Quantification of Fracture Roughness by Change Probabilities and Hurst Exponents

The objective of the current study is to utilize an innovative method called “change probabilities” for describing fracture roughness. In order to detect and visualize anisotropy of rock joint surfaces, the roughness of one-dimensional profiles taken in different directions is quantified. The central quantifiers, change probabilities, are based on counting monotonic changes in discretizations of a profile. These probabilities, which usually vary with the scale, can be reinterpreted as scale-dependent Hurst exponents. For a large class of Gaussian stochastic processes, change probabilities are shown to be directly related to the classical Hurst exponent, which generalizes a relationship known for fractional Brownian motion. While related to this classical roughness measure, the proposed method is more generally applicable, therefore increasing the flexibility of modeling and investigating surface profiles. In particular, it allows a quick and efficient visualization and detection of roughness anisotropy and scale dependence of roughness.

Ice ridge density signatures in high resolution SAR images

The statistics of ice ridging signatures was studied using a high (1.25 m) and a medium (20 m) resolution SAR image over the Baltic sea ice cover, acquired in 2016 and 2011, respectively. Ice surface profiles measured by a 2011 Baltic campaign was used as ground truth data for both. The images did not delineate well individual ridges as linear features. This was assigned to the random, intermittent occurrence of ridge rubble block arrangements with bright SAR return. Instead, the ridging signature was approached in terms of the density of bright pixels and relations with the corresponding surface profile quantity, ice ridge density, were studied. In order to apply discrete statistics, these densities were quantified by counting bright pixel numbers (BPN) in pixel blocks of side length L, and by counting ridge sail numbers (RSN) in profile segments of length L. The scale L is a variable parameter of the approach. The other variable parameter is the pixel intensity threshold defining bright pixels, equivalently bright pixel percentage (BPP), or the ridge sail height threshold used to select ridges from surface profiles, respectively. As a sliding image operation the BPN count resulted in enhanced ridging signature and better applicability of SAR in ice information production. A distribution model for BPN statistics was derived by considering how the BPN values change in BPP changes. The model was found to apply over wide range of values for BPP and L. The same distribution model was found to apply to RSN statistics. This reduces the problem of correspondence between the two density concepts to connections between the parameters of the respective distribution models. The correspondence was studied for the medium resolution image for which the 2011 surface data set has close temporal match. The comparison was done by estimating ridge rubble coverage in 1 km2 squares from surface profile data and, on the other hand, assuming that the bright pixel density can be used as a proxy for ridge rubble coverage. Apart from a scaling factor, both were found to follow the presented distribution model.

URANS PREDICTION OF THE SLAMMING COEFFICIENTS FOR PERFORATED PLATES DURING WATER ENTRY

The slamming coefficients for perforated plates of various perforation ratios and layout configurations were predicted using Unsteady Reynolds-Averaged Navier-Stokes (URANS) solver STAR-CCM+. The numerical model was validated by comparing with experimental measurements of slamming coefficient for a circular cylinder. The slamming coefficients and free surface profiles of perforated plates were then predicted at full-scale. It was found the air compressibility plays an important role by studying flat plate water entry phenomena. For perforated plates with small gap length/width ratios, the ability of the trapped air to evacuate through the space between the bottom of the plate and free surface is similar. For perforated plates with different gap number at a fixed perforation ratio, the slamming coefficient is increased with the increase in gap length/width ratio. However, a further increase in length/width ratio may impose a negative impact on the escape of trapped air due to the increase of gap number.

Related Works and Basic Considerations

The characterization and measurement of surface profiles is one of the most basic metrology tasks in industrial manufacturing. What started with mechanical stylus profilometers has developed with ultrasonic transducers towards optical instruments. These are capable of appropriate resolutions to enable nanometrology, [14].

Topographical evaluation of laser generated surfaces using statistical analysis of surface-normal vector distributions

Surfaces generated by Additive Manufacturing or laser texturing can involve the solidification of droplets of liquid, which can give rise to overhanging features on the solidified surface. Overhanging features add a layer of complexity to the surface topography and are undetectable by standard surface roughness measurement techniques such as profilometry. Such features are important because they can have a considerable effect on surface properties such as wettability. New techniques and algorithms are therefore required to analyse and quantify convoluted surfaces with overhanging (re-entrant) features. Earlier work by the authors introduced the concept of using X-ray micro-computed tomography (Micro-CT) to identify the directions of vectors normal to the surface at any point and thus indicate the presence or absence of overhanging features. This paper divides overhanging features into two types; simple and compound, and introduces new, size independent, analysis techniques which measure what proportion of each type is on the surface. Another extension of the analysis is the comparison of surface profiles taken in different directions in order to identify any surface roughness anisotropies.

Investigations on Practical Issues in Solid Immersion Lens Based Sub-Wavelength Terahertz Imaging Technique: System Stability Verification and Interference Pattern Removal

Terahertz (THz) imaging techniques are attractive for a wide range of applications, such as non-destructive testing, biological sensing, and security imaging. We investigate practical issues in THz imaging systems based on a solid immersion lens (SIL). The system stability in terms of longitudinal misalignment of the SIL is experimentally verified by showing that the diffraction-limited sub-wavelength beam size (0.7 λ) is maintained as long as the SIL is axially located within the depth-of-focus (~13 λ) of the objective lens. The origin of the fringe patterns, which are undesirable but inevitable in THz imaging systems that use continuous waves, is analytically studied, and a method for minimizing the interference patterns is proposed. By combining two THz images obtained at different axial positions of the object and separated by λ/4, the interference patterns are significantly reduced, and the information hidden under the interference patterns is unveiled. The broad applicability of the proposed method is demonstrated by imaging objects with different surface profiles. Our work proves that the resolution of conventional THz imaging systems can easily be enhanced by simply inserting a SIL in front of the object with high tolerance in the longitudinal misalignment and provides a method enabling THz imaging for objects with different surface profiles.