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Dieckmann, M. E., Folini, D., Hotz, I., Nordman, A., Dell'Acqua, P., Ynnerman, A. & Walder, R. (2019). Structure of a collisionless pair jet in a magnetized electron–proton plasma: flow-aligned magnetic field. Astronomy and Astrophysics, 621, Article ID A142.
Open this publication in new window or tab >>Structure of a collisionless pair jet in a magnetized electron–proton plasma: flow-aligned magnetic field
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A142Article in journal (Refereed) Published
Abstract [en]

Aims. We study the effect a guiding magnetic field has on the formation and structure of a pair jet that propagates through a collisionless electron–proton plasma at rest.

Methods. We model with a particle-in-cell (PIC) simulation a pair cloud with a temperature of 400 keV and a mean speed of 0.9c (c - light speed). Pair particles are continuously injected at the boundary. The cloud propagates through a spatially uniform, magnetized, and cool ambient electron–proton plasma at rest. The mean velocity vector of the pair cloud is aligned with the uniform background magnetic field. The pair cloud has a lateral extent of a few ion skin depths.

Results. A jet forms in time. Its outer cocoon consists of jet-accelerated ambient plasma and is separated from the inner cocoon by an electromagnetic piston with a thickness that is comparable to the local thermal gyroradius of jet particles. The inner cocoon consists of pair plasma, which lost its directed flow energy while it swept out the background magnetic field and compressed it into the electromagnetic piston. A beam of electrons and positrons moves along the jet spine at its initial speed. Its electrons are slowed down and some positrons are accelerated as they cross the head of the jet. The latter escape upstream along the magnetic field, which yields an excess of megaelectronvolt positrons ahead of the jet. A filamentation instability between positrons and protons accelerates some of the protons, which were located behind the electromagnetic piston at the time it formed, to megaelectronvolt energies.

Conclusions. A microscopic pair jet in collisionless plasma has a structure that is similar to that predicted by a hydrodynamic model of relativistic astrophysical pair jets. It is a source of megaelectronvolt positrons. An electromagnetic piston acts as the contact discontinuity between the inner and outer cocoons. It would form on subsecond timescales in a plasma with a density that is comparable to that of the interstellar medium in the rest frame of the latter. A supercritical fast magnetosonic shock will form between the pristine ambient plasma and the jet-accelerated plasma on a timescale that exceeds our simulation time by an order of magnitude.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
PIC simulation, collisionless plasma, relativistic jet
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:liu:diva-154054 (URN)10.1051/0004-6361/201834393 (DOI)000456274900002 ()
Note

Funding agencies; Ecole Nationale Superieure de Lyon, Universite de Lyon; French National Program of High Energy (PNHE); French supercomputing facilities GENCI [EP/P02212X/1, A0030406960]

Available from: 2019-01-24 Created: 2019-01-24 Last updated: 2019-02-06Bibliographically approved
Engelke, W., Lawonn, K., Preim, B. & Hotz, I. (2018). Autonomous Particles for Interactive Flow Visualization. Paper presented at EuroVis 2018, Conference on Visualization 4-8 June, Czech Republic. Computer graphics forum (Print), 1-12
Open this publication in new window or tab >>Autonomous Particles for Interactive Flow Visualization
2018 (English)In: Computer graphics forum (Print), ISSN 0167-7055, E-ISSN 1467-8659, p. 1-12Article in journal (Refereed) Published
Abstract [en]

We present an interactive approach to analyse flow fields using a new type of particle system, which is composed of autonomous particles exploring the flow. While particles provide a very intuitive way to visualize flows, it is a challenge to capture the important features with such systems. Particles tend to cluster in regions of low velocity and regions of interest are often sparsely populated. To overcome these disadvantages, we propose an automatic adaption of the particle density with respect to local importance measures. These measures are user defined and the systems sensitivity to them can be adjusted interactively. Together with the particle history, these measures define a probability for particles to multiply or die, respectively. There is no communication between the particles and no neighbourhood information has to be maintained. Thus, the particles can be handled in parallel and support a real‐time investigation of flow fields. To enhance the visualization, the particles' properties and selected field measures are also used to specify the systems rendering parameters, such as colour and size. We demonstrate the effectiveness of our approach on different simulated vector fields from technical and medical applications.

Place, publisher, year, edition, pages
John Wiley & Sons, 2018
Keywords
flow visualization, scientific visualization, visualization, real-time rendering, Visualization Human-centered computing: Visualization application domains–Scientific Visualization
National Category
Computer Sciences
Identifiers
urn:nbn:se:liu:diva-151724 (URN)10.1111/cgf.13528 (DOI)
Conference
EuroVis 2018, Conference on Visualization 4-8 June, Czech Republic
Available from: 2018-10-03 Created: 2018-10-03 Last updated: 2018-10-23Bibliographically approved
Engelke, W. & Hotz, I. (2018). Evolutionary Lines for Flow Visualization. In: Jimmy Johansson and Filip Sadlo and Tobias Schreck (Ed.), EuroVis 2018, 20th EG/VGTC Conference on Visualization, 4-8 June, Brno, Czech Republic: . Paper presented at EuroVis 2018, 20th EG/VGTC Conference on Visualization, 4-8 June, Brno, Czech Republic (pp. 7-11). The Eurographics Association
Open this publication in new window or tab >>Evolutionary Lines for Flow Visualization
2018 (English)In: EuroVis 2018, 20th EG/VGTC Conference on Visualization, 4-8 June, Brno, Czech Republic / [ed] Jimmy Johansson and Filip Sadlo and Tobias Schreck, The Eurographics Association , 2018, p. 7-11Conference paper, Published paper (Refereed)
Abstract [en]

In this work we explore evolutionary algorithms for selected a visualization application. We demonstrate its potential using an example from flow visualization showing promising first results. Evolutionary algorithms, as guided search approach, find close-to-optimal solutions with respect to some fitness function in an iterative process using biologically motivated mechanisms like selection, mutation and recombination. As such, they provide a powerful alternative to filtering methods commonly used in visualization where the space of possible candidates is densely sampled in a pre-processing step from which the best candidates are selected and visualized. This approach however tends to be increasingly inefficient with growing data size or expensive candidate computations resulting in large pre-processing times. We present an evolutionary algorithm for the problem of streamline selection to highlight features of interest in flow data. Our approach directly optimizes the solution candidates with respect to a user selected fitness function requiring significantly less computations. At the same time the problem of possible under-sampling is solved since we are not tied to a preset resolution. We demonstrate our approach on the well-known flow around an obstacle as case with a two-dimensional search space. The blood flow in an aneurysm serves as an example with a three-dimensional search space. For both, the achieved results are comparable to line filtering approaches with much less line computations.

Place, publisher, year, edition, pages
The Eurographics Association, 2018
National Category
Computer Sciences
Identifiers
urn:nbn:se:liu:diva-151725 (URN)10.2312/eurovisshort.20181070 (DOI)9783038680604 (ISBN)
Conference
EuroVis 2018, 20th EG/VGTC Conference on Visualization, 4-8 June, Brno, Czech Republic
Available from: 2018-10-03 Created: 2018-10-03 Last updated: 2018-10-23
Jankowai, J. & Hotz, I. (2018). Feature Level-Sets: Generalizing Iso-surfaces to Multi-variate Data. IEEE Transactions on Visualization and Computer Graphics
Open this publication in new window or tab >>Feature Level-Sets: Generalizing Iso-surfaces to Multi-variate Data
2018 (English)In: IEEE Transactions on Visualization and Computer Graphics, ISSN 1077-2626, E-ISSN 1941-0506Article in journal (Refereed) Epub ahead of print
Abstract [en]

Iso-surfaces or level-sets provide an effective and frequently used means for feature visualization. However, they are restricted to simple features for uni-variate data. The approach does not scale when moving to multi-variate data or when considering more complex feature definitions. In this paper, we introduce the concept of traits and feature level-sets, which can be understood as a generalization of level-sets as it includes iso-surfaces, and fiber surfaces as special cases. The concept is applicable to a large class of traits defined as subsets in attribute space, which can be arbitrary combinations of points, lines, surfaces and volumes.  It is implemented into a system that provides an interface to define traits in an interactive way and multiple rendering options. We demonstrate the effectiveness of the approach using multi-variate data sets of different nature, including vector and tensor data, from different application domains.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2018
National Category
Other Computer and Information Science
Identifiers
urn:nbn:se:liu:diva-151231 (URN)10.1109/TVCG.2018.2867488 (DOI)30183637 (PubMedID)2-s2.0-85052841941 (Scopus ID)
Funder
ELLIIT - The Linköping‐Lund Initiative on IT and Mobile Communications
Available from: 2018-09-13 Created: 2018-09-13 Last updated: 2018-09-18Bibliographically approved
Gustafsson, T., Engelke, W., Englund, R. & Hotz, I. (2017). Concepts of Hybrid Data Rendering. In: Ingrid Hotz and Martin Falk (Ed.), Proceedings of SIGRAD 2017, August 17-18, 2017 Norrköping, Sweden: . Paper presented at SIGRAD 2017, August 17-18, 2017 Norrköping, Sweden (pp. 32-39). Linköping: Linköping University Electronic Press, 143
Open this publication in new window or tab >>Concepts of Hybrid Data Rendering
2017 (English)In: Proceedings of SIGRAD 2017, August 17-18, 2017 Norrköping, Sweden / [ed] Ingrid Hotz and Martin Falk, Linköping: Linköping University Electronic Press, 2017, Vol. 143, p. 32-39Conference paper, Published paper (Refereed)
Abstract [en]

We present a concept for interactive rendering of multiple data sets of varying type, including geometry and volumetric data, in one scene with correct transparency. Typical visualization applications involve multiple data fields from various sources. A thorough understanding of such data often requires combined rendering of theses fields. The choice of the visualization concepts, and thus the rendering techniques, depends on the context and type of the individual fields. Efficiently combining different techniques in one scene, however, is not always a straightforward task. We tackle this problem by using an A-buffer based approach to gather color and transparency information from different sources, combine them and generate the final output image. Thereby we put special emphasis on efficiency and low memory consumption to allow a smooth exploration of the data. Therefore, we compare different A-buffer implementations with respect to memory consumption and memory access pattern. Additionally we introduce an early-fragment-discarding heuristic using inter-frame information to speed up the rendering..

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017
Series
Linköping Electronic Conference Proceedings, ISSN 1650-3686, E-ISSN 1650-3740 ; 143
Keywords
Hybrid Rendering, A-Buffer, Volume Rendering
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:liu:diva-143256 (URN)9789176853849 (ISBN)
Conference
SIGRAD 2017, August 17-18, 2017 Norrköping, Sweden
Available from: 2017-11-27 Created: 2017-11-27 Last updated: 2018-04-03Bibliographically approved
Schultz, T., Özarslan, E. & Hotz, I. (2017). Modeling, Analysis, and Visualization of Anisotropy. Cham: Springer
Open this publication in new window or tab >>Modeling, Analysis, and Visualization of Anisotropy
2017 (English)Book (Refereed)
Abstract [en]

This book focuses on the modeling, processing and visualization of anisotropy, irrespective of the context in which it emerges, using state-of-the-art mathematical tools. As such, it differs substantially from conventional reference works, which are centered on a particular application. It covers the following topics: (i) the geometric structure of tensors, (ii) statistical methods for tensor field processing, (iii) challenges in mapping neural connectivity and structural mechanics, (iv) processing of uncertainty, and (v) visualizing higher-order representations. In addition to original research contributions, it provides insightful reviews.This multidisciplinary book is the sixth in a series that aims to foster scientific exchange between communities employing tensors and other higher-order representations of directionally dependent data. A significant number of the chapters were co-authored by the participants of the workshop titled Multidisciplinary Approaches to Multivalued Data: Modeling, Visualization, Analysis, which was held in Dagstuhl, Germany in April 2016.

It offers a valuable resource for those working in the field of multi-directional data, vital inspirations for the development of new models, and essential analysis and visualization techniques, thus furthering the state-of-the-art in studies involving anisotropy.

Place, publisher, year, edition, pages
Cham: Springer, 2017. p. 407
Series
Mathematics and Visualization, ISSN 1612-3786, E-ISSN 2197-666X ; 2017
National Category
Telecommunications Social Sciences Interdisciplinary
Identifiers
urn:nbn:se:liu:diva-152348 (URN)10.1007/978-3-319-61358-1 (DOI)9783319613574 (ISBN)9783319613581 (ISBN)
Available from: 2018-10-29 Created: 2018-10-29 Last updated: 2018-11-23Bibliographically approved
Hotz, I. & Falk, M. (Eds.). (2017). Proceedings of SIGRAD 2017, August 17-18, 2017 Norrköping, Sweden. Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Proceedings of SIGRAD 2017, August 17-18, 2017 Norrköping, Sweden
2017 (English)Conference proceedings (editor) (Refereed)
Abstract [en]

The annual meeting 2017 of the Swedish Computer Graphics Association (SIGRAD) took place at Linköping University, Campus Norrköping in Norrköping, Sweden in August 2017. SIGRAD is an event where researchers and industry professionals meet to discuss novel visions and developments in the field of computer graphics and related areas, such as visualization and human-computer interaction (HCI). Since SIGRAD was started in 1976, it has developed into the major annual appointment for the Nordic community of graphics and visual computing experts with a broad range of backgrounds. It thereby addresses the increasing need for visual computing solutions in both commercial and academic areas. SIGRAD 2017 offered a strong scientific program consisting of international keynote speakers from research and industry, presentations of recent scientific achievements in the field within Sweden, and novel technological results from international contributors. The topics covered present a nice cross-section across the diverse research efforts in the domains.

Five original papers have been accepted for presentation after being peer-reviewed by an International Program Committee consisting of 22 highly qualified scientists. Each paper was reviewed, on average, by three reviewers from the committee. The accepted papers range from general computer graphics practices to practical applications and services that may benefit from the use of visualizations and computer graphics technologies. The extended participation of students at all levels of academia in research has been encouraged this year and 2 papers were selected which are first-authored by students studying at Master's Degree level.

This year, we continued the “Swedish Research Overview Session” introduced at last year’s conference. In this session, Swedish research groups are given the opportunity to present their academically outstanding, previously published work at the annual conference. All papers in this session have been published in an academically outstanding journals or conferences not more than two years prior to the SIGRAD conference.

We especially wish to thank our invited keynote speakers: Christoph Garth, University of Kaiserslautern, Germany, Ivan Viola, Vienna University of Technology, Austria, Claes Lundström, CMIV, Linköping University, and Samuel Ranta Eskola, Microsoft. Finally, we want to express our thanks to Gun-Britt Löfgren for helping us in organizing this event.

The SIGRAD 2017 organizers

Martin Falk, Daniel Jönsson, Ingrid Hotz

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. p. 39
Series
Linköping Electronic Conference Proceedings, ISSN 1650-3686, E-ISSN 1650-3740 ; 143
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:liu:diva-143255 (URN)9789176853849 (ISBN)
Available from: 2017-11-27 Created: 2017-11-27 Last updated: 2018-02-06Bibliographically approved
Hotz, I. (2016). Acceleration Feature Points of Unsteady Shear Flows. Journal Archives of Mechanics, 68(1), 55-80
Open this publication in new window or tab >>Acceleration Feature Points of Unsteady Shear Flows
2016 (English)In: Journal Archives of Mechanics, ISSN 0373-2029, Vol. 68, no 1, p. 55-80Article in journal (Refereed) Published
Abstract [en]

A framework for extracting features in 2D transient flows, based on the acceleration field to ensure Galilean invariance is proposed in this paper. The minima of the acceleration magnitude (a superset of acceleration zeros) are extracted and discriminated into vortices and saddle points, based on the spectral properties of the velocity Jacobian. The extraction of topological features is performed with purely combinatorial algorithms from discrete computational topology. The feature points are prioritized with persistence, as a physically meaningful importance measure. These feature points are tracked in time with a robust algorithm for tracking features. Thus, a space-time hierarchy of the minima is built and vortex merging events are detected. We apply the acceleration feature extraction strategy to three two-dimensional shear flows: (1) an incompressible periodic cylinder wake, (2) an incompressible planar mixing layer and (3) a weakly compressible planar jet. The vortex-like acceleration feature points are shown to be well aligned with acceleration zeros, maxima of the vorticity magnitude, minima of the pressure field and minima of λ2.

Place, publisher, year, edition, pages
Instytut Podstawowych Problemów Techniki, 2016
Keywords
Vortex extraction, topological feature analysis
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-127646 (URN)
Available from: 2016-05-06 Created: 2016-05-06 Last updated: 2016-05-16
Kasten, J., Reininghaus, J., Hotz, I., Hege, H.-C., Noack, B. R., Daviller, G. & Morzynski, M. (2016). Acceleration feature points of unsteady shear flows. ARCHIVES OF MECHANICS, 68(1), 55-80
Open this publication in new window or tab >>Acceleration feature points of unsteady shear flows
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2016 (English)In: ARCHIVES OF MECHANICS, ISSN 0373-2029, Vol. 68, no 1, p. 55-80Article in journal (Refereed) Published
Abstract [en]

A FRAMEWORK FOR EXTRACTING FEATURES IN 2D TRANSIENT FLOWS, based on the acceleration field to ensure Galilean invariance is proposed in this paper. The minima of the acceleration magnitude (a superset of acceleration zeros) are extracted and discriminated into vortices and saddle points, based on the spectral properties of the velocity Jacobian. The extraction of topological features is performed with purely combinatorial algorithms from discrete computational topology. The feature points are prioritized with persistence, as a physically meaningful importance mea sure. These feature points are tracked in time with a robust algorithm for tracking features. Thus, a space-time hierarchy of the minima is built and vortex merging events are detected. We apply the acceleration feature extraction strategy to three two-dimensional shear flows: (1) an incompressible periodic cylinder wake, (2) an incompressible planar mixing layer and (3) a weakly compressible planar jet. The vortex-like acceleration feature points are shown to be well aligned with acceleration zeros, maxima of the vorticity magnitude, minima of the pressure field and minima of lambda(2). Copyright (C) 2016 by IPPT PAN

Place, publisher, year, edition, pages
POLISH ACAD SCIENCES INST FUNDAMENTAL TECHNOLOGICAL RESEARCH, 2016
Keywords
visualization; feature extraction; flow topology
National Category
Fluid Mechanics and Acoustics Computer Vision and Robotics (Autonomous Systems)
Identifiers
urn:nbn:se:liu:diva-127064 (URN)000372097000003 ()
Note

Funding Agencies|German Research Foundation (DFG) via the Collaborative Research Center "Control of Complex Turbulent Shear Flows" [SFB 557]; Emmy Noether Program; Zuse Institute Berlin (ZIB); DFG-CNRS research group "Noise Generation in Turbulent Flows"; French Agence Nationale de la Recherche (ANR); European Social Fund (ESF) [100098251]; Polish National Centre of Science [2011/01/B/ST8/07264]; Ambrosys Ltd. Society for Complex Systems Management; Bernd R. Noack Cybernetics Foundation; GENCI-[CCRT/CINES/IDRIS] [2011-[x2011020912]]

Available from: 2016-04-13 Created: 2016-04-13 Last updated: 2018-01-10
Englund, R., Ropinski, T. & Hotz, I. (2016). Coherence Maps for Blood Flow Exploration. In: VCBM 16: Eurographics Workshop on Visual Computing for Biology and Medicine: . Paper presented at The 6th Eurographics Workshop on Visual Computing for Biology and Medicine, Bergen, Norway, September 7-9, 2016 (pp. 79-88). Eurographics - European Association for Computer Graphics
Open this publication in new window or tab >>Coherence Maps for Blood Flow Exploration
2016 (English)In: VCBM 16: Eurographics Workshop on Visual Computing for Biology and Medicine, Eurographics - European Association for Computer Graphics, 2016, p. 79-88Conference paper, Published paper (Refereed)
Abstract [en]

Blood flow data from direct measurements (4D flow MRI) or numerical simulations opens new possibilities for the understanding of the development of cardiac diseases. However, before this new data can be used in clinical studies or for diagnosis, it is important to develop a notion of the characteristics of typical flow structures. To support this process we developed a novel blood flow clustering and exploration method. The method builds on the concept of coherent flow structures. Coherence maps for cross-sectional slices are defined to show the overall degree of coherence of the flow. In coherent regions the method summarizes the dominant blood flow using a small number of pathline representatives. In contrast to other clustering approaches the clustering is restricted to coherent regions and pathlines with low coherence values, which are not suitable for clustering and thus are not forced into clusters. The coherence map is based on the Finite-time Lyapunov Exponent (FTLE). It is created on selected planes in the inflow respective outflow area of a region of interest. The FTLE value measures the rate of separation of pathlines originating from this plane. Different to previous work using FTLE we do not focus on separating extremal lines but on local minima and regions of low FTLE intensities to extract coherent flow. The coherence map and the extracted clusters serve as basis for the flow exploration. The extracted clusters can be selected and inspected individually. Their flow rate and coherence provide a measure for their significance. Switching off clusters reduces the amount of occlusion and reveals the remaining part of the flow. The non-coherent regions can also be explored by interactive manual pathline seeding in the coherence map.

Place, publisher, year, edition, pages
Eurographics - European Association for Computer Graphics, 2016
Series
Eurographics Workshop on Visual Computing for Biology and Medicine, ISSN 2070-5778, E-ISSN 2070-5786
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:liu:diva-130979 (URN)10.2312/vcbm.20161274 (DOI)978-3-03868-010-9 (ISBN)
Conference
The 6th Eurographics Workshop on Visual Computing for Biology and Medicine, Bergen, Norway, September 7-9, 2016
Available from: 2016-09-02 Created: 2016-09-02 Last updated: 2018-02-08Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7285-0483

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