PhD and Masters by research theses (SoE)

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Open Access
Optimal energy management for electric aircraft.
(Cranfield University, 2012-03) Omar, Baba S. A.; Savvaris, Al
Current technological advances in aviation are geared towards a more "electric aircraft"; seen as a step towards making the aircraft more efficient, reducing emissions and minimising environmental impact. This thesis investigates the feasibility of using an electric hybrid system consisting of a 1.2KW proton exchange membrane fuel cell (PEMFC), three 12V lead acid batteries, a unidirectional stepdown DC/DC converter, a bidirectional DC/DC converter to power a small aircraft. Using this hybrid configuration the desired power for different flight phases could be achieved. The advantage of PEMFCs is their high power density, low volume and light weight. They operate at relatively low temperatures that allow them to start up quickly without a warming up time. The thesis starts with a literature review of fuel cell technology and a description of a simulation model for the hybrid system developed using MATLAB/Simulink. Initial experimental results for model validation and the system components performance characteristics will be presented. The experimental data were obtained using 1.2kW Nexa power module FC. With regards to the power management aspect, two control strategies have been applied: (1) a traditional (PID) controller was designed and implemented in hardware-in- the-loop to control the battery current via the bidirectional DC/DC converter in the hybrid system, and, (2) replacing the PM controller with a fuzzy logic controller (FLC), because significant time delays were found under PID control. The FLC was implemented to manage the power between two sources in the hybrid system because there are many reports in the literature of successful application of FLC to distribute power between different sources in automotive applications. FLC design was based on the following principle: the PEMFC is the main power supply for the electric engine driving the propeller and it has to be operated at optimal efficiency. In the case that the fuel cell cannot completely meet the power demands in any of the flight phases, the battery will provide a short power burst demand as required. At the same time the FC should keep the battery adequately charged at all times. The controller's input variables are the electric engine power demand and battery state of charge represented by the battery voltage. The output variable is the bidirectional DC/DC converter output power. The use of a FLC allowed the system to operate at an optimal efficiency, satisfying the aircraft's power demand during different flight phases.
Open Access
Progressive damage analysis of composite layered plates and shells using finite strip methods
(2005-04) Zahari, Rizal; El-Zafrany, A.
As far as design of composite structures is concerned, it is important that we are able to accurately determine the failure modes and damage progression of structures made of composite materials. This will in turn, aid structural designers to develop reliable and safe designs which will exploit the advantages offered by composite materials. With the development of numerical analysis strategies towards non-linear progressive failure algorithms and the increase in computing capacity, it has now become possible to replace the existing experimental data with numerical results. Verified numerical models offer a practical method for exploring parametric study. Hence, it is essential to devise a reliable progressive damage scheme in order to predict the damage propagation in composite laminated structures using efficient computational tools such as the finite strip method. In this thesis, a new finite strip method, for non-linear stress analysis based on the tangential stiffness matrix has been developed using the new concept of polynomial and spline-type finite strip elements, with Mindlin and Reissner plate-bending theories for composite plates and shells. A progressive damage methodology and algorithm for composite laminates was successfully developed for the new finite strip methods using stress-based failure criteria. A finite strip analysis programming package which is capable of performing nonlinear progressive damage analysis for composite plates and facetted shells, has also been developed with Mindlin and Reissner plate bending elements. Validation of the developed finite strip package has been successfully carried out by comparing the results with corresponding results obtained with the finite element analysis using ABAQUS and with some published experimental results. Good comparison with the finite element results and experimental results were observed through various test cases, confirming the accuracy and reliability of the new derivations and the programming package.
Open Access
A new method using b-splines as shape functions and the knot insertion algorithm for shape optimization
(2007-09) Xu, Bo; Sherar, Peter A.; Thompson, Chris
A new method is developed to deal with shape optimization problems. The core idea of the method is to introduce the knot insertion algorithm which keeps the geometry unchanged while increasing the number of control points. In addition to this idea, the super-reduced idea and the mesh refinement are also employed to deal with the equality constrained optimization problem. The developed method has been tested on several applications. The first application is a Poisson equation problem. The result produced by the new method is compared with the result produced by the BFGS method because the BFGS method is considered to be one of the best methods in optimization. The result shows that the new method is more efficient than the BFGS method. The second application of the method is an airfoil design problem. The performance of using the new method is compared with the performance of using the EXTREM method in two design cases, RAE 2822 and NACA 0012. In both of these cases, the new method is much more efficient than the EXTREM method. The 3-dimensional tested case used is a mathematical problem. In this case, one finds that the discretization error is much bigger when compared with the 2-dimensional case. However the method still converges quickly to the optimum solution. As well as the above applications, the use of high order B-splines and multi-objective optimization have also been investigated. In summary, a new method is developed for shape optimization problems and validation has been carried out on several numerical examples. With the idea of the new method, it is possible to improve the efficiency of the method currently used as long as B-splines are used to describe the geometry.
Open Access
Development of a liquid-phase LPG MPI conversion system
(2004-10) Vuorenkoski, Anni Kaisa; Jermy, Mark; Harrison, M. F.
For decades Liquefied Petroleum Gas (LPG) has been considered as one o f the most prominent alternative fuels to petrol. LPG typically consists of propane, butane and propylene, but also smaller quantities o f methane and ethane. LPG has a low price compared to petrol, the potential o f low emissions and has indigenous availability. It possesses approximately the same energy density, on a gravimetric basis, as petrol, vaporises easily, mixes readily with air, is resistant to auto-ignition, has wide flammability limits and a good laminar burning speed. Therefore it is also possible to achieve an acceptable efficiency and power from a spark ignition engine run with LPG. The state-of-the-art LPG systems used in spark-ignition engines are either mono-fuel systems, where the vehicle is solely operated using LPG, or bi-fuel vehicles, capable o f using either petrol or LPG. The objectives o f this work were to develop an aftermarket conversion bi-fuel LPG system, which would improve the efficiency o f the engine during LPG operation, with further improvements in the mixture preparation and control, the methods for LPG fuelling calibration and the methods to prevent premature vaporisation in the fuel rail. An additional objective o f this study was to investigate the performance and combustion o f LPG in a non-optimised spark-ignition engine. A prototype system was developed and demonstrated in a 4-cylinder research engine. This novel system uses a liquid LPG injection system, in contrast to the conventional vapour injection systems used in aftermarket LPG bi-fuel conversions. A significant improvement in engine power output was shown, as well as an improvement in mixture control. An optical diagnostics method was applied in order to study the mixture preparation using two alternative LPG fuel injection configurations. The results from both the mixture formation study and the engine experiments showed that the charge cooling effect can be used to improve the efficiency o f a non-optimised bifuel engine. It was also shown that from the mixture control point o f view, injecting the fuel directly to the manifold gives a significant advantage over systems where the fuel is injected to the manifold through coupling pipes. This novel LPG system also uses a fuel pressurising method that improves the fuel system performance in extreme conditions. In addition, a control method to prevent the premature vaporisation of the LPG fuel in the fuel supply line was developed. The method comprises an optical sensor which can detect migrating vapour bubbles in addition to complete phase changes in the fuel line. It was noticed during validation o f the sensor that vaporisation in the fuel rail starts in local hot spots, before the global saturation conditions in the fuel rail are met. This work has demonstrated the potential o f using non-optimised LPG systems in bifuel vehicles. However, the final validation o f the novel control system requires extensive testing on a fleet of test vehicles, and this was not possible within the scope of the work.
Open Access
Tuneable and switchable dual wavelength laser diodes using fibre Bragg grating external cavities
(2003) Reilly, Simon Philip; James, Stephen W.; Tatam, Ralph P.
This thesis describes two external cavity laser diode designs. The first utilises a Bragg grating fabricated in highly biréfringent optical fibre and offers, through the use of a waveplate, the ability to switch between modes that are separated in both polarisation and wavelength, due to the differing refractive index in either eigenmode. The laser offers three stabilised states of operation, single mode operation for either axis of the fibre, or a third state in which both modes lase simultaneously with a wavelength separation of 0.3 nm. The application of transverse strain on the fibre Bragg grating was also demonstrated as a method of tuning the wavelength separation between these modes. The second external cavity laser design utilises two spatially and spectrally separate Bragg gratings fabricated in mono-mode fibre. This allows two longitudinal modes corresponding to the Bragg wavelengths to oscillate simultaneously. The application of longitudinal strain allowed either fibre Bragg grating to be tuned, thus generating a stabilised tuneable beat frequency, which was demonstrated between 130 GHz - 2.28 THz. A three fibre Bragg grating laser is also presented which allowed three modes to oscillate simultaneously. Both the laser based on a Bragg grating fabricated in highly biréfringent fibre and the laser based on multiple Bragg gratings fabricated in single mode fibre demonstrated reduced injection current threshold and mode hop free operation over the full injection current range. A practical application for the two fibre Bragg grating lasers is presented where the properties o f independently tuneable dual wavelength operation are used to interrogate a miniature fibre Fabry-Perot sensor. The wavelength separation o f the two oscillating modes can be tuned to reach a point of quadrature for the sensor cavity. A sensing system demonstrating this technique is presented which detected vibrations up to 500 Hz in cavities of 16-34 pm.
Open Access
Raman optical time domain reflectometry for aircraft fire-overheat detection and monitoring
(2004-07) Odic, Richard Marie; Tatam, Ralph P.
The analysis of the development context for a distributed temperature detection system in an aircraft cargo compartment and the review of candidate technologies stressed the advantages of using the spontaneous Raman scattering principle with a signal processing in the time domain. Simultaneously, the wished detection performance defines the hardware specifications of the system. For the "Cargo Compartment Fire Detection and Extinguishing" application, several design parameters were identified; namely: the spatial and sampling resolution, the temperature accuracy and its resolution, the possible sensing element length for a given peak optical power per pulse, the refreshment time to get a temperature profile over the sensing element length, the operating/survival temperature ranges and the mechanical constraints at the sensing element in operation or during installation and the possibility to discriminate an optical fibre break from an increase in temperature. To reject the noise generated and amplified by the Avalanche Photo-Diodes (APD) and by the cascaded amplification modules, the numerical signals were . processed using digital filters. The detection system has also to early detect a fire while rejecting fire-like events. Its detection performances are, namely: a temperature profile refreshment time of 3s, a temperature resolution of± 1 °C around the alarm signal, a spatial resolution of 1.3m for a sensing element length of 500m. The detection of a temperature increase is a spatial analysis organized through specific algorithms. These algorithms involve processing the temperature time derivative, the spatial gradient, the variation of danger surfaces, an analysis of the temperature spatial frequencies and the calculation of the power spectral density and of the temperature covariance matrix. Fire tests were performed and the optical fibre within a protective stainless steel tube was connected to a control unit to monitor the danger situation over an aircraft cargo compartment.
Open Access
The development of a design methodology for large cargo aircraft
(2008-11) Mederer, A.; Fielding, John
Detailed specialist knowledge of pre-design, design, aerodynamics, engine performances and flying performances is necessary for the pre-design of transport aeroplanes. In this paper, studies are described which provide the main detailed information on the above-mentioned specialist topics. All studies consists of a problem with a suitable solution and the associated results. The studies are part of the overall task “Systematic design study on a number of configurations for large aircraft types, in particular for cargo aeroplanes”. As part of the overall task, a design method was developed which is very user-friendly and which is suitable for the design of any aeroplane configurations, in particular for configurations of large passenger or transport aeroplanes. This thesis version shows how 11 transport aeroplane configurations of very different types can be drawn up. The configuration results are shown on a scale of 1 : 500 and can be directly used for more detailed studies, as they are based on calculations. 4 of the first 5 transport aeroplane configurations are basic configurations and were scaled between 200 to 10001 . Important flying performance characteristics were calculated for the configurations by the use of 8 PC programs additionally developed for this purpose. Validity studies have shown that the described design method provides useful results. The main result of the method is that very different transport aeroplane versions can be developed and compared in terms of flying performances. Currently, only a few very complex design methods are offered elsewhere which have the disadvantage that they are only intended for one aeroplane version. So far, the design method described in this method is fully applied to the first five configurations. A next step could be to also investigate the additional six configurations listed here with the method.
Open Access
A study of single and two-phase flows in devices with narrow flow passages
(2002-05) Laskey, S. J.; Yeung, Hoi
In many industries, the depressurisation of gas-saturated solutionseis controlled to regulate bubble formation. Carbonated drink dispensers need to depressurise solutions with minimum bubble formation, whereas dissolved air flotation nozzles need to produce the maximum number of micro-bubbles. Four commercial carbonated drink dispensers were tested. The dispenser predicted to retain the most dissolved carbon dioxide at the outlet had a narrow annular gap of 0.1mm at the narrowest point. The pressure drop across this device varied linearly with water flow rate. When tested with two-phase air and water flow, the pressure drop decreased with increasing air flow at given water flowrates. This unusual behaviour was thought to be due to the narrow flow path. Carbon dioxide-saturated water tests supported these results as the pressure drop was found to be lower than the single-phase water tests. Thus under similar conditions, devices that create less turbulence would retain more dissolved gas. Flow in coils was investigated, as they have flow characteristics that were potentially suitable for carbonated drink dispensing. Compared to straight pipes, flow in coils remained laminar until higher Reynolds numbers. The friction factors were also higher in coils than straight pipes. Coils made from 0.0025m internal diameter polyurethane tubing were tested, with coil diameters of 0.029m, 0.079m and 0.139m and lengths of 2, 3, 3.7, 5 and 7m. A method of estimating the friction factors in coils by treating them as a series of 90° bends was proposed. The calculated results agreed with the present small tube experiments and with data from published literature for a range of tube diameters. At a given pressure drop, the shortest coil with the smallest coil diameter had the greatest dissolved gas concentration at the outlet and the highest flowrate. Furthermore, the concentration of dissolved gas at the coil outlet was greater than at the nozzle outlet.
Open Access
The development of usability scales for flight management systems (volumes one and two)
(2002) Lane, Karen P.; Harris, Don
Usability is a multidimensional concept, and although a number of techniques have been developed for assessing usability, none have been developed specifically for the civil flight deck or its components. The requirement for Human Factors tools to support civil flight deck certification have been well documented (e.g. FAA, 1996). This thesis reports on a research programme that resulted in the development of a series of scales to assess the usability of Flight Management System (FMS) Control and Display Units (CDUs). Since the goal of this research was to produce scales that could be used by certification authorities and FMS manufacturers, demonstrating that the scales had appropriate psychometric properties was critical. A rigorous and systematic approach to the construction of the usability scales was therefore adopted. The research was conducted in three studies. The first study involved the development of a 49-item questionnaire that was distributed to airline pilots. A series of analyses on the data collected demonstrated that a reliable and valid usability factor structure was produced. The second study focused on the production of usability scales and examined the impact of flight phase on usability ratings. Analyses demonstrated that the factor structure produced in the first study was reliable, and was valid for data collected for the cruise and descent and approach phases of flight. A series of internally consistent usability scales was therefore produced which demonstrated evidence of discriminant, face, content, and construct validity. The third study involved a content analysis on a large volume of participant comments derived during studies one and two, and examined the usability problems associated with current FMSs and suggested improvements for future FMSs. The content analysis produced evidence of construct validity for the usability scales developed, and permitted recommendations for FMS designers and certification authorities to be made.
Open Access
Wet gas flow metering with pattern recognition techniques
(2004-04) Jama, Ali Ahmed; Sanderson, M. L.
The development of many gas condensate fields and the increasing number of marginal fields whose economics do not support conventional bulky separation and processing facilities means that new wet gas flow metering techniques are becoming of greater importance to the oil and gas industry worldwide. For the purpose of this research wet gas flow is defined as multiphase flow (gas-liquid) having in-situ gas volume fraction greater than 95 % at the point of measurement. This research presents a novel wet gas measurement technique involving the use of a standard Venturi meter together with advanced pattern (PR) recognition methods for the detection of liquid presence in wet gas flow conditions and the simultaneous measurement of gas and liquid flow rates without the need for preconditioning of the flow or prior knowledge of either phase. The technique involves four major steps: 1) collection of experimental data spanning the range of flow regimes likely to be encountered in wet gas flow conditions; 2) extraction of flow dependent variables from the Venturi pressure sensors in the form of features; 3) development of PR model for mapping between input features and corresponding gas and liquid flow rates; 4) generalisation test to new and previously unseen flow conditions to determine the accuracy of the Venturi-PR methods developed in this research work. Data was sampled at 50 Hz using two axial differential pressure sensors and one singleend absolute pressure sensor on a 2-inch horizontally mounted Venturi meter using airwater at normally atmospheric conditions. Extensive features were extracted from the time and frequency domains of the raw data and evaluated for their discriminatory ability between different flow conditions. A Bayesian multi layer perceptron (MLP) neural network was used to construct a non-linear mapping between the different feature vectors and the corresponding gas and liquid flow rates using a correctly labelled training data. When the generalisation performance of different measurement scenarios developed was tested, the cross-sensor data fusion of the amplitude features achieved 100 % of the test data to within ± 5 % error across the whole flow domain of interest. The Venturi-PR results also performed significantly better than published wet gas differential pressure flow correlations over the flow domain of interest.
Open Access
Distortion management effects on jet engine performance
(2005-11) Isseris, Christos; Pilidis, Pericles
Boundary layer separation studies have been going on for many years. The concept of flow control is not new, with boundary layer blowing or suction to delay separation known since Prandtl. Many flow control methods are considered nowadays to introduce improvements on flow characteristics both for internal and external applications. This work is concerned with the application of flow control methods for improving the flow characteristics within intake ducts. Flow control applications are presented with a challenge to improve the flow characteristics of intake ducts with complex geometries that are used to deliver airflow to buried propulsion systems, common for military engine applications. This work is analysing the flow control capabilities for improving s-shaped ducts flow characteristics and how these improvements translate onto engine performance. For the purpose of this study a methodology was created in order to simulate an intake/compressor/engine interaction and analyse the characteristics of all three components. Three-dimensional flow simulations have been carried out for the intake and compressor models and compressor performance results have been transferred to an engine performance tool to investigate performance parameters under different inflow conditions. The case studies involved axial inflow simulations and cases with inflow of different pitch and yaw angles. The influence of these conditions has been transferred from the intake model to the compressor model and the performance characteristics used to analyse the engine operation under these conditions. Flow control methods have been applied to the system in order to minimise the negative effects of the inflow and improve the flow characteristics at the intake duct. Furthermore, air or power off-takes requested from the engine in order to operate the flow control mechanisms have also been considered. The results showed that the application of flow control introduces improvements throughout the system components. These improvements are sufficient in order to compensate for the air and power off-takes requested by the engine for the use of the flow control mechanisms. Although the results support such a conclusion it is also important to point out that some case studies especially for low distorted flows showed a very small improvement in engine performance which can conclude that flow control may not be as effective when the cost and complexity introduced to the system is considered.
Open Access
FEEP thruster nano-satellite application
(2004-01) Fernando, W. C. P.; Bowling, Tom
Open Access
Novel ice protection system suitable for UAV composite wings
(2005-02) Bhakta, Besant; Jones, R. I.
The existence o f an atmospheric icing th re a t to a irc ra ft operating in moist, low altitudes at temperatures below freezing is well known. The objective o f this study was to develop an a irc ra ft ice protection system suitable fo r a UAV composite wing. Conventional wing leading edge ice protection systems were examined and found to either, necessitate significant electrical power, or were costly with respect to system mass. A low cost and low power technology capable o f protecting the UAV wing leading edge was identified. I t was proposed th a t a commercial magnetostrictive actuator fitte d to and in direct contact with the non-airflow wing surface would provide mechanical impulses to break the ice-wing surface bond. Assuming the accreted ice was o f a form expected o f te s t points in the FAR/ JAR Appendix C flight-icing envelope. Computational simulations demonstrated th a t pairs o f magnetostrictive actuators acting in unison a t a 0.3m span spacing, and deployed along the upper and lower wing leading edge surfaces a t around the 7% chord coordinate, would successfully break the ice-surface bond. I t was estimated th a t fo r a medium endurance UAV o f Predator B class, the proposed system power requirements is 500W at a 45kg total system mass. The proposed system would be more competitive than conventional systems if the use o f consumer grade electronics and control systems, harnessing etc. were permitted, together with the removal o f system redundancy and fail-safe provision requirements necessary fo r manned aircraft. Further work would require the demonstration o f a physical de-icing installation in the icing wind tunnel.
Open Access
An analysis of fibre optic interferometer configurations for optical coherence tomography applications
(2003-10) Beddows, Ruth; Tatam, Ralph P.
The Michelson Interferometer has been the principal interferometer configuration for Optical Coherence Tomography (OCT) imaging. For OCT applications, in particular endoscopy, there may be polarisation induced signal fading due to environmental changes within the sample arm. The most significant cause of polarisation induced fading within OCT systems is suggested to be due to changes within the optical fibres. The Fizeau interferometer has been shown to eliminate this polarisation induced fading as the sample and reference arm experience the same environmental effects. However the Signal to Noise Ratio (SNR) o f the Fizeau configuration is lower than that o f the Michelson because a second processing interferometer is required. An investigation into suitable interferometer configurations for OCT was carried out in order to increase the performance o f the Fizeau interferometer compared with Michelson configuration. It was shown theoretically that by using a circulator and balanced detection, a standard Fizeau interferometer could achieve a signal to noise ratio (SNR) up to I8dB greater than the conventional Michelson interferometer and a comparable SNR to the Michelson interferometer optimised for power conservation. An analysis into the optical components used in OCT systems was carried out and the optimal Fizeau interferometer design was constructed. The Optimal design was shown to give an increase o f 26dB compared to the standard Fizeau interferometer relative to theoretical results using the same optical components. It was experimentally shown that the use o f an optical circulator within the Fizeau interferometer improves the SNR compared to the standard configuration.
Open Access
Hypersonic interference aerothermodynamics
(2009-10) Estruch-Samper, David; Lawson, Nicholas J.; Garry, Kevin P.
When a vehicle travels at hypersonic speeds during launch, cruise or atmospheric re-entry it is subject to extremely high surface flow temperatures. As well as on the vehicle forebody, extreme heating can take place close to surface protuberances which are almost impossible to avoid in a real flight vehicle. These disturbances interfere with the freestream flow and result in complex viscous interactions which induce a local heat flux augmentation that can become detrimental to the integrity of the vehicle. A greater understanding of these flow phenomena is required. This thesis develops the understanding of the behaviour of the flow around surface protuberances in hypersonic vehicles and presents an engineering approach to predict the location and magnitude of the highest heat transfer rates in their vicinity. To this end, an experimental investigation was performed in a gun tunnel at freestream Mach numbers of 8.2 and 12.3 and Reynolds numbers ranging from Reoo/m=3.35xl0 ⁶ to Reꚙ /m=9.35xl0 ⁶. The effects of protuberance geometry, boundary layer state, freestream Reynolds number and freestream Mach number were assessed. Further understanding of the flowfield was obtained through oil-dot visualisations and highspeed schlieren videos taken at frame rates of up to 50 kHz. Results show the local interference interaction is strongly three-dimensional and is dominated by the incipient separation angle induced by the protuberance. In subcritical interactions - in which the incoming boundary layer remains unseparated upstream of the protuberance - the highest heating occurs adjacent to the device. In supercritical interactions - in which the incoming boundary layer is fully separated ahead of the protuberance - the highest heating generally occurs on the surface just upstream of it. An exception is for low-deflection protuberances under low-Reynolds freestream flow conditions in which case the heat flux to the side is greater.
Open Access
Tightly-wound vortex and self-generated intake distortion effects on turbomachinery performance
(2013-07) di Cugno, Domenico; Pachidis, Vassilios; MacManus, David G.
Current developments of civil and military applications may require the installation of aero-engines embedded into the aircraft structure. Consequently, complex diffusing Sducts are necessary to convey air from the intake to the compressor. In addition, civil applications require the reduction of specific fuel consumption and noise levels. These targets can be met by increasing the engine mass flow and reducing the specific thrust by increasing the bypass ratio. In underwing installations there is an increased tendency of ground vortex creation. Generally, the application of complex S-shaped intakes as well as the ingestion of vortices can lead to inlet flow distortion in terms of total pressure and swirl. Consequently, blade vibrations and changes in turbomachinery performance are likely to occur. The aim of this research is to provide qualitative and quantitative information regarding the effect on a fan rotor performance caused by the self-generated distortion of an S-duct and its combination with that attributed to a tightly-wound vortex. A purely numerical coupled system S-shaped intake/fan rotor configuration was defined in this research to analyze the effect of total pressure combined with swirl distortion on the fan rotor performance. Steady-state CFD simulations were carried out on this system by considering clean conditions and, for the first time, the vortex ingestion at the intake inlet and with the rotor operating at two different rotational speeds. Under clean inlet conditions, the self-generated distortion of the S-duct causes a degradation of rotor performance. Moreover, the rotor operability range reduces significantly due to a localized blade overloading. On the other hand, as a vortex is ingested in the system, this interacts with the self-generated distortion in different manners depending on the location and polarity of the vortex itself. Consequently, the level of flow distortion at the AIP changes accordingly. The sign of the change in rotor corrected mass flow is essentially established by the polarity of the vortex ingested. Therefore, the effect of the swirl is predominant compared to that of the total pressure distortion. In particular, the vortices ingested at the centre of the intake inlet plane cause the largest change in rotor corrected mass flow. Regarding the loss of stability pressure ratio, this is established by the swirl distortion even though the effect of total pressure distortion is also notable. Amongst the case studies characterized by low total pressure distortion, a swirl distortion correlation is defined between the loss of stability pressure ratio and the mass flow average of the relative rotor incidence change calculated at the aerodynamic interface plane. A scatter between the CFD results and the established correlation can be attributed to the variations in total pressure distortion. In addition, a CFD based methodology was assessed to determine the location of the aerodynamic interface plane for swirl distortion. This was applied on the datum NASA Rotor 67 configuration working with a vortex ingested at different span locations and for two relevant operating conditions. The outcome of this analysis confirms that, in the worst scenario, the location of the aerodynamic interface plane is located in a position that is an order of magnitude closer to the rotor face compared to what established by previous research for total pressure distortion. This finding would allow the application during the experiments of shorter upstream ducts than that required for total pressure distortion. However, the assessment of a methodology providing a more precise information on location of the aerodynamic interface plane for total pressure distortion would be necessary.
Open Access
Multiple site damage of aeronautical riveted joints
(2005-09) Garcia, Abilio Neves; Irving, Phil E.
Among all aeronautical structures prone to develop MSD, riveted lap joints in the fuselage have been identified as being the most susceptible. Recent recommendations by regulators to avoid MSD threat stipulate an Inspection Starting Point (ISP) and a Structural Modification Point (SMP) in the life of aircraft. These points can be defined in terms of MSD analysis and the capability to accurately calculate service life to MSD onset becomes of considerable importance. To investigate this failure mode, a probabilistic model for MSD assessment considering both fatigue crack initiation and crack propagation as random variables was proposed. The fatigue crack initiation stage of the model differed from other published models for incorporating continuing damage assumption instead of a damage accumulation technique for re-initiation of fatigue cracks at crack-free fatigue critical locations. The crack propagation stage of the model was firstly performed deterministically by means of a dual boundary element computer code; and, secondly, probabilistic crack growth treatment was incorporated in a simple way. The results from the MSD assessment model provided good agreement with published experimental work on fatigue of lap-splice joints and with other model for the same geometrical configuration. In order to fulfil a lack from the literature, the probabilistic MSD model was employed to investigate variables influencing MSD. The variables were related to possible changes in the standard deviation for fatigue crack initiation, uniform and non-uniform hole pin-loadings, nominal stress level and high rivet squeeze force. The parametric study showed that the ISP and the SMP can be considerably affected. Further results include the identification of a conflict between two different structural safety criteria and the proposition of a new one; the use of uniform pin-loading distribution with peak loads from a non-uniform pin-loading distribution was suggested to avoid non-conservatism specially at low cumulative probabilities of failure; a clear tendency to decrease the mean time for the lead crack propagation as the number of MSD-like scenarios increased was verified, but not always a bigger number of nucleated cracks per damage scenario gave the smallest time for crack propagation and the crack nucleation sequence was more important than the number of nucleated cracks per damage scenario. To investigate the effects of high rivet squeeze on MSD, an experimental work was carried out to obtain input S-N fatigue data. Recent findings from the literature established the benefits that high squeeze force can provide for the mean time to fatigue crack initiation, but no probabilistic analysis was undertaken comparing different squeeze force values. The results of such analysis reviewed that the whole MSD failure process was retarded and the number of MSD-like scenarios considerably reduced, demonstrating that high rivet squeeze force is extremely beneficial for MSD prevention. Finally, the probabilistic model was employed for comparison to in-service MSD data from pressurized fuselage panels. During the preliminary modelling stage, it was found clear evidences that S-N input data obtained from good quality riveted test specimens could not be used for MSD assessment of real pressurized fuselage panels, and it was assumed open hole quality input S-N fatigue data. The results demonstrated to be rationally conservative compared to in-service findings. In-service data indicated that both the ISP and the SMP were well established from the simulations, and failure due to MSD occurrence would not threaten structural safety during the monitoring period.
Open Access
Computational analysis of high-speed axial-flow compressors using two deterministic stress models
(2002-01) Amaral Teixeira, J. A.; Elder, R. L.; Tourlidakis, A.
The employment of CFD methods for the analysis of multistage turbomachinery flows is restricted for the foreseeable future to steady-state methods of which the most widespread one employs the mixing-plane approach. The last method nevertheless is known to introduce jumps in some quantities at the stage interface due to the unphysical pitchwise averaging process it models and when employed in the simulation of multi-stage machines leads to a progressive downstream degradation of the solution accuracy. Adamczk’s average-passage equation system is the steady-state approach which fundamentally overcomes these limitations but the requirement that a set of machine- wide domains be iteratively solved as part of the calculation of each of the rows of interest, entails large computational requirements. A far more economical solution is to combine some of Adamczyk’s equation terms accounting for the cyclic components of the unsteady stage effects, the deterministic stress terms, with a mixing-plane steady-state formulation. This is the approach taken by the two semi- empirical models which were employed in this work to study two high-speed axial- flow compressor configurations, the single-stage Rolls-Royce HP9 and the novel three stage Cranfield University High Speed Research Compressor. The Hall and the Bolger deterministic stress models were combined with the CFD turbomachinery code CFX-TASCflow via a Source Code Interface and results were obtained for the two cases with and without the application of the models. Although the impact of the addition of deterministic stress terms to the solution of HP9 is noticeable but small, these effects are in the Cranfield machine more noticeable and seen to offer some contribution towards the approximation of the computational to the experimental global performance results.
Open Access
Maturity framework for e-Government in Oman
(2004-05) Al-Rawahi, Hamed S.; Reeves, Richard
The aim of this research was to develop a model for successfully managing the development of e-government in Oman. E-government refers to the transaction of government business with citizens by electronic means, including email and the internet. The outcome of a series of workshops for managers involved in IT in Oman was that a number of issues would need to be investigated before starting to implement e- govenment on a large scale, and there was seen to be a need to develop a systematic roadmap. The management of the Y2K project in Oman was then evaluated, and practical lessons were learned which would be applicable to subsequent national-scale IT projects. Ways of measuring the progress of countries regarding e-government, and a number of basic management models for introducing e-government, were found by researching the literature. These models covered fewer management aspects than had been found in this research. One important model was the Maturity Framework, which had been applied with some success for managing IT in one international company and one UK Government agency. It was decided to develop a maturity framework for e-government, incorporating the best ideas from the literature, and adding new ideas from the research carried out so far. Detail was added to the new framework, called the POTIRDMO framework, by research involving a large number of IT stakeholders in Oman. The completed POTIRDMO framework has been tested in use and has been compared against frameworks developed by other Governments. The new framework is more comprehensive than previous frameworks because it includes People and Organisational aspects of management, in addition to Technology aspects. It also defines the Characteristics of each stage, the Tasks to be done, and the Measures of completion. Road mapping and benchmarking functions are implicit within the model. It is now being used to manage the development of e-government in Oman.
Open Access
Packaging and configuration design aspects of UCAV concept synthesis and optimization
(2002-11) Niyomthai, Nattapol; Smith, Howard
Uninhabited Combat Air Vehicle (UCAV) escapes the bounds of conventional ideas and opens a big window of new opportunities in combat aircraft design. A conceptual design methodology was produced to investigate the packaging and configuration design aspects of UCAV. To obtain first estimates of the major parameters that define the aircraft, an initial sizing program was developed to consider the primarydesign variables and constraints requirements. They are subsequently applied to9 a numerical design synthesis methodology for UCAVs. The tailless UCAV was high on the list of interesting concepts because of the inherent benefits from the simple construction of a flying wing, its efficient low-drag aerodynamics, the ease of weapon and engine installations, and natural low observability of the design. A variety of packaging aspects was set up comprising of avionics bay, internal weapons carriage, propulsion system with the use of two-dimensional nozzle, undercarriage bay, and fuel tanks. The conceptual design possessed a flexible design layout for a wide range of configurations. The aircraft sysnthesis models were described, and validation and comparison results were presented where possible. The design methodology incorporated sufficiently accurate and realistic algorithms for the estimation of the packaging, geometry, layout, aircraft mass, and aircraft centre of gravity. A series of results was generated allowing a demonstration of several trade studies that could be accomplished. Results from a variety of particular designs were presented and discussed. Overall research conclusions were derived, and recommendations for further work prescribed. Appendices contained results of initial sizing program; publish data for avionics instruments, weapons, and UCVAs/UAVs; methodology implementation; and detailed aircraft parameters, packaging and configuration aspects of UCAV output data.