| 1. |
- Grigoriev, Igor A., et al.
(författare)
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A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation
- 2013
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 25:10, s. 105112-
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Tidskriftsartikel (refereegranskat)abstract
- The explicit algebraic Reynolds stress model of Wallin and Johansson [J. Fluid Mech. 403, 89 (2000)] is extended to compressible and variable-density turbulent flows. This is achieved by correctly taking into account the influence of the mean dilatation on the rapid pressure-strain correlation. The resulting model is formally identical to the original model in the limit of constant density. For two-dimensional mean flows the model is analyzed and the physical root of the resulting quartic equation is identified. Using a fixed-point analysis of homogeneously sheared and strained compressible flows, we show that the new model is realizable, unlike the previous model. Application of the model together with a K - omega model to quasi one-dimensional plane nozzle flow, transcending from subsonic to supersonic regime, also demonstrates realizability. Negative "dilatational" production of turbulence kinetic energy competes with positive "incompressible" production, eventually making the total production negative during the spatial evolution of the nozzle flow. Finally, an approach to include the baroclinic effect into the dissipation equation is proposed and an algebraic model for density-velocity correlations is outlined to estimate the corrections associated with density fluctuations. All in all, the new model can become a significant tool for CFD (computational fluid dynamics) of compressible flows.
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| 2. |
- Grigoriev, Igor A., et al.
(författare)
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Capturing turbulent density flux effects in variable density flow by an explicit algebraic model
- 2015
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 27:4
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Tidskriftsartikel (refereegranskat)abstract
- The explicit algebraic Reynolds stress model of Grigoriev et al. ["A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation," Phys. Fluids 25, 105112 (2013)] is extended to account for the turbulent density flux in variable density flows. The influence of the mean dilatation and the variation of mean density on the rapid pressure-strain correlation are properly accounted for introducing terms balancing a so-called "baroclinic" production in the Reynolds stress tensor equation. Applying the weak-equilibrium assumption leads to a self-consistent formulation of the model. The model together with a K - ω model is applied to a quasi-one-dimensional plane nozzle flow transcending from subsonic to supersonic regimes. The model remains realizable with constraints put on the model parameters. When density fluxes are taken into account, the model is less likely to become unrealizable. The density variance coupled with a "local mean acceleration" also can influence the model acting to increase anisotropy. The general trends of the behaviour of the anisotropy and production components under the variation of model parameters are assessed. We show how the explicit model can be applied to two- and three-dimensional mean flows without previous knowledge of a tensor basis to obtain the general solution. Approaches are proposed in order to achieve an approximate solution to the consistency equation in cases when analytic solution is missing. In summary, the proposed model has the potential to significantly improve simulations of variable-density flows.
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| 3. |
- Grigoriev, I. A., et al.
(författare)
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Explicit algebraic and differential Reynolds stress model application to homogeneously sheared and compressed turbulence
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Konferensbidrag (refereegranskat)abstract
- An explicit algebraic and differential Reynolds stress models (EARSM and DRSM) are used to investigate the influence of homogeneous shear and compression on the behaviour of turbulence in the limit of rapid distortion theory (RDT). EARSM is shown to give realizable results and to preserve RDT regime, unlike the eddy-viscosity model (EVM). The DRSM version of our model is in reasonable agreement with RDT theory.
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| 4. |
- Izosimov, Viacheslav, et al.
(författare)
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Requirement Decomposition and Testability in Development of Safety-Critical Automotive Components
- 2012
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Konferensbidrag (refereegranskat)abstract
- 2ISO26262 is a recently approved standard for functional safety in road vehicles. It provides guidelines on minimization of unreasonable safety risks during development of embedded systems in road vehicles. However, the development process specified in ISO26262 involves a number of steps that will require changing traditional and well established development processes. In a transition phase, however, due to lack of tool support, the steps may be performed manually, increasing the risk for delays and increased cost. This paper describes a case study in which we have successfully worked with traceability and testability of functional safety requirements, as well as safety requirements assigned to a testing tool that automates integration and verification steps, leading to standard-compliant tool qualification. Our tool qualification method employs fault injection as a validation method to increase confidence in the tool. Our case study will help to avoid many of the new pitfalls that can arise when attempting to realize standard-compliant development.
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| 5. |
- Jirasek, A., et al.
(författare)
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Computational study of the high-lift a-airfoil
- 2001
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Ingår i: Journal of Aircraft. - : American Institute of Aeronautics and Astronautics (AIAA). - 0021-8669 .- 1533-3868. ; 38:4, s. 769-772
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Tidskriftsartikel (refereegranskat)
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| 6. |
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| 7. |
- Lundin, Daniel, et al.
(författare)
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Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering
- 2009
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Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 18:4
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Tidskriftsartikel (refereegranskat)abstract
- Current and voltage have been measured in a pulsed high power impulse magnetron sputtering (HiPIMS) system for discharge pulses longer than 100 mu s. Two different current regimes could clearly be distinguished during the pulses: (1) a high-current transient followed by (2) a plateau at lower currents. These results provide a link between the HiPIMS and the direct current magnetron sputtering (DCMS) discharge regimes. At high applied negative voltages the high-current transient had the characteristics of HiPIMS pulses, while at lower voltages the plateau values agreed with currents in DCMS using the same applied voltage. The current behavior was found to be strongly correlated with the chamber gas pressure, where increasing gas pressure resulted in increasing peak current and plateau current. Based on these experiments it is suggested here that the high-current transients cause a depletion of the working gas in the area in front of the target, and thereby a transition to a DCMS-like high-voltage, lower current regime.
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| 8. |
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| 9. |
- Skagerlund, J., et al.
(författare)
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Evaluation of an automatic method to extract the grating coupling coefficient in different types of fabricated DFB lasers
- 1998
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Ingår i: IEEE Journal of Quantum Electronics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9197 .- 1558-1713. ; 34, s. 141-146
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Tidskriftsartikel (refereegranskat)abstract
- Distributed feedback (DFB) laser parameters such as grating coupling coefficient, effective indices, facet reflectances, and the phases of facet reflectances have been determined using a method based on least-square fitting of theoretical spectra to measured, subthreshold DFB laser emission spectra. The only inputs needed are geometrical parameters such as length, grating period, and internal grating phase shifts. A larger number of devices have been successfully characterized, and consistent results have been obtained in both 1.3- mu;m multi-quantum-well (MQW) DFB lasers with both facets as-cleaved, and in 1.55- mu;m MQW DFB lasers with no, one, or two facets as-cleaved
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| 10. |
- Vestergren, A., et al.
(författare)
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Generalized anisotropic scaling theory and the transverse Meissner transition
- 2005
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 94:8
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Tidskriftsartikel (refereegranskat)abstract
- We consider a depinning transition in vortex systems with columnar disorder and tilted applied magnetic fields. From scaling arguments and Monte Carlo simulations, we find that this transverse Meissner transition is governed by a fixed point which is anisotropic in all three directions. This generalization of conventional anisotropic scaling means that the correlation length in different directions diverges with different rates, and we derive exact results for the anisotropy exponents. We make predictions which can be tested in experiments on superconductors with columnar disorder.
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