Total linkage of quaternion algebras and Pfister forms in characteristic two

We study the subfields of quaternion algebras that are quadratic extensions of their center in characteristic 2. We provide examples of the following: two non-isomorphic quaternion algebras that share all their quadratic subfields, two quaternion algebras that share all their inseparable but not all their separable quadratic subfields and two algebras that share all their separable but not all their inseparable quadratic subfields. We also discuss quaternion algebras over global fields and fields of Laurent series over a perfect field of characteristic 2 and show that the quaternion algebras over these fields are determined by their separable quadratic subfields. Throughout, these linkage questions are treated in the more general setting by considering the linkage of Pfister forms.Comment: 16 page

The descent of biquaternion algebras in characteristic two

In this paper we associate an invariant to a biquaternion algebra $B$ over a field $K$ with a subfield $F$ such that $K/F$ is a quadratic separable extension and $\operatorname{char}(F)=2$. We show that this invariant is trivial exactly when $B \cong B_0 \otimes K$ for some biquaternion algebra $B_0$ over $F$. We also study the behavior of this invariant under certain field extensions and provide several interesting examples

Hybrid and modular multilevel converter designs for isolated HVDC–DC converters

Efficient medium and high-voltage dc-dc conversion is critical for future dc grids. This paper proposes a hybrid multilevel dc-ac converter structure that is used as the kernel of dc-dc conversion systems. Operation of the proposed dc-ac converter is suited to trapezoidal ac-voltage waveforms. Quantitative and qualitative analyses show that said trapezoidal operation reduces converter footprint, active and passive components' size, and on-state losses relative to conventional modular multilevel converters. The proposed converter is scalable to high voltages with controllable ac-voltage slope; implying tolerable dv/dt stresses on the converter transformer. Structural variations of the proposed converter with enhanced modularity and improved efficiency will be presented and discussed with regards to application in front-to-front isolated dc-dc conversion stages, and in light of said trapezoidal operation. Numerical results provide deeper insight of the presented converter designs with emphasis on system design aspects. Results obtained from a proof-of-concept 1-kW experimental test rig confirm the validity of simulation results, theoretical analyses, and simplified design equations presented in this paper. - 2013 IEEE.Scopu

Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability

Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype

Agricultural Credit Under Economic Liberalization And Islamization In Sudan

This study uses survey data to examine the operations of the agrarian credit market,formal and informal, in Sudan under conditions of recent economic liberalization and Islamization; the latter does not allow interest rate fixing. In addition to descriptive analysis, the study specifies and estimates a model of farm household participation in the credit market. The survey results show a substantial increase in formal borrowing in agriculture, but relatively low informal credit. Implicit interest rates are found to be high in the formal segment compared with their previous levels, and the levels of formal and informal agrarian rates of interest are comparable. The research concludes that there is a need for enhanced institutional financial intermediation in the agrarian credit market as well as scope for the promotion of savings and credit associations among farmers.

Understanding the motivational and behavioural factors behind PSL fans in South Africa.

Masters Degree. University of KwaZulu-Natal, Durban.This study explored the motivation and behaviour of Premier Soccer League (PSL) fans in South Africa. A survey research method using convenience and purposive sampling was adopted in this study. A sample of 221 PSL fans selected from three research venues completed a short demographic questionnaire, the Sports Fan Motivation Scale (SFMS) (Wann, 1995), and the Fan Behaviour Questionnaire (FBQ) (Capella, 2002). In relation to motivation, the results show that PSL fans in South Africa are motivated primarily by entertainment, aesthetics, self-esteem, and group-affiliation. The results found that male fans are influenced by team success to support a PSL team in South Africa and that female PSL fans are influenced by family members. Male PSL fans of this sample were found to be more emotionally invested and more likely to portray negative behaviour than the female fans. Implications of the findings are discusse

Comprehensive steady state analysis of bidirectional dual active bridge DC/DC converter using triple phase shift control

Several papers have been published recently on TPS control of dual active bridge (DAB) converter, however, no complete study of the converter operation behaviour exists, that takes into account all switching modes in both charging and discharging (bidirectional) power transfer. In this paper, six switching modes and their complements with opposite power transfer direction are defined with their operational constraints. Exact expressions for power transferred are derived with no fundamental frequency assumptions and range of power transfer for each mode is also defined to characterize mode limitations. Detailed constraints for zero voltage switching (ZVS) are also obtained. A new definition for converter reactive power consumption is introduced. This is based on calculation of inductor apparent power which avoids fundamental frequency approximations as well as the vague negative (back flowing) power definitions in recent papers. All known DAB phase shift modulation techniques including conventional, dual and extended phase shift, represent special cases from triple phase shift, therefore the presented analysis provides a generalised theory for all phase shift based modulation techniques

A hybrid multilevel converter for medium and high voltage applications

This paper investigates the suitability of the hybrid multilevel converter for medium and high voltage application. The converter operation, modulation, and capacitor voltage balancing method are described in detail. The ability of the hybrid multilevel converter to operate with different modulation indices and load power factors is investigated. It has been established that the hybrid multilevel converter is capable of operating independent of load power factor. Operation with variable modulation index increases voltage stresses on the converter switches and does not alter the fundamental voltage magnitude as in all known voltage source converter topologies. The viability of the hybrid multilevel converter for medium and high voltage applications is confirmed by simulations

Effect of tube inclination angle on the thermal and fluid dynamic performance of flat tube heat exchanger

At the present time, the performance of finned and flat tube heat exchangers (HEs) has become a very important issue in the thermal industrial sector. Finned-and-flat tube heat exchangers have gained great interest from many researchers due to their role in any thermal engineering system. Some geometrical and process parameters such as fin spacing, tube spacing, tube inclination angle, etc. affects the performance of fin-and flat tube heat exchangers. Significant number of research have been done to study the effect of such geometrical and process parameters. However, the effect of flat tube inclination angle on the thermal-hydraulic performance of fin-and-tube HE is not fully examined. Thus, the aim of this study was to investigate the effect of flat tube inclination angle, air velocity and tube configuration on the thermal and flow characteristics of compact fin-and-flat tube heat exchangers. A series of experimental and numerical investigations were carried out to evaluate the influence of the aforementioned parameters on the thermal-hydraulic performance between the tube bundles. Moreover, Response Surface Methodology (RSM) was used to determine the optimum parameter condition for the fin-and–tube HE. The range of the parameters considered in the study were tube inclination angle from 0˚ to 150˚, inlet air flow velocity from 1.8 to 3.8 m/s and tube configuration (inline and staggered). For the experiments, the wind tunnel available at the Faculty of Mechanical Engineering, UMP was used. The wind tunnel was equipped with a blower of capacity 50 W, flow straightener, test section and measuring sensors. Twelve plate fin and nine flat tube heat exchangers test sections were designed and manufactured at various inclination angles and configurations. Temperature, velocity, and pressure measurements were recorded at various positions in the test section as well as before and after the test section. For the numerical analysis, the CFD commercial software called ANSYS FLUENT-15 was used to solve the Navier-Stoke and energy equations together with proper turbulent equations. The parameters are similar to the experimental investigation. The experiment and numerical analysis used Nusselt number, pressure drop, and area goodness factor to evaluate the thermal-hydraulic performance of fin-and-flat tube heat exchangers. The major findings showed that flat tube inclination angle has significant impact on the heat transfer enhancement. However, the results from both experimental and numerical analysis revealed that increasing the tube inclination angle from 0˚ to 90˚ augments the convective heat transfer coefficient. While 120˚ and 150˚ provide thermal performance close to 60˚ and 30˚, respectively. The average deviations of Nusselt number between experimental and numerical results were 5.42% and 4.44% for inline and staggered configurations respectively. Moreover, due to the air flow blockage caused by inclining the tube angle, the pressure drop increases dramatically for all cases studied. From all acquired results, the best thermal performance occurred at 0˚, while 90˚ provided the minimum thermal performance. Therefore, inclining the tube is beneficial for enhancing the heat transfer performance but is not beneficial in term of pressure drop as it requires higher pumping power. The developed correlations from the RSM can predict experimental data with average deviation of 2.78% for Nu for both configurations. Moreover, it can predict the numerical data with average deviation of 0.554% and 0.92% for inline and staggered configuration, respectively. The optimum parameters are found to be with high air velocity and low tube inclination angle which provide maximum heat transfer enhancement and low-pressure drop penalty. Thus, it is recommended for the design of fin-and-flat tube HEs