Echim M.M.,Belgian Institute for Space Aeronomy |
Echim M.M.,Romanian Space Science Institute |
Lemaire J.,Belgian Institute for Space Aeronomy |
Lie-Svendsen O.,Norwegian Defense Research Establishment FFI |
Lie-Svendsen O.,University of Oslo
Surveys in Geophysics | Year: 2011
The paper reviews the main advantages and limitations of the kinetic exospheric and fluid models of the solar wind (SW). The general theoretical background is outlined: the Boltzmann and Fokker-Planck equations, the Liouville and Vlasov equations, the plasma transport equations derived from an "equation of change". The paper provides a brief history of the solar wind modeling. It discusses the hydrostatic model imagined by Chapman, the first supersonic hydrodynamic models published by Parker and the first generation subsonic kinetic model proposed by Chamberlain. It is shown that a correct estimation of the electric field, as in the second generation kinetic exospheric models developed by Lemaire and Scherer, provides a supersonic expansion of the corona, reconciling the hydrodynamic and the kinetic approach. The modern developments are also reviewed emphasizing the characteristics of several generations of kinetic exospheric and multi-fluid models. The third generation kinetic exospheric models consider kappa velocity distribution function (VDF) instead of a Maxwellian at the exobase and in addition they treat a non-monotonic variation of the electric potential with the radial distance; the fourth generation exospheric models include Coulomb collisions based on the Fokker-Planck collision term. Multi-fluid models of the solar wind provide a coarse grained description of the system and reproduce with success the spatio-temporal variation of SW macroscopic properties (density, bulk velocity). The main categories of multi-fluid SW models are reviewed: the 5-moment, or Euler, models, originally proposed by Parker to describe the supersonic SW expansion; the 8-moment and 16-moment fluid models, the gyrotropic approach with improved collision terms as well as the gyrotropic models based on observed VDFs. The outstanding problem of collisions, including the long range Coulomb encounters, is also discussed, both in the kinetic and multi-fluid context. Although for decades the two approaches have been seen as opposed, in this paper we emphasize their complementarity. The review of the kinetic and fluid models of the solar wind contributes also to a better evaluation of the open questions still existent in SW modeling and suggests possible future developments. © 2010 The Author(s).
Elboth T.,Fugro |
Pettersson Reif B.A.,Norwegian Defense Research Establishment FFI |
Andreassen O.,Norwegian Defense Research Establishment FFI |
Martell M.B.,University of Massachusetts Amherst
Geophysics | Year: 2012
This work investigates how a highly (super)hydrophobic surface can be used to reduce turbulence-generated drag and noise on a towed streamer cable. The work is done by analyzing full-scale drag and flow noise measurements taken on a commercial seismic streamer in combination with direct numerical simulations of turbulence-generated flow noise. The main findings are that viscous drag and flow noise can be significantly reduced on a seismic streamer that is coated to make the surface highly hydrophobic. In an ocean towing test, a 4% reduction of drag on a streamer section was measured. In a separate test on a commercial seismic vessel, a reduction in the flow noise level of nearly 50% (6 dB) for frequencies below 10 Hz was found. Based upon an analysis of numerical simulation data, it is suggested that the reduction in drag and noise can be attributed to a reduced level of shear stress and change in the kinematic structure of the turbulence, both of which occur in the immediate vicinity of the highly hydrophobic surface. © 2012 Society of Exploration Geophysicists.
Cure C.,University of St. Andrews |
Antunes R.,University of St. Andrews |
Samarra F.,University of St. Andrews |
Alves A.C.,University of St. Andrews |
And 4 more authors.
PLoS ONE | Year: 2012
In cetaceans' communities, interactions between individuals of different species are often observed in the wild. Yet, due to methodological and technical challenges very little is known about the mediation of these interactions and their effect on cetaceans' behavior. Killer whales (Orcinus orca) are a highly vocal species and can be both food competitors and potential predators of many other cetaceans. Thus, the interception of their vocalizations by unintended cetacean receivers may be particularly important in mediating interspecific interactions. To address this hypothesis, we conducted playbacks of killer whale vocalizations recorded during herring-feeding activity to free-ranging long-finned pilot whales (Globicephala melas). Using a multi-sensor tag, we were able to track the whales and to monitor changes of their movements and social behavior in response to the playbacks. We demonstrated that the playback of killer whale sounds to pilot whales induced a clear increase in group size and a strong attraction of the animals towards the sound source. These findings provide the first experimental evidence that the interception of heterospecific vocalizations can mediate interactions between different cetacean species in previously unrecognized ways. © 2012 Curé et al.
Guzman H.M.,Smithsonian Tropical Research Institute |
Gomez C.G.,Smithsonian Tropical Research Institute |
Guevara C.A.,Smithsonian Tropical Research Institute |
Kleivane L.,Norwegian Defense Research Establishment FFI
Marine Mammal Science | Year: 2013
Vessel collision is a threat to many whale species, and the risk has increased with expanding maritime traffic. This compromises international conservation efforts and requires urgent attention from the world's maritime industry. Humpback whales (Megaptera novaeangliae) are at the top of the death toll, and although Central America is a wintering area for populations from both the Northern and Southern Hemispheres, existing efforts to reduce ship-whale collisions are meager. Herein, we evaluated the potential collisions between vessels and humpback whales wintering off Pacific Panama by following the movements of 15 whales tagged with satellite transmitters and comparing these data with tracks plotted using AIS real-time latitude-longitude points from nearly 1,000 commercial vessels. Movements of whales (adults and calves) in the Gulf of Panama coincide with major commercial maritime routes. AIS vessel data analyzed for individual whale satellite tracks showed that 53% (8 whales) of whales had 98 encounters within 200 m with 81 different vessels in just 11 d. We suggest implementing a 65 nmi Traffic Separation Scheme and a 10 kn maximum speed for vessel routing into the Gulf of Panama during the wintering season. In so doing, the area for potential whale-vessel collisions could be reduced by 93%. © 2012 by the Society for Marine Mammalogy.
Haakestad M.W.,Norwegian Defense Research Establishment FFI |
Marandi A.,Stanford University |
Leindecker N.,Stanford University |
Vodopyanov K.L.,University of Central Florida
Laser and Photonics Reviews | Year: 2013
Five-cycle (50 fs) mid-IR pulses at 80-MHz repetition rate are produced using a degenerate (subharmonic) optical parametric oscillator (OPO), synchronously pumped by an ultrafast 1560-nm fiber laser. The effects of cavity dispersion and the length of a periodically poled lithium niobate (PPLN) gain element on the output spectrum and pulse duration are investigated by taking advantage of a very broad (∼ 1000 cm-1) gain bandwidth near the 3.1-μm OPO degeneracy point. A new method of assessing the total OPO group delay dispersion across its entire spectrum is proposed, based on measuring spectral signatures of trace amounts of molecular gases injected into the OPO cavity. Five-cycle (50 fs) mid-IR pulses at 80-MHz repetition rate are produced using a degenerate (subharmonic) optical parametric oscillator (OPO), synchronously pumped by an ultrafast 1560-nm fiber laser. The effects of cavity dispersion and the length of a periodically poled lithium niobate (PPLN) gain element on the output spectrum and pulse duration are investigated by taking advantage of a very broad (∼ 1000 cm-1) gain bandwidth near the 3.1-μm OPO degeneracy point. A new method of assessing the total OPO group delay dispersion across its entire spectrum is proposed, based on measuring spectral signatures of trace amounts of molecular gases injected into the OPO cavity. © 2013 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Linge S.O.,Telemark University College |
Linge S.O.,Simula Research Laboratory |
Mardal K.-A.,Simula Research Laboratory |
Helgeland A.,Simula Research Laboratory |
And 4 more authors.
Journal of Neurosurgery: Spine | Year: 2014
Object. The effect of craniovertebral decompression surgery on CSF flow dynamics in patients with Chiari malformation Type I (CM-I) has been incompletely characterized. The authors 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 CM-I subarachnoid spaces (both previously described) and in 3 models of CM-I post-craniovertebral decompressions. The 3 postoperative models were created from the CM model by virtually modifying the CM 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/min. 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 postoperative models than in the preoperative CM model. With the 2 larger surgical defects, peak systolic velocities were brought closer to normal model velocities (equal values at C-3 and C-4) than with the smallest surgical defect. 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. Pressure gradients along the spinal canal in the normal and CM models were nonlinear, with steeper gradients below C3-4 than above. The CM 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 CM model above C2-3. The most conservative decompression had lower pressure gradients than the normal model above C2-3. The two larger decompression defects had CSF pressure gradients below those in the normal model above C2-3. These 2 models had a less steep gradient above C-3 and a steeper gradient below.Conclusions. In computer simulations, craniovertebral surgical defects generally diminished CSF velocities and CSF pressures. (http://thejns.org/doi/abs/10.3171/2014.6.SPINE13950). © AANS, 2014.
Haakestad M.W.,Norwegian Defense Research Establishment FFI |
Fonnum H.,Norwegian Defense Research Establishment FFI |
Lippert E.,Norwegian Defense Research Establishment FFI
Optics Express | Year: 2014
Mid-infrared (3-5 μm) pulses with high energy are produced using nonlinear conversion in a ZnGeP2-based master oscillator-power amplifier, pumped by a Q-switched cryogenic Ho:YLF oscillator. The master oscillator is based on an optical parametric oscillator with a V-shaped 3-mirror ring resonator, and the power amplifier is based on optical parametric amplification in large-aperture ZnGeP2 crystals. Pulses with up to 212 mJ energy at 1 Hz repetition rate are obtained, with FWHM duration 15 ns and beam quality M2 = 3. © 2014 Optical Society of America.
Ellingsaeter B.,Norwegian Defense Research Establishment FFI |
Maseng T.,Norwegian Defense Research Establishment FFI
IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC | Year: 2011
In this paper we consider an OFDM multiuser system with K orthogonal subcarriers. We assume each user uses M-QAM modulation in each subcarrier. The modulation must be adaptive to cope with propagation changes and to meet the bit error rate requirement of the user. We maximize the sum of information bits per user selfishly, and compare the performance with other power allocation algorithms. Surprisingly, measuring rate by constellation size instead of Shannon's capacity yields a power allocation algorithm that differ widely from previously known power allocation algorithms, such as the iterative water filling algorithm (IWFA). This is because we only accept bit error rates better than 10-3 and the rate achieved with modulation can only be discrete values, whereas other algorithms based on Shannon's capacity can achieve any rate. © 2011 IEEE.
Van Rheenen A.D.,Norwegian Defense Research Establishment FFI |
Haakestad M.W.,Norwegian Defense Research Establishment FFI
International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz | Year: 2014
Spectral angle mapping is used to identify materials in Terahertz images. We test the performance of several spectral characteristics derived from the measured spectra. Robustness is studied by investigating the Receiver-Operating-Characteristics. © 2014 IEEE.
Hjelmervik K.T.,Norwegian Defense Research Establishment FFI |
Jensen J.K.,Norwegian Defense Research Establishment FFI |
Ostenstad P.,Norwegian Defense Research Establishment FFI |
Ommundsen A.,Norwegian Defense Research Establishment FFI
Ocean Dynamics | Year: 2012
Sonar performance modeling is crucial for submarine and anti-submarine operations. The validity of sonar performance models is generally limited by environmental uncertainty, and particularly uncertainty in the vertical sound speed profile (SSP). Rapid environmental assessment (REA) products, such as oceanographic surveys and ocean models may be used to reduce this uncertainty prior to sonar operations. Empirical orthogonal functions (EOF) applied on the SSPs inherently take into account the vertical gradients and therefore the acoustic properties. We present a method that employs EOFs and a grouping algorithm to divide a large group of SSPs from an ocean model simulation into smaller groups with similar SSP characteristics. Such groups are henceforth called acoustically stable groups. Each group represents a subset in space and time within the ocean model domain. Regions with low acoustic variability contain large and geographically contiguous acoustically stable groups. In contrast, small or fragmented acoustically stable groups are found in regions with high acoustic variability. The main output is a map of the group distribution. This is a REA product in itself, but the map may also be used as a planning aid for REA survey missions. © 2011 Springer Science+Business Media, LLC.