    Sjögreen, Björn
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis and Computer Science, NADA.
    Adaptive filtering and limiting in compact high order methods for multiscale gas dynamics and MHD systems2008In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 37, no 5, p. 593-619Article in journal (Refereed)
    Abstract [en]

    The adaptive multistep linear and nonlinear filters for multiscale shock/turbulence gas dynamics and magnetohydrodynamics (MHD) flows of the authors are extended to include compact high order central differencing as the spatial base scheme. The adaptive mechanism makes used of multiresolution wavelet decomposition of the computed flow data as sensors for numerical dissipative control. The objective is to expand the work initiated in [Yee HC, Sjogreen B. Nonlinear filtering in compact high order schemes. In: Proceedings of the 19th ICNSP and 7th APPTC conference; 2005; J Plasma Phys 2006;72:833-36] and compare the performance of adaptive multistep filtering in compact high order schemes with adaptive filtering in standard central (non-compact) schemes for multiscale problems containing shock waves.

  • 27.
    Åsén, Per-Olov
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kreiss, Gunilla
    Uppsala Univ, Dept Informat Technol..
    Rempfer, Dietmar
    IIT, Chicago.
    Direct numerical simulations of localized disturbances in pipe Poiseuille flow2010In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 39, no 6, p. 926-935Article in journal (Refereed)
    Abstract [en]

    We consider pipe Poiseuille flow subjected to a disturbance which is highly localized in space. Experiments by Peixinho and Mullin have shown this disturbance to be efficient in triggering turbulence, yielding a threshold dependence on the required amplitude as R-1.5 on the Reynolds number, R. The experiments also indicate an initial formation of hairpin vortices, with each hairpin having a length of approximately one pipe radius, independent of the Reynolds number in the range of R = 2000-3000. We perform direct numerical simulations for R = 5000. The results show a packet of hairpin vortices traveling downstream, each having a length of approximately one pipe radius. The perturbation remains highly localized in space while being advected downstream for approximately 10 pipe diameters. Beyond that distance from the disturbance origin, the flow becomes severely disordered.

1 - 27 of 27
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