Turbulent convection in stars

Thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy at the University of the Witwatersrand, Johannesburg, 2017.This thesis investigates in detail the structure of models of turbulent convec tion with phenomenological closures for the eddy-viscosity. It explores the merits of replacing the canonical Mixing Length Theory of stellar convection with more realistic models of fluid turbulence that take into account the full spectrum of eddy sizes. The author provides a detailed exposition of the fun damental assumptions and the modus operandi of various approaches to the treatment of convective energy-transfer in stars. He focuses in particular on spectral descriptions of the convective process. The structure of several clo sure models developed by various authors are investigated, and he identifies and elucidates those aspects of these closures that lead to an improved descrip tion of convective turbulence in the stellar interior. The author also develops an implementation within the public-domain code, called Modules for Experi ments in Stellar Astrophysics, of two of these models and reports and discusses the results of his numerical experiments.XL201

Electronic band structure of calcium oxide

We employed electron momentum spectroscopy (EMS) to measure the bulk electronic structure of calcium oxide. We extracted the electron momentum density (EMD), density of occupied states (DOS), band dispersions, bandwidths and intervalence bandgaps from the data. The results are compared with calculations based on the full potential linear muffin-tin orbital(FP-LMTO) approximation. While the bandwidths of 0.6±0.2 and 1.2±0.1 eV for the s- and p-bands, respectively, and their dispersions agree well with the LMTO calculation, the relative intensity of the two bands is at odds with the theory. The measured intervalence bandgap at the Γ-point of 16.5±0.2 eV is larger by 2.1 eV than that from the LMTO calculation. The experimental bandwidth of the Ca 3p semi-core level of 0.7±0.1 eV agrees with the LMTO prediction. The measured bandgap between this level and the s-band is 3.6±0.2 eV. The Ca 3s-3p level splitting is in excellent agreement with the literature. © 2004 Elsevier B.V. All rights reserved

The role of convection in stellar models

This thesis reviews the essential ingredients of local, time-independent mixing-length theory, the convective stability of uid elements in the stellar interior, the origins and in uence of convection zones in stars, and the numerical implementation of convection in some popular stellar evolution codes. The thesis concludes with a brief discussion of the future role of mixing-length theory in stellar modelling

Refining prediction accuracy for pest blackfly outbreaks using Bayesian networks, Orange River, Northern Cape, South Africa.

Master of Science in Geography. University of KwaZulu-Natal, Pietermaritzburg, 2018Construction of dams, impoundments and Inter-Basin Transfer schemes (IBTs) along the Orange River are aimed to provide useable water for multiple sectors. However, operation of these water schemes had led to changes in flow regimes, seston concentrations and water temperatures, which has led to an escalation of pest blackfly (S.chutteri) outbreaks along the lower to middle reaches of the Orange River. Pest blackfly bite livestock, poultry and humans asthey require a blood meal to complete ovarian development. During outbreak periods, livestock farming and the grape industries are affected negatively by pest blackflies along the Orange River. The blackfly control programme has been operating for over twenty years, and aims to control blackfly outbreaks by applying larvicides along the Orange River. Although this programme is in place, periodic outbreaks occur and losses in livestock and productivity can amount to an estimated R300 million during an outbreak (2013). Therefore, other methods should need to be integrated with this programme to achieve blackfly control. Predictive modelling was identified as a method to assist the blackfly problem. Being able to predict when, where and the severity of an outbreak, will assist management in control planning. Bayesian network (Bn) models were identified as a suitable predictive model,as multiple variables can be used in understanding the cause and effects of a response variable.The aim of the research was to refine prediction accuracy of blackfly outbreaks along the middle to lower reaches of the Orange River, using Bns. Fourteen sites were sampled along the Orange River, for which abiotic and biotic data were collected during four sampling periods. These data were used in assisting quantitative components of the Bns, whilst the qualitative components were based of previous Bns with additions on new nodes that were identified as affecting blackfly outbreaks. Water temperature data showed that sites were split into two distinct groupings, for which Bns were constructed.These were termed the upper and lower stream models. The upper stream model had the higher outbreak probabilities, whilst it was predicted for both models that summer would be the season most likely for an outbreak to occur. The species most likely to cause an outbreak was identified to be either S.chutteri or S.damnosum, with switching in dominance throughout sampling periods potentially due to switching in seston concentrations. Future outbreak probabilities based on scenarios of increased discharge and water temperatures indicate that the blackfly problem is likely to worsen, with increases in discharge resulting in greater habitat availability for pest species and increases in water temperature resulting in shorter life cycles and more rapid reproduction.The Bns constructed show promise in assisting management as blackfly outbreak probabilities were refined on a spatial and temporal scale along the middle to lower reaches of the Orange River

The Transition from Heavy Fermion to Mixed Valence in Ce1-xYxAl3: A Quantitative Comparison with the Anderson Impurity Model

We present a neutron scattering investigation of Ce1-xYxAl3 as a function of chemical pressure, which induces a transition from heavy-fermion behavior in CeAl3 (TK=5 K) to a mixed-valence state at x=0.5 (TK=150 K). The crossover can be modeled accurately on an absolute intensity scale by an increase in the k-f hybridization, Vkf, within the Anderson impurity model. Surprisingly, the principal effect of the increasing Vkf is not to broaden the low-energy components of the dynamic magnetic susceptibility but to transfer spectral weight to high energy.Comment: 4 pages, 5 figure

CONSERVATION STRATEGIES FOR NEPENTHES KHASIANA IN THE NOKREK BIOSPHERE RESERVE OF GARO HILLS, NORTHEAST, INDIA

The present paper focuses on the various disturbance agents such as coal mining, limestone extraction, stone quarrying, jhum cultivation, fire, grazing, over-exploitation of resources, road constructions etc., affecting the natural growth of Nepethes khasiana in the Nokrek Biosphere Reserve of India. N. khasiana is the prominent insectivorous scandent shrubs species of this biosphere reserve and is an important source of medicine and basic ornamental uses for the local garo tribal people of north-east India. The inevitable pressure due to commercialization of the N. khasiana is leading to severe destruction of the species and may create the scarcity of that species in the near future. Therefore, joint efforts need to be implemented by the local garo villagers with governmental and non-governmental agencies for conservation and sustainable use of N. khasiana. The government may also take initiative by allotting demarcated forests areas to the villagers as village forest, thus motivating the villagers to take special care for its protection and rehabilitation and for a sustainable output

Parameters of the NERA spectrometer for cold and thermal moderators of the IBR-2 pulsed reactor

The inverted geometry time-of-flight spectrometer NERA designed for simultaneous investigation of elastic and inelastic neutron scattering was constructed on a 100 meter long flight path of the IBR-2 reactor at JINR, Dubna, Russia. Spectrometer parameters were optimized for the thermal neutron spectrum of a water moderator at 330 K. Recently, a new type of cold neutron source operating at 30 K was installed in the sector of six horizontal channels. The cold source in combination with the water moderator allows one to effectively use incident neutrons in wide range of wavelengths

Intracranial halo pin penetration causing brain injury secondary to poor halo care technique: a case report and literature review

This is a case report of intra cranial penetration by halo pins resulting in cerebritis and fits secondary to incorrect halo care by the patient and his family. Halo pin penetration into the skull with brain injury is itself a rare incident. Previously documented case reports were in patients with a previous cranioplasties and they were highlight the fact that halo not to be used in cranioplasty patients. Cranial penetration of the halo pins has generally been secondary to a fall/medical condition as epilepsy. This incident how ever highlights the fact the halo care itself along with proper techniques used for tightening the halo pins by the carer plays a crucial role in preventing complications such as this