Cavity formation resulting from the water-entry of solid objects has been the subject of extensive research owing to its practical relevance in naval, military, industrial, sports and biological applications. The cavity formed by an impacting hydrophobic sphere normally seals at two places, one below (deep seal) and the other above the water surface (surface seal). For Froude numbers , the air flow into the resulting cavity is strong enough to suck the splash crown above the surface and disrupt the cavity dynamics before it deep seals. In this research work we eliminate surface seals by means of a novel practice of using cone splash-guards and examine the undisturbed transient cavity dynamics by impact of hydrophobic spheres for Froude numbers ranging . This enabled the measurement of extremely accurate pinch-off heights, pinch-off times, radial cavity collapse rates, and jet speeds in an extended range of Froude numbers compared to the previous work of Duclaux et al. (2007). Results in the extended regime were in remarkable agreement with the theoretical prediction of scaled pinch-off depth, and experimentally derived pinch-off time for . Furthermore, we investigated the influence of confinement on cavity formation by varying the cross-sectional area of the tank of liquid. In conjunction with surface seal elimination we observed the formation of multiple pinch-off points where a maximum of four deep seals were obtained in a sequential order for the Froude number range investigated. The presence of an elongated cavity beneath the first pinch-off point 5 resulted in evident "kinks" primarily related to the greatly diminished air pressure at the necking region caused by supersonic air flows (Gekle et al. 2010). Such flows passing through second pinch-offs were also found to choke the cavities beneath the first pinch- off depths causing radial expansion and hence disappearance of downward jets.

This research introduces a new application of the three microphones method, which was originally developed to analyse standing waves, to measure the aeroacoustic power of a duct housing a shallow cavity coupled with a longitudinal acoustic mode. In addition, this work provides, for the first time, the spatial distribution of the aeroacoustic sound sources over the cavity region for this type of flow-sound-structure interaction pattern. Furthermore, this research includes a comprehensive study of the effect of cavity geometrical parameters on the characteristics of the cavity aeroacoustic source. An experimental investigation of the aeroacoustic source of an axisymetric cavity in a pipeline is presented. This aeroacoustic source is generated due to the interaction of the cavity shear layer oscillation with the resonant acoustic field in the pipe. The source is determined under high Reynolds number, fully developed turbulent pipe flow. The experimental technique (Sound Wave Method, SWM) employs six microphones distributed upstream and downstream of the cavity to evaluate the fluctuating pressure difference generated by the oscillating cavity shear layer in the presence of externally imposed sound waves. The results of the dimensionless aeroacoustic sources are in good agreement with the concepts of free shear layer instability and the fluid resonant oscillation behavior. A validation study is performed in order to validate the measurement technique and the measured source term from the SWM. The validation methodology consisted of comparing the self-excited resonance response obtained from self-excitation measurements with that estimated from an acoustic model supplemented with the measured source term using the SWM. The comparison depicts a very good agreement for the resonance frequency, lock-in ranges, and the resonance amplitude. Extensive PIV flow measurements are performed to clarify the non-linear behavior of the aeroacoustic source at high levels of the acoustic particle velocity, and to understand the dependence of the flow-sound interaction patterns on the main system parameters such as the Strouhal number and excitation level. The results of a finite element analysis of the resonant sound field are combined with those of the PIV flow measurements into Howe’s aeroacoustic integrand to compute the spatial and temporal distributions of the aeroacoustic sources. The results are also compared with the measured aeroacoustic source strength obtained by means of the SWM. This comparison highlights the superior efficiency of the SWM technique. Identification of the aeroacoustic source distributions as function of the acoustic excitation level showed that the non-linear behaviour of the source strength, which occurs at moderate sound levels, is caused by a gradual transition in the vorticity field oscillation pattern; from a distributed vorticity cloud over the whole cavity length at small excitation amplitudes to a pattern involving rapid formation of (discrete) vortices at the leading edge which becomes dominant at large excitation levels. The spatial distribution of the acoustic power over the cavity length at resonance condition shows sources of sound generation at the first and last thirds of the cavity mouth and an absorption sink in the middle third. This distribution is different from that observed for deep cavities and trapped modes of shallow cavities. Due to these differences in the aeroacoustic source distributions, the effects of cavity geometrical parameters for the present shallow cavity are not necessarily similar to those reported in the literature for deep cavities and trapped mode resonance cases. A comprehensive study of the effect of cavity geometrical parameters (including rounding-off the cavity edges) on the aeroacoustic sound sources is also included. Nine cavity sizes are studied in three different groups of length to depth ratios (L/H) with three different cavity volumes for each group of L/H. The aeroacoustic source strength and the Strouhal number corresponding to its maximum value are found to increase in a systematic manner as the cavity volume is increased for the same L/H ratio. These results indicate that the aeroacoustic sources of shallow cavities are affected not only by the ratio L/H, but also by the cavity volume. The effect of cavity edge curvatures on the resonance response is experimentally investigated by testing different sizes of curvatures at different locations (upstream, downstream or both edges). The results show that rounding-off the cavity edges causes a reduction in the vertical component of the acoustic particle velocity but also an increase in the cavity length. These two consequences have opposite effects on acoustic power generation and therefore, rounding-off the edges has no significant effect on the resonance amplitude in the present case, except for relatively large radius. / Thesis / Doctor of Philosophy (PhD)

We study the time evolution of light fields inside a double cavity which is comprised of two perfect end mirrors and a parametrically driven, partially transmissive central mirror in both a classical and a quantum mechanical framework. It is common practise in the field of optomechanics to take a Hamiltonian approach \cite{aspelmeyer2014cavity} ignoring non-linear coupling terms between the light field and the moving mechanical element. By contrast, we start from the Maxwell wave equation which is second order in time and find that a first order in time Schr\"{o}dinger-type wave equation (equivalent to neglecting the non-linear coupling) is a valid approximation for low enough mirror reflectivity and speed and for large light frequencies. We also study adiabatic dynamics for the Maxwell wave equation and find it differs from the more familiar adiabaticity in the Schr\"{o}dinger equation. Next, we numerically simulate the dynamical Casimir effect (DCE) in the double cavity with a sinusoidally driven central mirror following earlier numerical work on the perfect single cavity \cite{Ruser2006NumericalDCE,ruser2005vibrating,naylor2009dynamical}. Because our central mirror is partially transmissive it is physically more realistic and circumvents fundamental problems associated with having perfectly reflecting moving mirrors \cite{Moore1970DCESingleCavity,barton1993quantum}. The corresponding photon creation rates are drastically lower when compared to the perfectly reflective mirror case. Furthermore, if we make one of the cavities much longer than the other we can simulate the DCE for a single open cavity coupled to an environment without having to make the Markov approximation. The resultant asymmetric double cavity (ADC) model is valid for times short enough that only a negligible number of the photons that has leaked out of the open cavity has sloshed back in again. As for the symmetric case, one advantage of the ADC is that driven mirror is partially transmissive rather than perfectly reflecting. / Thesis / Doctor of Philosophy (PhD)

<p> This report contains the results of an exploratory investigation into the strength (compressive and tensile) of the most commonly used cavity wall ties. Z, Truss, Ladder and Corrugated metal ties were tested in the Applied Dynamics Laboratory at McMaster University.</p> <p> A total of 110 specimens were constructed and tested. Fifty-five specimens were tested in compression, another 55 specimens were tested in tension. The influence of different cavity widths and lengths of mortar imbedment on the strengths of the ties were also investigated.</p> <p> The ultimate load capacity, for each tie investigated, was related to loading on a unit of wall area by taking into consideration the recommended spacings for each tie.</p> <p> A strength comparison of the ties tested was made. Finally, conclusions were drawn and recommendations made.</p> / Thesis / Master of Engineering (MEngr)

No description available.

This thesis describes the development of a technique for measuring frequency noise of semiconductor lasers. Equivalent laser linewidths were calculated from frequency noise measurements on several InGaAsP lasers with short external cavities to give single mode operation. Conventional 250 um lasers demonstrated linewidths of about 125 MHz-mW, compressively strained quantum well lasers of commensurate length had linewidth of 37 MHz-mW, and 500 um strained quantum well lasers had linewidth of 18 to 28 MHz-mW with an apparent strain dependence. The short external configuration allowed selection of a number of laser modes. Measurement of linewidth variation with laser mode showed a 20% to 40% change over six to eight modes. The system was adapted to make measurements of the optical frequency tuning with fine external cavity length change. This measurement provided a novel means to estimate the linewidth enhancement factor and the reflectivity of the external cavity element. The estimated values of the linewidth enhancement factor for 250 um conventional and quantum well lasers were found to be in the correct ratio to account for the measured difference in linewidth. / Thesis / Master of Engineering (ME)

Quantification of cavity excavation produced strong empirical support for the ecological constraints model for the evolution of delayed dispersal in the cooperatively breeding red-cockaded woodpecker. The long times required for cavity excavation select for competition over breeding vacancies in established territories and against excavation of cavities in unoccupied habitat. Duration of excavation varies between woodpecker populations, but may require over 13 years in longleaf pine and over 10 years in loblolly pine. Duration of excavation is extremely variable. Much of the variation is due to variation in effort by excavating woodpeckers, which is in turn partially related to the need for new cavities in relation to the number of available cavities on a territory. An average of only 11 % of an individual's time budget is devoted to excavation, and only one individual per group makes significant contributions to excavation. Once completed, cavities are used for periods that may exceed fifteen years. Cavities in longleaf pine are used for significantly longer periods than cavities in loblolly. Whereas cavities no longer used as nests are abandoned altogether in loblolly, they are still roosted in for many years in longleaf. Final abandonment of longleaf cavities appears to be related to cavity loss. Quantification of cavity turnover revealed that three of the study populations were stable in cavity numbers over the study period, while a fourth underwent alarming declines. The continued use of restrictors and artificial cavities, and the protection of old-growth upon which the woodpeckers depend for excavation, are recommended. / Master of Science

Thesis (Ph. D.)--University of Toronto, 1997. / Two chapters co-authored with Kathy Martin. Includes bibliographical references.

The effects of scaling a cavity with respect to a fixed incoming boundary layer thickness on its flow dynamics and control was studied experimentally. Three cavity models with constant length-to-depth ratio of 5 and length-to-width ratio of 2 and with corresponding linear dimensions in the ratio 0.5 : 1 : 2 were tested at freestream Mach number 0.71. Additionally, the 0.5 and 1 scale models were tested at freestream Mach number of 0.85. The experiments involved timeaveraged pressure measurements, unsteady pressure measurements, and PIV measurements. Time-averaged pressure measurements made at the floor were used to study the ’flow-type’ of the cavities. Unsteady pressure measurements were used to study the acoustic characteristics of the cavity. The cavity length-to-boundary layer thickness ratios tested were 10, 20 and 40. The Cp distribution on the clean cavities indicated a change in the cavity flowtype with change in the cavity scale. Varying the L/δ from 10 to 40 changed the cavity flow-type from open to transitional. Analysis of the frequency spectra of the cavity revealed an increase in tonal amplitudes and OASPL with increasing L/δ . The PIV measurements indicated that this could be caused by an increase in energy exchange between the freestream and the cavity. The velocity magnitudes inside the cavities were found to increase with increase in L/δ . A comparative study of different passive control methods on the largest cavity showed that leading-edge spoilers were superior in cavity tone suppression. Of these, the effectiveness of a sawtooth spoiler on the three cavities of different scales was tested. The results showed that while the spoiler was effective in eliminating tones and suppression of noise for the smaller cavities, it was unable to eliminate the tones completely for the largest cavity. To find the correct method for scaling the spoilers with the cavity dimensions, different spoiler heights were tested on the three cavities. The results showed that the cavity noise suppression for a given cavity attains saturation level at a particular spoiler height, called the critical spoiler height. An increase in spoiler height beyond the critical spoiler height was found to have no effect on the noise suppression. It is also found that this critical spoiler height can be scaled with the length of the cavity (for given L/D, M and spoiler profile) irrespective of the boundary layer thickness.

published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy