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Parametric excitation can occur when the value of a parameter of an oscillator is modulated at twice the natural frequency of the oscillator. The response grows exponentially and is only limited by a nonlinearity of the system, so large response amplitudes typically occur. However, there is no response unless the parametric drive amplitude is above a threshold value that is dictated by the damping. We investigate parametric excitation in three physical systems. The first involves an acoustic standing wave in a pipe that is driven by a piston at one end. An analysis shows that parametric excitation is not feasible in this system unless one uses a very large-excursion piston (for example, from an aircraft engine). The second system is an inductor-capacitor circuit which can undergo oscillations of the current. An analysis of capacitance modulation with a bank of alternate rotating and stationary parallel plates shows that parametric excitation would be very difficult to achieve. Finally, we describe the construction of a torsional oscillator whose length is modulated. Parametric excitation is successfully demonstrated in this system. A comparison of data to predictions of the standard theory of parametric excitation reveals significant deviations.

Reducing the deleterious effect of Vortex Induced Vibrations (VIV) in marine risers is an important task for ocean engineers; and many competing factors exist in the design of VIV suppression devices. This thesis explores the experimental minimization of the drag force and the disruption of the vortex formation by utilizing VIV suppression devices. Two series of tests are conducted- both utilizing separate testing designs. The first tests are the flexible cylinder experiments, detailed in Chapter 2, which determine the drag force and vibration amplitude of numerous, original testing configurations. The second series of tests are the rigid cylinder, PIV experiments, detailed in Chapter 3. These rests measure both the drag force on the cylinder and the oscillating component of the lift force, the latter of which is a good indication of vortex formation. The Chapter 3 tests also image the test section wake- providing helpful insight into the physical process of vortex formation. / Contract number: N62271-97-G-0026.

A low-cost fibre-optic probe was constructed, and used to provide an optical link between an FT-Raman spectrometer and a Differential Scanning Calorimeter (DSC). The resulting Simultaneous Raman-DSC (SRD) method combines the vibrational structural information of FT-Raman spectroscopy with the thermal data of DSC, which is particularly useful for characterisation of samples exhibiting temperature- and thermal conditioning-dependent polymorphic (i. e. structural) changes. The samples chosen for method development and structural study were ammonium nitrate (NH4NO3), the triglyceride sn-1,3-distearoyl-2-oleoyl glycerol (SOS), cocoa butter (which is chiefly composed of a mixture of monounsaturated triglycerides), and some chocolates of differing composition. The bifurcated `6-around-i' parallel fibres probe design allowed Raman spectra to be collected from unencapsulateds amplesi n the normal DSC sample position without extensive mechanical modifications or the use of extra optical components. Method development using the above samples showed that the combined SRD sampling has various effects on collected data. With this probe design, a "glass background" spectrum is superimposed on the Raman spectrum, and must be subtracted using the spectrometer software. At raised temperatures, a thermal emission feature appears in the Raman spectrum at high wavenumber shift. The incident laser light was found to raise sample temperatures by around 1.5-3K. Raman spectra were therefore collected with samples held around 10K below transition onset temperatures. The necessary lack of encapsulation resulted in broadened thermal transitions in DSC heating curves, and potential heat loss due to radiative emission at raised temperatures. However, for samples analysed near room temperature, onset melting points for transitions with the laser off were not significantly altered by SRD sampling. Ammonium nitrate is often irreproducible in it's exact phase transition thermal behaviour. SRD analysis showed the expected phases IV, III, II and I at room temperature and above at normal atmospheric pressure. Raman spectra could then be unambiguously assigned to the respective phases. Comparison with previous literature work allows the spectral changes seen to be interpreted using symmetry-based rationalisations. The triglyceride SOS was found to conform well to the literature thermal behaviour. SRD analysis showed DSC heating curves with onset melting points and features identifying the polymorphs produced as a, , y, ß', ß2 and [il. A literature survey allows the corresponding Raman spectra to be interpreted in the light of structural packing suggestions made by workers using various other analytical techniques. Cocoa butter was successfully prepared in states 3,4,5 and 6 of the six polymorphs known to exist. DSC melting behaviour and heating curve shapes were used to unambiguously relate Raman spectra to the individual polymorphs. Interpretation of Raman spectra was based on the SOS triglyceride SRD results, together with structural packing suggestions from the literature. A selection of chocolate samples were also analysed by SRD. Those with a high cocoa butter content were found to approximate well to the thermal and structural behaviour of pure cocoa butter.

Approved for public release; distribution is unlimited / Motivation to expand the understanding of a helicopter rotor descending into the vortex ring state (VRS) stems from the aircraft mishaps that have plagued the helicopter community. The V-22 has become the most recent victim of encounters with VRS. The onset of VRS is associated with the collapse of the helical vortex wake in the plane of the rotor. The resulting wake disturbances develop an irregular and aperiodic flow. Rotor blade interaction with the disturbed vortices causes large variations in the blade spanwise aerodynamic load distribution. Harmonic analysis of the loading indicates that higher harmonic content becomes prevalent in this state. The dynamic flow similarities achieved in a water tunnel are used to explore flow visualization and conduct vibration analysis of a rotor system operating in the VRS. A scaled rotor system was operated in the NPS Aeronautical Engineering Department's water tunnel. Sensors were used to gather thrust and vibration power spectrum data when operating in VRS. Experimental results correlate with full scale flight data and show a significant increase in the vibration levels of the even multiples of the blade passage frequency. The relative strength of these higher harmonics can be used as an indicator of impending VRS encounters. / Major, United States Marine Corps

Approved for public release, distribution is unlimited / Damping of welded structures is a subject of great interest and application for the navy as relates to ship shock survivability and acoustic transmission of ship noise. The purpose of this research is to study the effects of welding on damping. A generic model of a warship's hull structure was used to study damping effects. The model's natural frequencies and mode shapes were calculated using a finite element model prior to model testing. The frequency response and natural frequencies of the model were determined experimentally by exciting the model and measuring the response throughout the structure using Frequency Response Functions (FRF's). The results were compared with the finite element modeling. The damping ratio of the model in relation to position from excitation was calculated using the half-power point method and then a more detailed analysis of frequency dependent damping versus position was made using modal parameter extraction using the Complex Exponential Method. / Lieutenant, United States Navy

This thesis deals with the analytical study of vibration of carbon nanotubes and graphene plates. First, a brief overview of the traditional Bresse-Timoshenko models for thick beams and Uflyand-Mindlin models for thick plates will be conducted. It has been shown in the literature that the conventionally utilized mechanical models overcorrect the shear effect and that of rotary inertia. To improve the situation, two alternative versions of theories of beams and plates are proposed. The first one is derived through the use of equilibrium equations and leads to a truncated governing differential equation in displacement. It is shown, by considering a power series expansion of the displacement, that this is asymptotically consistent at the second order. The second theory is based on slope inertia and results in the truncated equation with an additional sixth order derivative term. Then, these theories will be extended in order to take into account some scale effects such as interatomic interactions that cannot be neglected for nanomaterials. Thus, different approaches will be considered: phenomenological, asymptotic and continualized. The basic principle of continualized models is to build continuous equations starting from discrete equations and by using Taylor series expansions or Padé approximants. For each of the different models derived in this study, the natural frequencies will be determined, analytically when the closed-form solution is available, numerically when the solution is given through a characteristic equation. The objective of this work is to compare the models and to establish the eventual superiority of a model on others. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2018. / FAU Electronic Theses and Dissertations Collection

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