This paper presents powerful surface based techniques for the analysis of complex flow fields resulting from CFD simulations. Emphasis is put on the examination of vortical structures. An improved method for stream surface computation that delivers accurate results in regions of intricate flow is presented, along with a novel method to determine boundary surfaces of vortex cores. A number of surface techniques are presented that aid in understanding the flow behavior displayed by these surfaces. Furthermore, a scheme for phenomenological extraction of vortex core lines using stream surfaces is discussed and its accuracy is compared to one of the most established standard techniques.

Surface techniques for vortex visualization

We present a novel approach for the direct computation of integral surfaces in time-dependent vector fields. As opposed to previous work, which we analyze in detail, our approach is based on a separation of integral surface computation into two stages: surface approximation and generation of a graphical representation. This allows us to overcome several limitations of existing techniques. We first describe an algorithm for surface integration that approximates a series of time lines using iterative refinement and computes a skeleton of the integral surface. In a second step, we generate a well-conditioned triangulation. Our approach allows a highly accurate treatment of very large time-varying vector fields in an efficient, streaming fashion. We examine the properties of the presented methods on several example datasets and perform a numerical study of its correctness and accuracy. Finally, we investigate some visualization aspects of integral surfaces.

/pdf/generation-of-accurate-integral-surfaces-in-time-dependent-5bh81u0smp.pdf

Generation of Accurate Integral Surfaces in Time-Dependent Vector Fields

Vortex breakdowns and flow recirculation are essential phenomena in aeronautics where they appear as a limiting factor in the design of modern aircrafts. Because of the inherent intricacy of these features, standard flow visualization techniques typically yield cluttered depictions. The paper addresses the challenges raised by the visual exploration and validation of two CFD simulations involving vortex breakdown. To permit accurate and insightful visualization we propose a new approach that unfolds the geometry of the breakdown region by letting a plane travel through the structure along a curve. We track the continuous evolution of the associated projected vector field using the theoretical framework of parametric topology. To improve the understanding of the spatial relationship between the resulting curves and lines we use direct volume rendering and multi-dimensional transfer functions for the display of flow-derived scalar quantities. This enriches the visualization and provides an intuitive context for the extracted topological information. Our results offer clear, synthetic depictions that permit new insight into the structural properties of vortex breakdowns.

/pdf/visualization-of-intricate-flow-structures-for-vortex-59dr2332pf.pdf

Visualization of Intricate Flow Structures for Vortex Breakdown Analysis

Applying certain visualization techniques to datasets described on unstructured grids requires the interpolation of variables of interest at arbitrary locations within the dataset's domain of definition. Typical solutions to the problem of finding the grid element enclosing a given interpolation point make use of a variety of spatial subdivision schemes. However, existing solutions are memory- intensive, do not scale well to large grids, or do not work reliably on grids describing complex geometries. In this paper, we propose a data structure and associated construction algorithm for fast cell location in unstructured grids, and apply it to the interpolation problem. Based on the concept of bounding interval hierarchies, the proposed approach is memory-efficient, fast and numerically robust. We examine the performance characteristics of the proposed approach and compare it to existing approaches using a number of benchmark problems related to vector field visualization. Furthermore, we demonstrate that our approach can successfully accommodate large datasets, and discuss application to visualization on both CPUs and GPUs.

/pdf/fast-memory-efficient-cell-location-in-unstructured-grids-3hnsu9gc15.pdf

Fast, Memory-Efficient Cell Location in Unstructured Grids for Visualization

In order to understand complex vortical flows in large data sets, we must be able to detect and visualize vortices in an automated fashion. In this paper, we present a feature-based vortex detection and visualization technique that is appropriate for large computational fluid dynamics data sets computed on unstructured meshes. In particular, we focus on the application of this technique to visualization of the flow over a serrated wing and the flow field around a spinning missile with dithering canards. We have developed a core line extraction technique based on the observation that vortex cores coincide with local extrema in certain scalar fields. We also have developed a novel technique to handle complex vortex topology that is based on k-means clustering. These techniques facilitate visualization of vortices in simulation data that may not be optimally resolved or sampled. Results are included that highlight the strengths and weaknesses of our approach. We conclude by describing how our approach can be improved to enhance robustness and expand its range of applicability

https://www.cavs.msstate.edu/publications/docs/2006/09/1378vortexVis06.pdf

Vortex Visualization for Practical Engineering Applications

Visualization of data dened over large unstructured grids requires an efcient solution to the point location problem, since many visualization methods need values at arbitrary positions given in global coordinates. This paper presents a memory efcient, fast in-core solution to the problem using cell adjacency and a complete adaptive kd-tree based on the vertices. Since cell adjacency information is stored rather often (for example for fast ray-casting or streamline integration), the extra memory needed is very small compared to conventional solutions like octrees. The method is especially useful for highly non-uniform point distributions and extreme edge ratios in the cells. The data structure is tested on several large unstructured grids from computational uid dynamics (CFD) simulations for industrial applications.

/pdf/an-efficient-point-location-method-for-visualization-in-59tssawgas.pdf

An Efficient Point Location Method for Visualization in Large Unstructured Grids.

For object recognition in tensorfields and in pointclouds, the recognition of features in the object irrespective of their rotation is an important task Rotationally invariant features exist for 2d scalar fields and for 3d scalar fields as moments of a second order structure tensor For higher order structure tensors iterative algorithms for computing something similar to an eigenvector-decomposition exist In this paper, we introduce a method to compute a basis for analytical rotationally invariant moments of tensorfields of --- in principle --- any order and dimension and give an example using up to 4th-order structure tensors in 3d

A Generalization of Moment Invariants on 2D Vector Fields to Tensor Fields of Arbitrary Order and Dimension

Directed graphs are a useful model of many computational systems including software, hardware, fault trees, and of course the Internet. We present the TopoLayout-DG framework, an extension to the original TopoLayout algorithm, for visualizing the inside behaviour of large directed graphs. The proposed framework consists of: a feature-based multi-level algorithm, called ToF2DG, that detects topological features in large directed graphs in a hierarchical fashion; and visualization methods for the resulting levels of details of the graph’s topological structure. In this work-in-progress paper, we highlight the main steps of the proposed ToF2DG algorithm. Moreover, we show some preliminary visual representations of artificial directed graphs. These preliminary representations indicate that the framework promises to solve some scalability issues in the visualisation of large directed graphs.

https://www.ep.liu.se/ecp/106/012/ecp14106012.pdf

TopoLayout-DG: A Topological Feature-Based Framework for Visualizing Inside Behavior of Large Directed Graphs

To generate power from high-altitude winds, concepts using kites or planes linked to the ground with tether are in development. The most popular high-altitude wind generation concept is one using a flying wing attached to a single tether whose movement generates power by turning a winch. The usual trajectories for power generation consist of a period where the kite distance is increased, and the pulling force enlarged by figure-of- eight movements, interrupted by a pull-back phase where power is consumed. We compare that with a new concept we introduce here. It uses a triplet of tethers whose length sum is kept more or less constant using differential gears, resulting in a trajectory surface. It does not have a pull-back phase and allows to have similar power output in a closed trajectory. Moreover, starting and landing can be achieved without additional equipment when using a soft kite as wing, and keeping the wing flying without any wind is easier. Also the control can be easier, as one has more degrees of freedom in the force direction and the movement of the kite. Its disadvantages are an increased effort for the ground stations and more restrictions on the location. Also the tether's air drag is increased. Optimal power generation is compared using an example configuration and state with given wind speed under the assumption of an optimal steering of the generators and the kites. This is done for state snapshots, for example trajectories, wind speeds, and kites.

https://iopscience.iop.org/article/10.1088/1742-6596/1037/4/042023/pdf

Assessment of an Alternative Concept for a High-Altitude Wind-Power Generator

In Europe, urban land-use plans only give a very abstract representation of possible building forms. Combined with a written statement describing all constraints, the so-called building code, architects have to start planning. This can create many problems by violating the building codes. To overcome these problems we developed a system visualizing building code violations in real-time and giving a spatial impression of the planned area by an additional output on 3D printers and 3D displays.

Max Langbein

Papers

An Efficient Point Location Method for Visualization in Large Unstructured Grids.

A Generalization of Moment Invariants on 2D Vector Fields to Tensor Fields of Arbitrary Order and Dimension

TopoLayout-DG: A Topological Feature-Based Framework for Visualizing Inside Behavior of Large Directed Graphs

Assessment of an Alternative Concept for a High-Altitude Wind-Power Generator

B:3D - Visualize Land-use Plans Interactively