Acoustic observations of suspended participate matter in the ocean
CLOUDS of suspended particulate matter in the ocean have been observed by satellite1, sensed by nephelometers2, and sampled by water bottles. We present here evidence that it may be possible to observe oceanic suspended particulate matter acoustically. The particulate matter discussed is thought to have arisen from a dredging operation. It is also thought that the suspended matter is present at lower concentrations than any in any other case recorded acoustically3. Certain questions, as yet unresolved, regarding acoustic scattering strengths are also discussed.
An Evaluation of Target Strength Estimation from Echo Ensembles
Estimation of the acoustic target strength of point scatterers in the ocean calls for a calibrated acoustic system and a signal processing methodology based on either a single or multiple/modified sound beam geometry. A single beam sounding system acting on a population of point scatterers returns an ensemble of echoes which, once range corrected, may be regarded as arising from a combination of two stochastic processes, corresponding to the positions and attitudes of individual targets in the beam. An ensemble of returns from individual targets may be processed to yield a target strength estimate for an individual target provided a suite of assumptions associated with the scattering process are made. The validity of a suite of assumptions used in such processing has been evaluated using Monte Carlo methods for a set of non-ideal conditions representative of those expected in tank and field experiments involving marine biota and results from experiments on Antarctic krill are presented.
Numerical analysis of surface crack propagation in flexible pavements using XFEM and cohesive zone model
Surface initiated (top-down) cracking in the longitudinal direction of pavements has become a predominant mode of failure in thick asphalt pavements. Most previous researchers studied crack propagation using the traditional Linear Elastic Fracture Mechanics (LEFM) method by calculating the stress intensity factor around the crack tip. In this paper, Extended Finite Element Method (XFEM) was used to study the key factors causing surface initiated crack propagation under surface tension and/or shear. A two-dimensional (2-D) finite element (FE) model for a two-lane flexible pavement was built to predict pavement responses and crack propagation potential under various scenarios. It was found that surface tension is the major driving force for crack propagation when the crack is at some distance away from tire loading. The results show that the distance from the location of crack to tire loading, the depth and direction of the crack, and the modulus ratio between the hot-mix asphalt (HMA) layer and the base layer have significant influences on crack propagation under surface tension. On the other hand, this study shows that shear stress can induce surface initiated crack propagation near the loading area as the shear resistance of material becomes low at the high temperature.