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  • 101.
    Ashwear, Nasseradeen
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Vibration Frequencies as Status Indicators for Tensegrity Structures2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

     Applications of vibration structural health monitoring (VHM) techniques are increasing rapidly. This is because of the advances in sensors and instrumentation during the last decades. VHM uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.

    The stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.

    The first part of the  present work studies the improvement of current models for resonance frequency simulation of tensegrities by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness. From this, both local and global vibration modes are obtained. The resonance frequencies are seen as non-linearly dependent on the pre-stress level in the structure, thereby giving a basis for diagnosis of structural conditions from measured frequencies. The new aspects of tensegrity simulations are shown for simple, plane structures but the basic methods are easily used also for more complex structures.

    In the second part, the environmental temperature effects on vibration properties of tensegrity structures have been investigated, considering primarily seasonal temperature differences (uniform temperature differences). Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies made by temperature changes could be equivalent to the change made by damage (slacking). Different combinations of materials used and boundary conditions are also investigated. These are shown to have a significant impact on the pre-stress level and the natural frequencies of the tensegrity structures when the environment temperature is changed.

  • 102.
    Ashwear, Nasseradeen
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Vibration-based Assessment of Tensegrity Structures2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Vibration structural health monitoring (VHM) uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The whole function, expressed by stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.One of the aims of the thesis was to improve the current models for resonance frequency simulation of tensegrities. This has been achieved by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness.The environmental temperature effects on vibration properties of tensegrity structures have been also  investigated. Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies coming from temperature changes could of several magnitude as those from damage.Coinciding natural frequencies and low stiffness are known issues of tensegrity structures. The former can be an obstacle in VHM, while the later normally limits their uses in real engineering applications. It has been shown that the optimum self-stress vector of tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others.The environmental temperature effects on vibration properties of tensegrity structures were revisited to find a solution such that the natural frequencies of the tensegrity structures are not strongly affected by the changes in the environmental temperature. An asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution. The last part of this thesis, services as a summary of the work.

  • 103.
    Ashwear, Nasseradeen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Influence of Temperature on the Vibration Properties of Tensegrity Structures2015In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 99, p. 237-250Article in journal (Refereed)
    Abstract [en]

    Vibration health monitoring methods use the sensitivity of the natural frequencies to structural damage. Natural frequencies are sensitive to damage, but are also affected by environmental conditions like temperature changes. It is important to be able to distinguish between the effects of these different factors when using the vibration properties as a monitoring tool. This paper discusses the impact of damage and environment temperature changes on the natural frequencies of tensegrity ("tensile-integrity") structures, in particular noting that component bending is a prominent vibration mode, which motivates a use of non-linear beam elements with axial-bending coupling. The model considers not only thermal expansion effects, but also the change of the elastic modulus with temperature. Changes in natural frequencies produced by environment temperature changes are shown to be similar to the ones produced by damage. The geometry of tensegrity structures, the support conditions and the materials are found to be important factors. The sensitivity of the natural frequency to temperature changes is found to be dependent on pre-stress level.

  • 104.
    Ashwear, Nasseradeen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Natural frequencies describe the pre-stress in tensegrity structures2014In: Computers & structures, ISSN 0045-7949, E-ISSN 1879-2243, Vol. 136, p. 162-171Article in journal (Refereed)
    Abstract [en]

    This paper investigates the effect of pre-stress level on the natural frequencies of tensegrity structures. This has been established by using Euler–Bernoulli beam elements which include the effect of the axial force on the transversal stiffness. The axial-bending coupling emphasizes the non-linear dependence of the natural frequencies on the pre-stress state. Pre-stress is seen as either synchronous, considering a variable final pre-stress design or as tuning, when increasing pre-stress is followed in a planned construction sequence. It is shown that for a certain tensegrity structure, increasing the level of pre-stress may cause the natural frequencies to rise or fall. This effect is related to whether the structural behavior can be seen as compression or tension dominant. Vanishing of the lowest natural frequency of the system is shown to be related to the critical buckling load of one or several compressed components. Modes of vibration show that when the force in the compressed components approaches any type of critical buckling load, this results in lower vibration frequencies. The methods in this study can be used to plan the tuning of the considered tensegrity structure towards the design level of pre-stress, and as health monitoring tools.

  • 105.
    Ashwear, Nasseradeen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Reducing effects from environmental temperature on the natural frequencies of tensegrity structuresIn: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568Article in journal (Other (popular science, discussion, etc.))
    Abstract [en]

    n vibration health monitoring, dynamic properties such as natural frequencies and mode shapes are used as tools for assessing the structures health condition.~They are, however, also affected by environmental conditions like wind, humidity and temperature changes. Of particular importance is the change of the environmental temperature, and it is the most commonly considered environmental variable that influences the vibration health monitoring algorithms.~This paper discusses how the tensegrity structures can be designed such that some of their lowest natural frequencies are less sensitive to the temperature changes. A genetic algorithm is used to solve the optimization problem. In the form-finding stage, an asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution, particularly when a symmetric self-stress vector is chosen.

  • 106.
    Ashwear, Nasseradeen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Vibration health monitoring for tensegrity structures2017In: Mechanical systems and signal processing, ISSN 0888-3270, E-ISSN 1096-1216, Vol. 85, p. 625-637Article in journal (Refereed)
    Abstract [en]

    Tensegrities are assembly structures, getting their equilibrium from the interaction between tension in cables and compression in bars. During their service life, slacking'in their cables and nearness to buckling in their bars need to be monitored to avoid a sudden collapse. This paper discusses how to design the tensegrities to make them feasible for vibrational health monitoring methods. Four topics are discussed; suitable finite elements formulation, pre-measurements analysis to find the locations of excitation and sensors for the interesting modes, the effects from some environmental conditions, and the pre-understanding of the effects from different slacking scenarios.

  • 107.
    Ashwear, Nasseradeen
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Tamadapu, Ganesh
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Optimization of modular tensegrity structures for high stiffness and frequency separation requirements2016In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 80, p. 297-309Article in journal (Refereed)
    Abstract [en]

    Tensegrities are cable-strut assemblies which find their stiffness and self-equilibrium states from the integrity between tension and compression. Low stiffness and coinciding natural frequencies are known issues. Their stiffness can be regulated and improved by changing the level of pre-stress. In vibration health monitoring, the first natural frequency is used as an indicator of better stiffness, but coinciding natural frequencies will be an obstacle in measuring and analysing the correct resonance. In this paper, the above two issues have been considered for modular tensegrity structures. The finite element model used considers not only the axial vibration of the components, but also the transversal vibration where non-linear Euler-Bernoulli beam elements are used for simulations. A genetic algorithm is used to solve the optimization problem, with a multi-objective criterion combination. The optimum self-stress of the tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others. Two approaches are used to find the optimal self-stress vector: scaling from a base module or considering all modules at once. Both approaches give the same optimum solutions.

  • 108.
    Atzori, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lozano-Durán, A.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Characterization of turbulent coherent structures in square duct flow2018In: Journal of Physics: Conference Series, Institute of Physics Publishing (IOPP), 2018, Vol. 1001, no 1Conference paper (Refereed)
    Abstract [en]

    This work is aimed at a first characterization of coherent structures in turbulent square duct flows. Coherent structures are defined as connected components in the domain identified as places where a quantity of interest (such as Reynolds stress or vorticity) is larger than a prescribed non-uniform threshold. Firstly, we qualitatively discuss how a percolation analysis can be used to assess the effectiveness of the threshold function, and how it can be affected by statistical uncertainty. Secondly, various physical quantities that are expected to play an important role in the dynamics of the secondary flow of Prandtl's second kind are studied. Furthermore, a characterization of intense Reynolds-stress events in square duct flow, together with a comparison of their shape for analogous events in channel flow at the same Reynolds number, is presented.

  • 109.
    Augier, Pierre
    et al.
    LEGI, Université Grenoble Alpes.
    Bonamy, Cyrille
    LEGI, Université Grenoble Alpes.
    Campagne, Antoine
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    FluidImage, a libre framework for scientific treatments of large sets of images: A software for the fluid dynamic community, by the fluid dynamic community2016Conference paper (Other academic)
  • 110.
    Augier, Pierre
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A New Formulation of the Spectral Energy Budget of the Atmosphere, with Application to Two High-Resolution General Circulation Models2013In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 70, no 7, p. 2293-2308Article in journal (Refereed)
    Abstract [en]

    A new formulation of the spectral energy budget of kinetic and available potential energies of the atmosphere is derived, with spherical harmonics as base functions. Compared to previous formulations, there are three main improvements: (i) the topography is taken into account, (ii) the exact three-dimensional advection terms are considered, and (iii) the vertical flux is separated from the energy transfer between different spherical harmonics. Using this formulation, results from two different high-resolution GCMs are analyzed: the Atmospheric GCM for the Earth Simulator (AFES) T639L24 and the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) T1279L91. The spectral fluxes show that the AFES, which reproduces quite realistic horizontal spectra with a k(-5/3) inertial range at the mesoscales, simulates a strong downscale energy cascade. In contrast, neither the k(-5/3) vertically integrated spectra nor the downscale energy cascade are produced by the ECMWF IFS.

  • 111.
    Augier, Pierre
    et al.
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bonamy, Cyrille
    LEGI, Université Grenoble Alpes.
    FluidDyn: A Python Open-Source Framework for Research and Teaching in Fluid Dynamics by Simulations, Experiments and Data Processing2019In: Journal of Open Research Software, E-ISSN ‎2049-9647, Vol. 7, no 1Article in journal (Refereed)
    Abstract [en]

    FluidDyn is a project to foster open-science and open-source in the fluid dynamics community. It is thought of as a research project to channel open-source dynamics, methods and tools to do science. We propose a set of Python packages forming a framework to study fluid dynamics with different methods, in particular laboratory experiments (package fluidlab), simulations (packages fluidfft, fluidsim and fluidfoam) and data processing (package fluidimage). In the present article, we give an overview of the specialized packages of the project and then focus on the base package called fluiddyn, which contains common code used in the specialized packages. Packages fluidfft and fluidsim are described with greater detail in two companion papers [4, 5]. With the project FluidDyn, we demonstrate that specialized scientific code can be written with methods and good practices of the open-source community. The Mercurial repositories are available in Bitbucket (https://bitbucket.org/fluiddyn/). All codes are documented using Sphinx and Read the Docs, and tested with continuous integration run on Bitbucket Pipelines and Travis. To improve the reuse potential, the codes are as modular as possible, leveraging the simple object-oriented programming model of Python. All codes are also written to be highly efficient, using C++, Cython and Pythran to speedup the performance of critical functions.

  • 112.
    Augier, Pierre
    et al.
    LEGI, Université Grenoble Alpes.
    Mohanan, Ashwin Vishnu
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Shallow water wave turbulence2019In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 874, p. 1169-1196Article in journal (Refereed)
    Abstract [en]

    The dynamics of irrotational shallow water wave turbulence forced in large scales and dissipated at small scales is investigated. First, we derive the shallow water analogue of the `four-fifths law' of Kolmogorov turbulence for a third order structure function involving velocity and displacement increments. Using this relation and assuming that the flow is dominated by shocks we develop a simple model predicting that the shock amplitude scales as (ϵd)1/3, where ϵ is the mean dissipation rate and d the mean distance between the shocks, and that the pth order displacement and velocity structure functions scale as (ϵd)p/3r/d, where r is the separation. Then we carry out a series of forced simulations with resolutions up to 76802, varying the Froude number, Ff=ϵ1/3/ckf1/3, where kf is the forcing wave number and c is the wave speed. In all simulations a stationary state is reached in which there is a constant spectral energy flux and equipartition between kinetic and potential energy in the constant flux range. The third order structure function relation is satisfied with a high degree of accuracy. Mean energy is found to scale as E∼√(ϵc/kf), and is also dependent on resolution, indicating that shallow water wave turbulence does not fit into the paradigm of a Richardson-Kolmogorov cascade. In all simulations shocks develop, displayed as long thin bands of negative divergence in flow visualisations. The mean distance between the shocks is found to scale as dFf1/2/kf. Structure functions of second and higher order are found to scale in good agreement with the model. We conclude that in the weak limit, Ff→0, shocks will become denser and weaker and finally disappear for a finite Reynolds number. On the other hand, for a given Ff, no matter how small, shocks will prevail if the Reynolds number is sufficiently large.

  • 113.
    Axelsson, Alexander
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Unsteady 3D CFD analysis of a 11=2-stage turbine with focus on heat transfer validation2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The turbine mid-structure is a component located downstream of the high-pressure turbine in modern turbo-fan engines. It is exposed to high gas temperatures. It is therefore of intererst to predict heat transfer in this component. In this thesis unsteady and steady computational uid dynamics (CFD) simulations carried out with ANSYS CFX are used to predict heat transfer in the turbine mid-structure. Experimental data is used for validation of the results. In one of the unsteady simulations a 90 degree sector of the turbine is used. The other unsteady simulation uses a Transient Blade Row model and a smaller sector of the turbine. At 50% span the simulations underpredict the heat transfer on the pressure side and on the leading edge of the turbine mid-structure vane compared to measurements. On the suction side the agreement with measurements is better. The 90 degree sector simulation does not match the measurements much better than the results of the Time Transformation method and the steady simulation. Further unsteady simulations may advantageously be of the Time Transformation type due to its smaller computational cost.

  • 114.
    Ayaka, Kamada
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Assembly of whey protein nanofibrils into macroscopic filaments2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Recently, it has been reported that the increasing number of plastic are significantly floated in the sea, because they are difficult to be degraded naturally. This lack of biodegradability requires us to develop new environmental-friendly material replacing current plastics. One promising composite of bio-friendly material is proteins, since there are abundant in our nature. Here, this study aims to exploit such proteins to form macroscopic fibers, which could open the new path to use of protein-based materials. We approach this in ‘bottom-up’ method, which means to combine the small pieces to develop large and complex materials. First, we made protein nanofibrils from native proteins, turning them from nanometer scale into microscopic scale. Subsequently, these nanofibrils are assembled into macrofibers, achieving macroscopic scale. Since the properties of final material are strongly depending on the assembly of thier building blocks, it is required to control the assembly system. We realize this with flow-focusing spinning setup, which enable to control the orientation of nanofibrils by the acceleration of the flow. We have tested some conditions of the nanofibrils, especially the morphology of fibrils such as flexible and straight fibrils, and identified essential parameters for the assembly of nanofibrils. These results give us not only the insight of complex assembly mechanism of proteins, but also the opportunity to develop the new protein-based material.

  • 115.
    Back, Izabelle
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Rayleigh-Bénard convection.2012Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 116.
    Bagheri, Faranggis
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical Study of Polymers in Turbulent Channel Flow2010Independent thesis Advanced level (degree of Master (Two Years)), 30 credits / 45 HE creditsStudent thesis
    Abstract [en]

    The phenomenon of drag reduction by polymers in turbulent flow has beenstudied over the last 60 years. New insight have been recently gained by meansof numerical simulation of dilute polymer solution at moderate values of theturbulent Reynolds number and elasticity. In this thesis, we track elastic parti-cles in Lagrangian frame in turbulent channel flow at Reτ = 180, by tracking,where the single particle obeys the FENE (finite extendible nonlinear elastic)formulation for dumbbel model. The feedback from polymers to the flow is notconsidered, while the Lagrangian approach enables us to consider high valuesof polymer elasticity. In addition, the finite time Lyapunov exponent (FTLE)of the flow is computed tracking infinitesimal material elements advected bythe flow. Following the large deviation theory, the Cramer’s function of theprobability density function of the FTLE for large values of time intervals isstudied at different wall-normal positions. The one-way effect of the turbulentflow on polymers is investigated by looking at the elongation and orientation ofthe polymers, with different relaxation times, across the channel. The confor-mation tensor of the polymers deformation which is an important contributionin the momentum balance equation is calculated by averaging in wall-parallelplanes and compared to theories available in the literature.

  • 117.
    Bagheri, Faranggis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mitra, Dhrubaditya
    NORDITA.
    Perlekar, Prasad
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Statistics of polymer extensions in turbulent channel flow2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 86, no 5, p. 056314-Article in journal (Refereed)
    Abstract [en]

    We present direct numerical simulations of turbulent channel flow with passive Lagrangian polymers. To understand the polymer behavior we investigate the behavior of infinitesimal line elements and calculate the probability distribution function (PDF) of finite-time Lyapunov exponents and from them the corresponding Cramer's function for the channel flow. We study the statistics of polymer elongation for both the Oldroyd-B model (for Weissenberg number Wi<1) and the FENE model. We use the location of the minima of the Cramer's function to define the Weissenberg number precisely such that we observe coil-stretch transition at Wi1. We find agreement with earlier analytical predictions for PDF of polymer extensions made by Balkovsky, Fouxon, and Lebedev for linear polymers (Oldroyd-B model) with Wi <1 and by Chertkov for nonlinear FENE-P model of polymers. For Wi >1 (FENE model) the polymer are significantly more stretched near the wall than at the center of the channel where the flow is closer to homogenous isotropic turbulence. Furthermore near the wall the polymers show a strong tendency to orient along the streamwise direction of the flow, but near the center line the statistics of orientation of the polymers is consistent with analogous results obtained recently in homogeneous and isotropic flows.

  • 118.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Analysis and control of transitional shear flows using global modes2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis direct numerical simulations are used to investigate two phenomenain shear flows: laminar-turbulent transition over a flat plate and periodicvortex shedding induced by a jet in cross flow. The emphasis is on understanding and controlling the flow dynamics using tools from dynamical systems and control theory. In particular, the global behavior of complex flows is describedand low-dimensional models suitable for control design are developed; this isdone by decomposing the flow into global modes determined from spectral analysisof various linear operators associated with the Navier–Stokes equations.Two distinct self-sustained global oscillations, associated with the sheddingof vortices, are identified from direct numerical simulations of the jet incrossflow. The investigation is split into a linear stability analysis of the steadyflow and a nonlinear analysis of the unsteady flow. The eigenmodes of theNavier–Stokes equations, linearized about an unstable steady solution revealthe presence of elliptic, Kelvin-Helmholtz and von K´arm´an type instabilities.The unsteady nonlinear dynamics is decomposed into a sequence of Koopmanmodes, determined from the spectral analysis of the Koopman operator. Thesemodes represent spatial structures with periodic behavior in time. A shearlayermode and a wall mode are identified, corresponding to high-frequency andlow-frequency self-sustained oscillations in the jet in crossflow, respectively.The knowledge of global modes is also useful for transition control, wherethe objective is to reduce the growth of small-amplitude disturbances to delaythe transition to turbulence. Using a particular basis of global modes, knownas balanced modes, low-dimensional models that capture the behavior betweenactuator and sensor signals in a flat-plate boundary layer are constructed andused to design optimal feedback controllers. It is shown that by using controltheory in combination with sensing/actuation in small, localized, regionsnear the rigid wall, the energy of disturbances may be reduced by an order of magnitude.

  • 119.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Computational Hydrodynamic Stability and Flow Control Based on Spectral Analysis of Linear Operators2012In: Archives of Computational Methods in Engineering, ISSN 1134-3060, E-ISSN 1886-1784, Vol. 19, no 3, p. 341-379Article, review/survey (Refereed)
    Abstract [en]

    This paper considers the analysis and control of fluid flows using tools from dynamical systems and control theory. The employed tools are derived from the spectral analysis of various linear operators associated with the Navier-Stokes equations. Spectral decomposition of the linearized Navier-Stokes operator, the Koopman operator, the spatial correlation operator and the Hankel operator provide a means to gain physical insight into the dynamics of complex flows and enables the construction of low-dimensional models suitable for control design. Since the discretization of the Navier-Stokes equations often leads to very large-scale dynamical systems, matrix-free and in some cases iterative techniques have to be employed to solve the eigenvalue problem. The common theme of the numerical algorithms is the use of direct numerical simulations. The theory and the algorithms are exemplified on flow over a flat plate and a jet in crossflow, as prototypes for the laminar-turbulent transition and three-dimensional vortex shedding.

  • 120.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of weak noise on oscillating flows: Linking quality factor, Floquet modes, and Koopman spectrum2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 9, p. 094104-Article in journal (Refereed)
    Abstract [en]

    Many fluid flows, such as bluff body wakes, exhibit stable self-sustained oscillations for a wide range of parameters. Here we study the effect of weak noise on such flows. In the presence of noise, a flow with self-sustained oscillations is characterized not only by its period, but also by the quality factor. This measure gives an estimation of the number of oscillations over which periodicity is maintained. Using a recent theory [P. Gaspard, J. Stat. Phys. 106, 57 (2002)], we report on two observations. First, for weak noise the quality factor can be approximated using a linear Floquet analysis of the deterministic system; its size is inversely proportional to the inner-product between first direct and adjoint Floquet vectors. Second, the quality factor can readily be observed from the spectrum of evolution operators. This has consequences for Koopman/Dynamic mode decomposition analyses, which extract coherent structures associated with different frequencies from numerical or experimental flows. In particular, the presence of noise induces a damping on the eigenvalues, which increases quadratically with the frequency and linearly with the noise amplitude. (C) 2014 AIP Publishing LLC.

  • 121.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Koopman-mode decomposition of the cylinder wake2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 726, p. 596-623Article in journal (Refereed)
    Abstract [en]

    The Koopman operator provides a powerful way of analysing nonlinear flow dynamics using linear techniques. The operator defines how observables evolve in time along a nonlinear flow trajectory. In this paper, we perform a Koopman analysis of the first Hopf bifurcation of the flow past a circular cylinder. First, we decompose the flow into a sequence of Koopman modes, where each mode evolves in time with one single frequency/growth rate and amplitude/phase, corresponding to the complex eigenvalues and eigenfunctions of the Koopman operator, respectively. The analytical construction of these modes shows how the amplitudes and phases of nonlinear global modes oscillating with the vortex shedding frequency or its harmonics evolve as the flow develops and later sustains self-excited oscillations. Second, we compute the dynamic modes using the dynamic mode decomposition (DMD) algorithm, which fits a linear combination of exponential terms to a sequence of snapshots spaced equally in time. It is shown that under certain conditions the DMD algorithm approximates Koopman modes, and hence provides a viable method to decompose the flow into saturated and transient oscillatory modes. Finally, the relevance of the analysis to frequency selection, global modes and shift modes is discussed.

  • 122.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Stability analysis and control design of spatially developing flows2008Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Methods in hydrodynamic stability, systems and control theory are applied to spatially developing flows, where the flow is not required to vary slowly in the streamwise direction. A substantial part of the thesis presents a theoretical framework for the stability analysis, input-output behavior, model reduction and control design for fluid dynamical systems using examples on the linear complex Ginzburg-Landau equation. The framework is then applied to high dimensional systems arising from the discretized Navier–Stokes equations. In particular, global stability analysis of the three-dimensional jet in cross flow and control design of two-dimensional disturbances in the flat-plate boundary layer are performed. Finally, a parametric study of the passive control of two-dimensional disturbances in a flat-plate boundary layer using streamwise streaks is presented.

  • 123.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Input-output analysis, model reduction and control of the flat-plate boundary layer2009In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 620, p. 263-298Article in journal (Refereed)
    Abstract [en]

    The dynamics and control of two-dimensional disturbances in the spatially evolving boundary layer oil a flat plate are investigated from an input output viewpoint. A set-up of spatially localized inputs (external disturbances and actuators) and Outputs (objective functions and sensors) is introduced for the control design of convectively unstable flow configurations. From the linearized Navier Stokes equations with the inputs and outputs, controllable, observable and balanced modes are extracted using the snapshot method. A balanced reduced-order model (ROM) is constructed and shown to capture the input output behaviour of the linearized Navier Stokes equations. This model is finally used to design H-2-feedback controller to suppress the growth or two-dimensional perturbations inside the boundary layer.

  • 124.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Research on the interaction between streamwise streaks and Tollmien-Schlichting waves at KTH2007In: ERCOFTAC Bulletin, ISSN 2518-0991, Vol. 74, p. 37-43Article in journal (Refereed)
    Abstract [en]

    This paper summarises the experimental and numericalinvestigations on how two different types of disturbancesmay, in a positive way, interact in a flat plateboundary-layer flow. The project, which mainly hasbeen centered at KTH1, has been performed in collaborationwith colleagues from University of Bologna2and LadHyX CNRS Ecole Polytechnique3, duringthe last years. The main phenomena — the stabilisingeffect of streamwise boundary-layer streakson Tollmien-Schlichting waves (and other exponentialdisturbances) — have been captured both in experiments[1, 2] and with different numerical approachessuch as direct numerical simulations [3], parabolicstability equation calculations [5] and large-eddy simulations[6]. We will here briefly review the methodsand the main results of these studies, and discuss howthey correlate with each other. For related referencesoutside KTH the interested reader is referred to thejournal publications in the reference list.

  • 125.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The stabilizing effect of streaks on Tollmien-Schlichting and oblique waves: A parametric study2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 7, p. 078103-1-078103-4Article in journal (Refereed)
    Abstract [en]

    The stabilizing effect of finite amplitude streaks on the linear growth of unstable perturbations [Tollmien-Schlichting (TS) and oblique waves] is numerically investigated by means of the nonlinear parabolized stability equations. We have found that for stabilization of a TS-wave, there exists an optimal spanwise spacing of the streaks. These streaks reach their maximum amplitudes close to the first neutral point of the TS-wave and induce the largest distortion of the mean flow in the unstable region of the TS-wave. For such a distribution, the required streak amplitude for complete stabilization of a given TS-wave is considerably lower than for beta=0.45, which is the optimal for streak growth and used in previous studies. We have also observed a damping effect of streaks on the growth rate of oblique waves in Blasius boundary layer and for TS-waves in Falkner-Skan boundary layers.

  • 126.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hoepffner, J.
    Institut de Recherche sur les Phénomènes Hors Équilibre (IRPHÉ), CNRS-Université d'Aix-Marseille.
    Schmid, Peter
    Laboratoire d'Hydrodynamique (LadHyX), CNRS-École Polytechnique.
    Input-Output Analysis and Control Design Applied to a Linear Model of Spatially Developing Flows2009In: Applied Mechanics Review, ISSN 0003-6900, E-ISSN 1088-8535, Vol. 62, no 2Article in journal (Refereed)
    Abstract [en]

    This review presents a framework for the input-output analysis, model reduction, and control design for fluid dynamical systems using examples applied to the linear complex Ginzburg-Landau equation. Major advances in hydrodynamics stability, such as global modes in spatially inhomogeneous systems and transient growth of non-normal systems, are reviewed. Input-output analysis generalizes hydrodynamic stability analysis by considering a finite-time horizon over which energy amplification, driven by a specific input (disturbances/actuator) and measured at a specific output (sensor), is observed. In the control design the loop is closed between the output and the input through a feedback gain. Model reduction approximates the system with a low-order model, making modern control design computationally tractable for systems of large dimensions. Methods from control theory are reviewed and applied to the Ginzburg-Landau equation in a manner that is readily generalized to fluid mechanics problems, thus giving a fluid mechanics audience an accessible introduction to the subject.

  • 127.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Transition delay using control theory2011In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 369, no 1940, p. 1365-1381Article in journal (Refereed)
    Abstract [en]

    This review gives an account of recent research efforts to use feedback control for the delay of laminar-turbulent transition in wall-bounded shear flows. The emphasis is on reducing the growth of small-amplitude disturbances in the boundary layer using numerical simulations and a linear control approach. Starting with the application of classical control theory to two-dimensional perturbations developing in spatially invariant flows, flow control based on control theory has progressed towards more realistic three-dimensional, spatially inhomogeneous flow configurations with localized sensing/actuation. The development of low-dimensional models of the Navier-Stokes equations has played a key role in this progress. Moreover, shortcomings and future challenges, as well as recent experimental advances in this multi-disciplinary field, are discussed.

  • 128.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mazzino, A.
    Bottaro, A.
    Spontaneous symmetry breaking of a hinged flapping filament generates lift2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 15, p. 154502-Article in journal (Refereed)
    Abstract [en]

    Elastic filamentous structures found on swimming and flying organisms are versatile in function, rendering their precise contribution to locomotion difficult to assess. We show in this Letter that a single passive filament hinged on the rear of a bluff body placed in a stream can generate a net lift force without increasing the mean drag force on the body. This is a consequence of spontaneous symmetry breaking in the filament's flapping dynamics. The phenomenon is related to a resonance between the frequency associated with the von Kármán vortex street developing behind the bluff body and the natural frequency of the free bending vibrations of the filament.

  • 129.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The global stability of the jet in crossflow2008Report (Other academic)
  • 130.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schmid, Peter J.
    Laboratoire d'Hydrodynamique (LadHyX), CNRS-Ecole Polytechnique.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Global stability of a jet in crossflow2009In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 624, p. 33-44Article in journal (Refereed)
    Abstract [en]

    A linear stability analysis shows that the jet in crossflow is characterized by self-sustained global oscillations for a jet-to-crossflow velocity ratio of 3. A fully three-dimensional unstable steady-state solution and its associated global eigenmodes are computed by direct numerical simulations and iterative eigenvalue routines. The steady flow, obtained by means of selective frequency damping, consists mainly of a (steady) counter-rotating vortex pair (CVP) in the far field and horseshoe-shaped vortices close to the wall. High-frequency unstable global eigenmodes associated with shear-layer instabilities on the CVP and low-frequency modes associated with shedding vortices in the wake of the jet are identified. Furthermore, different spanwise symmetries of the global modes are discussed. This work constitutes the first simulation-based global stability analysis of a fully three-dimensional base flow.

  • 131.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Matrix-free methods for the stability and control of boundary layers2009In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 47, no 5, p. 1057-1068Article in journal (Refereed)
    Abstract [en]

    This paper presents matrix-free methods for the stability analysis and control design of high-dimensional systems arising from the discretized linearized Navier-Stokes equations. The methods are applied to the two-dimensional spatially developing Blasius boundary-layer. A critical step in the process of systematically investigating stability properties and designing feedback controllers is solving very large eigenvalue problems by storing only velocity fields at different times instead of large matrices. For stability analysis, where the entire dynamics of perturbations in space and time is of interest, iterative and adjoint-based optimization techniques are employed to compute the global eigenmodes and the optimal initial conditions. The latter are the initial conditions yielding the largest possible energy growth over a finite time interval. The leading global eigenmodes take the shape of Tollmien-Schlichting wavepackets located far downstream in streamwise direction, whereas the leading optimal disturbances are tilted structures located far upstream in the boundary layer. For control design on the other hand, the input-output behavior of the system is of interest and the snapshot-method is employed to compute balanced modes that correctly capture this behavior. The inputs are external disturbances and wall actuation and the outputs are sensors that extract wall shear stress. A low-dimensional model that capture the input-output behavior is constructed by projection onto balanced modes. The reduced-order model is then used to design a feedback control strategy such that the growth of disturbances are damped as they propagate downstream.

  • 132.
    Bagheri, Shervin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Åkervik, Espen
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Input-output analysis and control design of spatially developing shear flows2008In: 5th AIAA Theoretical Fluid Mechanics Conference, 2008Conference paper (Refereed)
    Abstract [en]

    A framework for the input-output analysis, model reduction and control design of spatially developing shear flows is presented using the Blasius boundary-layer flow as an example. An input-output formulation of the governing equations yields a flexible formulation for treating stability problems and for developing control strategies that optimize given objectives. Model reduction plays an important role in this process since the dynamical systems that describe most flows are discretized partial differential equations with a very large number of degrees of freedom. Moreover, as system theoretical tools, such as controllability, observability and balancing has become computationally tractable for large-scale systems, a systematic approach to model reduction is presented.

  • 133. Bailey, S. C. C.
    et al.
    Hultmark, M.
    Monty, J. P.
    Alfredsson, Per Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Chong, M. S.
    Duncan, R. D.
    Fransson, Jens
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hutchins, N.
    Marusic, I.
    McKeon, B. J.
    Nagib, H. M.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Smits, A. J.
    Vinuesa, R.
    Obtaining accurate mean velocity measurements in high Reynolds number turbulent boundary layers using Pitot tubes2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 715, p. 642-670Article in journal (Refereed)
    Abstract [en]

    This article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from Reθ = 11 100 to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here.

  • 134.
    Bakkman, Frida
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Calculation of mechanical energy in cross country skiing2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Cross country skiing is considered to be one of the most demanding sports in terms of endurance. Therefore the skiers are attractive subjects for physiological and biomechanical research whose interest has increased a lot during the 21 stcentury. The results are used to improve the mechanical knowledge about the body and to improve the capacity and technique for the competitors.

    The aim with this study is to implement a method for mechanical energy calculation in cross country skiing. This is based on data from 15 skiers using the double poling technique, where the potential, rotational and translational energies are calculated.

    The measurements are made in a lab using a treadmill with stepwise increased velocity. The system used is Vicon MX where the skiers wear re- ective markers, whose positions is calculated from data from infra-red light cameras. The positions of the joint centres are calculated used as input data to the program. Joint centres and marker data divide the body into segments where the energies of each segment are calculated and possible to sum up for the whole body.

    The results are examples of obtainable data from the model. It is possible to compare chosen subjects' total mechanical energy but also the energies and segments separately. The results can be used to analyse the dierent techniques to improve the capacity of the competitors.

  • 135. Balestra, G.
    et al.
    Zhu, Lailai
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Gallaire, F.
    Viscous Taylor droplets in axisymmetric and planar tubes: from Bretherton’s theory to empirical models2018In: Microfluidics and Nanofluidics, ISSN 1613-4982, E-ISSN 1613-4990, Vol. 22, no 6, article id 67Article in journal (Refereed)
    Abstract [en]

    The aim of this study is to derive accurate models for quantities characterizing the dynamics of droplets of non-vanishing viscosity in capillaries. In particular, we propose models for the uniform-film thickness separating the droplet from the tube walls, for the droplet front and rear curvatures and pressure jumps, and for the droplet velocity in a range of capillary numbers, Ca, from 10 - 4 to 1 and inner-to-outer viscosity ratios, λ, from 0, i.e. a bubble, to high-viscosity droplets. Theoretical asymptotic results obtained in the limit of small capillary number are combined with accurate numerical simulations at larger Ca. With these models at hand, we can compute the pressure drop induced by the droplet. The film thickness at low capillary numbers (Ca&lt; 10 - 3) agrees well with Bretherton’s scaling for bubbles as long as λ&lt; 1. For larger viscosity ratios, the film thickness increases monotonically, before saturating for λ&gt; 10 3 to a value 2 2 / 3 times larger than the film thickness of a bubble. At larger capillary numbers, the film thickness follows the rational function proposed by Aussillous and Quéré (Phys Fluids 12(10):2367–2371, 2000) for bubbles, with a fitting coefficient which is viscosity-ratio dependent. This coefficient modifies the value to which the film thickness saturates at large capillary numbers. The velocity of the droplet is found to be strongly dependent on the capillary number and viscosity ratio. We also show that the normal viscous stresses at the front and rear caps of the droplets cannot be neglected when calculating the pressure drop for Ca> 10 - 3.

  • 136. Ballarotta, M.
    et al.
    Brodeau, L.
    Brandefelt, Jenny
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Lundberg, P.
    Döös, K.
    Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions: do eddies contribute to a better consistency between models and palaeoproxies?2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, no 6, p. 2669-2686Article in journal (Refereed)
    Abstract [en]

    Most state-of-the-art climate models include a coarsely resolved oceanic component, which hardly captures detailed dynamics, whereas eddy-permitting and eddy-resolving simulations are developed to reproduce the observed ocean. In this study, an eddy-permitting and a coarse resolution numerical experiment are conducted to simulate the global ocean state for the period of the Last Glacial Maximum (LGM, similar to 26 500 to 19 000 yr ago) and to investigate the improvements due to taking into account the smaller spatial scales. The ocean state from each simulation is confronted with a data set from the Multiproxy Approach for the Reconstruction of the Glacial Ocean (MARGO) sea surface temperatures (SSTs), some reconstructions of the palaeo-circulations and a number of sea-ice reconstructions. The western boundary currents and the Southern Ocean dynamics are better resolved in the high-resolution experiment than in the coarse simulation, but, although these more detailed SST structures yield a locally improved consistency between model predictions and proxies, they do not contribute significantly to the global statistical score. The SSTs in the tropical coastal upwelling zones are also not significantly improved by the eddy-permitting regime. The models perform in the mid-latitudes but as in the majority of the Paleo-climate Modelling Intercomparison Project simulations, the modelled sea-ice conditions are inconsistent with the palaeo-reconstructions. The effects of observation locations on the comparison between observed and simulated SST suggest that more sediment cores may be required to draw reliable conclusions about the improvements introduced by the high resolution model for reproducing the global SSTs. One has to be careful with the interpretation of the deep ocean state which has not reached statistical equilibrium in our simulations. However, the results indicate that the meridional overturning circulations are different between the two regimes, suggesting that the model parametrizations might also play a key role for simulating past climate states.

  • 137.
    Banach, Patriq
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Deployment Simulations of the Space Tow Solar Sail: Utfällningssimuleringar av solseglet"rymdrepet"2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Solar sailing has been long in the coming, a notion almost a century old and only recently

    demonstrated to work. The idea of using ambient photons for space propulsion is an appealing

    one not only for the elegance of not having to carry heavy fuel, but also for the special physics

    of a continuously accelerating spacecraft.

    One of the proposed architectures for a light, modular solar sail is the Space Tow, consisting

    of hundreds of

    mm thin sheets suspended in several km of carbon filament, at the same time

    stowable in mere meter-scale height.

    This thesis investigates the deployment mechanics of the Space Tow for two passive deployment

    strategies, “drag along” and “leave behind” deployment. Simulations were made using a

    simple 3D model in ABAQUS/Explicit and compared to a 1D mechanical analysis.

    Many of the problems with these deployment schemes were of acceleration-rate changes

    and the damping thereof. The last part of the thesis touches upon the involved energies and

    how these could be dissipated by dry friction, as well as how this would be described in an

    acceleration-rate proportional damping constant for use in future models.

    The thesis concludes that the “drag along” scheme is sensitive to perturbations and that the

    “leave behind” scheme needs careful consideration of its parameters or risk that the undeployed

    stack accelerates to pass the deployed structure.

    The thesis is composed of two parts, section I is a background presenting the subject and

    available literature. After that follows an article which at the time of writing is to be presented

    at the 3rd International Symposium on Solar Sailing, 2013.

  • 138.
    Banaei, Arash Alizad
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Simulation of deformable objects transported in fluid flow2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Deformable particles suspended in a viscous fluid can be found in many industrial and biological applications. In this thesis, two different numerical tools have been developed to simulate suspensions of capsules, thin membranes enclosing a second fluid and a rigid nucleus so to work as model for ”Eukaryotic” cells, and flexible slender bodies known as filaments/fibres. Both tools use a semi-implicit fluid flow solver with different approaches for the deformable structure. The capsule membrane is modelled as a thin hyperelastic material and the elasticity equations are solved with an accurate spectral representation of the capsule shape as a truncated number of spherical harmonics. The filaments are considered as one dimensional inextensible slender bodies obeying Euler-Bernoulli beam equations which is solved by a two-step method using finite difference discretisation. The immersed boundary method is exploited to couple the fluid and solid motion using different versions for the two different objects considered. The nucleus inside the capsules is modelled either as a second stiffer capsule or as a rigid particle. In order to avoid membrane-membrane, membrane-wall and membrane-nucleus overlapping, a short range repulsive force is implemented in terms of a potential function of the distance between the approaching objects. For the short range interactions between the filaments, both lubrication correction and collision forces are considered and it is found that the inclusion of the lubrication correction has significant effect on the rheology in shear flow. Both codes are validated against the numerical and experimental data in the literature. We study the capsule behaviour in a simple shear flow created by with two walls moving in opposite directions. The membrane obeys the Neo-Hookean constitutive equations and, in the simulations with a rigid nucleus, its radius is fixed to half the capsule initial radius. The filaments, on the other hand, are studied in 4 different flow configurations: shear flow, channel flow, settling in quiescent fluid and homogeneous isotropic turbulence. The results indicate that for single capsule, the nucleus reduces the membrane deformation significantly and changes the deformed shaped when there is negligible bending resistance of the membrane. The rheological properties of nucleated capsule suspensions result from the competition between the capsule deformation and their orientation angle and similarly to the case of single capsules, the nucleus reduces the mean deformation. By increasing the capsule volume fraction, the relative viscosity increases and capsules become more oriented in the mean flow direction. Filament suspensions in shear flow exhibit shear thinning behaviour with respect to deformability; inertia has a significant effect on the rheological properties of the suspensions as documented here. For the case of settling fibres, we document the formation of columnar structures with higher settling velocity known as streamers, which are more pronounced at higher volume fractions and for flexible fibres. For a single filament in homogeneous isotropic turbulence, two distinct regimes for the filament motion are identified with a sharp transition from one to another at a critical bending stiffness. In turbulent channel flow, we demonstrate how finite-size filaments cause considerable drag reduction, of the order of 30% for volume fractions of the order of 1.5%, and that the main averaged quantities are almost independent of the filament flexibility for the bending rigidities studied here.

  • 139.
    Banaei, Arash Alizad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Loiseau, Jean-Christophe
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lashgari, Iman
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, L.uca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Numerical simulations of elastic capsules with nucleus in shear flow2017In: EUROPEAN JOURNAL OF COMPUTATIONAL MECHANICS, ISSN 1779-7179, Vol. 26, no 1-2, p. 131-153Article in journal (Refereed)
    Abstract [en]

    The shear-induced deformation of a capsule with a stiff nucleus, a model of eukaryotic cells, is studied numerically. The membrane of the cell and of its nucleus are modelled as a thin elastic material obeying a Neo-Hookean constitutive law. The fluid-structure coupling is obtained using an immersed boundary method. The variations induced by the presence of the nucleus on the cell deformation are investigated when varying the viscosity ratio between the inner and outer fluids, the membrane elasticity and its bending stiffness. The deformation of the eukaryotic cell is smaller than that of the prokaryotic one. The reduction in deformation increases for larger values of the capillary number. The eukaryotic cell remains thicker in itsmiddle part compared to the prokaryotic one, thus making it less flexible to pass through narrow capillaries. For a viscosity ratio of 5, the deformation of the cell is smaller than in the case of uniform viscosity. In addition, for non-zero bending stiffness of the membrane, the deformation decreases and the shape is closer to an ellipsoid. Finally, we compare the results obtained modelling the nucleus as an inner stiffer membrane with those obtained using a rigid particle.

  • 140.
    Banaei, Arash Alizad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Rahmani, Mona
    Martinez, Mark
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical study of settling of flexible fiber suspensionsManuscript (preprint) (Other academic)
  • 141.
    Banaei, Arash Alizad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical study of filament suspensions at finite inertiaIn: Article in journal (Other academic)
  • 142.
    Banaei, Arash Alizad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Shahmardi, Armin
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Suspensions of nucleated capsules at finite inertiaManuscript (preprint) (Other academic)
  • 143.
    Banerjee, Indradumna
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Protein Science, Nano Biotechnology.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Kumar, Tharagan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Protein Science, Nano Biotechnology.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Russom, Aman
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Protein Science, Nano Biotechnology.
    Particle focusing dynamics in extended elasto inertial flow2018Conference paper (Refereed)
    Abstract [en]

    Elasto Inertial microfluidics has been exploited recently for a number of industrial and biological applications. Recently, we experimentally showed that it is possible to achieve single stream focusing of particles even at higher flow rates in the elasto inertial regime, relevant to flow cytometry applications, and , based on this concept, built a silica fibre based micro flow cytometer.1 However, the physics behind the focusing of particles is still poorly understood, specially for combinations of higher Reynolds (Re) and Weissenberg numbers(Wi).

    In the present study, for the first time, we seek to understand both experimentally and with numerical simulations, particle focusing across elasticity regimes. We vary the concentration of PEO (200 ppm to upto 10000 ppm) in PBS solution at sufficiently high flow rates of 100!l/min or above. We introduce a parameter, focusing bandwidth (F) to evaluate the extent of single stream focusing of 15 !m particles in a 75 !m diameter circular channel. Fig.1 shows the flow setup(fig.1a) along with images demonstrating the focused (fig.1b) and unfocused cases(fig.1c), as well as how F is calculated(fig.1d). We evaluate particle focusing by identifying the flow conditions for each concentration that leads to the minimum value of F. Fig.2 shows the variation of the focusing bandwidth(fig.2a) when changing PEO concentration, and the variation in Re along with Wi (fig.2b) and Elasticity number(El). The results show that for identical mass flow conditions across the different regimes the focusing bandwidth slowly shifts to a narrow single stream with increasing elasticity. We validated our experimental results as well as gained new insights into particle focusing with 3D numerical simulations based on a FENE P model. We studied the decoupled effects of Reynolds number and Weissenberg number on particle focusing, as well as the particle trajectories and migration dynamics as the particles reach equilibrium. Interestingly, enough we find a combination of high Re(Re=400) and sufficiently high Wi(Wi=3) for which the particles achieve a single stream focusing (fig.3a). The entire dynamics of particle migration in a circular cross section is also shown (in fig.3b) by changing Wi for a constant Re(Re=200). It can be seen that the particle goes through a longer amount of oscillations to reach its final equilibrium position as Wi is increased. Fig.4a shows the equilibrium position of the particle moving closer to the center with an increase in Wi at the same Re(Re=200). However, in the Non Newtonian cases, the particle has a slight oscillatory behaviour as it reaches its equilibrium position as compared to the Newtonian one. We introduced the particle at two different positions(at Re=200, We=0 and 1) and observed the same equilibrium positions in both cases (Fig.4b).

  • 144.
    Banerjee, Indradumna
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Protein Science, Nano Biotechnology.
    Rosti, Marco Edoardo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Kumar, Tharagan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Protein Science, Nano Biotechnology.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Russom, Aman
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Protein Science, Nano Biotechnology.
    Analog particle position tuning in Elasto-inertial microfluidic flowsManuscript (preprint) (Other academic)
    Abstract [en]

    We observe for the first time an analog trend in particle focusing in a high throughput weakly viscoelastic regime, where it is possible to tune particles into multiple intermediate focusing positions that lie between the "Segre-Silberberg annulus" and the center of a circular microcapillary. The "Segre-Silberberg annulus" (0.6 times the pipe radius), that describes particle equilibrium in a predominantly inertial flow, shrinks consistently closer to the center for increasing elasticity in extremely dilute PEO concentrations (ranging from 0.001 wt% to 0.05wt%). The experimental observations are supported by direct numerical simulations, where an Immersed Boundary Method is used to account for the presence of particles and a FENE-P model is used to simulate the presence of polymers in a Non-Newtonian fluid. The numerical simulations study the dynamics and stability of finite size particles and are further used to analyze particle behavior at Reynolds number higher than what is allowed by the present experimental setup. In particular, we are able to report the entire migration trajectories of the particles as they reach their final equilibrium positions and extend our predictions to other geometries such as the square cross-section. We believe complex effects originate due to a combination of inertia and elasticity in a weakly viscoelastic regime, where neither inertia nor elasticity are able to mask each other's effect completely, thus leading to a number of intermediate focusing positions. The present study provides a new understanding into the mechanism of particle focusing in elasto-inertial flows and opens up new possibilities for exercising analog control in tuning the particle focusing positions.

  • 145.
    Banerjee, Indradumna
    et al.
    KTH, School of Biotechnology (BIO), Proteomics and Nanobiotechnology.
    Rosti, Marco Eduardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Niazi Ardekani, Mehdi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kumar, Tharagan
    KTH, School of Biotechnology (BIO), Proteomics and Nanobiotechnology.
    Lashgari, Iman
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Russom, Aman
    KTH, School of Biotechnology (BIO), Proteomics and Nanobiotechnology.
    Dynamics of Inertial migration of particles in straight channels2017Conference paper (Refereed)
    Abstract [en]

    SUMMARY

    We study numerically the entire migration dynamics of spherical and oblate particles in straight rectangular and square cross sectional ducts. The reported results can help in design of straight duct channel based microfluidic systems.

     

    KEYWORDS: Inertial microfluidics, Lateral migration, Oblate particles, Straight particles.

     

    INTRODUCTION

    We  simulate spherical and oblate rigid particles in straight ducts of different aspect ratios using an Immersed Boundary Method. To the best of our knowledge, this is the first time not only the equilibrium position of particles is described, but also the entire migration dynamics of the particle from the initial to final position, including particle trajectory, velocity, rotation and orientation, are investigated.

     

    EXPERIMENTAL

     The fluid is considered incompressible and its motion is governed by the Navier Stokes and Continuity equations. The numerical approach employed is an Immersed Boundary Method (IBM) with two sets of grid points: an equispaced Eulerian mesh for the fluid flow, and Lagrangian grid points uniformly distributed on the surface of the particle. The flow is set up in square and rectangular cross section ducts with no slip and no penetration boundary conditions (Fig.1).

     

    RESULTS AND DISCUSSION

    We examine the lateral motion of spherical and oblate particles using the IBM method mentioned above. While simulating three different spheres in a square duct of duct width to sphere diameter ratio H/Ds= [3.5, 5, 10], we find that the particles focus at closest face-cantered equilibrium position from their point of introduction(Fig.2a). We also show the downstream length needed for a sphere to focus, focusing length, as a function of the distance from the vertical duct symmetry line and as a function of Reynolds number(Fig.2b and c respectively). Spherical particles in rectangular duct tend to move laterally toward the longer length wall and then slowly moves towards the equilibrium position at the face-centre along the long wall(fig.3a). We also observe that the focusing length is longer for spherical particles in a rectangular duct, about three times longer than that in square duct (fig. 3b). In case of an oblate particle flowing through a square duct, the lateral motion towards the face centred equilibrium position is similar to that of a sphere (fig.4a), however there is significant tumbling motion of the particle as it tries to reach equilibrium(fig.4b).In a rectangular duct of aspect ratio 2, the oblate particle reaches a steady configuration on the duct symmetry line at the center of the different faces (fig.5a). The focusing length surprisingly is shorter in a rectangular duct for an oblate particle in contrast to its focusing length in a square duct. This is attributed to the higher lateral velocity of the oblate in the second stage of the migration, that with negligible tumbling(fig.5b). The behavior of three oblate particles in a square duct of duct width to longer diameter ratio H/Ds= [3.5, 5, 10] is different compared to a sphere as the largest oblate tend to focus at the duct cross section diagonals compared to the other two which are at face centred equilibrium as in case of a sphere(fig.6a). We attribute this to the rotation rate of the larger particle which is initially increasing and then decreasing(fig.6b).When it comes to focusing lengths, the smaller particles need longer times to reach their final equilibrium(fig.6c). Another interesting behavior we see is the effect of Reynolds number, where it can be seen that the oblate particles show a tilt of 21 degrees when focusing at equilibrium at certain high Reynolds number (fig.7).

     

    CONCLUSION

    The results presented employ a highly accurate interface-resolved numerical algorithm, based on the Immersed Boundary Method to study the entire inertial migration of an oblate particle in both square and rectangular ducts and compare it with that of a single sphere. Currently, we apply a volume penalization method and polymeric drag component to the code to solve for viscoelastic effects in circular microcapillaries.

     

    ACKNOWLEDGEMENTS

    This work was supported by the European Research Council Grant no. ERC-2013-CoG-616186, TRITOS and by the Swedish Research Council Grant no. VR 2014-5001, COST Action MP1305: Flowing matter, and computation time from SNIC.

     REFERENCES : Lashgari, Iman, et al. Journal of Fluid Mechanics 819 (2017): 540-561.

  • 146. Barcena, L. T.
    et al.
    Shiomi, J.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Control of oscillatory thermocapillary convection with local heating2006In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 286, no 2, p. 502-511Article in journal (Refereed)
    Abstract [en]

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

  • 147.
    Bark, Fritz H.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Vynnycky, M.
    A note on electrolysis with forced convection at large peclet number in a channel and an excess of supporting electrolyte2008In: Russian journal of electrochemistry, ISSN 1023-1935, E-ISSN 1608-3342, Vol. 44, no 4, p. 470-478Article in journal (Refereed)
    Abstract [en]

    Electrolysis of an aqueous solution of a metal salt with an excess of supporting electrolyte flowing in a two-dimensional channel is considered. The reaction kinetics is modeled by a Butler - Volmer law. The metal electrodes are symmetrically flush mounted in the channel walls, which are otherwise electrically insulating. Using the perturbation scheme originally proposed by Levich for electrolytes with an excess of supporting electrolyte, a solution in closed form, involving the root of a transcendental algebraic equation, is obtained for the polarization curve. For small and large values of the potential difference between the electrodes, explicit expressions for the polarization curve and the distributions of electric current and concentration on the electrodes are obtained. Particular attention is given to the conditions prevailing during the asymptotic approach to the limiting current.

  • 148.
    Barman, Emelie
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Aerodynamics of Flutter2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The unsteady ow around an aerofoil placed in a uniform ow stream with an angle of attack is investigated, under the assumption of inviscid, incompressible, two-dimensional flow. In particular, a function of the velocity jump over the wake is achieved, where this function depends on the horizontal displacement and time. The aerofoil geometry is represented by two arbitrary functions, one for the upper and one for the lower side of the aerofoil. These functions are dependent on time, hence the aerofoil can perform oscillating movement, which is the case when subjected to utter. The governing equations for the ow are the Euler equations. By assuming thin aerofoil, small angle of attack and that the perturbation of the wake is small, the problem is linearised. It is shown that the linearised Euler equations can be rewritten as the Cauchy-Riemann equations, and an analytic function exists where its real part is the horizontal velocity component and its imaginary part is the vertical velocity component with opposite sign. The ow eld is then investigated in the complex plane by making an appropriate branch cut removing all discontinuities, and with restrictions on the analytic function such that the kinematic and boundary conditions are satis ed. By using Cauchy's integral formula an expression for the anti-symmetric part of the analytic function is achieved. A general expression for the velocity jump over the wake is obtained, which is applied to the speci c case of harmonic oscillations for a symmetric aerofoil. In the end three types of utter is investigated; twisting oscillations around the centre of stiness, vertical oscillation, and aileron flutter.

  • 149. Bartonek, A.
    et al.
    Lidbeck, C. M.
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Influence of external visual focus on gait in children with bilateral cerebral palsy2016In: Pediatric Physical Therapy, ISSN 0898-5669, E-ISSN 1538-005X, Vol. 28, no 4, p. 393-399Article in journal (Refereed)
    Abstract [en]

    Purpose: To explore whether focusing a target influenced gait in children with cerebral palsy (CP) and typical development (TD). Methods: Thirty children with bilateral CP (Gross Motor Function Classification System [GMFCS] I-III) and 22 with TD looked at a light at walkway end (Gaze Target) while walking and returned (No Target). Results: During Gaze versus No Target, children with TD reduced temporal-spatial parameters and movements in the sagittal (SPM) and transverse planes. In comparison, during Gaze Target, children in CP1 (GMFCS I) had larger trunk SPM, children in CP2 (GMFCS II) larger neck (SPM), and children in CP3 (GMFCS III) greater head and neck frontal plane movements, and reduced cadence and single support. Conclusions: Focusing a target altered gait in children with CP. Children in CP1 reduced movements similar to children with TD, children in CP2 behaved nearly unchanged, whereas children in CP3 reduced movements and temporalspatial parameters, potentially as a consequence of lack of sensory information from lower limbs.

  • 150. Bartonek, Asa
    et al.
    Eriksson, Marie
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A new carbon fibre spring orthosis for children with plantarflexor weakness2007In: Gait & Posture, ISSN 0966-6362, E-ISSN 1879-2219, Vol. 25, no 4, p. 652-656Article in journal (Refereed)
    Abstract [en]

    We tested a new orthosis with a carbon fiber spring constructed to enable energy storing during increasing dorsiflexion in mid-stance, and to use the energy at the end of stance phase to aid push-off. The orthosis was tested on children with plantarflexor weakness due to motor disorders. All subjects were tested with 3D gait analysis with both the new orthosis and with their regularly used orthosis. In this technical note, the results of three individuals are reported. The preliminary findings show increased dorsiflexion, altered knee kinematics and improved kinetic and temporo-spatial parameters. Although the carbon spring orthosis influenced the subjects' gait in different ways, we conclude that the tested subjects with plantarflexion weakness benefit from the carbon fiber spring orthoses during walking. The parents' and children's subjective impressions as acquired from a questionnaire were also positive.

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