SwePub - sökning: WFRF:(Andersson Niklas 1976)

Numrering	Referens	Omslagsbild	Hitta
1.	Wallerstedt, Cecilia, 1976, et al. (författare) Socioculturally-informed Interaction Analysis (SIA): Methodology and theoretical and empirical contributions of an emerging research program in early childhood education 2022 Ingår i: International Research in Early Childhood Education. - 1838-0689. ; 12:1, s. 1-23 Tidskriftsartikel (refereegranskat)abstract Over the last decade, a Swedish research group has conducted several empirical studiesin the field of Early Childhood Education (ECE). These studies are examples of what is sometimes referred to as practice-based studies, and they are often conducted in collaboration with ECE personnel. In this meta-study, we review 37 publications from the research group to highlight key contributions in terms of methodological issues or challenges identified,as well as empirical findings and theoretical developments. We argue that these studies constitute an emerging research program, termed Socioculturally-informed Interaction Analysis(SIA). Key aspects of SIA are:examining learning as a process, using recordings to avoid bias, considering pragmatic validity when working with transcriptions, making claims closely aligned with what is studied (ecological validity), and viewing context as an analytical rather than as a descriptive concept,and generalising at a conceptual level, which affords empirical generalisation.
2.	Andersson, Ola, 1976-, et al. (författare) Spectral shaping of DAC nonlinearity errors through modulation of expected errors 2001 Ingår i: Circuits and Systems, 2001. ISCAS 2001. The 2001 IEEE International Symposium on. - : IEEE. - 0780366859 ; , s. 417-420 Konferensbidrag (refereegranskat)abstract Traditionally, delta-sigma modulation has been used for shaping of quantization noise. We present a modified version of delta-sigma modulation which also takes into account unwanted nonlinearities by feeding back not only the quantization error, but also the expected physical error. Behavioral-level simulations of a 5th-order structure showing an improvement of up to 4 effective bits are included
3.	Eriksson, Lars-Erik, 1950, et al. (författare) Development of a Cooled Radial Flameholder for the F404/RM12 Afterburner: Part III Afterburner Rumble Prediction and Suppression 2003 Ingår i: ISABE-2003-1060, proc. of 16th ISABE Conf, Cleveland Ohio, September 2003. Konferensbidrag (refereegranskat)abstract One-dimensional methods used for solving acoustic instabilities are of doubtful value in the design of afterburner systems. In this paper a three-dimensional method developed and used at Volvo Aero Corporation is described. The method deals with the mean flow and the perturbation flow separately. The perturbation part is based on the linearized Euler equations (time domain) for compressible multi-species flow together with a spectrum trans-formation and Arnoldis method for eigenmode extraction.
4.	Lai, Szhau, 1985, et al. (författare) Accurate Phase-Noise Prediction for a Balanced Colpitts GaN HEMT MMIC Oscillator 2013 Ingår i: IEEE Transactions on Microwave Theory and Techniques. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9480 .- 1557-9670. ; 61:11, s. 3916-3926 Tidskriftsartikel (refereegranskat)abstract This paper presents an X-band balanced Colpitts oscillator in GaN HEMT technology and a method to calculate its phase noise accurately. The method employs a low-frequency (LF) noise measurement and the oscillator waveforms from a harmonic-balance simulator. These data are post-calculated by Hajimiri's phase-noise model, in which the LF noise can be activated with a cyclo-stationary effect in the calculation of phase noise. Compared to commercial phase-noise simulation using predefined stationary noise, the calculation gives significantly improved phase-noise prediction in the 30-dB/decade region near carrier. The prediction is within 3-dB accuracy at 10-kHz, 100-kHz, and 1-MHz offset frequencies. In addition to the method used for phase-noise prediction, the potential for wideband tuning of this oscillator topology is analytically investigated. The measured phase noise of the oscillator is -102 dBc/Hz at 100-kHz offset from a 8.6-GHz carrier frequency for drain voltage and current of Vd = 15 V amd Id = 40 mA.
5.	Sudow, Mattias, 1980, et al. (författare) An AlGaN/GaN HEMT-Based Microstrip MMIC Process for Advanced Transceiver Design 2008 Ingår i: IEEE Transactions on Microwave Theory and Techniques. - 0018-9480 .- 1557-9670. ; 56:8, s. 1827-1833 Tidskriftsartikel (refereegranskat)abstract A MMIC process in AlGaN/GaN technology for advanced transceiver design has been developed. The process is based on microstrip technology with a complete model library of passive elements and AlGaN/GaN HEMTs. The transistor technology in this process is suitable for both power and low noise design, demonstrated with a power density of 5 W/mm, and an ${rm NF}_{min}$ of 1.4 dB at $X$ -band. Process stability of subcircuits, complementary to power amplifiers and LNAs, in a transceiver system have been investigated. The results indicate that an all AlGaN/GaN MMIC transceiver is realizable using this technology.
6.	Sudow, Mattias, 1980, et al. (författare) An SiC MESFET-based MMIC process 2006 Ingår i: IEEE Transactions on Microwave Theory and Techniques. ; 54:12, Part 1, s. 4072-4078 Tidskriftsartikel (refereegranskat)abstract A monolithic microwave integrated circuit (MMIC)process based on an in-house SiC MESFET technology has been developed. The process uses microstrip technology, and a complete set of passive components, including MIMcapacitors, spiral inductors,thin-film resistors, and via-holes, has been developed. The potential of the process is demonstrated by an 8-W power amplifierat 3 GHz, a high-linearity -band mixer showing a third-order intercept point of 38 dBm, and a high-power limiter.
7.	Sudow, Mattias, 1980, et al. (författare) The Chalmers microstrip SiC MMIC Process 2005 Ingår i: Conference Proceedings Gighahertz 2005. Konferensbidrag (refereegranskat)abstract A generic microstrip MMIC process targeted for SiC and GaN technology has beendeveloped. Passive components for high power operation were developed and verified. Circuit modelsfor both passive and active components have been formulated. Using the developed MMIC process anamplifier and a limiter have been manufactured.
8.	Thorsell, Mattias, 1982, et al. (författare) An X-Band AlGaN/GaN MMIC Receiver Front-End 2010 Ingår i: IEEE Microwave and Wireless Components Letters. - 1558-1764 .- 1531-1309. ; 20:1, s. 55-57 Tidskriftsartikel (refereegranskat)abstract This letter presents an integrated AlGaN/GaN X-band receiver front-end. This is to the authors knowledge the first published results of an integrated AlGaN/GaN MMIC receiver front-end. The receiver uses an integrated SPDT switch to reduce size, weight and cost compared to circulator based transceiver front-ends. The integrated front-end has more than 13 dB of gain and a noise figure of 3.5 dB at 11 GHz.
9.	ÖZEN, MUSTAFA, 1984, et al. (författare) High efficiency RF pulse width modulation with tunable load network class-E PA 2011 Ingår i: 2011 IEEE 12th Annual Wireless and Microwave Technology Conference, WAMICON 2011. - 9781612840819 Konferensbidrag (refereegranskat)abstract In this paper, a 10 W peak power 2 GHz highly efficient RF pulse width modulation (RF-PWM) based transmitter is presented. RF-PWM signals are generated with a dedicated 65 nm CMOS modulator and subsequently amplified with a GaN Class-E power amplifier (PA). The modulator use extended drain MOS (EDMOS) high voltage transistors to provide the required voltage swing to drive the GaN used as a switch. The imaginary load impedance of the Class-E is electronically tunable and is implemented with in-house high breakdown voltage SiC varactors. The tunable imaginary load impedance enables optimization of the Class-E versus the duty cycle (pulse width). The peak efficiency is therefore preserved over a wide range of output power levels. The measured drain efficiency of the Class-E output stage is above 70% over a 6.5 dB output power dynamic range. The overall transmitter efficiency is above 60% for the same dynamic range.
10.	Abou-Taouk, Abdallah, 1982, et al. (författare) CFD analysis of a SGT-800 burner in a combustion RIG 2016 Ingår i: ASME Turbo Expo, June 13 – 17, 2016, Seoul, South Korea. Konferensbidrag (refereegranskat)abstract This work focuses on 3D turbulent reacting flow modeling of a SGT-800 3rd generation dry low emission (DLE) burner at both atmospheric and engine-like conditions. At atmospheric pressure the burner is fitted in a test rig with high pre-heating of the incoming air. To reduce the computational cost, the M4 mechanism previously developed by Abou-Taouk et al. (2013) is used for operating pressure of 1 bar. A new novel optimized 4-step reaction mechanism for methane-air mixture is developed in the present work at an operating pressure of 20 bar. The mechanism is based on a large sample of detailed chemistry solutions that are processed by an iterative optimization procedure. This leads to a reduced 4-step mechanism, reproducing the targeted detailed chemistry solutions in terms of laminar flame speeds, species profiles and temperatures. The CFD simulations are performed using the combined eddy dissipation model / finite rate chemistry (EDM/FRC) turbulence chemistry interaction model. The turbulence is modeled using both the k-ω SST and the scale adaptive simulation (SAS) turbulence models. A comprehensive testing and measurement campaign carried out at atmospheric pressure for this burner was previously performed in a combustion test rig. The CFD results are compared to measurement data which includes for example flame position and pressure drop.
11.	Andersson, Kristoffer, 1976, et al. (författare) C-Band Linear Resistive Wide Bandgap FET Mixers 2003 Ingår i: IEEE MTT-S International Microwave Symposium Digest. ; 2, s. 8- Konferensbidrag (refereegranskat)
12.	Andersson, Kristoffer, 1976, et al. (författare) Fabrication and characterization of field-plated buried-gate SiC MESFETs 2006 Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 27:7, s. 573-575 Tidskriftsartikel (refereegranskat)abstract Silicon carbide (SiC) MESFETs were fabricated using a standard SiC MESFET structure with the application of the "buried-channel" and field-plate (FP) techniques in the process. FPs combined with a buried-gate are shown to be favorable concerning output power density and power-added efficiency (PAE), due to higher breakdown voltage and decreased output conductance. A very high power density of 7.8 W/mm was measured on-wafer at 3 GHz for a two-finger 400-/spl mu/m gate periphery SiC MESFET. The PAE for this device was 70% at class AB bias. Two-tone measurements at 3 GHz /spl plusmn/ 100 kHz indicate an optimum FP length for high linearity operation.
13.	Andersson, Kristoffer, 1976, et al. (författare) Resistive SiC-MESFET mixer 2002 Ingår i: IEEE Microwave and Wireless Components Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1531-1309 .- 1558-1764. ; 12:4, s. 119-121 Tidskriftsartikel (refereegranskat)abstract A single-ended silicon carbide resisitve MESFET mixer with minimum conversion loss (CL) of 10.2 dB and an input third order intercept point of 35.7 dB at 3.3 GHz was designed and characterized. A lumped-element, large-signal model was used for modeling the device. The drain-source resistance was measured by taking the real part of the output port impedence. Analysis suggested that the optimum gate bias for minimum CL was -6.7 V.
14.	Andersson, Niklas, 1976 (författare) A Non-Intrusive Acceleration Technique for Compressible Flow Solvers Based on Dynamic Mode Decomposition 2016 Ingår i: Computers and Fluids. - : Elsevier BV. - 0045-7930. ; 133, s. 32-42 Tidskriftsartikel (refereegranskat)abstract There are several well-established methods to achieve solver speed-up for finite-volume solvers for compressible flows. These methods are more or less difficult to implement and more or less suitable for implementation in a parallel unstructured type of solver. This article presents a non-intrusive solver acceleration technique applicable for steady-state problems. The proposed technique is based on Dynamic Mode Decomposition (DMD) and does not depend on data format or mesh structure and is thus as straightforward to implement in an unstructured parallel code as in a structured sequential one. The main idea behind the method is that it is possible to use the information available in global flow-field modes to find a correction that will bring the solution closer to a steady-state condition. The DMD-based acceleration technique has been implemented in a massively parallel block-structured finite-volume Navier-Stokes solver for compressible flows and tested on a turbine cascade case with promising results.
15.	Andersson, Niklas, 1976, et al. (författare) A Novel Solver Acceleration Technique Based on Dynamic Mode Decomposition 2014 Ingår i: 11th World Congress on Computational Mechanics, WCCM 2014. - 9788494284472 ; , s. 4832-4851 Konferensbidrag (refereegranskat)abstract The speed up of finite-volume solvers for compressible flows is a difficult task. There are several ways to achieve solver speed-up, more or less difficult to implement and more or less suitable for implementation in a parallel, unstructured type of solver. Examples of such techniques are the multi-grid method and implicit residual smoothening. In this article, a solver acceleration technique based on Dynamic Mode Decomposition (DMD) is proposed. The technique does not depend on data or mesh structure and is thus as straightforward to implement in an unstructured parallel code as in a structured sequential code. The main idea behind the proposed method is that one can use the information in flow field modes extracted using the DMD technique to find a correction that will bring the solution closer to a steady state condition, i.e. the method is only applicable to steady-state problems. In the presented work the DMD-based acceleration technique has been implemented in a massively parallel block-structured finite-volume Navier-Stokes solver for compressible flows. The method has been tested on a turbine cascade case with promising results. To the knowledge of the authors, the proposed method is not previously published in the open literature.
16.	Andersson, Niklas, 1976 (författare) A Study of Mach 0.75 Jets and Their Radiated Sound Using Large-Eddy Simulation 2003 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Large-Eddy Simulations (LES) of a compressible nozzle/jet configuration have been carried out. Two jets were simulated, an isothermal jet and a jet with a higher temperature than the surrounding air. The Mach number was in both cases 0.75 and the jet Reynolds number was 50,000. Sound pressure levels in far-field observer locations were evaluated using both Kirchhoff surface integration and Lighthill's acoustic analogy. The Favre filtered Navier-Stokes equations were solved using a finite volume method solver with a low dissipative third-order upwind scheme for the convective fluxes, a second-order centered difference approach for the viscous fluxes and a three-stage second-order Runge-Kutta time marching technique. The computational domain was discretized using a block structured boundary fitted mesh with approximately 3,000,000 cells. The calculations were performed on a parallel computer, using message-passing interface (MPI). A compressible form of Smagorinsky's subgrid scale model was used for computation of the subgrid scale stresses. Absorbing boundary conditions based on characteristic variables were adopted for all free boundaries. Velocity components specified at the entrainment boundaries were estimated from corresponding Reynolds Averaged Navier-Stokes (RANS) calculations, which enable the use of a rather narrow domain. This, furthermore, ensures that the correct amount of fluid is entrained into the domain. Two-point space-time correlations were obtained for locations in the shear layer center, from which length and time scales of turbulence structures were evaluated. Aerodynamic results and predicted sound pressure levels are both in good agreement with experiments. Experimental data were provided by Laboratoire dEtude Aeròdynamiques, Poiters, France.
17.	Andersson, Niklas, 1976, et al. (författare) A Study of Mach 0.75 Jets and Their Radiated Sound Using Large-Eddy Simulation 2004 Ingår i: AIAA 2004-3024, proc. of 10th AIAA/CEAS Aeroacoustics Conference, May 10-12, 2004. Konferensbidrag (refereegranskat)abstract Large-Eddy Simulations (LES) of a compressible nozzle/jet configuration have been carried out. Two jets were simulated, an isothermal jet and a jet with a higher temperature than the quiescent surrounding air. The Mach number was in both cases 0.75 and the jet Reynolds number was 50,000. Sound pressure levels in far-field observer locations were evaluated using Kirchhoff surface integration. The Favre filtered Navier-Stokes equations were solved using a finite volume method solver with a low-dissipation third-order upwind scheme for the convective fluxes, a second-order centered difference approach for the viscous fluxes and a three-stage second-order Runge-Kutta technique in time. The computational domain was discretized using a block structured boundary fitted mesh with approximately 3,000,000 cells. The calculations were performed on a parallel computer, using message-passing interface (MPI). A compressible form of Smagorinsky's subgrid scale model was used for computation of the subgrid scale stresses. Absorbing boundary conditions based on characteristic variables were adopted for all free boundaries. Velocity components specified at the entrainment boundaries were estimated from corresponding Reynolds Averaged Navier-Stokes (RANS) calculations, which enable the use of a rather narrow domain. This, furthermore, ensures that the correct amount of fluid is entrained into the domain. Two-point space-time correlations were obtained for locations in the shear layer center, from which length and time scales of turbulence structures were evaluated. Predicted near-field flow statistics and far-field sound pressure levels (SPL) are both in good agreement with experiments. Predicted (SPL) are for all observers locations, where evaluated, within a 3.0 [dB] deviation from measured levels and for most locations within a 1.0 [dB] deviation. Experimental data used for validation were provided by Laboratoire dEtude Aeròdynamiques, Poitiers, France.
18.	Andersson, Niklas, 1976 (författare) A Study of Subsonic Turbulent Jets and Their Radiated Sound Using Large-Eddy Simulation 2005 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract Stricter noise regulation for near-ground operations has made noise reduction in commercial aircraft a topic of growing interest in the aerospace industry. To meet airworthiness requirements new noise reduction technologies have to be developed and numerical methods for correct assessment of these technologies are desirable.This thesis deals with predictions of near-field flow and far-field acoustic signature of subsonic turbulent single-stream and dual-stream jets at isothermal and heated conditions. The flowfield predictions are obtained using large-eddy simulation (LES), and Kirchhoff's surface integration technique is used to extend the acoustic domain to far-field locations. In all cases studied, the nozzle geometry is included in the calculation domain.For the single-stream jet, predicted near-field flow statistics and far-field sound pressure levels (SPL) are both in good agreement with experiments. Predicted SPL for all observer locations, where evaluated, are within a deviation of 3.0 dB from measured levels and for most locations within a deviation of 1.0 dB. For the specific cases studied, Mach 0.75 jets, only small differences in radiated sound could be identified between an isothermal jet and a jet with temperature twice that of the surrounding fluid. The effects of changes in inflow conditions, Reynolds number and subgrid-scale (SGS) model on the flowfield and acoustic signature were investigated. Only minor changes could be identified in the predictions of flow statistics and radiated sound.For the dual-stream jet, changing the subgrid-scale filter width and introducing a TVD limiter gave significant changes in the shear layer flow. Sound radiated in the upstream direction was shown to depend appreciably on the initial shear layer development. Vortex generators placed on the outside wall of the inner nozzle were found to effectively break up ring-shaped vortical structures in the initial inner shear layer region and speed up the mixing between the core and bypass streams.
19.	Andersson, Niklas, 1976, et al. (författare) Effects of Inflow Conditions and Subgrid Model on LES for Turbulent Jets 2005 Ingår i: AIAA 2005-2925, proc. of 11th AIAA/CEAS Aeroacoustics Conference, May 23-25, 2005, Monterey, California. Konferensbidrag (refereegranskat)abstract The turbulent mixing process prescribing the spreading rate of the jet and the length of the potential core region is influenced by a number of factors. Using large-eddy simulation (LES), the four factors that are believed to be the most important in this respect are: subgrid-scale properties, the accuracy of the numerical scheme, the entrainment boundary conditions, and the inflow conditions. In a previously performed study of a subsonic (Mach 0.75) jet, the turbulence mixing was found to be too efficient and hence the length of the potential core region was underpredicted. In that study indications were found of that the overpredicted mixing was due to the inflow conditions. For a model nozzle, capturing the initial turbulent shear flow might not be of that great importance for accurate prediction of radiated sound since most of these effects will appear in the high-frequency range. When dealing with real engine geometries, however, it becomes quite important. Moreover, methods for industrial use have to cope with complex geometries and high temperature and velocity ratios making the ability to capture the initial flow physics even more important. In the present work LES has been used for the same Mach 0.75 jet. The acoustic field is extracted to the far field using Kirchhoff surface integration. The effects of inflow conditions, Reynolds number, and subgrid-scale model on flowfield and acoustic signature are investigated.The Favre-filtered Navier-Stokes equations were solved using a finite-volume method solver with a low-dissipation third-order upwind scheme for the convective fluxes, a second-order centered difference approach for the viscous fluxes and a three-stage second-order Runge-Kutta technique in time. The computational domain was discretized using a block-structured boundary-fitted mesh with approximately 3,000,000 cells. The calculations were performed on a parallel computer, using message-passing interface (MPI). A compressible form of Smagorinsky's subgrid-scale model was used to compute the subgrid-scale stresses. Absorbing boundary conditions based on characteristic variables were adopted for all free boundaries.
20.	Andersson, Niklas, 1976, et al. (författare) Investigation of an Isothermal Mach 0.75 Jet and its Radiated sound Using Large-Eddy Simulation and Kirchhoff Surface Integration 2005 Ingår i: International Journal of Heat and Fluid Flow. ; 26, s. 393-410 Tidskriftsartikel (refereegranskat)abstract A large-eddy simulation (LES) of a compressible nozzle/jet configuration has been carried out. An isothermal Mach 0.75 jet was simulated. The Reynolds number based on the jet velocity at the nozzle exit plane and the nozzle diameter was 50,000. The Favre filtered Navier-Stokes equations were solved using a finite volume method solver with a low-dissipation third-order upwind scheme for the convective fluxes, a second-order centered difference approach for the viscous fluxes and a three-stage second-order Runge-Kutta time marching technique. A compressible form of Smagorinsky's subgrid scale model was used for computation of the subgrid scale stresses. The computational domain was discretized using a block structured boundary fitted mesh with approximately 3,000,000 cells. The calculations were performed on a parallel computer, using message-passing interface (MPI). Absorbing boundary conditions based on characteristic variables were adopted for all free boundaries. Velocity components specified at the entrainment boundaries were estimated from a corresponding Reynolds Averaged Navier-Stokes (RANS) calculation, which enabled the use of a rather narrow domain. In order to diminish disturbances caused by the outlet boundary, a buffer layer was added at the domain outlet. Kirchhoff surface integration using instantaneous pressure data from the LES was utilized to obtain far-field sound pressure levels in a number of observer locations. The predicted sound pressure levels were for all observer locations within a 3dB deviation from the measured levels and for most observer locations within a 1dB deviation. Aerodynamic results and predicted sound pressure levels are both in good agreement with experiments. Experimental data were provided by Laboratoire dEtude Aeròdynamiques, Poiters, France.
21.	Andersson, Niklas, 1976, et al. (författare) Large-Eddy Simulation of a Mach 0.75 Jet 2003 Ingår i: AIAA 2003-3312, proc. of 9th AIAA/CEAS Aeroacoustic Conference, May 12-14, 2003. Konferensbidrag (refereegranskat)abstract A large-eddy simulation (LES) of a compressible nozzle/jet configuration has been carried out. A cold Mach 0.75 jet was simulated. The Reynolds number based on the jet velocity at the nozzle exit plane and the nozzle diameter was 50,000. The Favre filtered Navier-Stokes equations were solved using a finite volume method with a low dissipative third order upwind scheme for the convective fluxes, a second order centered difference approach for the viscous fluxes and a three-stage second order Runge-Kutta time marching technique. A compressible form of Smagorinsky's sub-grid scale model was used for computation of the sub-grid scale stresses. The calculations were performed using a block structured boundary fitted mesh with approximately 3,000,000 cells. The calculations have been performed on a parallel computer, using message-passing interface (MPI). Absorbing boundary conditions based on characteristic variables were adopted for all free boundaries. Velocity components specified at the entrainment boundaries were estimated from a corresponding Reynolds Averaged Navier-Stokes (RANS) calculation. In order to diminish disturbances caused by the outlet boundary a buffer layer was added at the domain outlet. Kirchhoff surface integration has been utilized to obtain far-field sound pressure levels in a number of observer locations using instantaneous pressure from the LES. Aerodynamic results and predicted sound pressure levels are both in good agreement with experiments. Experimental data were provided by Laboratoire dEtude Aeròdynamiques, Poiters, France.
22.	Andersson, Niklas, 1976, et al. (författare) Large-Eddy Simulation of Subsonic Turbulent Jets and Their Radiated Sound 2005 Ingår i: AIAA Journal. ; 43:9, s. 1899-1912 Tidskriftsartikel (refereegranskat)abstract Large-Eddy Simulations (LES) of a compressible nozzle/jet configuration have been carried out. Two jets were simulated, an isothermal jet and a jet with a higher temperature than the quiescent surrounding air. The Mach number was in both cases 0.75 and the jet Reynolds number was 50,000. Sound pressure levels in far-field observer locations were evaluated using Kirchhoff surface integration. The Favre filtered Navier-Stokes equations were solved using a finite volume method solver with a low-dissipation third-order upwind scheme for the convective fluxes, a second-order centered difference approach for the viscous fluxes and a three-stage second-order Runge-Kutta technique in time. The computational domain was discretized using a block structured boundary fitted mesh with approximately 3,000,000 cells. The calculations were performed on a parallel computer, using message-passing interface (MPI). A compressible form of Smagorinsky's subgrid scale model was used for computation of the subgrid scale stresses. Absorbing boundary conditions based on characteristic variables were adopted for all free boundaries. Velocity components specified at the entrainment boundaries were estimated from corresponding Reynolds Averaged Navier-Stokes (RANS) calculations, which enable the use of a rather narrow domain. This, furthermore, ensures that the correct amount of fluid is entrained into the domain. Two-point space-time correlations were obtained for locations in the shear layer center, from which length and time scales of turbulence structures were evaluated. Predicted near-field flow statistics and far-field sound pressure levels (SPL) are both in good agreement with experiments. Predicted (SPL) are for all observers locations, where evaluated, within a 3.0 [dB] deviation from measured levels and for most locations within a 1.0 [dB] deviation. Experimental data used for validation were provided by Laboratoire dEtude Aeròdynamiques, Poiters, France.
23.	Andersson, Niklas, 1976, et al. (författare) LES Prediction of Flow and Acoustic Field of a Coaxial Jet 2005 Ingår i: AIAA 2005-2884, proc. of 11th AIAA/CEAS Aeroacoustics Conference, May 23-25, 2005, Monterey, California. Konferensbidrag (refereegranskat)abstract A compressible high-subsonic coaxial jet has been simulated using large-eddy simulation (LES). The acoustic field was extended to the far field using Kirchhoff surface integration. The jet Mach number based on the local speed of sound is approximately 0.9 for both the primary and secondary stream. The static temperature in the primary stream is three times that of the secondary stream. In order to resolve the acoustic field, it is desirable to have a computational domain with a rather large radial extent and a mesh that is relatively fine even in the far-field regions. Furthermore, the mesh should be as equidistant as possible so as to minimize the introduction of numerical errors. In order to keep the number of cells down, the computational domain was divided into three regions: a well resolved near-wall LES region, a medium-resolution LES region optimized for propagation of acoustic waves, and a coarse LES region. Over the interfaces between these regions, the number of cells is increased by factor two in each direction. Special treatment of the interfaces between the regions is utilized in order to minimize undesirable numerical errors. The radial extent of the computational domain increases downstream such that the flow in the outer boundary region can be assumed to be irrotational and axisymmetric. Hence, the flow outside the three-dimensional computational domain can be represented by a less expensive two-dimensional axisymmetric calculation. The interface between the full 3D LES region and the 2D region is based on azimuthally averaged quantities and acts as an absorbing boundary condition. The Favre-filtered Navier-Stokes equations were solved using a finite-volume method solver with a low-dissipation third-order upwind scheme for the convective fluxes, a second-order centered difference approach for the viscous fluxes and a three-stage second-order Runge-Kutta technique in time. The computational domain was discretized using a block-structured boundary-fitted mesh with approximately 20,000,000 nodes. The calculations were performed on a parallel computer, using message-passing interface (MPI). A compressible form of Smagorinsky's subgrid-scale model was used to compute the subgrid-scale stresses.
24.	Andersson, Niklas, 1976, et al. (författare) Numerical Simulation of Stirling Engines Using an Unsteady Quasi-One-Dimensional Approach 2015 Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 137:5, s. Art. no. 051104- Tidskriftsartikel (refereegranskat)abstract An existing computer code for solving the quasi-one-dimensional flow equations governing unsteady compressible flow in tubes with smoothly varying cross-section areas, has been adapted to the simulation of the oscillatory flow in Stirling engines for engine design purposes. By utilizing an efficient smoothing algorithm for the area function that preserves the total volume of the tube, it has been possible to achieve a highly accurate and fully conservative numerical scheme. Sub-models for wall friction and heat transfer have been added, enabling the simulation of gas heaters, gas coolers, and regenerators. The code has been used for the modeling of an alpha-type Stirling engine and validated for a range of operating conditions with good results.
25.	Andersson, Niklas, 1976, et al. (författare) Prediction of flowfield and acoustic signature of a coaxial jet using RANS-based methods and large-eddy simulation 2008 Ingår i: International Journal of Aeroacoustics. - : SAGE Publications. - 1475-472X .- 2048-4003. ; 7:1, s. 23-40 Tidskriftsartikel (refereegranskat)abstract The feasibility of using large-eddy simulation (LES) for jet noise predictions has been discussed in several publications. The vast majority of these studies are, however, restricted to fairly simple geometries and moderate Reynolds numbers flows. Recent studies have shown that LES-based methods can be used for realistic flows and complex geometries with promising results. With continuously increased computer capacity and with the possibility to perform computations in parallel on PC-clusters, the possibility of using LES for industrial applications is increasing. However, although, the complexity of configurations for which LES may be used is approaching that of a real jet engine, due to high demands for computational power and long turn-over times LES is still not feasible for industrial use. For low-noise nozzle design, engineering tools with short turn-over time, accurate enough for prediction of noise trends are therefore needed. In the development of such methods, LES can provide data suitable for method validation. In the present work, the high-subsonic flow in an axisymmetric coaxial nozzle/jet configuration is studied. Flowfield predictions obtained using an axisymmetric RANS solver for compressible flows and predicted acoustic signature obtained using a method based on RANS and Lighthill's acoustic analogy are compared with flowfield and radiated sound predictions obtained using LES combined with Kirchhoff surface integration. In the predicted flow statistics, the most noticeable difference between RANS and LES results is that the RANS calculation gives somewhat lower levels of turbulence kinetic energy in the shear layers in the near-nozzle regions and hence the LES jet mixes somewhat faster and thus the potential core region is shorter in this simulation.
26.	Andersson, Svante, 1962-, et al. (författare) Sustainable development considerations in supply chains : Firms' relationships with stakeholders in their business sustainability practices—A triangular comparison 2022 Ingår i: Business Strategy and the Environment. - Oxford : John Wiley & Sons. - 0964-4733 .- 1099-0836. ; 32:4, s. 1885-1899 Tidskriftsartikel (refereegranskat)abstract This study aims to investigate focal firms' business sustainability relationships in connection with their stakeholders in supply chain networks. A questionnaire survey was sent to a sample consisting of large Swedish firms, with 107 usable questionnaires returned. The results reveal which stakeholders are of interest for firms in sustainability efforts. The results are compared with earlier findings from Norway and Spain in a triangular approach. The former is a similar country, while the latter is different to Sweden in several ways. In addition, the study uncovers which stakeholders in the supply chain network should be considered. The study also demonstrates how firms can implement business sustainability in their supply chain networks and shows the extent to which different stakeholders are considered in sustainability efforts. The study contributes to sustainability research and stakeholder theory in supply chain networks. Opposite to earlier findings, this study showed only minor influences from national culture and institutions on firms' sustainable business practices in supply chain networks. © 2022 The Authors. Business Strategy and The Environment published by ERP Environment and John Wiley & Sons Ltd.
27.	Andersson, Svante, 1962-, et al. (författare) Sustainable development—Direct and indirect effects between economic, social, and environmental dimensions in business practices 2022 Ingår i: Corporate Social Responsibility and Environmental Management. - Oxford : John Wiley & Sons. - 1535-3958 .- 1535-3966. ; 29:5, s. 1158-1172 Tidskriftsartikel (refereegranskat)abstract This study investigates the direct and indirect effects between economic, social and environmental dimensions of triple bottom line (TBL), based on a questionnaire survey and cross-industrial sample in Sweden. The analyses apply partial least squares structural equation models. The study tests the direct and indirect effects between economic, social, and environmental dimensions of TBL and offers additional validity and reliability to establish the measurement and structural properties between the dimensions of TBL. The study extends earlier findings by explicitly discussing how the three TBL goals relate to each other and shows how the dynamic capability view can be a fruitful lens to investigate business sustainability. Some differences in sustainability business practices caused by differences in national cultures are identified. Sustainability reporting in a strong uncertainty avoidance (UA) country happens in accordance with regulations and laws. Conversely, for weak UA cultures, reporting and compliance with regulations are ways to build trust with stakeholders. That is, reporting is more transparent and widespread in weak UA countries. The study also provides a foundation to guide companies' actions of business sustainability. The model shows companies how to establish the order of actions undertaken across economic, social, and environmental dimensions. In addition, it clarifies that the economic dimension exerts an effect on the social and environmental dimensions. The model also grasps long-term economic performance by including competitiveness and brand value, while earlier research mainly has focused on more short-term measurements as return on assets. © 2022 The Authors. Corporate Social Responsibility and Environmental Management published by ERP Environment and John Wiley & Sons Ltd.
28.	Angelov, Iltcho, 1943, et al. (författare) Large-Signal Modelling and Comparison of AlGaN/GaN HEMTs and SiC MESFETs 2006 Ingår i: IEEE Asia PAcific Microwave Conference, 2006, Yokohama, Japan. - 9784902339116 ; , s. 279-282 Konferensbidrag (refereegranskat)abstract The Large Signal (LS) model for GaN and SiC FET devices was developed and evaluated with DC, S, and LS measurements. Special attention was paid to improve the management of harmonics and provide. a more physical treatment of the dispersion. The model was implemented in a commercial CAD tool and exhibit good overall accuracy.
29.	Angelov, Iltcho, 1943, et al. (författare) On the large-signal modeling of High Power AlGaN/GaN HEMTs 2012 Ingår i: IEEE MTT-S International Microwave Symposium Digest. - 0149-645X. - 9781467310871 Konferensbidrag (refereegranskat)abstract In this paper are given some recent results on modeling of High Power GaN HEMT devices. The GaN HEMT is very promising for high power application, but we push device to the limits, so many issues are becoming critical. For example, access resistances Rs, Rd in high power GAN HEMT are bias and temperature dependent-their extraction from cold FET measurements can lead to over optimistic prediction for output power. Thermal management, self-heating modeling are another very important issue-they influence reliability, power and PAE. Models without dynamic self-heating are not practical for GaN. The models without breakdown can easily predict world records for PAE, output power/mm etc. Some examples are given using vectorial Large Signal Measurements (LSNA/NVNA) to provide useful, global info about device behavior, influence of traps, knee voltage walkout etc.
30.	Barregård, Lars, 1948, et al. (författare) Cadmium Exposure and Coronary Artery Atherosclerosis: A Cross-Sectional Population-Based Study of Swedish Middle-Aged Adults 2021 Ingår i: Environmental health perspectives. - 1552-9924 .- 0091-6765. ; 129:6 Tidskriftsartikel (refereegranskat)abstract The general population is ubiquitously exposed to the toxic metal cadmium through the diet and smoking. Cadmium exposure is associated with increased morbidity and mortality in myocardial infarction and stroke. Atherosclerosis is the main underlying mechanism of myocardial infarction. However, associations between cadmium and coronary artery atherosclerosis have not been examined.Our study sought to examine the hypothesis that blood cadmium (B-Cd) is positively associated with coronary artery calcification, as a measure of coronary artery atherosclerosis in the population-based Swedish SCAPIS study.Our analysis included 5,627 individuals (51% women), age 50-64 y, enrolled from 2013 to 2018. The coronary artery calcium score (CACS) was obtained from computed tomography. Blood cadmium was determined by inductively coupled plasma mass spectrometry (ICP-MS). Associations between B-Cd and coronary artery calcium score (CACS Agatston score) were evaluated using prevalence ratios (PRs) in models adjusted for sex, age, smoking, hypertension, diabetes, low-density cholesterol/high-density cholesterol ratio, and family history.The median B-Cd concentration was 0.24 μ g / L . The prevalence of positive coronary artery calcium ( CACS > 0 ) was 41% and the prevalence of CACS ≥ 100 was 13%. Relative to the lowest quartile (Q) of B-Cd ( < 0.16 μ g / L ), the highest quartile (median 0.63 μ g / L ) was associated with a small but significant increase in CACS > 0 (PR 1.1; 95% CI: 1.0, 1.3), and a greater relative increase in CACS ≥ 100 (PR 1.6; 95% CI: 1.3, 2.0). When restricted to 2,446 never-smokers, corresponding PRs were 1.1 (95% CI 0.9, 1.3) for CACS > 0 (63 cases in Q4) and 1.7 (95% CI 1.1, 2.7) for CACS ≥ 100 (17 cases in Q4).Blood cadmium in the highest quartile was associated with CACS in a general population sample with low to moderate cadmium exposure. This supports the hypothesis that atherosclerosis is an important mechanism underlying the associations between cadmium and incident cardiovascular disease. The findings suggest that public health measures to reduce cadmium exposure are warranted. https://doi.org/10.1289/EHP8523.
31.	Bremer, Johan, 1991, et al. (författare) Electric-Based Thermal Characterization of GaN Technologies Affected by Trapping Effects 2020 Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:5, s. 1952-1958 Tidskriftsartikel (refereegranskat)abstract This article presents an electric-based methodology for thermal characterization of semiconductor technologies. It is shown that for technologies such as gallium nitride (GaN) high electron mobility transistors, which exhibit several field induced electron trapping effects, the thermal characterization has to be performed under specific conditions. The electric field is limited to low levels to avoid activation of trap states. At the same time, the dissipated power needs to be high enough to change the operating temperature of the device. The method is demonstrated on a test structure implemented as a GaN resistor with large contact separation. It is used to evaluate the thermal properties of samples with different silicon carbide suppliers and buffer thickness.
32.	Chehrenegar, Pirooz, 1964, et al. (författare) Design and characterization of a highly linear 3 GHz GaN HEMT amplifier 2011 Ingår i: 2011 Workshop on Integrated Nonlinear Microwave and Millimetre-Wave Circuits, INMMiC 2011. Vienna, 18-19 April 2011. - 9781457706493 Konferensbidrag (refereegranskat)abstract In this paper a highly linear amplifier using an in-house gallium nitride (GaN) high electron mobility transistor (HEMT) technology is presented. A 3 dB bandwidth of 2.7-3.6 GHz with a maximum gain of 18 dB was measured. The output third-order intercept point (OIP3) was measured to 39 dBm with a maximum power consumption of 2.1 W. With a reduction of power consumption to 1 W the noise figure was improved by 0.6 dB while the OIP3 was degraded 3 dB.
33.	Cheregi, Otilia, et al. (författare) Transcriptome analysis reveals insights into adaptive responses of two marine microalgae species to Nordic seasons 2023 Ingår i: Algal Research. - 2211-9264. ; 74 Tidskriftsartikel (refereegranskat)abstract There is an increasing interest in algae-based biomass produced outdoors in natural and industrial settings for biotechnological applications. To predict the yield and biochemical composition of the biomass, it is important to understand how the transcriptome of species and strains of interest is affected by seasonal changes. Here we studied the effects of Nordic winter and summer on the transcriptome of two phytoplankton species, namely the diatom Skeletonema marinoi (Sm) and the eustigmatophyte Nannochloropsis granulata (Ng), recently identified as potentially important for biomass production on the west coast of Sweden. Cultures were grown in photobioreactors in simulated Nordic summer and winter, and the gene expression in two phases was quantified by Illumina RNA-sequencing. Five paired comparisons were made among the four conditions. Sm was overall more responsive to seasons since 70 % of the total transcriptome (14,783 genes) showed differential expression in at least one comparison as compared to 1.6 % (1403 genes) for Ng. For both species, we observed larger differences between the seasons than between the phases of the same season. In summer phase 1, Sm cells focused on photosynthesis and polysaccharide biosynthesis. Nitrate assimilation and recycling of intracellular nitrogen for protein biosynthesis were more active in summer phase 2 and throughout winter. Lipid catabolism was upregulated in winter relative to summer to supply carbon for respiration. Ng favored lipid accumulation in summer, while in winter activated different lipid remodeling pathways as compared to Sm. To cope with winter, Ng upregulated breakdown and transport of carbohydrates for energy production. Taken together, our transcriptome data reveal insights into adaptive seasonal responses of Sm and Ng important for biotechnological applications on the west coast of Sweden, but more work is required to decipher the molecular mechanisms behind these responses.
34.	Costa, Fabíola Paula, et al. (författare) Aerodynamic Analysis of Conventional and Boundary Layer Ingesting Propellers 2023 Ingår i: Journal of Engineering for Gas Turbines and Power. - : ASME International. - 1528-8919 .- 0742-4795. ; 145:1 Tidskriftsartikel (refereegranskat)abstract The boundary layer ingestion (BLI) concept has emerged as a novel technology for reducing aircraft fuel consumption. Several studies designed BLI-fans for aircraft. BLI-propellers, although, have still received little attention, and the choice of open-rotors or ducted propellers is still an open question regarding the best performance. The blade design is also challenging because the BLI-propulsors ingest a nonuniform flow. These aspects emphasize further investigation of unducted and ducted BLI-propulsors and the use of optimization frameworks, coupled with computational fluid dynamics simulations, to design the propeller to adapt to the incoming flow. This paper uses a multi-objective NSGA-II optimization framework, coupled with three-dimensional RANS simulations and radial basis function (RBF) metamodeling, used for the design and optimization of three propeller configurations at cruise conditions: (a) conventional propeller operating in the freestream, (b) unducted BLI-propeller, and (c) ducted BLI-propeller, both ingesting the airframe boundary layer. The optimization results showed a significant increase in chord and a decrease in the blade angles in the BLI configurations, emphasizing that these geometric parameters optimization highly affects the BLI-blade design. The unducted BLI-propeller needs approximately 40% less shaft power than the conventional propeller to generate the same amount of propeller force. The ducted BLI-propeller needs even less power, 47%. The duct contributes to the tip vortex weakening, recovering the swirl, and turning into propeller force, as noticed from 80% of the blade span to the tip. However, the unducted and ducted BLI-configurations presented a higher backward force, 26% and 46%, respectively, compared to the conventional propeller, which can be detrimental and narrow the use of these configurations.
35.	Costa, Fabíola Paula, et al. (författare) Coupled unsteady RANS and FW-H methodology for aeroacoustics prediction of high-speed propellers 2022 Ingår i: 28th AIAA/CEAS Aeroacoustics Conference, 2022. - Reston, Virginia : American Institute of Aeronautics and Astronautics. Konferensbidrag (refereegranskat)abstract This work investigates the use of solid and permeable surfaces in the Ffowcs WilliamsHawkings (FW-H) analogy for predicting high-speed propeller noise. The CFD/CAA methodology encompasses unsteady Reynolds-Averaged Navier-Stokes simulations to compute the flowfield on the acoustic surface applied in the FWH analogy to obtain the noise signatures in the far-field. Furthermore, this manuscript also investigates the effects of the downstream end-cap position, on the propeller noise prediction, by using two permeable surfaces with different lengths to assess the propeller noise levels in each case. The former is a short SFW-H surface placed near the rotor, and the latter, namely the LFW-H, is a surface larger in length where the end-cap grid is placed farther downstream from the rotor. The results showed the capability of the permeable surface technique for predicting the noise with higher accuracy than the solid formulation, especially at the first blade passing frequency. Also, the larger LFW-H surface performed better than the SFW-H surface. A reason that could justify this is that the LFW-H end-cap surface is placed at a suitable distance downstream from the propeller. Therefore, the LFW-H surface can include more of the contributions of the non-linear effects or quadrupole sources enclosed within the permeable source surface region.
36.	Ericson, Y, et al. (författare) Faktablad från Integrerad kustfiskövervakning: Torhamn (södra Egentliga Östersjön) 2002-2015 2016 Ingår i: Faktablad 2016 från Inst för Akvatiska resurser, SLU. ; :2016:4 Annan publikation (övrigt vetenskapligt/konstnärligt)abstract Faktabladet presenterar resultat och bedömningar från den integrerade kustfiskövervakningen i Torhamnsområdet under tidsperioden 2002-2015. Resultaten från tretton års undersökningar visar en alltmer tydlig påverkan på hälsotillståndet hos abborre liknande den som observeras hos abborre och tånglake i andra kustreferensområden. Denna tydliga och successiva påverkan som ses på abborrens hälsa på individnivå har ännu inte resulterat i några påtagliga förändringar på bestånds- eller samhällsnivå.
37.	Fagerlind, Martin, 1980, et al. (författare) GaN Device and MMIC development at Chalmers 2008 Ingår i: Gigahertz Symposium 2008, Abstract book. ; , s. 86- Konferensbidrag (refereegranskat)
38.	Flyckt, Jonatan, et al. (författare) Detecting ditches using supervised learning on high-resolution digital elevation models 2022 Ingår i: Expert systems with applications. - : Elsevier Ltd. - 0957-4174 .- 1873-6793. ; 201 Tidskriftsartikel (refereegranskat)abstract Drained wetlands can constitute a large source of greenhouse gas emissions, but the drainage networks in these wetlands are largely unmapped, and better maps are needed to aid in forest production and to better understand the climate consequences. We develop a method for detecting ditches in high resolution digital elevation models derived from LiDAR scans. Thresholding methods using digital terrain indices can be used to detect ditches. However, a single threshold generally does not capture the variability in the landscape, and generates many false positives and negatives. We hypothesise that, by combining the digital terrain indices using supervised learning, we can improve ditch detection at a landscape-scale. In addition to digital terrain indices, additional features are generated by transforming the data to include neighbouring cells for better ditch predictions. A Random Forests classifier is used to locate the ditches, and its probability output is processed to remove noise, and binarised to produce the final ditch prediction. The confidence interval for the Cohen's Kappa index ranges [0.655, 0.781] between the evaluation plots with a confidence level of 95%. The study demonstrates that combining information from a suite of digital terrain indices using machine learning provides an effective technique for automatic ditch detection at a landscape-scale, aiding in both practical forest management and in combatting climate change. © 2022 The Authors
39.	Flyckt, Jonatan, et al. (författare) Explaining rifle shooting factors through multi-sensor body tracking 2023 Ingår i: Intelligent Data Analysis. - : IOS Press. - 1088-467X .- 1571-4128. ; 27:2, s. 535-554 Tidskriftsartikel (refereegranskat)abstract There is a lack of data-driven training instructions for sports shooters, as instruction has commonly been based on subjective assessments. Many studies have correlated body posture and balance to shooting performance in rifle shooting tasks, but have mostly focused on single aspects of postural control. This study has focused on finding relevant rifle shooting factors by examining the entire body over sequences of time. A data collection was performed with 13 human participants carrying out live rifle shooting scenarios while being recorded with multiple body tracking sensors. A pre-processing pipeline produced a novel skeleton sequence representation, which was used to train a transformer model. The predictions from this model could be explained on a per sample basis using the attention mechanism, and visualised in an interactive format for humans to interpret. It was possible to separate the different phases of a shooting scenario from body posture with a high classification accuracy (80%). Shooting performance could be detected to an extent by separating participants using their strong and weak shooting hand. The dataset and pre-processing pipeline, as well as the techniques for generating explainable predictions presented in this study have laid the groundwork for future research in the sports shooting domain.
40.	Ghosh, Debarshee, 1995, et al. (författare) Aerodynamic Analysis of Low-Pressure Axial Fans Installed in Parallel 2024 Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - 1528-901X .- 0098-2202. ; 146:5 Tidskriftsartikel (refereegranskat)abstract Ducted rotor-only low-pressure axial fans play an integral role in automotive thermal management. The tightly packed under-hood region and down-stream heat-exchanger shape limit the fan diameter. In order to circumvent this limitation, multiple cooling fans of small diameters are tightly packaged and placed in parallel. Currently, there is limited scientific work, that study the aerodynamics of low-pressure axial fans when installed in parallel. This work aims to quantify the aerodynamic performance and the flow-field as a result of installing low-pressure axial fans in parallel through computational fluid dynamics (CFD). Publicly available experimental data from Friedrich-Alexander University is used for the validation of the numerical setup. Three-dimensional, full-annulus, unsteady Reynolds-averaged Navier‐Stokes (URANS) analysis has been performed for both a single-fan and two-fans installed in parallel and their respective aerodynamic performance has been compared for the operation condition identified as the best efficiency point in experiments. Only small differences are observed in the overall aerodynamic performance of the two-fans in parallel compared to a single-fan. A circumferential nonuniformity in the form of a local high-pressure zone at the inlet of the fan is observed when the two-fans are placed in parallel. The resulting circumferential nonuniformity is quantified, both in space and time. A strong correlation is found between the pressure fields of the two-fans installed in parallel.
41.	Ghosh, Debarshee, 1995, et al. (författare) Computational Fluid Dynamics Simulations of Aerodynamic Performance of Low-Pressure Axial Fans with Upstream Blockage 2024 Ingår i: SAE Technical Papers. - 0148-7191 .- 2688-3627. Konferensbidrag (refereegranskat)abstract Rotor-only ducted low-pressure axial fans play a crucial role in automotive thermal management of the tightly packed under-hood region. Most current scientific work concerning low-pressure axial fans investigate the aerodynamic performance of these fans while operating with uniform inlet flow conditions. This is rarely the case in real-world applications. This work aims to investigate the aerodynamic performance of low-pressure axial fans operating with upstream blockages. First, a validation study is performed in the absence of any upstream blockage. Numerical results are compared against publicly available experimental data. Steady-state, Reynolds-Averaged Navier Stokes (RANS) analysis is performed on a single-blade passage. The validation study also evaluates the choice of turbulence model and suggests the use of the k- ε turbulence model with wall functions for the best comparison against experimental data. To study the effect of upstream blockage, a generic blockage disc is positioned upstream of the fan inlet. Three different radial extents of the blockage disc is evaluated, such that different radial extents of the blade span is blocked. A strong influence of the upstream blockage is observed on the fan performance and flow distribution along the blade span. The total-to-static pressure coefficient and the total-to-static efficiency decrease proportionately to the extent of blockage in the radial direction. The peak total-to-static efficiency moves to a lower flow coefficient with increase in upstream flow blockage. This is deemed undesirable for automotive applications where it is desirable to have maximum aerodynamic efficiency at the highest possible flow coefficient.
42.	Hafsteinsson, Haukur, 1984, et al. (författare) Acoustic Signature of a Supersonic Jet Emanating from a Rectangular C-D Nozzle 2016 Ingår i: 54th AIAA Aerospace Sciences Meeting, 2016, San Diego, United States, 4-8 January 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103933 ; 0 Konferensbidrag (refereegranskat)abstract We live in a world with ever increasing air traffic and the demand for fuel efficient low noise emitting aircraft is high. The use of blended wing bodies (BWB) has gained interests within the aerospace industry due to its potential for reduced fuel consumption. These type of aircraft are generally equipped with rectangular nozzles. The drawback of such nozzles is increased instability of the emanating jet which increases the risk of higher noise radiation. Understanding the instability patterns and the underlying flow physics is therefore the key to improved stability and reduced noise. In the presented paper, an LES/CAA approach is utilized to predict the flow dynamics and the radiated noise from a rectangular nozzle. The nozzle is operated at underexpanded conditions. The simulations are compared with experiments and are used as a complement to the experimental data for improved understanding of the flow physics. The supersonic jet is found to exhibit an intense flapping motion followed by a large jet spreading in the minor-axis plane. In general, the prediction of the most amplified frequency and higher harmonics observed in the near-field and far-field spectra is in agreement with the experiment. Two types of flow events associated with the generation of high amplitude acoustic waves are detected. These events are identified as vortex-collision and shock-leakage through the shear layer.
43.	Hafsteinsson, Haukur, 1984, et al. (författare) Exploration of temperature effects on the far-field acoustic radiation from a supersonic jet 2014 Ingår i: 20th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102851 Konferensbidrag (refereegranskat)abstract Jet engines designed for high-speed aircraft commonly include C-D nozzles to obtain supersonic speeds. The radiated noise from the jet exhaust reaches acoustic levels which may cause hearing damage to the pilot and the air-field personnel even though state of the art noise protection such as noise-canceling ear muffs are employed. It is therefore extremely important to keep the noise levels as low as possible. Understanding the noise generation mechanism is of great importance in order to reduce strength of the noise sources. Typical far-field noise spectral characteristics from the supersonic jet exhaust consist of turbulent mixing noise and shock-associated noise. Another noise component named'crackle' is radiated from the jet under certain circumstances. Although it does not appear in the noise spectra due to its characteristics, its rasping character is perceived as a dominant annoyance factor by the human ear. Since it does not appear in the spectrum other measures are needed to identify the existence of'crackle'. Statistical tools like Skewness and Kurtosis applied to the far- and near-field pressure signals and the time derivate of the pressure signal have been shown in literature to be useful for identification of'crackle' events. In this paper the near-field and far-field acoustic radiation from a supersonic jet is analyzed using LES with a code developed at Chalmers University of Technology. The code has previously shown to accurately capture far-field noise spectra of supersonic jets under a variety of moderately cool jet conditions. In the present study we move towards more realistic high-speed aircraft conditions with higher jet exhaust temperatures. The nozzle is operated at slightly underexpanded conditions (NPR = 4.0) and three different stagnation temperature ratios NTR = 1.0, NTR = 2.0 and NTR = 3.0. The LES results are compared with experiments conducted at the Gas Dynamics and Propulsion Laboratory at the University of Cincinnati.
44.	Hafsteinsson, Haukur, 1984, et al. (författare) Near-field and far-field spectral analyzis of supersonic jet with and without fluidic injection 2014 Ingår i: 52nd AIAA Aerospace Sciences Meeting - AIAA Science and Technology Forum and Exposition, SciTech 2014; National Harbor, MD; United States; 13 January 2014 through 17 January 2014. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102561 Konferensbidrag (refereegranskat)abstract In the presented study the time-dependent flow features of a supersonic jet with and without steady microjet injection are investigated. The flow field is sampled at various axial and radial locations in the supersonic region and its near surroundings. The jet is emitted from a sharp-throat converging diverging nozzle operated at a nozzle pressure ratio (NPR) of 4.0, which gives a jet exitMach number of M = 1.56 and a Reynolds number of Re = 2.46×106 based on the jet exit diameter. Large Eddy Simulation (LES) is used to obtain the fully three dimensional instantenous turbulent flow field and the Kirchhoff surface integral method is applied to obtain the far-field radiated noise. Both the near-field flow dynamics and the far-field noise obtained from the LES are in good agreement with experimental data. The noise components in the far-field noise are identified and compared with the spectra obtained from the probe-locations within the jet. The effect of micro-jet injection on the spectral characteristics within the jet and the far-field noise is analyzed. The screech tone appearing in the far-field noise is clearly established also in the jet-plume. Two point cross-correlations within and outside the supersonic region of the jet-plume revealed two types of moving phenomenon. These where found to be turbulent structures and acoustic waves. The odd thing at first sight was that the acoustic waves appeared to be traveling upstream within the supersonic region, which sounds contradictory. However, it was showed that the acoustic wave was traveling in the form of a helical mode which allows the phase velocity of the pressure wave to be higher than the flow velocity, even at supersonic flow speeds. The fluidic injection was shown to disrupt and weaken the helical pattern which resulted in a lower far-field screech tone noise. Upon sufficient dissipation of the injection, a few nozzle diameters downstream of the nozzle exit, the helical pattern picks up strength again. However, the feed-back loop mechanism associated with the screech tone is still disabled.
45.	Hafsteinsson, Haukur, 1984, et al. (författare) Noise Control of Supersonic Jet with Steady and Flapping Fluidic Injection 2015 Ingår i: AIAA Journal. - 1533-385X .- 0001-1452. ; 53:11, s. 3251-3272 Tidskriftsartikel (refereegranskat)abstract Large-eddy simulation is used to investigate steady-state mass flow injection into a supersonic jet stream with and without flapping motion of the microjets. The results are validated with particle image velocimetry and acoustic measurements. The effect of microjet penetration on the far-field acoustics is studied by altering the number of injectors, the cross-sectional area of each injector, and the injection mass flow. The injectors are evenly distributed around the nozzle exit. The injection angle is 90 deg relative to the main jet flow. This research is a continuation of a previous large-eddy simulation study of pulsed injection that showed that the unsteady injection-induced pressure pulses in the jet caused increased tonal noise for far-field observers at low angles. Flapping jet injection was shown to minimize the creation of the pressure pulses, except for high-amplitude flapping angles and high injection mass flows, where the injections divert out of the shear layer and introduce periodic superposition of the double shock-cell structure. Furthermore, the flapping injection did not show improved noise reduction compared with the steady injection, which is essentially promising because steady injection proves to be a more practical solution for implementation in real jet engine applications.
46.	Hafsteinsson, Haukur, 1984, et al. (författare) Reduction of supersonic jet noise using micro-jets in the diverging part of a conical CD-nozzle 2015 Ingår i: The International Conference on Jets, Wakes and Separated Flows (ICJWSF2015), June 16-18, 2015, Stockholm, Sweden. Konferensbidrag (refereegranskat)abstract Reducing the radiated noise from supersonic jets with maintainedthrust is a major challenge. An LES/CAA method and experiments are uti-lized to investigate the flow field and the far-field acoustics of a supersonic jetemitted from a circular CD-nozzle with a sharp throat. Air is injected into thedivergent section of the nozzle using 12 evenly distributed micro-jets aroundthe nozzle circumference and the change in total thrust and acoustic signa-ture is investigated. The micro-jets are shown to decrease shock-strength withincreased injection until an optimum point where further increase in injectionmass flow caused increased shock-strength. As a consequence of the reducedshock strength, the far-field shock-noise was decreased. The net thrust wasshown to increase with increased injection mass flow, whereas the specificthrust was slightly decreased. Micro-jet injection into the divergent sectionof a CD-nozzle appears, therefore, to be an effective configuration for reducedshock-noise radiation from supersonic jets.
47.	Hafsteinsson, Haukur, 1984, et al. (författare) Supersonic Jet Excitation using Flapping Injection 2013 Ingår i: American Physical Society, Division of Fluid Dynamics 66th Annual Meeting, November 24–26, 2013 Pittsburgh, Pennsylvania. Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Supersonic jet noise reduction is important for high speed military aircraft. Lower acoustic levels would reduce structural fatigue leading to longer lifetime of the jet aircraft. It is not solely structural aspects which are of importance, health issues of the pilot and the airfield personnel are also very important, as high acoustic levels may result in severe hearing damage. It remains a major challenge to reduce the overall noise levels of the aircraft, where the supersonic exhaust is the main noise source for near ground operation. Fluidic injection into the supersonic jet at the nozzle exhaust has been shown as a promising method for noise reduction. It has been shown to speed up the mixing process of the main jet, hence reducing the kinetic energy level of the jet and the power of the total acoustic radiation. Furthermore, the interaction mechanism between the fluidic injection and the shock structure in the jet exhaust plays a crucial role in the total noise radia- tion. In this study, LES is used to investigate the change in flow structures of a supersonic (M=1.56) jet from a converging-diverging nozzle. Six fluidic actuators, evenly distributed around the nozzle exit, inject air in a radial direction towards the main flow axis with a total mass flow ratio of 3%. Steady injection is compared with flapping injection. With flapping injection turned on, the injection angle of each injector is varied sinusoidally in the nozzle exit plane and the variation is the same for all injectors. This fluid dynamics video is submitted to the APS DFD Gallery of Fluid Motion 2013 at the 66 the Annual Meeting of the American Physical Society, Division of Fluid Dynamics (24-26 November, Pittsburgh, PA, USA).
48.	Hafsteinsson, Haukur, 1984, et al. (författare) Supersonic Jet Noise Reduction Using Steady Injection and Flapping Injection 2013 Ingår i: 19th AIAA/CEAS Aeroacoustics Conference, Berlin, Germany, May 27-29, 2013. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134 ; , s. 140- Konferensbidrag (refereegranskat)
49.	Hjelmgren, Hans, 1960, et al. (författare) Electro-thermal simulations of a microwave 4H-SiC MESFET on high purity semi-insulating substrate 2007 Ingår i: Solid-State Electronics. - : Elsevier BV. - 0038-1101. ; 51:8, s. 1144-1152 Tidskriftsartikel (refereegranskat)abstract DC and small-signal electro-thermal simulations of a 4H-SiC microwave power MESFET on a high purity semi-insulating substrate are compared to measurements. The focus is on the electron transport, substrate traps, and thermal heating. The doping concentrations are described by measured SIMS profiles, and the material parameters are in accordance with published results. Although the simulated MESFET has a p-buffer and a high purity substrate, the simulations show that the density of shallow traps affects the device characteristics.The very good agreement between simulated and measured DC and small-signal characteristics indicates that the models for electron mobility, substrate traps, and heating are the most important to achieve good agreement with measured data.
50.	Hjelmgren, Hans, 1960, et al. (författare) Extraction of an Electrothermal Mobility Model for AlGaN/GaN Heterostructures 2012 Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 59:12, s. 3344-3349 Tidskriftsartikel (refereegranskat)abstract An electron mobility model for AlGaN/GaN heterostructures taking both the lattice temperature and electric field into account is proposed. Numerical device simulation of an ungated Al0.25Ga0.75N/GaN HEMT structure on 4H-SiC substrate is compared to measured electrical characteristics. Mobility model parameters are extracted by comparing iso-thermal numerical simulations with microwave (6 GHz) large signal measurements. The extracted model was used in static simulations, showing good agreement with measurements.

Skapa referenser, mejla, bekava och länka

Länka till träfflistan

Träfflista för sökning "WFRF:(Andersson Niklas 1976) "

Avgränsa träffmängd

År