4,507 research outputs found
A direct A.C. to A.C. regenerative frequency and voltage converter
No full textIncludes bibliographical references.The reliable variable speed operation of an a. c. machine over a wide speed range is a problem that has received attention for some time. In this thesis a system to permit such operation from a fixed frequency fixed voltage supply is proposed, under the name of the asynchronous modulation converter. This converter is of the cycloconverter family, but is force-commutated and is not synchronised with the supply frequency. The power switching element comprises a power transistor in a diode bridge, coupled to the control circuitry by opto-isolators. The output of the a-mod converter is produced by a "chopper" modulation process and this results in the harmonics present being at frequencies well removed from the fundamental. A feedback system to ensure smooth commutation of the output current from the power switches is described. This feedback system operates by sensing the rate of rise of the inductive voltage surge on the load when a switch is opened to operate a freewheel path switch
Polymer multimode waveguide optical and electronic PCB manufacturing
No full textThe paper describes the research in the £1.3 million IeMRC Integrated Optical and Electronic Interconnect PCB Manufacturing (OPCB) Flagship Project in which 8 companies and 3 universities carry out collaborative research and which was formed and is technically led by the author. The consortium’s research is aimed at investigating a range of fabrication techniques, some established and some novel, for fabricating polymer multimode waveguides from several polymers, some formulations of which are being developed within the project. The challenge is to develop low cost waveguide manufacturing techniques compatible with commercial PCB manufacturing and to reduce their alignment cost. The project aims to take the first steps in making this hybrid optical waveguide and electrical copper track printed circuit board disruptive technology widely available by establishing and incorporating waveguide design rules into commercial PCB layout software and transferring the technology for fabricating such boards to a commercial PCB manufacturer. To focus the research the project is designing an optical waveguide backplane to tight realistic constraints, using commercial layout software with the new optical design rules, for a demonstrator into which 4 daughter cards are plugged, each carrying an aggregate of 80 Gb/s data so that each waveguide carries 10 Gb/s
Zane Grey with Orange A.C. baseball team
No full textThis photograph shows Ohio native Zane Grey with the Orange A.C. Baseball Team, ca. 1896-1904. Prior to becoming a well-known author of western novels, Zane Grey was also a dentist and a semi-professional baseball player, with teams including the Orange Athletic Club and the Newark (New Jersey) Colts
Experimental research on spatial distribution of overtopping
No full textThe overtopping empirical formulas calculate the discharge only at the top of the crest of a coastal protection structure. On the other hand, the tolerable overtopping discharges are defined at certain points behind the crest where the total overtopping is reduced. The scope of this thesis is to find an empirical formula to describe the distribution of overtopping at the space behind the crest. This thesis comes as a further investigation on the work conducted by v.Kester [2009] for regular waves. In this research, a physical model was developed on which irregular waves are tested. Because of the duration of the tests and the amount of collected water (significant lose of water during the test), a completely new measuring system was designed. Five influencing parameters (variables) are considered on this research: wave height, wave period/steepness, slope angle, crest freeboard and crest permeability. The entire overtopping process is analysed separately for the total overtopping discharge, the overtopping discharge directly behind the crest and the distribution of overtopping behind the structure. In the analysis of the data collected from the measurements, the impact of the varying parameters is investigated leading to useful conclusions and better understanding of the entire process. Additionally, the experimental findings are analysed and compared to the relative existing methods. Based on the TAW [2002] method which is proposed by the EurOtop Manual [2007], a prediction formula is developed. This formula is a generic version of TAW [2002] formula in which a new reduction factor ?c is introduced in order to describe the decay of the overtopping and thus predict the discharge at any certain distance behind the crest. Other relevant methods are also analysed (Juul Jensen [1984], Steenaard [2002], Besley [1999] and v.Kester [2009]) and conclusions for their applicability are drawn leading to suggested improvements or corrections. Apart from the distribution of overtopping, on this thesis the determination of crest freeboard (which is an ambiguous issue) is also investigated. Finally, suggestions of further research on this topic are discussed. The entire work has been perfomed in close cooperation with van Oord.Hydraulic EngineeringCivil Engineering and Geoscience
Flow over partially liquid filled cavity
No full textExperiments have been carried out to investigate the effect of liquid cavity filling on the behavior of the gas flow over a flat plate cavity. PIV measurements in the gas phase reveal that cavity filling can affect vortex shedding in the cavity mouth. Shear layer vortices can break-up into smaller vortices, thereby losing their periodic interaction with the aft wall and, hence, their sound producing potential. Expected is that this is one of the mechanisms causing sound mitigation in corrugated pipes with liquid addition, observed in literature
On modular lattices generated by chains
No full textDe La Maza, A.C. (reprint author). Instituto de Matematica y Fisica, Universidad de Talca. Casilla 747, Talca, Chile.We describe the free modular lattice generated by two chains and a single point, under the assumption that there are few meets
Effectiveness of management strategies for uninvestigated dyspepsia: systematic review and network meta-analysis
No full textOBJECTIVE: To determine the effectiveness of management strategies for uninvestigated dyspepsia. DESIGN: Systematic review and network meta-analysis. DATA SOURCES: Medline, Embase, Embase Classic, the Cochrane Central Register of Controlled Trials, and clinicaltrials.gov from inception to September 2019, with no language restrictions. Conference proceedings between 2001 and 2019. ELIGIBILITY CRITERIA FOR SELECTING STUDIES: Randomised controlled trials that assessed the effectiveness of management strategies for uninvestigated dyspepsia in adult participants (age ≥18 years). Strategies of interest were prompt endoscopy; test for Helicobacter pylori and perform endoscopy in participants who test positive; test for H pylori and eradication treatment in those who test positive ("test and treat"); empirical acid suppression; or symptom based management. Trials reported dichotomous assessment of symptom status at final follow-up (≥12 months). RESULTS: The review identified 15 eligible randomised controlled trials that comprised 6162 adult participants. Data were pooled using a random effects model. Strategies were ranked according to P score, which is the mean extent of certainty that one management strategy is better than another, averaged over all competing strategies. "Test and treat" ranked first (relative risk of remaining symptomatic 0.89, 95% confidence interval 0.78 to 1.02, P score 0.79) and prompt endoscopy ranked second, but performed similarly (0.90, 0.80 to 1.02, P score 0.71). However, no strategy was significantly less effective than "test and treat." Participants assigned to "test and treat" were significantly less likely to receive endoscopy (relative risk v prompt endoscopy 0.23, 95% confidence interval 0.17 to 0.31, P score 0.98) than all other strategies, except symptom based management (relative risk v symptom based management 0.60, 0.30 to 1.18). Dissatisfaction with management was significantly lower with prompt endoscopy (P score 0.95) than with "test and treat" (relative risk v "test and treat" 0.67, 0.46 to 0.98), and empirical acid suppression (relative risk v empirical acid suppression 0.58, 0.37 to 0.91). Upper gastrointestinal cancer rates were low in all trials. Results remained stable in sensitivity analyses, with minimal inconsistencies between direct and indirect results. Risk of bias of individual trials was high; blinding was not possible because of the pragmatic trial design. CONCLUSIONS: "Test and treat" was ranked first, although it performed similarly to prompt endoscopy and was not superior to any of the other strategies. "Test and treat" led to fewer endoscopies than all other approaches, except symptom based management. However, participants showed a preference for prompt endoscopy as a management strategy for their symptoms. SYSTEMATIC REVIEW REGISTRATION: PROSPERO registration number CRD42019132528
Quantized current steps due to the a.c. coherent quantum phase-slip effect
No full text| openaire: EC/H2020/862660/EU//QUANTUM E-LEAPS Funding Information: This work was supported by European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 862660/QUANTUM E-LEAPS and Engineering and Physical Sciences Research Council (EPSRC) grant no. EP/T004088/1. Publisher Copyright: © 2022, The Author(s), under exclusive licence to Springer Nature Limited.The a.c. Josephson effect predicted in 19621 and observed experimentally in 19632 as quantized ‘voltage steps’ (the Shapiro steps) from photon-assisted tunnelling of Cooper pairs is among the most fundamental phenomena of quantum mechanics and is vital for metrological quantum voltage standards. The physically dual effect, the a.c. coherent quantum phase slip (CQPS), photon-assisted tunnelling of magnetic fluxes through a superconducting nanowire, is envisaged to reveal itself as quantized ‘current steps’3,4. The basic physical significance of the a.c. CQPS is also complemented by practical importance in future current standards, a missing element for closing the quantum metrology triangle5,6. In 2012, the CQPS was demonstrated as superposition of magnetic flux quanta in superconducting nanowires 7. However, the direct flat current steps in superconductors, the only unavailable basic effect of superconductivity to date, was unattainable due to lack of appropriate materials and challenges in circuit engineering. Here we report the direct observation of the dual Shapiro steps in a superconducting nanowire. The sharp steps are clear up to 26 GHz frequency with current values 8.3 nA and limited by the present set-up bandwidth. The current steps were theoretically predicted in small Josephson junctions 30 years ago5. However, unavoidable broadening in Josephson junctions prevents their direct experimental observation8,9. We solve this problem by placing a thin NbN nanowire in an inductive environment.Peer reviewe
Systematic review with meta-analysis: association of Helicobacter pylori infection with gastro-oesophageal reflux and its complications
No full textBackground: Conflicting results exist on the association between Helicobacter pylori infection and gastro-oesophageal reflux (GOR), and its complications, such as erosive oesophagitis (EO) and Barrett's oesophagus (BO). Aims: To explore the association of H. pylori infection with GOR symptoms and their complications. Methods: We searched Embase, PubMed, Web of Science and Scopus databases through December 2020 for relevant articles. Regarding the association between H. pylori and GOR symptoms (heartburn, regurgitation or reflux), we included observational studies comparing the prevalence of GOR symptoms between H. pylori-positive and -negative individuals. Concerning the association between H. pylori and complications of GOR, we included studies comparing the prevalence of EO or BO between H. pylori-positive and -negative individuals. Results: In total, 36 papers were eligible. Based on seven cross-sectional surveys, H. pylori infection was associated with a lower odds of GOR symptoms (odds ratio [OR] 0.74, 95% confidence interval [CI] 0.61-0.90). However, in four case-control studies, H. pylori infection was not associated with odds of GOR symptoms (OR 1.10, 95% CI 0.85-0.1.43). In 26 cross-sectional studies in patients with GOR symptoms, the OR for EO was 0.70 (95% CI 0.58-0.84) in H. pylori-positive vs -negative cases. Based on nine cross-sectional studies in subjects with GOR complications, no significant association was found between H. pylori infection and either endoscopically-diagnosed (OR 1.84, 95% CI 0.67-5.02) or histologically confirmed (OR 0.85, 95% CI 0.60-1.20) BO. Conclusions: Helicobacter pylori infection appears to be associated with a decreased odds of GOR symptoms and EO. In contrast, H. pylori infection did not seem to affect the odds of BO in patients with GER complications
Stabiliteit van het achtertalud van een rubble mound golfbreker
No full textCivil Engineering and Geoscience
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