title: von Kármán self-preservation hypothesis for magnetohydrodynamic turbulence and its consequences for universality

authors: Wan, Minping

subjects: homogeneous turbulence

abstract: We argue that the hypothesis of preservation of shape of dimensionless second- and third-order correlations during decay of incompressible homogeneous magnetohydrodynamic (MHD) turbulence requires, in general, at least two independent similarity length scales. These are associated with the two Elsässer energies. The existence of similarity solutions for the decay of turbulence with varying cross-helicity implies that these length scales cannot remain in proportion, opening the possibility for a wide variety of decay behaviour, in contrast to the simpler classic hydrodynamics case. Although the evolution equations for the second-order correlations lack explicit dependence on either the mean magnetic field or the magnetic helicity, there is inherent implicit dependence on these (and other) quantities through the third-order correlations. The self-similar inertial range, a subclass of the general similarity case, inherits this complexity so that a single universal energy spectral law cannot be anticipated, even though the same pair of third-order laws holds for arbitrary cross-helicity and magnetic helicity. The straightforward notion of universality associated with Kolmogorov theory in hydrodynamics therefore requires careful generalization and reformulation in MHD.

text: von Kármán self-preservation hypothesis for magnetohydrodynamic turbulence and its consequences for universality We argue that the hypothesis of preservation of shape of dimensionless second- and third-order correlations during decay of incompressible homogeneous magnetohydrodynamic (MHD) turbulence requires, in general, at least two independent similarity length scales. These are associated with the two Elsässer energies. The existence of similarity solutions for the decay of turbulence with varying cross-helicity implies that these length scales cannot remain in proportion, opening the possibility for a wide variety of decay behaviour, in contrast to the simpler classic hydrodynamics case. Although the evolution equations for the second-order correlations lack explicit dependence on either the mean magnetic field or the magnetic helicity, there is inherent implicit dependence on these (and other) quantities through the third-order correlations. The self-similar inertial range, a subclass of the general similarity case, inherits this complexity so that a single universal energy spectral law cannot be anticipated, even though the same pair of third-order laws holds for arbitrary cross-helicity and magnetic helicity. The straightforward notion of universality associated with Kolmogorov theory in hydrodynamics therefore requires careful generalization and reformulation in MHD.

year: 2012

title: The links between poverty and the environment in Malawi

authors: Mkwara, Bentry

subjects: poverty

abstract: Deforestation arising from conversion of forest areas into agriculture is a serious problem in Malawi. Cultivation of subsistence and cash crops is often cited as a major cause of this problem. This paper applies the von Thunen model to firstly, discuss competition for agricultural land and secondly, establish why the poor are closely associated with forests. Further, a regression analysis is conducted to examine the effects of changes in crop land use on changes in forest cover. Results indicate that cultivation of different crops has varying effects on deforestation. Cultivation of maize, primarily by the poor, appears to be the principal cause of deforestation while tobacco and pulses stand at second and third positions, respectively. Finally, a simple methodology is developed to estimate the extent of poverty-driven deforestation in Malawi.

text: The links between poverty and the environment in Malawi Deforestation arising from conversion of forest areas into agriculture is a serious problem in Malawi. Cultivation of subsistence and cash crops is often cited as a major cause of this problem. This paper applies the von Thunen model to firstly, discuss competition for agricultural land and secondly, establish why the poor are closely associated with forests. Further, a regression analysis is conducted to examine the effects of changes in crop land use on changes in forest cover. Results indicate that cultivation of different crops has varying effects on deforestation. Cultivation of maize, primarily by the poor, appears to be the principal cause of deforestation while tobacco and pulses stand at second and third positions, respectively. Finally, a simple methodology is developed to estimate the extent of poverty-driven deforestation in Malawi.

year: 2009-11

title: Turangawaewae/ Tu'ungava'e: Echoes of a place to stand and belong

authors: Brown Pulu, Teena J.

subjects: Maori

abstract: This paper examines the ways in which Pacific communities in Auckland are located and conceptualised by others in contrast to the ways in which they locate and conceptualise themselves. In particular, it looks at the ways in which, in the face of homeland fracture and socio-economic marginality, neighbourhoods are being reclaimed as places of belonging-ness, places in which informal alliances among Māori and Pacific groups are being forged. It also explores the problematic, often ambiguous, nature of these alliances.

text: Turangawaewae/ Tu'ungava'e: Echoes of a place to stand and belong This paper examines the ways in which Pacific communities in Auckland are located and conceptualised by others in contrast to the ways in which they locate and conceptualise themselves. In particular, it looks at the ways in which, in the face of homeland fracture and socio-economic marginality, neighbourhoods are being reclaimed as places of belonging-ness, places in which informal alliances among Māori and Pacific groups are being forged. It also explores the problematic, often ambiguous, nature of these alliances.

year: 2002-09