Laser Machining Of Glass Fibre Reinforced Plastics (GFRP)

Glass fibre reinforced plastics (GFRP) composite materials are in increasingly high demand, particularly in marine industries for reduced weight. This is due to their superior structural characteristics (in fatigue and static conditions) as well as light weight. Anisotropic and heterogeneous features of these materials, however, have posed serious challenges in machining of GFRP. Hence, a new machining technology needs to be investigated. Laser is a non-contact process which is identified as being satisfactory for this research project. A major quality challenge in terms of the laser cutting quality of these materials includes dimensional accuracy. Various laser parameters and cutting techniques are investigated in this study to minimise these defects. In order to improve the cutting quality and dimensional accuracy, design parameters and responses were correlated, modelled, analysed, optimized and experimentally validated to meet the requirements of marine engineering sponsored industry. The objective of this research work is to study the different aspects of GFRP composite cutting using CO2 laser and to establish the relationship between the kerf width, taper and roundness with the process parameters like laser power, cutting speed, gas pressure, frequency and duty cycle. The experimental plans were conducted according to the design of experiment (DOE) to accommodate a full range of experimental analysis. Identification of the important parameter effects presented using analysis of variance (ANOVA) technique combined with graphical representation provides a clearer picture of the whole laser profiling phenomenon. The results show that, the interaction between lower level laser power (2600 Watt), higher level cutting speed (1200 mm/min), higher level gas pressure (8 Bar), medium level frequency (1825 Hz) and medium level duty cycle (96 %) gives better cutting performance towards three responses. Finally, the predictive mathematical model that was established to predict the responses were also validated and are found to be promising in resolving the cut quality issues of industrial GFRP laminates with the error value 16.12 % for kerf width, 18.60 % for taper and 16.28 % for roundness. It was demonstrated that, the response surface methodology (RSM) has played a valuable role to identify the interaction factors of design parameters in attaining industrial desired cut quality response

Whither 215? limitations on the use of discretionary funding incentives as a means of inducing local land use policy reform

Thesis (M.C.P.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 1983.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ROTCH.Includes bibliographical references.by James Robert Milkey.M.C.P

Energy transfer by the scattering of resonant photons

A formal derivation is presented of the energy transfer rate between radiation and matter due to the scattering of an isotropic distribution of resonant photons. The derivation is developed in the context of the two-level atom in the absence of collisions and radiative transitions to and from the continuum, but includes the full angle-averaged redistribution function for photon scattering. The result is compared with previous derivations, all of which have been based on the Fokker-Planck approximation to the radiative transfer equation. A new Fokker-Planck approximation, including an extension to higher (post-diffusive) orders, is derived to solve the radiative transfer equation, and time-dependent numerical solutions are found. The relaxation of the colour temperature to the matter temperature is computed as the radiation field approaches statistical equilibrium through scattering. The results are discussed in the context of the Wouthuysen-Field mechanism for coupling the 21cm spin temperature of neutral hydrogen to the kinetic temperature of the gas through LyA scattering. The evolution of the heating rate is also computed, and shown to diminish as the gas approaches statistical equilibrium.Comment: 13 pages, 4 figures. Submitted to MNRAS. RT eq. simplified to generalise results including stimulated emissio

Constraints on the ecomorphological convergence of zooplanktivorous butterflyfishes

Whether distantly related organisms evolve similar strategies to meet the demands of a shared ecological niche depends on their evolutionary history and the nature of form–function relationships. In fishes, the visual identification and consumption of microscopic zooplankters, selective zooplanktivory, is a distinct type of foraging often associated with a suite of morphological specializations. Previous work has identified inconsistencies in the trajectory and magnitude of morphological change following transitions to selective zooplanktivory, alluding to the diversity and importance of ancestral effects. Here we investigate whether transitions to selective zooplanktivory have influenced the morphological evolution of marine butterflyfishes (family Chaetodontidae), a group of small-prey specialists well known for several types of high-precision benthivory. Using Bayesian ancestral state estimation, we inferred the recent evolution of zooplanktivory among benthivorous ancestors that hunted small invertebrates and browsed by picking or scraping coral polyps. Traits related to the capture of prey appear to be functionally versatile, with little morphological distinction between species with benthivorous and planktivorous foraging modes. In contrast, multiple traits related to prey detection or swimming performance are evolving toward novel, zooplanktivore-specific optima. Despite a relatively short evolutionary history, general morphological indistinctiveness, and evidence of constraint on the evolution of body size, convergent evolution has closed a near significant amount of the morphological distance between zooplanktivorous species. Overall, our findings describe the extent to which the functional demands associated with selective zooplanktivory have led to generalizable morphological features among butterflyfishes and highlight the importance of ancestral effects in shaping patterns of morphological convergence

Approximations for radiative cooling and heating in the solar chromosphere

Context. The radiative energy balance in the solar chromosphere is dominated by strong spectral lines that are formed out of LTE. It is computationally prohibitive to solve the full equations of radiative transfer and statistical equilibrium in 3D time dependent MHD simulations. Aims. To find simple recipes to compute the radiative energy balance in the dominant lines under solar chromospheric conditions. Methods. We use detailed calculations in time-dependent and 2D MHD snapshots to derive empirical formulae for the radiative cooling and heating. Results. The radiative cooling in neutral hydrogen lines and the Lyman continuum, the H and K and intrared triplet lines of singly ionized calcium and the h and k lines of singly ionized magnesium can be written as a product of an optically thin emission (dependent on temperature), an escape probability (dependent on column mass) and an ionization fraction (dependent on temperature). In the cool pockets of the chromosphere the same transitions contribute to the heating of the gas and similar formulae can be derived for these processes. We finally derive a simple recipe for the radiative heating of the chromosphere from incoming coronal radiation. We compare our recipes with the detailed results and comment on the accuracy and applicability of the recipes.Comment: accepted for publication in Astronomy & Astrophysic

Enhancement of the helium resonance lines in the solar atmosphere by suprathermal electron excitation I: non-thermal transport of helium ions

Models of the solar transition region made from lines other than those of helium cannot account for the strength of the helium lines. However, the collisional excitation rates of the helium resonance lines are unusually sensitive to the energy of the exciting electrons. Non-thermal motions in the transition region could drive slowly-ionizing helium ions rapidly through the steep temperature gradient, exposing them to excitation by electrons characteristic of higher temperatures than those describing their ionization state. We present the results of calculations which use a more physical representation of the lifetimes of the ground states of He I and He II than was adopted in earlier work on this process. New emission measure distributions are used to calculate the temperature variation with height. The results show that non-thermal motions can lead to enhancements of the He I and He II resonance line intensities by factors that are comparable with those required. Excitation by non-Maxwellian electron distributions would reduce the effects of non-thermal transport. The effects of non-thermal motions are more consistent with the observed spatial distribution of helium emission than are those of excitation by non-Maxwellian electron distributions alone. In particular, they account better for the observed line intensity ratio I(537.0 A)/I(584.3 A), and its variation with location.Comment: 12 pages, 7 figures, accepted to appear in MNRAS, LaTeX uses mn.st

Interpreting the Mg II h and k Line Profiles of Mira Variables

We use radiative transfer calculations to reproduce the basic appearance of Mg II lines observed from Mira variables. These lines have centroids that are blueshifted by at least 30 km/s from the stellar rest frame. It is unlikely that flow velocities in the stellar atmospheres are this fast, so radiative transfer effects must be responsible for this behavior. Published hydrodynamic models predict the existence of cool, downflowing material above the shocked material responsible for the Mg II emission, and we demonstrate that scattering in this layer can result in Mg II profiles as highly blueshifted as those that are observed. However, our models also show that scattering within the shock plays an equally strong role in shaping the Mg II profiles, and our calculations illustrate the importance of partial redistribution and the effects of being out of ionization equilibrium.Comment: 14 pages, 3 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; to appear in Ap

Radiative transfer in disc galaxies I - A comparison of four methods to solve the transfer equation in plane-parallel geometry

Accurate photometric and kinematic modelling of disc galaxies requires the inclusion of radiative transfer models. Due to the complexity of the radiative transfer equation (RTE), sophisticated techniques are required. Various techniques have been employed for the attenuation in disc galaxies, but a quantitative comparison of them is difficult, because of the differing assumptions, approximations and accuracy requirements which are adopted in the literature. In this paper, we present an unbiased comparison of four methods to solve the RTE, in terms of accuracy, efficiency and flexibility. We apply them all on one problem that can serve as a first approximation of large portions of disc galaxies: a one-dimensional plane-parallel geometry, with both absorption and multiple scattering taken into account, with an arbitrary vertical distributions of stars and dust and an arbitrary angular redistribution of the scattering. We find that the spherical harmonics method is by far the most efficient way to solve the RTE, whereas both Monte Carlo simulations and the iteration method, which are straightforward to extend to more complex geometries, have a cost which is about 170 times larger.Comment: 12 pages, 4 figures, accepted for publication in MNRA

Explanation of the activity sensitivity of Mn I 5394.7 \AA

There is a long-standing controversy concerning the reason why the Mn I 5394.7 A line in the solar irradiance spectrum brightens more at larger activity than most other photospheric lines. The claim that this activity sensitivity is caused by spectral interlocking to chromospheric emission in Mg II h & k is disputed. Classical one-dimensional modeling is used for demonstration; modern three-dimensional MHD simulation for verification and analysis. The Mn I 5394.7 A line thanks its unusual sensitivity to solar activity to its hyperfine structure. This overrides the thermal and granular Doppler smearing through which the other, narrower, photospheric lines lose such sensitivity. We take the nearby Fe I 5395.2 A line as example of the latter and analyze the formation of both lines in detail to demonstrate and explain granular Doppler brightening. We show that this affects all narrow lines. Neither the chromosphere nor Mg II h & k play a role, nor is it correct to describe the activity sensitivity of Mn I 5394.7 A through plage models with outward increasing temperature contrast. The Mn I 5394.7 A line represents a proxy diagnostic of strong-field magnetic concentrations in the deep solar photosphere comparable to the G band and the blue wing of H-alpha, but not a better one than these. The Mn I lines are more promising as diagnostic of weak fields in high-resolution Stokes polarimetry.Comment: 12 pages, 8 figures, accepted by A&