| 1. |
- Aryal, Pradip, et al.
(författare)
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Comparative study of the main electromagnetic models applied to melt pool prediction with gas metal arc : Effect on flow, ripples from drop impact, and geometry
- 2022
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier. - 0017-9310 .- 1879-2189. ; 194
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Tidskriftsartikel (refereegranskat)abstract
- The present work concerns the electromagnetic force models in computational fluid dynamics simulations of melt pools produced with electric arcs. These are commonly applied to gas metal arcs with metal transfer, in welding and additive manufacturing. Metal drop impact on the melt pool is thus included in this study. The electromagnetic force models applied in literature use either numerical solutions of Poisson equations or one of the two analytical models developed by Kou and Sun, or Tsao and Wu. These models rely on assumptions for which the effect on the melt pool predictions remains to be understood. The present work thoroughly investigates those assumptions and their effects. It has been supported by dedicated experimental tests that did provide estimates of unknown model parameters and validation data. The obtained results show that the assumptions that fundamentally distinguish these three models change the electromagnetic force, including the relation between its components. These changes, which can also be spatially non-uniform, are large. As a result, these models lead to significantly different recirculation flow pattern, thermal convection, melt pool morphology, bead dimensions, and free surface response to the metal transfer. We conclude by proposing conditions in which each of these models is suited or questionable.
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| 2. |
- Aryal, Pradip, et al.
(författare)
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Effect of Substrate Orientation on Melt Pool during Multi-Layer Deposition in V-Groove with Gas Metal Arc
- 2021
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Ingår i: Proceedings of the World Congress on Mechanical, Chemical, and Material Engineering. - : Avestia Publishing. - 2369-8136. - 9781927877616
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Konferensbidrag (refereegranskat)abstract
- Thermo-fluid dynamic and experimental approaches are used to investigate the influence of 20° uphill, downhill and sideway substrate orientation during metal deposition over a previously deposited bead in a V-groove. The computational fluid dynamic model with free surface deformation and metal transfer gives insight into the melt pool flow and causes of defect formation observed on the solidified beads. The experimental metallographs, high-speed images and computational results show good agreement. It is found that the deposition of a second layer on a smooth first layer cooled down to room temperature leads to large changes in melt pool flow pattern at 20° substrate inclination compared to flat condition. It results in undercut and humps with the uphill orientation and undercut with the side inclination. Therefore, lower angle range is necessary for multilayer gas metal arc deposition for these two last configurations.
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| 3. |
- Buntic-Ogor, Ivana, et al.
(författare)
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An Adaptive Turbulence Model for Swirling Flow
- 2006
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Ingår i: Conference on Turbulence and Interactions, TI2006, May 29 - June 2, 2006, Porquerolles, France.
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Konferensbidrag (refereegranskat)abstract
- Swirling flows are very common in technical applications, especially in hydraulic machinery, and they require rather sophisticated modeling. At present, an applicative method for simulating unsteady flows is Very Large Eddy Simulation (VLES). In VLES, large turbulence structures are resolved in time and space, while the small scales are modeled with an adequate turbulence model. Therefore the turbulence model must be able to distinguish between the resolved and the unresolved scales. In order to accomplish this the method used in this work employs an adaptive and dynamic filtering technique that restricts the turbulence model to only predict the effect of non-resolvable turbulent scales. The results obtained from using the standard k-ε model is compared to those using the extended model of Chen and Kim, with and without the filtering approach. The modified k-ε model of Reif et al. is also investigated together with the filtering approach. The models are implemented in the FENFLOSS and the CALC-PMB CFD codes. The chosen test cases are swirling flow in a straight pipe and swirling flow through a straight conical diffuser.
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| 4. |
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| 5. |
- Gyllenram, Walter, 1971, et al.
(författare)
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Design and Validation of a Scale-Adaptive Filtering Technique for LRN Turbulence Modeling of Unsteady Flow
- 2008
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Ingår i: Journal of Fluid Engineering. ; Volume 130:Issue 5, s. 10-
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Tidskriftsartikel (refereegranskat)abstract
- An adaptive low-pass filtering procedure for the modeled turbulent length and time scales is derived and applied to Wilcox' original low reynolds number k-omega turbulence model. It is shown that the method is suitable for complex industrial unsteady flows in cases where full large eddy simulations (LESs) are unfeasible. During the simulation, the modeled length and time scales are compared to what can potentially be resolved by the computational grid and time step. If the modeled scales are larger than the resolvable scales, the resolvable scales will replace the modeled scales in the formulation of the eddy viscosity. The filtered k-omega model is implemented in an in-house computational fluid dynamics (CFD) code, and numerical simulations have been made of strongly swirling flow through a sudden expansion. The new model surpasses the original model in predicting unsteady effects and producing accurate time-averaged results. It is shown to be superior to the wall-adpating local eddy-viscosity (WALE) model on the computational grids considered here, since the turbulence may not be sufficiently resolved for an accurate LES. Because of the adaptive formulation, the filtered k-omega model has the potential to be successfully used in any engineering case where an LES is unfeasible and a Reynolds (ensemble) averaged Navier–Stokes simulation is insufficient.
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| 6. |
- Gyllenram, Walter, 1971, et al.
(författare)
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Large Eddy Simulation of Turbulent Swirling Flow Through a Sudden Expansion
- 2006
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Ingår i: 23rd IAHR Symposium, Yokohama, Japan, October 2006.
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Konferensbidrag (refereegranskat)abstract
- Turbulent swirling flow through a sudden expansion is investigated numerically using Large Eddy Simulation (LES). The flow resembles the flow in a draft tube of a water turbine that is working at part load. The swirling inflow is subject to a strong adverse pressure gradient and the symmetry of the flow breaks down close to the inlet. This gives rise to an oscillating, helicoidal vortex core which in turn creates a highly unsteady and turbulent flow field. In this work, the large-scale turbulent structures are numerically resolved, and detailed information about the flow characteristics is obtained. The oscillating flow is analysed using Fourier transforms of the wall pressure at different downstream locations. The most dominant frequency corresponds to the rotational rate of the precessing vortex core, and it is found that this frequency is constant throughout the domain. The results of two simulations using numerical discretization schemes of different order are compared. It is shown that the frequency of the precessing vortex core is not sensitive to the choice of discretization. However, the lower frequencies of the flow depend to a higher extent on the numerical accuracy. To validate the results, the computed velocities are averaged and compared to experimental data. The agreement is good. The Reynolds stress tensor is also computed and analysed. It is found that large degrees of turbulent anisotropy are found only in the region that is dominated by the oscillating vortex core. Further downstream, the degree of turbulent anisotropy is almost negligible despite the relatively higher level of swirl.
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| 7. |
- Gyllenram, Walter, 1971, et al.
(författare)
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Numerical Investigations of Swirling flow in a Conical Diffuser
- 2004
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Ingår i: 22nd IAHR Symposium, Stockholm, Sweden, June 29 - July 2, 2004. ; A, s. A09-2 (1-10)-
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Konferensbidrag (refereegranskat)abstract
- When a hydraulic power plant is operating at off-design, a swirling flow exits the runner and is convected through the draft tube. The swirling flow gives rise to several features that decrease the efficiency and/or may damage the construction. As a step towards making reliable numerical predictions of the swirling flow in draft tubes, steady computations in an idealised model have been carried out. The model geometry is a straight conical diffuser and O. G. Dahlhaug of NTNU, Trondheim, Norway, has provided experimental data. In this paper, numerical 3D RANS investigations are quantitatively compared to the available experimental data. Good agreement with experimental data was obtained. The discrepancies are partly reminiscent of the nature of the k-omega turbulence model that was used in this work. Given symmetric geometry and boundary conditions, a fluid flow is most often thought to behave in an equally symmetric manner. If the flow is swirling, this is not generally true. Due to the unstable properties of the symmetric solutions for the averaged Navier-Stokes equations, the flow will collapse into an asymmetric mode. In the first computational cases of this paper, the disturbance that triggers the instability is shown to be imperfections in the CAD-geometry. Also included is a discussion concerning the development of counter-rotating vortices in the boundary layer of swirling flow in a circular pipe.
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| 8. |
- Gyllenram, Walter, 1971, et al.
(författare)
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On the failure of the quasicylindrical approximation and the connection to vortex breakdown in turbulent swirling flow
- 2007
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:4
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Tidskriftsartikel (refereegranskat)abstract
- This paper analyses the properties of viscous swirling flow in a pipe. The analysis is based on the time-averaged quasi-cylindrical Navier-Stokes equations and is applicable to steady, unsteady and turbulent swirling flow. A method is developed to determine the critical level of swirl (vortex breakdown) for an arbitrary vortex. The method can also be used for an estimation of the radial velocity profile if the other components are given or measured along a single radial line. The quasi-cylindrical equations are rearranged to yield a single ordinary differential equation for the radial distribution of the radial velocity component. The equation is singular for certain levels of swirl. It is shown that the lowest swirl level at which the equation is singular corresponds exactly to the sufficient condition for axisymmetric vortex breakdown as derived by Wang and Rusak [J. Fluid Mech. {340}, 177-223 (1997)] and Rusak et al. [AIAA J. {36}, 1848-1853 (1998)]. In narrow regions around the critical levels of swirl, the solution violates the quasi-cylindrical assumptions and the flow must undergo a drastic change of structure. The critical swirl level is determined by the sign change of the smallest eigenvalue of the discrete linear operator which relates the radial velocities to effects of viscosity and turbulence. It is shown that neither viscosity nor turbulence directly alters the critical level of swirl.
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| 9. |
- Gyllenram, Walter, 1971, et al.
(författare)
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Very Large Eddy Simulations of Draft Tube Flow
- 2006
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Ingår i: 23rd IAHR Symposium, Yokohama, Japan, October 2006.
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Konferensbidrag (refereegranskat)abstract
- The objective of this work is to improve numerical predictions of unsteady turbulent flows in the draft tubes of hydraulic power plants. The standard two-equation turbulence models are known to have a strong damping effect on the resolved turbulence in this type of flow. In order to reduce this negative influence of the model, while retaining the usually satisfying near-wall behaviour, a dynamic filtering technique of the turbulent length and time scales is generalised, employed and evaluated. The filter limits the influence of the modeled turbulent length and time scales on the mean flow in regions where unsteadiness can potentially be resolved. The Wilcox (1988) $k-\omega$ turbulence model was chosen as basis for the investigations, and the effects of five different filter widths were examined. The original non-filtered model is also evaluated. A swirling flow through a straight axisymmetric diffuser was chosen as a test case and detailed measurements carried out by Clausen et al. were used to validate the numerical results. The influence of the filtering approach on the resolved frequencies and the time averaged solutions were analysed. It is shown that the filtering procedure gives better predictions of the time-averaged velocity field and more information on the large scale unsteadiness.
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| 10. |
- Lehr, C., et al.
(författare)
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Unveiling the two-proton halo character of 17 Ne: Exclusive measurement of quasi-free proton-knockout reactions
- 2022
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Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 827
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Tidskriftsartikel (refereegranskat)abstract
- The proton drip-line nucleus 17Ne is investigated experimentally in order to determine its two-proton halo character. A fully exclusive measurement of the 17Ne(p,2p)16F→15⁎O+p quasi-free one-proton knockout reaction has been performed at GSI at around 500 MeV/nucleon beam energy. All particles resulting from the scattering process have been detected. The relevant reconstructed quantities are the angles of the two protons scattered in quasi-elastic kinematics, the decay of 16F into 15O (including γ decays from excited states) and a proton, as well as the 15O+p relative-energy spectrum and the 16F momentum distributions. The latter two quantities allow an independent and consistent determination of the fractions of l=0 and l=2 motion of the valence protons in 17Ne. With a resulting relatively small l=0 component of only around 35(3)%, it is concluded that 17Ne exhibits a rather modest halo character only. The quantitative agreement of the two values deduced from the energy spectrum and the momentum distributions supports the theoretical treatment of the calculation of momentum distributions after quasi-free knockout reactions at high energies by taking into account distortions based on the Glauber theory. Moreover, the experimental data allow the separation of valence-proton knockout and knockout from the 15O core. The latter process contributes with 11.8(3.1) mb around 40% to the total proton-knockout cross section of 30.3(2.3) mb, which explains previously reported contradicting conclusions derived from inclusive cross sections.
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| 11. |
- Marganiec, J., et al.
(författare)
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Coulomb breakup of 17Ne from the viewpoint of nuclear astrophysics
- 2012
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Ingår i: Proceedings of Science. - Proceedings of Science : Sissa. - 1824-8039.
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Konferensbidrag (refereegranskat)abstract
- By the Coulomb breakup of 17Ne, the time-reversed reaction 15O(2p,γ)17Ne has been studied. This reaction might play an important role in the rp process, as a break-out reaction of the hot CNO cycle. The secondary 17Ne ion beam with an energy of 500 MeV/nucleon has been dissociated in a Pb target. The reaction products have been detected with the LAND-R3B experimental setup at GSI. The preliminary differential and integral Coulomb dissociation cross section sCoul has been determined, which then will be converted into a photo-absorption cross section sphoto, and a two-proton radiative capture cross section σcap. Additionally, information about the structure of the 17Ne, a potential two-proton halo nucleus, will be received. The analysis is in progress. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
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| 12. |
- Nilsson, Anders, et al.
(författare)
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Temporally fluctuating prey and coexistence among unequal conspecific interferers
- 2003
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Ingår i: Oikos. - : Wiley. ; 101:2, s. 411-415
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Tidskriftsartikel (refereegranskat)abstract
- Coexistence among unequal conspecific interferers should be unlikely to persist if stronger interferers always experience a relative fitness increment from their higher foraging rates. In this study, we suggest that decreased relative costs to weaker interferers with increasing temporal fluctuations in prey availability may be a mechanism enhancing coexistence of unequal conspecific interferers. Previous work on fluctuation and coexistence has dealt with oscillations over a time-scale measured in generations of competitor species and their resources, while our work shows that fluctuations in prey availability facilitate coexistence of different phenotypic strategies within species and generations, and over short time-scales. With increasing amplitude of temporal fluctuation about an average prey density, cumulative intakes for differently strong interferers are affected differently. Because of the prey-dependent effect of interference, high amplitudes of fluctuation allow for relatively lower foraging-rate costs in weaker interferers, which decreases the difference in foraging success between strong and weaker interferers. This decreased difference in foraging success could thus significantly relax the conditions allowing for unequal interferer coexistence.
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| 13. |
- Roos, Håkan, et al.
(författare)
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Air bubble release and flow-induced forces in stent grafts
- 2017
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Ingår i: Engineering Health.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Background: Stent grafts are used to treat aortic aneurysms, as a minimal invasive alternative to open surgery. The aorta is accessed via percutaneous punctures in the groins and insertion of guide wires. A stent graft is advanced over the guide wire and deployed in the desired position, using x-ray fluoroscopic guidance. The procedure significantly reduces short-term morbidity and mortality compared to open repair, but has a higher rate of late complications and re-interventions. Two possible complications are: release of air bubbles during thoracic stent graft deployment may lead to cerebral embolization and immediate stroke, and long-term migration due to flow-induced forces may lead to leakage into the aneurysm sack and remaining risk of aortic rupture. Objectives: Stent grafts are compressed in a long, soft and thin delivery catheter. Once the stent graft is in position, the surgeon withdraws the catheter, allowing the self-expanding stent graft to expand radially to the artery wall, or to a previously inserted stent graft. The stent graft is kept in place by the expansion force of the over-sized stent, and in some cases by anchoring hooks. Despite a strict preparation procedure, there is a risk that air is trapped in the compressed stent graft and released into the aorta during deployment. The present work describes two of our studies. In the first experimental study we measure the total amount of air released by two different brands of thoracic stent grafts and investigate the size and origin of the air bubbles. In the second experimental and numerical study angulated iliac limbs stent grafts are mounted in a pulsatile flow model to determine the flow-induced displacement forces under a variety of conditions.Methods: The air release measurements and visualizations are performed by inserting and deploying stent grafts in a closed water-filled container. A high-resolution and high-speed camera is used to capture the air release process, and the total amount of air collected in the container is measured. An experimental pulsatile flow model, with conditions similar to aortic in vivo conditions, is used to measure the proximal and distal displacement forces of iliac stent grafts under different angulations, pulsation frequencies, pressures, and configurations (tapered, tubular, bellbottom). Fluid-structure interaction simulations are used to determine the forces numerically.Results: We found that air is released during deployment of thoracic stent grafts and that the largest bubbles come from the proximal end. There is a significant difference between the two brands of stent grafts that have been investigated. The flow-induced forces are under some conditions comparable to the forces required to extract the iliac limb stent grafts. Both the forces and the graft movements increase with angulation and with perfusion pressure, but not with stroke rate. The distal displacement forces are particularly large for the bellbottom configuration. The origin of the forces is mainly the pressure, while the contributions of shear force and flow redirection are insignificant. The flexibility of the stent graft material reduces the forces (by 15% under the conditions in the numerical simulations in the present work).Conclusions: New packing and/or preparation procedures should be developed to reduce the amount of air released during deployment of thoracic stent grafts. Improved anchoring of the distal ends of stent grafts should be considered.References[1] Displacement Forces in Iliac Landing Zones and Stent Graft Interconnections in Endovascular Aortic Repair: An Experimental Study. Roos H, Ghaffari M, Falkenberg M, Chernoray V, Jeppsson A, Nilsson H.European Journal of Vascular and Endovascular Surgery 2014:47(3):262-267.
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| 14. |
- Roos, Håkan, 1967, et al.
(författare)
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Displacement Forces in Iliac Landing Zones and Stent Graft Interconnections in Endovascular Aortic Repair: An Experimental Study
- 2014
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 47:3, s. 262-267
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: Stent graft migration influences the long-term durability of endovascular aortic repair. Flow-induced displacement forces acting on the attachment zones may contribute to migration. Proximal fixation of aortic stent grafts has been improved by using hooks, while distal fixation and stent graft interconnections depend on self-expansion forces only. We hypothesized that flow-induced displacement forces would be significant at the distal end, and would correlate with graft movements. Methods: As part of an experimental study, an iliac limb stent graft was inserted in a pulsatile flow model similar to aortic invivo conditions, and fixed-mounted at its proximal and distal ends to strain gauge load cells. Peak displacement forces at both ends and pulsatile graft movement were recorded at different graft angulations (0-90°), perfusion pressures (145/80, 170/90, or 195/100mmHg), and stroke frequencies (60-100b.p.m.). Results: Flow-induced forces were of the same magnitude at the proximal and distal end of the stent graft (peak 1.8N). Both the forces and graft movement increased with angulation and perfusion pressure, but not with stroke rate. Graft movement reached a maximum of 0.29±0.01mm per stroke despite fixed ends. There were strong correlations between proximal and distal displacement forces (r=0.97, p<.001), and between displacement forces and graft movement (r=0.98, p<.001). Conclusions: Pulsatile flow through a tubular untapered stent graft causes forces of similar magnitude at both ends and induces pulsatile graft movements in its unsupported mid-section. Peak forces are close to those previously reported to be required to extract a stent graft. The forces and movements increase with increasing graft angulation and perfusion pressure. Improved anchoring of the distal end of stent grafts may be considered. © 2013 European Society for Vascular Surgery.
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| 15. |
- Roos, Håkan, 1967, et al.
(författare)
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Displacement Forces in Stent Grafts: Influence of Diameter Variation and Curvature Asymmetry
- 2016
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 52:2, s. 150-156
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: Long-term durability after endovascular aortic repair is influenced by stent graft migration causing types I and III endoleaks. Flow induced displacement forces have been shown to have the potential to cause migration. In this study, the influence of the distal diameter of iliac limb stent grafts and the shape of graft curvature on flow induced displacement forces, were investigated. Methods: In an experimental pulsatile flow model mimicking aortic conditions in vivo, flow induced displacement forces at the proximal and distal ends of iliac limb stent grafts were studied at different angles (0-90 degrees) and perfusion pressures (145/80, 170/90, 195/100 mmHg). Bell-bottomed, tapered, and non-tapered stent grafts and also asymmetric stent graft curvatures at 90 bend were studied. Measurements of graft movement were performed at all studied angulations and graft shapes. Results: For all stent graft diameters, flow induced displacement forces increased with higher pressure and increased stent graft angulation. Forces in the bell-bottom graft were considerably higher than in tapered and non-tapered grafts, with a markedly elevated peak force at the distal end (proximal end, 2.3 +/- 0.06 N and distal end, 6.9 +/- 0.05 N compared with 1.7 +/- 0.08 N and 1.6 +/- 0.08 N in non-tapered grafts; p <.001 both). Peak forces in tapered and non-tapered grafts were not significantly different between the proximal and distal end. In asymmetric stent graft curvatures, a significant increase in displacement forces was observed in the attachment zone that was closest to the stent graft bend. Graft movement increased with greater displacement forces. Conclusion: Flow induced displacement forces in iliac limb stent grafts are significant and are influenced by distal stent graft diameter and the shape of the graft curvature. The displacement forces are particularly high at the large distal end of bell-bottom grafts. Wide iliac arteries treated with bell-bottom stent grafts may require more vigilant surveillance and improved stent graft fixation.
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| 16. |
- Roos, Håkan, et al.
(författare)
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Fluid Pressure Derived Force is the Main Contributor to Iliac Limb Displacement Forces – Shear Force and Redirection of Flow are Negligible
- 2019
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 58:6 (supplement 1), s. e18-e19
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Tidskriftsartikel (refereegranskat)abstract
- Introduction - Complications after EVAR, including aortic rupture, continue to be a problem also in long term follow-up1. There is still a need for life-long surveillance and late re-interventions raises the question about long term EVAR-durability. Additional iliac stent grafts due to distal endoleak type I are among the more frequent among these late re-interventions2,3,4. There are indications that graft migration at iliac landing zones and graft interconnections are among the most common causes and that wide iliac arteries predispose for these complications5. Flow induced displacement forces have, in an experimental model, been shown to be significant also at distal landing zones in EVAR stent grafts6 and particularly high in grafts with large distal diameters7. These findings indicate that late EVAR failures are linked to flow induced displacement forces and subsequent graft migration. We have therefore conducted a study of fluid structure interaction with the aim to differentiate the magnitude of the different force components and numerically evaluate the forces acting on iliac limb stent grafts in different graft configurations. Methods - The displacement forces in iliac limb stent grafts were numerically evaluated using a finite volume approach for fluid-structure interaction (FSI) with the open source tool FOAM-extend-3.1. The grafts were modelled with homogeneous properties in three configurations; tapered (16 mm proximal diameter, 12 mm distal diameter), non tapered (16-16 mm) and bellbottom (16-27 mm), all at 90° angulation (see 16-27 in figure). Experimentally determined pulsatile conditions were applied for different pressures; 145/80, 170/90, 195/100 mmHg. Results - Stent graft displacement forces increased with higher fluid pressure in all graft configurations. Maximum forces in bellbottom grafts (proximal end 2.3 N, distal end 8.1 N) were particularly high compared to tapered (proximal end 2.5 N, distal end 1.2 N) and non tapered grafts (proximal end 2.5 N, distal end 2.5 N). The effects of shear stress and redirection of flow were together less than 2.8 % of the total forces in all graft configurations, whereas pressure derived forces were the main contributor to the forces acting on the stent graft. The flexibility of the stent graft absorbed up to 15 % of the forces. Conclusion - The forces acting on stent grafts during pulsatile flow are significant and particularly high in the distal end of grafts with large distal diameter. The main contributor to these displacement forces is pressure while the contribution of shear and redirection of flow are negligible. The flexibility of the stent graft tends to absorb a significant part of the forces.
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| 17. |
- Sundström, Johan, Professor, 1971-, et al.
(författare)
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Risk factors for subarachnoid haemorrhage : a nationwide cohort of 950 000 adults
- 2019
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Ingår i: International Journal of Epidemiology. - : Oxford University Press. - 0300-5771 .- 1464-3685. ; 48:6, s. 2018-2025
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: Subarachnoid haemorrhage (SAH) is a devastating disease, with high mortality rate and substantial disability among survivors. Its causes are poorly understood. We aimed to investigate risk factors for SAH using a novel nationwide cohort consortium.METHODS: We obtained individual participant data of 949 683 persons (330 334 women) between 25 and 90 years old, with no history of SAH at baseline, from 21 population-based cohorts. Outcomes were obtained from the Swedish Patient and Causes of Death Registries.RESULTS: During 13 704 959 person-years of follow-up, 2659 cases of first-ever fatal or non-fatal SAH occurred, with an age-standardized incidence rate of 9.0 [95% confidence interval (CI) (7.4-10.6)/100 000 person-years] in men and 13.8 [(11.4-16.2)/100 000 person-years] in women. The incidence rate increased exponentially with higher age. In multivariable-adjusted Poisson models, marked sex interactions for current smoking and body mass index (BMI) were observed. Current smoking conferred a rate ratio (RR) of 2.24 (95% CI 1.95-2.57) in women and 1.62 (1.47-1.79) in men. One standard deviation higher BMI was associated with an RR of 0.86 (0.81-0.92) in women and 1.02 (0.96-1.08) in men. Higher blood pressure and lower education level were also associated with higher risk of SAH.CONCLUSIONS: The risk of SAH is 45% higher in women than in men, with substantial sex differences in risk factor strengths. In particular, a markedly stronger adverse effect of smoking in women may motivate targeted public health initiatives.
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| 18. |
- Tokarev, Mikhail, 1982, et al.
(författare)
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DIC for Surface Motion Analysis Applied to Displacement of a Stent Graft for Abdominal Aortic Repair in a Pulsating Flow
- 2015
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Ingår i: PIV15; 11th International Symposium on Particle Image Velocimetry, Santa Barbara, California, USA, September 14-16, 2015. ; , s. 1-12
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Stent graft migration has been recognized to influence the long-term durability of endovascular aortic repair. Flow-induced displacement forces acting on the attachment zones may contribute to this migration. An experimental perfusion model consisting of the flow loop described by Roos et al. 2014 was used for further characterization of the pulsating flow induced stent graft movements with monocular and stereoscopic configurations of an optical imaging system. This paper adds new information on displacement measurement accuracy and 3D deformation analysis of the stent graft, which is used for abdominal aortic aneurysm treatment. The work describes used modification of Soloff’s Stereo PIV reconstruction algorithm for surface motion analysis. It was found that the oscillation of the stent graft’s body in the perpendicular direction to the front plane was 5 times less than side movements of the bent stent graft. These results can be used for further studies on different stent graft geometrical configurations and CFD simulations using fluid-structure interaction approach.
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| 19. |
- Wamers, F., et al.
(författare)
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Diverse mechanisms in proton knockout reactions from the Borromean nucleus Ne-17
- 2023
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Ingår i: European Physical Journal A. - : Springer. - 1434-6001 .- 1434-601X. ; 59
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Tidskriftsartikel (refereegranskat)abstract
- Nucleon knockout experiments using beryllium or carbon targets reveal a strong dependence of the quenching factors, i.e., the ratio (Rs) of theoretical to the experimental spectroscopic factors (C2S), on the proton-neutron asymmetry in the nucleus under study. However, this dependence is greatly reduced when a hydrogen target is used. To understand this phenomenon, exclusive H-1(Ne-17, 2p F-16) and inclusive C-12(Ne-17, 2p 1(6)F)X, 1(2)C(Ne-17, F-16)X as well as 1H(Ne-17,(16) F)X(X-denotes undetected reaction products) reactions with F-16 in the ground and excited states were anal- ysed. The longitudinal momentum distribution of F-16 and the correlations between the detached protons were studied. In the case of the carbon target, there is a significant deviation from the predictions of the eikonal model. The eikonal approximation was used to extract spectroscopic factor values (CS)-S-2. The experimental (CS)-S-2 value obtained with C target is markedly lower than that for H target. This is interpreted as rescattering due to simultaneous nucleon knockout from both reaction partners, Ne-17 and C-12.
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| 20. |
- Wamers, F., et al.
(författare)
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First Observation of the Unbound Nucleus Ne-15
- 2014
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 112:13
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Tidskriftsartikel (refereegranskat)abstract
- We report on the first observation of the unbound proton-rich nucleus Ne-15. Its ground state and first excited state were populated in two-neutron knockout reactions from a beam of 500 MeV/u Ne-17. The Ne-15 ground state is found to be unbound by 2.522(66) MeV. The decay proceeds directly to O-13 with simultaneous two-proton emission. No evidence for sequential decay via the energetically allowed 2(-) and 1(-) states in F-14 is observed. The Ne-15 ground state is shown to have a strong configuration with two protons in the (sd) shell around O-13 with a 63(5)% (1s(1/2))(2) component.
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| 21. |
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| 22. |
- Zhang, Renyun, et al.
(författare)
-
Soap-film coating : High-speed deposition of multilayer nanofilms
- 2013
-
Ingår i: Scientific Reports. - Nature Publishing Group : Springer Science and Business Media LLC. - 2045-2322. ; 3, s. Art. no. 1477-
-
Tidskriftsartikel (refereegranskat)abstract
- The coating of thin films is applied in numerous fields and many methods are employed for the deposition of these films. Some coating techniques may deposit films at high speed; for example, ordinary printing paper is coated with micrometre-thick layers of clay at a speed of tens of meters per second. However, to coat nanometre thin films at high speed, vacuum techniques are typically required, which increases the complexity of the process. Here, we report a simple wet chemical method for the high-speed coating of films with thicknesses at the nanometre level. This soap-film coating technique is based on forcing a substrate through a soap film that contains nanomaterials. Molecules and nanomaterials can be deposited at a thickness ranging from less than a monolayer to several layers at speeds up to meters per second. We believe that the soap-film coating method is potentially important for industrial-scale nanotechnology.
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| 23. |
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| 24. |
- Abedi, Hamidreza, 1979, et al.
(författare)
-
Development of blade element momentum (BEM) method for hydropower
- 2022
-
Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 1079:1
-
Konferensbidrag (refereegranskat)abstract
- The BEM method is extensively used for analyzing the aerodynamic performance of wind turbines and marine propellers. It is computationally fast and is easily implemented while it can give fairly accurate results. Application of the BEM method to predict the forces acting on rotor blades for a model scale axial shaft-driven Counter-Rotating Pump-Turbine (CRPT) is investigated. Some modifications have been proposed to adopt the classical BEM method for CRPT machine and the results are validated against results from Computational Fluid Dynamics (CFD). The results display that the proposed modifications can improve the loading predicted by BEM. However, the improvements are more pronounced in pump mode rather than turbine mode.
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| 25. |
- Aksouh, F., et al.
(författare)
-
STUDY OF THE O-15(2p,gamma)Ne-17 CROSS SECTION BY COULOMB DISSOCIATION OF Ne-17 FOR THE rp PROCESS OF NUCLEOSYNTHESIS
- 2014
-
Ingår i: Acta Physica Polonica, Series B.. - 1509-5770 .- 0587-4254. ; 45:2, s. 229-234
-
Tidskriftsartikel (refereegranskat)abstract
- The O-15(2p, gamma)Ne-17 cross section has been studied by the inverse reaction, the Coulomb dissociation of Ne-17. The experiment has been performed at the GSI. The Ne-17 excitation energy prior to decay has been reconstructed by using the invariant-mass method. The preliminary differential and integral Coulomb dissociation cross sections (sigma(Coul)) have been extracted, which provide a photoabsorption (sigma(photo)) and a radiative capture cross section (sigma(cap)). Additionally, important information about the Ne-17 nuclear structure will be obtained. The analysis is in progress.
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| 26. |
- Aksyutina, Yu, et al.
(författare)
-
Lithium isotopes beyond the drip line
- 2008
-
Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693. ; 666:5, s. 430-434
-
Tidskriftsartikel (refereegranskat)abstract
- The unbound isotopes 10Li, 12Li and 13Li have been observed after nucleon-knockout reactions at relativistic energies with 11Li and 14Be beams impinging on a liquid hydrogen target. The channels , and were analysed in the ALADIN-LAND setup at GSI. The 10Li data confirm earlier findings, while the 12Li and 13Li nuclei were observed for the first time. The relative-energy spectrum shows that the ground state of 12Li can be described as a virtual s-state with a scattering length of -13.7(1.6) fm. A broad energy spectrum was found for the channel. Based on the assumption that the relative-energy spectrum is dominated by a correlated background presumably stemming from initial correlations in the 14Be ground-state, evidence for a 13Li resonance at 1.47(31) MeV above the threshold with a width around 2 MeV has been found.
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| 27. |
- Aksyutina, Yuliya, 1983, et al.
(författare)
-
Properties of the 7He ground state from 8He neutron knockout
- 2009
-
Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693. ; 679:3, s. 191-196
-
Tidskriftsartikel (refereegranskat)abstract
- The unbound nucleus 7He, produced in neutron-knockout reactions with a 240 MeV/u 8He beam in a liquid-hydrogen target, has been studied in an experiment at the ALADIN-LAND setup at GSI. From an R-matrix analysis the resonance parameters for 7He as well as the spectroscopic factor for the 6He(0+) + n configuration in its ground-state have been obtained. The spectroscopic factor is 0.61 confirming that 7He is not a pure single-particle state. An analysis of 5He data from neutron-knockout reactions of 6He in a carbon target reveals the presence of an s-wave component at low energies in the α+n relative energy spectrum. A possible low-lying exited state in 7He observed in neutron knockout data from 8He in a carbon target and tentatively interpreted as a Iπ=1/2− state, could not be observed in the present experiment. Possible explanations of the shape difference between the 7He resonance obtained in the two knockout reactions are discussed in terms of target-dependence or different reaction mechanisms at relativistic energies.
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| 28. |
- Andric, Jelena, 1979, et al.
(författare)
-
A particle-level rigid fiber model for high-Reynolds number flow, implemented in a general-purpose CFD code
- 2013
-
Ingår i: 8th International Conference on Multiphase Flow ICMF 2013, Korea.
-
Konferensbidrag (refereegranskat)abstract
- A particle-level rigid fiber model has been integrated into a general-purpose, open source computational fluid dynamics code to carry out detailed studies of fiber–flow interactions in realistic flow fields. The fibers are modeled as chains of cylindrical segments, and their translational and rotational degrees of freedom are considered. The equations of motion contain the contributions from hydrodynamic forces and torques, and the segment inertia is taken into account. The model is validated for the rotational motion of isolated fibers in simple shear flow, and the computed period of rotation is in good agreement with the one computed using Jeffery’s equation for a prolate spheroid with an equivalent aspect ratio. The model is applied by suspending a number of fibers in the swirling flow of a conical diffuser, resembling one stage in the dry-forming of pulp mats. The Reynolds-averaged Navier–Stokes equations with an eddy-viscosity turbulence model are employed to describe the fluid motion, and a one-way coupling between the fibers and the fluid phase is included. The dependence of the fiber motion on initial position and density is analyzed.
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| 29. |
- Andric, Jelena, 1979, et al.
(författare)
-
A study of a flexible fiber model and its behavior in DNS of turbulent channel flow
- 2013
-
Ingår i: Acta Mechanica. - : Springer Science and Business Media LLC. - 0001-5970 .- 1619-6937. ; 224:10, s. 2359-2374
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamics of individual flexible fibers in a turbulent flow field have been analyzed, varying their initial position, density and length. A particlelevel fiber model has been integrated into a general-purpose, open source Computational Fluid Dynamics (CFD) code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joints. The equations of motion of the fibers contain the inertia of the segments, the contributions from hydrodynamic forces and torques, and the connectivity forces at the joints. Direct Numerical Simulation (DNS) of the incompressible Navier–Stokes equations is used to describe the fluid flow in a plane channel and a one-way coupling is considered between the fibers and the fluid phase. We investigate the translational motion of fibers by considering the mean square displacement of their trajectories. We find that the fiber motion is primarily governed by velocity correlations of the flow fluctuations. In addition, we show that there is a clear tendency of the thread-like fibers to evolve into complex geometrical configurations in a turbulent flow field, in fashion similar to random conformations of polymer strands subjected to thermal fluctuations in a suspension. Finally, we show that fiber inertia has a significant impact on reorientation time-scales of fibers suspended in a turbulent flow field.
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| 30. |
- Andric, Jelena, 1979, et al.
(författare)
-
Ballistic deflection of fibres in decelerating flow
- 2016
-
Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 85, s. 57-66
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the motion of inertial, rod-like fibres in the decelerating flow of a wedge-shaped channel with non-creeping fibre-flow interactions. We consider the trajectories of isolated fibres to identify the conditions for which these trajectories deflect from the streamlines of the flow as well as a rectilinear path. We carry out analytical and numerical studies under the assumption of an infinite fibre hydrodynamic resistance to transverse flow, and we expand the numerical study by taking into account a finite transverse hydrodynamic resistance. The analytical analysis identifies a longitudinal ballistic number Bℓ and a transverse ballistic number Bt as two dimensionless parameters that govern the fibre dynamics. It is found that Bℓ is the product of the Stokes number Stℓ in the longitudinal direction of the fibre and the channel opening angle β. As anticipated, a fibre moves along the streamlines in the viscosity-dominated regime (Bℓ « 1, Bt « 1), while it moves in a straight line without being rotated in the inertia-dominated regime (Bt » 1). The focus of the present study is on the intermediate regime (Bℓ » 1, Bt
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| 31. |
- Andric, Jelena, 1979, et al.
(författare)
-
Description and validation of a flexible fiber model, implemented in a general-purpose CFD code
- 2013
-
Ingår i: Proceedings of the 8th International Conference on Multiphase Flow ICMF 2013, Korea.
-
Konferensbidrag (refereegranskat)abstract
- A flexible fiber model has been implemented in a general purpose open-source computational fluid dynamics code. The fibers are modeled as chains of cylindrical segments. Each segment is tracked individually and their equations of motion account for the hydrodynamic forces and torques from the interaction with the fluid, the elastic bending and twisting torques, and the connectivity forces and moments that ensure the fiber integrity. The segment inertia is taken into account and a one-way coupling with the fluid phase is considered. The model is applied to the rotational motion of an isolated fiber in a low segment Reynolds number shear flow. In the case of a stiff fiber, the computed period of rotation is in good agreement with the one computed using Jeffery's equation for an equivalent spheroid aspect ratio. A qualitative comparison is made with experimental data for flexible fibers. These results show that the implemented model can reproduce the known dynamical behavior of rigid and flexible fibers successfully.
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| 32. |
- Andric, Jelena, 1979, et al.
(författare)
-
Numerical investigation of fiber flocculation in the air flow of an asymmetric diffuser
- 2014
-
Ingår i: Proceedings of the ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting and 12th International Conference on Nanochannels, Microchannels and Minichannels, FEDSM 201, August 3-7, Chicago, Illinois, USA..
-
Konferensbidrag (refereegranskat)abstract
- A particle-level rigid fiber model is used to studyflocculation in an asymmetric planar diffuser with a turbulent Newtonian fluid flow, resembling one stage in dry-forming process of pulp mats. The fibers are modeled as chains of rigid cylindrical segments. The equations of motion incorporatehydrodynamic forces and torques from the interaction with thefluid, and the fiber inertia is taken into account. The flow isgoverned by the Reynolds-averaged Navier ̶ Stokes equationswith the standard k-omega turbulence model. A one-waycoupling between the fibers and the flow is considered. Astochastic model is employed for the flow fluctuations tocapture the fiber dispersion. The fibers are assumed to interactthrough short-range attractive forces, causing them to interlockas the fiber-fiber contacts occur during the flow. It is found thatthe formation of fiber flocs is driven by both the turbulenceinduceddispersion and the gradient of the averaged flow field
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| 33. |
- Andric, Jelena, 1979, et al.
(författare)
-
Particle-level simulations of flocculation in a fiber suspension flowing through a diffuser
- 2017
-
Ingår i: Thermal Science. - : VINCA INST NUCLEAR SCI. - 0354-9836 .- 2334-7163. ; 21, s. S573-S583
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate flocculation in dilute suspensions of rigid, straight fibers in a decelerating flow field of a diffuser. We carry out numerical studies using a particle-level simulation technique that takes into account the fiber inertia and the non-creeping fiber-flow interactions. The fluid flow is governed by the Reynolds-averaged Navier-Stokes equations with the standard k-omega eddy-viscosity turbulence model. A one-way coupling between the fibers and the flow is considered with a stochastic model for the fiber dispersion due to turbulence. The fibers interact through short-range attractive forces that cause them to aggregate into flocs when fiber-fiber collisions occur. We show that ballistic deflection of fibers greatly increases the flocculation in the diffuser. The inlet fiber kinematics and the fiber inertia are the main parameters that affect fiber flocculation in the pre-diffuser region.
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| 34. |
- Andric, Jelena, 1979, et al.
(författare)
-
Rheological properties of dilute suspensions of rigid and flexible fibers
- 2014
-
Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 212, s. 36-46
-
Tidskriftsartikel (refereegranskat)abstract
- Particle-level simulations are used to study the rheology of monodispersed suspensions of rigid and flexible fibers in a creeping, simple shear flow of a Newtonian fluid. We also investigate the influence of different equilibrium shapes (straight and curved) of the fibers on the behavior of the suspension. A parametric study of the impacts of fiber flexural rigidity and morphology on rheology quantifies the effects of these realistic fiber features on the experimentally accessible rheological properties. A fiber is modeled as a chain of rigid cylindrical segments, interacting through a two-way coupling with the fluid described by the incompressible three-dimensional Navier--Stokes equations. The initial fiber configuration is in the flow--gradient plane. We show that, when the shear rate is increased, straight flexible fibers undergo a buckling transition, leading to the development of finite first and second normal stress differences and a reduction of the viscosity. These effects, triggered by shape fluctuations, are dissimilar to the effects induced by the curvature of stiff, curved fibers, for which the viscosity increases with the curvature of the fiber. An analysis of the orbital drift of fibers initially oriented at an angle to the flow--gradient plane provides an estimate for the time-scale within which the prediction of the rheological behavior is valid. The information obtained in this work can be used in the experimental characterization of fiber morphology and mechanics through rheology.
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| 35. |
- Arabnejad Khanouki, Mohammad Hossein, 1988, et al.
(författare)
-
A novel single-fluid cavitation model with gas content and slip velocity, applied to cavitating tip leakage vortex
- 2023
-
Ingår i: International Journal of Multiphase Flow. - 0301-9322. ; 169
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we develop a new model based on the single-fluid approach which consider the effects of gas content and slip velocity between the gas/vapor phase and the liquid phase in cavitation modeling. The derivation of the mathematical formulation of the model is presented in the context of Large Eddy Simulation (LES), although a similar derivation can be developed for a Reynolds-averaged Navier–Stokes (RANS) framework. To validate the model and to study the effect of including this physics, a cavitating Tip Leakage Vortex (TLV) formed around the NACA0009 foil is simulated. The results of the simulation are compared with the available experimental data and the results obtained from a conventional single-fluid approach. This comparison shows that the new model predicts cavitation regions in significantly better agreement with the experimental data, compared to the conventional single-fluid approach. This better agreement is observed for both the cavitation regions in the TLV and the cavitation regions at the leading edge. Analyzing the results, the reason for this significant improvement is shown to be the accumulation of non-dissolved gas due to the consideration of the slip velocity, and the effect of this accumulation in the cavitation formation. Furthermore, the effects of gas content and the size of nuclei in the incoming flow to the TLV are investigated, and these effects are discussed in detail.
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| 36. |
- Arabnejad Khanouki, Mohammad Hossein, 1988, et al.
(författare)
-
Investigation of Flow-Induced Instabilities in a Francis Turbine Operating in Non-Cavitating and Cavitating Part-Load Conditions
- 2023
-
Ingår i: Fluids. - : MDPI AG. - 2311-5521. ; 8:2
-
Tidskriftsartikel (refereegranskat)abstract
- The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft tube. In this paper, we investigate the formation of this vortex using the scale-resolving methods SST-SAS, wall-modeled LES (WMLES), and zonal WMLES. The numerical results are first validated against the available experimental data, and then analyzed to explain the effect of using different scale-resolving methods in detail. It is revealed that although all methods can capture the main features of the RVRs, the WMLES method provides the best quantitative agreement between the simulation results and experiment. Furthermore, cavitating simulations are performed using WMLES method to study the effect of cavitation on the flow in the turbine. These effects of cavitation are shown to be highly dependent on the amount of vapor in the RVR. If the amount of vapor is small, cavitation induces broadband high-frequency fluctuations in the pressure and forces exerted on the turbine. As the amount of cavitation increases, these fluctuations tend to have a distinct dominant frequency which is different from the frequency of the RVR.
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| 37. |
- Arabnejad Khanouki, Mohammad Hossein, 1988, et al.
(författare)
-
Large-eddy Simulation of Cavitating Tip Leakage Vortex
- 2022
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Tip Leakage Vortices (TLV) are very common in axial turbomachines. This type of vortex forms as the result of an interaction between the leakage flow, a flow from the pressure side of a blade to its suction side, and the upstream flow. In hydraulic machines, the high rotation in TLVs can cause a significant pressure drop at the core of the vortex leading to formation of cavitating structures. Previous experimental studies have shown that the formation of these cavitating structures is responsible for many adverse effects such as cavitation breakdown and a high level of noise and vibration [1, 2, 3]. To avoid such adverse effects, a better understanding of the flow dynamics in TLVs is needed. Numerical simulations can be a useful tool for this purpose as they provide a complete access to the flow field, especially near the cavitating regions where it is difficult to get optical access in the experiments. In this study, we numerically investigate non-cavitating and cavitating TLVs around a NACA0009 foil using a large-eddy simulation approach. The results of the non-cavitating simulation are validated against the experimental data provided in Dreyer et al. [4]. This comparison, shown in Fig. 1, indicates that the simulation can capture the main features of the flow field. Furthermore, a cavitating condition is simulated and the effect of cavitation on the structure of the TLV is discussed in detail.
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| 38. |
- Arabnejad Khanouki, Mohammad Hossein, 1988, et al.
(författare)
-
Scale-resolving simulations of the flow in the Francis-99 turbine at part-load condition
- 2022
-
Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 1079:1
-
Konferensbidrag (refereegranskat)abstract
- In this paper, we investigate the formation of the Rotating Vortex Rope (RVR) using scale-resolving methods, SAS and Wall-Modeled LES (WMLES). We compare the results from these simulation methods with the experimental data of the Francis-99 workshop. This comparison shows that the general features of the RVR can be captured with both methods. However, using WMLES methods would lead to a better quantitative agreement between the velocity profiles in the draft tube in the simulation and the experiment. The reasons for this better agreement are discussed in detail. A comparison of the pressure fluctuations in the draft tube captured in the simulations and the experiment is also presented. This comparison shows that all simulations under-predict the Root Mean Square (RMS) of these pressure fluctuations, although the RMS values predicted by the WMLES simulation are closer to the experimental values.
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| 39. |
- Beaudoin, Martin, et al.
(författare)
-
Evaluation of an improved mixing plane interface for OpenFOAM
- 2014
-
Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 22
-
Konferensbidrag (refereegranskat)abstract
- A mixing plane interface provides a circumferentially averaging rotor-statorcoupling interface, which is extremely useful in practical turbomachinery simulations. Itallows fundamentally transient problems to be studied in steady-state, using simplified meshcomponents having periodic properties, and with the help of a multiple reference frames(MRF) approach. An improved version of the mixing plane interface for the community-drivenversion of OpenFOAM is presented. This new version of the mixing plane introduces a perfield,user-selectable mixing option for the flow fields at the interface, including the possibilityto use a mass-flow averaging algorithm for the velocity field. We show that the quality of themass-flow transfer can be improved by a proper selection of the mixing options at theinterface. This paper focuses on the evaluation of the improved mixing plane interface forvarious steady-state simulations of incompressible flows, applied to a simple 2D validation testcase, and to more complex 3D turbomachinery cases.
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| 40. |
- Berríos-Negrón, Luis, 1971-
(författare)
-
Breathtaking Greenhouse Parastructures : a supplement to the Arcades Project from a Caribbean Perspective [and a call for a careful practice of epistemológica].
- 2020
-
Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- Breathtaking Greenhouse Parastructures is a doctoral work that supplements the unfinished modern opus The Arcades Project [in German Das Passagen-Werk (Mit Bindestrich und Werk mit Capital W) ]. The supplement takes the form of a sculptural, historical, and technological deposition of ‘greenhouse’ that presently oscillates between a past-background and future-foreground to Walter Benjamin's ‘theatrical’ handling of the Parisian arcades. From my Caribbean perspective, that oscillating treatment of ‘greenhouse’ affords me a prop from which to activate the following question: is colonial memory the drive of Global Warming? That core question has led me to retrospectively hypothesise that the technology of ‘greenhouse’ is—beyond metaphor—the illusory (dis)embodiment of the toxic binaries of interior & exterior that are still shaping various influential frameworks such as the modern & Marxian ideas of superstructure, as well as the past & future of Western natural sciences (and their histories). Because of that illusory, spectral, if paranormal power, ‘greenhouse’ becomes at once the Western colonial enframing to both the messianic promise for conserving biological history, as well as the messianic remedy to suppress the traumæ that are destining Global Warming. Also, because of that potent (dis)embodied character, the robust analysis of the manifold instrumentalisation of ‘greenhouse’ is set to play a primary role in deposing the geological timeline of the Anthropocene. Now, while the book-supplement is itself the dissertation (as an unpacking of the aforementioned hypothetical findings), the PhD also relies on two other research devices that are worth mentioning: an installation (titled Anarquivo Negantrópico), and an online journal (titled Intransitive Journal). Altogether, the dissertation offers a field from which to asses my broader experimentation with infrastructures, epistemic things, and social pedestals. Moreover, the dissertation revises the so-called forgotten list of ‘Epistemologica’ (the categorical of Western display preparations and phenomenotechnologies) so to format my study and practice into an adaptable, careful, and differentiated kind of epistemológica…a careful study and practice that we may share as object-relations that proportionately (dis)play more-than-human perspectives for (re)mediating the forms and forces of the climate crises.
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| 41. |
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| 42. |
- Choquet, Isabelle, et al.
(författare)
-
A physically based model for thermal plasma arc attachment on a W-ThO2 cathode
- 2016
-
Ingår i: 2016 IEEE International Conference on Plasma Science (ICOPS).
-
Konferensbidrag (refereegranskat)abstract
- Summary form only given. Arc attachment radius imposed a priori when modelling the coupling between cathode, cathode layer and thermal plasma still hinders models from being predictive, as underlined in a recent review1. The aim of this work was to find a physical element, still lacking in the models, which could contribute in governing the arc attachment. In this study the cathode layer is modeled within the frame of the partial local thermal equilibrium approach1 including the space charge layer, the Knudsen layer and the ionization layer, while the plasma column is assumed to be in local thermal equilibrium. Several modeling assumptions were questioned based on e.g. contradictory assumptions in the literature, or oversimplified physics compared to experimental observations. For testing model and assumptions, 5 mm argon arc test cases with a sharp cathode geometry that have been investigated experimentally in the literature were calculated. Within this framework, the following conclusions were drawn. The space charge emitted electrons is negligible. The Richardson-Dushman emission law supplemented with Schottky correction is used within its domain of validity when applied to thorium doped tungsten cathodes, which are mainly characterized by a field enhanced thermionic emission regime. The radiative heat absorption from the plasma at the cathode surface is not negligible compared to the radiative emission. Ignoring the non-homogeneous structure and composition of a doped tungsten cathode operated in these conditions leads to a large over-estimation of the extent of the arc attachment, and results in an under-estimation of the arc temperature. A cathode model based on physical criteria for taking into account a first level of the cathode inhomogeneity has a significant effect on the arc attachment and on arc properties such as temperature and pressure. The cathode physics is thus an important element to include for obtaining a comprehensive and predictive arc model.
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| 43. |
- Choquet, Isabelle, et al.
(författare)
-
A predictive model for gas tungsten arc heat source
- 2016
-
Ingår i: The 7th International Swedish Production Symposium, SPS16, Conference Proceedings. - Lund : Swedish Production Academy. ; , s. 1-10
-
Konferensbidrag (refereegranskat)abstract
- Gas tungsten arcs are used as heat sources in production processes such as welding and metal deposition.However, the most advanced of the existing gas tungsten arc models still lack predicting the arc temperature observed experimentally, unless imposing a priori the extent of the cathode arc attachment.Possible causes of this problem were investigated. It was concluded that the physical state of the arcing gas tungsten cathode was too simplified by the existing models. This oversimplification results in an overestimation of the cathode arc attachment area and an underestimation of the arc temperature field.An improved model was developed based only on physical criteria. It was tested by comparison with experimental measurements available in the literature. Good agreement with the temperature measured on the cathode surface and within the arc were obtained.
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| 44. |
- Choquet, Isabelle, et al.
(författare)
-
Analysis of the Influence of the Composition of the Shielding Gas on Pressure Force and Heat Fluxes in Arc Welding
- 2014
-
Ingår i: Proceedings of The 6<sup>th</sup> International Swedish Production Symposium 2014. - 9789198097412 ; , s. 1-8
-
Konferensbidrag (refereegranskat)abstract
- A main problem raised by arc welding manufacturing is the determination ofthe optimal process parameters to ensure weld quality as well as resource efficient andsustainable production. To address this problem a better process understanding is required.In this study thermal magneto hydrodynamic modeling of a welding arc is used to reacha deeper insight into the influence of the composition of the shielding gas on the pressureforce and the heat fluxes to a workpiece. The model was implemented in the open sourcesimulation software OpenFOAM. Four different shielding gas mixtures combining argonand carbon dioxide were studied. When increasing the fraction of carbon dioxide the resultsshow a significant increase of the arc velocity and temperature with constriction of thetemperature field, a decrease of the pressure force and a significant increase of the heatfluxes on the base metal.
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| 45. |
- Choquet, Isabelle, et al.
(författare)
-
Electric welding arc modeling with the solver OpenFOAM - A comparison of different electromagnetic models
- 2011
-
Ingår i: International Institute of Welding Document No 212-1189-11, July 2011..
-
Konferensbidrag (refereegranskat)abstract
- This study focuses on the modeling of a plasma arc heat source in the context of electric arc welding. The model was implemented in the open source CFD software OpenFOAM-1.6.x, coupling thermal fluid mechanics in three dimensions with electro magnetics. Four different approaches were considered for modeling the electromagnetic fields: i) the three-dimensional approach, ii) the two-dimensional axi-symmetric approach, iii) the electric potential formulation, and iv) the magnetic field formulation as described by Ramírez et al. [1]. The underlying assumptions and the differences between these models are described in detail. Models i) to iii) reduce to the same quasi one-dimensional limit for an axi-symmetric configuration with negligible radial current density, contrary to model iv). Models ii) to iv) do not represent the same physics when the radial current density is significant, such as or an electrode with a conical tip. Models i) to iii) were retained for the numerical simulations. The corresponding results were validated against the analytic solution of an infinite electric rod. Perfect agreement was obtained for all the models tested. The results from the coupled solver (thermal fluid mechanics coupled with electromagnetics) were compared with experimental measurements for Gas Tungsten Arc Welding (GTAW). The shielding gas was argon, the arc was short (2mm), the electrode tip was conical, and the configuration was axi-symmetric. The boundary conditions were specified at the anode and cathode surfaces. Models i) and ii) lead to the same results, but not the model iii). Model iii) neglects the radial current density component, resulting in a poor estimation of the magnetic field, and in turn of the arc fluid velocity. The limitations of the coupled solver were investigated changing the gas composition, and using different boundary conditions. The boundary conditions, difficult to measure and to estimate a priori, significantly affect the simulation results.
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| 46. |
- Choquet, Isabelle, et al.
(författare)
-
Magnetic field models for high intensity arcs, applied to welding - A comparison between three different formulations
- 2013
-
Ingår i: 9th International Conference on Trends in Welding Research; Chicago, IL; United States; 4 June 2012 through 8 June 2012. - 9781627089982 ; , s. 876-885
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Konferensbidrag (refereegranskat)abstract
- Most simulation studies done to deeper understand high-intensity welding arcs address axi-symmetric configurations and use the electric potential formulation. This formulation involves the assumption of a one-dimensional magnetic field. The assumption is justified in its original frame: rather long arcs (about 10 mm), and when the electrode tip is excluded from the computational domain. However, arcs applied to welding are shorter, and the electrode geometry is important to take into account. The present work questions the assumption of a one-dimensional magnetic field for simulating short welding arcs. We have compared three different approaches for modeling the magnetic field: three-dimensional, two-dimensional axi-symmetric, and the electric potential formulation. These models have been applied to water cooled anode Gas Tungsten Arc Welding (GTAW) test cases with truncated conical electrode tip (tip radius of 0.5 and 0.2 mm) and various arc lengths (2, 3 and 5 mm). For the axi-symmetric cases studied in the present work, the three- and two-dimensional models give exactly the same results. The one-dimensional simplification of the magnetic field turns out to have a significant unfavorable effect on the simulation results. For axi-symmetric welding applications, it is argued that the two-dimensional axi-symmetric formulation should be used.
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| 47. |
- Choquet, Isabelle, 1965-, et al.
(författare)
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Modeling and simulation of a heat source in electric arc welding
- 2011
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Ingår i: Proceedings from the 4th Swedish Production Symposium, Lund, Sweden. ; , s. 202-211, s. 201-211
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Konferensbidrag (refereegranskat)abstract
- This study focused on the modeling and simulation of a plasma heat source applied toelectric arc welding. The heat source was modeled in three space dimensions couplingthermal fluid mechanics with electromagnetism. Two approaches were considered forcalculating the magnetic field: i) three-dimensional, and ii) axi-symmetric. The anodeand cathode were treated as boundary conditions. The model was implemented in theopen source CFD software OpenFOAM-1.6.x. The electromagnetic part of the solverwas tested against analytic solution for an infinite electric rod. Perfect agreement wasobtained. The complete solver was tested against experimental measurements for GasTungsten Arc Welding (GTAW) with an axi-symmetric configuration. The shielding gaswas argon with thermodynamic and transport properties covering a temperature rangefrom 200 to 30 000 K. The numerical solutions then depend greatly on the approachused for calculating the magnetic field. The axi-symmetric approach indeed neglectsthe radial current density component, mainly resulting in a poor estimation of the arcvelocity. Various boundary conditions were set on the anode and cathode. Theseconditions, difficult to measure and to estimate a priori, significantly affect the plasmaheat source simulation results. Solution of the temperature and electromagnetic fieldsin the electrodes will thus be included in the forthcoming developments.
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| 48. |
- Choquet, Isabelle, 1965-, et al.
(författare)
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Numerical simulation of Ar-x%CO2 shielding gas and its effect on an electric welding arc
- 2011
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Ingår i: IIW Commission XII / SG 212 Intermediate meeting, University West, Trollhättan, Sweden, 21 - 23 March 2011, IIW Doc. XII-2017-11. ; , s. 1-12
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This study focuses on the simulation of a plasma arc heat source in the context of electric arc welding. The simulation model was implemented in the open source CFD software OpenFOAM-1.6.x, in three space dimensions, coupling thermal fluid mechanics with electromagnetism. Two approaches were considered for calculating the magnetic field: i) the three-dimensional approach, and ii) the so-called axisymmetric approach. The electromagnetic part of the solver was tested against analytic solution for an infinite electric rod. Perfect agreement was obtained. The complete solver was tested against experimental measurements for Gas Tungsten Arc Welding (GTAW) with an axisymmetric configuration. The shielding gas was argon, and the anode and cathode were treated as boundary conditions. The numerical solutions then depend significantly on the approach used for calculating the magnetic field. The so-called axisymmetric approach indeed neglects the radial current density component, mainly resulting in a poor estimation of the arc velocity. Plasma arc simulations were done for various Ar-x%CO2 shielding gas compositions: pure argon ( x =0), pure carbon dioxide ( x =100), and mixtures of these two gases with x =1 and 10% in mole. The simulation results clearly show that the presence of carbon dioxide results in thermal arc constriction, and increased maximum arc temperature and velocity. Various boundary conditions were set on the anode and cathode (using argon as shielding gas) to evaluate their influence on the plasma arc. These conditions, difficult to measure and to estimate a priori, significantly affect the heat source simulation results. Solution of the temperature and electromagnetic fields in the anode and cathode will thus be included in the forthcoming developments.
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| 49. |
- Choquet, Isabelle, 1965-, et al.
(författare)
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On the choice of electromagnetic model for shorthigh-intensity arcs, applied to welding
- 2012
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 45:20, s. 205203-
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Tidskriftsartikel (refereegranskat)abstract
- Four different approaches were considered for modelling the electromagneticfields of high-intensity electric arcs: i) the three-dimensional model, ii) the twodimensionalaxi-symmetric model, iii) the electric potential formulation, and iv) themagnetic field formulation. The underlying assumptions and the differences betweenthese models are described in detail. Models i) to iii) reduce to the same limit for anaxi-symmetric configuration with negligible radial current density, contrary to modeliv). Models i) to iii) were retained and implemented in the open source CFD softwareOpenFOAM. The simulation results were first validated against the analytic solutionof an infinite electric rod. Perfect agreement was obtained for all the models tested.The electromagnetic models i) to iii) were then coupled with thermal fluid mechanicsin OpenFOAM, and applied to the calculation of an axi-symmetric Gas Tungsten ArcWelding (GTAW) test case with short arc (2mm) and truncated conical electrode tip.Models i) and ii) lead to the same simulation results, but not model iii). Model iii)is suited in the specific limit of long axi-symmetric arc, with negligible electrode tipeffect. For short axi-symmetric arc, the more general axi-symmetric formulation ofmodel ii) should instead be used.
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| 50. |
- Cintolesi, Carlo, et al.
(författare)
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Numerical simulation of conjugate heat transfer and surface radiative heat transfer using the P 1 thermal radiation model: Parametric study in benchmark cases.
- 2017
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 107, s. 956-971
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Tidskriftsartikel (refereegranskat)abstract
- A parametric investigation of radiative heat transfer is carried out, including the effects of conjugate heattransfer between fluid and solid media. The thermal radiation is simulated using the P1 -model. Thenumerical model and the thermal coupling strategy, suitable for a transient solver, is described. Suchnumerical coupling requires that the radiative equation is solved several times at each iteration; hence,the computational cost of the radiative model is a crucial issue. The P1 -model is adopted because of itsparticularly fast computation. First, a collection of benchmark cases is presented and used to carefullyvalidate the radiation model against literature results and to analyse the model prediction limits.Despite the simplicity of the model, it satisfactorily reproduces the thermal radiation effects. Some lackof accuracy is identified in particular cases. Second, a number of benchmark cases are described andadopted to investigate fluid–solid thermal interaction in the presence of radiation. Three cases aredesigned, to couple radiation with: pure conduction, conduction and forced convection, conductionand natural convection. In all the cases, the surface radiative heat transfer strongly influences the systemthermodynamics, leading to a significant increase of the fluid–solid interface temperature. The main non-dimensional numbers, related to the mutual influence of the different heat transfer modes, are intro-duced and employed in the analyses. A new conduction-radiation parameter is derived in order to studythe conductive boundary layer in absence of convective heat transfer.
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