reif

• Osnes, Andreas Nygård; Vartdal, Magnus; Omang, Marianne Gjestvold & Reif, Bjørn Anders Pettersson (2020). Particle-resolved simulations of shock-induced flow through particle clouds at different Reynolds numbers. Physical Review Fluids. doi: 10.1103/PhysRevFluids.5.014305. Full text in Research Archive Show summary

  This study investigates the Reynolds-number dependence of shock-induced flow through stationary particle clouds at 10% volume fraction, using ensemble-averaged results from three-dimensional particle-resolved large eddy simulations. The advantage of using large eddy simulations to study this problem is that they capture the strong velocity shears and flow separation caused by the no-slip condition at the particle surfaces. The shock particle cloud interaction produces a reflected shock wave, whose strength increases with decreasing particle Reynolds number. This results in important changes to the flow field that enters the particle cloud. The results show an approximate proportionality between the mean flow velocity and the flow fluctuation magnitudes. Maximum particle drag forces are in excellent agreement with previous inviscid studies, and we complement these results with statistics of time-averaged particle forces as well as the variation of temporal oscillations. The results of this work provide a basis for development of improved simplified dispersed flow models.

• Osnes, Andreas Nygård; Vartdal, Magnus; Omang, Marianne Gjestvold & Reif, Bjørn Anders Pettersson (2019). Numerical investigation of shock wave particle cloud interaction in cylindrical geometries. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT’19 - 10th National Conference on Computational Mechanics (Trondheim, Norway, 3-4 June 2019). International Center for Numerical Methods in Engineering (CIMNE). ISSN 9788494919497. p. 285–310. Full text in Research Archive
• Osnes, Andreas Nygård; Vartdal, Magnus; Omang, Marianne Gjestvold & Reif, Bjørn Anders Pettersson (2019). Computational analysis of shock-induced flow through stationary particle clouds. International Journal of Multiphase Flow. ISSN 0301-9322. doi: 10.1016/j.ijmultiphaseflow.2019.03.010. Full text in Research Archive Show summary

  We investigate the shock-induced flow through random particle arrays using particle-resolved Large Eddy Simulations for different incident shock wave Mach numbers, particle volume fractions and particle sizes. We analyze trends in mean flow quantities and the unresolved terms in the volume averaged momentum equation, as we vary the three parameters. We find that the shock wave attenuation and certain mean flow trends can be predicted by the opacity of the particle cloud, which is a function of particle size and particle volume fraction. We show that the Reynolds stress field plays an important role in the momentum balance at the particle cloud edges, and therefore strongly affects the reflected shock wave strength. The Reynolds stress was found to be insensitive to particle size, but strongly dependent on particle volume fraction. It is in better agreement with results from simulations of flow through particle clouds at fixed mean slip Reynolds numbers in the incompressible regime, than with results from other shock wave particle cloud studies, which have utilized either inviscid or two-dimensional approaches. We propose an algebraic model for the streamwise Reynolds stress based on the observation that the separated flow regions are the primary contributions to the Reynolds stress.

• Osnes, Andreas Nygård; Vartdal, Magnus & Reif, Bjørn Anders Pettersson (2017). Numerical simulation of particle jet formation induced by shock wave acceleration in a Hele-Shaw cell. Shock Waves. ISSN 0938-1287. p. 1–11. doi: 10.1007/s00193-017-0778-9. Full text in Research Archive Show summary

  The formation of jets from a shock-accelerated cylindrical shell of particles, confined in a Hele-Shaw cell, is studied by means of numerical simulation. A number of simulations have been performed, systematically varying the coupling between the gas and solid phases in an effort to identify the primary mechanism(s) responsible for jet formation. We find that coupling through drag is sufficient for the formation of jets. Including the effect of particle volume fraction and particle collisions did not alter the general behaviour, but had some influence on the length, spacing and number of jets. Furthermore, we find that the jet selection process starts early in the dispersal process, during the initial expansion of the particle layer.

• Wingstedt, Emma My Maria; Vartdal, Magnus & Reif, Bjørn Anders Pettersson (2017). Large-eddy simulations of dense-gas dispersion within a high-Reynolds number turbulent boundary layer. Physics of Fluids. ISSN 1070-6631. doi: 10.1063/1.4999466. Full text in Research Archive Show summary

  Large-eddy simulations of flows over a backward-facing step with release of four different gases with increasing densities have been performed. The results have been analysed with particular emphasis on the part of the flow field sufficiently far downstream where the local effect of the source dynamics is no longer dominating the flow field. The dense gas plumes maintain high concentrations close to the ground and become approximately twice as shallow in comparison with a neutrally buoyant plume. The vertical mixing is significantly reduced close to the ground, and both momentum and scalar flux reversal are found in the two densest cases, indicating negative shear and buoyancy production of turbulence kinetic energy. Examination of the gradient Richardson number indicates that all dense gases are significantly affected by buoyancy. Interestingly, a narrow layer within the plumes is identified where the impact of stratification is expected to be reduced. It is found that relaminarization is most likely occurring and that fluctuations found might originate from interfacial wave modes rather than from turbulence. The large-scale structures are seen to resemble those found in the plane channel flow close to the wall and further away from it. In the middle layer, significant differences are found due to buoyancy effects. The large-scale structures become more pancake-shaped with large-scale vorticity almost exclusively about the spanwise direction in the dense gas cases.

• Wingstedt, Emma My Maria; Osnes, Andreas Nygård; Åkervik, Espen; Eriksson, Daniel & Reif, Bjørn Anders Pettersson (2017). Large-eddy simulation of dense gas dispersion over a simplified urban area. Atmospheric Environment. ISSN 1352-2310. doi: 10.1016/j.atmosenv.2016.12.039. Full text in Research Archive Show summary

  Dispersion of neutral and dense gas over a simplified urban area, comprising four cubes, has been investigated by the means of large-eddy simulations (LES). The results have been compared to wind tunnel experiments and both mean and fluctuating quantities of velocity and concentration are in very good agreement. High-quality inflow profiles are necessary to achieve physically realistic LES results. In this study, profiles matching the atmospheric boundary layer flow in the wind tunnel, are generated by means of a separate precursor simulation. Emission of dense gas dramatically alters the flow in the near source region and introduces an upstream dispersion. The resulting dispersion patterns of neutral and dense gas differ significantly, where the plume in the latter case is wider and shallower. The dense gas is highly affected by the cube array, which seems to act as a barrier, effectively deflecting the plume. This leads to higher concentrations outside of the array than inside. On the contrary, the neutral gas plume has a Gaussian-type shape, with highest concentrations along the centreline. It is found that the dense gas reduces the vertical and spanwise turbulent momentum transport and, as a consequence, the turbulence kinetic energy. The reduction coincides with the area where the gradient Richardson number exceeds its critical value, i.e. where the flow may be characterized as stably stratified. Interestingly, this region does not correspond to where the concentration of dense gas is the highest (close to the ground), as this is also where the largest velocity gradients are to be found. Instead there is a layer in the middle of the dense gas cloud where buoyancy is dynamically dominant.

• Wingstedt, Emma My Maria; Fossum, Hannibal Eie; Reif, Bjørn Anders Pettersson & Werne, Joseph (2015). Anisotropy and shear-layer edge dynamics of statistically unsteady,stratified turbulence. Physics of Fluids. ISSN 1070-6631. doi: 10.1063/1.4922238. Full text in Research Archive Show summary

  Direct numerical simulation data of an evolving Kelvin-Helmholtz instability have been analyzed in order to characterize the dynamic and kinematic response of shear-generated turbulent flow to imposed stable stratification. Particular emphasis was put on anisotropy and shear-layer edge dynamics in the net kinetic energy decay phase of the Kelvin-Helmholtz evolution. Results indicate a faster increase of small-scale anisotropy compared to large-scale anisotropy. Also, the anisotropy of thermal dissipation differs significantly from that of viscous dissipation. It is found that the Reynolds stress anisotropy increases up to a stratification level roughly corresponding to Ri g≈ 0.4, but subsequently decreases for higher levels of stratification, most likely due to relaminarization. Coherent large-scale turbulence structures are cylindrical in the center of the shear layer, whereas they become ellipsoidal in the strongly stratified edge-layer region. The structures of the Reynolds stresses are highly one-componental in the center and turn two-componental as stratification increases. Stratification affects all scales, but it seems to affect larger scales to a higher degree than smaller scales and thermal scales more strongly than momentum scales. The effect of strong stable stratification at the edge of the shear layer is highly reminiscent of the non-local pressure effects of solid walls. However, the kinematic blocking inherently associated with impermeable walls is not observed in the edge layer. Vertical momentum flux reversal is found in part of the shear layer. The roles of shear and buoyant production of turbulence kinetic energy are exchanged, and shear production is transferring energy into the mean flow field, which contributes to relaminarization. The change in dynamics near the edge of the shear layer has important implications for predictive turbulence model formulations.© 2015 AIP Publishing LLC

• 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), p. 1082–1090. doi: 10.1016/j.jbiomech.2013.12.023. Full text in Research Archive
• Abdul-Karim, Nadia; Blackman, Christopher S.; Gill, Philip P.; Wingstedt, Emma My Maria & Reif, Bjørn Anders Pettersson (2014). Post-blast explosive residue-a review of formation and dispersion theories and experimental research. RSC Advances. doi: 10.1039/c4ra04195j. Full text in Research Archive Show summary

  The presence of undetonated explosive residues following high order detonations is not uncommon, however the mechanism of their formation, or survival, is unknown. The existence of these residues impacts on various scenarios, for example their detection at a bomb scene allows for the identification of the explosive charge used, whilst their persistence during industrial explosions can affect the safety and environmental remediation efforts at these sites. This review article outlines the theoretical constructs regarding the formation of explosive residues during detonation and their subsequent dispersal and deposition in the surrounding media. This includes the chemical and physical aspects of detonation and how they could allow for undetonated particles to remain. The experimental and computational research conducted to date is presented and compared to the theory in order to provide a holistic review of the phenomenon.

• Fossum, Hannibal Eie; Wingstedt, Emma My Maria & Reif, Bjørn Anders Pettersson (2013). A model for the viscous dissipation rate in stably stratified, sheared turbulence. Geophysical Research Letters. ISSN 0094-8276. doi: 10.1002/grl.50663. Full text in Research Archive Show summary

  A model for the turbulence dissipation rate in stably stratified shear turbulence is developed and validated. The functional dependence of the model is derived from first principles and it represents a conceptually new approach in that it depends on the background temperature field rather than on the fluctuating velocity field. This novel feature makes the proposed model a viable candidate for dissipation rate estimates in measured real-life flows. Direct numerical simulation data are used for a priori assessment of the proposed model. It is demonstrated that the proposed model performs very well, particularly in cases where the background stratification becomes dynamically important. Also, a generalized expression for the mixing coefficient has been rigorously derived from first principles assuming local isotropy of incompressible turbulent flows. The mixing coefficient is shown to depend on the Prandtl number and values are in correspondence with previous studies.

• Fossum, Hannibal Eie; Reif, Bjørn Anders Pettersson; Tutkun, Murat & Gjesdal, Thor (2012). On the Use of Computational Fluid Dynamics to Investigate Aerosol Dispersion in an Industrial Environment: A Case Study. Boundary-Layer Meteorology. ISSN 0006-8314. doi: 10.1007/s10546-012-9711-z. Full text in Research Archive Show summary

  Aerosol dispersion in the area surrounding an existing biological treatment facility is investigated using large-eddy simulation, with the objective to investigate the applicability of computational fluid dynamics to complex real-life problems. The aerosol sources consist of two large aeration ponds that slowly diffuse aerosols into the atmosphere. These sources are modelled as dilute concentrations of a non-buoyant non-reacting pollutant diffusing from two horizontal surfaces. The time frame of the aerosol release is restricted to the order of minutes, justifying a statistically steady inlet boundary condition. The numerical results are compared to wind-tunnel experiments for validation. The wind-tunnel flow characteristics resemble neutral atmospheric conditions with a Reynolds number, based on the boundary-layer thickness, of Reδ ≈ 2 × 105. The numerical inflow conditions are based upon the wind-tunnel flow field. The predicted decay of both the mean and root-mean-square concentrations are in good agreement with experimental data; at 3 m from the ground, the plume mean concentration 200 m downwind of the source is approximately 2% of the source strength. The numerical data in the near-surface layer (0–50 m from the ground) correspond particularly well with the wind-tunnel data. Tentative deposition simulations suggest that there seems to be little difference in the deposition rates of large (1.8 × 10−5 m) and small (3 × 10−6 m) particles in the near-field under the flow conditions considered.

• Elboth, Thomas; Reif, Bjørn Anders Pettersson; Andreassen, Øyvind & Martell, Michael B. (2012). Flow noise reduction from superhydrophobic surfaces. Geophysics. ISSN 0016-8033. doi: 10.1190/GEO2011-0001.1. Full text in Research Archive Show summary

  This work investigates how a highly (super)hydrophobicsurface can be used to reduce turbulence-generated drag andnoise on a towed streamer cable. The work is done by analyzingfull-scale drag and flow noise measurements taken ona commercial seismic streamer in combination with directnumerical simulations of turbulence-generated flow noise.The main findings are that viscous drag and flow noisecan be significantly reduced on a seismic streamer that iscoated to make the surface highly hydrophobic. In an oceantowing test, a 4% reduction of drag on a streamer sectionwas measured. In a separate test on a commercial seismicvessel, a reduction in the flow noise level of nearly 50%(6 dB) for frequencies below 10 Hz was found. Based uponan analysis of numerical simulation data, it is suggested thatthe reduction in drag and noise can be attributed to a reducedlevel of shear stress and change in the kinematic structure ofthe turbulence, both of which occur in the immediate vicinityof the highly hydrophobic surface.

• Fossum, Hannibal Eie & Reif, Bjørn Anders Pettersson (2011). Flow Patterns and Aerosol Dispersion inside an Aerosol Test Chamber. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'11: Sixth National Conference on Computational Mechanics, Trondheim 23-24 May 2011. Tapir Akademisk Forlag. ISSN 9788251927987. p. 73–87. Full text in Research Archive
• Blatny, Janet Martha; Olsen, Jaran Strand; Andreassen, Øyvind; Waagen, Viggo & Reif, Bjørn Anders Pettersson (2011). Tracking Legionella in air generated from a biological treatment plant - A case study of the outbreak of legionellosis in Norway. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. 8018. doi: 10.1117/12.886932. Full text in Research Archive
• Valen-Senstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Reif, Bjørn Anders Pettersson & Langtangen, Hans Petter (2011). On The Complexity of Stationary Flow in Patient–Specific Aneurysms. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'11: Sixth National Conference on Computational Mechanics, Trondheim 23-24 May 2011. Tapir Akademisk Forlag. ISSN 9788251927987. Full text in Research Archive Show summary

  Summary Sound or bruits in the vasculatory system at the range of 1Hz-1kHz is known to occur, and frequency peaks at high frequencies are associated with sick blood vessels. The exact reason for these sounds are not known, but the sound may be generated by turbulence. Turbulence generated sound is typically associated with high frequencies but also with low energy levels. Unfortunately, most blood flow simulations are performed with space and time resolutions where rapid fluctuations typ- ically associated with non-laminar flow cannot be captured. Complex non-laminar flow and the subsequent oscillatory wall stresses can po- tentially be of importance in understanding aneurysm rupture through the local arterial wall response. To investigate the possible presence of complex non-laminar blood flow in intracranial aneurysms, we per- formed high resolution computational fluid dynamics (CFD) simulations for patient specific middle cerebral artery (MCA) aneurysms with sta- tionary inflow conditions. The current study confirms, through high resolution simulations, that complex oscillatory flow may occur in in- tracranial aneurysms. CFD practitioners should therefore be aware of the potentially strong demands to space-time computational resolution in blood flow simulations.

• Reif, Bjørn Anders Pettersson & Mortensen, Mikael (2011). A Nonlinear Eddy-Viscosity Model for Near-Wall Turbulence. ERCOFTAC Series. ISSN 1382-4309. 14, p. 269–276. doi: 10.1007/978-90-481-9603-6_28. Full text in Research Archive
• Grafsrønningen, Stig; Jensen, Atle & Reif, Bjørn Anders Pettersson (2011). PIV investigation of buoyant plume from natural convection heat transfer above a horizontal heated cylinder. International Journal of Heat and Mass Transfer. ISSN 0017-9310. 54(23-24), p. 4975–4987. doi: 10.1016/j.ijheatmasstransfer.2011.07.011. Full text in Research Archive
• Vik, Thomas & Reif, Bjørn Anders Pettersson (2011). Large eddy simulations of the release of liquified chlorine from a pressurized tank into the atmosphere. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'11: Sixth National Conference on Computational Mechanics, Trondheim 23-24 May 2011. Tapir Akademisk Forlag. ISSN 9788251927987. Full text in Research Archive
• Elboth, Thomas; Reif, Bjørn Anders Pettersson & Andreassen, Øyvind (2011). Hydrodynamic flow noise reduction from superhydrophobic surfaces. Geophysics. ISSN 0016-8033. doi: 10.1190/geo2011-0001.1. Full text in Research Archive
• Valen-Senstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Reif, Bjørn Anders Pettersson & Langtangen, Hans Petter (2011). Direct numerical simulation of transitional flow in a patient-specific intracranial aneurysm. Journal of Biomechanics. ISSN 0021-9290. 44(16), p. 2826–2832. doi: 10.1016/j.jbiomech.2011.08.015. Full text in Research Archive
• Wingstedt, Emma My Maria; Vik, Thomas & Reif, Bjørn Anders Pettersson (2011). Numerical simulation of non-neutral gas dispersion in a complex urban area. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'11: Sixth National Conference on Computational Mechanics, Trondheim 23-24 May 2011. Tapir Akademisk Forlag. ISSN 9788251927987. p. 463–471. Full text in Research Archive
• Blatny, Janet Martha; Tutkun, Murat; Fossum, Hannibal Eie; Skogan, Gunnar; Fykse, Else-Marie & Andreassen, Øyvind [Show all 8 contributors for this article] (2011). Assessment of the dispersion of Legionella-containing aerosols from a biological treatment plant. Frontiers in Bioscience. ISSN 1093-9946. doi: 10.2741/e333. Full text in Research Archive Show summary

  Legionella was detected in aeration ponds (biological treatment plant) at Borregaard Ind. Ltd., Norway, and in air samples harvested directly above these ponds. Since 2005, three outbreaks of legionellosis occurred within a 10 km radius from this plant. This work addresses the dispersion patterns of Legionella-containing particles by characterizing the aerosol plume emitted from these ponds (outbreak source) < 500 meters using wind-tunnel measurements, CFD simulations, and real-life measurements. The most abundant particles directly over the ponds were < 6 and >15 ľm. The results showed that the aerosol plume remained narrow; 180 meters wide at 350 meters downwind of the ponds, and that 2 and 18 ľm aerosols were mainly deposited in the vicinity of the ponds ( 150 - 200 meters). Furthermore, the maximum aerosol concentration level appeared 5-10 meters above ground level and the maximum concentration 500 meters downwind was approximately 2% of the concentration level directly above the ponds. Our study demonstrates the strength of combining modeling with real-life aerosol analyses increasing the understanding of dispersion of airborne (pathogenic) microorganisms.

• Elboth, Thomas; Lilja, Didrik; Reif, Bjørn Anders Pettersson & Andreassen, Øyvind (2010). Investigation of flow and flow noise around a seismic streamer cable. Geophysics. ISSN 0016-8033. 75(1), p. Q1–Q9. doi: 10.1190/1.3294639. Full text in Research Archive
• Mortensen, Mikael; Reif, Bjørn Anders Pettersson & Wasberg, Carl Erik (2010). Assessment of the finite volume method applied to the nu(2)-f model. International Journal for Numerical Methods in Fluids. ISSN 0271-2091. 63(4), p. 495–516. doi: 10.1002/fld.2091. Full text in Research Archive Show summary

  The objective of this paper is to assess the accuracy of low-order finite volume (FV) methods applied to the nu(2)-f turbulence model of Durbin (Theoret. Comput. Fluid Dyn. 1991; 3:1-13) in the near vicinity of solid walls. We are not (like many others) concerned with the stability of solvers - the topic at hand is simply whether the mathematical properties of the nu(2)-f model can be captured by the given, widespread, numerical method. The nu(2)-f model is integrated all the way up to solid walls, where steep gradients in turbulence parameters are observed. The full resolution of wall gradients imposes quite high demands on the numerical schemes and it is not evident that common (second order) FV codes can fully cope with such demands. The nu(2)-f model is studied in a statistically one-dimensional, fully developed channel flow where we compare FV schemes with a highly accurate spectral element reference implementation. For the FV method a higher-order face interpolation scheme, using Lagrange interpolation polynomials up to arbitrary order, is described. It is concluded that a regular second-order FV scheme cannot give an accurate representation of all model parameters, independent of mesh density. To match the spectral element solution an extended source treatment (we use three-point Gauss-Lobatto quadrature), as well as a higher-order discretization of diffusion is required. Furthermore, it is found that the location of the first internal node need to be well within y(+) = 1. Copyright (C) 2009 John Wiley & Sons, Ltd.

• Endregard, Monica; Reif, Bjørn Anders Pettersson; Vik, Thomas & Busmundrud, Odd (2010). Consequence assessment of indoor dispersion of sarin-A hypothetical scenario. Journal of Hazardous Materials. ISSN 0304-3894. doi: 10.1016/j.jhazmat.2009.11.041. Full text in Research Archive Show summary

  The objective of this study is to provide a hypothetical scenario of indoor dispersion of the highly toxic nerve agent sarin in a large building, which can be used as a starting point for discussion, planning, training and exercises for emergency services and responsible authorities. The indoor dispersion has been simulated using a Computational Fluid Dynamics (CFD) approach. Possible consequences have been calculated based on concentration and dose profiles. Mild intoxication effects appear within minutes, while serious injuries and fatalities occur approximately 20 min after release. Anticipated key emergency response challenges are: (i) the time factor due to rapid onset of symptoms, (ii) the large number of casualties, and (iii) the contaminated hazard scene. (C) 2009 Elsevier B.V. All rights reserved.

• Reif, Bjørn Anders Pettersson; Mortensen, Mikael & Langer, Carlos A. (2009). Towards Sensitizing the Nonlinear v(2)-f Model to Turbulence Structures. Flow Turbulence and Combustion. ISSN 1386-6184. 83(2), p. 185–203. doi: 10.1007/s10494-008-9194-7. Full text in Research Archive Show summary

  In this paper a one-way coupling between the nonlinear v (2) -aEuro parts per thousand f model by Pettersson Reif (Flow Turbul Combust 76:241-256, 2006) and an algebraic structure-based model have been investigated. Comparisons with available experimental and numerical data indicate that the compatibility between the two models is good and that their joint performance is satisfactory in the cases considered here. A full coupling between the models seems therefore a potentially viable route towards a significant advancement of engineering turbulence models and their predictive capabilities.

• Fossum, Hannibal Eie & Reif, Bjørn Anders Pettersson (2009). Predicting particle deposition in the human airways with RANS turbulence modeling. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'09 : Fifth National Conference on Computational Mechanics, Trondheim 26-27 May 2009. Tapir Akademisk Forlag. ISSN 9788251924214. p. 169–186. Full text in Research Archive
• Elboth, Thomas; Reif, Bjørn Anders Pettersson & Andreassen, Øyvind (2009). Flow and swell noise in marine seismic data. Geophysics. ISSN 0016-8033. 74(2), p. Q17–Q25. doi: 10.1190/1.3078403. Full text in Research Archive
• Elboth, Thomas; Helgeland, Anders; Andreassen, Øyvind & Reif, Bjørn Anders Pettersson (2009). Flow noise simulation around a cylinder. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'09 : Fifth National Conference on Computational Mechanics, Trondheim 26-27 May 2009. Tapir Akademisk Forlag. ISSN 9788251924214. p. 151–168. Full text in Research Archive
• Vartdal, Magnus & Reif, Bjørn Anders Pettersson (2009). Numerical simulation of aerosol dispersion inside a rotating aerosol chamber. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'09 : Fifth National Conference on Computational Mechanics, Trondheim 26-27 May 2009. Tapir Akademisk Forlag. ISSN 9788251924214. p. 459–470. Full text in Research Archive
• Valen-Sendstad, Kristian; Mortensen, M.; Langtangen, Hans Petter; Reif, Bjørn Anders Pettersson & Mardal, Kent-Andre (2009). Implementing a k-e Turbulence Model in the FEniCS Finite Element Programming Environment. In Skallerud, Bjørn & Andersson, Helge (Ed.), Proceedings of MekIT'09. Tapir Akademisk Forlag. ISSN 9788251924214. Full text in Research Archive
• Wasberg, Carl Erik; Gjesdal, Thor; Reif, Bjørn Anders Pettersson & Andreassen, Øyvind (2009). Variational multiscale turbulence modelling in a high order spectral element method. Journal of Computational Physics. ISSN 0021-9991. 228(19), p. 7333–7356. doi: 10.1016/j.jcp.2009.06.029. Full text in Research Archive Show summary

  In the variational multiscale (VMS) approach to large eddy simulation (LES), the governing equations are projected onto an a priori scale partitioning of the solution space. This gives an alternative framework for designing and analyzing turbulence models. We describe the implementation of the VMS LES methodology in a high order spectral element method with a nodal basis, and discuss the properties of the proposed scale partitioning. The spectral element code is first validated by doing a direct numerical simulation of fully developed plane channel flow. The performance of the turbulence model is then assessed by several coarse grid simulations of channel flow at different Reynolds numbers. (C) 2009 Elsevier Inc. All rights reserved.

• Elboth, Thomas; Wasberg, Carl Erik; Helgeland, Anders; Andreassen, Øyvind & Reif, Bjørn Anders Pettersson (2009). Flow noise simulation around a cylinder. In Skallerud, Bjørn Helge & Andersson, Helge Ingolf (Ed.), MekIT'09 : Fifth National Conference on Computational Mechanics, Trondheim 26-27 May 2009. Tapir Akademisk Forlag. ISSN 9788251924214. p. 151–168. Full text in Research Archive
• Reif, Bjørn Anders Pettersson; Mortensen, Mikael & Langer, Carlos A. (2008). Towards Sensitizing the Nonlinear v^2-f Model to Turbulence Structures. Flow Turbulence and Combustion. ISSN 1386-6184. doi: 10.1007/s10494-008-9194-7. Full text in Research Archive
• Blatny, Janet Martha; Reif, Bjørn Anders Pettersson; Skogan, Gunnar; Andreassen, Øyvind; Høiby, Ernst Arne & Ask, Eirik [Show all 10 contributors for this article] (2008). Tracking airborne Legionella and Legionella pneumophila at a biological treatment plant. Environmental Science and Technology. ISSN 0013-936X. 42(19). doi: 10.1021/es800306m. Full text in Research Archive Show summary

  Biological treatment plants are frequently used to degrade organic substances in wastewater from wood refinement processes. Aeration ponds in such plants provide an optimal growth environment for many microorganisms, including Legionella species. To investigate whether legionellae could be dispersed as aerosols from the ponds and transported by the wind, the wetted-wall cyclone SASS 2000PLUS and the impactors MAS-100 and STA-204 were used to collect air samples directly above, upwind, and downwind of aeration ponds during a 4-month period. Computational fluid dynamics was used a priori to estimate the aerosol paths and to determine suitable air-sampling locations. Several Legionella species, including Legionella pneumophila, were identified in air samples at the biological treatment plant using microbiological and molecular methods. L. pneumophila was identified up to distances of 200 m downwind from the ponds, but, in general, not upwind nor outside the predicted aerosol paths. The highest concentration level of viable legionellae was identified directly above the aeration ponds (3300 CFU/m3). This level decreased as the distance from the aeration ponds increased. Molecular typing indicated that a single clone of L. pneumophila was dispersed from the ponds during the period of the study. Thus, our study demonstrated that aerosols generated at aeration ponds of biological treatment facilities may contain L. pneumophila, which then can be transported by the wind to the surroundings. The methods used in this study may be generically applied to trace biological aerosols that may pose a challenge to environmental occupational health.

• 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), p. 1055–1066. doi: 10.1109/TVCG.2007.1062. Full text in Research Archive Show summary

  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. In Palma, J.M.L.M. & Lopes, A. Silva (Ed.), Advances in Turbulence XI, Proceedings of the 11th European Turbulence Conference. Springer. ISSN 9783540726036. p. 767–767. Full text in Research Archive
• Helgeland, Anders; Andreassen, Øyvind; Reif, Bjørn Anders Pettersson & Wasberg, Carl Erik (2007). On Vorticity and Vortices in Wall-Bounded Turbulent Flows. In Skallerud, B. & Andersson, H.I. (Ed.), MEKIT'07 Fourth National Conference on Computational Mechanics. Tapir Akademisk Forlag. ISSN 9788251922357. p. 163–174. Full text in Research Archive
• Gjesdal, Thor; Wasberg, Carl Erik & Reif, Bjørn Anders Pettersson (2005). Application of a variational multiscale turbulence model in a high order spectral element solver. In Skallerud, B & Andersson, H. I. (Ed.), MEkIT'05. Third national conference on Computational Mechanics. Tapir Akademisk Forlag. ISSN 9788251920520. p. 101–116. Full text in Research Archive
• Wasberg, Carl Erik & Reif, Bjørn Anders Pettersson (2003). Large-eddy simulation of flow around a wall-mounted rounded cylinder, MekIT'03, Second national conference on Computational Mechanics, Trondheim 8-9 May 2003. Tapir Akademisk Forlag. ISSN 9788251918688. p. 391–405. Full text in Research Archive
• Wasberg, Carl Erik; Andreassen, Øyvind & Reif, Bjørn Anders Pettersson (2001). Numerical simulation of turbulence by spectral element methods, MekIT'01 - First national conference on Computational Mechanics. Tapir Akademisk Forlag. ISSN 9788251916622. p. 387–402. Full text in Research Archive Show summary

  Proceedings from the first national conference on computational mechanics includes 30 papers presented during the conference, which took place in Trondheim 3-4 May 2001. The Proceedings, which contains contributions both from industry and academia, gives the state-of-the-art in the field in Norway.

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• Durbin, Paul & Reif, Bjørn Anders Pettersson (2010). Statistical Theory and Modeling for Turbulent Flows. John Wiley & Sons. ISBN 9780470689318. 344 p. Full text in Research Archive

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• Osnes, Andreas Nygård; Vartdal, Magnus & Reif, Bjørn Anders Pettersson (2018). Analysis of carrier phase fluctuations during the passage of a shock wave through a particle cloud. Full text in Research Archive
• Osnes, Andreas Nygård; Vartdal, Magnus & Reif, Bjørn Anders Pettersson (2016). Jetting instability of a shocked cylindrical shell of solid particles. Full text in Research Archive
• Wingstedt, Emma My Maria; Eriksson, Daniel; Parmhed, Oskar; Leroy, Guillaume; Osnes, Andreas Nygård & Reif, Bjørn Anders Pettersson [Show all 7 contributors for this article] (2016). MODITIC - Large-eddy simulations of dispersion of neutral and non-neutral scalar fields in complex urban-like geometries. Full text in Research Archive
• Vik, Thomas & Reif, Bjørn Anders Pettersson (2014). Numerical Simulation of the Dispersion of Chlorine Following the Release from a Pressurized Tank. Full text in Research Archive
• Wingstedt, Emma My Maria; Fossum, Hannibal Eie & Reif, Bjørn Anders Pettersson (2013). Modelling the viscous dissipation rate in stably stratified turbulence. Full text in Research Archive
• Vik, Thomas & Reif, Bjørn Anders Pettersson (2013). Evaporation of a thin liquid surface beneath a turbulent boundary layer. Full text in Research Archive
• Vik, Thomas; Wingstedt, Emma My Maria; Tørnes, John Aasulf; Reif, Bjørn Anders Pettersson & Endregard, Monica (2013). Numerical simulations of the dispersion of non-neutral toxic industrial chemicals in an urban environment. Full text in Research Archive
• Grafsrønningen, Stig; Jensen, Atle & Reif, Bjørn Anders Pettersson (2012). PIV investigation of a buoyant plume above heated horizontal cylinder. Full text in Research Archive
• Vik, Thomas; Wingstedt, Emma My Maria & Reif, Bjørn Anders Pettersson (2012). Numerical simulation of the dispersion of ammonia in complex urban environment. Full text in Research Archive
• Valen-Sendstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Langtangen, Hans; Reif, Bjørn & Steinman, David (2011). 'Turbulence' in Cerebral Aneurysms. Full text in Research Archive
• Valen-Sendstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Reif, Bjørn & Langtangen, Hans (2011). Direct Numerical Simulation of Transitional Flow in Patient-Specific Intracranial Aneurysms. Full text in Research Archive
• Valen-Sendstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Reif, Bjørn & Langtangen, Hans (2011). Simulation Methodology for Cerebral Blood Flow. Full text in Research Archive Show summary

  In experiments turbulence has previously been shown to occur in intracranial aneurysms, but the turbulent effects on the arterial wall have not been investigated. The effects of turbulence induced oscillatory wall stresses could be of great importance in understanding aneurysm rupture due to the local arterial wall response to such hemodynamical forces. To investigate the effects of turbulence on blood flow in an intracranial aneurysm, we performed a high resolution computational fluid dynamics (CFD) simulations in a patient specific middle cerebral artery (MCA) aneurysm using a realistic, pulsatile inflow velocity. The flow showed transition to turbulence just after peak systole, and the turbulent fluctuations increased in intensity until mid deceleration, before relaminarization occurred during diastole. The flow impinged with an angle nearly normal onto the aneurysm dome, and the turbulence structures greatly affected both the frequency of change of wall shear stress (WSS) direction and WSS magnitude, which reached a maximum value of 41.5 Pa. The recorded frequencies were predominantly in the range of 1-500 Hz. The current study confirms, through properly resolved CFD simulations, that turbulence can occur in intracranial aneurysms. Because of the local arterial wall response to hemodynamical forces, the effects of oscillatory WSS caused by turbulence, could be correlated with aneurysm rupture, and should be investigated experimentally.

• Valen-Sendstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Reif, Bjørn & Langtangen, Hans (2011). Computational Cerebral Hemodynamics. Full text in Research Archive
• Valen-Sendstad, Kristian; Mortensen, Mikael; Mardal, Kent-Andre; Reif, Bjørn & Langtangen, Hans (2011). Investigations of Transistional Flows Using Image Based Hemodynamics. Full text in Research Archive Show summary

  We investigated the complexity of blood flow in intracranial aneurysms in terms of the frequency spectrum of pointwise velocity and pressure fluctuations. The investigations were performed using high-resolution computational fluid dynamics (CFD) simulations for patient-specific middle cerebral artery (MCA) aneurysms, using realistic inflow profile. Through these simulations, the study confirmed that complex transitional flow can occur in intracranial aneurysms. In particular, we found fluctuations of the order of 500- 1000 Hz.

• Valen-Sendstad, Kristian; Mardal, Kent-Andre; Mortensen, Mikael; Reif, Bjørn; Langtangen, Hans & Skallerud, Bjørn [Show all 7 contributors for this article] (2011). On the Complexity of Stationary Flow in Patient-Specific Aneurysms. Full text in Research Archive Show summary

  Summary Sound or bruits in the vasculatory system at the range of 1Hz-1kHz is known to occur, and frequency peaks at high frequencies are associated with sick blood vessels. The exact reason for these sounds are not known, but the sound may be generated by turbulence. Turbulence generated sound is typically associated with high frequencies but also with low energy levels. Unfortunately, most blood flow simulations are performed with space and time resolutions where rapid fluctuations typ- ically associated with non-laminar flow cannot be captured. Complex non-laminar flow and the subsequent oscillatory wall stresses can po- tentially be of importance in understanding aneurysm rupture through the local arterial wall response. To investigate the possible presence of complex non-laminar blood flow in intracranial aneurysms, we per- formed high resolution computational fluid dynamics (CFD) simulations for patient specific middle cerebral artery (MCA) aneurysms with sta- tionary inflow conditions. The current study confirms, through high resolution simulations, that complex oscillatory flow may occur in in- tracranial aneurysms. CFD practitioners should therefore be aware of the potentially strong demands to space-time computational resolution in blood flow simulations.

• Mardal, Kent-Andre; Valen-Sendstad, Kristian; Alnæs, Martin; Mortensen, Mikael; Langtangen, Hans & Reif, Bjørn (2011). The Finite Element Library FEniCS and Simulation of Blood Flow in Cerebral Aneurysms. Full text in Research Archive
• Vik, Thomas & Reif, Bjørn Anders Pettersson (2011). Large Eddy Simulations of the Release of Liquified Chlorine from a Pressurized Tank into the Atmosphere. Full text in Research Archive
• Vik, Thomas & Reif, Bjørn Anders Pettersson (2011). Large eddy simulations of the release of liquified chlorine from a pressurized tank into the atmosphere. Full text in Research Archive
• Endregard, Monica; Reif, Bjørn Anders Pettersson; Vik, Thomas & Busmundrud, Odd (2009). Consequence management challenges after indoor dispersion of a toxic chemical. Full text in Research Archive
• Vik, Thomas; Reif, Bjørn Anders Pettersson & Endregard, Monica (2009). CFD simulations of the evaporation from a liquid pool in a turbulent flow. Full text in Research Archive
• Vik, Thomas & Reif, Bjørn Anders Pettersson (2009). Large eddy simulations of the evaporation from a liquid pool beneath a turbulent air flow. Full text in Research Archive
• Elboth, Thomas; Andreassen, Øyvind & Reif, Bjørn Anders Pettersson (2008). Experiments and simulations of flow noise inside a cylinder aligned with the flow. Full text in Research Archive
• Mortensen, Mikael; Reif, Bjørn Anders Pettersson & Langer, Carlos A. (2008). Modelling adverse pressure-gradient boundary layers using the nonlinear V2F model in combination with a structure based model. Full text in Research Archive
• Helgeland, Anders; Andreassen, Øyvind; Ommundsen, Atle; Reif, Bjørn Anders Pettersson; Werne, Joseph & Gaarder, Trond (2004). Visualization of the energy-containing turbulent scales. Full text in Research Archive
• Osnes, Andreas Nygård; Vartdal, Magnus; Reif, Bjørn Anders Pettersson & Omang, Marianne Gjestvold (2019). Shock-induced flow through particle clouds. Universitetet i Oslo. Full text in Research Archive Show summary

  High-speed flows through dense particle clouds occur in many technological applications such as explosion mitigation systems, combustion engines, drug-delivery systems, as well as natural phenomena such as volcanic eruptions and meteoroid breakup. This thesis studies such flows by means of numerical simulations. Typical flows of interest feature trillions of particles, and it is therefore necessary to use simplified models to describe the behaviour of the particles and the flow around them. In this thesis, we use very large, accurate, simulations to study the details of shock-induced flows with a moderate number of particles. In addition, full-scale simulations with a large number of particles are used to determine the sensitivity of such simulations to modelling choices. The simulations with few particles show that high particle concentration necessitates modifications to the drag-laws and flow model approximations that are used to simulate full-scale problems. Small-scale flow fluctuations affect the average flow statistics, and must be modelled in simulations where they are not directly captured. The full-scale simulations show that the shock-accelerated particles self-organise and form particle jets, in agreement with experiments. To capture this process, the simulations must account for the interchange of momentum and energy between the particles and the surrounding air.

• Endregard, Monica; Burkhart, Stephane; Burman, Jan; Gentilhomme, Olivier; Robins, Alan & Wingstedt, Emma My Maria [Show all 16 contributors for this article] (2016). MODITIC : modelling the dispersion of toxic industrial chemicals in urban environments. Forsvarets forskningsinstitutt . ISSN 9788246428437. Full text in Research Archive Show summary

  Hovedmålet med MODITIC-prosjektet (Modelling the disperison of toxic industrial chemicals inurban environments) er å forbedre vår grunnleggende forståelse av spredning av ikke-nøytralegasser i bebygde miljøer. Prosjektet har som mål å legge til rette for fremtidige forbedringer avspredningsmodeller som brukes i krisesituasjoner av militært personell og sivile nødetater, og dermedgi bedre beredskap og respons.Atmosfæriske vindtunneleksperimenter har blitt systematisk gjennomført, og nye eksperimentelledatasett for en rekke nøye utvalgte spredningscenarier er blitt produsert. De samme konfigurasjoneneer også blitt modellert ved bruk av både avansert Computational Fluid Dynamics (CFD) og enkleregaussiske modeller. Prosjektet har også fått tillgang på eksperimentelle data for utslipp av giftigekjemikalier fra trykksatte tanker som har blitt brukt for å gi realistiske kildekarakteristikker veddenne type hendelser. Beregninger av disse utslippseksperimentene har blitt utført for å validereberegningsmodellene.Prosjektet har generert en stor database med eksperimentelle og numeriske resultater for spredningav nøytrale og tunge gasser i konfigurasjoner som spenner fra enkle til komplekse geometrier.Denne databasen vil bli gjort tilgjengelig. Den gir referansedata som er nødvendige for å forbedreden grunnleggende forståelsen av spredning av gasser i urbane omgivelser og for modellering avspredningen. Databasen kan anvendes for utvikling, forbedring og validering av spredningsmodellerfor farlige kjemikalier i bymiljø.Den vitenskapelige metode som er brukt i MODITIC, der numeriske simuleringer er utført sammenmed vindtunnelforsøk og feltforsøk , har vist seg å være svært vellykket. Denne tilnærmingenanbefales også i fremtidige prosjekter, deriblant oppfølgingsprosjektet MODISAFE (CRN modellingof sources and agent fate).

• Vik, Thomas & Reif, Bjørn Anders Pettersson (2010). Modeling the evaporation from a thin liquid surface beneath a turbulent boundary layer. Forsvarets forskningsinstitutt . ISSN 9788246418216. doi: 10.1016/j.ijthermalsci.2011.07.006. Full text in Research Archive Show summary

  Utslipp og spredning av giftige kjemikalier kan utgjøre en trussel mot militært personell og befolkningeni allmennhet. Modellering, simulering og analyse av hypotetiske scenarier og historiskehendelser en svært verdifull og mye brukt metode for å utvikle og implementere passende beskyttelseskapabiliteterog planlegge beskyttelses- og mottiltak. Dette krever at prosesser der giftigekjemikalier slippes ut og spres i luft kan modelleres og simuleres på en troverdig måte.En fysisk og matematisk modellering av en spredningshendelse kan grovt sett deles inn i tre deler:kildemodellering, transportmodellering og effektmodellering. Denne studien tar fokuserer på kildemodellering.I en NATO-studie nylig gjennomført, SAS-061, analyserte forskningsgrupper i USA, Nederland ogFFI hver for seg et scenario som involverte spredning av det kjemiske trusselstoffet sarin. De ulikegruppene benyttet fordampingsrater som varierte med en faktor 10. Dette medfører tilsvarende storevariasjoner i beregninger av konsekvenser og skadeomfang. Dette gir en uakseptabel usikkerhet ikonsekvensvurderingene og demonstrerer videre nødvendigheten av å forbedre vår fundamentalekunnskap om fordamingsprossesser.Dette arbeidet er en videreføring av tidligere arbeid på FFI. Odd Busmundrud utviklet en modellfor fordamping fra dråper og overflater. Hans modell baserer seg i hovedsak på molekylær diffusjongjennom et tenkt stillestående luftlag over overflaten, og tar kun indirekte hensyn til strømningstekniskeeffekter på fordampningsprosessen. Studien som presenteres i denne rapporten tar forseg fordampning av en nøytral kontaminant fra en tynn væskeoverflate under et turbulent grensesjikt.Analysen er basert på det turbulente hastighetsfeltet og fluktuasjoner nær en overflate. Måletmed studien er å utvikle og verifisere en algebraisk fordampningsmodell som er følsom for grensesjiktsturbulens.Avhengigheten av friksjonshastigheten vises å være naturlig inkludert i analysen,og modellen avhenger ikke av noen a priori antagelser om eksistensen av en logaritmisk grensesjiktetsregioni likevekt. Det turbulente hastighetsfeltet og fluktuasjonene nær vegg er forholdsvisuniverselle og dermed gyldig for et stort spekter av eksterne strømningsforhold. Modellen er validertved bruk av eksperimentelle data fra vindtunneleksperimenter. Arbeidet vil ble videreført ved åinkludere modellen i CFD-software.Det er svært viktig å ha tilgang til gode eksperimentelle data. For å undersøke påvirkningen avulike aspekter på fordampningen (termiske effekter, turbulens, væskeoverflatens størrelse osv), erdet nødvendig med gode målinger der de ulike parametrene varieres på en systematisk måte. Detvil være av stor nytte å selv kunne foreta slike målinger. Spesielt for skarpe stridsmidler er detvanskelig å få tak i gode eksperimentelle data.

• Gjesdal, Thor; Wasberg, Carl Erik & Reif, Bjørn Anders Pettersson (2004). Spectral element benchmark simulations of natural convection in two-dimensional cavities. Forsvarets forskningsinstitutt . ISSN 9788246408422. doi: 10.1002/%01d.1121. Full text in Research Archive Show summary

  This report presents the results of spectral element simulations of natural convection in two-dimensional cavities. Inparticular, a detailed comparison is performed with the reference data recently described by Christon et. al. [Int .J.Numer. Meth. Fluids, vol. 40, pp. 953-980, 2002]. The Navier-Stokes equations augmented by the Boussinesqapproximation to represent buoyancy effects are solved by a numerical method based on spectral element discretisationsand operator splitting. The computed results agree closely with the reference data for both the square and the rectangularcavity configurations.

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