title: Exact models for hall current reconnection with axial guide fields

authors: Craig, Ian J.D.

subjects: mathematics

abstract: This paper employs an analytic reconnection model to investigate the conditions under which Hall currents can influence reconnection and Ohmic dissipation rates. It is first noted that time dependent magnetohydrodynamic systems can be analyzed by decomposing the magnetic and velocity fields into guide field and reconnecting field components. A formally exact solution shows that Hall currents can speed up or slow down the reconnection rate depending on the strength and orientation of the axial guide field. In particular, merging solutions are developed in which the axial guide field is the dominant driver of the reconnection. The extent to which Hall currents can alleviate the buildup of back pressures in flux pile-up reconnection models is also examined. The analysis shows that, although enhancements of the merging rate can be expected under certain conditions, it is unlikely that Hall currents can completely undo the fundamental pressure limitations associated with flux pile-up reconnection.

text: Exact models for hall current reconnection with axial guide fields This paper employs an analytic reconnection model to investigate the conditions under which Hall currents can influence reconnection and Ohmic dissipation rates. It is first noted that time dependent magnetohydrodynamic systems can be analyzed by decomposing the magnetic and velocity fields into guide field and reconnecting field components. A formally exact solution shows that Hall currents can speed up or slow down the reconnection rate depending on the strength and orientation of the axial guide field. In particular, merging solutions are developed in which the axial guide field is the dominant driver of the reconnection. The extent to which Hall currents can alleviate the buildup of back pressures in flux pile-up reconnection models is also examined. The analysis shows that, although enhancements of the merging rate can be expected under certain conditions, it is unlikely that Hall currents can completely undo the fundamental pressure limitations associated with flux pile-up reconnection.

year: 2004-12-06

title: Effective thermal conductivity prediction of foods using composition and temperature data

authors: Carson, James K.

subjects: Science & Technology

abstract: Thermal conductivity data are important for food process modelling and design. Where reliable thermal conductivity data are not available, they need to be predicted. The most accurate ‘first approximation’ methodology for predicting the isotropic thermal conductivity of foods based only on data for composition, initial freezing temperature and temperature dependent thermal conductivity of the major food components was sought. A key feature of the methodology was that no experimental measurements were to be required. A multi-step prediction procedure employing the Parallel, Levy and Effective Medium Theory models sequentially for the components other than ice and air, ice and then air respectively is recommended. It was found to provide the most accurate predictions over the range of foods considered (both frozen and unfrozen, porous and non-porous). The Co-Continuous model applied in a single step also provided prediction accuracy within ±20% (on average), except for the porous frozen foods considered. For greater accuracy more rigorous modelling approaches based on knowledge of the foods structure would be required.

text: Effective thermal conductivity prediction of foods using composition and temperature data Thermal conductivity data are important for food process modelling and design. Where reliable thermal conductivity data are not available, they need to be predicted. The most accurate ‘first approximation’ methodology for predicting the isotropic thermal conductivity of foods based only on data for composition, initial freezing temperature and temperature dependent thermal conductivity of the major food components was sought. A key feature of the methodology was that no experimental measurements were to be required. A multi-step prediction procedure employing the Parallel, Levy and Effective Medium Theory models sequentially for the components other than ice and air, ice and then air respectively is recommended. It was found to provide the most accurate predictions over the range of foods considered (both frozen and unfrozen, porous and non-porous). The Co-Continuous model applied in a single step also provided prediction accuracy within ±20% (on average), except for the porous frozen foods considered. For greater accuracy more rigorous modelling approaches based on knowledge of the foods structure would be required.

year: 2016

title: Boat electrofishing survey of fish abundance in the Ohau Channel, Rotorua, in 2015

authors: Hicks, Brendan J.

abstract: The aim of the survey was to provide on-going monitoring of the fish communities and abundance by boat electrofishing in the Ohau Channel, especially fish species that are taonga to Maori (eels, goldfish, and koura). In the current study, we present the findings from the ninth year of sampling (2015) and a summary of previous surveys.
We used the University of Waikato's 4.5 m-long, aluminium-hulled electrofishing boat to catch a total of total of 1,198 fish and koura (18.9 kg) at 13 sites on 2 December 2015, which comprised 2,671 lineal m and 10,684 m² in area. Koura (freshwater crayfish) and 6 fish species were present, with common bully the most abundant species (up to 45.5 fish 100 m⁻² at the site 6, edge habitat). Goldfish (up to 4.85 100 m⁻²) was the next most abundant species, with most goldfish at sites 7 and 12-13 in and around the side channel. Rainbow trout densities were up to 0.28 fish 100 m⁻². Mean bully density (11.41 fish 100 m⁻²) was much higher than for smelt (0.29 fish 100 m⁻²). Koura had a patchy distribution; only 3 individuals were caught at one site.
Comparing catches over the 9 years of sampling, the mean abundance of common bullies in 2015 was consistent with densities in most post-wall years (after 2007), but lower than in 2007 before wall closure (ANOVA P = 0.001). The cause of fluctuating bully abundance is not known, and was not accounted for by changes in water clarity expressed as black disc distance (BDD), water temperature, or water conductivity. Poor water clarity can reduce the efficiency of electrofishing, but high BDD did not correspond with high common bully densities. In 2015, smelt abundance had recovered somewhat from the low catch in 2014.
Goldfish biomass increased initially (2009-2010) because of targeted fishing in the excavated side branch (site 11), which has dense macrophytes and offers good habitat for goldfish. The continued rise in density from 2012 on, however, suggests a real increase in goldfish numbers. In 2012 and 2013 shortfin eels were caught, but no eels were caught in 2014 and only a single longfin eel was caught in 2015.
Analysis of fish densities before and after wall closure is hampered by the single data point before closure. However, we now have 9 years of post-wall data, and comparison of means suggest that the number of bullies has been lower since 2007. An obvious cause could be interruption of bully migration into the Ohau Channel from Lake Rotoiti by the wall. This suggests that the bully population in the Ohau Channel before wall construction and closure was a mixture of fish from lakes Rotorua and Rotoiti, and that recruitment from Rotoiti is now restricted. This hypothesis is testable with otolith microchemistry.

text: Boat electrofishing survey of fish abundance in the Ohau Channel, Rotorua, in 2015 The aim of the survey was to provide on-going monitoring of the fish communities and abundance by boat electrofishing in the Ohau Channel, especially fish species that are taonga to Maori (eels, goldfish, and koura). In the current study, we present the findings from the ninth year of sampling (2015) and a summary of previous surveys.
We used the University of Waikato's 4.5 m-long, aluminium-hulled electrofishing boat to catch a total of total of 1,198 fish and koura (18.9 kg) at 13 sites on 2 December 2015, which comprised 2,671 lineal m and 10,684 m² in area. Koura (freshwater crayfish) and 6 fish species were present, with common bully the most abundant species (up to 45.5 fish 100 m⁻² at the site 6, edge habitat). Goldfish (up to 4.85 100 m⁻²) was the next most abundant species, with most goldfish at sites 7 and 12-13 in and around the side channel. Rainbow trout densities were up to 0.28 fish 100 m⁻². Mean bully density (11.41 fish 100 m⁻²) was much higher than for smelt (0.29 fish 100 m⁻²). Koura had a patchy distribution; only 3 individuals were caught at one site.
Comparing catches over the 9 years of sampling, the mean abundance of common bullies in 2015 was consistent with densities in most post-wall years (after 2007), but lower than in 2007 before wall closure (ANOVA P = 0.001). The cause of fluctuating bully abundance is not known, and was not accounted for by changes in water clarity expressed as black disc distance (BDD), water temperature, or water conductivity. Poor water clarity can reduce the efficiency of electrofishing, but high BDD did not correspond with high common bully densities. In 2015, smelt abundance had recovered somewhat from the low catch in 2014.
Goldfish biomass increased initially (2009-2010) because of targeted fishing in the excavated side branch (site 11), which has dense macrophytes and offers good habitat for goldfish. The continued rise in density from 2012 on, however, suggests a real increase in goldfish numbers. In 2012 and 2013 shortfin eels were caught, but no eels were caught in 2014 and only a single longfin eel was caught in 2015.
Analysis of fish densities before and after wall closure is hampered by the single data point before closure. However, we now have 9 years of post-wall data, and comparison of means suggest that the number of bullies has been lower since 2007. An obvious cause could be interruption of bully migration into the Ohau Channel from Lake Rotoiti by the wall. This suggests that the bully population in the Ohau Channel before wall construction and closure was a mixture of fish from lakes Rotorua and Rotoiti, and that recruitment from Rotoiti is now restricted. This hypothesis is testable with otolith microchemistry.

year: 2016