The study of anti-stall techniques for fans and compressors has never been more significant, since the potential applications were recognised in gas turbines, tunnel ventilation (jet fans) and industrial process where fans/compressors would benefit from the devices. This thesis will discuss the techniques achieving competitiveness by modifying the conventional casing design of the fan and compressor, which is referred to as casing treatments, taking into account the change of maximum efficiency and flow range. An experimental investigation to examine the influence of the vaned recess casing treatment on stall margin, operating efficiency and flow field of a low speed axial flow fan with aerospace type blade loading is presented. Different geometrical designs of the vaned passages were examined and more than 65% of stall margin improvements and over twice pressure rise with insignificant peak efficiency change were obtained. Experiments to examine the effect of casing treatments on the flow field were carried out using the same fan rig with a tip clearance of 1.2% of the blade height. A high frequency data acquisition system including both hardware and software was developed and the 3-D flow measurements with a slanted hot-wire were undertaken. The first detailed results of flow measurements associated with the vaned recess casing treatment are presented, including both time-averaged and ensemble-averaged measurement results. The flow features in both the solid casing and treated casing builds are captured and comparison between the builds presented. The results revealed that the stall margin improvement due to the casing treatment was not achieved by reducing the incidence nor by increasing the total pressure in the tip region. It appeared that the combined functions of elimination of the whirling flow, the removal of the randomness of the inlet flow and modification of the tip clearance flow are salient features associated with the mechanism of the treatment. The steady flow field of the test fan with various tip clearances has also been simulated in the rotating frame with computational fluid dynamics (CFD) to investigate the clearance effect on the end-wall flow development and to elucidate the behaviour of the tip leakage flow, and hopefully shed more light onto the flow phenomena involved.

Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / Dissertation presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering at Stellenbosch University. / ENGLISH ABSTRACT: Airworthiness certification of unmanned aircraft systems (UAS) is normally considered to be a regulatory function. In the absence of comprehensive UAS airworthiness regulations, the development of new and unique UAS, and their introduction into non-segregated airspace, remain major challenges for the UAS industry and regulators. Thus, in response, the objective of this research was to establish a framework and guidelines, within the scope of the typical regulatory regime, that can be used by the UAS engineering domain to ensure the safe and reliable functioning of a UAS, whether regulated or not. UAS airworthiness is currently mainly based on manned aircraft regulations, and the focus is on the unmanned aircraft and the 'airworthiness' of the remote control station. The typical UAS as a system, however, consists of more than just these elements and a broader approach to the 'airworthiness' of a UAS is required. This study investigated and introduces the concept of UAS operability, where the term 'operability' addresses the safe and reliable functioning of the UAS as a system, the airworthiness of its airborne sub-systems, and the safe and reliable functioning of its non-airborne subsystems and functional payloads. To ensure that the results of this study are aligned with typical aviation regulatory systems, a regulatory basis was defined within which UAS operability guidelines could be developed. Based on the operability concept, and in the scope of the regulatory basis, a UAS operability framework was developed for the UAS engineering domain. This framework is an index and reference source from which appropriate operability elements can be selected for a particular UAS. The scope of the framework is generic, rather than UAS-type or -class specific, and includes operability elements for the UAS as a system, for its airborne and non-airborne sub-systems, and for its payloads. The framework was validated by developing lower hierarchical levels for the framework and by populating each operability element of the framework with appropriate engineering guidance criteria. The guidance criteria were derived and/or developed from industry 'best practices' found in the literature, or were newly developed where no existing practices were found. The significance of this study is found in its establishing of a generic UAS operability framework that not only focuses on the airworthiness of the unmanned aircraft, but addresses the operability of the UAS as a system, as well as the operability of its airborne sub-systems, its non-airborne sub-systems and its payloads. In practice, the UAS operability framework can be used in the UAS engineering domain as an index and reference source to select relevant operability elements for a particular UAS. The guidance criteria for the selected elements can subsequently be used to develop the appropriate processes, procedures, requirements and specifications to achieve initial operability of the UAS, and to maintain its continued operability. Although the objective of the research was achieved, the UAS operability framework must still be applied and tested in real-life UAS projects and, where necessary, revised to eliminate shortcomings and to provide for new and novel developments in UAS engineering technologies. / AFRIKAANSE OPSOMMING: Die lugwaardigheidsertifisering van onbemande vliegtuigstelsels (OVS) word normaalweg beskou as 'n reguleringsfunksie. In die afwesigheid van omvattende OVS lugwaardigheidsregulasies bly die ontwikkeling van nuwe en unieke OVS, en die inbedryfstelling daarvan in onafgesonderde lugruim, besonderse uitdagings vir beide die OVS nywerheid en reguleerders. Die doelwit van hierdie navorsing was dus om riglyne binne die bestek van die tipiese reguleringsregime te vestig wat deur die OVS ingenieursdomein benut kan word om die veilige en betroubare funksionering van 'n OVS te verseker, of dit gereguleer word aldan nie. OVS lugwaardigheid word tans hoofsaaklik gebaseer op lugwaardigheidsvereistes vir bemande vliegtuie. Die fokus is dan ook meerendeels op die onbemande vliegtuig en die 'lugwaardigheid' van die afstandbeheerstasie. Die tipiese OVS bestaan egter uit meer sub-stelsels en 'n weier beskouing van die 'lugwaardigheid' van 'n OVS is nodig. Die konsep van OVS bedryfbaarheid is in hierdie studie ondersoek en voorgestel. 'Bedryfbaarheid' beteken in hierdie konteks die veilige en betroubare funksionering van die OVS as 'n stelsel, die lugwaardigheid van die lug sub-stelsels, die veilige en betroubare funksionering van die nie-lug sub-stelsels, asook die veilige en betroubare funksionering van funksionele loonvragte. Om te verseker dat die resultate van hierdie studie versoenbaar is met tipiese lugvaart reguleringstelsels, is 'n reguleringsbasis omskryf vir die ontwikkeling van OVS bedryfbaarheidsriglyne. Gebaseer op die bedryfbaarheidskonsep, en binne die riglyne van die reguleringsbasis, is 'n OVS bedryfbaarheidsraamwerk ontwikkel vir die OVS ingenieursdomein. Die raamwerk is 'n indeks en verwysingsbron waaruit gepaste bedryfbaarheids-elemente gekies kan word vir 'n bepaalde OVS. Die bestek van die raamwerk is generies en nie beperk tot spesifieke OVS tipes of klasse nie. Die raamwerk sluit bedryfbaarheidselemente in vir die OVS as stelsel, asook vir die lug en nie-lug sub-stelsels van die OVS, en vir die loonvragte van die OVS. Die raamwerk se geldigheid was bevestig deur die struktuur van die raamwerk tot laer vlakke uit te brei en gepaste ingenieursriglyne vir elke bedryfbaarheids-element in die raamwerk te ontwikkel. Die riglyne was gebaseer op 'beste praktyke' soos beskryf in die literatuur, of was van nuuts af ontwikkel waar geen bestaande praktyke gevind kon word nie. Die bydrae van hierdie studie is gesetel in die vestiging van 'n generiese OVS bedryfbaarheidsraamwerk wat nie net gemik is op die lugwaardigheid van die onbemande vliegtuig nie, maar wat die bedryfbaarheid in geheel van die OVS as stelsel aanspreek, asook die bedryfbaarheid van die OVS se lug sub-stelsels, nie-lug sub-stelsels en loonvragte. In die praktyk kan die raamwerk in die OVS ingenieursdomein gebruik word om gepaste bedryfbaarheids-elemente vir 'n OVS te kies. Daarna kan die bedryfbaarheidsriglyne gebruik word om gepaste prosesse, prosedures, vereistes en spesifikasies te ontwikkel om die OVS se aanvanklike en voortgesette bedryfbaarheid te bewerkstellig. Alhoewel die doelwit vir die navorsing bereik is, moet die OVS bedryfbaarheidsraamwerk nog op werklike OVS projekte getoets word. Waar nodig, moet die raamwerk dan hersien word om tekortkominge, asook nuwe en unieke ontwikkelinge in OVS ingenieurstegnologie, aan te spreek.

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