The finite element method is applied to the analysis of rotating disks. The equation governing the transverse deflection of a rotating disk is presented in both body-fixed and space-fixed coordinate systems. The quadratic stress triangle is used to determine the in-plane stress distribution which contributes to the out-of-plane stiffness. The out-of-plane problem is analyzed using elements based on the high precision 18 degree of freedom compatible plate bending triangle. Free vibration problems of both axisymmetric and non-axisymmetric disks are solved in the body-fixed coordinate system. The numerical results obtained for axisymmetric disks show good agreement with results from other analyses. In a space-fixed coordinate system, the response of a rotating axisymmetric disk to a space-fixed transverse load is examined in some detail. It is found that there exists a fundamental critical rotation speed at which a component of a natural vibration mode becomes stationary in space, leading to an instability. This critical speed may be related to a non-dimensional stiffness parameter which, together with the non-dimensional hub radius, completely defines the disk geometry and stiffness. Thus, it appears that the critical value of the stiffness parameter is a function only of the non-dimensional hub radius. The response of the disk past this critical stiffness is also examined. The results provide evidence of the existence of the higher critical speeds and indicate that the response is very sensitive to any approximations made with respect to the geometry of the disk or the in-plane stress distributions. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

No description available.

Thesis (M.S.)--Ohio University, November, 2002. / Title from PDF t.p. Includes bibliographical references (leaves 81-82).

Thesis (Ph.D.) - University of St Andrews, November 2008.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

No description available.

Circumstellar debris disks are the extrasolar analogues of the asteroid belt and the Kuiper belt. These disks consist of comets and leftover planetesimals that continuously collide to produce copious amounts of circumstellar dust that can be observed as infrared excess or in resolved imaging. As an obvious outcome of the planet formation process, debris disks can help us constrain planet formation theories and learn about the history of our own solar system. Structures in the disks such as gaps or warps can hint at the presence of planets. Thus, the study of debris disks is an important branch of exoplanetary science. In this thesis, some aspects of debris disks are considered in detail. A handful of debris disks show observable amounts of gas besides the dust. One such case is the edge-on debris disk around the young A-type star β Pictoris, where the gas is thought to be of secondary origin, i.e. derived from the dust itself. By observing this gas, we can thus learn something about the dust, and therefore about the building blocks of planets. In paper I, spectrally resolved observations of C II emission with Herschel/HIFI are presented. The line profile is used to constrain the spatial distribution of carbon gas in the disk, which helps understanding the gas producing mechanism. In paper II, we analyse C II and O I emission detected with Herschel/PACS and find that the oxygen must be located in a relatively dense region, possibly similar to the CO clump seen by ALMA. An upcoming analysis of our ALMA C I observations will give us a clearer picture of the system. Another famous debris disk is found around the nearby, 440 Myr old A-star Fomalhaut. Its morphology is that of an eccentric debris belt with sharp edges, suggesting shaping by a planet. However, gas-dust interactions may result in a similar morphology without the need to invoke planets. We test this possibility in paper III by analysing non-detections of C II and O I emission by Herschel/PACS. We find that there is not enough gas present to efficiently sustain gas-dust interactions, implying that the morphology of the Fomalhaut belt is due to a yet unseen planet or alternatively stellar encounters. One of the biggest challenges in exoplanetary research is to answer the question whether there are inhabited worlds other than the Earth. With the number of known rocky exoplanets in the habitable zone increasing rapidly, we might actually be able to answer this question in the coming decades. Different approaches exist to detect the presence of life remotely, for example by studying exoplanetary atmospheres or by analysing light reflected off the surface of an exoplanet. In paper IV, we study whether biosignatures (for example, certain minerals or microorganisms) ejected into a circumstellar debris disk by an impact event could be detected. We consider an impact similar to the Chicxulub event and model the collisional evolution of the ejected debris. Dust from such an event can potentially be detected by current telescopes, but analysis of the debris composition has to wait for future, advanced instruments. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Submitted.</p>

abstract: The composition of planets and their volatile contents are intimately connected to the structure and evolution of their parent protoplanetary disks. The transport of momentum and volatiles is often parameterized by a turbulent viscosity parameter $\alpha$, which is usually assumed to be spatially and temporally uniform across the disk. I show that variable $\alpha$(r,z) (where $r$ is radius, and $z$ is height from the midplane) attributable to angular momentum transport due to MRI can yield disks with significantly different structure, as mass piles up in the 1-10 AU region resulting in steep slopes of p $>$ 2 here (where p is the power law exponent in $\Sigma \propto r^{-p}$). I also show that the transition radius (where bulk mass flow switches from inward to outward) can move as close in as 3 AU; this effect (especially prominent in externally photoevaporated disks) may significantly influence the radial water content available during planet formation. I then investigate the transport of water in disks with different variable α profiles. While radial temperature profile sets the location of the water snowline (i.e., inside of which water is present as vapor; outside of which, as ice on solids), it is the rates of diffusion and drift of small icy solids and diffusion of vapor across the snow line that determine the radial water distribution. All of these processes are highly sensitive to local $\alpha$. I calculate the effect of radially varying α on water transport, by tracking the abundance of vapor in the inner disk, and fraction of ice in particles and larger asteroids beyond the snow line. I find one α profile attributable to winds and hydrodynamical instabilities, and motivated by meteoritic constraints, to show considerable agreement with inferred water contents observed in solar system asteroids. Finally, I calculate the timing of gap formation due to the formation of a planet in disks around different stars. Here, I assume that pebble accretion is the dominant mechanism for planetary growth and that the core of the first protoplanet forms at the water snow line. I discuss the dependence of gap timing to various stellar and disk properties. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2018

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 106-115). Available also in a digital version from Dissertation Abstracts.

Thesis--Ph. D., University of Hong Kong. / Mimeographed.