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  • 1
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    Kinematic dynamo action in a network of screw motions; application to the core of a fast breeder reactor (1999)
    Cambridge University Press
    Details
    Publication Date: 1999-03-10
    Description: Most of the studies concerning the dynamo effect are motivated by astrophysical and geophysical applications. The dynamo effect is also the subject of some experimental studies in fast breeder reactors (FBR) for they contain liquid sodium in motion with magnetic Reynolds numbers larger than unity. In this paper, we are concerned with the flow of sodium inside the core of an FBR, characterized by a strong helicity. The sodium in the core flows through a network of vertical cylinders. In each cylinder assembly, the flow can be approximated by a smooth upwards helical motion with no-slip conditions at the boundary. As the core contains a large number of assemblies, the global flow is considered to be two-dimensionally periodic. We investigate the self-excitation of a two-dimensionally periodic magnetic field using an instability analysis of the induction equation which leads to an eigenvalue problem. Advantage is taken of the flow symmetries to reduce the size of the problem. The growth rate of the magnetic field is found as a function of the flow pitch, the magnetic Reynolds number (Rm) and the vertical magnetic wavenumber (k). An α-effect is shown to operate for moderate values of Rm, supporting a mean magnetic field. The large-Rm limit is investigated numerically. It is found that α = O(Rm-2/3), which can be explained through appropriate dynamo mechanisms. Either a smooth Ponomarenko or a Roberts type of dynamo is operating in each periodic cell, depending on k. The standard power regime of an industrial FBR is found to be subcritical.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 2
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    Reconnaissance Study of glacier energy balance in North Greenland, 1993–94 (1998)
    Cambridge University Press
    Details
    Publication Date: 1998-01-01
    Description: Reconnaissance energy-balance studies were made for the first time at two sites in North Greenland to compare with conditions in West Greenland. The field experiments were planned to save weight because it is expensive to operate in North Greenland. The larger energy components (incoming radiation and ablation) were measured for 55 days altogether, and the smaller components were evaluated by indirect methods, e.g. turbulent fluxes are calculated from air temperature, humidity and wind speed, to save the weight of instruments. The energy-balance model is “tuned" by choosing surface roughness and albedo to reduce the mean error between measured ablation and modelled daily melting. The error standard deviation for ablation is only ± 5 kg m−2d−1’, which is much lower than found in West Greenland, due to better instruments and modelling in the present study. Net radiation is the main energy source for melting in North Greenland but ablation is relatively low because sublimation and conductive-heat fluxes use energy that would otherwise be available for melting. There is a strong diurnal variation in ablation, mainly forced by variations in shortwave radiation and reinforced by nocturnal cooling of the ice surface by outgoing longwave radiation and sublimation. The model frequently predicts a frozen glacier surface at night even when air temperatures are positive.
    Print ISSN: 0022-1430
    Electronic ISSN: 1727-5652
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of International Glaciological Society.
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  • 3
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    Reconnaissance Study of glacier energy balance in North Greenland, 1993–94 (1998)
    Cambridge University Press
    Details
    Publication Date: 1998-01-01
    Description: Reconnaissance energy-balance studies were made for the first time at two sites in North Greenland to compare with conditions in West Greenland. The field experiments were planned to save weight because it is expensive to operate in North Greenland. The larger energy components (incoming radiation and ablation) were measured for 55 days altogether, and the smaller components were evaluated by indirect methods, e.g. turbulent fluxes are calculated from air temperature, humidity and wind speed, to save the weight of instruments. The energy-balance model is “tuned" by choosing surface roughness and albedo to reduce the mean error between measured ablation and modelled daily melting. The error standard deviation for ablation is only ± 5 kg m −2 d−1’, which is much lower than found in West Greenland, due to better instruments and modelling in the present study. Net radiation is the main energy source for melting in North Greenland but ablation is relatively low because sublimation and conductive-heat fluxes use energy that would otherwise be available for melting. There is a strong diurnal variation in ablation, mainly forced by variations in shortwave radiation and reinforced by nocturnal cooling of the ice surface by outgoing longwave radiation and sublimation. The model frequently predicts a frozen glacier surface at night even when air temperatures are positive.
    Print ISSN: 0022-1430
    Electronic ISSN: 1727-5652
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of International Glaciological Society.
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  • 4
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    Errors in daily ablation measurements in northern Greenland, 1993-94, and their implications for glacier climate studies (1998)
    Cambridge University Press
    Details
    Publication Date: 1998-01-01
    Description: Ablation climate studies were made at two locations in northern Greenland in the summers of 1993 and 1994, respectively. Daily ablation was measured at ten stakes within a small area, and the data were compared with each other to detect gross errors. For example, high standard deviations for data taken on the same day, or low correlations between data series at different stakes, indicate erroneous data. After discarding data for one stake in 1993 and two stakes in 1994, random errors in daily ablation data for individual stakes are ± 5 kg m2d−1, which is further reduced to only about db 2 kg m−2d−1by averaging over eight or nine stakes. Random errors in calculated energy balances using the present ablation data are much lower than found in earlier stuthes in West Greenland where ablation was only measured on three stakes without any attempt to detect gross errors. Aside from day-to-day errors, there are ±10% differences in mean ablation at different stakes, which are probably caused by small-scale variations in surface albedo. Such interstate differences give a ± 10% uncertainty in positive degree-day factors, which are 9.8 ± 0.9 and 5.9 ± 0.6 kg m2d−1deg−1for the two sites.
    Print ISSN: 0022-1430
    Electronic ISSN: 1727-5652
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of International Glaciological Society.
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  • 5
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    Errors in daily ablation measurements in northern Greenland, 1993-94, and their implications for glacier climate studies (1998)
    Cambridge University Press
    Details
    Publication Date: 1998-01-01
    Description: Ablation climate studies were made at two locations in northern Greenland in the summers of 1993 and 1994, respectively. Daily ablation was measured at ten stakes within a small area, and the data were compared with each other to detect gross errors. For example, high standard deviations for data taken on the same day, or low correlations between data series at different stakes, indicate erroneous data. After discarding data for one stake in 1993 and two stakes in 1994, random errors in daily ablation data for individual stakes are ± 5 kg m 2 d−1, which is further reduced to only about db 2 kg m−2d−1 by averaging over eight or nine stakes. Random errors in calculated energy balances using the present ablation data are much lower than found in earlier stuthes in West Greenland where ablation was only measured on three stakes without any attempt to detect gross errors. Aside from day-to-day errors, there are ±10% differences in mean ablation at different stakes, which are probably caused by small-scale variations in surface albedo. Such interstate differences give a ± 10% uncertainty in positive degree-day factors, which are 9.8 ± 0.9 and 5.9 ± 0.6 kg m 2 d −1 deg −1 for the two sites.
    Print ISSN: 0022-1430
    Electronic ISSN: 1727-5652
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of International Glaciological Society.
    Location Call Number Expected Availability
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