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  • 1. Adolfsson, J.
    et al.
    Dankowicz, H.
    Nordmark, Arne B.
    KTH, Superseded Departments, Mechanics.
    3D passive walkers: Finding periodic gaits in the presence of discontinuities2001In: Nonlinear dynamics, ISSN 0924-090X, E-ISSN 1573-269X, Vol. 24, no 2, p. 205-229Article in journal (Refereed)
    Abstract [en]

    This paper studies repetitive gaits found in a 3D passive walking mechanism descending an inclined plane. By using direct numerical integration and implementing a semi-analytical scheme for stability analysis and root finding, we follow the corresponding limit cycles under parameter variations. The 3D walking model, which is fully described in the paper, contains both force discontinuities and impact-like instantaneous changes of state variables. As a result, the standard use of the variational equations is suitably modified. The problem of finding initial conditions for the 3D walker is solved by starting in an almost planar configuration, making it possible to use parameters and initial conditions found for planar walkers. The walker is gradually transformed into a 3D walker having dynamics in all spatial directions. We present such a parameter variation showing the stability and the amplitude of the hip sway motion. We also show the dependence of gait cycle measurements, such as stride time, stride length, average velocity, and power consumption, on the plane inclination. The paper concludes with a discussion of some ideas on how to extend the present 3D walker using the tools derived in this paper.

  • 2.
    Adolfsson, Jesper
    KTH, Superseded Departments, Mechanics.
    Passive control of mechanical systems2001Doctoral thesis, comprehensive summary (Other scientific)
  • 3.
    Ahlström, Anders
    KTH, Superseded Departments, Mechanics.
    Simulating Dynamical Behaviour of Wind Power Structures2002Licentiate thesis, monograph (Other scientific)
    Abstract [en]

    The workin this thesis deals with the development of anaeroelastic simulation tool for horizontal axis wind turbineapplications.

    Horizontal axiswind turbines can experience significanttime varying aerodynamic loads, potentially causing adverseeffects on structures, mechanical components, and powerproduction. The need of computational and experimentalprocedures for investigating aeroelastic stability and dynamicresponse have increased as wind turbines become lighter andmore flexible.

    A finite element model for simulation of the dynamicresponse of horizontal axis wind turbines has been developed.The simulations are performed using the commercial finiteelement software SOLVIA, which is a program developed forgeneral analyses, linear as well as non-linear, static as wellas dynamic. The aerodynamic model, used to transform the windflow field to loads on the blades, is a Blade- Element/Momentummodel. The aerodynamic code is developed by FFA (TheAeronautical Research Institute of Sweden) and is astate-of-the-art code incorporating a number of extensions tothe Blade-Element/Momentum formulation. SOSIS-W, developed byTeknikgruppen AB was used to develop wind time series formodelling different wind conditions.

    The model is rather general, and different configurations ofthe structural model and various type of wind conditions couldeasily be simulated. The model is primarily intended for use asa research tool when influences of specific dynamic effects areinvestigated.

    Simulation results for the three-bladed wind turbine Danwin180 kW are presented as a verification example.

    Keywords:aeroelastic modelling, rotor aerodynamics,structural dynamics, wind turbine, AERFORCE, SOSIS-W,SOLVIA

  • 4.
    Alvelius, Krister
    KTH, Superseded Departments, Mechanics.
    Large eddy simulation of homogeneous turbulence1997Licentiate thesis, comprehensive summary (Other scientific)
  • 5.
    Alvelius, Krister
    KTH, Superseded Departments, Mechanics.
    Studies of turbulence and its modelling through large eddy- and direct numerical simulation1999Doctoral thesis, comprehensive summary (Other scientific)
  • 6.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Semisharp phase field method for quantitative phase change simulations2003In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 91, no 26Article 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.

  • 7.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Solidification microstructure, dendrites and convection2004In: PHASE CHANGE WITH CONVECTION: MODELLING AND VALIDATION / [ed] Kowalewski, TA., Gobin, D., 2004, no 449, p. 1-53Conference paper (Refereed)
  • 8.
    Andersson, Andreas
    et al.
    KTH, Superseded Departments, Mechanics.
    Malm, Richard
    KTH, Superseded Departments, Mechanics.
    Measurement evaluation and FEM simulation of bridge dynamics2004Independent thesis Basic level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The aim of this thesis is to analyse the effects of train induced vibrations in a steel Langer beam bridge. A case study of a bridge over the river Ljungan in Ånge has been made by analysing measurements and comparing the results with a finite element model in ABAQUS. The critical details of the bridge are the hangers that are connected to the arches and the main beams. A stabilising system has been made in order to reduce the vibrations which would lead to increased life length of the bridge.

    Initially, the background to this thesis and a description of the studied bridge are presented. An introduction of the theories that has been applied is given and a description of the modelling procedure in ABAQUS is presented.

    The performed measurements investigated the induced strain and accelerations in the hangers. The natural frequency, the corresponding damping coefficients and the displacement these vibrations leads to has been evaluated. The vibration-induced stresses, which could lead to fatigue, have been evaluated. The measurement was made after the existing stabilising system has been dismantled and this results in that the risk of fatigue is excessive. The results were separated into two parts: train passage and free vibrations. This shows that the free vibrations contribute more and longer life expectancy could be achieved by introducing dampers, to reduce the amplitude of the amplitude of free vibrations.

    The finite element modelling is divided into four categories: general static analysis, eigenvalue analysis, dynamic analysis and detailed analysis of the turn buckle in the hangers. The deflection of the bridge and the initial stresses due to gravity load were evaluated in the static analysis. The eigenfrequencies were extracted in an eigenvalue analysis, both concerning eigenfrequencies in the hangers as well as global modes of the bridge. The main part of the finite element modelling involves the dynamic simulation of the train passing the bridge. The model shows that the longer hangers vibrate excessively during the train passage because of resonance. An analysis of a model with a stabilising system shows that the vibrations are damped in the direction along the bridge but are instead increased in the perpendicular direction. The results from the model agree with the measured data when dealing with stresses. When comparing the results concerning the displacement of the hangers, accurate filtering must be applied to obtain similar results.

  • 9.
    Andersson, Paul
    KTH, Superseded Departments, Mechanics.
    Modelling of boundary layer stability1999Doctoral thesis, comprehensive summary (Other scientific)
  • 10.
    Andersson, Paul
    et al.
    KTH, Superseded Departments, Mechanics.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    Bottaro, A
    Henningson, Dan Stefan
    KTH, Superseded Departments, Mechanics.
    On the breakdown of boundary layer streaks2001In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 428, p. 29-60Article in journal (Refereed)
    Abstract [en]

    A scenario of transition to turbulence likely to occur during the development of natural disturbances in a flat-plate boundary layer is studied. The perturbations at the leading edge of the flat plate that show the highest potential for transient energy amplification consist of streamwise aligned vortices. Due to the lift-up mechanism these optimal disturbances lead to elongated streamwise streaks downstream, with significant spanwise modulation, Direct numerical simulations are used to follow the nonlinear evolution of these streaks and to verify secondary instability calculations. The theory is based on a linear Floquet expansion and focuses on the temporal, inviscid instability of these flow structures. The procedure requires integration in the complex plane, in the coordinate direction normal to the wall, to properly identify neutral modes belonging to the discrete spectrum. The streak critical amplitude, beyond which streamwise travelling waves are excited, is about 26% of the free-stream velocity. The sinuous instability mode (either the fundamental or the subharmonic, depending on the streak amplitude) represents the most dangerous disturbance. Varicose waves are more stable, and are characterized by a critical amplitude of about 37%. Stability calculations of streamwise streaks employing the shape assumption, carried out in a parallel investigation, are compared to the results obtained here using the nonlinearly modified mean fields; the need to consider a base flow which includes mean flow modification and harmonics of the fundamental streak is clearly demonstrated.

  • 11.
    Angele, Kristian
    KTH, Superseded Departments, Mechanics.
    Experimental studies of turbulent boundary layer separation and control2003Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    The object ofthe present work is to experimentally study thecase ofa turbulent boundary layer subjected to an AdversePressure Gradient (APG) with separation and reattachment. Thisconstitutes a good test case for advanced turbulence modeling.The work consists ofde sign of a wind-tunnel setup, developmentofP article Image Velocimetry (PIV) measurements and evaluationtechniques for boundary layer flows, investigations ofs calingofb oundary layers with APG and separation and studies oftheturbulence structure ofthe separating boundary layer withcontrol by means ofs treamwise vortices. The accuracy ofP IV isinvestigated in the near-wall region ofa zero pressure-gradientturbulent boundary layer at high Reynolds number. It is shownthat, by careful design oft he experiment and correctly appliedvalidation criteria, PIV is a serious alternative toconventional techniques for well-resolved accurate turbulencemeasurements. The results from peak-locking simulationsconstitute useful guide-lines for the effect on the turbulencestatistics. Its symptoms are identified and criteria for whenthis needs to be considered are presented. Different velocityscalings are tested against the new data base on a separatingAPG boundary layer. It is shown that a velocity scale relatedto the local pressure gradient gives similarity not only forthe mean velocity but also to some extent for the Reynoldsshear-stress. Another velocity scale, which is claimed to berelated to the maximum Reynolds shear-stress, gives the samedegree of similarity which connects the two scalings. However,profile similarity achieved within an experiment is notuniversal and this flow is obviously governed by parameterswhich are still not accounted for. Turbulent boundary layerseparation control by means ofs treamwise vortices isinvestigated. The instantaneous interaction between thevortices and the boundary layer and the change in the boundarylayer and turbulence structure is presented. The vortices aregrowing with the boundary layer and the maximum vorticity isdecreased as the circulation is conserved. The vortices arenon-stationary and subjected to vortex stretching. Themovements contribute to large levels ofthe Reynolds stresses.Initially non-equidistant vortices become and remainequidistant and are con- fined to the boundary layer. Theamount ofi nitial streamwise circulationwas found to be acrucial parameter for successful separation control whereas thevortex generator position and size is ofseco ndary importance.At symmetry planes the turbulence is relaxed to a nearisotropic state and the turbulence kinetic energy is decreasedcompared to the case without vortices.

    Keywords:Turbulence, Boundary layer, Separation,Adverse Pressure Gradient (APG), PIV, control, streamwisevortices, velocity scaling.

  • 12.
    Angele, Kristian
    KTH, Superseded Departments, Mechanics.
    PIV measurements in a separating turbulent APG boundary layer2000Licentiate thesis, monograph (Other scientific)
  • 13.
    Apazidis, Nicholas
    KTH, Superseded Departments, Mechanics.
    Focusing of strong shocks in an elliptic cavity2003In: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 13, no 2, p. 91-101Article in journal (Refereed)
    Abstract [en]

    Focusing of strong shock waves in a gas-filled thin chamber with an elliptic boundary is investigated theoretically and numerically. The process of reflection and convergence of cylindrical shocks generated at one of the focal points of the chamber is studied by means of Whitham's theory of geometrical shock dynamics (1957, 1959). The current calculations are based on the modified version of this theory, which takes into account the non-homogeneous flow ahead of the advancing shock, Apazidis and Lesser (1996). There are two main results of the present study indicating a qualitatively different behavior of strong shocks as compared to the linear acoustic case. (1) - the form of the reflected shock is not cylindrical and (2) - the reflected, converging shock does not focus at the geometrical second focal point of the chamber. The dependence of the form of the reflected shock and the location of its focusing point on the strength of the shock and the eccentricity of the chamber is investigated.

  • 14.
    Apazidis, Nicholas
    et al.
    KTH, Superseded Departments, Mechanics.
    Lesser, M. B.
    Tillmark, Nils
    KTH, Superseded Departments, Mechanics.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    An experimental and theoretical study of converging polygonal shock waves2002In: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 12, no 1, p. 39-58Article in journal (Refereed)
    Abstract [en]

    An experimental investigation was carried out to explore the possibility of producing converging polygonal shocks in an essentially two-dimensional cavity. Previous calculations by Apazidis and Lesser (1996) suggested that such configurations could be produced by reflecting a cylindrical outgoing shock from a smoothly altered circular boundary, the alteration having n-gonal symmetry. In the experiments the outgoing shock was produced by a spark discharge which yielded shocks in the Mach number range from 1.1 to 1.7 at a radius just prior to the reflection. Polygonal shocks were observed as predicted by using a modified form of geometrical shock dynamics, derived in the above paper. In addition, the modified theory was used to calculate the results of an experiment carried out by Sturtevant and Kulkarny (1976). The results of the numerical calculations were found to be in substantial agreement with both experiments, suggesting that the modifications in geometrical shock dynamics for non-uniform flow ahead of an advancing shock are useful in the case of shock focusing. The experiment. also showed that the polygonal shapes were stable in the examined range of shock Mach numbers. a result that may be of importance for a number of practical situations in which shock focusing is present.

  • 15.
    Audenis, Gérald
    KTH, Superseded Departments, Mechanics.
    On the impingement of a plane liquid jet on the wires of a paper machine1999Licentiate thesis, monograph (Other scientific)
  • 16.
    Battini, Jean-Marc
    et al.
    KTH, Superseded Departments, Mechanics.
    Pacoste, Costin
    On the choice of local element frame for corotational triangular shell elements2004In: Communications in Numerical Methods in Engineering, ISSN 1069-8299, E-ISSN 1099-0887, Vol. 20, no 10, p. 819-825Article in journal (Refereed)
    Abstract [en]

    In the context of corotational triangular shell elements, the objective of this paper is to show that for certain stability problems it is interesting to choose a local element frame invariant to the element node ordering. Two methods of obtaining such a local frame are presented. These two methods, already proposed by other authors, are reformulated. For the first one, based on the minimisation of local nodal displacements, it is shown that the iterative process can be avoided.

  • 17.
    Berlin, Stellan
    KTH, Superseded Departments, Mechanics.
    Oblique waves in boundary layer transition1998Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Traditional research on laminar-turbulent transition has focused on scenarios that are caused by the exponential growth of eigensolutions to the linearized disturbance equations, e.g. two-dimensional Tollmien-Schlichting waves. Recent research has reveled the existence of other non-modal growth mechanisms, for example associated with the transient growth of streamwise streaks.

    Oblique waves may trigger transition in which the new mechanisms is an important ingredient. We have investigated the role of oblique waves in boundary layer transition, using an efficient spectral code for direct numerical simulations.

    In the initial stage of this transition scenario oblique waves have been found to interact nonlinearly and forte streamwise vortices, which in turn forte growing stream wise streaks. If the streak amplitude reaches a threshold value, transition from laminar to turbulent flow Will take place.

    In the late transition stage, large velocity fluctuations are found at flow positions associated with steep spanwise gradients between the streaks. At those positions we have also found h-vortices, structures that are also characteristic for traditional sec ondary instability transition. The h-vortices are shown to be due to the interaction of oblique waves and streaks that seem to play a more important role in the late stage of transition than previously appreciated.

    The numerical results are compared in detail with experimental results on oblique transition and good agreement is found.

    A new nonlinear receptivity mechanism is found that can trigger boundary layer transition from oblique waves in the free-stream. The mechanism continuously interact with the boundary layer and the resulting transition scenario is characterized by the growth of streamwise streaks. The same structures that are observed in experiments on transition caused by free-stream turbulente. A linear receptivity mechanism that interact with the boundary layer downstream of the leading edge is also identified. It is related to linear receptivity mechanisms previously studied at the leading edge. The nonlinear and linear mechanisms are of comparable strength for moderate free-stream disturbance levels.

    Two strategies for control of oblique transition are investigated, both based on spanwise flow oscillations. The longest transition delay was found when the flow oscil lations were generated by a body forte. When the control was applied to a transition scenario initiated by a random disturbance it was more successful and transition was prevented.

  • 18.
    Birgersson, Erik
    KTH, Superseded Departments, Mechanics.
    Mathematical Modeling of Transport Phenomena in Polymer Electrolyte and Direct Methanol Fuel Cells2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    This thesis deals with modeling of two types of fuel cells:the polymer electrolyte fuel cell (PEFC) and the directmethanol fuel cell (DMFC), for which we address four majorissues: a) mass transport limitations; b) water management(PEFC); c) gas management (DMFC); d) thermal management.

    Four models have been derived and studied for the PEFC,focusing on the cathode. The first exploits the slenderness ofthe cathode for a two-dimensional geometry, leading to areduced model, where several nondimensional parameters capturethe behavior of the cathode. The model was extended to threedimensions, where four di.erent flow distributors were studiedfor the cathode. A quantitative comparison shows that theinterdigitated channels can sustain the highest currentdensities. These two models, comprising isothermal gasphaseflow, limit the studies to (a). Returning to a two-dimensionalgeometry of the PEFC, the liquid phase was introduced via aseparate flow model approach for the cathode. In addition toconservation of mass, momentum and species, the model wasextended to consider simultaneous charge and heat transfer forthe whole cell. Di.erent thermal, flow fields, and hydrodynamicconditions were studied, addressing (a), (b) and (d). A scaleanalysis allowed for predictions of the cell performance priorto any computations. Good agreement between experiments with asegmented cell and the model was obtained.

    A liquid-phase model, comprising conservation of mass,momentum and species, was derived and analyzed for the anode ofthe DMFC. The impact of hydrodynamic, electrochemical andgeometrical features on the fuel cell performance were studied,mainly focusing on (a). The slenderness of the anode allows theuse of a narrow-gap approximation, leading to a reduced model,with benefits such as reduced computational cost andunderstanding of the physical trends prior to any numericalcomputations. Adding the gas-phase via a multiphase mixtureapproach, the gas management (c) could also be studied.Experiments with a cell, equipped with a transparent end plate,allowed for visualization of the flow in the anode, as well asvalidation of the two-phase model. Good agreement betweenexperiments and the model was achieved.

    Keywords:Fuel cell; DMFC; PEFC; one-phase; two-phase;model; visual cell; segmented cell; scale analysis; asymptoticanalysis.

  • 19.
    Birgersson, Erik
    et al.
    KTH, Superseded Departments, Mechanics.
    Nordlund, Joakim
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Vynnycky, Michael
    KTH, Superseded Departments, Mechanics.
    Picard, Cyril
    KTH, Superseded Departments, Mechanics.
    Lindbergh, Göran
    KTH, Superseded Departments, Chemical Engineering and Technology.
    Reduced two-phase model for analysis of the anode of a DMFC2004In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 151, no 12, p. A2157-A2172Article in journal (Refereed)
    Abstract [en]

    An isothermal two-phase ternary mixture model that takes into account conservation of momentum, mass, and species in the anode of a direct methanol fuel cell (DMFC) is presented and analyzed. The slenderness of the anode allows a considerable reduction of the mathematical formulation, without sacrificing the essential physics. The reduced model is then verified and validated against data obtained from an experimental DMFC outfitted with a transparent end plate. Good agreement is achieved. The effect of mass-transfer resistances in the flow field and porous backing are highlighted. The presence of a gas phase is shown to improve the mass transfer of methanol at higher temperatures (>30 degreesC). It is also found that at a temperature of around 30 degreesC, a one-phase model predicts the same current density distribution as a more sophisticated two-phase model. Analysis of the results from the two-phase model, in combination with the experiments, results in a suggestion for an optimal flow field for the liquid-fed DMFC.

  • 20. Borg, K I
    et al.
    Soderholm, Lars
    KTH, Superseded Departments, Mechanics.
    Essen, Hanno
    KTH, Superseded Departments, Mechanics.
    Response to "Comment on 'Force on a spinning sphere moving in a rarefied gas' and 'On the inverse Magnus effect in free molecular flow'" [Phys. Fluids 16, 3832 (2004)]2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, p. 3833-3833Article in journal (Other academic)
  • 21. Borg, K. I.
    et al.
    Söderholm, Lars H.
    KTH, Superseded Departments, Mechanics.
    Essén, Hanno
    KTH, Superseded Departments, Mechanics.
    Force on a spinning sphere moving in a rarefied gas2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 3, p. 736-741Article in journal (Refereed)
    Abstract [en]

    The force acting on a spinning sphere moving in a rarefied gas is calculated. It is found to have three contributions with different directions. The transversal contribution is of opposite direction compared to the so-called Magnus force normally exerted on a sphere by a dense gas. It is given by F=-alpha(tau)xi2/3piR(3)mnomegaxv, where alpha(tau) is the accommodation coefficient of tangential momentum, R is the radius of the sphere, m is the mass of a gas molecule, n is the number density of the surrounding gas, omega is the angular velocity, and v is the velocity of the center of the sphere relative to the gas. The dimensionless factor xi is close to unity, but depends on omega and kappa, the heat conductivity of the body.

  • 22.
    Borg, Karl
    KTH, Superseded Departments, Mechanics.
    Dynamics of bodies small compared to the mean free path in gases2003Doctoral thesis, comprehensive summary (Other scientific)
  • 23.
    Borg, Karl
    KTH, Superseded Departments, Mechanics.
    Transport of bodies small compared to the mean free path in non-uniform gases2001Licentiate thesis, comprehensive summary (Other scientific)
  • 24.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    Numerical studies of bypass transition in the Blasius boundary layer2003Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak.

    Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed.

    Keywords:Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.

  • 25.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    Study of generation, growth and breakdown of streamwise streaks in a Blasius boundary layer.2001Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Transition from laminar to turbulent flow has beentraditionally studied in terms of exponentially growingeigensolutions to the linearized disturbance equations.However, experimental findings show that transition may occuralso for parameters combinations such that these eigensolutionsare damped. An alternative non-modal growth mechanism has beenrecently identified, also based on the linear approximation.This consists of the transient growth of streamwise elongateddisturbances, mainly in the streamwise velocity component,called streaks. If the streak amplitude reaches a thresholdvalue, secondary instabilities can take place and provoketransition. This scenario is most likely to occur in boundarylayer flows subject to high levels of free-stream turbulenceand is the object of this thesis. Different stages of theprocess are isolated and studied with different approaches,considering the boundary layer flow over a flat plate. Thereceptivity to free-stream disturbances has been studiedthrough a weakly non-linear model which allows to disentanglethe features involved in the generation of streaks. It is shownthat the non-linear interaction of oblique waves in thefree-stream is able to induce strong streamwise vortices insidethe boundary layer, which, in turn, generate streaks by thelift-up effect. The growth of steady streaks is followed bymeans of Direct Numerical Simulation. After the streaks havereached a finite amplitude, they saturate and a new laminarflow, characterized by a strong spanwise modulation isestablished. Using Floquet theory, the instability of thesestreaks is studied to determine the features of theirbreakdown. The streak critical amplitude, beyond which unstablewaves are excited, is 26% of the free-stream velocity. Theinstability appears as spanwise (sinuous-type) oscillations ofthe streak. The late stages of the transition, originating fromthis type of secondary instability, are also studied. We foundthat the main structures observed during the transition processconsist of elongated quasi-streamwise vortices located on theflanks of the low speed streak. Vortices of alternating signare overlapping in the streamwise direction in a staggeredpattern.

    Descriptors:Fluid mechanics, laminar-turbulenttransition, boundary layer flow, transient growth, streamwisestreaks, lift-up effect, receptivity, free-stream turbulence,nonlinear mechanism, streak instability, secondary instability,Direct Numerical Simulation.

  • 26.
    Brandt, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Cossu, C.
    Chomaz, J. M.
    Huerre, P.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    On the convectively unstable nature of optimal streaks in boundary layers2003In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 485, p. 221-242Article in journal (Refereed)
    Abstract [en]

    The objective of the study is to determine the absolute/convective nature of the secondary instability experienced by finite-amplitude streaks in the flat-plate boundary layer. A family of parallel streaky base flows is defined by extracting velocity profiles from direct numerical simulations of nonlinearly saturated optimal streaks. The computed impulse response of the streaky base flows is then determined as a function of streak amplitude and streamwise station. Both the temporal and spatio-temporal instability properties are directly retrieved from the impulse response wave packet, without solving the dispersion relation or applying the pinching point criterion in the complex wavenumber plane. The instability of optimal streaks is found to be unambiguously convective for all streak amplitudes and streamwise stations. It is more convective than the Blasius boundary layer in the absence of streaks; the trailing edge-velocity of a Tollmien-Schlichting wave packet in the Blasius boundary layer is around 35% of the free-stream velocity, while that of the wave packet riding on the streaky base flow is around 70%. This is because the streak instability is primarily induced by the spanwise shear and the associated Reynolds stress production term is located further away from the wall, in a larger velocity region, than for the Tollmien-Schlichting instability. The streak impulse response consists of the sinuous mode of instability triggered by the spanwise wake-like profile, as confirmed by comparing the numerical results with the absolute/convective instability properties of the family of two-dimensional wakes introduced by Monkewitz (1988). The convective nature of the secondary streak instability implies that the type of bypass transition studied here involves streaks that behave as amplifiers of external noise.

  • 27.
    Brandt, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    Direct numerical simulations of streak breakdown in boundary layers2004In: Direct and Large-Eddy Simulation V, Proceedings / [ed] Friedrich, R; Geurts, BJ; Metais, O, 2004, Vol. 9, p. 175-196Conference paper (Refereed)
    Abstract [en]

    Numerical simulations of bypass transition in Blasius boundary layers are presented. The breakdown of streamwise streaks is first considered in the case of the steady, spanwise periodic basic flows arising from the nonlinear saturation of optimal perturbations and then in the case of transition in boundary layers subject to free-stream turbulence. Similarity and differences with previous work are discussed.

  • 28.
    Brandt, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Henningson, Dan Stefan
    KTH, Superseded Departments, Mechanics.
    Transition of streamwise streaks in zero-pressure-gradient boundary layers2002In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 472, p. 229-261Article in journal (Refereed)
    Abstract [en]

    A transition scenario initiated by streamwise low- and high-speed streaks in a flat-plate boundary layer is studied. In many shear flows, the perturbations that show the highest potential for transient energy amplification consist of streamwise-aligned vortices. Due to the lift-up mechanism these optimal disturbances lead to elongated streamwise streaks downstream, with significant spanwise modulation. In a previous investigation (Andersson et al. 2001), the stability of these streaks in a zero-pressure-gradient boundary layer was studied by means of Floquet theory and numerical simulations. The sinuous instability mode was found to be the most dangerous disturbance. We present here the first simulation of the breakdown to turbulence originating from the sinuous instability of streamwise streaks. The main structures observed during the transition process consist of elongated quasi-streamwise vortices located on the flanks of the low-speed streak. Vortices of alternating sign are overlapping in the streamwise direction in a staggered pattern. The present scenario is compared with transition initiated by Tollmien-Schlichting waves and their secondary instability and by-pass transition initiated by a pair of oblique waves. The relevance of this scenario to transition induced by free-stream turbulence is also discussed.

  • 29.
    Brandt, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Henningson, Dan Stefan
    KTH, Superseded Departments, Mechanics.
    Ponziani, D
    Weakly nonlinear analysis of boundary layer receptivity to free-stream disturbances2002In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 14, no 4, p. 1426-1441Article in journal (Refereed)
    Abstract [en]

    The intent of the present paper is to study the receptivity of a zero pressure gradient boundary layer to free-stream disturbances with the aim to isolate the essential features involved in the generation of streamwise streaks. A weakly nonlinear formulation based on a perturbation expansion in the amplitude of the disturbance truncated at second order is used. It is shown that the perturbation model provide an efficient tool able to disentangle the sequence of events in the receptivity process. Two types of solutions are investigated: the first case amounts to the receptivity to oblique waves generated by a wave-like external forcing term oscillating in the free stream, the second the receptivity to free-stream turbulence-like disturbances, represented as a superposition of modes of the continuous spectrum of the Orr-Sommerfeld and Squire operators. A scaling property of the governing equations with the Reynolds number is also shown to be valid. The relation between this nonlinear receptivity process and previously investigated linear ones is also discussed.

  • 30.
    Brandt, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Schlatter, Philipp
    KTH, Superseded Departments, Mechanics.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    Transition in boundary layers subject to free-stream turbulence2004In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 517, p. 167-198Article in journal (Refereed)
    Abstract [en]

    The effect of high levels of free-stream turbulence on the transition in a Blasius boundary layer is studied by means of direct numerical simulations, where a synthetic turbulent inflow is obtained as superposition of modes of the continuous spectrum of the Orr-Sommerfeld and Squire operators. In the present bypass scenario the flow in the boundary layer develops streamwise elongated regions of high and low streamwise velocity and it is suggested that the breakdown into turbulent spots is related to local instabilities of the strong shear layers associated with these streaks. Flow structures typical of the spot precursors are presented and these show important similarities with the flow structures observed in previous studies on the secondary instability and breakdown of steady symmetric streaks. Numerical experiments are performed by varying the energy spectrum of the incoming perturbation. It is shown that the transition location moves to lower Reynolds numbers by increasing the integral length scale of the free-stream turbulence. The receptivity to free-stream turbulence is also analysed and it is found that two distinct physical mechanisms are active depending on the energy content of the external disturbance. If low-frequency modes diffuse into the boundary layer, presumably at the leading edge, the streaks Lire induced by streamwise vorticity through the linear lift-up effect. If, conversely, the free-stream perturbations are mainly located above the boundary layer a nonlinear process is needed to create streamwise vortices inside the shear layer. The relevance of the two mechanisms is discussed.

  • 31.
    Brunet, Philippe
    et al.
    KTH, Superseded Departments, Mechanics.
    Limat, L
    Defects and spatiotemporal disorder in a pattern of falling liquid columns2004In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 70, no 4, p. 046207-Article in journal (Refereed)
    Abstract [en]

    Disordered regimes of a one-dimensional pattern of liquid columns hanging below an overflowing circular dish are investigated experimentally. The interaction of two basic dynamical modes (oscillations and drift) combined with the occurrence of defects (birth of new columns, disappearances by coalescences of two columns) leads to spatiotemporal chaos. When the flow rate is progressively increased, a continuous transition between transient and permanent chaos is pointed into evidence. We introduce the rate of defects as the sole relevant quantity to quantify this "turbulence" without ambiguity. Statistics on both transient and endlessly chaotic regimes enable to define a critical flow rate around which exponents are extracted. Comparisons are drawn with other interfacial pattern-forming systems, where transition towards chaos follows similar steps. Qualitatively, careful examinations of the global dynamics show that the contamination processes are nonlocal and involve the propagation of blocks of elementary laminar states (such as propagative domains or local oscillations), emitted near the defects, which turn out to be essential ingredients of this self-sustained disorder.

  • 32.
    Brüger, Arnim
    KTH, Superseded Departments, Mechanics.
    A hybrid high order method for simulation of turbulent flow in complex geometries2004Doctoral thesis, comprehensive summary (Other scientific)
  • 33.
    Brüger, Arnim
    KTH, Superseded Departments, Mechanics.
    Higher order methods suitable for direct numerical simulation of flows in complex geometries2002Licentiate thesis, comprehensive summary (Other scientific)
  • 34.
    Caraghiaur, Diana
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.
    Frid, Wiktor
    KTH, Superseded Departments, Energy Technology.
    Tillmark, Nils
    KTH, Superseded Departments, Mechanics.
     Detailed pressure drop measurements in single- and two-phase adiabatic air-water turbulent flows in realistic BWR fuel assembly geometry with spacer grids2004In: The 6th International Conference on Nuclear Thermal Hydraulics, Operations and Safety (NUTHOS-6) Nara, Japan, October 4-8, 2004, 2004Conference paper (Refereed)
    Abstract [en]

    In recent years, advanced numerical simulation tools based on CFD methods have been increasingly used in various multi-phase flow applications. One of these is two-phase flow in fuel assemblies of Boiling Water Reactors. The important and often missing aspect of this development is the validation of CFD codes against proper experimental data. The purpose of the current paper is to present detailed pressure measurements over a spacer grid in adiabatic single- and two-phase flow, which will be used to validate and further develop a CFD code for BWR fuel bundle analysis. The experiments have been carried out in an asymmetric 24-rod sub-bundle, representing ¼ of Westinghouse SVEA-96 nuclear reactor fuel assembly. Single-phase measurements have been performed at superficial velocities comprised between jliq: 0.90 – 4.50 m/s and in the two-phase, which was simulated by air-water mixture, measurements have been performed at void fractions ranging from 4 to 12% and liquid superficial velocity jliq : 4.50 m/s. In order to increase the number of the measured points, five pressure taps were drilled in one of the rods, which was easily moved vertically by a traverse system, covering most of the points in axial direction. The possibility to substitute any of the rods in the fuel bundle by the pressure sensing rod and the possibility to change the pressure taps facing-angle provides more measuring points inside the subchannels. A detailed pressure distribution comparison between single- and two-phase flows for different subchannel positions and different flow conditions was performed over one of the spacers.  In addition, single-phase pressure drop measurements on the upper part of the test section comprising two spacer grids has been carried out.

  • 35.
    Chevalier, Mattias
    KTH, Superseded Departments, Mechanics.
    Adjoint based control and optimization of aerodynamic flows2002Licentiate thesis, comprehensive summary (Other scientific)
  • 36.
    Chevalier, Mattias
    KTH, Superseded Departments, Mechanics.
    Feedback and adjoint based control of boundary layer flows2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Linear and nonlinear optimal control have been investigated in transitional channel and boundary layer .ows. The flow phenomena that we study are governed by the incompressible Navier–Stokes equations and the main aim with the control is to prevent transition from laminar to turbulent flows. A linear model-based feedback control approach, that minimizes an objective function which measures the perturbation energy, can be formulated where the Orr– Sommerfeld/Squire equations model the flow dynamics. A limitation with the formulation is that it requires complete state information. However, the control problem can be combined with a state estimator to relax this requirement. The estimator requires only wall measurements to reconstruct the flow in an optimal manner.

    Physically relevant stochastic models are suggested for the estimation problem which turns out to be crucial for fast convergence. Based on these models the estimator is shown to work for both in.nitesimal as well as finite amplitude perturbations in direct numerical simulations of a channel flow at Recl = 3000.

    A stochastic model for external disturbances is also constructed based on statistical data from a turbulent channel flow at ReT = 100. The model is successfully applied to estimate a turbulent channel flow at the same Reynolds number.

    The combined control and estimation problem, also known as a compensator, is applied to spatially developing boundary layers. The compensator is shown to successfully reduce the perturbation energy for Tollmien–Schlichting waves and optimal perturbations in the Blasius boundary layer. In a Falkner– Skan–Cooke boundary layer the perturbation energy of traveling and stationary cross-flow disturbances are also reduced.

    A nonlinear control approach using the Navier–Stokes equations and the associated adjoint equations are derived and implemented in the context of direct numerical simulations of spatially-developing three-dimensional boundary layer .ows and the gradient computation is veri.ed with .nite-di.erences. The nonlinear optimal control is shown to be more e.cient in reducing the disturbance energy than feedback control when nonlinear interactions are becoming signi.cant in the boundary layer. For weaker disturbances the two methods are almost indistinguishable.

  • 37. Cho, J. Y. N.
    et al.
    Lindborg, Erik
    KTH, Superseded Departments, Mechanics.
    Horizontal velocity structure functions in the upper troposphere and lower stratosphere 1. Observations2001In: Journal of Geophysical Research-Atmospheres, ISSN 0747-7309, Vol. 106, no D10, p. 10223-10232Article in journal (Refereed)
    Abstract [en]

    We compute horizontal velocity structure functions using quasiglobal data accumulated by specially equipped commercial aircraft on 7630 flights from August 1994 to December 1997. Using the ozone concentration measurements, we classify the results as tropospheric or stratospheric. We further divide the results into four absolute latitude bands. For separation distance r between 10 and 100 km, the lower stratospheric diagonal third-order structure functions are proportional to negative r. This implies a downscale energy cascade, and we estimate the mean energy dissipation rate to be (E) approximate to 6 x 10(-5) m(2) s(-3). For r between 300 and 1500 km a positive r(3) dependence was visible for the polar stratospheric data. This may be the result of a two-dimensional (2D) turbulence downscale enstrophy cascade, and we estimate the average enstrophy flux to be IIomega approximate to 2 x 10(-15) s(-3) and the energy spectral constant to be K approximate to 2. The negative sign of these third-order functions at mesoscales in both the upper troposphere and lower stratosphere provide no support for an inverse energy cascade 2D turbulence. At scales above similar to 100 km, the second-order structure functions increase with latitude in the troposphere and decrease with latitude in the stratosphere. The off-diagonal third-order functions in the stratosphere show a remarkably clean negative r(2) dependency from 10 to 1000 km in scale.

  • 38. Cossu, C.
    et al.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    On Tollmien-Schlichting-like waves in streaky boundary layers2004In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 23, no 6, p. 815-833Article in journal (Refereed)
    Abstract [en]

    The linear stability of the boundary layer developing on a flat plate in the presence of finite-amplitude, steady and spanwise periodic streamwise streaks is investigated. The streak amplitudes considered here are below the threshold for onset of the inviscid inflectional instability of sinuous perturbations. It is found that, as the amplitude of the streaks is increased, the most unstable viscous waves evolve from two-dimensional Tollmien-Schlichting waves into three-dimensional varicose fundamental modes which compare well with early experimental findings. The analysis of the growth rates of these modes confirms the stabilising effect of the streaks on the viscous instability and that this stabilising effect increases with the streak amplitude. Varicose subharmonic modes are also found to be unstable but they have growth rates which typically are an order of magnitude lower than those of fundamental modes. The perturbation kinetic energy production associated with the spanwise shear of the streaky flow is found to play an essential role in the observed stabilisation. The possible relevance of the streak stabilising role for applications in boundary layer transition delay is discussed.

  • 39. Cossu, C.
    et al.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    Stabilization of Tollmien-Schlichting waves by finite amplitude optimal streaks in the Blasius boundary layer2002In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 14, no 8, p. L57-L60Article in journal (Refereed)
    Abstract [en]

    In this Letter we show by numerical simulation that streamwise streaks of sufficiently large amplitude are able to stabilize Tollmien-Schlichting waves in zero pressure gradient boundary layers at least up to Re=1000. This stabilization is due to the spanwise averaged part of the nonlinear basic flow distortion induced by the streaks and occurs for streak amplitudes lower than the critical threshold beyond which secondary inflectional instability is observed. A new control strategy is implemented using optimal perturbations in order to generate the streaks.

  • 40.
    Danielsson, Carl-Ola
    KTH, Superseded Departments, Mechanics.
    Continuous electropermutation using ion-exchange textile2004Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Increased levels of nitrate in ground water has made many wells unsuitable as sources for drinking water. In this thesis an ion-exchange assisted electromembrane process, suitable for nitrate removal, is investigated both theoretically and experimentally. A new ion-exchange textile material is introduced as a conducting spacer in the feed compartment of an continuous electropermutation cell. The ion-exchange textile have a high permeability and provides faster ion-exchange kinetics compared to ion-exchange resins. The sheet shaped structure of the textile makes it easy to incorporate into the cell. A report on the development of a new electro-membrane module, capable of incorporating an ion-exchange textile spacer, is presented. A theoretical study of the flow field through the electro-membrane module was performed using two different 2-D models. The calculated flow distributions provided by different proposed module designs were compared and a prototype module was produced. The flow field obtained with the prototype cell was visualised in a experimental cell with a transparent plexiglass cover. A steady-state model based on the conservation of the ionic species is developed. The governing equations on the microscopic level are presented and volume averaged to give macro-homogeneous equations. The model equations are analysed and relevant simplifications are motivated and introduced. The dimensionless parameters governing the continuous electropermutation process are identified and their influence on the process are discussed. The mathematical model can be used as a tool when optimising the process parameters and designing equipment. An experimental study that aimed to show the positive influence of using the ion-exchange textile in the feed compartment of an continuous electropermutation process is presented. The incorporation of the ion-exchange textile significantly improves the nitrate removal rate at the same time as the power consumption is decreased. A superficial solution of sodium nitrate with a initial nitrate concentration of 105 ppm was treated. A product stream with less than 20 ppm nitrate could be obtained, in a single pass mode of operation. Its concluded from these experiments that continuous electropermutation using ion-exchange textile provides an interesting alternative for nitrate removal, in drinking water production. The predictions of the mathematical model are compared with experimental results and a good agreement is obtained

  • 41. Dankowicz, H.
    et al.
    Adolfsson, J.
    Nordmark, Arne B.
    KTH, Superseded Departments, Mechanics.
    Repetitive gait of passive bipedal mechanisms in a three-dimensional environment2001In: Journal of Biomechanical Engineering, ISSN 0148-0731, E-ISSN 1528-8951, Vol. 123, no 1, p. 40-46Article in journal (Refereed)
    Abstract [en]

    The inherent dynamics of bipedal, kneed mechanisms are studied with emphasis on the existence and stability of repetitive gait in a three-dimensional environment, in the absence of external, active control. The investigation is motivated by observations that sustained anthropomorphic locomotion is largely a consequence of geometric and inertial properties of the mechanism. While the modeling excludes active control, the energy dissipated in ground and knee collisions is continuously re-injected by considering gait down slight inclines. The paper describes the dependence of the resulting passive gait in vertically constrained and unconstrained mechanisms on model parameters, such as ground compliance and ground slope. We also show the possibility of achieving statically unstable gait with appropriate parameter choices.

  • 42. Dankowicz, H.
    et al.
    Nordmark, Arne B.
    KTH, Superseded Departments, Mechanics.
    On the origin and bifurcations of stick-slip oscillations2000In: Physica D: Non-linear phenomena, ISSN 0167-2789, E-ISSN 1872-8022, Vol. 136, no 04-mar, p. 280-302Article in journal (Refereed)
    Abstract [en]

    A recently proposed model of macroscopic friction is investigated using methods of dynamical systems analysis. Particular emphasis is put on the bifurcations associated with the appearance of stick-slip oscillations. In the model it is found that the existence of these oscillations is a result of a periodic orbit straddling a discontinuity in the first derivative of the vector field. A local analysis tool is developed to discuss the stability of such an orbit and its bifurcations due to changes in system parameters. The analysis tool is found to be highly efficient at quantitatively predicting the location and type of bifurcations. It is argued that the method and the general results are applicable to a large class of friction models containing similar discontinuities and thus, hopefully, to actual frictional dynamics. (C)2000 Elsevier Science B.V. All rights reserved.

  • 43. Dankowicz, H.
    et al.
    Piiroinen, P.
    Nordmark, Arne B.
    KTH, Superseded Departments, Mechanics.
    Low-velocity impacts of quasiperiodic oscillations2002In: Chaos, Solitons & Fractals, ISSN 0960-0779, E-ISSN 1873-2887, Vol. 14, no 2, p. 241-255Article in journal (Refereed)
    Abstract [en]

    A method based on the idea of a discontinuity mapping is derived for predicting the characteristics of system attractors that occur following a grazing intersection of a two-frequency, quasiperiodic oscillation with a two-dimensional impact surface in a three-dimensional state space. Within certain restrictions, the correction to the non-impacting flow afforded by the discontinuity mapping is computable using quantities determined solely by the non-impacting flow and the properties of the impact surface and the associated impact mapping in the immediate vicinity of the initial grazing contact. A model example is discussed to illustrate the quantitative predictive power of the discontinuity-mapping approach even relatively far away in parameter space from the original grazing intersection. Finally, constraints on the applicability of the methodology are described in detail with suggestions for suitable modifications.

  • 44.
    De Magistris, Federica
    KTH, Superseded Departments, Mechanics.
    Combined shear and compression analysis using a modification of the iosipescu shear test device2003Licentiate thesis, comprehensive summary (Other scientific)
  • 45. di Bernardo, M.
    et al.
    Kowalczyk, P.
    Nordmark, Arne B.
    KTH, Superseded Departments, Mechanics.
    Bifurcations of dynamical systems with sliding: derivation of normal-form mappings2002In: Physica D: Non-linear phenomena, ISSN 0167-2789, E-ISSN 1872-8022, Vol. 170, no 04-mar, p. 175-205Article in journal (Refereed)
    Abstract [en]

    This paper is concerned with the analysis of so-called sliding bifurcations in n-dimensional piecewise-smooth dynamical systems with discontinuous vector field. These novel bifurcations occur when the system trajectory interacts with regions on the discontinuity set where sliding is possible. The derivation of appropriate normal-form maps is detailed. It is shown that the leading-order term in the map depends on the particular bifurcation scenario considered. This is in turn related to the possible bifurcation scenarios exhibited by a periodic orbit undergoing one of the sliding bifurcations discussed in the paper. A third-order relay system serves as a numerical example.

  • 46. Di Bernardo, M.
    et al.
    Kowalczyk, P.
    Nordmark, Arne B.
    KTH, Superseded Departments, Mechanics.
    Sliding bifurcations: A novel mechanism for the sudden onset of chaos in dry friction oscillators2003In: International Journal of Bifurcation and Chaos in Applied Sciences and Engineering, ISSN 0218-1274, Vol. 13, no 10, p. 2935-2948Article in journal (Refereed)
    Abstract [en]

    Recent investigations of nonsmooth dynamical systems have resulted in the study of a class of novel bifurcations termed as sliding bifurcations. These bifurcations are a characteristic feature of so-called Filippov systems, that is, systems of ordinary differential equations (ODEs) with discontinuous right-hand sides. In this paper we show that sliding bifurcations also play an important role in organizing the dynamics of dry friction oscillators, which are a subclass of nonsmooth systems. After introducing the possible codimension-1 sliding bifurcations of limit cycles, we show that these bifurcations organize different types of slip to stick-slip transitions in dry friction oscillators. In particular, we show both numerically and analytically that a sliding bifurcation is an important mechanism causing the sudden jump to chaos previously unexplained in the literature on friction systems. To analyze such bifurcations we make use of a new analytical method based on the study of appropriate normal form maps describing sliding bifurcations. Also, we explain the circumstances under which the theory of so-called border-collision bifurcations can be used in order to explain the onset of complex behavior in stick-slip systems.

  • 47.
    Do-Quang, Minh
    KTH, Superseded Departments, Mechanics.
    Melt convection in welding and crystal growth2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

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

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

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

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

  • 48.
    Do-Quang, Minh
    KTH, Superseded Departments, Mechanics.
    Parallel computations on fusion welding and floating zones2003Licentiate thesis, comprehensive summary (Other scientific)
  • 49.
    Do-Quang, Minh
    et al.
    KTH, Superseded Departments, Mechanics.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Modeling of Time-Dependent 3D Weld Pool Flow Due to a Moving Arc2003In: Proceedings of High Performance Scientific Computing, 2003Conference paper (Other academic)
  • 50.
    Do-Quang, Minh
    et al.
    KTH, Superseded Departments, Mechanics.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Modelling of time-dependent 3D weld pool flow2003In: Mathematical modelling of weld Phenomena 7 / [ed] Cerjak, H, 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.

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