1,118 research outputs found
Differential rotation and angular momentum
Differential rotation not only occurs in astrophysical plasmas like accretion disks, it is also measured in laboratory plasmas as manifested in the toroidal rotation of tokamak plasmas. A re-examination of the Lagrangian of the system shows that the inclusion of the angular momentumâs radial variation in the derivation of the equations of motion produces a force term that couples the angular velocity gradient with the angular momentum. This force term is a property of the angular velocity field, so that the results are valid wherever differential rotation is present
The effect of sheared diamagnetic flow on turbulent structures generated by the CharneyâHasegawaâMima equation
The generation of electrostatic drift wave turbulence is modelled by the CharneyâHasegawaâMima equation. The equilibrium density gradient n0=n0(x) is chosen so that dn0 /dx is nonzero and spatially variable (i.e., v*e is sheared). It is shown that this sheared diamagnetic flow leads to localized turbulence which is concentrated at max(grad n0), with a large dv*e/dx inhibiting the spread of the turbulence in the x direction. Coherent structures form which propagate with the local v*e in the y direction. Movement in the x direction is accompanied by a change in their amplitudes. When the numerical code is initialized with a single wave, the plasma behaviour is dominated by the initial mode and its harmonics
Numerical simulations of sunspots
The origin, structure and evolution of sunspots are investigated using a numerical model. The compressible MHD equations are solved with physical parameter values that approximate the top layer of the solar convection zone. A three dimensional (3D) numerical code is used to solve the set of equations in cylindrical geometry, with the numerical domain in the form of a wedge. The linear evolution of the 3D solution is studied by perturbing an axisymmetric solution in the azimuthal direction. Steady and oscillating linear modes are obtained
A South African review of harmonic emission level assessment as per IEC61000-3-6
Large-scale renewable power producing plants are being integrated into South African networks.
Network operators need to ensure that Renewable Power Plants (RPP) do not negatively affect the
power quality levels of their networks, as harmonics amongst others could become a concern.
IEC 61000-3-6 details a method for allocating voltage harmonic emission limits for distorting loads.
This method works well for the allocation of emission limits; however it does not address the
management of harmonic emissions once a plant is connected to the network. The management of
harmonic emissions requires that network operators measure or quantify the emissions from loads and
generators to determine compliance. Post-connection quantification of harmonic levels and
compliance is a challenge for network operators. The question asked is âHow should a network
operator measure/quantify the harmonic emissions of a load/generator to establish compliance with the
calculated limits as per IEC 61000-3-6â.
This paper reviews within a South African context methods of assessing harmonic emission levels and
then evaluates these methods by means of field data. Opportunities for improvement are identified
and operational requirements discussed
Observational signatures of the coronal kink instability with thermal conduction
It is known from numerical simulations that thermal conduction along magnetic
field lines plays an important role in the evolution of the kink instability in
coronal loops. This study presents the observational signatures of the kink
instability in long coronal loops when parallel thermal conduction is included.
The 3D nonlinear magnetohydrodynamic equations are solved numerically to
simulate the evolution of a coronal loop that is initially in an unstable
equilibrium. The loop has length 80 Mm, width 8 Mm and an initial maximum twist
of Phi = 11.5 pi, where Phi is a function of the radius. The initial loop
parameters are obtained from a highly twisted loop observed in the TRACE 171 A
waveband. Synthetic observables are generated from the data. These observables
include spatial and temporal averaging to account for the resolution and
exposure times of TRACE images. Parallel thermal conduction reduces the maximum
local temperature by up to an order of magnitude. This means that different
spectral lines are formed and different internal loop structures are visible
with or without the inclusion of thermal conduction. However, the response
functions sample a broad range of temperatures. The result is that the
inclusion of parallel thermal conductivity does not have as large an impact on
observational signatures as the order of magnitude reduction in the maximum
temperature would suggest; the net effect is a blurring of internal features of
the loop structure.Comment: To be published in Astrophysical Journal, 14 pages, 9 figure
Validating accelerometry-derived proxies of energy expenditure using the doubly-labelled water method in the smallest penguin species
Acknowledgements We are grateful to Dr Catherine Hambly and Peter Thompson for technical assistance with the isotope analysis for the doubly labelled water measurements. We thank Parks Victoria (in particular, the rangers at Gabo Island and Port Campbell Offices) and Kevin Lotte for logistical support. Funding This project was funded by the Holsworth Wildlife Research Endowment â Equity Trustees Charitable Foundation and the Ecological Society of Australia and Deakin University internal funds.Peer reviewedPublisher PD
Reconstruction of an Extensive Midfacial Defect Using Additive Manufacturing Techniques
Published ArticleMalignant peripheral nerve sheath tumors are extremely rare tumors arising in peripheral nerves. Only 17 cases involving the trigeminal nerve have ever been reported. These tumors have a very poor prognosis and very high rates of recurrence and metastases. Their recommended treatment involves complete tumor resection followed by radiation. This can be problematic in the head and neck region. We present a clinical case involving a 33-year-old female patient presenting with a slow-growing, exophytic mass of the anterior maxilla. Incisional biopsy and subsequent histological examination revealed a diagnosis of a malignant peripheral nerve sheath tumor. Surgical resection involved a complete maxillectomy, rhinectomy, and resection of the upper lip and aspects of the left and right cheeks. Reconstruction of the subsequent defect incorporated the placement of four zygomatic oncology implants to aid in retention of a facial prosthesis. These implants, however, were subsequently lost; and an anatomical model of the hard tissues was manufactured via 3D printing. This model was used to design and manufacture a titanium frame (customized implant) for the patient. The frame was then fixated and secured intraoperative with 21 cortical screws. A maxillary denture and silicone facial prosthesis were also made to fit onto this frame. This is the first known case where additive manufacturing, via the use of rapid prototyping and 3D printing, was employed to manufacture a facial prosthesis
Chromospheric seismology above sunspot umbrae
The acoustic resonator is an important model for explaining the three-minute
oscillations in the chromosphere above sunspot umbrae. The steep temperature
gradients at the photosphere and transition region provide the cavity for the
acoustic resonator, which allows waves to be both partially transmitted and
partially reflected. In this paper, a new method of estimating the size and
temperature profile of the chromospheric cavity above a sunspot umbra is
developed. The magnetic field above umbrae is modelled numerically in 1.5D with
slow magnetoacoustic wave trains travelling along magnetic fieldlines.
Resonances are driven by applying the random noise of three different
colours---white, pink and brown---as small velocity perturbations to the upper
convection zone. Energy escapes the resonating cavity and generates wave trains
moving into the corona. Line of sight (LOS) integration is also performed to
determine the observable spectra through SDO/AIA. The numerical results show
that the gradient of the coronal spectra is directly correlated with the
chromosperic temperature configuration. As the chromospheric cavity size
increases, the spectral gradient becomes shallower. When LOS integrations is
performed, the resulting spectra demonstrate a broadband of excited frequencies
that is correlated with the chromospheric cavity size. The broadband of excited
frequencies becomes narrower as the chromospheric cavity size increases. These
two results provide a potentially useful diagnostic for the chromospheric
temperature profile by considering coronal velocity oscillations
School-based human papillomavirus vaccination: An opportunity to increase knowledge about cervical cancer and improve uptake of screening
Background. Poor knowledge about cervical cancer plays a role in limiting screening uptake. HPV vaccination provides an untested platform to distribute information that could possibly improve knowledge and screening coverage.Objective. To measure changes in knowledge and screening uptake when information and screening opportunities were provided to mothers of adolescent HPV vaccine recipients.Methods. During an HPV vaccine implementation project in the Western Cape (WC) and Gauteng Province (GP), South Africa, information about cervical cancer was provided to parents during a lecture, written information was distributed, and mothers were then invited to either screen at their clinic (WC) or use a self-screening kit (GP). A structured questionnaire was used to test cervical cancer knowledge and screening practices, comparing these before and after the project and between the two screening groups.Results. Complete data for both questionnaires were available for 777 of 906 recruited women. Initial knowledge was poor, but on retesting 6 months later, knowledge about symptoms (p<0.005), screening (p<0.005) and vaccination (p<0.05) improved significantly after the information session and school-based HPV vaccination. In the second questionnaire, women reported significantly more screening and the last reported screening test was more recent. This improvement was more favourable in GP than in the WC (41% v. 26% reporting screening in the past 12 months).Conclusion. These results demonstrate how adolescent HPV vaccine programmes can help to control cervical cancer among mothers by offering information and screening. It is important not to lose this opportunity to educate mothers and their daughters and offer effective methods to prevent cervical cancer in both generations
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