Anders Helgeland

• Fossum, Hannibal Eie; Vik, Thomas; Helgeland, Anders; Gjesdal, Thor; Deebank, Charles & Placidi, Marco [Vis alle 7 forfattere av denne artikkelen] (2026). LES and RANS simulations of wind and dispersion in a complex urban environment: Comparison of momentum and scalar fluxes. Building and Environment. ISSN 0360-1323. 303. doi: 10.1016/j.buildenv.2026.114962. Fulltekst i vitenarkiv
• Gant, Simon; Chang, Joseph; Hetherington, Rory; Hanna, Steven; Tickle, Gemma & Spicer, Tom [Vis alle 46 forfattere av denne artikkelen] (2025). Pressure-liquefied ammonia jet dispersion: Multi-model intercomparison using Desert Tortoise and FLADIS field data. Atmospheric Environment: X. 28. doi: 10.1016/j.aeaoa.2025.100389. Fulltekst i vitenarkiv Vis sammendrag

  This paper presents the findings of an international model inter-comparison exercise that was undertaken in the period 2021–2024 to assess the performance of atmospheric dispersion models for simulating releases of pressure-liquefied ammonia. The exercise used data from ammonia field trials dating from the 1980s and 1990s: the Desert Tortoise and the FLADIS trials. Concentration data from two arcs of sensors in the Desert Tortoise trials and three arcs of sensors in the FLADIS trials were used. Twenty-one independent modelling teams from North America and Europe participated in the exercise and provided in total twenty-seven sets of results from a range of different models, including empirically-based nomograms, integral, Gaussian puff, Lagrangian particle, and Computational Fluid Dynamics (CFD) models. The work is novel in presenting the results from such a large cohort of models, examining specifically the dispersion behaviour of ammonia. This is particularly relevant at the current time, given the growing international interest in using ammonia as a clean energy vector and shipping fuel. The study found that the agreement between model predictions and measurements (as determined by performance measures such as geometric mean bias and geometric variance) varied between different models. At any downwind distance, the range in predicted plume arc-max concentrations spanned a range of up to one or two orders of magnitude about the measurements. Several modelling teams used the same models and, in most cases, their predictions differed. Given appropriate inputs, most models generally predicted concentrations that agreed with the data within commonly-used model acceptance criteria. There was no single class of model that provided superior predictions to others; predictions from several empirically-based nomograms, integral, Gaussian puff, Lagrangian particle, and CFD models were all in close agreement with the data (as defined by the model acceptance criteria). The findings of the exercise are being used to help plan a programme of future ammonia experiments in the USA, called the Jack Rabbit III trials. The results are also useful for assessing the performance of models that may be applied to assess risks at ammonia facilities, and for emergency planning and response. Copyright © 2025. Published by Elsevier Ltd.

• Linge, Svein; Mardal, Kent-Andre; Helgeland, Anders & Heiss, John (2014). Effect of Cranio-Vertebral Decompression on CSF Dynamics in Chiari I Malformation Studied With Computational Fluid Dynamics. Journal of Neurosurgery. ISSN 0022-3085. 21(4), s. 559–564. Fulltekst i vitenarkiv Vis sammendrag

  Object. The effect of craniovertebral decompression surgery on CSF flow dynamics in Chiari I patients has been incompletely characterized. We used computational fluid dynamics to calculate the effect of decompression surgery on CSF flow dynamics in the posterior fossa and upper cervical spinal canal.Methods. Oscillatory flow was simulated in idealized 3D models of the normal adult and the Chiari I subarachnoid spaces (both previously described) and in three models of the Chiari malformation post cranio-vertebral decompressions. The three postoperative models were created from the Chiari model by virtually modifying the Chiari model subarachnoid space to simulate surgical decompressions of different magnitudes. Velocities and pressures were computed with the Navier-Stokes equations in Star-CD for multiple cycles of CSF flow oscillating at 80 cycles per minute. Pressure gradients and velocities were compared for 8 levels extending from the posterior fossa to the C3/4 level. Relative pressures and peak velocities were plotted by level from the posterior fossa to C3/4. The heterogeneity of flow velocity distribution around the spinal cord was compared between models.Results. Peak systolic velocities were generally lower in the post-operative models, than in the pre-operative Chiari model. The two larger surgical defects had peak systolic velocities that were closer to normal model velocities than the smallest surgical defect and equal to those in the normal model at the C3 and C4 levels. For the smallest defect, peak velocities were decreased, but not to levels in the normal model. In the postoperative models, heterogeneity in flow velocity distribution around the spinal cord increased from normal model levels as the degree of decompression increased. Pressures in the 5 models differed in magnitude and in pattern. Pressures gradients along the spinal canal in the normal and Chiari models were non-linear with steeper gradients below C3/4 than above. The Chiari model had a steeper pressure gradient than the normal model above C3/4 and the same gradient below. The postoperative models had lower pressure gradients than the Chiari model above C2/3. The most conservative decompression had smaller than normal model pressure gradients above C2/3 The two larger decompression defects had CSF pressure gradients below those in the normal model above C2/3. These two models had a less steep gradient above C3 and a steeper gradient below.Conclusions. In computer simulations, cranio-vertebral surgical defects generally diminished CSF velocities and CSF pressures.

• Helgeland, Anders; Mardal, Kent-Andre; Haughton, Victor & Reif, Bjørn Anders Pettersson (2014). Numerical simulations of the pulsating flow of cerebrospinal fluid flow in the cervical spinal canal of a Chiari patient. Journal of Biomechanics. ISSN 0021-9290. 47(5), s. 1082–1090. doi: 10.1016/j.jbiomech.2013.12.023. Fulltekst i vitenarkiv
• Linge, Svein; Mardal, Kent-Andre; Helgeland, Anders; Heiss, John D. & Haughton, Victor (2014). Effect of craniovertebral decompression on CSF dynamics in Chiari malformation Type I studied with computational fluid dynamics. Journal of Neurosurgery : Spine. ISSN 1547-5654. 21(4), s. 559–564. doi: 10.3171/2014.6.SPINE13950. Fulltekst i vitenarkiv
• Linge, Svein; Mardal, Kent-Andre; Haughton, Victor & Helgeland, Anders (2013). Simulating CSF flow dynamics in the normal and chiari I subarachnoid space during rest and exertion. American Journal of Neuroradiology. ISSN 0195-6108. doi: 10.3174/ajnr.A3282. Fulltekst i vitenarkiv Vis sammendrag

  BACKGROUND AND PURPOSE: CSF fluid dynamics in healthy subjects and patients with Chiari I have been characterized during rest with phase-contrast MR imaging and CFD. CSF flow velocities and pressures in the nonresting state have not been adequately characterized. We used computer simulations to study CSF dynamics during increased heart rates in the normal and Chiari I subarachnoid space. MATERIALS AND METHODS: Cyclic CSF flow was simulated for multiple cycles in idealized 3D models of the subarachnoid space for normal and Chiari I malformation subarachnoid spaces, with flow cycles corresponding to 80 or 120 heart beats per minute. Flow velocities and pressures were computed by the Navier-Stokes equations. Synchronous bidirectional flow and flow patterns were displayed in Star-CD and inspected visually. Peak velocities and pressure differences in the 2 models were compared for the 2-cycle frequencies. RESULTS: Elevating the cycle rate from 80 to 120 cpm increased peak superior-inferior pressure gradients (top-bottom) by just 0.01% in the normal model and 2% in the Chiari model. Corresponding average pressure gradients increased by 92% and 100%, respectively. In addition, in both models, the range of synchronous bidirectional flow velocities increased. Systolic velocities had smaller increases with faster cycling. For each cycle rate, peak and average pressure gradients in the Chiari model were greater than in the normal model by 11%–16%. CONCLUSIONS: Raising the cycle rate from 80 to 120 cpm increased superior-inferior average pressure gradients and the range of synchronous bidirectional flow velocities in the normal and Chiari I models.

• Linge, Svein; Haughton, Victor; Løvgren, Alf; Mardal, Kent-Andre; Helgeland, Anders & Langtangen, Hans (2011). Effect of Tonsilar Herniation on Cyclic CSF Flow Studied With Computational Flow Analysis. American Journal of Neuroradiology. ISSN 0195-6108. 32, s. 1474–1481. Fulltekst i vitenarkiv Vis sammendrag

  Purpose: The Chiari I malformation, characterized by tonsils extending below the foramen magnum, has increased cerebrospinal fluid (CSF) velocities compared to normal subjects. Measuring the effect of tonsilar herniation on CSF flow in humans is confounded by inter-individual variation. The goal of this study was to determine the effect of herniated tonsils on flow velocity and pressure dynamics. Materials and Methods: A previously described 3D mathematical model of the normal subarachnoid space was modified by extending the tonsils inferiorly. The chamber created was compared with the anatomy of the subarachnoid space. Pressures and velocities were calculated by Computational Fluid Analysis (CFA) methods for sinusoidal flow of a Newtonian fluid. Results were displayed as 2D color-coded plots and 3D animations. Pressure gradients and flow velocities were compared with those in the normal model. Velocity distributions were also compared with those in clinical images of CSF flow. Results: The model represented grossly the subarachnoid space of a Chiari patient. Fluid flow patterns in the Chiari model were complex with jets in some locations and stagnant flow in others. Flow jets, synchronous bi-directional flow, and pressure gradients were greater in the Chiari model than in the normal model. The distribution of flow velocities in the model corresponded well with those observed in clinical images of CSF flow in Chiari I patients. Conclusions: Tonsilar herniation per se increases the pressure gradients and the complexity of flow patterns associated with oscillatory CSF flow.

• Helgeland, Anders (2011). VoluViz: a multi-field volume visualization framework for three-dimensional data analysis. I Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Red.), MekIT'11: Sixth National Conference on Computational Mechanics, Trondheim 23-24 May 2011. Tapir Akademisk Forlag. ISSN 9788251927987. s. 127–138. Fulltekst i vitenarkiv
• Linge, Svein; Haughton, Victor; Løvgren, Alf Emil; Mardal, Kent-Andre; Helgeland, Anders & Langtangen, Hans Petter (2011). Effect of Tonsillar Herniation on Cyclic CSF Flow Studied with Computational Flow Analysis. American Journal of Neuroradiology. ISSN 0195-6108. 32. doi: 10.3174/ajnr.A2496. Fulltekst i vitenarkiv Vis sammendrag

  BACKGROUND AND PURPOSE: The Chiari I malformation, characterized by tonsils extending below the foramen magnum, has increased CSF velocities compared with those in healthy subjects. Measuring the effect of tonsillar herniation on CSF flow in humans is confounded by interindividual variation. The goal of this study was to determine the effect of herniated tonsils on flow velocity and pressure dynamics by using 3D computational models. MATERIALS AND METHODS: A previously described 3D mathematic model of the normal subarachnoid space was modified by extending the tonsils inferiorly. The chamber created was compared with the anatomy of the subarachnoid space. Pressures and velocities were calculated by CFA methods for sinusoidal flow of a Newtonian fluid. Results were displayed as 2D color-coded plots and 3D animations. Pressure gradients and flow velocities were compared with those in the normal model. Velocity distributions were also compared with those in clinical images of CSF flow. RESULTS: The model represented grossly the subarachnoid space of a patient with Chiari I malformation. Fluid flow patterns in the Chiari model were complex, with jets in some locations and stagnant flow in others. Flow jets, synchronous bidirectional flow, and pressure gradients were greater in the Chiari model than in the normal model. The distribution of flow velocities in the model corresponded well with those observed in clinical images of CSF flow in patients with Chiari I. CONCLUSIONS: Tonsillar herniation per se increases the pressure gradients and the complexity of flow patterns associated with oscillatory CSF flow.

• Elboth, Thomas; Helgeland, Anders; Andreassen, Øyvind & Reif, Bjørn Anders Pettersson (2009). Flow noise simulation around a cylinder. I Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Red.), MekIT'09 : Fifth National Conference on Computational Mechanics, Trondheim 26-27 May 2009. Tapir Akademisk Forlag. ISSN 9788251924214. s. 151–168. Fulltekst i vitenarkiv
• Elboth, Thomas; Wasberg, Carl Erik; Helgeland, Anders; Andreassen, Øyvind & Reif, Bjørn Anders Pettersson (2009). Flow noise simulation around a cylinder. I Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Red.), MekIT'09 : Fifth National Conference on Computational Mechanics, Trondheim 26-27 May 2009. Tapir Akademisk Forlag. ISSN 9788251924214. s. 151–168. Fulltekst i vitenarkiv
• Helgeland, Anders; Reif, Bjørn Anders Pettersson; Andreassen, Øyvind & Wasberg, Carl Erik (2007). Visualization of vorticity and vortices in wall-bounded turbulent flows. IEEE Transactions on Visualization and Computer Graphics (TVCG). ISSN 1077-2626. 13(5), s. 1055–1066. doi: 10.1109/TVCG.2007.1062. Fulltekst i vitenarkiv Vis sammendrag

  This study was initiated by the scientifically interesting prospect of applying advanced visualization techniques to gain further insight into various spatio-temporal characteristics of turbulent flows. The ability to study complex kinematical and dynamical features of turbulence provides means of extracting the underlying physics of turbulent fluid motion. The objective is to analyze the use of a vorticity field line approach to study numerically generated incompressible turbulent flows. In order to study the vorticity field, we present a field line animation technique that uses a specialized particle advection and seeding strategy. Efficient analysis is achieved by decoupling the rendering stage from the preceding stages of the visualization method. This allows interactive exploration of multiple fields simultaneously, which sets the stage for a more complete analysis of the flow field. Multifield visualizations are obtained using a flexible volume rendering framework, which is presented in this paper. Vorticity field lines have been employed as indicators to provide a means to identify "ejection" and "sweep" regions, two particularly important spatio-temporal events in wall-bounded turbulent flows. Their relation to the rate of turbulent kinetic energy production and viscous dissipation, respectively, has been identified.

• Helgeland, Anders; Reif, Bjørn Anders Pettersson; Andreassen, Øyvind & Wasberg, Carl Erik (2007). On Vorticity, vortices and material lines in turbulent channel flow. I Palma, J.M.L.M. & Lopes, A. Silva (Red.), Advances in Turbulence XI, Proceedings of the 11th European Turbulence Conference. Springer. ISSN 9783540726036. s. 767–767. Fulltekst i vitenarkiv
• Helgeland, Anders; Andreassen, Øyvind; Reif, Bjørn Anders Pettersson & Wasberg, Carl Erik (2007). On Vorticity and Vortices in Wall-Bounded Turbulent Flows. I Skallerud, B. & Andersson, H.I. (Red.), MEKIT'07 Fourth National Conference on Computational Mechanics. Tapir Akademisk Forlag. ISSN 9788251922357. s. 163–174. Fulltekst i vitenarkiv
• Helgeland, Anders & Elboth, Thomas (2006). High-quality and interactive animations of 3D time-varying vector fields. IEEE Transactions on Visualization and Computer Graphics (TVCG). ISSN 1077-2626. 12, s. 1535–1546. Fulltekst i vitenarkiv
• Helgeland, Anders & Andreassen, Øyvind (2004). Visualization of Vector Fields Using Seed LIC and Volume Rendering. IEEE Transactions on Visualization and Computer Graphics (TVCG). ISSN 1077-2626. 10(6), s. 673–682. doi: 10.1109/TVCG.2004.49. Fulltekst i vitenarkiv

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• Wasberg, Carl Erik; Andreassen, Øyvind; Gjesdal, Thor; Greni, Pål Martin; Helgeland, Anders & Vartdal, Magnus (2019). Modelling of flow-induced vibrations in a thin towed antenna. Fulltekst i vitenarkiv
• Mardal, Kent-Andre; Helgeland, Anders; Haugthon, Victor; Reif, Anders & Støverud, Karen-Helene (2014). On the assumption of laminar flow of CSF in Chiari patients. Fulltekst i vitenarkiv
• Linge, Svein; Haughton, Victor; Løvgren, Alf; Mardal, Kent-Andre; Helgeland, Anders & Langtangen, Hans [Vis alle 7 forfattere av denne artikkelen] (2011). Cyclic Cerebrospinal Fluid Flow (I): Towards the Understanding of Cerebrospinal Fluid Flow Dynamics in Chiari I Malformation. Fulltekst i vitenarkiv Vis sammendrag

  Purpose Chiari I malformation leads to abnormal cerebrospinal fluid (CSF) flow dynamics and often is accompanied by syrinx formation. Phase constrast MR investigations reveal that peak flow velocities increase, whereas the altered pressure gradients are much more difficult to assess noninvasively. In this study, we use computer simulations to investigate how tonsil herniation changes pressure dynamics and flow in the cervical subarachnoid space. Materials and Methods. A 3D mathematical model of the cervical subarachnoid space, with and without herniated tonsils, was used to simulate CSF flow dynamics over several cycles of sinusoidal flow. Simulation results for the case with herniated tonsils were compared with corresponing results for normally positioned tonsils, while keeping all other parameters the same. Results Tonsil herniation caused pressure gradients to increse both in the superior-inferior direction and in axial planes of the model. The change in pressure dynamics caused a more complex flow velocity pattern with higher peak velocities, pronounced synchronous bi-directional flow, and larger velocity components within axial planes. Conclusion Chiari I malformation increases pressure gradients and flow complexity in the cervical subarachnoid space. This implies, e.g., that the spinal cord-CSF interaction dynamics changes following tonsil herniation, something which might have a role to play in syrinx formation.

• Linge, Svein; Mardal, Kent-Andre; Haughton, Victor & Helgeland, Anders (2011). Cerebrospinal fluid flow characteristics at double heart rate. Fulltekst i vitenarkiv
• Gibson, Max; Helgeland, Anders; Tutkun, Murat & Cal, Raul B. (2010). Classification of critical points in a turbulent boundary layer. Fulltekst i vitenarkiv
• Mardal, Kent-Andre; Helgeland, Anders; Hentschel, Susanne; Langtangen, Hans; Logg, Anders & Linge, Svein [Vis alle 8 forfattere av denne artikkelen] (2009). Some Biomedical Applications at Simula. Fulltekst i vitenarkiv
• Mardal, Kent-Andre; Hentschel, Susanne; Helgeland, Anders; Langtangen, Hans; Linge, Svein & Løvgren, Alf [Vis alle 8 forfattere av denne artikkelen] (2009). Patient-Specific Simulations of Stroke and Syringomyelia. Fulltekst i vitenarkiv
• Helgeland, Anders; Andreassen, Øyvind; Ommundsen, Atle; Reif, Bjørn Anders Pettersson; Werne, Joseph & Gaarder, Trond (2004). Visualization of the energy-containing turbulent scales. Fulltekst i vitenarkiv
• Waaler, Hans Thomas; Helgeland, Anders; Hjort, Peter Fredrik; Lund-Johansen, Per; Lund-Larsen, Per & Mathisen, Reidar [Vis alle 7 forfattere av denne artikkelen] (1978). Høyt blodtrykk : behandlingsprogram, utbytte, kostnader. Norges allmennvitenskapelige forskningsråd's gruppe for helsetjenesteforskning. ISSN 9788272160318. Fulltekst i vitenarkiv

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