PhD and Masters by research theses (SoE)
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Item Open Access Discharge coefficient and jet deflection studies for combustor liner air-entry holes(Cranfield University, 1964-06) Kaddah, K. S. M. I.In the simple tubular combustor system for turbojets and ramjets, figure 1, at the primary zone it is required to achieve a high heat release and stable flame. At the dilution cone to have a good air flow distribution (Ref. I ), and adequate penetration. In both zones these requirements needs to be with the minimum total pressure loss for high overall efficiency, short liner length, and light simple construction (Ref. 2 ). The air flow distribution between these two zones is still the main problem of the design engineer. But the flow of annular air to these zones is governed by the geometry of the eyetem, hole coonetry and dicehargo coefficient of these holes. For light construction and adequate penetration, the main governing factor is the deflection angle of the liner jots. Cont/d.Item Open Access Weak compression waves in relaxing gases(Cranfield University, 1972) Scott, E. M.; Clarke, J. F.; Busing, J. R.Studies have been made of the structure of weak compression (shock) waves in relaxing gases. These studies have been primarily concerned with the development of separate theoretical and experimental techniques. These techniques are not inter-related in general except in the overall context of Vibrational relaxation. The theoretical studies have been concerned with the influence on the structure of weak normal shock waves of translational non-equilibrium. bimodal relaxation and second order unimodal relaxation. The affect of translational nonequilibrium on the relaxation process has been studied by forming an asymptotic expansion in the ratio of the viscous length to the relaxation length. The perturbation scheme was singular and required the application at the method of matched asymptotic expansions. Bimodal relaxation has been studied by forming an asymptotic expansion in the ratio of the energy of the secondary vibrational mode to the total Vibrational energy. The addition of a second order term to the rate equation describing the behaviour of a single vibrational mode has also been studied by forming an asymptotic expansion. In this case. the perturbation parameter was the ratio of the two relaxation times concerned. The experimental studies have been concerned with the production and study or weak normal shock waves in the Cranfield Institute or Technology 2" shock tube. A time resolved quantitative schlieren system has been used for the study of the weak normal shock waves. This particular system had been developed previously for this purpose. and further developments and refinements have been made to it. Experimental studies have been made with the schlieren system of the structure or strong incident shock waves in carbon dioxide. The vibrational relaxation time of carbon dioxide determined in this way for translational temperatures from 300 o K to 1200 o K has been found to be in reasonable agreement with measurements made elsewhere. A technique has been developed for the production of weak incident shock waves in t.i.1e shock tube, which involved the positioning of a perforated plate in the channel of the shock tube. The vibrational relaxation time of carbon dioxide determined in this way for translational o temperatures of approximately 300 K has been found to be in good agreement with measurements made elsewhere. Good agreement has also been obtained between the experimentally measured density gradient profiles and theoretical profiles. The curvature of the shock waves obscured the transition from a fully dispersed to a partly dispersed shook wave. Further improvements and refinements have been made to a technique which had been developed previously for the propagation of weak normal shock waves through the reflected shock region of the shock tube flow. This technique was used to study the behaviour of fully dispersed shock waves at high temperatures. The relaxation time of carbon dioxide determined in this way for temperatures from 300 o K to 600o K has been found to be in good agreement with measurements made elsewhere.Item Open Access Theoretical and experimental studies of the effect of clearance and geometry in hydrodynamic journal bearings(Cranfield University, 1973-11) Dyer, D.; Reason, B. R.This study is concerned with oil lubricated, hydrodynamic journal bearings. A test rig was developed which allowed measurements of film pressure, film thickness and journal surface temperature, from the rotating journal. In addition to this it allowed the more commonly measured variables of bearing oil flow and some characteristic bearing temperatures to be friction, monitored. It has been shown that an optimum clearance exists, which maximizes the minimum film thickness. This experimentally measured optimum has been compared with various theoretical estimates, with varying degrees of correlation. An optimum bearing length has been observed which minimizes the bearing friction. This was not predicted by any theoretical results, but a reason for its existence has been suggested. Small errors in bearing geometry were experimentally observedâ to give rise to large distortions in the film pressure A simple theoretical model for an infinitely long non-geometric profiles. bearing was constructed and measured trends. â º found to support the experimentally The infrequently observed phenomenon of tensile stresses was . measured in the oil film under certain stresses, at times of the order of 100lbf/in. , have been accounted conditåons. These tensile for by consideration of the In order to permit the parameters it was necessary A film thickness inductance nature of liquids under tension. _ measurement of journal bearing to develop certain instrumentation. transducer of low temperature and pressure sensitivity was developed. In addition to this a pressure transducer was constructed and its frequency response characteristics were evaluated in conjunction with various entrance tubes. This particular study highlighted the region of applicability of the ' available theoretical models for predicting pressure transducer/ tubing response. -Item Open Access Spark ignition studies in flowing liquid fuel-air mixtures(Cranfield University, 1974-03) Rao, K. V. L.; Lefebvre, A. H.The thesis describes a programme of research to determine ignition energy requirements in flowing kerosene spray air mixtures under various flow conditions such as velocity, mixture strength, spray mean drop size and drop distribution. In order to determine the spray characteristics of mixture ratio, mean drop size and drop distribution in the flowing fuel-air spray accurately, new designs have been evolved and employed. A single rectangular pulse of constant power was employed for initiating ignition in the mixture. The flow parameters were found to have considerable influence on spark characteristics and hence on the energy released in the spark. The breakdown voltage requirements of the spark in the two phase flow differed from they of premixed gaseous mixtures. In the range of weak mixtures investigated, optimum spark duration was found to vary depending on spray drop size between 30 and 60 microseconds. Minimum ignition energy increased with increase in velocity, but decreased with reduction in drop diameter and with increase in equivalence ratio. Low flow number atomisers giving small drop sizes extended the weak ignition limits for a given spark energy. Spray drop size and distribution were found to have a singularly large influence on all aspects of ignition. The thesis includes a detailed account of the designs and procedures employed in carrying out the investigation, and also a discussion on the significance of the results on the practical aspects of liquid fuel ignition in aero engine combustion system.Item Open Access Development of a modular-type computer program for the calculation of gas turbine off-design performance(Cranfield University, 1974-09) MacMillan, W. L.; Palmer, J. R.This thesis presents a computer program written (in Fortran IV) to simulate the steady state off design point performance of virtually any gas turbine configuration (dealing primarily with aerospace applications). The performance is calculated using component characteristic maps for compressors, combustion chambers, turbines (both compressor turbines and free turbines) and a map giving the velocity coefficient for exhaust nozzles. The off design point performance is determined by using a modified Newton-Raphson method. Cont/d.Item Open Access The aerodynamic behaviour of a compressor rotor with steady and unsteady inlet flow(Cranfield University, 1974-11) Overli, J. M.; Peacock, R. E.Recent developments in have made possible the mic behaviour of rotor stage compressor using I pressure sensing and transfer techniques detailed investigation of the aerodyna- blades. Tests on a low speed single this instrumentation have given new evidence on rotor behaviour. The rotor blade performance with steady inlet flow condition was assessed from surveys of total pressure, static pressure and flow direction made upstream and downstreäm of the rotor hub, mid-height and tip section. In addition, measurements of the static pressure distributions of the same blade sections were completed. It is concluded that the combined effect of centrifugal force and annulus wall boundary layer skewing aid the performance of the rotor. This suggests that the efficiency of compressors would be improved if they were designed for reactions of rather more than 50%. The presence of a tip shroud has the effect of enhancing the overall compressor performance. A comparison of compressor rotor and two-dimensional cascade blade results is given, and predictions of the boundary layer development and the pressure distribution around the blades under various flow conditions are also presented. A series of experiments were also conducted at the mid-height section in which the compressor was subjected to 'sine-wave' and 'square-wave' circumferential stagnation pressure distortions including tests with the compressor operating in the surge regime. The deteriorating effect of total pressure distortion on overall compressor performance is clearly demonstrated, and a critical sector of spoiling is found. The effect of the level of reduced frequency on the unsteady normal force response of rotor blades is shown. II In all cases of distorted inlet flow, stallwinception is dela ed until values of normal force coefficient are reached well in excess of those encountered at steady state maximum. For 'square-wave' type distortion this is ensued by a complex stall situation with separation bubbles apparently traversing the rotor blade section and promoting dynamic lift patterns which pass through secondary and tertiary peaks. One may conclude that the aerodynamic rotor behaviour in the presence of such an unsteady flow environment is not predictable by current mathematical models. A theoretical analysis of the transient flow applicable at low values of reduced frequency is given. The transient response of the pressure-transmitting system was examined thoroughly theoretically, and experimental frequency response tests were conducted on the rotor blades as well as on a subsidiary rig. Careful examination of the fluctuating pressure signal was performed including time and frequency analyses.Item Open Access A general solution for distorted flows in aerofoil cascades and contributions to distortion sensitivity reductions(Cranfield University, 1975) El-Attar, M. A. R. A. A.; Peacock, R. E.This thesis presents a number of new solutions to reduce the sensitivity of an existing compressor stage to a specified inlet flow distortion._ The interacting effects of flow vorticity, the change of rotor incidence and the time rate of change of íncidence are then studied, yielding a new design criterion to reduce the sensitivity of a compressor stage to any circumferential inlet flow maldistribution. Furthermore, the application of the unsteady thin a stator or inlet guide vane and a rotor, results unsteady fluctuating lift function which contains .functions as special cases. - Minimisation of the aerofoil theory to inia new generalised Sears and Hor1ock's resultant fluctuating ' lift leads to a design criterion for inlet guide vanes in agreement with the above conceptwhich is obtained by a different approach. The thesis also discusses a number of possible mathematical models for Cascade shear flow investígating their advantages and disadvantages and'the possibilities°of obtaining solutions. gA linearised theory for-_ nonuniform cascade flow is presented.i This fs'main1y an extension of Schlichting's method,`ref;(27) for uniform flow,i ' The main contribution of this work is the development of a nonlinear theory for nonuniform~cascade shear flow.â The.theory-accepts large amplitudes of distortion of any smooth shape, that is, cusps and 'g sharp corners in the Velocity distribution should be rounded. It is also capable of treating cascades of thick and highly cambered blades. The solution of Poisson's equation is obtained in the form of Fredholm'sV integral equation of the second kind which reduces to Martensen's equations, ref.(29), when the vorticity is zero everywhere in the flow field (clean flow). .The vorticity transport equation is satisfied in the flow field and its simultaneous solution with the solution of Poisson equation is carried out through an iteration process. Good agreement is obtained between the theoretical predictions and experimental test resultItem Open Access Design considerations of future short haul transport aircraft(Cranfield University, 1976-05) Howe, D.The rapid escalation of air transport in the past two decades has resulted in difficulties associated with airport congestion and ground trip delays. These have been particularly acute for short haul operations. Suggestions have been made that they might be alleviated by the use of smaller airfields located nearer to population centres than existing large airports which operate aircraft specifically designed for the purpose. This paper reports a systematic investigation of the requirements, design parameters and performance characteristics of short haul transport aircraft. Designs intended for operation- in the VTOL, STOL, RTOL or CTOL modes have been considered and compared in an attempt to predict the more promising prospects for the future. A number of specific design studies were used as the basis for the comparison of the weight, economic and noise characteristics of the aircraft concepts which fall into the categories investigated.Item Open Access The influence of swirler design parameters on the aerodynamics of downstream recirculation region(Cranfield University, 1976-05) Kilik, Erol; Lefebvre, A. H.; Ramsden, K. W.The influence of swirler design parameters namely, vane shape (flat or curved), vane outlet angle, aspect ratio and space-to-chord ratio (number of vanes), on the pressure drop-mass flow characteristics and the size and turbulence characteristics of the downstream recirculation region has been determined experimentally. A five-hole spherical pressure probe and hot-wire anemometry have been used for the investigation. A technique for simple and reasonably accurate interpretation of the signals from the hot-wire anemometer has been formulated. Its performance has been tested at first in round jets for which experimental turbulence data are already available for comparison purposes. The method has been applied subsequently to swirling flows issuing from the swirlers; the mean and turbulence characteristics of the flow fields have been determined in this way. In addition to these, the turbulent viscosities Ĭrr, Ĭrz, Ĭrθ and Ĭθθ have been calculated. Also, detailed accounts of the previous experimental work on swirling flows and of some finite difference procedures for the computation of weakly and strongly swirling flows have been given. It has been shown that the hot-wire formulation developed provides a simple, economic and efficient procedure for the measurement of the turbulence quantities in swirling flows. The results have shown that curved vane type swirlers operate more efficiently than flat vane types such that a larger recirculation region and a stronger shear zone are induced at a lower pressure drop. The pressure drop through both the flat and curved vane type swirlers increases with increasing vane angle, decreasing aspect ratio and decreasing space-to-chord ratio. The size of the downstream recirculation region, the reverse mass flow and the decay rates of the axial and swirl velocities increase with increasing vane angle and decreasing aspect ratio in the case of curved vane type swirlers. The effect of the space-to-chord ratio has not been found very significant in the cases of I6 and I2 vanes for the vane angle of 60° and the aspect ratio 0.4. Higher turbulence and stronger shear zones can be obtained in swirling flows with increasing vane angle, decreasing aspect ratio and decreasing space-to-chord ratios. Turbulence is anisotropic in strongly swirling jets issuing from vane type swirlers. The most dominant stresses are in turn ww, uw, and uu; all the other stresses are smaller. In parallel to the anisotropy in the stresses, the turbulent viscosities are also anisotropic; the turbulent viscosity Ĭθθ has shown domination among the four viscosities which have been calculated. Ĭrr values have shown too much scatter. For these reasons, curved vane type swirlers should prove much more efficient than flat vane types in the application to combustion process.Item Open Access Aerodynamic interactions between compressor blades and circumferential temperature distortions(Cranfield University, 1976-09) Hashem, A. A. F.; Peacock, R. E.A unifying principle was discovered to treat square-wave, steady state, circumferential temperature distortions at arbitrary levels of distorted stagnation pressure, applicable to the low speed range., ' ' A velocity transformation was found to relate the physical 4 properties of a two-dimensional, non-viscous flow field collectively, to the\introduced modified velocity. Furthermore, the flow in the modified plane retains the characteristics of classical incompressible flow and, in general, is rotational. The flow was divided into regions of either constant or linearly â º varying modified velocities. Each two regions were separated by a free streamline initíally of unknown position. A non-linear,large disturbance,mathematical model was set-up to find out the correct free streamlines location in the presence of a single aerofoil. Considering first order derivatives of the induced modified velocities of the flow modified vortices and the aerofoil,.when applying a modified continuity equation,_the model converges rapidly. - .Results showed the aerofoil lift coefficient to fluctuate above and below its undistorted value, near the regions of shear flow,~º - depending on the relative directions of the flow modified vortices, and the circulation around the aerofoil - « . . Experimental investigations included tests on«a single blade' 'mounted near the intake of an axial compressor of constant annulus - height. Results showed that at this level of temperature distortions _ the stagnation pressure of the distorted flow had a dominant effect on the behaviour of the lift coefficient.Item Open Access The application of perturbation methods to nonlinear problems in flight mechanics(Cranfield University, 1976-09) Padfield, Gareth D.; Christopher, P. A. T.Analytical-techniques are developed for deriving approximate solutions to a class of problems that arise in nonlinear flight mechanics typically when an aeroplane is flying close to a stability boundary. The techniques are based on pertoz-bation methods and the method of multiple scales is used to correct for_nonuriformities otherwise present in the asymptotic expai!,. 3ions. Applications are mainly concerned with the lateral dynamics of slender aircraft flying at high incidence. Approximations are derived for both linear and nonlinear lateral motions that serve to illustrate the importance of particular parameters in the problem. For the linear theory the approximations are achieved by reducing the fourth order system to two. weakly coupled second order systems. The nonlinear theory is mainly concerned with the effect of aerodynamic nonlinearity on the lateral oscillation when the latter has marginal stability or instability, as predicted by the linear theory. The analytic approximations are extended for use in the problem of estimating damping moments on aircraft in oscillatory motion. For this purpose approximations are constructed for the logarithmic decrement of a nonlinear oscillation that can be used to fit experimental measurements. Some thoughtis given to-the issue of memory effects in the representation of aerodynamic forces and moments.Item Open Access Studies on liquid sheet disintegration in airblast atomizers(Cranfield University, 1976-10) Rizk, N. K.; Lifebvre, A. H.The work in this thesis has been devoted to studying the influence of liquid film thickness on the characteristics of airblast atomizers. The research was carried out using two special atomizers that were both designed to produce flat liquid sheets which were atomized by high velocity air streams acting on both sides of the shoot. The liquid was injected through sintered plates in the first design, while the film issued from a variable gap, slot in the second design.Item Open Access Supersonic flow over low aspect-ratio wings(Cranfield University, 1976-11) Richards, I. C.An attempt has been made, by means of exp": rimental work, to consolidate and augment existing knowledge of high-speed flows over delta wings. Particular emphasis . is placed on the investigation of flows which did not confirm to the 'accepted' pattern. In Part 1 the flow regimes on a caret wing are discussed with particular emphasis placed on the occurrence of 'strong' oblique shocks. Results are presented which demonstrate that when viewed in direction normal to the leading edge, shocks of both the 'weak' and the 'strong' families can be said to exist. An experiment designed to produce a single strong shock by means of a caret wingýof large anhedral, instead gave rise to a complex multiple shock pattern which could not be adequately explained by exact inviscid theory. In Part 2 the unexpected pressure rises reported on the lee surfaces of various delta wings (References 22 and 23) are shown to be, at least in part, the result of interference from the model support and base-mounted instrumentation. The physical reason for the pressure rise is discussed. 'Correct' leeside pressure distributions are pre- sented for one of the models used in Reference 23 for angles of attack up to 50 degrees. In Part 3 the different flow regimes on delta wings are discussed together with the methods of defining the boundaries between them. The conjecture that thin shock-layer theory can be used to predict the on- set of leading-edge separation is carefully investigated by means of tunnel tests on a particular wing with triangular cross-section.Item Open Access The minimum weight design of compression structures including plasticity effects(Cranfield University, 1977-11) Richards, D. M.; Howe, D.The minimum weight design of a wide range of structures required to resist compressive loading is considered. Items which have been analysed in detail include struts of various sorts, thin plates, honeycomb core sandwich panels, wide column stiffened panels, stiffened panels with optimised support locations, and cylindrical shells stiffened by axial stiffners and rings. A further study is concerned with the effects of imperfections on a tower with corrugation stiffened walls, loaded in compression. - Except for the corrugated tower, each analysis includes the effects of plasticity in a direct and realistic way by characterising stress-strain behaviour in the manner suggested by Ramberg and Osgood. Results for each type of structure are presented, together with the appropriate computer programmes.Item Open Access Flow and heat transfers associated with impinging jets in crossflows(Cranfield University, 1977-11) Kabari, L.; Ward, J.This thesis reports the results of an experimental study into the flow and heat transfers associated with both inclined and orthogonally impinging axisymmetric air jets. The majority of previously reported studies have been mainly confined to orthogonally impinging jets in stagnant surroundings. In this investigation, free jets as well as the effects of crossflows are considered. This investigation is Primarily concerned with local heat transfer variations. The experimental tests were. conducted with a single 12.7 mm diameter jet impinging on a flat surface, and heat transfers were evaluated using a heat-mass transfer analogy (t e Chilton-Colburn analogy). The sublimation of naphthalene was employed as the mass transfer technique. The flowfield associated with impinging jets has a significant influence on their heat transfer characteristics., In view of the present limited level of understanding of this 'complex' flowfield, extensive flow visualisation techniques were employed in this present investigation. Those were primarily intended to aid interpretation of the experimental heat transfer results, and also to provide further physical understanding of the flowfields resulting from the interactions between impinging jets and crossflowing streams. The flow and heat transfer tests conducted in the programme of work reported in this thesis covered typical ranges of flow parameters of interest in many practical applications of jet impingement systems. Jet inclinations of 45°, 600, and 90°, nozzle to target spacings of 2,11, and 8 nozzle diameters were studied. The Reynolds nuinbers were 30,200,32,700 and 55,100 and mass velocity ratios In the range 4.0 to 3.8 were studied. The effects of these parameters on the flow and heal transfers associated with impinging jets are reported. Comparisons were drawn between the heat transfer results and those of previously reported studies where appropriate.Item Open Access Thermal design of a compact recuperative heat exchanger for a stirling engine(Cranfield University, 1977-12) Jewad, M. A.; Ward, J.This thesis is concerned with factors affecting the thermal design of a compact recuperative type crossflowing heat exchanger for the primary heater of a Stirling engine. The exchanger is constructed of small diameter metal tubes (in the range of 3.0 mm to 6.0 mm) and close spacings are maintained between the tubes (i. e. in the range of 0.30 mm to 1.80 mm). These small slender tubes are usually arranged in single or double rows and are mounted around the combustion chamber. The exchanger is used to heat hydrogen or helium which act as a working fluid for the Stirling cycle. A survey of the published literature indicated that the available data does not include results for the tube Geometries of interest in this study. Consequently the heat transfers and hydraulic resistances were measured experimentally for a single row of small diameter, closelyspaced tubes situated in a crossflowing fluid stream. The Reynolds numbers (bc,.s ed on the mainstream fluid velocity and the tube diameter) ranged between 300 and 6500. Where possible the accuracy of the experimental procedure was checked by comparing the present results where possible with those obtained by previous workers. Several arrangements of both bare tubes and tubes fitted with extended surfaces were studied, The results were analysed and discussed, and where appropriate compared with those published in the open technical literature. In most comparative cases excellent agreement was experienced and any departure, could be explained. For the bare tube arrangements the influences of flow blockage ratio, mainstream turbulence intensity and surface roughness on the average heat transfer performance were investigated. A comparison of the heat transfers and pressure drops characteristics of the different tube arrangements led to proposals for an optimal exchanger geometry. The validity of the empirical corrections suggested by previous workers to account for the influence of flow blockage on average heat transfers was examinod. An alternative modified empirical expression was then proposed for the particularly high flow blockage situations (D/1I>0.85). It was found that at those high flow blockages the overall average heat transfers were independent of the mainstream turbulence intensity. However preliminary tests suggested that an increase in the tube surface roughness increases the average tube heat transfers. Since the proposed heat exchanger operates at the higheet possible mean metal temperatures, it is likely that 'hot spots' occurring due to variations in local heat transfers can lead to premature failure. Consequently a detailed study of the local heat transfer distributions is presented for various Geometrical conditions. The influences of blockage ratio and mainstream Reynolds numbers are examined and the results are analysed, and discussed, and where possible compared with other published data. The accuracy of experimental procedure employed in these tests was checked by comparing the results for a single cylinder ease with those reported by other investigators. The influence of fitting a single longitudinal fin to the rear of tubes on both the heat transfer and pumping power was studied. The tube diameter- in these tests was kept constant, at 6.0 mm, but the angle of inclination of this longitudinal fin was varied incrementally so that an optimal angle for maximum performance is recommended. In a similar manner, transverse finned tubes with two different fin spacings were also investigated. The heat transfers and pressure losses obtained for the different finned tube arrangements were compared with each other and with those obtained for the bare tube geometries so that an optimal tube configuration was proposed . The data presented in this thesis were generalized, where possible, so that the results should be useful for future work. They should thus contribute to an understanding of the basic phenomena associated with modern compact heat exchangers. Recommendations for further work are also presented.Item Open Access The stress distribution in the joints of vehicles chassis frames subject to torsion(Cranfield University, 1978-02) Alvi, M. S. I.; Tidbury, G. H.The problem of warping inhibition in the joints, of commercial vehicle chassis frames is very important for the estimation. of stress distribution in the joints and the overall torsional stiffness of frames. The joints being the weakest parts in the entire frame require that the stress levels should be estimated when designing a frame for required torsional stiffness. Plate theory has been applied to-analyse stress distributions in the joints of a ladder frame-in torsion. The frame consists of channel section cross members-welded to the web: of the side members. The degree of cross member warping inhibition in the welded joints has also been estimated analytically by using the concept of the strip beam theory. - The analytical values of stresses have been compared with-photoelastic measurements on photoelastic models of isolated joints and a complete ladder frame.Item Open Access Application of finite element methods to the post-buckling behaviour of thin plates containing imperfections(Cranfield University, 1978-02) Allahyari, H.; Rothwell, A.A finite element program, applicable to the pre- and post-buckling behaviour of plates with imperfections, is developed. Suitable incremental stiffness matrices are generated for a plate element with four nodes and twenty degrees of freedom. Preliminary work, together with a prototype pro- gram, is carried out on a simple strut in order to compare various nonlinear solution techniques, both incremental and iterative. Th6 plate program is veri- fied by large deflection calculations for a square plate under lateral pressure, and by comparison with theoretical buckling loads for a perfect plate, closely agreeing with previous theoretical work. Experimental results in compression, both with and without an artificially introduced imperfection, are used to demonstrate real plate behaviour, and they enable a comparison to be made with computed results. Measure- ments of deflection are made by the Moire fringe tech- nique, as well as by dial gauges. The program is used to investigate the effect of buckling on the compressive and shear stiffness of plates with various degrees of imperfection, including the compressive stiffness of a square plate after buckling in shear.Item Open Access Vibration of structures with non-linear damping(Cranfield University, 1978-03) Gabri, B. S.; Kirk, Colin L.The work reported here is directed towards the problem of measuring and modeling non-linear damping in steel space frame civil engineering structures. In order to gain a better understanding of the damping mechanism in such structures, tests were performed on a full scale bolted lattice structure and the results of section A show that the joint' damping can be represented approximately in the form: FD =C (1 +e IXIn) where n=1. Other situations. where this form of non-linear damping occurs is in the case of fluid flow past a solid body. In offshore structures the hydro-dynamic force due to drag effects are found to be proportional to velocity squared. For non linear material damping, n, takes a value of 6. In section B. various methods of solution to the non-linear dynamical system under random excitation were studied and it was found that the 'Equivalent Non- Linear Differential Equationl, (ENL) method yields a better measure of the response of the non-linear system to white noise excitation than other approximate methods. Analytical expressions are derived for the mean-square response, probability density function and level crossing rates. With the modified IENLI method (MENL) it is shown how one can extract the non-linear damping and excitation power spectral density. A good correlation is obtained between experimental and theoretical results.Item Open Access The influence of fuel structure on CO and NOx formation in lean premixed hydrocarbon flames(Cranfield University, 1978-09) Saleh, H. S. M.; Fletcher, R. S.A study is made of the influence chemical structure of fuel has upon combustion performance through tracing the generation of carbon monoxide and oxides of nitrogen in lean, premixed, hydrocarbon-air flames. The study includes both analytical and experimental investi- gations. On the analytical side, a kinetic model is developed to predict both CO and NO time-histories in one-dimensional, premixed flames. The model is based upon the assumption of partial equilibrium in the post-flame zone while the fuel oxidation in the main reaction zone is allowed for by using a global oxidation rate equation. NO formation is assumed to be entirely via the Zeldovitch mechanism and to start in the post-flame zone. The utility of the model is judged through comparison between theoretical results and experimental data. On the experimental side, a simple burner system, supporting a one-dimensional premixed flame was designed and built. All fuels selected for investigation were pure hydrocarbons representing the main hydrocarbon types usually found in practical fuels; namely paraffins, olefins, naphthenes and aromatics. The hydrogen-to-carbon ratio ranged from 1 to 2.67 and the carbon number from 3 to 12. The experiments were performed at 1,2 and 3 atm pressure levels and 140°C inlet temperature, while the equivalence ratio was in the range 0.6 to 0.9. Flames were sampled for most stable species by a water- cooled stainless steel sampling probe. The experimental results show that the fuel structure signifi- cantly affects CO time-histories in the investigated flames mainly through influencing its generation rather than its burnout. CO burnout is shown to be mainly controlled by radical recombination processes, and the experimentally derived CO global oxidation rate equations are found not to be universally applicable. The results also show that the fuel structure influences prompt NOx formation within, and very near, the main reaction zone but that it does not influence post-equilibrium NOx formation if account is taken of differences in the flame temperatures. N02 is found to constitute a large percentage of total NOx measured especially at lower temperature and equivalence ratios. Comparison between experimental and theoretical results show that the prescribed kinetic model can satisfactorily predict CO levels for different fuels and under different conditions if the fuel oxidation global rate equation is correctly defined for different fuels. On the other hand, agreement between predicted and measured NO profiles has been obtained at atmospheric pressure only. At high pressure, the predicted levels were much smaller than those measured experimentally, and this disagreement is attributed to the fact that proper account is not taken of the NO and N02 formation kinetics in the main reaction zone.