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Thesis (MScEng)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: An experimental investigation was conducted usmg a cantilever beam to illustrate an understanding of vibration techniques and phenomena. The [Tee and forced vibration of the cantilever beam was investigated using various modelling strategies. Theoretical models such as the Lumped parameter, Myklestad, Distributed Parameter and Finite Element Methods were analysed and compared with experimental measurements. Excellent agreement of the natural frequencies, mode shapes and the harmonic transfer functions were found. To investigate tail boom vibration, a Finite Element Model of a simplified helicopter tail boom design was tested against experimentally measured data. The results correlated accurately and the model was used to evaluate the effectiveness of a vibration suppression system designed for the tail boom. The vibration suppression system was designed using two, parallel dynamic absorbers to reduce tail boom vibrations at its two dominant frequencies. For the purposes of this thesis, the dynamic absorbers were tuned to reduce the resonant peaks at the dominant excitation frequencies of the excitation force in-line with the thrust of the tail rotor only. This excitation force was estimated using a simplified tail rotor dynamic model, which when compared with actual vibration data was assumed to predict the basic frequency trends of the force, reliably. Incorporating the designed components into an accurate Lumped Parameter model of a cantilever beam structure and simulating the response of the structure at the dominant excitation frequencies of the calculated excitation force, tested the vibration suppression system. The results of this investigation proved the effectiveness of the design procedure and optimisation process of the design parameters. A similarly designed suppression system was incorporated into the Finite Element tail boom model and from the analytical results obtained it was shown to cause a fair reduction in the vibration response at the two dominant frequencies of the excitation force. Finally, two possible applications were suggested from the research presented in this thesis. The first application was the simplified configuration of a vibration monitoring system for the tail boom. The second application was the possible development of a system using the response of vibration suppression absorbers to predict the magnitudes of the excitation forces on the tail boom. / AFRIKAANSE OPSOMMING: 'n Experimentele ondersoek is ondemeem deur gebruik te maak van 'n ingeklemde balk om vibrasie tegnieke te illustreer. Die vrye en geforseerde vibrasie van die ingeklemde balk is ondersoek deur verskillende model tegnieke te gebruik. Teoretiese modelle soos die Lumped parameter, Myklestad, Distributed Parameter en Eindige Element Metodes was geanaliseer en vergelyk met die eksperimentele waardes. Uitstekende ooreenstemmings was gevind tussen die natuurlike frekwensies, die eievektore en die harmoniese oordragsfunk:sie. Om stert vibrasie te ondersoek, was'n eindige element model van 'n helikopter stert getoets teenoor die eksperimentele data. Die resultate het akuraat gekorreleer en dus was die model gebruik om die effektiwiteit van 'n vibrasie supressie sisteem te evalueer. Die vibrasie supressie sisteem was ontwerp deur gebruik te maak van twee parallele dinamiese absorbeerders om die helikopter stert se eerste twee dominante vibrasie frekwensies te minimeer. Vir die doel van hierdie tesis was die dinamiese absorbeerders ingestel om net die resonante pieke van die opgewende krag wat in lyn is met die drykrag van die stert rotor te minimeer Hierdie opgewende krag was, deur gebruik te maak van 'n basiese stert rotor dinamiese model, aanvaar om die basiese frekwensie tendense van die krag te voorspel. Die supressie sisteem wat ontwerp was, was getoets op die lumped parameter balk en het veroorsaak dat die voorspelde opgewende krag by al twee van sy dominante . frekwensies baie geminirneer was. Die effektiwiteit van die sisteem was dus bewys en die sisteem was gebruik om die respons van die eindige element model van die stert te verbeter. Die resultate het 'n aanvaarbare reduksie in die respons by die twee dominante frekwensies van die opgewende krag, gewys. Laastens, deur gebruik te maak van die resultate in hierdie tesis. was twee moontlikhede voorgestel. Die eerste een was die konfigurasie van 'n vibrasie moniterings sisteem vir die helikopter stert. Die tweede mmontlikheid was die moontlike ontwikkeling van 'n sisteem wat die grootte van die opgewende krag by die stert rotor kon voorspel, deur gebruik te maak van die vibrasie suppressie absorbeerders.

The lack of methods for rigorous dynamic analysis of foundations and structures for vibrating machines has resulted in below optimum performance and in some cases reduction of life of machines, structures and foundations. The costs and complexities of these machines make it necessary to conduct proper geotechnical site investigations. and dynamic analyses to obtain the response of the soil, foundation and structure as a system to excitation. In order to highlight the use of dynamic analyses, the response of the foundations and structures were compared to the "rule of thumb" which is based on mass ratio. Furthermore sensitivity analyses were carried out comprising the following variables: • Shear modulus of soil, G • Poisson's ratio of soil,Y • Type of structure (ie raft, table top and multi-storey) • Stiffness of structure • Stiffness of foundation The fundamentals of structural dynamics have not been dealt with in this dissertation. The dynamic analyses were carried out using a finite element analysis program called Strand 7. The results were typical of a finite element analysis, giving stresses, strains, deflections, amplitudes, frequencies and velocities of vibration. The traditional "elastic halfspace model" is deficient as it does not account for soil comprising various layers. The theory has been based on an isolated circular footing. Most foundations are located in soils with layered mediums, are rectangular and in some cases are affected by the interaction of foundations in close proximity. Furthermore there is a need to account for the non-linear effects and properties of soil. It is therefore becoming more attractive to adopt mathematical models of soils using finite elements, where the visco-elastoplastic properties of soils can be realized and modeled. Furthermore the finite element method overcomes limitations such as layering and shapes or foot-prints of foundations. The "rule of thumb" or mass ratio method of design procedure is as follows:- • firstly the requirements of stresses and serviceability must be satisfied. This is usual in a statically loaded system. the ratio of the machine mass to that of the foundation together with the structure should be greater than 3 in the case of a revolving machine and 5 in the case of a reciprocating machine. • in order to obtain uniform settlement of the foundation the distance of the combined centre of gravity of the machine and foundation from the centre of area in contact with the soil is limited to 5% of the corresponding dimension of the foundation. It is evident that the mass ratio lacks accuracy in that there are several parameters that are required to describe the satisfactory performance of a system such as amplitude, frequency and velocity of vibration. The finite element method allows for calibration of the model to account for the real behaviour of the system. Calibration is generally conducted using sensitive transducers called accelerometers. The accelerometers produce power spectral density (PSD) graphs from which deflections and stresses can be back calculated. The deflections and stresses are compared with calculated deflections and stresses. Descriptions of the methods of analysis followed by presentation of results, discussions and interpretations have been included. To motivate the use of dynamic analyses case histories have been presented and discussed. Finally the dissertation concludes with findings of the study together with recommendations for the way forward in terms of research. / Thesis (M.Sc.)-University of Natal, Durban, 2002.

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No description available.

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Turbomachines are exposed to several environmental factors which may cause failure of components. One of these factors, high cycle fatigue, is often caused by blade utter. This thesis forms part of a project of the European Seventh Framework Programme (FP7), called project Future. Project Future is doing theoretical and experimental investigation into the occurrence of utter in turbomachinery. The objective of this thesis was to evaluate the effectiveness of a gas injection system as a means of exciting vibrations on the rst stage rotor blades of a compressor. Unsteady simulations of the excitation velocity perturbations were performed in the Computational Fluid Dynamics (CFD) software, Numeca FINE/Turbo. Experimental testing on the in-house Rofanco compressor test bench, using one prototype of the 15 injector system, provided data that was used to implement boundary conditions and to verify certain aspects of the unsteady simulation results. The simulation results revealed the following: the injector bypass frequency was so dominant that the excitation frequency was hardly detectable in the majority of cases. Furthermore, several secondary frequencies were consistently present. The injector bypass frequency, as well as the secondary frequencies, occurred as a result of the convolution of Fast Fourier Transforms. While the injector bypass frequencies can theoretically be eliminated, it will not be possible to eliminate the secondary frequencies from the blade response. In conclusion, according to the CFD results, it will not be possible to excite a single excitation frequency by making use of a nite number of gas injector vibration exciters. / AFRIKAANSE OPSOMMING: Turbomasjiene word onderwerp aan verskeie omgewingsfaktore wat falings van komponente kan veroorsaak. Een van hierdie faktore, naamlik hoëfrekwensie vermoeidheid, word onder andere veroorsaak deur lem adder. Hierdie tesis is deel van 'n projek in die Sewende Europese Raamwerk Program (European Seventh Framework Programme - FP7), projek Future. Projek Future doen teoretiese en eksperimentele ondersoek na die voorkoms van lemfl adder in turbomasjienerie. Die doelwit van hierdie tesis was om die effektiwiteit van 'n gasinspuiter vibrasie-opwekkingstelsel te evalueer, deur gebruik te maak van onbestendige simulasie in die berekenings vloei-meganika sagtewarepakket, Numeca FINE/Turbo. Eksperimentele toetswerk op die plaaslike Rofanco kompressortoetsbank, met 'n prototipe van die 15 inspuiter stelsel, het inligting verskaf wat gebruik is om die inlaattoestande te spesi seer en simulasieresultate te korreleer. Die simulasieresultate het getoon dat die frekwensie waarteen 'n lem by die inspuiters verbybeweeg, so prominent is, dat dit in die meerderheid van gevalle baie meer prominent is as die opwekkingsfrekwensie. Verder was daar ook deurgaans 'n aantal sekondêre frekwensies teenwoordig. Die teenwoordigheid van die inspuiter verbybeweeg frekwensie en die sekondêre frekwensies is die resultaat van die konvolusie van Vinnige Fourier Transforme. Alhoewel dit in teorie moontlik sal wees om die inspuiter verbybeweeg frekwensie te elimineer, is dit onmoontlik om die sekondêre frekwensies uit die lem vibrasie te elimineer. Ter opsomming, volgens die berekenings vloei-meganika resultate, is dit nie moontlik om met 'n stelsel van 'n eindige aantal inspuiters, 'n enkele vibrasie frekwensie op te wek nie.

To understand the phenomena associated with such stochastic processes and to predict, at least qualitatively, the behavior of mechanical systems within environments which are completely random in time, new mechanical tools are necessary. Fortunately, the derivation of these tools does not necessitate a complete departure from existing theories. In fact, they may be considered as an extension of the well-defined theory of the integral transform, in particular, the exponential Fourier integral transform.

Vibration signals generated from spalled elements in rolling element bearings (REBs) are investigated in this thesis. A novel signal-processing algorithm to diagnose localized faults in rolling element bearings has been developed and tested on a variety of signals. The algorithm is based on Spectral Kurtosis (SK), which has special qualities for detecting REBs faults. The algorithm includes three steps. It starts by pre-whitening the signal's power spectral density using an autoregressive (AR) model. The impulses, which are contained in the residual of the AR model, are then enhanced using the minimum entropy deconvolution (MED) technique, which effectively deconvolves the effect of the transmission path and clarifies the impulses. Finally the output of the MED filter is decomposed using complex Morlet wavelets and the SK is calculated to select the best filter for the envelope analysis. Results show the superiority of the developed algorithm and its effectiveness in extracting fault features from the raw vibration signal. The problem of modelling the vibration signals from a spalled bearing in a gearbox environment is discussed. This problem has been addressed through the incorporation of a time varying, non-linear stiffness bearing model into a previously developed gear model. It has the new capacity of modeling localized faults and extended faults in the different components of the bearing. The simulated signals were found to have the same basic characteristics as measured signals, and moreover were found to have a characteristic seen in the measured signals, and also referred to in the literature, of double pulses corresponding to entry into and exit from a localized fault, which could be made more evident by the MED technique. The simulation model is useful for producing typical fault signals from gearboxes to test new diagnostic algorithms, and also prognostic algorithms. The thesis provides two main tools (SK algorithm and the gear bearing simulation model), which could be effectively employed to develop a successful prognostic model.

Vibration signals generated from spalled elements in rolling element bearings (REBs) are investigated in this thesis. A novel signal-processing algorithm to diagnose localized faults in rolling element bearings has been developed and tested on a variety of signals. The algorithm is based on Spectral Kurtosis (SK), which has special qualities for detecting REBs faults. The algorithm includes three steps. It starts by pre-whitening the signal's power spectral density using an autoregressive (AR) model. The impulses, which are contained in the residual of the AR model, are then enhanced using the minimum entropy deconvolution (MED) technique, which effectively deconvolves the effect of the transmission path and clarifies the impulses. Finally the output of the MED filter is decomposed using complex Morlet wavelets and the SK is calculated to select the best filter for the envelope analysis. Results show the superiority of the developed algorithm and its effectiveness in extracting fault features from the raw vibration signal. The problem of modelling the vibration signals from a spalled bearing in a gearbox environment is discussed. This problem has been addressed through the incorporation of a time varying, non-linear stiffness bearing model into a previously developed gear model. It has the new capacity of modeling localized faults and extended faults in the different components of the bearing. The simulated signals were found to have the same basic characteristics as measured signals, and moreover were found to have a characteristic seen in the measured signals, and also referred to in the literature, of double pulses corresponding to entry into and exit from a localized fault, which could be made more evident by the MED technique. The simulation model is useful for producing typical fault signals from gearboxes to test new diagnostic algorithms, and also prognostic algorithms. The thesis provides two main tools (SK algorithm and the gear bearing simulation model), which could be effectively employed to develop a successful prognostic model.