Static quark potential and effective string corrections in the (2+1)-d SU(2) Yang-Mills theory

We report on a very accurate measurement of the static quark potential in SU(2) Yang-Mills theory in (2+1) dimensions in order to study the corrections to the linear behaviour. We perform numerical simulations at zero and finite temperature comparing our results with the corrections given by the effective string picture in these two regimes. We also check for universal features discussing our results together with those recently published for the (2+1)-d Z(2) and SU(3) pure gauge theories.Comment: 29 pages, 6 figure

String-like behaviour of 4d SU(3) Yang-Mills flux tubes

We present here results on the fine structure of the static q\bar q potential in d=4 SU(3) Yang-Mills theory. The potential is obtained from Polyakov loop correlators having separations between 0.3 and 1.2 fermi. Measurements were carried out on lattices of spatial extents of about 4 and 5.4 fermi. The temporal extent was 5.4 fermi in both cases. The results are analyzed in terms of the force between a q\bar q pair as well as in terms of a scaled second derivative of the potential. The data is accurate enough to distinguish between different effective string models and it seems to favour the expression for ground state energy of a Nambu-Goto string.Comment: 9 pages in LaTeX with 2 figures and 2 tables in JHEP style. Replaced to match with shortened published versio

Methods and protocols for incremental exercise testing in tetraplegia, using arm-crank ergometry assisted by Functional Electrical Stimulation

Cervical spinal cord injury (SCI) leads to tetraplegia, with paralysis and loss of sensation in the upper and lower limbs. The associated sedentary lifestyle results in an increased risk of cardiovascular disease. To address this, we require the design of exercise modalities aimed specifically at tetraplegia and methods to assess their efficacy. This paper describes methods for arm-crank ergometry (ACE) assisted by Functional Electrical Stimulation (FES) applied to the biceps and triceps. The instrumented ergometer enables work-rate control during exercise, implemented here for incremental exercise testing during FES-ACE. Detailed protocols for the tests are given. Experimental data collected during exercise tests with tetraplegic volunteers are provided to illustrate the feasibility of the proposed approach to testing and data analysis. Incremental tests enabled calculation of peak power output and peak oxygen uptake. We propose that the high-precision exercise testing protocols described here are appropriate to assess the efficacy of the novel exercise modality, FES-ACE, in tetraplegia

Central solar receiver CFD modelling utilising generated heliostat field heat flux maps

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.A numerical method to couple Monte Carlo ray tracing data to a Finite Volume (FV) semi-transparent surface to allow for the determination of thermal efficiency due to an input heat flux profile and corresponding ray directions within a central cavity receiver is presented. A sample Biomass cavity receiver[1, 2] is used as a 2-D validation case to demonstrate that a CFD FV approach can be used as an accurate solution to the Radiative Transfer Equation (RTE). A 3-D representation of this cavity allows for the approximation of cavity thermal efficiency to be compared between various input heat flux profiles due to the addition of conjugate heat transfer. Results allow for deductions to be made on the benefits of more accurate representations of heat flux maps due to the point concentration of solar energy from a heliostat field. These representations of heat profiles can be used in future applications such as cavity and heliostat field optimization by creating the critical link between ray tracing and conjugate heat transfer solution methods to evaluate central tower cavity receiver designs.cf201

A computational approach to simulate the optical and thermal performance of a novel complex geometry solar tower molten salt cavity receiver

A novel complex geometry solar tower molten salt cavity receiver is presented and investigated with regard to its optical and thermal performance. The receiver’s design consists of a collector with the goal of limiting the concentrated rays from escaping, which is further enhanced by an absorber design that consists of an array of hexagonal pyramid elements inspired by Garbrecht et al. (2013) that limits re-radiative and convective losses. The performance analysis considers the solar position, DNI and sun shape with an existing heliostat field (PS-10 field) to analyse the receiver, rather than assuming a flux. The optical analysis is conducted with the Monte Carlo ray-tracing approach, while the thermal analysis is conducted using computational fluid dynamics (CFD). The initial design showed impractical receiver efficiencies of 32.8%, while preliminary sensitivity studies on selected parameters increased efficiencies up to 69.9%. In the process a design with improved optics was developed and proposed, with initial results increasing efficiencies up to 82.4%. The study indicates that the design is promising from a heat transfer point of view, although many improvements are still to be made to the design to make it competitive.The University of Pretoria (South Africa) and the South African National Research Foundation ((DST-NRF Solar Spoke)).http://www.elsevier.com/locate/solener2020-07-15hj2019Mechanical and Aeronautical Engineerin

A novel computational approach to combine the optical and thermal modelling of Linear Fresnel Collectors using the finite volume method

A computational approach is presented, which uses the finite volume (FV) method in the Computational Fluid Dynamics (CFD) solver ANSYS Fluent to conduct the ray tracing required to quantify the optical performance of a line concentration Concentrated Solar Power (CSP) receiver, as well as the conjugate heat transfer modelling required to estimate the thermal efficiency of such a receiver. A Linear Fresnel Collector (LFC) implementation is used to illustrate the approach. It is shown that the Discrete Ordinates method can provide an accurate solution to the Radiative Transfer Equation (RTE) if the shortcomings of its solution are resolved appropriately in the FV CFD solver. The shortcomings are due to false scattering and the so-called ray effect inherent in the FV solution. The approach is first evaluated for a 2-D test case involving oblique collimated radiation and then for a more complex 2-D LFC optical domain based on the FRESDEMO project. For the latter, results are compared with and validated against those obtained with the Monte Carlo ray tracer, SolTrace. The outcome of the FV ray tracing in the LFC optical domain is mapped as a non-uniform heat flux distribution in the 3-D cavity receiver domain and this distribution is included in the FV conjugate heat transfer CFD model as a volumetric source. The result of this latter model is the determination of the heat transferred to the heat transfer fluid running in the collector tubes, thereby providing an estimation of the overall thermal efficiency. To evaluate the effectiveness of the phased approach in terms of accuracy and computational cost, the novel 2-D:3-D phased approach is compared with results of a fully integrated, but expensive 3-D optical and thermal model. It is shown that the less expensive model provides similar results and hence a large cost saving. The novel approach also provides the benefit of working in one simulation environment, i.e. ANSYS Workbench, where optimisation studies can be carried out to maximise the performance of linear CSP reflector layout and receiver configurations.University of Pretoria (South Africa) and the South African National Research Foundation (DST-NRF Solar Spoke).http://www.elsevier.com/locate/solener2016-06-30hb201

A novel computational approach to the combine optical and thermal modelling of a linear fresnel collector receiver

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.A computational approach is presented that uses the finite volume (FV) method in the Computational Fluid Dynamics (CFD) solver ANSYS Fluent to perform both the ray tracing required to quantify the optical performance of a line-concentration Linear Fresnel Collector (LFC) receiver, as well as the conjugate heat transfer modelling required to estimate the thermal efficiency of such a receiver. It is shown that the Discrete Ordinates method can provide an accurate solution of the Radiative Transfer Equation (RTE) if the shortcomings of its solution are addressed appropriately in the FV CFD solver. This approach is evaluated for a 2-D sample test case that includes a 2-D LFC optical domain of which the results are compared to those obtained with the Monte Carlo ray tracer, SolTrace. The outcome of the FV ray tracing in the LFC optical domain is mapped as a non-uniform heat flux distribution in the 3-D cavity receiver domain and this distribution is included in the FV conjugate heat transfer CFD model as a volumetric resource. The result of this latter model is the determination of the heat transferred to the heat transfer fluid running in the collector tubes, thereby providing an estimation of the overall thermal efficiency. To evaluate the effectiveness of the phased approach, the 2-D:3-D approach is compared to results of a fully integrated, but expensive, 3-D optical and thermal model. It is shown that the less expensive model provides similar results and that it provides the benefit of working in one simulation environment, i.e., ANSYS Workbench, where additionally optimization studies can be performed in future work.dc201

Computational investigation of worst-case wind loads on a heliostat pod for different reflector aspect ratios

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.Heliostat structures contribute significantly to the cost of a central receiver power plant. This study was concerned with obtaining wind loadings on a modular heliostat pod (HelioPOD) that houses six reflectors as developed by Stellenbosch University’s Solar Thermal Research Group (STERG) using computational fluid dynamics (CFD). Mean wind loadings were obtained for different combinations of elevation and azimuth angle, as well as different reflector aspect ratios, meshing and parameterizing the domain with ANSYS Workbench v15.0 and solving the steady-state flow field using ANSYS Fluentcf201