Physics and Astronomy

1] The measurement of turbulence dissipation rates and length scales associated with three-dimensional isotropic structures under spilling waves breaking in a laboratory surf zone is presented. Dissipation rates were estimated from the spectral characteristics of the turbulence velocities in the inertial subrange, and length scales were estimated using measurements of the turbulent kinetic energy and dissipation rate. The spatial velocity flow fields for the above analysis were measured using digital correlation image velocimetry. A unique set of measurements that spans the entire water column, including the aerated portion near the crest of the wave, is presented. Dissipation rates were found to reach a maximum above the effective trough level, with over 80% of the depth integrated dissipation occurring in this upper zone. The total depth integrated turbulence dissipation rate is found to be up to an order of magnitude smaller than the local rate of wave energy dissipation due to breaking, the primary turbulence production source. The length scale is found to increase in magnitude below the surface, consistent with the idea that turbulence production occurs above the trough level in the vicinity of the wave roller and is transported downward toward the bed.

Observations of coherent electrostatic ion cyclotron (EIC) waves associated with a strong, magnetized double layer are presented. The double layers are produced in a weakly ionized argon plasma by applying a positive potential to an electrode plate located in the diverging magnetic field region of a cylindrical plasma column. Ionization within the electrode sheath is essential to the formation of these double layers. The resulting V-shaped potential structures have extended parallel, oblique and perpendicular (to B) electric field components. The frequency of the ion cyclotron instability is dependent upon the magnetic field strength at the position of the parallel potential structure. The properties of EIC waves in the presence of the double layer are discussed in relation to the possible excitation mechanisms (field-aligned currents, ion and electron beams, and perpendicular E fields).

The cost-e¡ective measurement of the velocity £ow ¢elds in breaking water waves, using particle and correlation image velocimetry, is described. The £uid velocities are estimated by tracking the motion of neutrally buoyant particles and aeration structures. Results of the phase-ensemble-averaged velocity £ow ¢elds, time-averaged horizontal velocities and turbulence results are presented for laboratory waves breaking on a beach of slope 1:20 and for waves breaking over a submerged bar. A comparison of measurements using video and LDA techniques is also presented. The measurement of water levels and wave roller structures is accomplished by generating a video time series of the wave at a given horizontal position, from which the water levels and rollers structures are extracted.

Time variations of the geomagnetic field can be classified into two main categories of internal and external origin with respect to the surface of the Earth. It has been found that the variations that take place on longer time scales (commonly known as secular variation (SV)) are of internal origin. There is a need to develop SV models using satellite data as the use of ground data is not always possible with many limitations including the limited data points and lack of data over ocean areas that are not easily accessible. Two regional geomagnetic field modelling techniques namely polynomial surface modelling (PolyM) and Spherical Cap Harmonic Analysis (SCHA) were applied to CHAMP satellite data recorded between 2001 and 2005 to investigate the use of satellite data to develop a geomagnetic SV model over southern Africa. The restricted area of investigation is between 10 • and 40 • South in latitude and between 10 • and 40 • East in longitude. The resulting regional models of this investigation were validated against the two widely used global field models IGRF 10 and CHAOS-2 using the available ground survey data obtained during the same period over southern Africa. The results suggest that the regional field models can be derived based entirely on satellite data. However, the regional SV models can be improved using both high quality satellite and ground survey data, since they lack the high quality of a global field model like CHAOS-2.

DC arc furnaces operate with a graphite cathode positioned above a molten slag and metal bath contained inside a furnace vessel. The plasma arc, consisting of ionised particles at extremely high temperatur es, forms a conducting path between the graphite cathode and the molten bath. The power provided by the arc can be controlled by adjusting the arc length and the current.

The identification of normal breast tissues in mammograms is an important step in identifying abnormal tissues, or masses. The major focus of this study is to use image-processing techniques to characterize the normal breast tissues. Clustering techniques was developed to classify the breast tissue into one of 5 different tissue types including fat, glandular, connective, dense tissues and pectoral muscle. The classification was achieved by using a novel technique involving a subset of the elements (the values on the diagonals) in the Spatial Grey Level Dependence (SGLD) matrix which was calculated using texture features, thus it was called the diagonal SGLD method (dSGLD). The ultimate goal of this study was therefore to design and implement a computerized characterization method that mimics the radiologist's characterization of tissue composition in digitized mammograms. The classification results using dSGLD and SGLD features achieved an accuracy of 80% and 71% , respectively...

This paper discusses the development of a system for the automated real-time measurement of crystal size distributions during crystallisation. An optimised selection of the Daubechies wavelet coefficients is used as input to a Multi-Layer Perceptron artificial neural network to characterise the crystal scale lengths. This technique gives significant advantages over the sampling-based measurements, which require an individual measurement of single crystals. Test results obtained using simulated crystals, and actual images from a crystallisation pan and a laboratory crystaloscope are presented.