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  • 1.
    Alfredsson, P. Henrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Instability, transition and turbulence in plane Couette flow with system rotation2005Ingår i: IUTAM Symposium on Laminar-Turbulent Transition and Finite Amplitude Solutions / [ed] Mullin, T; Kerswell, R, Springer Netherlands, 2005, Vol. 77, s. 173-193Konferensbidrag (Refereegranskat)
    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.

  • 2.
    Apazidis, Nicholas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Kjellander, Malte
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    High energy concentration by symmetric shock focusing2013Ingår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 23, nr 4, s. 361-368Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-energy concentrations in gas are achieved experimentally in a specially constructed shock tube facility at KTH Mechanics. The high-energy concentration is manifested by a formation of a hot, light-emitting gas core. Experimental, numerical and theoretical investigations show that the shape of the imploding shock is of pivotal importance for the final energy concentration. Cylindrical shocks are unstable. Symmetric polygonal shocks are shown to be dynamically stable and are produced by various methods, e.g. thin wing profiles placed radially in the test section. Such symmetric polygonal shocks are able to produce extremely high energy levels at the focal point. Spectral data from 60 nanosecond short intervals of 8 microsecond light pulse give temperatures in the range of 6,000 K.

  • 3.
    Apazidis, Nicholas
    et al.
    KTH, Tidigare Institutioner                               , Mekanik.
    Lesser, M. B.
    Tillmark, Nils
    KTH, Tidigare Institutioner                               , Mekanik.
    Johansson, Börje
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    An experimental and theoretical study of converging polygonal shock waves2002Ingår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 12, nr 1, s. 39-58Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 4.
    Caraghiaur, Diana
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorteknologi.
    Frid, Wiktor
    KTH, Tidigare Institutioner                               , Energiteknik.
    Tillmark, Nils
    KTH, Tidigare Institutioner                               , Mekanik.
     Detailed pressure drop measurements in single- and two-phase adiabatic air-water turbulent flows in realistic BWR fuel assembly geometry with spacer grids2004Ingår i: The 6th International Conference on Nuclear Thermal Hydraulics, Operations and Safety (NUTHOS-6) Nara, Japan, October 4-8, 2004, 2004Konferensbidrag (Refereegranskat)
    Abstract [en]

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

  • 5.
    Eliasson, V
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, N.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Shaping converging shock waves by means of obstacles2006Ingår i: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975, Vol. 9, nr 3, s. 240-240Artikel i tidskrift (Övrigt vetenskapligt)
  • 6.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Controlling the form of strong converging shocks by means of disturbances.2007Ingår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 17, nr 1-2, s. 29-42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The influence of artificial disturbances on the behavior of strong converging cylindrical shocks is investigated experimentally and numerically. Ring-shaped shocks, generated in an annular cross sectional shock tube are transformed to converging cylindrical shocks in a thin cylindrical test section, mounted at the rear end of the shock tube. The converging cylindrical shocks are perturbed by small cylinders placed at different locations and in various patterns in the test section. Their influence on the shock convergence and reflection process is investigated. It is found that disturbances arranged in a symmetrical pattern will produce a symmetrical deformation of the converging shockfront. For example, a square formation produces a square-like shock and an octagon formation a shock with an octagonal front. This introduces an alternative way of tailoring the form of a converging shock, instead of using a specific form of a reflector boundary. The influence of disturbances arranged in non-symmetric patterns on the shape of the shockfront is also investigated.

  • 7.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lesser, Martin
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Focusing of strong shocks in an annular shock tube2007Ingår i: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 15, nr 3-4, s. 205-217Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Focusing of strong shock waves in a gas-filled thin convergence chamber with various forms of the reflector boundary is investigated experimentally and numerically. The convergence chamber is mounted at the end of the horizontal co-axial shock tube. The construction of the convergence chamber allows the assembly of the outer chamber boundaries of various shapes. Boundaries with three different shapes have been used in the present investigation-a circle, an octagon and a smooth pentagon. The shock tube in the current study was able to produce annular shocks with the initial Mach number in the range M-s = 2.3-3.6. The influence of the shape of the boundary on the shape and properties of the converging and reflected shock waves in the chamber has then been investigated both experimentally and numerically. It was found that the form of the converging shock is initially governed by the shape of the reflector and the nonlinear interaction between the shape of the shock and velocity of shock propagation. Very close to the center of convergence the shock obtains a square-like form in case of a circular and octagonal reflector boundary. This is believed to stem from the instability of the converging shock front triggered by the disturbances in the flow field. The outgoing, reflected shocks were also observed to be influenced by the shape of the boundary through the flow ahead as created by the converging shocks.

  • 8.
    Eliasson, Veronica
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    Szeri, Andrew
    University of California, Department of Mechanical Engineering, Berkeley.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Light emission during shock wave focusing in air and argon2007Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, nr 10, s. 106106-1-106106-17Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The light emission from a converging shock wave was investigated experimentally. Results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The light emission appears as the shock wave collapses. The full width at half maximum of the light pulse is about 200 ns for all geometrical shapes. It was also found that argon as a test gas produces more light than air. Numerical simulations showed good agreement with experimental results regarding the shape of the shock and the flow field behind the shock. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there.

  • 9. Facciolo, Luca
    et al.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Talamelli, Alessandro
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    A study of swirling turbulent pipe and jet flows2007Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Axially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress.

  • 10. Hiwatashi, Kazuaki
    et al.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Nagata, M.
    Experimental observations of instabilities in rotating plane Couette flow2007Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, nr 4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The transition from the two-dimensional (2D) longitudinal roll cell state to 3D flows in the rotating plane Couette system, predicted by the theoretical investigation [M. Nagata, J. Fluid Mech. 358, 357 (1998)], is examined experimentally. The streamwise and spanwise wave numbers of observed steady 3D flows seem to agree with those predicted by the theory when the rotation rate is relatively large. However, we observe unsteady 3D states in the region where the theory predicts stable steady 3D flows when the rotation rate is small.

  • 11.
    Kalpakli, Athanasia
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Experimental investigation on the effect of pulsations on exhaust manifold-related flows aiming at improved efficiency2012Ingår i: Institution of Mechanical Engineers - 10th International Conference on Turbochargers and Turbocharging, 2012, s. 377-387Konferensbidrag (Refereegranskat)
    Abstract [en]

    The gas flowing through the exhaust manifold of the internal combustion engine to the inlet of the turbine side of a turbocharger is highly pulsating and turbulent. The gas enters the turbine after travelling through a complex curved and branched pipe system where the effect of centrifugal (from the acute curvature), inertia and viscous forces result in a three-dimensional, non-symmetric flow field. Additionally, vortical structures are being formed and dissolved due to the co-existence of these forces that change in magnitude under a pulse period. This complex flow field, typical for the inflow condition to the turbine, is the focus of the present study. Instantaneous mass flow rate and pressure measurements that provide information on changes in the turbine map when a sharp bend is mounted at the inlet of the turbine are performed and complemented with time-resolved stereoscopic particle image velocimetry measurements of the pulsating turbulent flow downstream a 90° pipe bend. The results indicate, that the time-averaged operation point in a turbine map is only marginally affected by the inflow conditions and the pulsation frequency. The hysteresis loops, on the other hand, exhibit differences not only for different pulsation frequencies, but also for different inflow conditions as a comparison between a straight and a curved pipe section upstream the turbine shows.

  • 12.
    Kalpakli, Athanasia
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Centra, Centrum för förbränningsteknik, CICERO (stängd 20101231).
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Centra, Centrum för förbränningsteknik, CICERO (stängd 20101231).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Centrum för förbränningsteknik, CICERO (stängd 20101231).
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Centra, Centrum för förbränningsteknik, CICERO (stängd 20101231).
    Experimental investigation on the effect of pulsations on turbulent flow through a 90degrees pipe bend2010Ingår i: Proc. of 3rd Int. Conf. on Jets, Wakes & Separated Flows 2010, 2010Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Pulsatile turbulent flows in curved pipes at high Dean numbers are prevalent in various components of internal combustion engines, particularly the intake of exhaust manifolds. Despite their technological importance, there is a clear lack of experimental data. The present paper provides preliminary, albeit unique, data from an experimental investigation, thereby addressing this gap and depicts impressions of the phase evolution of the complex flow including a back flow region. It is also shown, that due to the scale separation of the pulsations and the turbulence, the pulsatile flow can statistically be decomposed into its large-scale pulsations and the steady case.

  • 13.
    Kalpakli, Athanasia
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Pulsatile turbulent flow through pipe bends at high Dean and Womersley numbers2011Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 318, s. 092023-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Turbulent pulsatile flows through pipe bends are prevalent in internal combustion engine components which consist of bent pipe sections and branching conduits. Nonetheless, most of the studies related to pulsatile flows in pipe bends focus on incompressible, low Womersley and low Dean number flows, primarily because they aim in modeling blood flow, while internal combustion engine related flows have mainly been addressed in terms of integral quantities and consist of single point measurements. The present study aims at bridging the gap between these two fields by means of time-resolved stereoscopic particle image velocimetry measurements in a pipe bend with conditions that are close to those encountered in exhaust manifolds. The time/phase-resolved three-dimensional cross-sectional flow-field 3 pipe diameters downstream the pipe bend is captured and the interplay between different secondary motions throughout a pulse cycle is discussed.

  • 14.
    Kalpakli Vester, Athanasia
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Some observations of pulsating, curved pipe flow and its influence on turbine maps2014Rapport (Övrigt vetenskapligt)
  • 15.
    Kjellander, Malte
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Energy concentration by spherical converging shocks generated in a shock tube2012Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, nr 12, s. 126103-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Spherical converging shock waves are produced in a conventional shock tube with a circular cross-section. Initially, plane shocks are transformed into the shape of a spherical cap by means of a smoothly convergent cross-section. The wall shape in the transformation section is designed to gradually change the form of the shock wave until it approaches a spherical shape. Thereafter, the shock enters a conical section where it converges towards the apex of the cone. Numerical calculations with the axisymmetric Euler equations show that the spherical form is only slightly dependent on the initial Mach number of the plane shock within the range 1.5 < MS < 5.5, and is preserved to a close vicinity of the focal point. The test gas is heated to very high temperatures as a result of shock convergence and emits a bright light pulse at the tip of the test section. The light radiation is collected by optical fibers mounted at the tip of the convergence chamber and investigated by photometric and spectroscopic measurements. Experiments are performed with argon and nitrogen and with different initial Mach numbers. The radiation of the shock-heated argon closely resembles blackbody radiation. Fits to the experimental data result in apparent blackbody temperatures in argon of up to ~27 000 K, some 250 ns after the focusing instant. The initial Mach number in these spectrometric runs is MS = 3.9, indicating an efficient amplification of the shock wave strength.

  • 16.
    Kjellander, Malte
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Experimental determination of self-similarity constant for converging cylindrical shocks2011Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, nr 11, s. 116103-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Guderley's self-similarity solution r = r(0)(1 - t/t(0)) a for strong converging cylindrical shocks is investigated experimentally for three different gases with adiabatic exponents gamma = 1.13; 1.40; and 1.66 and various values of the initial Mach number. Corresponding values of the similarity exponent a which determines the strength of shock convergence are obtained for each gas thus giving the variation of alpha with gamma. Schlieren imaging with multiple exposure technique is used to track the propagation of a single shock front during convergence. The present experimental results are compared with previous experimental, numerical, and theoretical investigations.

  • 17.
    Kjellander, Malte
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Generation of spherical converging shocks in a shock tube by wall shapingManuskript (preprint) (Övrigt vetenskapligt)
  • 18.
    Kjellander, Malte
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Polygonal shock waves: comparison between experiments and geometrical shock dynamics2012Ingår i: Proceedings of the 28th International Symposium on Shock Waves held in Manchester UK, 17 to 22 July 2011, Vol 2 / [ed] Konstantinos Kontis, Springer Berlin/Heidelberg, 2012Konferensbidrag (Övrigt vetenskapligt)
  • 19.
    Kjellander, Malte
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Shock dynamics of imploding spherical and cylindrical shock waves with real gas effectsManuskript (preprint) (Övrig (populärvetenskap, debatt, mm))
  • 20.
    Kjellander, Malte
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Thermal radiation from a converging shock implosion2010Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, nr 4, s. 046102-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High energy concentration in gas is produced experimentally by focusing cylindrical shock waves in a specially constructed shock tube. The energy concentration is manifested by the formation of a hot gas core emitting light at the center of a test chamber at the instant of shock focus. Experimental and numerical investigations show that the shape of the shock wave close to the center of convergence has a large influence on the energy concentration level. Circular shocks are unstable and the resulting light emission varies greatly from run to run. Symmetry and stability of the converging shock are achieved by wing-shaped flow dividers mounted radially in the test chamber, forming the shock into a more stable polygonal shape. Photometric and spectroscopic analysis of the implosion light flash from a polygonal shock wave in argon is performed. A series of 60 ns time-resolved spectra spread over the 8 mu s light flash shows the emission variation over the flash duration. Blackbody fits of the spectroscopic data give a maximum measured gas temperature of 5800 K in the beginning of the light flash. Line emissions originating in transitions in neutral argon atoms from energy levels of up to 14.7 eV were also detected.

  • 21.
    Kjellander, Melte
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Shock dynamics of strong imploding cylindrical and spherical shock waves with real gas effects2010Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, nr 11, s. 116102-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Strong cylindrical and spherical shock implosion in a monatomic gas is considered A simple solution is obtained by Whitham's geometrical shock dynamics approach modified to account for the real gas effects The real gas effects are introduced by jump relations over the shock and include several levels of ionization, Coulomb interaction as well as internal energy of the excited electrons It is shown that ionization has a major effect on temperature and density behind the converging shock as well as on the shock acceleration The temperature and acceleration being substantially reduced and density substantially increased as compared to the ideal nonionizing case The ionization effect on the pressure behind the converging shock is less pronounced It is also shown that for the considered test case of initial Mach number M-0=8 the gas becomes completely ionized behind the spherical shock at approximately 1% of the initial radius from the focal point and its speed being decreased by a factor of 1 8 as compared to the ideal case

  • 22.
    Laurantzon, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    A pulsating flow rig for analyzing turbocharger performance2010Konferensbidrag (Refereegranskat)
    Abstract [en]

     

    Drastic improved performance of IC-engines requires efforts along many lines, where gas management is one important area. Although simulation intensive research and design tools are becoming more and more in use, they still need to be verified through accurate experiments. This is especially so for the turbocharger, an essential component for improved performance that is working in a highly complex environment. In this paper we describe a new test facility where a pulsed flow can be generated and the pulses accurately quantified trough a traversed hot-wire flow meter.

  • 23.
    Laurantzon, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    The corona mass flow meter2008Konferensbidrag (Refereegranskat)
  • 24.
    Laurantzon, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    What does the hot-wire measure?2012Rapport (Refereegranskat)
    Abstract [en]

     

  • 25.
    Laurantzon, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A flow facility for the characterization of pulsatile flows2012Ingår i: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 26, s. 10-17Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper a new flow facility for studies of pulsating flows is described. The pulsating flow is generated by means of a rotating valve and the flow is quantified through a newly developed flow measurement module accurately measuring the phase averaged flow distribution. The measuring probe of the module consists of a hot/cold-wire pair that enables both the mass flux and recovery temperature to be measured simultaneously. Calibrations of both the hot and cold wire are done in-situ in steady flow by means of a Pitot tube and a thermocouple. Moreover, the phase averaged flow distribution can be obtained by an automatic traverse of the measuring probe across the pipe cross section in both the radial and azimuthal directions. In the paper we exemplify and describe the flow properties, using the new flow measurement module, both in steady and pulsating flow.

  • 26.
    Laurantzon, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A pulsating flow rig for analyzing turbocharger performanceArtikel i tidskrift (Övrigt vetenskapligt)
  • 27.
    Laurantzon, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Experimental analysis of turbocharger interaction with a pulsatile flow through time-resolved flow measurements upstream and downstream of the turbine2012Ingår i: Institution of Mechanical Engineers - 10th International Conference on Turbochargers and Turbocharging, 2012, s. 405-415Konferensbidrag (Refereegranskat)
    Abstract [en]

    The inflow to and outflow from turbochargers are highly complex and, in particular, pulsating. Nevertheless, most studies of turbocharger performance are conducted under steady conditions. Hence, there is a great interest in determining and understanding turbocharger performance maps under pulsatile conditions. The highly complex flow field constitutes a challenge for time-resolved flow measurements by means of conventional measurement techniques. In a recent paper by Laurantzon et al [Meas. Sci. Technol. 20 123001 (2010)], time-resolved bulk flow measurements under pulsatile conditions have been obtained via wavelet analysis of the signal from a vortex flow meter. Here, this method has been used in order to obtain time-resolved performance maps based on the mass flow both upstream and down-stream of the turbine. The results show that the turbine has a large damping effect on the mass flow pulsations, but that the pulse shape is to a high degree preserved while passing through the turbine, and that the time-dependent filling and emptying of the turbine case make the quasi-steady assumption invalid, if the whole turbine stage is considered.

  • 28.
    Laurantzon, Fredrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Response of common flowmeters to unsteady flow2012Rapport (Övrigt vetenskapligt)
  • 29.
    Pastuhoff, Markus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Dynamic Calibration of Polymer/Ceramic Pressure Sensitive Paint Using a Shock Tube2010Ingår i: Seventh International Conference on Flow Dynamics: Proceedings, 2010, s. 132-133Konferensbidrag (Refereegranskat)
    Abstract [en]

    This work deals with the topic of dynamic calibration of pressure sensitive paint (PSP) using methods of system identification in z-space, suitable for discrete time analysis. The aim is to be able to reconstruct time varying pressures from (phase and amplitude distorted) measurement data. The calibration method is useful when a well known pressure source is available, such as moving shock in a shock tube. 

  • 30.
    Pastuhoff, Markus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Measuring surface pressure on rotating compressor blades using pressure sensitive paint2016Ingår i: Sensors, ISSN 1424-8220, E-ISSN 1424-8220, Vol. 16, nr 3, artikel-id 344Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Pressure sensitive paint (PSP) was used to measure pressure on the blades of a radial compressor with a 51 mm inlet diameter rotating at speeds up to 50 krpm using the so called lifetime method. A diode laser with a scanning-mirror system was used to illuminate the paint and the luminescent lifetime was registered using a photo multiplier. With the described technique the surface-pressure fields were acquired for eight points in the compressor map, useful for general understanding of the flow field and for CFD validation. The PSP was of so called fast type, which makes it possible to observe pressure variations with frequencies up to several kHz. Through frequency spectrum analysis we were able to detect the pulsating flow frequency when the compressor was driven to surge.

  • 31.
    Pastuhoff, Markus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Wall Pressure Measurements in a Y-Junction at Pulsating Flow using Polymer/Ceramic Pressure Sensitive Paint2011Ingår i: Proceedings of the 10th International Symposium on Experimental and Computational Aerothermodynamics of Internal Flows, 2011, s. 1-6Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this experiment ruthenium based polymer/ceramic pressure sensitive paint (PC-PSP) has been used to study the pressure inside a y-junction at pulsating flow conditions. Pressure has been measured using the intensity based method and through phase locked averages. The aim has been to investigate the potential of PC-PSP at mass flows and pulse frequencies typical of those in the exhaust manifold of internal combustion engines. 

  • 32.
    Pastuhoff, Markus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Yorita, Daisuke
    Tohoku University.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Asai, Keisuke
    Tohoku University.
    Modal Analysis of Pressure Sensitive Paint Data2012Ingår i: Proceedings of Ninth International Conferenceon Flow Dynamics, 2012, s. 164-165Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this work, proper orthogonal decomposition (POD) was used to separate coherent structures from noise in pressure sensitive paint (PSP) data acquired by high-speed photography on the side of a square cylinder in a natural periodically fluctuating flow.

  • 33.
    Reifarth, Simon
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.), Förbränningsmotorteknik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Tillmark, Nils
    KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Ångström, Hans-Erik
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.), Förbränningsmotorteknik. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Exergy and energy analysis of high-pressure and low-pressure exhaust gas recirculation system of a diesel engine2015Ingår i: International Journal of Exergy, ISSN 1742-8297, E-ISSN 1742-8300, Vol. 17, nr 3, s. 313-334Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The emission legislation for internal combustion engines is becoming increasingly stringent. Exhaust gas recirculation (EGR) is an important tool for emission control in modern diesel engines. This study compares the most common EGR-systems, high-pressure and low-pressure EGR, and focuses on single components. To analyse the gas exchange system, both energy and exergy analysis methods can be used. In this study, both methods are compared and specific advantages and disadvantages are shown. It is shown that the exergy analysis contains useful information for engine development regarding the efficiency of single components and their influence on the entire systems performance.

  • 34.
    Reifarth, Simon
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.), Förbränningsmotorteknik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Ångström, Hans-Erik
    KTH, Skolan för industriell teknik och management (ITM), Maskinkonstruktion (Inst.), Förbränningsmotorteknik.
    Exergy and Energy Analysis of HP and LP EGR-systemArtikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    The emission legislation throughout the world is getting moreand more stringent. Especially in the US and Europe, dieselengine manufacturers are facing big challenges in order tokeep their engine emissions within the limits. In addition totraditional legislation of harmful emissions, the emission ofCO2, i.e. the fuel consumption, is starting to be subject of newlegislation. Many measures that control the harmful emissionscounteract the reduction of fuel consumption. After treatmentsystems for example increase backpressure, thus lowering theengine efficiency.Exhaust gas recirculation (EGR) is an important tool foremission control in modern diesel engines. The EGR-loopinduces some pumping losses thus decreasing the overallengine efficiency. Many studies have been published that aimat minimizing these losses. The use of a low-pressure EGRloop is one of the most common ways to alternate the system.This study compares a low-pressure (LP) and a high-pressureHP) EGR-system with focus on single components.To analyze the gas exchange system, both energy and exergybalance methods can be used. In this study, both methods arecompared and specific advantages and disadvantages areshown. It is shown that the exergy analysis contains usefulinformation for engine development regarding the efficiencyof single components and their influence on the entire systems performance.

  • 35. Romano, D. G.
    et al.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Borodulin, V. I.
    Ivanov, A. V.
    Kachanov, Y. S.
    Minervino, M.
    Design and tests of wind-tunnel sidewalls for receptivity experiments on a swept wing2013Ingår i: Mechanical and Aerospace Engineering IV: Volume 390, Trans Tech Publications Inc., 2013, s. 96-102Konferensbidrag (Refereegranskat)
    Abstract [en]

    This document explains in its first part the design procedure adopted to design the contoured sidewalls of a swept-wing airfoil section mounted in a wind tunnel in order to satisfy the infinite swept-wing approximation. In the second part, the experimental set-up is described as well as the first results of the experimental campaign. The sidewalls are shown to play their role properly and satisfactorily provide the infinite swept-wing conditions required for subsequent investigations of the most important vortex receptivity mechanisms responsible for excitation of crossflow and Tollmien-Schlichting instability modes in the airfoil boundary layer.

  • 36.
    Sembian, Sundarapandian
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Liverts, Michael
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Apazidis, Nicholas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Plane shock wave interaction with a cylindrical water column2016Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, nr 5, artikel-id 056102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A complex system of waves propagating inside a water column due to the impact of plane shock wave is investigated both experimentally and numerically. Flow features, such as, focusing of expansion waves generating large negative pressure, nucleation of cavitation bubbles, and a re-circulation zone are observed and discussed qualitatively and quantitatively. Experiments are conducted on a 22 mm diametrical water column hit by shock waves with Mach numbers 1.75 and 2.4 in a newly constructed exploding wire facility. A new technique to create a properly shaped, repeatable, large diameter water column with straight walls is presented. Qualitative features of the flow are captured using the shadowgraph technique. With the aid of numerical simulations the wave motions inside the column are analyzed; the spatial location of the expansion wave focusing point and the corresponding negative peak pressures is estimated.

  • 37.
    Suryadi, Alexandre
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.). KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Velocity measurements of streamwise roll cells in rotating plane Couette flow2013Ingår i: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, nr 11, s. 1617-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    For the first time, quantitative velocity measurements in rotating plane Couette flow are demonstrated. Particle image velocimetry is used at a low Reynolds number with anti-cyclonic, i.e. destabilising rotation, where the instability is expected to give rise to steady streamwise-oriented roll cells. The streamwise and span-wise velocities of the roll cells were measured on the centreplane of the flow and at two planes on either side. The streamwise velocity is spanwise periodic with an amplitude variation approximately +/- 42 % of half the velocity difference between the moving walls. The wall-normal velocity was estimated by assuming steady, laminar and streamwise-independent flow. Despite the large amplitude of the disturbance, both the spanwise wave length and amplitude ratio between the streamwise and wall-normal components were close to what is obtained from linear theory. A splitting event of a roll cell was also captured by the velocity measurements and its development followed in time.

  • 38. Tofighi, R.
    et al.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Dare, E.
    Aberg, A. M.
    Larsson, J. E.
    Ceccatelli, S.
    Hypoxia-independent apoptosis in neural cells exposed to carbon monoxide in vitro2006Ingår i: Brain Research, ISSN 0006-8993, E-ISSN 1872-6240, Vol. 1098, s. 1-8Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The neurotoxic effects of carbon monoxide (CO) are well known. Brain hypoxia due to the binding of CO to hemoglobin is a recognized cause of CO neurotoxicity, while the direct effect of CO on intracellular targets remains poorly understood. In the present study, we have investigated the pathways leading to neural cell death induced by in vitro exposure to CO using a gas exposure chamber that we have developed. Mouse hippocampal neurons (HT22) and human glial cells (D384) were exposed to concentrations of CO ranging from 300 to 1000 ppm in the presence of 20% oxygen. Cytotoxicity was observed after 48 h exposure to 1000 ppm, corresponding to approximately 1 mu M CO in the cultured medium, as measured by gas chromatography. CO induced cell death with characteristic features of apoptosis. Exposed cells exhibited loss of mitochondrial membrane potential, release of cytochrome c into the cytosol, nuclei with chromatin condensation, and exposure of phosphatidyl serine on the external leaflet of the plasma membrane. CO also triggered activation of caspase and calpain proteases. Pre-incubation with either the pancaspase inhibitor Z-VAD-fmk (20 mu M) or the calpain inhibitor E64d (25 mu M) reduced by SO% the occurrence of apoptosis. When pre-cubating the cells with the two inhibitors together there was an additional reduction in the number of cells with apoptotic nuclei. These data suggest that CO causes apoptosis via activation of parallel proteolytic pathways involving both caspases and calpains. Furthermore, pre-treatment with the antioxidant MnTBAP (100 mu M) significantly reduced the number of apoptotic nuclei, pointing to a critical role of oxidative stress in CO toxicity.

  • 39. Tsukahara, T.
    et al.
    Kawaguchi, Y.
    Kawamura, H.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulence stripe in transitional channel flow with/without system rotation2010Ingår i: 7th IUTAM Symposium on Laminar-Turbulent Transition: Proceedings of the seventh IUTAM Symposium on Laminar-Turbulent Transition, Stockholm, Sweden, 2009, Springer Science+Business Media B.V., 2010, s. 421-426Konferensbidrag (Refereegranskat)
    Abstract [en]

    We report direct numerical simulations and experiments conducted in two types of plane channel flows-plane Poiseuille flow and plane Couette flows without/ with system rotation-considering the subcritical-transition regime with using large aspect-ratio computational domains and channels. Both flows give rise to coexisting laminar and turbulent equilibrium regions in the form of oblique stripes, which are tilted by a certain angle with respect to the mean flow. When subjected to a stabilizing rotation, the Couette turbulence is locally quenched and exhibits the stripe pattern as similar to non-rotating cases. In this context, the turbulence stripe can be seen as an intrinsic phenomenon in the reverse transition in a channel flow.

  • 40.
    Tsukahara, Takahiro
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Flow regimes in a plane Couette flow with system rotation2010Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 648, s. 5-33Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Flow states in plane Couette flow in a spanwise rotating frame of reference have been mapped experimentally in the parameter space spanned by the Reynolds number and rotation rate. Depending on the direction of rotation, the flow is either stabilized or destabilized. The experiments were made through flow visualization in a Couette flow apparatus mounted on a rotating table, where reflected flakes are mixed with the water to visualize the flow. Both short- and long-time exposures have been used: the short-time exposure gives an instantaneous picture of the turbulent flow field, whereas the long-time exposure averages the small, rapidly varying scales and gives a clearer representation of the large scales. A correlation technique involving the light intensity of the photographs made it possible to obtain, in an objective manner, both the spanwise and streamwise wavelengths of the flow structures. During these experiments 17 different flow regimes have been identified, both laminar and turbulent with and without roll cells, as well as states that can be described as transitional, i.e. states that contain both laminar and turbulent regions at the same time. Many of these flow states seem to be similar to those observed in Taylor Couette flow.

  • 41.
    Vester, Athanasia
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Tillmark, Nils
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    The characteristics of turbulence in curved pipes under highly pulsatile flow conditions2014Ingår i: Springer Proceedings in Physics, Springer, 2014, s. 183-187Konferensbidrag (Refereegranskat)
    Abstract [en]

    High speed stereoscopic particle image velocimetry has been employed to provide unique data from a steady and highly pulsatile turbulent flow at the exit of a 90 degree pipe bend. Both the unsteady behaviour of the Dean cells under steady conditions, the so called “swirl switching” phenomenon, as well as the secondary flow under pulsations have been reconstructed through proper orthogonal decomposition. The present data set constitutes – to the authors’ knowledge – the first detailed investigation of a turbulent, pulsatile flow through a pipe bend.

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