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http://hdl.handle.net/2117/3965
20140417T06:56:31Z

Numerical bifurcation methods and their application to fluid dynamics: analysis beyond simulation
http://hdl.handle.net/2117/21331
Title: Numerical bifurcation methods and their application to fluid dynamics: analysis beyond simulation
Authors: Dijkstra, Hendrik; Wubs, Fred W.; Cliffe, Andrew K.; Doedel, Eusebius J.; Dragomirescu, Ioana Florica; Eckhardt, Bruno; Gelfgat, Alexander Yu; Hazel, Andrew L.; Lucarini, Valerio; Salinger, Andrew G.; Phipps, Erik T.; Sánchez Umbría, Juan; Schuttelaars, Henk M.; Tuckerman, Laurette S.; Thiele, Uwe
Abstract: We provide an overview of current techniques and typical applications of numerical bifurcation analysis in fluid dynamical problems. Many of these problems are characterized by highdimensional dynamical systems which undergo transitions as parameters are changed. The computation of the critical conditions associated with these transitions, popularly referred to as 'tipping points', is important for understanding the transition mechanisms. We describe the two basic classes of methods of numerical bifurcation analysis, which differ in the explicit or implicit use of the Jacobian matrix of the dynamical system. The numerical challenges involved in both methods are mentioned and possible solutions to current bottlenecks are given. To demonstrate that numerical bifurcation techniques are not restricted to relatively lowdimensional dynamical systems, we provide several examples of the application of the modern techniques to a diverse set of fluid mechanical problems.

On the entrainment coefficient in a forced plume: quantitative effects of source parameter
http://hdl.handle.net/2117/22461
Title: On the entrainment coefficient in a forced plume: quantitative effects of source parameter
Authors: Matulka, Anna Magdalena; Lopez GonzalezNieto, Pilar; Redondo Apraiz, José Manuel; Tarquis Alfonso, Ana Maria
Abstract: The behavior of a forced plume is mainly controlled by the source buoyancy and momentum fluxes and the efficiency of turbulent mixing between the plume and the ambient fluid (stratified or not). The interaction between the plume and the ambient fluid controls the plume dynamics and is usually represented by the entrainment coefficient aE. Commonly used onedimensional models incorporating a constant entrainment coefficient are fundamental and very useful for predictions in geophysical flows and industrial situations. Nevertheless, if the basic geometry of the flow changes, or the type of source or the environmental fluid conditions (e.g., level of turbulence, shear, ambient stratification, presence of internal waves), new models allowing for variable entrainment are necessary. The presented paper is an experimental study based on a set of turbulent plume experiments in a calm unstratified ambient fluid under different source conditions (represented by different buoyancy and momentum fluxes). The main result is that the entrainment coefficient is not a constant and clearly varies in time within the same plume independently of the buoyancy and the source position. This paper also analyzes the influence of the source conditions on the mentioned time evolution. The measured entrainment coefficient aE has considerable variability. It ranges between 0.26 and 0.9 for variable Atwood number experiments and between 0.16 and 0.55 for variable source position experiments. As is observed, values are greater than the traditional standard value of Morton et al. (1956) for plumes and jets, which is about 0.13
20140331T16:27:40Z

Transition of the stellar initial mass function explored using binary population synthesis
http://hdl.handle.net/2117/22447
Title: Transition of the stellar initial mass function explored using binary population synthesis
Authors: Suda, Takuma; Komiya, Yutaka; Yamada, Shimako; Katsuta, Yutaka; Aoki, Wako; Gil Pons, Pilar; Doherty, Carolyn L.; Campbell, Simon W.; Wood, Peter R.; Fujimoto, Masayuki Y.
Abstract: The stellar initial mass function (IMF) plays a crucial role in determining the number of surviving stars in galaxies, the chemical composition of the interstellar medium, and the distribution of light in galaxies. A key unsolved question is whether the IMF is universal in time and space. Here we use stateoftheart results of stellar evolution to show that the IMF of our Galaxy made a transition from an IMF dominated by massive stars to the presentday IMF at an early phase of the Galaxy formation. Updated results from stellar evolution in a wide range of metallicities have been implemented in a binary population synthesis code, and compared with the observations of carbonenhanced metalpoor (CEMP) stars in our Galaxy. We find that applying the presentday IMF to Galactic halo stars causes serious contradictions with four observable quantities connected with the evolution of AGB stars. Furthermore, a comparison between our calculations and the observations of CEMP stars may help us to constrain the transition metallicity for the IMF which we tentatively set at [Fe/H] = 2. A novelty of the current study is the inclusion of mass loss suppression in intermediatemass AGB stars at lowmetallicity. This significantly reduces the overproduction of nitrogenenhanced stars that was a major problem in using the highmass star dominated IMF in previous studies. Our results also demonstrate that the use of the present day IMF for all time in chemical evolution models results in the overproduction of Type I.5 supernovae. More data on stellar abundances will help to understand how the IMF has changed and what caused such a transition.
20140331T11:59:46Z

Secondary flows in a laterally heated horizontal cylinder
http://hdl.handle.net/2117/22429
Title: Secondary flows in a laterally heated horizontal cylinder
Authors: Mercader Calvo, María Isabel; Sánchez Casals, Odalys de la Caridad; Batiste Boleda, Oriol
Abstract: In this paper we study the problem of thermal convection in a laterally heated, finite, horizontal cylinder. We consider cylinders of moderate aspect ratio (height/diameter approximate to 2) containing a small Prandtl number fluid (sigma < 0.026) typical of molten metals and molten semiconductors. We use the NavierStokes and energy equations in the Boussinesq approximation to calculate numerically the basic steady states, analyze their linear stability, and compute some nonlinear secondary flows originated from the instabilities. All the calculated flows and the stability analysis are characterized by their symmetry properties. Due to the confined cylindrical geometry, presence of lateral walls and lids, all the flows are completely three dimensional even for the basic steady states. In the range of Prandtl numbers studied, we have identified four different types of instabilities, either oscillatory or stationary. The physical mechanisms, shear or buoyancy, of the corresponding flow transitions have been analyzed. As the value of the Prandtl number approaches sigma = 0.026 the scenario of bifurcations becomes more complicated due to the existence of two different stable basic states originated in a saddlenode bifurcation; a fact that had been overlooked in previous works. (C) 2014 AIP Publishing LLC.
20140328T13:41:16Z

Rapidly rotating cylinder flow with an oscillating sidewall
http://hdl.handle.net/2117/22419
Title: Rapidly rotating cylinder flow with an oscillating sidewall
Authors: López Moscat, Juan Manuel; Marqués Truyol, Francisco
Abstract: We present numerical simulations of a flow in a rapidly rotating cylinder subjected to a timeperiodic forcing via axial oscillations of the sidewall. When the axial oscillation frequency is less than twice the rotation frequency, inertial waves in the form of shear layers are present. For very fast rotations, these waves approach the form of the characteristics predicted from the linearized inviscid problem first studied by Lord Kelvin. The driving mechanism for the inertial waves is the oscillating Stokes layer on the sidewall and the corner discontinuities where the sidewall meets the top and bottom end walls. A detailed numerical and theoretical analysis of the internal shear layers is presented. The system is physically realizable, and attractive because of the robustness of the Stokes layer that drives the inertial waves but beyond that does not interfere with them. We show that the system loses stability to complicated threedimensional flow when the sidewall oscillation displacement amplitude is very large (of the order of the cylinder radius), but this is far removed from the displacement amplitudes of interest, and there is a large range of governing parameters which are physically realizable in experiments in which the inertial waves are robust. This is in contrast to many other physical realizations of inertial waves where the driving mechanisms tend to lead to instabilities and complicate the study of the waves. We have computed the response diagram of the system for a large range of forcing frequencies and compared the results with inviscid eigenmodes and ray tracing techniques.
20140327T16:17:45Z

Confined rotating convection with large Prandtl number: centrifugal effects on wall modes
http://hdl.handle.net/2117/22400
Title: Confined rotating convection with large Prandtl number: centrifugal effects on wall modes
Authors: Curbelo Hernández, Jezabel; López Moscat, Juan Manuel; Mancho Sánchez, Ana María; Marqués Truyol, Francisco
Abstract: Thermal convection in a rotating cylinder with a radiustoheight aspect ratio of G=4 for fluids with large Prandtl number is studied numerically. Centrifugal buoyancy effects are investigated in a regime where the Coriolis force is relatively large and the onset of thermal convection is in the socalled wall modes regime, where pairs of hot and cold thermal plumes ascend and descend in the cylinder sidewall boundary layer, forming an essentially onedimensional pattern characterized by the number of hot and cold plume pairs. In our numerical study, we use the physical parameters corresponding to aqueous mixtures of glycerine with mass concentration in the range of 60%90% glycerine and a Rayleigh number range that extends from the threshold for wall modes up to values where the bulk fluid region is also convecting. The study shows that for the range of Rayleigh numbers considered, the local variations in viscosity due to temperature variation in the flow are negligible. However, the mean viscosity, which varies faster than exponentially with variations in the percentage of glycerine, leads to a faster than exponential increase in the Froude number for a fixed Coriolis force, and hence an enhancement of the centrifugal buoyancy effects with significant dynamical consequences, which are detailed. © 2014 American Physical Society.
20140326T18:46:26Z

Fast Precise Point Positioning based on realtime ionospheric modelling
http://hdl.handle.net/2117/22338
Title: Fast Precise Point Positioning based on realtime ionospheric modelling
Authors: Juan Zornoza, José Miguel; Hernández Pajares, Manuel; Sanz Subirana, Jaume; Samson, Jaron; Tossaint, Michel Mathias Maria
Abstract: Summary of main results of new technique Fast Precise Point Positioning developed in the framework of the “Enhanced Precise Point Positioning (EPPP) GNSS multifrequency user algorithm” ESA funded project.
The precise ionospheric corrections facilitate the resolution of undifferenced carrier phase ambiguities, ambiguity validation and integrity monitoring.
The FPPP performance is shown in terms of accuracy, convergence time and integrity, with actual GPS and simulated Galileo data.
Very limited bandwidth requirements for future EPPP users (less than 300 bps for dualfrequency GPS data).
20140321T16:20:20Z

Fast Precise Point Positioning for decimetererrorlevel navigation for multi and singlefrequency users of Global Navigation Satellite Systems
http://hdl.handle.net/2117/22336
Title: Fast Precise Point Positioning for decimetererrorlevel navigation for multi and singlefrequency users of Global Navigation Satellite Systems
Authors: Juan Zornoza, José Miguel; Hernández Pajares, Manuel; Sanz Subirana, Jaume; Samson, J.; Tossaint, M.
Abstract: This manuscript summarizes the new algorithm of Fast Precise Point Positioning (FPPP) the developed during the projects "Enhanced PPP GNSS multifrequency user algorithm" nd “Precise Real Time Orbit Determination and Time synchronisation”, both funded by the European Space Agency (ESA). The main innovations achieved during the overall project comprise the application of precise ionospheric corrections to facilitate the fast resolution of undifferenced carrier phase ambiguities, ambiguity validation and integrity monitoring for both multi and singlefrequency users. Among the integrity, detailed in previous works, the performance of the FPPP algorithm in terms of improved accuracy and convergence time is demonstrated with actual GPS and simulated Galileo data. The 10centimeters error level realtime kinematic positioning can be achieved in few minutes for dualand singlefrequency users, almost instantaneous for threefrequency users (or once the tropospheric delay is well estimated in few minutes in cold start), and with very limited bandwidth requirements for the FPPP users (less than 300 bps for dualfrequency GPS).
20140321T16:01:06Z

ICASES scenarios Final Report
http://hdl.handle.net/2117/22332
Title: ICASES scenarios Final Report
Authors: Juan Zornoza, José Miguel; Sanz Subirana, Jaume
20140321T14:17:23Z

Shoreline sand waves along the catalan coast
http://hdl.handle.net/2117/22299
Title: Shoreline sand waves along the catalan coast
Authors: Falqués Serra, Albert; Caballería, Miquel; Ribas Prats, Francesca; Van den Berg, Niels
Abstract: The beach of Calella, north of Barcelona, in the Catalan coast, features a series of shoreline sand waves with wavelengths ranging from 700 to 1400 m that match with similar undulations in the 5 m bathymetric line. Historical satellite images from 2002 till 2010 show that these undulations slightly change in time. The wave climate on that stretch of the Catalan coast has a large proportion of waves from the ENE and from the SW, i.e., with high angles with respect to shore normal rending the shoreline potentially unstable. Here we show that those sand waves might be due to that instability. Model results, both Linear Stability Analysis and nonlinear time evolution, show that the shoreline is nearly at the threshold for instability and that the emergent wavelengths are roughly consistent with the observed ones.
20140319T17:41:51Z

Rapid and sudden advection of warm and dry air in the Mediterranean Basin
http://hdl.handle.net/2117/22280
Title: Rapid and sudden advection of warm and dry air in the Mediterranean Basin
Authors: Mazón Bueso, Jordi; Pino González, David; Barriendos, Mariano
Abstract: Rapid advection of extremely warm and dry air is studied during two events in the Mediterranean Basin. On 27 August 2010 a rapid advection of extremely warm and dry air affected the northeast Iberian Peninsula during a few hours. At the Barcelona city center, the temperature reached 39.3 ° C, which is the maximum temperature value recorded during 230 yr of daily data series. On 23 March 2008 a rapid increase of temperature and drop of relative humidity were recorded for a few hours in Heraklion (Crete). During the morning on that day, the recorded temperature reached 34 °C for several hours on the northern coastline of this island.
According to the World Meteorological Organization none of these events can be classified as a heat wave, which requires at least two days of abnormally high temperatures; neither are they a heat burst as defined by the American Meteorological Society, where abnormal temperatures take place during a few minutes. For this reason, we suggest naming this type of event flash heat.
By using data from automatic weather stations in the Barcelona and Heraklion area and WRF mesoscale numerical simulations, these events are analyzed. Additionally, the primary risks and possible impacts on several fields are presented.
20140319T12:33:29Z

Experimental study on interparticle acoustic forces
http://hdl.handle.net/2117/22246
Title: Experimental study on interparticle acoustic forces
Authors: Garcia Sabaté, Anna; González Cinca, Ricardo; Hoyos, Mauricio; Castro, Angelica
Abstract: A method for the experimental measurement of interparticle forces (secondary Bjerknes force) generated by the action of an acoustic field in a resonator microchannel is presented. The acoustic radiation force created by an ultrasonic standing wave moves suspended particles towards the pressure nodes and the acoustic pressure induces particle volume oscillations. Once particles are in the levitation plane, transverse and secondary Bjerknes forces become important. Experiments were carried out in a resonator filled with a suspension composed of water and latex particles of different size (5–15 lm) at different concentrations. Ultrasound was generated by means of a 2.5 MHz nominal frequency transducer. For the first time the acoustic force generated by oscillating
particles acting on other particles has been measured, and the critical interaction distance in various cases has been determined. Interparticle forces on the order of 10 14 N have been measured by
using this method
20140318T12:26:31Z

Solar EUV flux rate estimation during mid and strong flares from the ionospheric electron content response signature in GNSS observations
http://hdl.handle.net/2117/22153
Title: Solar EUV flux rate estimation during mid and strong flares from the ionospheric electron content response signature in GNSS observations
Authors: Hernández Pajares, Manuel; García Rigo, Alberto; Juan Zornoza, José Miguel; Sanz Subirana, Jaume; Monte Moreno, Enrique; Aragón Ángel, María Ángeles
Abstract: A simple and precise technique to measure the sudden Extreme Ultraviolet (EUV) radiation increase of the Sun, during mid and strong flares, has been formulated and demonstrated for the most active part of the last Solar Cycle. On the one hand, it is based on the short time scale of these events, which allows the validity of a simple global overionization model. And on the other hand on the prompt ionospheric response to the EUV ionization, which signature (in terms of the Global Navigation Satellite Systems GNSSSolar Flare Activity Indicator, GSFLAI) is accurately measured in realtime from the existing global networks of dualfrequency GNSS receivers, and with a time resolution higher that those of dedicated space probes. Moreover the sensitivity of this approach enables the detection of not only extreme Xclass flares, but also the detection of variations of one order of magnitude lower or even smaller (such as for Mclass flares): 100% successful detection for all the Xclass solar flares during 20002006 with registered location outside of the solar limb (i.e. detection of 94% of all of Xclass solarflares) and about 65% for Mclass ones, obtained with the associated SISTED detector. In summary, its full availability, continuity, high precision and integrity for mid and high solar flare effects on Ionosphere, can make GSFLAI an useful indicator of potential Space Weather activity for many users in radio propagation. These results, which have been recently published by the authors, are extended in this work up to one complete solar cycle, and using a lower sampling rate (30 seconds), demonstrating the better behaviour of this indirect solar EUV variability proxy (GSFLAI30), when comparisons are made with direct measurements from space probes, providing readings which can be affected by the late enhancement of particles.
20140317T13:19:16Z

Definition of an SBAS ionospheric activity indicator and its assessment over Europe and Africa During the last solar cycle
http://hdl.handle.net/2117/22085
Title: Definition of an SBAS ionospheric activity indicator and its assessment over Europe and Africa During the last solar cycle
Authors: Sanz Subirana, Jaume; Juan Zornoza, José Miguel; Hernández Pajares, Manuel; Prieto Cerdeira, Roberto; Schlueter, Stefan
Abstract: In this work, an ionospheric activity indicator is defined based in the “weighted” Along Arc TEC rate (AATR). It is shown that this indicator, which can be easily computed from the GPS carrier phases, is well correlated with the ionospheric activity and, unlike other global indicators linked to the geomagnetic activity, can be sensitive to regional behaviours of ionospheric activity
20140314T16:48:26Z

Characterization and modelling of the ionosphere for EGNOS development and qualification
http://hdl.handle.net/2117/22082
Title: Characterization and modelling of the ionosphere for EGNOS development and qualification
Authors: Schlüter, S.; Prieto Cerdeira, Roberto; Orús Pérez, Raul; Lam, J.P.; Juan Zornoza, José Miguel; Sanz Subirana, Jaume; Hernández Pajares, Manuel
Abstract: The availability of representative data samples, models and statistical information derived from the analysis of parameters that adequately describe the state of the ionosphere are key elements for the design, verification and qualification of EGNOS algorithms. As part of the ongoing evolutions of EGNOS V2 releases, and also in
support to the development of a future multiconstellation and dualfrequency SBAS (EGNOS V3), ESA has started a refinement process of the tools, data and precise definition of the ionospheric operational conditions. A particular driver for this process is the improvement of the availability performance of the current and future system especially under solar max and severe ionospheric conditions, without
degradation of integrity and accuracy performances. The paper provides an overview about the concept and the data and models used for EGNOS development and qualification. It outlines the approaches to generate reference models and describes the parameter used for the characterization of nominal and perturbed ionospheric conditions from an SBAS perspective.
20140314T15:53:04Z