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  • 1.
    Alenius, Emma
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Large eddy simulations of acoustic-flow interaction at an orifice plate2015In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 345, p. 162-177Article in journal (Refereed)
    Abstract [en]

    The scattering of plane waves by an orifice plate with a strong bias flow, placed in a circular or square duct, is studied through large eddy simulations and dynamic mode decomposition. The acoustic-flow interaction is illustrated, showing that incoming sound waves at a Strouhal number of 0.43 trigger a strong axisymmetric flow structure in the orifice in the square duct, and interact with a self-sustained axisymmetric oscillation in the circular duct orifice. These structures then generate a strong sound, increasing the acoustic energy at the frequency of the incoming wave. The structure triggered in the square duct is weaker than that present in the circular duct, but stronger than structures triggered by waves at other frequencies. Comparing the scattering matrix with measurements, there is a good agreement. However, the results are found to be sensitive to the inflow, where the self-sustained oscillation in the circular duct simulation is an artefact of an axisymmetric, undisturbed inflow. This illustrates a problem with using an undisturbed inflow for studying vortex-sound effects, and can be of interest when considering musical instruments, where the aim is to get maximum amplification of specific tones. Further, it illustrates that at the frequency where an amplification of acoustic energy is found for the orifice plate, the flow has a natural instability, which is suppressed by non-axisymmetry and incoming disturbances.

  • 2.
    Alenius, Emma
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    LES of Acoustic-Flow Interaction at an Orifice Plate2012In: 18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference), 2012Conference paper (Other academic)
    Abstract [en]

    The scattering of plane waves by a thick orifice plate, placed in a circular or square duct with flow, is studied through Large Eddy Simulation. The scattering matrix is computed and compared to measurements, showing reasonably good agreement except around one frequency ($St \approx 0.4$). Here a stronger amplification of acoustic energy is observed in the circular duct simulations than in the measurements and the square duct simulations. In order to improve the understanding of the interaction between an incoming wave, the flow, and the plate, a few frequencies are studied in more detail. A Dynamic Mode Decomposition is performed to identify flow structures at significant frequencies. This shows that the amplification of acoustic energy occurs at the frequency where the jet in the circular duct has an axisymmetric instability. Furthermore, the incoming wave slightly amplifies this instability, and suppresses background flow fluctuations.

  • 3.
    Alenius, Emma
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Scattering of Plane Waves by a Constriction2011In: Proceedings of ASME Turbo Expo 2011, Vol 7, Parts A-C, American Society Of Mechanical Engineers , 2011, p. 1043-1052Conference paper (Refereed)
    Abstract [en]

    Liner scattering of low frequency waves by an orifice plate has been studied using Large Eddy Simulation and an acoustic two-port model. The results have been compared to measurements with good agreement for waves coming from the downstream side. For waves coming from the upstream side the reflection is over-predicted, indicating that not enough of the acoustic energy is converted to vorticity at the upstream edge of the plate. Furthermore, the sensitivity to the amplitude of the acoustic waves has been studied, showing difficulties to simultaneously keep the amplitude low enough for linearity and high enough to suppress flow noise with the relatively short times series available in LES.

  • 4. Altimira, M.
    et al.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Corrigendum to Numerical investigation of throttle flow under cavitating conditions (International Journal of Multiphase Flow 75 (2015) 124–136) (S0301932215001238) (10.1016/j.ijmultiphaseflow.2015.05.006))2017In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 93, p. 216-217Article in journal (Refereed)
    Abstract [en]

    The authors regret that the figures that were included in the final version of their paper were incorrect. Corrected Figures 3, 4, 5, and 6 are included here. The authors would like to apologise for any inconvenience caused. 

  • 5. Altimira, M.
    et al.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical investigation of throttle flow under cavitating conditions2015In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 75, p. 124-136Article in journal (Refereed)
    Abstract [en]

    The present paper shows the importance of the resolution of large unsteady flow structures in numerical simulations of cavitating flows. Three-dimensional simulations of the flow through a throttle geometry representative for fuel injectors have been performed to characterise the inception and development of cavitation, adopting the implicit Large Eddy Simulation approach. The two-phase flow has been handled by the Volume of Fluid method; whilst the simplified Rayleigh equation has been adopted to handle bubble dynamics. The mathematical model has been solved in the open source C++ toolbox OpenFOAM 2.0.1. Results obtained with the mathematical model are compared with those from RANS-based simulations and validated against experimental measurements. The performed Large Eddy Simulations not only are able to reproduce vortex cavitation, but also give further insight into the complex interaction between cavitation and turbulence through the assessment of the different terms of the vorticity equation.

  • 6.
    Altimira, Mireia
    et al.
    Lund University.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Effect of fuel flexibility on cavitation in injector-like flows2014In: Proceedings of the 26th ILASS-Europe 2014, 2014Conference paper (Refereed)
  • 7. Arlov, D.
    et al.
    Revstedt, J.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Numerical simulation of a gas-liquid Rushton stirred reactor - LES and LPT2008In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 37, no 7, p. 793-801Conference paper (Refereed)
    Abstract [en]

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

  • 8. Bai, X. S.
    et al.
    Fuchs, Laszlo
    Mauss, F.
    Laminar flamelet structure at low and vanishing scalar dissipation rate2000In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 120, no 3, p. 285-300Article in journal (Refereed)
    Abstract [en]

    The laminar flamelet structures of methane/air, propane/air, and hydrogen/air nonpremixed combustion at low and vanishing scalar dissipation rates are investigated, by numerical calculations of a system of conservation equations in a counterflow diffusion flame configuration, together with a transport equation defining the mixture fraction and scalar dissipation rate. The chemical reaction mechanisms consist of 82 elementary reactions up to C-3 species. In the limit of vanishing scalar dissipation rate, two types of structures are shown to appear. In one structure fuel and oxygen are consumed in a thin layer located near the stoichiometric mixture fraction, Z(st), where the temperature and the major products reach their peaks. This is similar to the so-called Burke-Schumann single layer flame sheet structure. One example is the hydrogen/air diffusion flame. The second structure consists of multilayers. Fuel and oxygen are consumed at different locations. Oxygen is consumed at Z(l) (near Z(st)), where the temperature and the major products reach their peaks. Fuel is consumed at Z(r) (> Z(st)). Between Z(l) and Z(r) some intermediate and radical species are found in high concentrations. Hydrocarbon/air nonpremixed flames are of this type. It is shown that for methane/air diffusion flames, some chemical reactions which are negligible at large scalar dissipation rate near flame quenching conditions, play essential roles for the existence of the multilayer structure. Examples of such reactions are, CH4 --> CH3 + H, H2O + O-2 --> HO2 + OH, H2O + M --> H + OH + M and CHO + H-2 --> O + H. The sensitivity of the species distributions in the flamelet to the scalar dissipation rate varies for different species. The most sensitive species are the intermediates and radicals at the fuel-rich side. At low scalar dissipation rate the radiative heat transfer can significantly move the fuel consumption layer to the oxygen consumption layer, increase the oxygen leakage to fuel side, and even quench the flame. Differential diffusion modifies the species and temperature profiles in the flamelet, but does not affect the multilayer nature of the flamelet. This result is used to successfully explain the high CO emissions in a turbulent methane/air diffusion flame.

  • 9.
    Berg, Niclas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Blood flow simulations of the renal arteries - effect of segmentation and stenosis removalIn: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987Article in journal (Refereed)
    Abstract [en]

    Patient specic based simulation of blood flows in arteries has been proposed as a future approach for better diagnostics and treatment of arterial diseases.The outcome of theoretical simulations strongly depends on the accuracy in describing the problem (the geometry, material properties of the artery and of the blood, flow conditions and the boundary conditions). In this study, the uncertainties associated with the approach for a priori assessment of reconstructive surgery of stenoted arteries are investigated. It is shown that strong curvature in the reconstructed artery leads to large spatial- and temporal-peaks in the wall shear-stress. Such peaks can be removed by appropriate reconstruction that also handles the post-stenotic dilatation of the artery. Moreover, it is shown that the effects of the segmentation approach can be equally important as the effects of using advanced rheological models. Unfortunately, this fact has not been recognized in the literature up to this point, making patient specic simulations potentially less reliable.

  • 10.
    Berg, Niclas
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, BioMEx.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Centres, BioMEx. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, BioMEx. KTH Mech, Linne FLOW Ctr, BioMEx, SE-10044 Stockholm, Sweden..
    Blood Flow Simulations of the Renal Arteries - Effect of Segmentation and Stenosis Removal2019In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 102, no 1, p. 27-41Article in journal (Refereed)
    Abstract [en]

    Patient specific based simulation of blood flows in arteries has been proposed as a future approach for better diagnostics and treatment of arterial diseases. The outcome of theoretical simulations strongly depends on the accuracy in describing the problem (the geometry, material properties of the artery and of the blood, flow conditions and the boundary conditions). In this study, the uncertainties associated with the approach for a priori assessment of reconstructive surgery of stenoted arteries are investigated. It is shown that strong curvature in the reconstructed artery leads to large spatial- and temporal-peaks in the wall shear-stress. Such peaks can be removed by appropriate reconstruction that also handles the post-stenotic dilatation of the artery. Moreover, it is shown that the effects of the segmentation approach can be equally important as the effects of using advanced rheological models. This fact has not been recognized in the literature up to this point, making patient specific simulations potentially less reliable.

  • 11.
    Berg, Niclas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Flow characteristics and coherent structures in a centrifugal blood pump2019In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 102, no 2, p. 469-483Article in journal (Refereed)
    Abstract [en]

    Blood clot formation can be initiated by local flow conditions where regions of high shear and long residence time regions, such as flow separation and stagnation, have been identified as risk factors. This study highlights coherent structures,some of which not yet considered in the literature that may contribute to blood clot formation in the ECMO (Extra Corporeal Membrane Oxygenator) circuit. The centrifugal ECMO pump investigated in this study is compact and delivers adequate volume of blood with relatively high pressure in order to compensate for the large pressure drop in the membrane oxygenator. These requirements lead to regions with high shear in several different parts of the pump. In the narrow gap between the pump house and the impeller body (the magnet) a Taylor-Couette-like flow is observed with azimuthally aligned wavy vortices, which are also pushed towards the bottom of the pump-house by the flow generated by the blades. At the bottom gap between the impeller house and the pump house one finds spiraling flow structures, due to the rotation of the former structure. Separation bubbles are found near the tongue of the pump and at the lee sides of the blades. Such vortical structures have in literature been identified as regions where platelets may be activated whereby clots may develop.

  • 12.
    Berg, Niclas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Influence of red blood cell polydispersity on blood rheology and platelet marginationManuscript (preprint) (Other academic)
  • 13.
    Bodin, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Shock motion and shock induced separation in oscillating pressure driven tran-sonic ow over a bump2008In: 38th AIAA Fluid Dynamics Conference and Exhibit, 2008Conference paper (Refereed)
  • 14.
    Bodin, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Shock Unsteadiness and Shock Induced Separation at Transonic Flow Over a Bump2008In: 38th AIAA Fluid Dynamics Conference and Exhibit, 2008Conference paper (Refereed)
    Abstract [en]

    Numerical simulations of internal, transonic flow has been carried out using Large Eddy Simulation. The motion of the unsteady shock and separation has been studied by using Large Eddy Simulations for steady and unsteady boundary conditions. The shock position is highly sensitive to small changes in boundary conditions in the transonic flow range. The shock position and the extent of the separated flow behind it exhibit hysteretic behavior. For the steady-state inflow conditions one observes the presence of certain modes. One of these can be related to the acoustics of the channel, where as another can be related to the shear-layer instability associated with the separation bubble.

  • 15.
    Bodin, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Transonic flow in IC engine exhaust valves2008Conference paper (Refereed)
  • 16.
    Bodin, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Transonic flows in IC engine exhaust valves2008In: ICJWSF 2, 2008Conference paper (Refereed)
  • 17.
    Bodin, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Analysis of transonic shock/boundary-layer interaction by spectral methodsArticle in journal (Other academic)
  • 18.
    Bodin, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Wang, Yue
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Analysis of the flow structures in the exhaust manifold of a heavy duty engineArticle in journal (Other academic)
  • 19.
    Bodin, Olle
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wang, Yue
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    LES of the Exhaust Flow in a Heavy-Duty Engine2014In: Oil & gas science and technology, ISSN 1294-4475, E-ISSN 1953-8189, Vol. 69, no 1, p. 177-188Article in journal (Refereed)
    Abstract [en]

    The flow in the exhaust port and the exhaust manifold of a heavy-duty Diesel engine has been studied using the Large Eddy Simulation approach. Some of the flow characteristics in these components are: flow unsteadiness and separation combined with significant geometry-induced secondary flow motion. Detailed analysis of these features may add understanding which can be used to decrease the flow losses and increase the eciency of downstream components such as turbochargers and EGR coolers. Few LES studies of the flow in these components have been conducted in the past and this, together with the complexity of the flow are the motivations for this work. This paper shows that in the exhaust port, even global parameters like total pressure losses are handled better by LES than RANS. Flow structures of the type that afect both turbine performance and EGR cooler efficiency are generated in the manifold and these are found to vary significantly during the exhaust pulse. This paper also clearly illustrates the need to make coupled simulations in order to handle the complicated boundary conditions of these gas exchange components.

  • 20. Borg, A.
    et al.
    Bolinder, J.
    Fuchs, Laszlo
    Simultaneous velocity and concentration measurements in the near field of a turbulent low-pressure jet by digital particle image velocimetry-planar laser-induced fluorescence2001In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 31, no 2, p. 140-152Article in journal (Refereed)
    Abstract [en]

    The main purpose of this work is to develop a method for simultaneous measurement of velocity and passive scalar concentration by means of digital particle image velocimetry and planar laser-induced fluorescence. Details of the implementation of the method are given, and the technique is applied to measurements of concentration and velocity in the centre-plane of a liquid jet with a Reynolds number of 6,000. The measurements are compared with large eddy simulations. Mean velocities and concentrations, fluctuating velocities and concentrations, and correlation between fluctuating velocities and concentrations. are analysed for the first six diameters downstream of the jet exit. The general agreement between measured and simulated results was found to be good, in particular for mean quantities. Mean profiles are also found to be in good agreement with other experimental work on jets reported in the literature. The whole-plane measurement method was found to be very useful for detailed comparisons of turbulent statistics with simulated data. The inadequacy of models for turbulent mass transport based on the standard gradient diffusion concept is demonstrated through the experimental data.

  • 21. Borg, A.
    et al.
    Fuchs, Laszlo
    LIF study of mixing in a model of a vein punctured by a cannula2002In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 23, no 5, p. 664-670Article in journal (Refereed)
    Abstract [en]

    Steady flow and mixing in a model of an arterialized vein punctured by a cannula as occurs during hemodialysis has been investigated in vitro. The motivation is that a major cause of vascular access dysfunction is the development of venous stenoses. This phenomenon lacks physiological explanation. However, one may attribute this quick process to the chemical content of the dialyzed blood and its flow near the point of infusion. The interest in mixing of chemical compounds in the dialyzed blood supplied through the cannula is, therefore, genuine due to the clinical impacts of haemodyalysis. We are interested in understanding the mixing of the two streams; namely, the untreated blood through the vein and the treated blood through the cannula. This mixing affects the local pH, which in turn can affect the solubility of several salts used for dialysis. In addition, since the blood contains molecules of widely different diffusivity properties, the local composition of blood near the point of injection is of interest. The hypothesis is that concentration non-uniformities may lead to undesired chemical or bio-chemical reactions leading to the pathological processes in the region around the needle. The mixing of a high Schmidt number substance in the stream entering from the cannula with the base flow in the vein is studied by laser induced fluorescence (LIF). The investigations are performed for a range of typical Reynolds numbers in the cannula and the vein found during hemodialysis. The study shows complicated mixing patterns around the cannula, and that non-uniformities in the blood persist over long distances for the lower flow rates found in vivo. For the higher flow rates, the flow loses its stability and mixing is enhanced. The nature of this instability is shown, and quantitative data of concentration fluctuations are given. We have further considered the effect of rotating the cannula. This leads to a significant change in the mixing process. The significance of the non-uniformities in mixing of solvents in the blood for the development of venous stenoses should be further studied not only in fluid dynamical terms but also in terms of endothelial (cellular) effects.

  • 22. Caraeni, D.
    et al.
    Bergstrom, C.
    Fuchs, Laszlo
    Modeling of liquid fuel injection, evaporation and mixing in a gas turbine burner using large eddy simulations2000In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 65, no 2, p. 223-244Article in journal (Refereed)
    Abstract [en]

    The interaction of turbulence, temperature fluctuation, liquid fuel transport, mixing and evaporation is studied by using Large Eddy Simulations (LES). To assess the accuracy of the different components of the methods we consider first isothermal, single phase flow in a straight duct. The results using different numerical methods incorporating dynamic Sub-Grid-Scale (SGS) models are compared with DNS and experimental data. The effects of the interactions among turbulence, temperature fluctuation, spray transport, evaporation and mixing of the gaseous fuel are studied by using different assumptions on the temperature field. It has been found that there are strong non-linear interactions among temperature-fluctuation, evaporation and turbulent mixing which require additional modeling if not full LES is used. The developed models and methods have been applied to a gas turbine burner into which liquid fuel is injected. The dispersion of the droplets in the burner is described.

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

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

  • 24. Caraeni, D.
    et al.
    Fuchs, Laszlo
    Compact third-order multidimensional upwind scheme for Navier-Stokes simulations2002In: Theoretical and Computational Fluid Dynamics, ISSN 0935-4964, E-ISSN 1432-2250, Vol. 15, no 6, p. 373-401Article in journal (Refereed)
    Abstract [en]

    A new compact third-order scheme for the solution of the unsteady Navier-Stokes equations on unstructured grids is proposed. The scheme is a cell-based algorithm, belonging to the class of Multidimensional Upwind schemes, which uses a finite-element reconstruction procedure over the cell to achieve third order (spatial) accuracy. Derivation of the scheme is given. The asymptotic accuracy, for steady/unsteady inviscid or viscous flow situations, is proved using numerical experiments. Those results are compared with the performances of a second-order multidimensional upwind scheme. The new compact high-order discretization proves to have excellent parallel scalability, which makes it well suited for large-scale computations on parallel supercomputers. Our studies show clearly the advantages of the new compact third-order scheme compared with the classical second-order Multidimensional Upwind scheme.

  • 25. Caraeni, Mirela
    et al.
    Fuchs, Laszlo
    Investigation of nonreflective boundary conditions for computational aeroacoustics2006In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 44, no 9, p. 1932-1940Article in journal (Refereed)
    Abstract [en]

    Direct aeroacoustic computations require nonreflective boundary conditions that allow disturbances to leave the domain freely without anomalous reflections. In the present paper we analyze a series of nonreflective boundary conditions already published in the literature and propose an improved outflow nonreflective boundary condition with reflection characteristic that is reduced greatly. For the solution of the linearized Euler equations, a sixth-order compact finite-difference algorithm is used together with a sixth-order explicit digital filter that suppresses the high-frequency spurious oscillations in the solution. A number of representative test cases are presented. The new outflow boundary condition is recommended for the simulation of sound produced by turbulence.

  • 26.
    Carlsson, C.
    et al.
    Division of Fluid Mechanics, Department of Energy Sciences, Lund University, Sweden.
    Alenius, Emma
    Division of Fluid Mechanics, Department of Energy Sciences, Lund University, Sweden.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. Lund University, Sweden.
    Swirl switching in turbulent flow through 90 degrees pipe bends2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 8, article id 085112Article in journal (Refereed)
    Abstract [en]

    Turbulent flow through 90 degrees pipe bends, for four different curvatures, has been investigated using large eddy simulations. In particular, the origin of the so-called swirl switching phenomenon, which is a large scale oscillation of the flow after the bend, has been studied for different bend curvature ratios. A classification of the phenomenon into a high and a low frequency switching, with two distinct physical origins, is proposed. While the high frequency switching stems from modes formed at the bend, and becomes increasingly important for sharp curvatures, the low frequency switching originates from very-large-scale motions created in the upstream pipe flow.

  • 27. Conway, S.
    et al.
    Caraeni, D.
    Fuchs, Laszlo
    Large eddy simulation of the flow through the blades of a swirl generator2000In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 21, no 5, p. 664-673Article in journal (Refereed)
    Abstract [en]

    Subsonic turbulent flow between the blades of a swirl generator is considered using large eddy simulations (LES). The boundary layer exhibits transition in the upstream parts of the blade. Further downstream, the Row separates both on the suction and the pressure surfaces. In the boundary layer, one may note the streaky vortices. In addition, due to the blade surface curvature large, time-dependent streamwise vortices are formed. The wake of the blades also contains large scale, time-dependent structures. These phenomena are captured directly by LES. In our study, we use two different subgrid scale (SGS) models: the first model is an implicit model and the second is a novel dynamic model dubbed the dynamic divergence model (DDM). The effects of the SGS model and the grid resolution are also investigated.

  • 28. Duwig, C.
    et al.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Large eddy simulation of a H-2/N-2 lifted flame in a vitiated co-flow2008In: Combustion Science and Technology, ISSN 0010-2202, E-ISSN 1563-521X, Vol. 180, no 3, p. 453-480Article in journal (Refereed)
    Abstract [en]

    A lifted turbulent H-2/N-2 flame in a vitiated co-flow is studied using Large Eddy Simulation together with a closure based on perfectly stirred reactors. A part of the closure, chemical look-up tables, are generated to close the filtered temperature equations and to compute local radical concentrations throughout the computational domain. The approach has been used to simulate a lifted turbulent flame. The results have been found to be insensitive to the combustion model employed and to the grid resolution. However, the results are very sensitive to the temperature of the co-flow stream and this result is well in line with previous findings. The numerical predictions were further compared to detailed experimental data obtained by Cabra et al. (2002). The agreement between the two sets of data is very good, indicating that the present approach predicts successfully the combustion process including the OH mass fractions. Finally, the LES data were used to study the flame dynamics and stabilization mechanisms.

  • 29. Duwig, C.
    et al.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Lund University, Sweden .
    Numerical simulation of forced laminar flame2009In: 47th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2009, p. 2009-1186-Conference paper (Refereed)
    Abstract [en]

    The simulation of laminar flames consists of capturing the evolution of a very large number of species that may react within a broad range of time scales. It therefore results in a highly non-linear stiff numerical problem that requires large computational resources. In the present paper, an alternative approach for the simulation of unsteady premixed flames is proposed and is applied for simulating an excited laminar flame. The approach is to using a single scalar (i.e. a progress variable) on relatively coarse grids. The unresolved details of the flame structure are expressed in terms of the spatial filtering. A model that has originally been developed for Large Eddy Simulation is applied for the simulation of unsteady laminar flames. The ability of the present approach to represent the filtered flame structure is compared qualitatively to experimental data.

  • 30. Duwig, C.
    et al.
    Fuchs, Laszlo
    Study of flame stabilization in a swirling combustor using a new flamelet formulation2005In: Combustion Science and Technology, ISSN 0010-2202, E-ISSN 1563-521X, Vol. 177, no 8, p. 1485-1510Article in journal (Refereed)
    Abstract [en]

    The dynamics in a swirl-stabilized flame is studied using large eddy simulation (LES). We account for the effect of turbulence on the flame through a model based on a filtered flamelet technique. The model provides a consistent and robust reaction-diffusion expression for simulating the correct propagation of premixed flames. The filtered flamelet formulation has been implemented into a high-order-accurate LES code and used to study the flame stabilization and the combustion dynamics in a gas-turbine combustion chamber. The effects of inlet boundary conditions, in terms of velocity and equivalence ratio radial profiles, have been studied. The flow is found to be very sensitive to small changes in terms of flame shapes and anchoring position. The sensitivity of the results to the subgrid-scale flame thickness has also been investigated. The influence on the flame position is not significant. However, a too-large subgridscale flame thickness leads to different flame dynamics.

  • 31. Duwig, C.
    et al.
    Fuchs, Laszlo
    Griebel, P.
    Siewert, P.
    Boschek, E.
    Study of a confined turbulent jet: Influence of combustion and pressure2007In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 45, no 3, p. 624-639Article in journal (Refereed)
    Abstract [en]

    Today, environmental issues play an important role in the viability of modern low-emission power plants. As a consequence, gas turbine combustors need to be operated at lean premixed conditions. However, successful designs require a detailed knowledge of the combustion process under realistic operating conditions. The present study focuses on nonreacting and reacting jets at operating pressures from 1 to 14 bar. First, an isothermal confined jet has been studied experimentally and numerically using particle image velocimetry and large eddy simulation. The flow is highly turbulent and includes large-scale unsteady structures. The comparison of the numerical results and the experimental velocity data showed an excellent agreement that was generally below the numerical or experimental uncertainty. Second, the influences of combustion and operating pressure on the flowfield were investigated. The large eddy simulation results showed that the jet core was lengthened, due to the density jump across the flame. The effect of pressure on the flame was studied using planar laser-induced fluorescence and large eddy simulation at a constant Mach number. The flame brush and the velocity fields were found to be relatively insensitive to an increase of pressure from 1 to 14 bar (and, correspondingly, to an increase of Reynolds and Karlovitz numbers). The numerical results suggest that increasing pressure decreases the laminar flame speed and increases the flame-front wrinkling, causing the turbulent flame speed to be less sensitive to pressure.

  • 32. Duwig, C.
    et al.
    Salewski, M.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Simulations of a turbulent flow past a sudden expansion: A sensitivity analysis2008In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 46, no 2, p. 408-419Conference paper (Refereed)
    Abstract [en]

    Large eddy simulation is used to study the flow behind a pair of symmetric backward-facing steps. As reported in the literature, the flow exhibits an asymmetric pattern characterized by the deflection of the jet toward one of the walls. The large eddy simulation results are compared with laser Doppler anemometry measurements showing the ability of the present numerical tool to capture the complex features of the flow. Furthermore, a sensitivity study is conducted to assess the influence of the grid resolution, the inflow boundary, the channel width, and the step size on the flowfield. The flow was found to be only weakly sensitive to the grid, assuring the quality of the simulation results. The inflow boundary influences the mean results only marginally unless low-frequency fluctuations are applied. In this case, the flowfield recovers a mean symmetry with suppression of the jet bending. The jet mean bending has also been shown to increase with the step size h and to decrease with increasing channel width.

  • 33. Duwig, C.
    et al.
    Stankovic, D.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Li, G.
    Gutmark, E.
    Experimental and numerical study of flameless combustion in a model gas turbine combustor2008In: Combustion Science and Technology, ISSN 0010-2202, E-ISSN 1563-521X, Vol. 180, no 2, p. 279-295Article in journal (Refereed)
    Abstract [en]

    Flameless combustion is an attractive solution to address existing problems of emissions and stability when operating gas turbine combustors. Theoretical, numerical and experimental approaches were used to study the flameless gas turbine combustor. The emissions and combustion stability were measured and the limits of the flameless regime are discussed. Using experimental techniques and Large Eddy Simulation (LES), detailed knowledge of the flow field and the oxidation dynamics was obtained. In particular the relation between the turbulent coherent structures dynamics and the flameless oxidation was highlighted. A model for flameless combustion simulations including detailed chemistry was derived. The theoretical analysis of the flameless combustion provides 2 non-dimensional numbers that define the range of the flameless mode. It was determined that the mixture that is ignited and burnt is composed of similar to 50% of fresh gases and similar to 50% vitiated gases.

  • 34. Duwig, Christophe
    et al.
    Fuchs, Laszlo
    Large eddy simulation of turbulent premixed combustion using a marker field2007In: Combustion Science and Technology, ISSN 0010-2202, E-ISSN 1563-521X, Vol. 179, no 10, p. 2135-2152Article in journal (Refereed)
    Abstract [en]

    A new laminar flamelet model is presented for Large Eddy Simulation ( LES) of premixed turbulent combustion. The model uses a marker field S+ to capture the flame. Compared to previous work on marker fields, a modification was introduced to simplify the boundary conditions. Although the scalar S+ is smooth in space, it allows describing steep flame fronts. The S+ equation captures the laminar or turbulent flame propagation via a reactive diffusive balance. The present LES model was used to simulate a turbulent premixed flame stabilized behind a sudden expansion. The LES predictions agree well with available experimental data in term of reproducing the time averaged velocity field as well as for capturing the flame dynamics and flame response to a perturbation.

  • 35. Duwig, Christophe
    et al.
    Fuchs, Laszlo
    Large eddy simulation of vortex breakdown/flame interaction2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 7Article in journal (Refereed)
    Abstract [en]

    The dynamics of a swirl-stabilized premixed flame is studied using large eddy simulation (LES). A filtered flamelet model is used to account for the subgrid combustion. The model provides a consistent and robust reaction-diffusion expression for simulating the propagation of turbulent premixed flames correctly. The numerical results were found to be relatively insensitive to small changes in the inflow boundary conditions and to the numerical mesh employed. Furthermore, the results were found to agree well with the available experimental data both for velocity and scalar fields. In addition, unsteady flame features [i.e., precessing vortex core (PVC)] were identified and compared with experimental data. The agreement between LES results and experimental data, in terms of flame dynamics, was also good. Increasing swirl did not affect the flame strongly but a decrease of swirl number was shown to change the flame shape and suppress the PVC. The PVC and flame dynamics were studied using proper orthogonal decomposition (POD) allowing us to identify and isolate the PVC from smaller-scale turbulence. The POD results indicate that the PVC corresponds to a helical wave consisting of two counter-rotating helices. A dynamical reduced model was also derived do describe the flame response to the PVC.

  • 36.
    Enfors, Sven-Olof
    et al.
    KTH, Superseded Departments, Biotechnology.
    Jahic, M.
    Rozkov, A.
    Xu, B.
    Hecker, M.
    Jurgen, B.
    Kruger, E.
    Schweder, T.
    Hamer, G.
    O'Beirne, D.
    Noisommit-Rizzi, N.
    Reuss, M.
    Boone, L.
    Hewitt, C.
    McFarlane, C.
    Nienow, A.
    Kovacs, T.
    Tragardh, C.
    Fuchs, Laszlo
    Revstedt, J.
    Friberg, P. C.
    Hjertager, B.
    Blomsten, G.
    Skogman, H.
    Hjort, S.
    Hoeks, F.
    Lin, H. Y.
    Neubauer, P.
    van der Lans, R.
    Luyben, K.
    Vrabel, P.
    Manelius, A.
    Physiological responses to mixing in large scale bioreactors2001In: Journal of Biotechnology, ISSN 0168-1656, E-ISSN 1873-4863, Vol. 85, no 2, p. 175-185Article in journal (Refereed)
    Abstract [en]

    Escherichia coli fed-batch cultivations at 22 m(3) scale were compared to corresponding laboratory scale processes and cultivations using a scale-down reactor furnished with a high-glucose concentration zone to mimic the conditions in a feed zone of the large bioreactor. Formate accumulated in the large reactor, indicating the existence of oxygen limitation zones. It is suggested that the reduced biomass yield at large scale partly is due to repeated production/reassimilation of acetate from overflow metabolism and mixed acid fermentation products due to local moving zones with oxygen limitation. The conditions that generated mixed-acid fermentation in the scale-down reactor also induced a number of stress responses, monitored by analysis of mRNA of selected stress induced genes. The stress responses were relaxed when the cells returned to the substrate limited and oxygen sufficient compartment of the reactor. Corresponding analysis in the large reactor showed that the concentration of mRNA of four stress induced genes was lowest at the sampling port most distant from the feed zone. It is assumed that repeated induction/relaxation of stress responses in a large bioreactor may contribute to altered physiological properties of the cells grown in large-scale bioreactor. Flow cytometric analysis revealed reduced damage with respect to cytoplasmic membrane potential and integrity in cells grown in the dynamic environments of the large scale reactor and the scale-down reactor.

  • 37. Evegren, Philip
    et al.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Revstedt, Johan
    Wall shear stress variations in a 90-degree bifurcation in 3D pulsating flows2010In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 32, no 2, p. 189-202Article in journal (Refereed)
    Abstract [en]

    The exact role of fluid mechanics in the patho-physiological process of atherosclerosis has been a research topic over many years, yet without clear conclusive result. One has observed that morphological manifestations of the disease are found at some well-defined locations: certain vessel bifurcations and in curvatures. The flow in these regions is characterized by unsteadiness and often separation. Currently there are no complete theories that can explain the process since the different components in the process are not fully understood. Here we carry out detailed computations of the unsteady flow in an arterial segment typical to location of early appearance of arterial lesions. We study the wall shear stress (WSS) field variations near a junction with the purpose of identifying fluid-mechanical parameters that can be related to sites of atheroslcerosis. The results show that regions associated with atherosclerosis experience highly elevated temporal- and spatial-derivatives of the WSS, also at less commonly known locations. Thus, large derivatives in time and space do not seem unique for the most common areas of atherosclerosis. Differences in WSS character between these locations are identified as differences in the time period of back flow as well as differences in the magnitude of the WSS derivatives. The data is presented in a way that facilitates understanding of the variations in WSS.

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

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

  • 39.
    Fjällman, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Analysis of 3 Dimensional Turbine Flow by using Mode Decomposition Techniques2014In: Proceedings of the ASME Turbo Expo, 2014, p. GT2014-26963-Conference paper (Refereed)
    Abstract [en]

     Today one of the most popular ways of lowering the fuel consumption and emissions of the Internal Combustion Engine (ICE) is by downsizing the engine. Downsizing means that the swept volumes of the cylinders are decreased; this lowers the frictional and thermal losses. By combining the downsizing with a well matched turbocharger system the performance is preserved while the advantages are retained. Since more and more of the development work is being performed by simulations there is an increasing need for more accurate methods. These methods are more complex and require more resources than the simpler, faster and more robust models used today. In this study Large Eddy Simulations (LES) of the unsteady flow in a radial turbine designed for a gasoline ICE has been performed and analysed. The flow inside the turbine is highly 3 dimensional, pulsating and characterized by secondary flow motions and high curvatures. All these are reasons for which the method of choice should be LES. LES is able to resolve a large range of scales and capture the flow dynamics. The considered case concerns a non-pulsating flow condition but with engine like mass flow and temperature. Post-processing tools based on Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are used to analyse the large amount of LES based flow data. The POD method is used to investigate the energy content of the dominant, large structures present in the flow. The DMD method on the other hand is used to reveal the flow structures responsible for specific frequencies found in the flow field. Preliminary data show a fair agreement between experimental data and LES results in terms of predicting the turbine performance parameters.

  • 40.
    Fjällman, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Analysis of Secondary Flow Induced by a 90 Bend in a Pipe Using Mode Decomposition Techniques2013Conference paper (Refereed)
    Abstract [en]

    In this study unsteady simulations of the flow through and after a 90 pipe bend has been performed by Large Eddy Simulations (LES). In the passenger car engine there is an abundance of pipes and pipe bends. Since pipes and bends are often situated upstream of important engine components the flow in these needs to be well predicted. This entails that there is a need for accurate pipe flow simulations in order to ensure that the inflow conditions, to the e.g. cylinders or turbocharger, are as close to the experimental values as possible. The flow field is further studied by the use of Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). It has been found that there is a low frequency oscillation in the strength of the alternately dominant dean vortex at the exit of the pipe bend. This phenomenon is analysed and the mechanism for it is discussed.

  • 41.
    Fjällman, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Effects of inlet geometry on turbine performanceManuscript (preprint) (Other academic)
    Abstract [en]

    In this study comparisons have been performed between gas-stand experiments,Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations, and LargeEddy Simulations (LES) of the ow through a radial turbine of a turbochargerdesigned for an internal combustion engine. The long term goal for the projectis to improve the prediction capabilities for the simpler computational modelsused by industry in the research and development of new products. At thepresent stage the eects of using simplied geometries and methods for turbineperformance predictions are assessed. Additionally, data obtained from URANScalculations is compared against experimental data and against unsteady LESresults. The comparisons are made in order to evaluate the employed methodologies,to know when to use simplied models with reasonable accuracy, as wellas to justify the use of more advanced methods when the models are inadequate.It was found out that the LES results are closer to the gas-stand experimentsthan the URANS predictions are.

  • 42.
    Fjällman, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Effects of Pulsation Frequency and Pulse Shape on Turbine PerformanceManuscript (preprint) (Other academic)
    Abstract [en]

      The current paper studies the pulsating flow in the exhaust manifold and turbine of a passenger car engine. The study focuses on three engine RPMs and two different pulse shapes, one normal shape and one shorter DEP like shape. By simulating different valve strategies one can investigate how to maximize the available exhaust flow energy to the turbine. The simulations have been performed with the Large Eddy Simulation (LES) method with boundary conditions received from GT-Power simulations. The study focuses on kinetic energy and turbine torque analysis of the incoming flow and of the flow in the turbine wheel region. It is found that the pulse shape is affecting the wheel torque significantly and that the kinetic energy entering the wheel region is different depending on which cylinder fired.

  • 43.
    Fjällman, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Exhaust flow pulsation effect on radial turbine performance2015In: 11th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2015, European Conference on Turbomachinery (ETC) , 2015Conference paper (Refereed)
    Abstract [en]

    In the current vehicle manufacturing world the chase for a better fuel economy and better driveability is in high gear. One way of doing so is to investigate and optimize the turbocharger. In this paper the flow in a radial turbine of a passenger car turbocharger has been analysed by Large Eddy Simulations. The current simulations have investigated the effects of changing the inlet pulse frequency and inlet pulse shape on turbine's performance parameters (e.g. efficiency, shaft power, pressure distributions). Three different engine speeds and two different pulse shapes were chosen to be compared and analysed. With the total mass flow per pulse being constant for all cases there is a clear dependence of both pulse shape and frequency when it comes to e.g. wheel momentum. Higher frequency increases the peak momentum more than the minimum level. A reduction in pulse duration also increases peak momentum more than the minimum.

  • 44.
    Fjällman, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Numerical investigations of a free turbulent jet after a 90° pipe bendManuscript (preprint) (Other academic)
    Abstract [en]

    In this study the ow out of pipe after a 90pipe bend has been investigated byperforming Large Eddy Simulations. Comparisons of velocity proles, ow eldsand power spectral densities have been performed between numerical simulationsand experiments. Pipe bends exists in many industrial applications especiallyin the internal combustion engine. Performing accurate and predictive pipe owsimulations is becoming more important when more of the development work isbeing performed by simulations. The long term goal of this project is to helpwith development of simpler models that are faster to run but give the sameaccuracy. During this study it was found that the method used works well bothin predicting mean values and in resolving dynamic motions such as vortexswitching. LES calculations without a SGS model has been proven to work wellwith ows of this type as long as the grid resolution is suciently ne.

  • 45.
    Fjällman, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    On the Importance of Turbulence Modelling of the Flow after a 90 Pipe BendManuscript (preprint) (Other academic)
    Abstract [en]

    The flow in a free jet after a 90 pipe bend has been investigated by Large Eddy Simulations (LES) and Reynolds Averaged Navier-Stokes (RANS) simulations. The numerical results for the mean velocity profiles, flow fields, and power spectral densities have been compared to experimental data. The results show that LES has been able to predict the mean components of the velocity field and in resolving dynamic motions such as vortex switching. LES without an explicit SGS model (termed as ILES in the following) has been found to work well with flows of this type as long as the grid resolution is sufficiently fine. Different Sub-Grid-Scale (SGS) models have also been used. LES with different SGS models result in very similar results when a fine enough grid is used. The ILES approach also gives reasonably accurate results for the mean values even for coarser grids.

  • 46.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Some Characteristics of Non-Reacting and Reacting Low Swirl Number Jets2010In: TURBULENCE AND INTERACTIONS  , 2010, Vol. 110, p. 1-1Conference paper (Refereed)
    Abstract [en]

    The paper considers low swirl turbulent number jets. Swirling jets are used to stabilize premixed flames in gas turbines. Normally, the swirl number is large enough to allow vortex break-down and thereby flame stabilization along the upstream edge of the back-flow bubble. With decreasing swirl the vortex-breakdown may disappear altogether. However, it has been found that under certain conditions the flame may be kept at a certain (mean) distance away from the nozzle even without vortex break-down. The mechanism for the flame holding under such conditions is discussed. The discussion is based upon LES results and some experimental data. We discuss also the precession of the central core both under non-reacting and reacting condition. LES and experimental results show that the precession of the central core is normally in the same direction as the swirl. However, for certain range of swirl numbers and at some axial distances one may find precession in the counter direction. The mechanism for this effect is discussed.

  • 47.
    Fuchs, Laszlo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hällqvist, T.
    Numerical study of impinging jets with heat transfer-inlet conditions effects2009In: 47th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2009, p. 2009-1578-Conference paper (Refereed)
    Abstract [en]

    Inflow conditions for time dependent flow configurations are of major importance for the developing flow field and mixing characteristics. The wall heat transfer rates for impinging jets may be significantly affected by perturbations at the jet nozzle. This effect, however, depends on the plate distance to nozzle spacing. For large enough distance to nozzle spacing the influence of the inlet conditions is less significant at the target plate. By applying active forcing at the velocity inlet the wall heat transfer at the impingement plate may be controlled. The forcing here is sinusoidal and it is applied only in the streamwise direction at the velocity inlet of a circular nozzle. The computations included a range of Reynolds numbers (4500 to 30 000) and a range of nozzle-to-plate spacings (0.5 to 12). Here, we consider mainly a case of Re = 20000 and plate distance of two jet diameters. The Turbulent flow and mixing is modeled using Large Eddy Simulation (LES). The results are obtained on rather fine grids and some of the results are compared to relevant experimental data. The sensitivity of the results to inlet conditions shows that direct comparison with experimental results may not be easy or conclussive. However, the LES results agree well with the experimental ones, i.e. within the expected variations (due to uncertainties in inlet conditions) in the different results.

  • 48.
    Fuchs, Laszlo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Revstedt, J.
    Ding, R.
    Direct measurement of the turbulent fluxes of a passive scalar in impinging jets using simultaneous PIV AND LIF2009In: 39th AIAA Fluid Dynamics Conference, 2009, p. 2009-4010-Conference paper (Refereed)
    Abstract [en]

    Turbulent mixing of passive scalars in an impinging jet are studied experimentally and numerically. The experiments use simultaneous PIV/PLIF data to obtain data on the velocity and concentration fields in a circular impinging jet with nozzle to plate distance of five diameters. The distributions of the mean velocity, turbulent kinetic energy, mean concentration, concentration fluctuation, turbulent fluxes, turbulent diffusivity are obtained from the measured data. The information enables one to compare directly the validity of different models for the turbulent fluxes, e.g. GDM model, GGDM model. Large Eddy Simulation are used as complimentary for studying Schmidt number (Sc) effects. The results show that the simpler models for turbulent fluxes are inadequate for use in flows with large coherent structures. Also, under conditions turbulence that enhances mixing is not always adequate to maintain the homogeneity of the mixture, whereby Sc-number effects become evident. For such flows, models for turbulent fluxes that neglect Sc-effects are apparently inadequate.

  • 49. Grosshans, H.
    et al.
    Cao, L.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Szász, R. -Z
    Computational sensitivity study of spray dispersion and mixing on the fuel properties in a gas turbine combustor2017In: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 49, no 2, article id 025506Article in journal (Refereed)
    Abstract [en]

    A swirl stabilized gas turbine burner has been simulated in order to assess the effects of the fuel properties on spray dispersion and fuel-air mixing. The properties under consideration include fuel surface tension, viscosity and density. The turbulence of the gas phase is modeled applying the methodology of large eddy simulation whereas the dispersed liquid phase is described by Lagrangian particle tracking. The exchange of mass, momentum and energy between the two phases is accounted for by two-way coupling. Bag and stripping breakup regimes are considered for secondary droplet breakup, using the Reitz-Diwakar and the Taylor analogy breakup models. Moreover, a model for droplet evaporation is included. The results reveal a high sensitivity of the spray structure to variations of all investigated parameters. In particular, a decrease in the surface tension or the fuel viscosity, or an increase in the fuel density, lead to less stable liquid structures. As a consequence, smaller droplets are generated and the overall spray surface area increases, leading to faster evaporation and mixing. Furthermore, with the trajectories of the small droplets being strongly influenced by aerodynamic forces (and less by their own inertia), the spray is more affected by the turbulent structures of the gaseous phase and the spray dispersion is enhanced.

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

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

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