Computer Scientist

Center for Applied Scientific Computing,
Lawrence Livermore National Laboratory

Lawrence Livermore National Laboratory, P.O. Box 808, L-478, Livermore, CA - 94551

The inclusion of temporal dimension in data greatly increases the complexity of analysis, especially in the case of flows, i.e., unsteady (time-varying) flows. The current techniques for analyzing such flows are computationally prohibitive, and typically, cause data management issues by requiring simultaneous access to a large amount of data. For example, the pathline-based techniques have high computational cost, and typically use numerical integration which is prone to errors. These techniques must also require the data for a finite amount of data for pathline-integration.

The focus of this work is to simplify the analysis of unsteady flows by means of new mathematical models and robust computations allowing for lower cost and better data management. In particular, we are working on finding new reference frames which can enable the much-simpler analysis techniques for steady flows for the unsteady case.

The Helmholtz-Hodge decomposition (HHD) is a fundamental theorem in fluid dynamics, describing a vector field as a sum of a divergence-free, a curl-free, and a harmonic vector field. The HHD, however, describes an underspecified system on domains with boundary, and therefore, is not unique. The traditional way to obtain uniqueness is to impose boundary conditions. For example, the most common boundary conditions demand that the divergence-free and curl-free components must be normal and parallel to the boundary. While these boundary conditions work well for certain cases, imposing a standard set of boundary conditions irrespective of the data can produce aribtrary artifacts. Although the issue of boundary conditions has been acknowledged by many, currently, there exists no better solution to obtain uniqueness.

From the potential function theory, we know that harmonic functions are always created due to external influences with respect to the domain. Using these concepts, we have developed a new technique to obtain uniqueness, where one does not need to assume any boundary conditions a priori. In contrast, the decomposition naturally determines the flow on the boundary.

Computational tools for data analysis are based on mathematical foundations that assume smooth functions defined over smooth domains, and require the availibility of infinite-precision real numbers to represent the data. However, in practice, these computational framworks are limited to finite-precision floating point numbers, and can only represent interpolated functions over sampled domains. As a result, the computational realization of the underlying mathematical theory suffers from unavoidable numerical errors.

Understanding, capturing, and visualizing these errors during the analysis and visualization pipeline of scientific data is extremely important, especially because such errors can cause the results to become inconsistent with the fundamental theory, and therefore, the analysis may not be physically meaningful. For example, in an experiment measuring the gravitational acceleration, we may get the result to be 10, or 9.8, or 9.807, depending upon the accuracy of the system. While it is important to quantify this error, an even more serious problem appears if the system shows direction of this acceleration to be upward - clearly, a physically incorrect result. In the context of analysis, we call this result to be "inconsistent" with the theory.

The focus of this project is the development of new computational tools that provide consistency guarantees during analysis. The goal is to eliminate most numerical errors through more sophisticated representations of the data, and to capture the unavoidable errors to visualize them as well as to enforce consistent results. The achieved consistency guarantees lead to a greater confidence in analysis results when they are used for deriving scientific insights.

Consistent Streamline Tracing in Sampled Vector Fields

Streamlines are curves that are everywhere tangent to the direction of vector field. The theory of vector fields (and dynamical systems) dictate that streamlines must be mutually non-intersecting, meaning two distinct streamlines must not cross. However, since streamlines are most commonly computed using numerical integration, the associated errors get compounded and can become unbounded, possibly resulting in intersecting streamlines. Such analysis is scientifically misleading and must be avoided.

In a series of papers, we developed a new representation of sampeld vector fields, which we call the "Edge Maps", to overcome these inconsistency issues. The Edge Maps represent the streamlines as they travel from edge to edge in a triangulated domain using a highly accurate streamline tracer, thus bypassing most of the numerical errors in an offline computation. In the runtime, the system can use Edge Maps to trace streamlines interactively with an additional benefit of visualizing the unavoidable errors. Most importantly, it becomes possible to extend the Edge Maps to a combinatorial representation by finely discretizing the triangles' edges. This allows to replaced floating-point arithmetic with integer arithmetic, enabling the system to enforce consistency.

Consistent Singularity Detection in Sampled Vector Fields

Detection of singularities is one of the most important aspects of analysis, since they represent important features in the data. In the case of vector fields, the singularities must follow certain rules with respect to their spatial distribution, for example, between two sources (that create the flow), there must be a sink (that destroys the flow) or a saddle (that reroutes the flow). Formally, these laws can be summarized using the Poincare-Hopf index formula. The use of numerical techniques to detect singularities can, however, violate these laws, resulting in physically meaningless results.

To address these issues, we developed mathematical conditions required for the existence of a singularity in a simplex (a triangle in 2D, or a tetrahedron in 3D). Using a combinatorial representation and integer arithmetic, we develped a computational framework that consistenty detects the singularities in a vector field.

For many applications, it is important to study multiple scalar functions defined on a common domain in order to understand their combined effect on a system, or onto each other. The Jacobi set of two (or more) scalar functions describe the relation between them by capturing the points in the domain where their gradients are aligned. However, depending upon the relative complexity of the functions, presence of noise, or the required level of abstraction, the Jacobi set can be arbitrarliy complex or too detailed.

This project aims at developing a new simplification framework for Jacobi sets to provide a multi-resolution hierarchical analysis. The goal is to perform the simplification by modifying the Jacobi set directly, rather than modifying the underlying functions and anticipating a chance in the Jacobi set.