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  • 1.
    Aare, Magnus
    et al.
    KTH, Superseded Departments (pre-2005), Aeronautical and Vehicle Engineering.
    Kleiven, Svein
    KTH, Superseded Departments (pre-2005), Aeronautical and Vehicle Engineering.
    Halldin, Peter
    KTH, Superseded Departments (pre-2005), Aeronautical and Vehicle Engineering.
    Injury tolerances for oblique impact helmet testing2004In: International Journal of Crashworthiness, ISSN 1358-8265, E-ISSN 1754-2111, Vol. 9, no 1, p. 15-23Article in journal (Refereed)
    Abstract [en]

    The most frequently sustained severe injuries in motorcycle crashes are injuries to the head, and many of these are caused by rotational force. Rotational force is most commonly the result of oblique impacts to the head. Good testing methods for evaluating the effects of such impacts are currently lacking. There is also a need for improving our understanding of the effects of oblique impacts on the human head. Helmet standards currently in use today do not measure rotational effects in test dummy heads. However rotational force to the head results in large shear strains arising in the brain, which has been proposed as a cause of traumatic brain injuries like diffuse axonal injuries (DAI). This paper investigates a number of well-defined impacts, simulated using a detailed finite element (FE) model of the human head, an FE model of the Hybrid III dummy head and an FE model of a helmet. The same simulations were performed on both the FE human head model and the FE Hybrid III head model, both fitted with helmets. Simulations on both these heads were performed to describe the relationship between load levels in the FE Hybrid III head model and strains in the brain tissue in the FE human head model. In this study, the change in rotational velocity and the head injury criterion (HIC) value were chosen as appropriate measurements. It was concluded that both rotational and translational effects are important when predicting the strain levels in the human brain.

  • 2.
    Abbasi, Saeed
    Sharif university of Technology, Iran.
    A feasibilty study to establish freight cars overhaul center (Master thesis), Sharif university of technology2005Independent thesis Advanced level (degree of Master (Two Years)), 80 credits / 120 HE creditsStudent thesis
  • 3. Abbasi, Saeed
    Implementing theory of constraint in choosing six sigma project2008Conference paper (Refereed)
  • 4.
    Abbasi, Saeed
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements.
    Non-exhaust Nano particle emission in Rail traffic2010Conference paper (Refereed)
  • 5.
    Abbasi, Saeed
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements.
    Moslemi, Kianoush
    A new approach for optimization of heating system in tank wagons2003Conference paper (Refereed)
  • 6.
    Abbasi, Saeed
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements.
    Olander, Lars
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Services Engineering.
    Larsson, Christina
    Bombardier Transportation Sweden AB, Västerås, Sweden.
    Jansson, Anders
    Stockholm University, Applied Environmental Science, Stockholm, Sweden.
    Olofsson, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Tribologi.
    Sellgren, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements.
    A field test study of airborne wear particles from a running regional train2012In: IMechE, Part F: Journal of Rail and Rapid Transit, ISSN 0954-4097, Vol. 226, no 1, p. 95-109Article in journal (Refereed)
    Abstract [en]

    Inhalable airborne particles have inverse health affect. In railways, mechanical brakes, the wheel–rail contact, current collectors, ballast, sleepers, and masonry structures yield particulate matter. Field tests examined a Swedish track using a train instrumented with particle measurement devices, brake pad temperature sensors, and speed and brake sensors. The main objective of this field test was to study the characteristics of particles generated from disc brakes on a running train with an on-board measuring set-up.

    Two airborne particle sampling points were designated, one near a pad–rotor disc brake contact and a second under the frame, not near a mechanical brake or the wheel–rail contact; the numbers and size distributions of the particles detected were registered and evaluated under various conditions (e.g. activating/deactivating electrical brakes or negotiating curves). During braking, three speed/temperature-dependent particle peaks were identified in the fine region, representing particles 280 nm, 350 nm, and 600 nm in diameter. In the coarse region, a peak was discerned for particles 3–6 μm in diameter. Effects of brake pad temperature on particle size distribution were also investigated. Results indicate that the 280 nm peak increased with increasing temperature, and that electrical braking significantly reduced airborne particle numbers. FESEM images captured particles sizing down to 50 nm. The ICP-MS results indicated that Fe, Cu, Zn, Al, Ca, and Mg were the main elements constituting the particles.

     

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  • 7.
    Abbasi, Saeed
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements. Department of Mechanical engineering, Golpayegan University of Technology.
    Teimourimanesh, Shahab
    Chalmers.
    Vernersson, Tore
    Chalmers.
    Sellgren, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements.
    Olofsson, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements.
    Lunden, Roger
    Chalmers.
    Temperature and Thermoelastic Instability at Tread Braking Using Cast Iron Friction Material2013In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 314, no 1–2, p. 171-180Article in journal (Refereed)
    Abstract [en]

    Braking events in railway traffic often induce high frictional heating and thermoelastic instability (TEI) at the interfacing surfaces. In the present paper, two approaches are adopted to analyse the thermomechanical interaction in a pin-on-disc experimental study of railway braking materials. In a first part, the thermal problem is studied to find the heat partitioning between pin and disc motivated by the fact that wear mechanisms can be explained with a better understanding of the prevailing thermal conditions. The numerical model is calibrated using the experimental results. In a second part, the frictionally induced thermoelastic instabilities at the pin-disc contact are studied using a numerical method and comparing them with the phenomena observed in the experiments. The effects of temperature on material properties and on material wear are considered. It is found from the thermal analysis that the pin temperature and the heat flux to the pin increase with increasing disc temperatures up to a transition stage. This agrees with the behaviour found in the experiments. Furthermore, the thermoelastic analysis displays calculated pressure and the temperature distributions at the contact interface that are in agreement with the hot spot behaviour observed in the experiments.

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  • 8.
    Abbasi, Saeed
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements. KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Teimourimanesh, Shahab
    Chalmers.
    Vernersson, Tore
    Chalmers.
    Sellgren, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements.
    Olofsson, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Machine Elements. KTH, School of Engineering Sciences (SCI), Centres, The KTH Railway Group.
    Lunden, Roger
    Chalmers.
    Temperature and thermo-elastic instability of tread braking friction materials2012Conference paper (Refereed)
  • 9.
    Abbaszadeh Shahri, Abbas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Larsson, Stefan
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Johansson, Fredrik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Updated relations for the uniaxial compressive strength of marlstones based on P-wave velocity and point load index test2016In: INNOVATIVE INFRASTRUCTURE SOLUTIONS, ISSN 2364-4176, Vol. 1, no 1, article id UNSP 17Article in journal (Refereed)
    Abstract [en]

    Although there are many proposed relations for different rock types to predict the uniaxial compressive strength (UCS) as a function of P-wave velocity (V-P) and point load index (Is), only a few of them are focused on marlstones. However, these studies have limitations in applicability since they are mainly based on local studies. In this paper, an attempt is therefore made to present updated relations for two previous proposed correlations for marlstones in Iran. The modification process is executed through multivariate regression analysis techniques using a provided comprehensive database for marlstones in Iran, including UCS, V-P and Is from publications and validated relevant sources comprising 119 datasets. The accuracy, appropriateness and applicability of the obtained modifications were tested by means of different statistical criteria and graph analyses. The conducted comparison between updated and previous proposed relations highlighted better applicability in the prediction of UCS using the updated correlations introduced in this study. However, the derived updated predictive models are dependent on rock types and test conditions, as they are in this study.

  • 10.
    Abed, Kason
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    FEM-analys av torkapparat2011Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 11. Adolfsson, E
    et al.
    Gudmundson, Peter
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Matrix crack closure effects on the thermoelastic properties of [(0/90/+q/-q)s]M composite laminates1994Conference paper (Refereed)
  • 12.
    af Klinteberg, Ludvig
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA.
    Sorgentone, Chiara
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA.
    Tornberg, Anna-Karin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA.
    Quadrature error estimates for layer potentials evaluated near curved surfaces in three dimensions2022In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 111, p. 1-19Article in journal (Refereed)
    Abstract [en]

    The quadrature error associated with a regular quadrature rule for evaluation of a layer potential increases rapidly when the evaluation point approaches the surface and the integral becomes nearly singular. Error estimates are needed to determine when the accuracy is insufficient and a more costly special quadrature method should be utilized.& nbsp;The final result of this paper are such quadrature error estimates for the composite Gauss-Legendre rule and the global trapezoidal rule, when applied to evaluate layer potentials defined over smooth curved surfaces in R-3. The estimates have no unknown coefficients and can be efficiently evaluated given the discretization of the surface, invoking a local one-dimensional root-finding procedure. They are derived starting with integrals over curves, using complex analysis involving contour integrals, residue calculus and branch cuts. By complexifying the parameter plane, the theory can be used to derive estimates also for curves in R3. These results are then used in the derivation of the estimates for integrals over surfaces. In this procedure, we also obtain error estimates for layer potentials evaluated over curves in R2. Such estimates combined with a local root-finding procedure for their evaluation were earlier derived for the composite Gauss-Legendre rule for layer potentials written in complex form [4]. This is here extended to provide quadrature error estimates for both complex and real formulations of layer potentials, both for the Gauss-Legendre and the trapezoidal rule.& nbsp;Numerical examples are given to illustrate the performance of the quadrature error estimates. The estimates for integration over curves are in many cases remarkably precise, and the estimates for curved surfaces in R-3 are also sufficiently precise, with sufficiently low computational cost, to be practically useful.

  • 13.
    Afzal, Mohammad
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Numerical modelling and analysis of friction contact for turbine blades2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    High cycle fatigue failure of turbine and compressor blades due to resonance in the operating frequency range is one of the main problems in the design of gas turbine engines. To suppress excessive vibrations in the blades and prevent high cycle fatigue, dry friction dampers are used by the engine manufacturers. However, due to the nonlinear nature of friction contact, analysis of such systems becomes complicated.

    This work focuses on the numerical modelling of friction contact and a 3D friction contact model is developed. To reduce the computation time in the Newton-iteration steps, a method to compute the Jacobian matrix in parallel to the contact forces is proposed. The developed numerical scheme is successfully applied on turbine blades with shroud contact having an arbitrary 3D relative displacement. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of the cyclic symmetry boundary condition for a periodic structure.

    The developed 3D coupled numerical contact model is compared with a 3D contact model having uncoupled tangential motion and drawback of the uncoupled contact model is discussed. Furthermore, presence of higher harmonics in the nonlinear contact forces is analyzed and their effect on the excitation of the different harmonic indices (nodal diameters) of the bladed disk are systematically presented. Moreover, due to the quasi-analytical computation of the Jacobian matrix, the developed scheme is proved to be effective in solving the equations of motion and significant reduction in time is achieved without loss of accuracy.

     

     

     

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  • 14.
    Afzal, Mohammad
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH.
    On efficient and adaptive modelling of friction damping in bladed disks2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This work focuses on efficient modelling and adaptive control of friction damping in bladed disks. To efficiently simulate the friction contact, a full-3D time-discrete contact model is reformulated and an analytical expression for the Jacobian matrix is derived that reduces the computation time drastically with respect to the classical finite difference method. The developed numerical solver is applied on bladed disks with shroud contact and the advantage of full-3D contact model compared to a quasi-3D contact model is presented. The developed numerical solver is also applied on bladed disks with strip damper and multiple friction contacts and obtained results are discussed. Furthermore, presence of higher harmonics in the nonlinear contact forces is analyzed and their effect on the excitation of the different nodal diameters of the bladed disk are systematically presented. The main parameters that influence the effectiveness of friction damping in bladed disks are engine excitation order,  contact stiffnesses,  friction coefficient, relative motion at the friction interface and the normal contact load. Due to variation in these parameters during operation, the obtained friction damping in practice may differ from the optimum value. Therefore, to control the normal load adaptively that will lead to an optimum damping in the system despite these variations, use of magnetostrictive actuator is proposed. The magnetostrictive material that develops an internal strain under the influence of an external magnetic field is employed to increase and decrease the normal contact load. A linearized model of the magnetostrictive actuator is used to characterize the magnetoelastic behavior of the actuator.  A nonlinear static contact analysis of the bladed disk reveals that a change of normal load more than 700 N can be achieved using a reasonable size of the actuator. This will give a very good control on friction damping once applied in practice.

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  • 15.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Arteaga, Ines Lopez
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Eindhoven University of Technology, Netherlands.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Numerical analysis of multiple friction contacts in bladed disks2018In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 137, p. 224-237Article in journal (Refereed)
    Abstract [en]

    The damping potential of multiple friction contacts in a bladed disk is investigated. Friction contacts at tip shrouds and strip dampers are considered. It is shown that friction damping effectiveness can be potentially increased by using multiple friction contact interfaces. Friction damping depends on many parameters such as rotational speed, engine excitation order and mode family and therefore it is not possible to damp all the critical resonances using a single kind of friction contact interface. For example, a strip damper is more effective for the low nodal diameters, where blade/disk coupling is strong. The equations of motion of the bladed disk with multiple friction contacts are derived in the frequency domain for a cyclic structure with rotating excitations. A highly accurate method is used to generate the frequency response function (FRF) matrix. Furthermore, a finite element contact analysis is performed to compute the normal contact load and the contact area of the shroud interface at operating rotational speed. The multiharmonic balance method is employed in combination with the alternate frequency time domain method to find the steady state periodic solution. A low-pressure turbine bladed disk is considered and the effect of the engine excitation level, strip mass, thickness and the accuracy of FRF matrix on the nonlinear response curve are investigated in detail.

  • 16.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Adaptive control of normal load at the friction interface of bladed disks using giant magnetostrictive material2020In: Journal of Intelligent Materials Systems and Structures, ISSN 1045-389X, E-ISSN 1530-8138, Vol. 31, no 8, p. 1111-1125Article in journal (Refereed)
    Abstract [en]

    A novel application of magnetostrictive actuators in underplatform dampers of bladed disks is proposed for adaptive control of the normal load at the friction interface to achieve the desired friction damping in the structure. Friction damping in a bladed disk depends on operating parameters, such as rotational speed, engine excitation order, nodal diameter normal contact load, and contact interface parameters, such as contact stiffness and friction coefficient. The operating parameters have a fixed value, whereas the contact interface parameters vary in an unpredictable way at an operating point. However, the ability to vary some of these parameters such as the normal contact load in a controlled manner is desirable to attain an optimum damping in the bladed disk at different operating conditions. Under the influence of an external magnetic field, magnetostrictive materials develop an internal strain that can be exploited to vary the normal contact load at the friction interface, which makes them a potentially good candidate for this application. A commercially available magnetostrictive alloy, Terfenol-D is considered in this analysis that is capable of providing magnetostrain up to 2 × 10-3 under prestress and a blocked force over 1500 N. A linearized model of the magnetostrictive material, which is accurate enough for a direct current application, is employed to compute the output force of the actuator. A nonlinear finite element contact analysis is performed to compute the normal contact load between the blade platform and the underplatform damper as a result of magnetostrictive actuation. The nonlinear contact analysis is performed for different actuator mounting configurations and the obtained results are discussed. The proposed solution is potentially applicable to adaptively control vibratory stresses in bladed disks and consequently to reduce failure due to high-cycle fatigue. Finally, the practical challenges in employing magnetostrictive actuators in underplatform dampers are discussed.

  • 17.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics. kth.
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics. Eindhoven University of Technology, the Netherlands.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    A formulation of the Jacobian matrixfor 3D numerical friction contact model applied to turbine blade shroud contactIn: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568Article in journal (Other academic)
    Abstract [en]

    An analytical expression is formulated to compute the Jacobian matrix for 3D friction contact modelling that eciently evaluates the matrix while computing the friction contact forces in the time domain by means of the alternate frequency time domain approach. The developed expression is successfully used for thecalculation of the friction damping on a turbine blade with shroud contact interface having an arbitrary 3Drelative displacement. The analytical expression drastically reduces the computation time of the Jacobian matrix with respect to the classical finite dierence method, with many points at the contact interface. Therefore,it also significantly reduces the overall computation time for the solution of the equations of motion,since the formulation of the Jacobian matrix is the most time consuming step in solving the large set of nonlinear algebraic equations when a finite dierence approach is employed. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of cyclic symmetry boundary condition for aperiodic structure. Implementation of the developed scheme in solving the equations of motion is proved to be effective and significant reduction in time is achieved without loss of accuracy.

  • 18.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Lopez Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    An analytical calculation of the Jacobian matrix for 3D friction contact model applied to turbine blade shroud contact2016In: Computers & structures, ISSN 0045-7949, E-ISSN 1879-2243, Vol. 177, p. 204-217Article in journal (Refereed)
    Abstract [en]

    An analytical expression is formulated to compute the Jacobian matrix for 3D friction contact modeling that efficiently evaluates the matrix while computing the friction contact forces in the time domain by means of the alternate frequency time domain approach. The developed expression is successfully used for the calculation of the friction damping on a turbine blade with shroud contact interface having an arbitrary 3D relative displacement. The analytical expression drastically reduces the computation time of the Jacobian matrix with respect to the classical finite difference method, with many points at the contact interface. Therefore, it also significantly reduces the overall computation time for the solution of the equations of motion, since the formulation of the Jacobian matrix is the most time consuming step in solving the large set of nonlinear algebraic equations when a finite difference approach is employed. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of cyclic symmetry boundary condition for a periodic structure. Implementation of the developed scheme in solving the equations of motion is proved to be effective and significant reduction in time is achieved without loss of accuracy.

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  • 19.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Lopez-Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Adaptive control of normal load at the friction interface of bladed disks using giant magnetostrictive materialIn: Journal of Vibration and Control, ISSN 1077-5463, E-ISSN 1741-2986Article in journal (Other academic)
    Abstract [en]

    A novel application of magnetostrictive actuators in underplatform dampers of bladed disks is proposed for adaptive control of the normal load at the friction interface in order to achieve the desired friction damping in the structure. Friction damping in a bladed disk depends on many parameters such as rotational speed, engine excitation order, nodal diameter, contact stiffness, friction coefficient and normal contact load. However, all these parameters have a fixed value at an operating point. On the other hand, the ability to vary some of these parameters such as the normal contact load is desirable in order to obtain an optimum damping in the bladed disk at different operating conditions. Under the influence of an external magnetic field, magnetostrictive materials develop an internal strain that can be exploited to vary the normal contact load at the friction interface, which makes them a potentially good candidate for this application. A commercially available magnetostrictive alloy, Terfenol-D is considered in this analysis that is capable of providing magnetostrain up to 0.002 under prestress and a blocked force over 1500 N. A linearized model of the magnetostrictive material, which is accurate enough for a DC application, is employed to compute the output displacement and the blocked force of the actuator. A nonlinear finite element contact analysis is performed to compute the normal contact load between the blade platform and the underplatform damper as a result of magnetostrictive actuation. The contact analysis is performed for different mounting configurations of the actuator and the obtained results are discussed. The proposed solution is potentially applicable to adaptively control vibratory stresses in bladed disks and consequently to reduce failure due to high-cycle fatigue. Finally, the practical challenges in employing magnetostrictive actuators in underplatform dampers are discussed.

  • 20.
    Afzal, Mohammad
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Lopez-Arteaga, Ines
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Eindhoven University of Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Numerical analysis of multiple friction contacts in bladed disksIn: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162Article in journal (Other academic)
    Abstract [en]

    The damping potential of multiple friction contacts in a bladed disk, tip shroud and strip damper is investigated, showing that friction damping effectiveness can be potentially increased by using multiple friction contact interfaces. Friction damping depends on many parameters such as rotational speed, engine excitation order and mode family and therefore it is not possible to damp all the critical resonances using a single friction contact interface. For example, a strip damper is more effective for the low nodal diameters, where blade/disk coupling is strong. The equations of motion of the bladed disk with multiple friction contacts are derived in the frequency domain for a cyclic structure with rotating excitations and a highly accurate method is used to generate the frequency response function (FRF) matrix. Furthermore, a finite element contact analysis is performed to compute the normal contact load and the contact area of the shroud interface at operating rotational speed. The multiharmonic balance method is employed in combination with the alternate frequency time domain method to find the approximate steady state periodic solution. A low-pressure turbine bladed disk is considered and the effect of the engine excitation level, strip mass, thickness and the accuracy of FRF matrix on the nonlinear response curve are investigated in detail.

  • 21.
    Aghaali, Habib
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Exhaust Heat Utilisation and Losses in Internal Combustion Engines with Focus on the Gas Exchange System2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Exhaust gas energy recovery should be considered in improving fuel economy of internal combustion engines. A large portion of fuel energy is wasted through the exhaust of internal combustion engines. Turbocharger and turbocompound can, however, recover part of this wasted heat. The energy recovery depends on the efficiency and mass flow of the turbine(s) as well as the exhaust gas state and properties such as pressure, temperature and specific heat capacity. The exhaust gas pressure is the principal parameter which is required for the turbine energy recovery, but higher exhaust back-pressures on the engines create higher pumping losses. This is in addition to the heat losses in the turbochargers what makes any measurement and simulation of the engines more complex.

    This thesis consists of two major parts. First of all, the importance of heat losses in turbochargers has been shown theoretically and experimentally with the aim of including heat transfer of the turbochargers in engine simulations. Secondly, different concepts have been examined to extract exhaust heat energy including turbocompounding and divided exhaust period (DEP) with the aim of improved exhaust heat utilisation and reduced pumping losses.

    In the study of heat transfer in turbochargers, the turbocharged engine simulation was improved by including heat transfer of the turbocharger in the simulation. Next, the heat transfer modelling of the turbochargers was improved by introducing a new method for convection heat transfer calculation with the support of on-engine turbocharger measurements under different heat transfer conditions. Then, two different turbocharger performance maps were assessed concerning the heat transfer conditions in the engine simulation. Finally, the temperatures of turbocharger’s surfaces were predicted according to the measurements under different heat transfer conditions and their effects are studied on the turbocharger performance. The present study shows that the heat transfer in the turbochargers is very crucial to take into account in the engine simulations, especially in transient operations.

    In the study of exhaust heat utilisation, important parameters concerning turbine and gas exchange system that can influence the waste heat recovery were discussed. In addition to exhaust back-pressure, turbine speed and turbine efficiency, the role of the air-fuel equivalence ratio was demonstrated in details, because lower air-fuel equivalence ratio in a Diesel engine can provide higher exhaust gas temperature. The results of this study indicate that turbocompound engine efficiency is relatively insensitive to the air-fuel equivalence ratio.

    To decrease the influence of the increased exhaust back-pressure of a turbocompound engine, a new architecture was developed by combining the turbocompound engine with DEP. The aim of this study was to utilise the earlier phase (blowdown) of the exhaust stroke in the turbine(s) and let the later phase (scavenging) of the exhaust stroke bypass the turbine(s). To decouple the blowdown phase from the scavenging phase, the exhaust flow was divided between two different exhaust manifolds with different valve timing.

    According to this study, this combination improves the fuel consumption in low engine speeds and deteriorates it at high engine speeds. This is mainly due to long duration of choked flow in the exhaust valves because this approach is using only one of the two exhaust valves on each cylinder at a time.

    Therefore, the effects of enlarged effective flow areas of the exhaust valves were studied. Two methods were used to enlarge the effective flow area i.e. increasing the diameters of the blowdown and scavenging valves by 4 mm; and modifying the valve lift curves of the exhaust valves to fast opening and closing. Both methods improved BSFC in the same order even though they were different in nature. Fast opening and closing of the exhaust valves required shorter blowdown duration and longer scavenging duration. The modified lift curves provided less pumping losses, less available energy into the turbine and larger amplitude of the pulsating flow through the turbine.

    In order for defining a set of important parameters that should be examined in experimental studies, a sensitivity analysis was performed on the turbocompound DEP engine in terms of break specific fuel consumption to different parameters concerning the gas exchange such as blowdown valve timing, scavenging valve timing, blowdown valve size, scavenging valve size, discharge coefficients of blowdown and scavenging ports, turbine efficiency, turbine size and power transmission efficiency.

    Finally, to overcome the restriction in the effective flow areas of the exhaust valves, DEP was implemented externally on the exhaust manifold instead of engine exhaust valves, which is called externally DEP (ExDEP). This innovative engine architecture, which benefits from supercharging, turbocharging and turbocompounding, has a great fuel-saving potential in almost all load points up to 4%.

  • 22.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Ångstrom, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Demonstration of Air-Fuel Ratio Role in One-Stage Turbocompound Diesel Engines2013In: SAE Technical Papers, 2013, Vol. 11Conference paper (Refereed)
    Abstract [en]

    A large portion of fuel energy is wasted through the exhaust of internal combustion engines. Turbocompound can, however, recover part of this wasted heat. The energy recovery depends on the turbine efficiency and mass flow as well as the exhaust gas state and properties such as pressure, temperature and specific heat capacity.

    The main parameter influencing the turbocompound energy recovery is the exhaust gas pressure which leads to higher pumping loss of the engine and consequently lower engine crankshaft power. Each air-fuel equivalence ratio (λ) gives different engine power, exhaust gas temperature and pressure. Decreasing λ toward 1 in a Diesel engine results in higher exhaust gas temperatures of the engine.  λ can be varied by changing the intake air pressure or the amount of injected fuel which changes the available energy into the turbine. Thus, there is a compromise between gross engine power, created pumping power, recovered turbocompound power and consumed compressor power.

    In this study, the effects of different λ values and exhaust back-pressure have been investigated on the efficiency of a heavy-duty Diesel engine equipped with a single-stage electric turbocompounding. A one-dimensional gas dynamics model of a turbocharged engine was utilized that was validated against measurements at different load points. Two configurations of turbocompound engine were made. In one configuration an electric turbocharger was used and the amount of fuel was varied with constant intake air pressure. In another configuration the turbocharger turbine and compressor were disconnected to be able to control the turbine speed and the compressor speed independently; then the compressor pressure ratio was varied with constant engine fuelling and the exhaust back-pressure was optimized for each compressor pressure ratio.

    At each constant turbine efficiency there is a linear relation between the optimum exhaust back-pressure and ideally expanded cylinder pressure until bottom dead center with closed exhaust valves. There is an optimum λ for the turbocharged engine with regard to the fuel consumption. In the turbocompound engine, this will be moved to a richer λ that gives the best total specific fuel consumption; however, the results of this study indicates that turbocompound engine efficiency is relatively insensitive to the air-fuel ratio.

  • 23.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    A review of turbocompounding as a waste heat recovery system for internal combustion engines2015In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 49, p. 813-824Article in journal (Refereed)
    Abstract [en]

    Internal combustion engines waste a large amount of fuel energy through their exhausts. Various technologies have been developed for waste heat recovery such as turbocompounds, Rankine bottoming cycles, and thermoelectric generators that reduce fuel consumption and CO2 emissions. Turbocompounding is still not widely applied to vehicular use despite the improved fuel economy, lower cost, volume, and complexity higher exhaust gas recirculation driving capability and improved transient response. This paper comprehensively reviews the latest developments and research on turbocompounding to discover important variables and provide insights into the implementation of a high-efficiency turbocompound engine. Attention should be paid to the optimization of turbocompound engines and their configurations because the major drawback of this technology is additional exhaust back-pressure, which leads to higher pumping loss in the engines. Applying different technologies and concepts on turbocompound engines makes the exhaust energy recovery more efficient and provides more freedom in the design and optimization of the engines. Turbine efficiency plays an important role in the recovery of the wasted heat so turbine design is a crucial issue in turbocompounding. In addition, variability in geometry and rotational speed of power turbines allows for more efficient turbocompound engines in different operating conditions. The conclusion drawn from this review is that turbocompounding is a promising technology for reducing fuel consumption in the coming decades in both light- and heavy-duty engines.

  • 24.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Effects of Effective Flow Areas of Exhaust Valves on a Turbocompound Diesel Engine Combined With Divided Exhaust Period2014In: Proceedings from the FISITA 2014 World Automotive Congress, 2014Conference paper (Refereed)
    Abstract [en]

    Research and /or Engineering Questions/Objective: Exhaust gas energy recovery in internal combustion engines is one of the key challenges in the future developments. The objective of this study is to reveal the fuel-saving potential of a turbocompound Diesel engine combined with divided exhaust period (DEP). The exhaust flow is provided for two different manifolds via separate valves, blowdown and scavenging, at different timings. The main challenge in this combination is choked flow through the exhaust valves due to the restricted effective flow areas. Therefore, the effects of enlarged effective flow areas of the exhaust valves are studied.

    Methodology: A commercial 1D gas dynamics code, GT-POWER, was used to simulate a turbocharged Diesel engine which was validated against measurements. Then the turbocharged engine model was modified to a turbocompound engine with DEP. Using statistical analysis in the simulation (design of experiment), the performance of this engine was studied at different sizes, lift curves and timings of the exhaust valves and turbine swallowing capacity.

    Results: In the paper the effects of the effective flow areas of the exhaust valves are presented on the break specific fuel consumption, pumping mean effective pressure and the turbine energy recovery by increasing the valve size and modifying valve lift curve to fast opening and closing. This has been done in a low engine speed and full load. The main finding is that the flow characteristics of the exhaust valves in the turbocompound DEP engine are very important for gaining the full efficiency benefit of the DEP concept.  The turbocompound DEP engine with modified valve lift shape of the exhaust valves could improve the overall brake specific fuel consumption by 3.44% in which 0.64% of the improvement is due to the valve lift curve. Modified valve lift curves contribute mainly in decreasing the period of choked flow through the exhaust valves.

    Limitations of this study: The simulations were not validated against measurements; however, the mechanical and geometrical limitations were tried to keep realistic when manipulating the valve flow area events.

    What does the paper offer that is new in the field in comparison to other works of the author: In addition to the novelty of the engine architecture that combines turbocompound with DEP, the statistical analysis and comparison presented in this paper is new especially with demonstrating the importance of crank angle coupled flow characteristics of the valves.

    Conclusion: To achieve full fuel-saving potential of turbocompound DEP engines, the flow characteristics of the exhaust valves must be considered. The effective flow areas of the exhaust valves play important roles in the choked flow through the valves, the pumping work and the brake specific fuel consumption of the engine.

  • 25.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Externally divided exhaust period on a turbocompound engine for fuel-saving2014Conference paper (Other academic)
    Abstract [en]

    To improve exhaust heat utilization of a turbocharged engine, divided exhaust period (DEP) and turbocompound are integrated. The DEP concept decreases pumping loss created by the turbocompound. In the DEP concept the exhaust flow is divided between two different exhaust manifolds, blowdown and scavenging. One of the two exhaust valves on each engine cylinder is opened to the blowdown manifold at the first phase of exhaust stroke and the other valve is opened to the scavenging manifold at the later phase of exhaust stroke. This leads to lower exhaust back pressure and pumping loss. The combination of turbocompound engine with DEP has been examined previously and the result showed that this combination reduces the fuel consumption in low engine speeds and deteriorates it in high engine speeds. The main restriction of this combination was the low effective flow areas of the exhaust valves at high engine speeds.

    To overcome this restriction and increase the effective flow areas of the exhaust valves, DEP is employed externally on the exhaust manifold instead of engine exhaust valves. In externally DEP (ExDEP), both exhaust valves will be opened and closed similar to the corresponding turbocharged engine and the exhaust flow is divided by flow splits on the exhaust manifold. Two valves on the outlet ports of each flow split are added. One of them is a non-return valve (check valve) and the other one is synchronized with the cam shaft.

    In this study, the fuel-saving potential of ExDEP is analysed on the turbocompound engine at different engine speeds and loads and compared with the corresponding turbocharged engine, turbocompound engine and turbocompound DEP engine equipped. The results show that ExDEP has a great fuel-saving potential in almost all load points.

    ExDEP concept, itself, is a novel concept that there is no available literature about it. Moreover, combination of this new gas exchange system with turbocompound engines is an innovative extension of combined turbocompound DEP engines.

  • 26.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Performance Sensitivity to Exhaust Valves and Turbine Parameters on a Turbocompound Engine with Divided Exhaust Period2014In: SAE International Journal of Engines, ISSN 1946-3936, E-ISSN 1946-3944, Vol. 7, no 4, p. 1722-1733Article in journal (Refereed)
    Abstract [en]

    Turbocompound can utilize part of the exhaust energy on internal combustion engines; however, it increases exhaust back pressure, and pumping loss.  To avoid such drawbacks, divided exhaust period (DEP) technology is combined with the turbocompound engine. In the DEP concept the exhaust flow is divided between two different exhaust manifolds, blowdown and scavenging, with different valve timings. This leads to lower exhaust back pressure and improves engine performance.

    Combining turbocompound engine with DEP has been theoretically investigated previously and shown that this reduces the fuel consumption and there is a compromise between the turbine energy recovery and the pumping work in the engine optimization. However, the sensitivity of the engine performance has not been investigated for all relevant parameters. The main aim of this study is to analyze the sensitivity of this engine architecture in terms of break specific fuel consumption to different parameters concerning the gas exchange such as blowdown valve timing, scavenging valve timing, blowdown valve size, scavenging valve size, discharge coefficients of blowdown and scavenging ports, turbine efficiency, turbine size and power transmission efficiency. This study presents the sensitivity analysis of the turbocompound DEP engine to these parameters and defines a set of important parameters that should be examined in experimental studies.

  • 27.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Temperature Estimation of Turbocharger Working Fluids and Walls under Different Engine Loads and Heat Transfer Conditions2013In: SAE Technical Papers, 2013Conference paper (Refereed)
    Abstract [en]

    Turbocharger performance maps, which are used in engine simulations, are usually measured on a gas-stand where the temperatures distributions on the turbocharger walls are entirely different from that under real engine operation. This should be taken into account in the simulation of a turbocharged engine. Dissimilar wall temperatures of turbochargers give different air temperature after the compressor and different exhaust gas temperature after the turbine at a same load point. The efficiencies are consequently affected. This can lead to deviations between the simulated and measured outlet temperatures of the turbocharger turbine and compressor. This deviation is larger during a transient load step because the temperatures of turbocharger walls change slowly due to the thermal inertia. Therefore, it is important to predict the temperatures of turbocharger walls and the outlet temperatures of the turbocharger working fluids in a turbocharged engine simulation.

    In the work described in this paper, a water-oil-cooled turbocharger was extensively instrumented with several thermocouples on reachable walls. The turbocharger was installed on a 2-liter gasoline engine that was run under different loads and different heat transfer conditions on the turbocharger by using insulators, an extra cooling fan, radiation shields and water-cooling settings. The turbine inlet temperature varied between 550 and 850 °C at different engine loads.

    The results of this study show that the temperatures of turbocharger walls are predictable from the experiment. They are dependent on the load point and the heat transfer condition of the turbocharger. The heat transfer condition of an on-engine turbocharger could be defined by the turbine inlet temperature, ambient temperature, oil heat flux, water heat flux and the velocity of the air around the turbocharger. Thus, defining the heat transfer condition and rotational speed of the turbocharger provides temperatures predictions of the turbocharger walls and the working fluids. This prediction enables increased precision in engine simulation for future work in transient operation.

  • 28.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    The Exhaust Energy Utilization of a Turbocompound Engine Combined with Divided Exhaust Period2014Conference paper (Refereed)
    Abstract [en]

    To decrease the influence of the increased exhaust pressure of a turbocompound engine, a new architecture is developed by combining the turbocompound engine with divided exhaust period (DEP). The aim of this study is to utilize the earlier stage (blowdown) of the exhaust stroke in the turbine(s) and let the later stage (scavenging) of the exhaust stroke bypass the turbine(s). To decouple the blowdown phase from the scavenging phase, the exhaust flow is divided between two different exhaust manifolds with different valve timing. A variable valve train system is assumed to enable optimization at different load points. The fuel-saving potential of this architecture have been theoretically investigated by examining different parameters such as turbine flow capacity, blowdown valve timing and scavenging valve timing. Many combinations of these parameters are considered in the optimization of the engine for different engine loads and speeds.

    This architecture produces less negative pumping work for the same engine load point due to lower exhaust back pressure; however, the exhaust mass flow into the turbine(s) is decreased. Therefore, there is a compromise between the turbine energy recovery and the pumping work. According to this study, this combination shows fuel-saving potential in low engine speeds and limitations at high engine speeds. This is mainly due to the choked flow in the exhaust valves because this approach is using only one of the two exhaust valves at a time. To reveal the full potential of this approach, increasing the effective flow area of the valves should be studied.

  • 29.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
    Turbocharged SI-Engine Simulation with Cold and Hot-Measured Turbocharger Performance Maps2012In: Proceedings of ASME Turbo Expo 2012, Vol 5, ASME Press, 2012, p. 671-679Conference paper (Refereed)
    Abstract [en]

    Heat transfer within the turbocharger is an issue in engine simulation based on zero and one-dimensional gas dynamics. Turbocharged engine simulation is often done without taking into account the heat transfer in the turbocharger. In the simulation, using multipliers is the common way of adjusting turbocharger speed and parameters downstream of the compressor and upstream of the turbine. However, they do not represent the physical reality. The multipliers change the maps and need often to be different for different load points. The aim of this paper is to simulate a turbocharged engine and also consider heat transfer in the turbocharger. To be able to consider heat transfer in the turbine and compressor, heat is transferred from the turbine volute and into the compressor scroll. Additionally, the engine simulation was done by using two different turbocharger performance maps of a turbocharger measured under cold and hot conditions. The turbine inlet temperatures were 100 and 600°C, respectively. The turbocharged engine experiment was performed on a water-oil-cooled turbocharger (closed waste-gate), which was installed on a 2-liter gasoline direct-injected engine with variable valve timing, for different load points of the engine. In the work described in this paper, the difference between cold and hot-measured turbocharger performance maps is discussed and the quantified heat transfers from the turbine and to/from the compressor are interpreted and related to the maps.

  • 30.
    Aghaali, Habib
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Ångström, Hans-Erik
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Serrano, Jose R
    Universitat Politècnica de València.
    Evaluation of different heat transfer conditions on an automotive turbocharger2014In: International Journal of Engine Research, ISSN 1468-0874, E-ISSN 2041-3149, Vol. 16, no 2, p. 137-151Article in journal (Refereed)
    Abstract [en]

    This paper presents a combination of theoretical and experimental investigations for determining the main heat fluxes within a turbocharger. These investigations consider several engine speeds and loads as well as different methods of conduction, convection, and radiation heat transfer on the turbocharger. A one-dimensional heat transfer model of the turbocharger has been developed in combination with simulation of a turbocharged engine that includes the heat transfer of the turbocharger. Both the heat transfer model and the simulation were validated against experimental measurements. Various methods were compared for calculating heat transfer from the external surfaces of the turbocharger, and one new method was suggested.

    The effects of different heat transfer conditions were studied on the heat fluxes of the turbocharger using experimental techniques. The different heat transfer conditions on the turbocharger created dissimilar temperature gradients across the turbocharger. The results show that changing the convection heat transfer condition around the turbocharger affects the heat fluxes more noticeably than changing the radiation and conduction heat transfer conditions. Moreover, the internal heat transfers from the turbine to the bearing housing and from the bearing housing to the compressor are significant, but there is an order of magnitude difference between these heat transfer rates.

  • 31.
    Agrawal, Vipin
    et al.
    Nordita SU; Department of Physics, Stockholm University, AlbaNova University Centre, Fysikum, 106 91 Stockholm, Sweden, Fysikum.
    Pandey, Vikash
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Mitra, Dhrubaditya
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Active buckling of pressurized spherical shells: Monte Carlo simulation2023In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 108, no 3, article id L032601Article in journal (Refereed)
    Abstract [en]

    We study the buckling of pressurized spherical shells by Monte Carlo simulations in which the detailed balance is explicitly broken - thereby driving the shell to be active, out of thermal equilibrium. Such a shell typically has either higher (active) or lower (sedate) fluctuations compared to one in thermal equilibrium depending on how the detailed balance is broken. We show that, for the same set of elastic parameters, a shell that is not buckled in thermal equilibrium can be buckled if turned active. Similarly a shell that is buckled in thermal equilibrium can unbuckle if sedated. Based on this result, we suggest that it is possible to experimentally design microscopic elastic shells whose buckling can be optically controlled.

  • 32.
    Agrawal, Vishal
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Scapin, Nicolo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway.
    An efficient isogeometric/finite-difference immersed boundary method for the fluid–structure interactions of slender flexible structures2024In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 418, article id 116495Article in journal (Refereed)
    Abstract [en]

    In this contribution, we present a robust and efficient computational framework capable of accurately capturing the dynamic motion and large deformation/deflection responses of highly-flexible rods interacting with an incompressible viscous flow. Within the partitioned approach, we adopt separate field solvers to compute the dynamics of the immersed structures and the evolution of the flow field over time, considering finite Reynolds numbers. We employ a geometrically exact, nonlinear Cosserat rod formulation in the context of the isogeometric analysis (IGA) technique to model the elastic responses of each rod in three dimensions (3D). The Navier–Stokes equations are resolved using a pressure projection method on a standard staggered Cartesian grid. The direct-forcing immersed boundary method is utilized for coupling the IGA-based structural solver with the finite-difference fluid solver. In order to fully exploit the accuracy of the IGA technique for FSI simulations, the proposed framework introduces a new procedure that decouples the resolution of the structural domain from the fluid grid. Uniformly distributed Lagrangian markers with density relative to the Eulerian grid are generated to communicate between Lagrangian and Eulerian grids consistently with IGA. We successfully validate the proposed computational framework against two- and three-dimensional FSI benchmarks involving flexible filaments undergoing large deflections/motions in an incompressible flow. We show that six times coarser structural mesh than the flow Eulerian grid delivers accurate results for classic benchmarks, leading to a major gain in computational efficiency. The simultaneous spatial and temporal convergence studies demonstrate the consistent performance of the proposed framework, showing that it conserves the order of the convergence, which is the same as that of the fluid solver.

  • 33.
    Aihara, Aya
    et al.
    Division of Electricity, Department of Electrical Engineering, Uppsala University, Uppsala, Sweden.
    Bolin, Karl
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Marcus Wallenberg Laboratory MWL.
    Goude, Anders
    Division of Electricity, Department of Electrical Engineering, Uppsala University, Uppsala, Sweden.
    Bernhoff, Hans
    Division of Electricity, Department of Electrical Engineering, Uppsala University, Uppsala, Sweden.
    Aeroacoustic noise prediction of a vertical axis wind turbine using large eddy simulation2021In: International Journal of Aeroacoustics, ISSN 1475-472X, E-ISSN 2048-4003, Vol. 20, no 8, p. 959-978Article in journal (Refereed)
    Abstract [en]

    This study investigates the numerical prediction for the aerodynamic noise of the vertical axis wind turbine using large eddy simulation and the acoustic analogy. Low noise designs are required especially in residential areas, and sound level generated by the wind turbine is therefore important to estimate. In this paper, the incompressible flow field around the 12 kW straight-bladed vertical axis wind turbine with the rotor diameter of 6.5 m is solved, and the sound propagation is calculated based on the Ffowcs Williams and Hawkings acoustic analogy. The sound pressure for the turbine operating at high tip speed ratio is predicted, and it is validated by comparing with measurement. The measured spectra of the sound pressure observed at several azimuth angles show the broadband characteristics, and the prediction is able to reproduce the shape of these spectra. While previous works studying small-scaled vertical axis wind turbines found that the thickness noise is the dominant sound source, the loading noise can be considered to be a main contribution to the total sound for this turbine. The simulation also indicates that the received noise level is higher when the blade moves in the downwind than in the upwind side.

  • 34. Alam, Minhaj M
    et al.
    Barsoum, Zuheir
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Jonsén, Pär
    Häggblad, Hans Åke
    Kaplan, Alexander
    Geometrical Aspects of The Fatigue Behaviour of Laser Hybrid Fillet Welds2009In: Proceedings of the Fatigue Design Conference, 2009Conference paper (Refereed)
  • 35.
    Alaniz, Monica
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Belyayev, Serhiy
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Bergman, David
    Casselbrant, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Honeth, Mark
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Huang, Jiangwei
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Ivchenko, Nickolay
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Laukkanen, Mikko
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Michelsen, Jacob
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Pronenko, Vira
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Paulson, Malin
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Schlick, Georg
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Tibert, Gunnar
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Valle, Mario
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    The SQUID sounding rocket experiment2011In: Proceedings of the 20th ESA Symposium on European Rocket and Balloon Programmes and Related Research, European Space Agency, 2011, p. 159-166Conference paper (Refereed)
    Abstract [en]

    The objective of the SQUID project is to develop and in flight verify a miniature version of a wire boom deployment mechanism to be used for electric field measurements in the ionosphere. In February 2011 a small ejectable payload, built by a team of students from The Royal Institute of Technology (KTH), was launched from Esrange on-board the REXUS-10 sounding rocket. The payload separated from the rocket, deployed and retracted the wire booms, landed with a parachute and was subsequently recovered. Here the design of the experiment and post fight analysis are presented.

  • 36. Alberdi, A.
    et al.
    Gil-Negrete, N.
    Vinolas, J.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Nieto, F. J.
    Dynamic characterisation of different magneto-sensitive natural rubbers for application in vibration isolation2010In: Proceedings of ISMA 2010 - International Conference on Noise and Vibration Engineering, including USD 2010, Katholieke Universiteit Leuven , 2010, p. 227-231Conference paper (Refereed)
    Abstract [en]

    Dynamic properties of magneto-sensitive natural rubber components were experimentally studied. Different magneto-sensitive rubbers were manufactured, consisting of irregularly shaped micron-sized iron particles embedded in a natural rubber matrix, and the influence of the hardness of the matrix material and the particle volume concentration were analyzed. Vibration isolators consisting of magneto-sensitive elastomers promise to have more functionality than conventional isolators as they can change their dynamic stiffness rapidly, continuously and reversibly under the application of an external magnetic field. Experimental measurements on MS components show that a better performance may be obtained at applications where small amplitudes are required, using soft matrix materials and with concentration close to a critical particle volume fraction.

  • 37.
    Alberdi-Muniain, Ane
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Magneto-sensitive elastomers in vibration isolation2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Vibration isolators made of rubber are used in numerous engineeringapplications to isolate structures from undesirable effects of vibrations.However, once a vibration isolator is installed in an application, it is not possible to modify its characteristics to adjust to changing conditions. An alternative to obtain more adaptive characteristics is touse magneto-sensitive (MS) elastomers. MS elastomers are a type of smart material consisting of an elastomer matrix, such as natural or synthetic rubber, to which iron particles are added displaying properties that vary rapidly, continuously and reversibly by applying an external magnetic field.The aim of this thesis is to investigate the possibility to use MS natural rubber in vibration isolation.Firstly, dynamic shear properties of MS natural rubber are experimentally studied at various frequencies, dynamic amplitudes and magnetic fields. In addition, the influence on the dynamic properties of adding carbon black and plasticisers to MS rubber is investigated. Carbon black is the most popular reinforcing filler that rubber usually contains in engineering applications to improve mechanical properties where as plasticisers simplify the filler blending process.Furthermore, the effectiveness of MS rubber applied in a vibration isolation system is experimentally investigated by measuring the energy flow into the foundation. The energy flow, including both force and velocity of the foundation, is a suitable measure of the effectiveness of a real vibration isolation system where the foundation is not perfectly rigid. The vibration isolation system in this study consists of a solid aluminium mass excitedby an electro-dynamic shaker and mounted upon four nonlinear frequency,amplitude and magnetic field dependent MS isolators being connected to a relatively stiff foundation. The energy flow through the MS isolators is directly measured by inserting a force transducer below each isolator andan accelerometer on the foundation close to each isolator. MS isolators are shown to be more useful than conventional rubber isolators since the dynamic stiffness varies with the application of an external magnetic field,thus resulting in more effective vibration isolation. In addition, the indirect technique is employed to measure the energy flow while requiring only accelerometers since it is usually difficult to directly measure the force in a real application. The indirect technique is validated by direct measurements.Finally, a model of the energy flow through the nonlinear frequency,amplitude and magnetic field dependent MS isolators is developed for the tested vibration isolation system. Vibration isolators are usually only a small connecting component within a more complex system. Hence, simple discrete models are frequently used to characterise the frequency and dynamic amplitude dependence of rubber. Recently, a model of this type has been modified to include magneto-sensitivity and thus model MS rubber. In this study, this novel MS rubber model is incorporated into the full system to model the MS isolators while the foundation is characterised by its driving-point and transfer inertances at and between the connection points.The energy flow model results are compared to those of measurements,showing good agreement. The developed energy flow model provides a basis to design vibration isolator systems made of MS isolators.

  • 38.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Direct energy flow measurement in magneto-sensitive vibration isolator systems2012In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 331, no 9, p. 1994-2006Article in journal (Refereed)
    Abstract [en]

    The effectiveness of highly nonlinear, frequency, amplitude and magnetic field dependent magneto-sensitive natural rubber components applied in a vibration isolation system is experimentally investigated by measuring the energy flow into the foundation. The energy flow, including both force and velocity of the foundation, is a suitable measure of the effectiveness of a real vibration isolation system where the foundation is not perfectly rigid. The vibration isolation system in this study consists of a solid aluminium mass supported on four magneto-sensitive rubber components and is excited by an electro-dynamic shaker while applying various excitation signals, amplitudes and positions in the frequency range of 20-200 Hz and using magneto-sensitive components at zero-field and at magnetic saturation. The energy flow through the magneto-sensitive rubber isolators is directly measured by inserting a force transducer below each isolator and an accelerometer on the foundation close to each isolator. This investigation provides novel practical insights into the potential of using magneto-sensitive material isolators in noise and vibration control, including their advantages compared to traditional vibration isolators. Finally, nonlinear features of magneto-sensitive components are experimentally verified.

  • 39.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Indirect energy flow measurement in magneto-sensitive vibration isolator systems2013In: Applied Acoustics, ISSN 0003-682X, E-ISSN 1872-910X, Vol. 74, no 4, p. 575-584Article in journal (Other academic)
    Abstract [en]

    The indirect energy flow measurement method is extended to cover highly nonlinear, frequency, amplitude and magnetic field dependent magneto-sensitive natural rubber isolators applied in a real vibration isolation system. Energy flow is an effective measure of vibration isolation while being a single quantity that considers both force and velocity. The use of the indirect technique is of interest while requiring only accelerometers since it is usually difficult to directly measure the force in a real application. The vibration isolation system is composed of four magneto-sensitive rubber isolators that are inserted under a vibrating source consisting of a solid aluminium mass excited by an electro-dynamic shaker. Magneto-sensitive rubber isolators are more useful than conventional rubber isolators since the dynamic stiffness varies with the application of an external magnetic field, thus resulting in more effective vibration isolation. Various approximations regarding the indirect technique are investigated, concluding that average stiffness of magneto-sensitive isolators can be used and auto-spectrum of the foundation velocity ignored. In addition, various error analyses are performed. Finally, the indirect measurement of the energy flow is validated by direct measurements, showing very good agreement.

  • 40.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Modelling energy flow through magneto-sensitive vibration isolators2013In: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 65, p. 22-39Article in journal (Refereed)
    Abstract [en]

    A highly nonlinear model of the energy flow in a magneto-sensitive (MS) vibration isolation system is developed where it is possible to investigate the influences of MS rubber material parameters; magnetic field strength; MS isolator dimension and position; excitation force magnitude, position and frequency; engine mass, inertia and dimension and, finally, foundation inertance. The MS vibration isolation system consists of an engine modelled by a solid mass, excited by a vertical force and mounted upon four MS isolators being connected to a relatively stiff foundation characterised by its driving-point and transfer inertances at and between the connection points. The energy flow into the foundation is the most appropriate indicator of the effectiveness of a real vibration isolation system while considering both foundation velocity and force. The MS isolator model applied is a nonlinear MS rubber model including frequency, dynamic amplitude and magnetic field dependence. The energy flow model results are compared to those of measurements, showing good agreement. Finally, parameter studies are carried out. The developed energy flow model provides a basis for designing MS vibration isolation systems to meet specific requirements.

  • 41.
    Alberdi-Muniain, Ane
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Gil-Negrete, N.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Nieto, F.J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Vinolas, J.
    Department of Applied Mechanics, CEIT and Tecnun (University of Navarra).
    An experimental study of magneto-sensitive natural rubber components applied in a vibration isolation system2009In: CONSTITUTIVE MODELS FOR RUBBER VI / [ed] Gert Heinrich, Michael Kaliske, Alexander Lion, London: Taylor & Francis, 2009, p. 99-104Conference paper (Refereed)
    Abstract [en]

    The effectiveness of magneto-sensitive natural rubber components applied in a vibration isolation system is experimentally investigated, where influences of excitation position, amplitude, frequency and magnetic field are examined. The magneto-sensitive elastomer consists of micron-sized, irregularly shaped iron particles blended in soft natural rubber at a concentration close to the critical particle volume fraction, shown to be the most favorable composition for optimum behaviour. A rigid aluminium mass supported on four vibration isolators is excited by an electro-dynamic shaker. Each component of this vibration isolation system is composed of two thin, square shaped, symmetrically positioned magneto-sensitive elements excited in simple shear with a magnetic field applied perpendicularly to the motion by an electromagnet. The magnetic field is varied by applying different intensities through the coil. The excitation position is either on the centre or on the edge of the surface of the mass, using step-sine excitation of various amplitudes in the frequency range of 0 to 300 Hz. The results show that it is possible to use magneto-sensitive rubber for vibration control purposes.

  • 42.
    Albernaz, Daniel L.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hermanson, J. C.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Droplet deformation and heat transfer in isotropic turbulence2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 820, p. 61-85Article in journal (Refereed)
    Abstract [en]

    The heat and mass transfer of deformable droplets in turbulent flows is crucial. to a wide range of applications, such as cloud dynamics and internal combustion engines. This study investigates a single droplet undergoing phase change in isotropic turbulence using numerical simulations with a hybrid lattice Boltzmann scheme. Phase separation is controlled by a non-ideal equation of state and density contrast is taken into consideration. Droplet deformation is caused by pressure and shear stress at the droplet interface. The statistics of thermodynamic variables are quantified and averaged over both the liquid and vapour phases. The occurrence of evaporation and condensation is correlated to temperature fluctuations, surface tension variation and turbulence intensity. The temporal spectra of droplet deformations are analysed and related to the droplet surface area. Different modes of oscillation are clearly identified from the deformation power spectrum for low Taylor Reynolds number Re, whereas nonlinearities are produced with the increase of Re A, as intermediate frequencies are seen to overlap. As an outcome, a continuous spectrum is observed, which shows a decrease in the power spectrum that scales as similar to f(-3) Correlations between the droplet Weber number, deformation parameter, fluctuations of the droplet volume and thermodynamic variables are also developed.

  • 43. Alegre-Martínez, C.
    et al.
    Choi, K. -S
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    McNally, D.
    On the axial distribution of plaque stress: Influence of stenosis severity, lipid core stiffness, lipid core length and fibrous cap stiffness2019In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 68, p. 76-84Article in journal (Refereed)
    Abstract [en]

    Numerical simulations of blood flow through a partially-blocked axisymmetric artery are performed to investigate the stress distributions in the plaque. We show that the combined effect of stenosis severity and the stiffness of the lipid core can drastically change the axial stress distribution, strongly affecting the potential sites of plaque rupture. The core stiffness is also an important factor when assessing plaque vulnerability, where a mild stenosis with a lipid-filled core presents higher stress levels than a severe stenosis with a calcified plaque. A shorter lipid core gives rise to an increase in the stress levels. However, the fibrous cap stiffness does not influence the stress distributions for the range of values considered in this work. Based on these mechanical analyses, we identify potential sites of rupture in the axial direction for each case: the midpoints of the upstream and downstream regions of the stenosis (for severe, lipid-filled plaques), the ends of the lipid core (for short cores), and the middle of the stenosis (for mild stenoses with positive remodelling of the arterial wall). 

  • 44.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Öberg, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, J.
    Propagation of physically short cracks in a bainitic high strength bearing steel due to fatigue load2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 90, p. 166-180Article in journal (Refereed)
    Abstract [en]

    Physically short cracks in a bainitic high strength bearing steel were fatigue loaded. The rapid propagation rate of early open short cracks agreed with that of long closure free cracks. After some rapid growth, the short cracks entered a transition period to the rate of growth limited long cracks. Potential drop showed that the short cracks were open to the tip throughout the growth sequence, which excluded crack face closure in the wake as the growth limiting mechanism in this material. Instead the short crack effect was related to residual stresses and other mechanisms at the crack tip. Crack manufacturing procedures were determined for straight long and short start cracks in the present material. LEFM with effective material parameters and limit compensation predicted the short crack lives.

  • 45.
    Alfredsson, P. Henrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Instability, transition and turbulence in plane Couette flow with system rotation2005In: IUTAM Symposium on Laminar-Turbulent Transition and Finite Amplitude Solutions / [ed] Mullin, T; Kerswell, R, Springer Netherlands, 2005, Vol. 77, p. 173-193Conference paper (Refereed)
    Abstract [en]

    System rotation may have either stabilizing or destabilizing effects on shear flows depending on the direction of rotation vector as compared to the vorticity vector of mean flow. This study describes experimental results of laminar, transitional and turbulent plane Couette flow with both stabilizing and destabilizing system rotation. For laminar flow with destabilizing rotation roll cells appear in the flow which may undergo several different types of secondary instabilities, especially interesting is a repeating pattern of wavy structures followed by breakdown, thereafter roll cells reappear in a cyclic pattern. For higher Reynolds number roll cells appear also in a turbulent environment. It is also shown how stabilizing rotation may quench the turbulence completely.

  • 46.
    Alfredsson, P. Henrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH Mech, Linne FLOW Ctr, SE-10044 Stockholm, Sweden..
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Large-Eddy BreakUp Devices - a 40 Years Perspective from a Stockholm Horizon2018In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 100, no 4, p. 877-888Article in journal (Refereed)
    Abstract [en]

    In the beginning of the 1980's Large Eddy BreakUp (LEBU) devices, thin plates or airfoils mounted in the outer part of turbulent boundary layers, were shown to be able to change the turbulent structure and intermittency as well as reduce turbulent skin friction. In some wind-tunnel studies it was also claimed that a net drag reduction was obtained, i.e. the reduction in skin-friction drag was larger than the drag on the devices. However, towing-tank experiments with a flat plate at high Reynolds numbers as well as with an axisymmetric body showed no net reduction, but instead an increase in total drag. Recent large-eddy simulations have explored the effect of LEBUs on the turbulent boundary layer and evaluations of the total drag show similar results as in the towing tank experiments. Despite these negative results in terms of net drag reduction, LEBUs manipulate the boundary layer in an interesting way which explains why they still attract some interest. The reason for the positive results in the wind-tunnel studies as compared to drag measurements are discussed here, although no definite answer for the differences can be given.

  • 47. Alfthan, J.
    et al.
    Gudmundson, Peter
    Östlund, S.
    Micro-mechanical model for mechanosorptive creep2000Conference paper (Other academic)
  • 48.
    Ali, Asad
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Michael, Danay
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    In-plane bi-axial testing of thin paper2022Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    During its lifetime, a beverage package is subjected to a complex loading history where the loading, in general, is multiaxial. Hence, to optimize the performance of the package, knowledge of the material’s anisotropic multi-axial deformation and subsequent failure is necessary. This report presents a way to investigate the anisotropic loading and fracture of thin paper using bi-axial testing and to analyse the bi-axial test using the Finite Element Method.

    Getting thin paper to fracture in the central region, where the deformation and loading are bi-axial, proved difficult. To overcome this challenge, a new bi-axial specimen, with a specific sample preparation technique, is proposed and implemented in this specific project. This cruciform-shaped thin paper specimen was reinforced with laminating plastic everywhere but in the central region. This was done to avoid material failure in the notched radius area or the clamps, rather to facilitate a material failure in the central region of the material specimen. In order to simulate the bi-axial test, a Hill elastic-plastic material model was calibrated, and the material parameters were obtained from uniaxial tensile tests.

    When subjected to bi-axial loading, the proposed cruciform-shaped specimen fractured systematically and repetitively for the different load cases studied. Moreover, the Hill material model captured the force vs. displacement curve from the experimental results well but overestimated its values. The overestimation was mainly due to sliding during the experimental tests.

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  • 49.
    Alimadadi, Majid
    et al.
    Mid Sweden Univ, Dept Nat Sci NAT, Sundsvall, Sweden..
    Lindström, Stefan B.
    Linköping Univ, Dept Management & Engn, Div Solid Mech, Linkoping, Sweden..
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Role of microstructures in the compression response of three-dimensional foam-formed wood fiber networks2018In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 14, no 44, p. 8945-8955Article in journal (Refereed)
    Abstract [en]

    High-porosity, three-dimensional wood fiber networks made by foam forming present experimentally accessible instances of hierarchically structured, athermal fiber networks. We investigate the large deformation compression behavior of these networks using fiber-resolved finite element analyses to elucidate the role of microstructures in the mechanical response to compression. Three-dimensional network structures are acquired using micro-computed tomography and subsequent skeletonization into a Euclidean graph representation. By using a fitting procedure to the geometrical graph data, we are able to identify nine independent statistical parameters needed for the regeneration of artificial networks with the observed statistics. The compression response of these artificially generated networks and the physical network is then investigated using implicit finite element analysis. A direct comparison of the simulation results from the reconstructed and artificial network reveals remarkable differences already in the elastic region. These can neither be fully explained by density scaling, the size effect nor the boundary conditions. The only factor which provides the consistent explanation of the observed difference is the density and fiber orientation nonuniformities; these contribute to strain-localization so that the network becomes more compliant than expected for statistically uniform microstructures. We also demonstrate that the experimentally manifested strain-stiffening of such networks is due to development of new inter-fiber contacts during compression.

  • 50.
    Alizadeh, Amir
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fracture Simulation of Electrofusion Joining.2015Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This Master thesis investigates mechanical failures of electrofusion joints. This type of joints are used for weld high density polyethylene pipe systems where tting and pipe will be welded together by the heat caused by the resistance of the copper cables to electric current. There have been some failures where a brittle crack has grown through the fusion zone. Polyethylene as a material has a ductile character but brittle behavior occurs due to an unsuccessful welding by a poor fusion interface. In this study, we have chosen to investigate the plausible load cases that can cause the failure and studied the e ect of the geometry of the tting on the fracture toughness of the welded structure. We used the nite element method numerical analysis. We have approached unsuccessful welding (brittle) with a linear model and non-linear (CZM) model and successful welding (ductile) with a non-linear XFEM model. The material parameters needed for these models are gathered by series of experiments. The results shows that the inside pressure is the critical load case. The linear model and CZM model are consistent in terms of predicted responses to the geometrical parameters for the unsuccessful welding. Decreasing the inner cold zone length, increasing the fusion length and the thickness of the tting will improve the fracture toughness of the welded structure.

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