Mani Thomas

The mean and turbulent flow structure of a weak hydraulic jump

The turbulent air–water interface and flow structure of a weak, turbulent hydraulic jump are analyzed in detail using particle image velocimetry measurements. The study is motivated by the need to understand the detailed dynamics of turbulence generated in steady spilling breakers and the relative importance of the reverse-flow and breaker shear layer regions with attention to their topology, mean flow, and turbulence structure. The intermittency factor derived from turbulent fluctuations of the air–water interface in the breaker region is found to fit theoretical distributions of turbulent interfaces well. A conditional averaging technique is used to calculate ensemble-averaged properties of the flow. The computed mean velocity field accurately satisfies mass conservation. A thin, curved shear layer oriented parallel to the surface is responsible for most of the turbulence production with the turbulence intensity decaying rapidly away from the toe of the breaker (location of largest surface curvature) wi...

Motion Tracking of Discontinuous Sea Ice

The purpose of this paper is twofold: 1) to develop a high-resolution sea ice motion tracking system at the geospatial mesoscale (1-100 km2) and 2) to propose an algorithm that measures motion at close proximity to discontinuous regions. Here, we present a motion tracking system that computes differential motion at 400 m resolution and validate the accuracy/precision of this system via four studies. The first study measures the accuracy against displacements measured from in situ Global Positioning System (GPS) buoys deployed at the Sea-ice Experiment: Dynamic Nature of the Arctic (SEDNA) and the Surface HEat Budget of the Arctic Ocean (SHEBA) experiments. The estimates are found to be statistically comparable with GPS, with an average error of 361.9 and 600.6 m for the experiments, respectively. The second study compares the estimated displacements to those measured by the RADARSAT Geophysical Processing System. A precision error of 75.7 m is found between the two motion tracking systems. The third study uses intensity warping of randomly sampled measurements to evaluate discontinuous motion tracking. A one-tailed Wilcoxon signed rank test is used to validate these measurements at α = 0.01. Results from this paper prove that anisotropic smoothing produces significantly smaller errors at discontinuous locations (W = 4240 and p <; 0.001) over conventional isotropic smoothing. The fourth study compares displacements measured by anisotropic smoothing against manual measurements. This paper demonstrates an average reduction of the estimation error by 50 m with the use of anisotropic smoothing over the conventional isotropic smoothing.

Face Recognition Using a Color Subspace LDA Approach

This paper delves into the problem of face recognition using color as an important cue in improving the accuracy of recognition. To perform recognition of color images, we use the characteristics of a 3D color tensor to generate a color LDA subspace, which in turn can be used to recognize a new probe image. To test the accuracy of our methodology, we computed the recognition rate across two color face databases. We observe that the use of the LDA color subspace significantly improves recognition accuracy over the standard gray scale approach without sacrificing computational efficiency.

Discontinuous Non-Rigid Motion Analysis of Sea Ice using C-Band Synthetic Aperture Radar Satellite Imagery

Sea-ice motion consists of complex non-rigid motions involving continuous, piece-wise continuous and discrete particle motion. Techniques for estimating non-rigid motion of sea ice from pairs of satellite images (generally spaced three days apart) are still in the developmental stages. For interior Arctic and Antarctic pack ice, the continuum assumption begins to fail below the 5 km scale with evidence of discontinuities already revealed in models and remote sensing products in the form of abrupt changes in magnitude and direction of the differential velocity. Using a hierarchical multi-scale phase-correlation method and profiting from known limitations of cross correlation methods, we incorporate the identification of discontinuities into our motion estimation algorithm, thereby descending below the continuum threshold to examine the phenomenon of discontinuous non-rigid sea-ice motion.

Face recognition using a color PCA framework

This paper delves into the problem of face recognition using color as an important cue in improving recognition accuracy. To perform recognition of color images, we use the characteristics of a 3D color tensor to generate a subspace, which in turn can be used to recognize a new probe image. To test the accuracy of our methodology, we computed the recognition rate across two color face databases and also compared our results against a multi-class neural network model. We observe that the use of the color subspace improved recognition accuracy over the standard gray scale 2D-PCA approach [17] and the 2-layer feed forward neural network model with 15 hidden nodes. Additionally, due to the computational efficiency of this algorithm, the entire system can be deployed with a considerably short turn around time between the training and testing stages.

Role of Ice Dynamics in the Sea Ice Mass Balance

Over the past decade, the Arctic Ocean and Beaufort Sea ice pack has been less extensive and thinner than has been observed during the previous 35 years [e.g., Wadhams and Davis, 2000; Tucker et al., 2001; Rothrock et al., 1999; Parkinson and Cavalieri, 2002; Comiso, 2002]. During the summers of 2007 and 2008, the ice extents for both the Beaufort Sea and the Northern Hemisphere were the lowest on record. Mechanisms causing recent sea ice change in the Pacific Arctic and the Beaufort Sea are under investigation on many fronts [e.g., Drobot and Maslanik, 2003; Shimada et al., 2006]; the mechanisms include increased ocean surface warming due to Pacific Ocean water inflow to the region and variability in meteorological and surface conditions. However, in most studies addressing these events, the impact of sea ice dynamics, specifically deformation, has not been measured in detail.

Streamline regularization for large discontinuous motion of sea ice

Non-rigid motion has to sometimes contend with the presence of discontinuous structures when it is estimated under a non-topology preserving deformation. In this paper, we propose an algorithm that estimates large scale non-rigid motion in the presence of these discontinuous structures. We have developed a streamline regularization framework that uses particle streamlines to compute a plausible flow at discontinuities, thereby enabling us to predict the motion more accurately. To quantitatively validate the accuracy of our results, we applied the Wilcoxon Signed Rank Test, which shows an improvement in estimation accuracy using our proposed scheme.

Vector field characterization in ERS-1 imagery of sea ice

The nonrigid motion of sea ice is an essential component when describing global climatology models. With the availability of sequential ERS-1 satellite imagery, we estimate high resolution motion and describe the differential characteristics of motion. This characterization is subsequently used to locate critical points, also known as coherent structures, in the flow, thereby reducing the large vector field into a collection of important feature points which essentially describe the flow pattern. In this paper, we build on previous work in an attempt to enhance the characterization of the flow field via visualization and quantification of these coherent structures. We show that the statistical quantification of these structures can provide a greater clarity in our understanding of the physical dynamics taking place within the sea ice

Vector field resampling using local streamline approximation

In this paper, we propose an algorithm to resample coarse vector fields in order to obtain vector fields of a higher density. Unlike the typical linear interpolation scheme, our algorithm attempts to identify streamline characteristics in the flow field, and uses local polynomial parameterization of the flow to perform interpolation. Quantitative validation of the streamline oriented algorithm indicate that the Mean Square Error of the flow field obtained is more accurate than those obtained by bilinear interpolation schemes.

Dynamic open contours using particle swarm optimization with application to fluid interface extraction

This paper describes a method for the estimation of a dynamic open contour by incorporating a modified particle swarm optimization technique. This scheme has been applied to a Particle Image Velocimetry experiment for the analysis of fluid turbulence during a hydraulic jump. Due to inter reflections within the medium and refractions across different media interfaces, the imagery contains spurious regions, which have to be eliminated prior to the estimation of turbulence statistics at the fluid surface. The PIV image sequences provide a strict test bed for the performance analysis of this estimation mechanism due to the occurrence of intense specularity and extreme non-rigid motion dynamics.