| 1. |
- Löfgren, Hans B., et al.
(author)
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Damping mechanisms of perturbations in electromagnetically braked horizontal film flows
- 2000
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In: Fluid Dynamics Research. - 0169-5983 .- 1873-7005. ; 26:1, s. 53-68
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Journal article (peer-reviewed)abstract
- The stability of shear and surface modes in an electromagnetically braked free surface liquid metal flow over a horizontally moving boundary is considered. The main objective is to investigate the stabilizing effect of a transverse magnetic field of the film flow appearing in connection with the horizontal belt strip casting. The flow is two-dimensional and the upper boundary is a free surface. In the stability analysis the film flow is approximated as parallel and is therefore based on the MHD-modified Orr-Sommerfeld equation. Numerical calculations present the linear growth rate for shear and surface mode instabilities. A transverse magnetic field is found to have two damping mechanisms. For low Hartmann numbers the main mechanism is due to the braking of the flow, reducing the shear stress. The other mechanism is due to dissipation by the Joule effect.
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| 2. |
- Löfgren, Hans B., et al.
(author)
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Electromagnetic braking of the flow of a liquid metal with a free surface
- 1998
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In: Fluid Dynamics Research. - 0169-5983 .- 1873-7005. ; 23:1, s. 1-25
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Journal article (peer-reviewed)abstract
- The electromagnetic braking of liquid metal flowing over a moving boundary is considered. The flow is two dimensional and the upper boundary is a free surface. The main interest of this flow originates from the application of horizontal belt strip casting. In this process, the liquid metal is fed onto a single endless horizontal belt that runs between two rollers. The bottom of the belt is cooled by water and the liquid is fed onto the belt through a slit. This results in a flow that can be modelled as a flat free jet impinging onto a horizontally moving boundary. The flow after the impingement region is analysed using the boundary layer equations. Here, the flow depends on the Reynolds number R, the Froude number F, the Hartman number M, and the ratio of the belt velocity to jet velocity, β. For βF-2/3, there is no hydraulic jump and the film thickness grows smoothly into the final state with constant thickness and uniform velocity. The braking length for the case without hydraulic jump is shown to be of order O(F2/3Ra/2) without magnetic field and of order O(M-2Ra/2) with magnetic field, where a is the jet width. Thus, the magnetic field decreases the braking distance considerably. This result is favourable for both the stability of the flow and for the solidification process in the horizontal belt strip casting process.
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| 3. |
- Löfgren, Hans B., et al.
(author)
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Initial solidification in liquid metal film flow over a moving bondary
- 2001
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In: International Journal of Heat and Mass Transfer. - 0017-9310 .- 1879-2189. ; 44:4, s. 837-842
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Journal article (peer-reviewed)abstract
- The initial solidification problem of a two-dimensional liquid metal film flow over a heat extracting moving boundary is studied. Analytical solutions in the limit of large Peclet numbers are found. It is shown that the point of initial solidification depends on the Peclet number, the Biot number and the superheat. The initial growth of the solidified phase is found to have a quadratic dependence of the distance from the point of initial solidification. The results are applicable to continuous strip casters.
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| 4. |
- Löfgren, Hans B., et al.
(author)
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The laminar stagnation-point flow against a solidifying moving shell
- 2000
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In: Acta Mechanica. - 0001-5970 .- 1619-6937. ; 142:1-4, s. 33-46
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Journal article (peer-reviewed)abstract
- This paper considers the two-dimensional laminar stagnation-point flow due to a jet impinging onto a solidifying moving boundary. The flow is of interest in connection with the horizontal belt strip casting process. An exact solution to the Navier-Stokes equations is found that is shown to depend on a single ordinary differential equation. The solution is useful in the study of morphological and hydrodynamic instabilities within the impingement region. Solutions for the steady-state shape of the initial stages as well as the asymptotic behavior of the solidifying interface are also discussed in a perturbative manner.
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| 5. |
- Tan, Xiao, et al.
(author)
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Interactive association between insomnia symptoms and sleep duration for the risk of dementia : a prospective study in the Swedish National March Cohort
- 2023
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In: Age and Ageing. - : Oxford University Press. - 0002-0729 .- 1468-2834. ; 52:9
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Journal article (peer-reviewed)abstract
- Objective: Given the importance of sleep in maintaining neurocognitive health, both sleep duration and quality might be component causes of dementia. However, the possible role of insomnia symptoms as risk factors for dementia remain uncertain. Methods: We prospectively studied 22,078 participants in the Swedish National March Cohort who were free from dementia and stroke at baseline. Occurrence of dementia was documented by national registers during a median follow-up period of 19.2 years. Insomnia symptoms and sleep duration were ascertained by Karolinska Sleep Questionnaire. Multivariable Cox proportional hazards models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI). Results: Compared to participants without insomnia at baseline, those who reported any insomnia symptom experienced a greater incidence of dementia during follow-up (HR 1.08, 95% CI: 1.03, 1.35). Difficulty initiating sleep versus non insomnia (HR 1.24, 95% CI: 1.02, 1.52), but not difficulty maintaining sleep or early morning awakening was associated with an increased risk of dementia. Short sleep duration was associated with increased risk of dementia (6 h vs. 8 h, HR 1.29, 95% CI: 1.11-1.51; 5 h vs. 8 h, HR 1.26, 95% CI: 1.00-1.57). Stratified analyses suggested that insomnia symptoms increased the risk of dementia only amongst participants with =7 h sleep (vs. non-insomnia HR 1.24, 95% CI: 1.00-1.54, P=0.05), but not amongst short sleepers (<7 h). Short sleep duration also did not further inflate the risk of dementia amongst insomniacs. Conclusion: Insomnia and short sleep duration increase the risk of dementia amongst middle-aged to older adults.
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| 6. |
- Westerberg, Lars-Göran, et al.
(author)
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3D MHD reconnection model coupled with Cluster multi-spacecraft data : 3D reconnection model
- 2008
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113
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Journal article (peer-reviewed)abstract
- We present a model where a three-dimensional viscous/resistive analytical model of the large scale plasma flow in the outflow region about a reconnection site at the dayside magnetopause, is coupled with multi-spacecraft measurements from the Cluster satellite armada. A total of ten magnetopause crossings have been analyzed during the period 10:29-11:05 UT January 26, 2001, where nine are shown to be suitable to be coupled with the analytical model. Five of these nine reconnection events results in a successful coupling. By fitting the analytical expressions of the plasma velocity and magnetic field behaviour during the transition of the magnetopause, with data from the Cluster spacecraft we obtain estimates of the location of the X-line together with estimates on the anomalous transport coeffcients of kinematic viscosity (v) and magnetic diffusivity (ηd). We also obtain the development of the magnetopause transition layer away from the X-line. It is shown that the distance from the spacecraft to the X-line varies between 0.01 and 1 Earth radii, while v + ηd have a value between 4 . 104 km2/ s and 4 . 106 km2/ s. Furthermore it is shown that the transition layer thickness follows the analytical pattern where it grows proportional to the square root of the distance from the reconnection site. The estimated thickness obtained from each spacecraft is shown to follow the analytical structure well.
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| 7. |
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| 8. |
- Åkerstedt, Hans O., et al.
(author)
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Free surface magnetohydrodynamic flow with solidification
- 2003
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In: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 22:6, s. 581-601
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Journal article (peer-reviewed)abstract
- The paper considers an analysis of a liquid metal flow, occurring in the horizontal belt strip casting process. The liquid metal flows over a moving copper belt with a growing solidifying phase beneath the melt. The effect of applying a transverse magnetic field is investigated. A set of three-dimensional shallow water equations is derived. Supercritical flow is assumed and the shallow water equations are solved numerically using a shock-capturing method, which automatically takes care of the possibility of oblique hydraulic jumps.It is shown that non-uniform conditions introduced in the feeding region give a pattern of steady hydraulic jumps, which propagate downstream and are reflected at the sidewalls. The effect of the magnetic field is to brake the flow and damp the standing hydraulic jump pattern. Different feeding methods are compared and it is shown that the magnetic field erases the initial differences in liquid distribution using different feeding techniques.
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| 9. |
- Barcot, Ana, et al.
(author)
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Stormwater Uptake in Sponge-Like Porous Bodies Surrounded by a Pond: A Fluid Mechanics Analysis
- 2023
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In: Water. - : MDPI. - 2073-4441. ; 15:18
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Journal article (peer-reviewed)abstract
- In this work, a previously published model for the water up take of stormwater in sponge-like porous bodies by the group is further developed. This is done by investigating the highest-performing model and considering the water uptake from the surroundings of a pond and rain-infiltrated soil. This implies that water uptake from impermeable to partially permeable surfaces is examined. Hence, the following cases are considered: (1) impervious bottom surface and no precipitation, (2) impervious bottom surface with precipitation, (3) permeable soil with no precipitation, and (4) permeable soil with precipitation. A mathematical model covering all these cases is presented, where the governing equations are the mass conservation and Darcy’s law together with an assumption of a sharp wetting front being a first-order approximation of the complete Richard’s equation. Results for the water uptake height, pond depth, and wetting front are computed numerically and plotted against time. Analytical solutions are also presented in certain cases, and critical values are obtained. The parametric study includes variations in the ratio of the model- to the surrounding ground surface area, initial pond depth, precipitation, and soil characteristics. To exemplify, the time it takes to absorb the water from the pond after a precipitation period is presented. The results are related to the Swedish rainfall data of 1 h duration with a return period of 10 years. When evaluating efficiency, the focus is on the absorption time. Results vary considerably, demonstrating a general trend that with soil infiltration, the water absorption rate is higher. For most cases, the considered water amount is absorbed completely, although depending on the parameters and conditions. These results serve to optimize the model for each of the cases. The main focus of the research lies in the theoretical aspect.
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| 10. |
- Ekstedt, Mirjam, et al.
(author)
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Disturbed sleep and fatigue in occupational burnout
- 2006
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In: Scandinavian Journal of Work, Environment and Health. - : Scandinavian Journal of Work, Environment and Health. - 0355-3140 .- 1795-990X. ; 32:2, s. 121-31
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Journal article (peer-reviewed)abstract
- OBJECTIVES: The purpose of this study was to investigate sleep with polysomnography and self-ratings and the diurnal pattern of sleepiness and fatigue in a group suffering from severe occupational burnout.METHOD: Twelve white-collar workers on long-term sick leave (>3 months) and 12 healthy controls with high and low scores on the Shirom Melamed Burnout Questionnaire (SMBQ) were included. A 1-night polysomnographic recording (after habituation) was carried out at home, and sleepiness and mental fatigue were rated at different times of the day for weekdays and the weekend. Precipitating factors at the time of the illness at work and real life were considered, and different dimensions of occupational fatigue were described. A repeated-measures analysis of variance using two or three within group factors was used to analyze the data.RESULTS: The main polysomnographic findings were more arousals and sleep fragmentation, more wake time and stage-1 sleep, lower sleep efficiency, less slow wave sleep and rapid eye movement sleep, and a lower delta power density in non-rapid eye movement sleep in the burnout group. The burnout patients showed pronounced sleepiness and mental fatigue at most times of the day for weekdays without reduction during weekends. The precipitating factor was occupational stress (psychiatric interview), and work stress indicators were increased.CONCLUSIONS: Occupational burnout is characterized by impaired sleep. It is suggested that impaired sleep may play a role in the development of fatigue or exhaustion in burnout.
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