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http://hdl.handle.net/2117/1085
2015-01-30T20:20:48ZNumerical 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 high-dimensional 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 low-dimensional dynamical systems, we provide several examples of the application of the modern techniques to a diverse set of fluid mechanical problems.Singularity-free gravitational collapse and asymptotic safety
http://hdl.handle.net/2117/25170
Title: Singularity-free gravitational collapse and asymptotic safety
Authors: Torres Herrera, Ramon
Abstract: A general class of quantum improved stellar models with interiors composed of non-interacting (dust) particles is obtained and analyzed in a framework compatible with asymptotic safety. First, the effective exterior, based on the Quantum Einstein Gravity approach to asymptotic safety is presented and, second, its effective compatible dust interiors are deduced. The resulting stellar models appear to be devoid of shell-focusing singularities. (C) 2014 The Author. Published by Elsevier B.V.2015-01-08T11:26:16ZPressure effects on the performance of external gear pumps under cavitation
http://hdl.handle.net/2117/25113
Title: Pressure effects on the performance of external gear pumps under cavitation
Authors: Campo Sud, David del; Castilla López, Roberto; Raush Alviach, Gustavo Adolfo; Gámez Montero, Pedro Javier; Codina Macià, Esteban
Abstract: The numerical analysis of an external gear pump with cavitation effects has been validated with experimental data obtained by applying Time-Resolved Particle Image Velocimetry. The effect of inlet and outlet pressure on volumetric efficiency has been studied numerically. First, the Particle Image Velocimetry method was used to analyze the two-dimensional velocity field in the middle plane of the suction chamber of the gear pump. The main improvement, with respect to previous similar analysis is the use of alginate micro particles as tracers. It is seen that the two-dimensional model is able to characterize the flow field of the real pump in the region of the inlet chamber in which cavitation is expected. In a previous study, it was seen that a cavitation cloud acted as a virtual contact point at low pressure, being responsible for an increase on the volumetric efficiency. The first set of simulations represents the pump working with high outlet pressure. Now, the cavitation cloud is not present and cavitation no longer helps to improve the efficiency of the pump. The second set of simulations represents the pump with an inlet loss factor, which implies a mean inlet pressure below atmospheric conditions. This allows cavitation clouds to propagate upstream. Despite the larger cavitation clouds, volumetric efficiency only drops at high operating velocities, when some clouds become trapped between gears and casing and are transported to the pressure side.2014-12-19T16:57:53ZNon-linear fractal percolation experiments in carstic and aggregation soils
http://hdl.handle.net/2117/25088
Title: Non-linear fractal percolation experiments in carstic and aggregation soils
Authors: Chierubini, Claudia; Tarquis Alfonso, Ana Maria; Redondo Apraiz, José Manuel; Pastore, Nicola
Abstract: The present study compares the performances and reliabilities of the classical Mobile – Immobile Model (MIM)
and the Explicit Network Model (ENM) that takes expressly into account the network geometry for describing
tracer transport behaviour in a fractured sample at bench scale. Though ENM shows better fitting results than
MIM, the latter remains still valid as it proves to describe the observed curves quite well.
The results show that the presence of nonlinear flow plays an important role in the behaviour of solute
transport. Firstly the distribution of solute according to different pathways is not constant but it is related to the
flow rate. Secondly nonlinear flow influences advection, in that it leads to a delay in solute transport respect to the
linear flow assumption. Whereas nonlinear flow does not show to be related with dispersion. The experimental
results show that in the study case the geometrical dispersion dominates the Taylor dispersion. However the
interpretation with the ENM model shows a weak transitional regime from geometrical dispersion to Taylor
dispersion for high flow rates.2014-12-18T17:05:57ZSediment and plankton lift off recirculations in strong synthetic turbulence (KS)
http://hdl.handle.net/2117/25087
Title: Sediment and plankton lift off recirculations in strong synthetic turbulence (KS)
Authors: Redondo Apraiz, José Manuel; Castilla López, Roberto; Sekula, Emil; Furmanek, Petr
Abstract: The study of particle diffusion and of turbulent sedimentation is of great importance in many geophysical fields, such as in Environmental Science or Oceanography as well as in Bio-environmental and industrial processes. For a long time, the study of diffusion was numerically computed with random free paths, which gives Brownian behavior.
These stochastics methods have the objection that do not take into account the flow profile. On the other hand, there are many ways to simulate a fluid flow, but when this is turbulent our aim is to simulate the behaviour of neutral or heavy and inertial particles of biological or geological nature in a turbulent flow, in a simple way with a kinematically simulated model and to validate the results. We use the Kinematic Simulation (KS) model, also known as Synthetic Turbulence to model several environmental situations, both in the Atmosphere and the Ocean.2014-12-18T16:57:16ZThe role of vorticity in maximal swimming propulsion
http://hdl.handle.net/2117/25086
Title: The role of vorticity in maximal swimming propulsion
Authors: Redondo Apraiz, José Manuel; Arellano, Raul; Carrillo Cortés, José Alejandro; Terrés Nicoli, José María
Abstract: Experimental and theoretical results on the propulsion of swimrners are presented with an
emphasis on the specific characteristics of the wakes produced by the swimmers in the
water. We introduce the subject revising sorne of the previous work on the fruitless
controversy between Drag and Lift cornponents of the forces induced by swirnmers hands.
After presenting sorne experimental results on Underwater Oscillatory Swimrning we
discuss in detail the complex nature of the swimrners body, feet and hand wakes and show
the role of vortices. Severa! basic recornmendations rnay be deduced comparing animal and
human swimming. One is the need to reduce Strouhal numbers to increase propulsion, the
other one is to generate wakes that maximize momenturn and rninimize energy, this is
di scussed in terms of the fluid topological properties of basic vortical flow configurations.2014-12-18T16:35:59ZPattern selection near the onset of convection in binary mixtures in cylindrical cells
http://hdl.handle.net/2117/24514
Title: Pattern selection near the onset of convection in binary mixtures in cylindrical cells
Authors: Alonso Maleta, Arantxa; Mercader Calvo, María Isabel; Batiste Boleda, Oriol
Abstract: We report numerical investigations of three-dimensional pattern formation of binary mixtures in a vertical cylindrical container heated from below. Negative separation ratio mixtures, for which the onset of convection occurs via a subcritical Hopf bifurcation, are considered. We focus on the dynamics in the neighbourhood of the initial oscillatory instability and analyze the spatio-temporal properties of the patterns for different values of the aspect ratio of the cell, 0.25 less than or similar to Gamma less than or similar to 11 (Gamma equivalent to R/d, where R is the radius of the cell and d its height). Despite the oscillatory nature of the primary instability, for highly constrained geometries, Gamma less than or similar to 2.5, only pure thermal stationary modes are selected after long transients. As the aspect ratio of the cell increases, for intermediate aspect ratio cells such as Gamma = 3, multistability and coexistence of stationary and time-dependent patterns is observed. In highly extended cylinders, Gamma approximate to 11, the dynamics near the onset is completely different from the pure fluid case, and a startling diversity of confined patterns is observed. Many of these patterns are consistent with experimental observations. Remarkably, though, we have obtained persistent large amplitude highly localized states not reported previously.2014-10-30T14:06:50ZTurbulent diffusion and mixing in non-homogeneous environments
http://hdl.handle.net/2117/24385
Title: Turbulent diffusion and mixing in non-homogeneous environments
Authors: Redondo Apraiz, José Manuel; Díez Rilova, Margarita; Sekula, Emil; Mahjoub, Otman
Abstract: Many experimental and field studies have been devoted to the understanding of non-homogeneous turbulent dynamics. Activity in this area intensified when the basic Kolmogorov self-similar theory was extended to two-dimensional
or quasi 2D turbulent flows such as those appearing in the environment, that seem to control mixing [1,2]. The statistical description and the dynamics of these geophysical flows depend strongly on the distribution of long lived organized (coherent) structures. These flows show a complex topology, but may be subdivided in terms of strongly elliptical domains (high vorticity regions), strong hyperbolic domains (deformation cells with high energy condensations) and the background turbulent field of moderate elliptic and hyperbolic characteristics. It is of fundamental importance to investigate the different influence of these topological diverse regions.2014-10-16T15:44:43ZObservational and Numerical Simulation Study of a Sequence of Eight Atmospheric Density Currents in Northern Spain
http://hdl.handle.net/2117/24264
Title: Observational and Numerical Simulation Study of a Sequence of Eight Atmospheric Density Currents in Northern Spain
Authors: Soler Duffour, Maria Rosa; Udina, Mireia; Ferreres Soler, Enriqueta
Abstract: A sequence of eight atmospheric density current fronts occurred in consecutive days are identified and analyzed using micrometeorological time series and numerical simulations. Observations were collected in the context of the INTERCLE project, which took place from September 2002 to November 2003 at the CIBA (Research Centre for the Lower Atmosphere) site located over the northern Spanish plateau. Numerical simulations used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km). Both observations and simulations agree that the arrival of the density currents are characterized by a sharp change in temperature, wind velocity, wind direction and specific humidity and a source of intermittent turbulence. However, comparison between model and observations shows that the model predicts the intrusion of the density currents earlier than is observed. In addition, wavelet techniques applied to the data help distinguish the different scales present in the events, and therefore can reveal traces of gravity waves induced by the arrival of the density currents.2014-10-06T08:16:38ZThree-dimensional instabilities in a discretely heated annular flow: onset of spatio-temporal complexity via defect dynamics
http://hdl.handle.net/2117/24190
Title: Three-dimensional instabilities in a discretely heated annular flow: onset of spatio-temporal complexity via defect dynamics
Authors: Marqués Truyol, Francisco; Lopez, Juan M
Abstract: The transition to three-dimensional and unsteady flow in an annulus with a discrete heat source on the inner cylinder is studied numerically. For large applied heat flux through the heater (large Grashof number Gr), there is a strong wall plume originating at the heater that reaches the top and forms a large scale axisymmetric wavy structure along the top. For Gr approximate to 6 x 109, this wavy structure becomes unstable to three-dimensional instabilities with high azimuthal wavenumbers m similar to 30, influenced by mode competition within an Eckhaus band of wavenumbers. Coexisting with some of these steady three-dimensional states, solution branches with localized defects break parity and result in spatio-temporal dynamics. We have identified two such time dependent states. One is a limit cycle that while breaking spatial parity, retains spatio-temporal parity. The other branch corresponds to quasi-periodic states that have globally broken parity. (C) 2014 AIP Publishing LLC.2014-09-30T17:35:55ZEvolution and CNO yields of Z = 10-5 stars and possible effects on carbon-enhanced metal-poor production
http://hdl.handle.net/2117/24107
Title: Evolution and CNO yields of Z = 10-5 stars and possible effects on carbon-enhanced metal-poor production
Authors: Gil Pons, Pilar; Doherty, Carolyn L.; Lau, Herbert B.; Campbell, Simon W; Suda, Takuma; Guilani, Shervin Mansouri; Gutierrez Cabello, Jorge Luis; Lattanzio, John C.
Abstract: Aims. Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate the C, N, and O yields of these stars.
Methods. Using the Monash University Stellar Evolution code MONSTAR we computed and analysed the evolution of stars of metallicity Z = 10-5 and masses between 4 and 9 M¿, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase.
Results. Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up process, the dredge-out phenomenon, or the third dredge-up early during the TP-(S)AGB phase. Their late evolution is therefore similar to that of higher metallicity objects. When using a standard prescription for the mass loss rates during the TP-(S)AGB phase, the computed stars are able to lose most of their envelopes before their cores reach the Chandrasekhar mass (mCh), so our standard models do not predict the occurrence of SNI1/2 for Z = 10-5 stars. However, we find that the reduction of only one order of magnitude in the mass-loss rates, which are particularly uncertain at this metallicity, would prevent the complete ejection of the envelope, allowing the stars to either explode as an SNI1/2 or become an electron-capture SN. Our calculations stop due to an instability near the base of the convective envelope that hampers further convergence and leaves remnant envelope masses between 0.25 M¿ for our 4 M¿ model and 1.5 M¿ for our 9 M¿ model. We present two sets of C, N, and O yields derived from our full calculations and computed under two different assumptions, namely, that the instability causes a practically instant loss of the remnant envelope or that the stars recover and proceed with further thermal pulses.
Conclusions. Our results have implications for the early chemical evolution of the Universe and might provide another piece for the puzzle of the carbon-enhanced EMP star problem.2014-09-18T17:16:10ZLa rubinada de Santa Tecla a Tàrrega (23 de Setembre de 1874)
http://hdl.handle.net/2117/24102
Title: La rubinada de Santa Tecla a Tàrrega (23 de Setembre de 1874)
Authors: Barriendos Valve, Mariano; Tuset Mestre, Jordi; Mazón Bueso, Jordi; Pino González, David; Ruiz-Bellet, Josep Lluis; Balasch Solanes, Josep Carles
Abstract: A partir de la recopilación histórica de los datos documentales disponibles y aplicando una metodología multidisci-
plinar con modelos de simulación hidráulica, hidrológica y meteorológica, se han reconstruido las características prin-
cipales de la devastadora crecida hidrológica (o
rubinada
de Santa Tecla) que padeció la ciudad de Tàrrega en la
madrugada del 23 de septiembre de 1874, y se describe el contexto meteorológico que condicionó la tormenta. El
caudal punta fue excepcional, causando una de las mayores mortandades de la historia de Tàrrega. Las circunstan-
cias actuales no han cambiado, lo que no permite pensar que una avenida similar no pueda volver a ocurrir.
The main characteristics of the Santa Tecla’s flash flood that took place in Tàrrega the 23rd of September of 1874 are
reconstructed in this paper. For this, we use historical data from available contemporary documents and apply a mul-
tidisciplinary approach based on hydraulic, hydrological and meteorological simulation models. The meteorological
conditions that determined the storm are also described. The peak discharge was the overriding cause of one of the
major human tragedies in the history of Tàrrega. Since the circumstances have not changed, rather the opposite, such
a flood is likely to occur again2014-09-18T16:11:31ZStreamwise-localized solutions at the onset of turbulence in pipe flow
http://hdl.handle.net/2117/24091
Title: Streamwise-localized solutions at the onset of turbulence in pipe flow
Authors: Àvila Cañellas, Marc; Mellibovsky Elstein, Fernando; Roland, N.; Hof, Björn
Abstract: Although the equations governing fluid flow are well known, there are no analytical expressions that describe the complexity of turbulent motion. A recent proposition is that in analogy to low dimensional chaotic systems, turbulence is organized around unstable solutions of the governing equations which provide the building blocks of the disordered dynamics. We report the discovery of periodic solutions which just like intermittent turbulence are spatially localized and show that turbulent transients arise from one such solution branch. 2013 American Physical Society.2014-09-17T16:53:27ZSubcritical equilibria in Taylor-Couette flow
http://hdl.handle.net/2117/23990
Title: Subcritical equilibria in Taylor-Couette flow
Authors: Deguchi, Kengo; Meseguer Serrano, Álvaro; Mellibovsky Elstein, Fernando
Abstract: Nonlinear equilibrium states characterized by strongly localized vortex pairs are calculated in the linearly stable parameter region of counterrotating Taylor-Couette flow. These subcritical states are rotating waves whose region of existence is consistent with the critical threshold for relaminarization observed in experiments. For sufficiently rapid outer cylinder rotation the solutions extend beyond the static inner cylinder case to corotation, thus exceeding, for the first time, the boundary defined by the inviscid Rayleigh's stability criterion.2014-09-05T15:22:03ZRole of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer
http://hdl.handle.net/2117/23501
Title: Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer
Authors: Blay Carreras, Estel; Pino González, David; Vilà Guerau de Arellano, Jordi; van de Boer, Anneke; de Coster, Olivier; Darbieu, Clara; Hartogensis, Oskar; Lohou, Fabienne; Lothon, Marie; Pietersen, Henk
Abstract: Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations.; The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget.; Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations.; Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.2014-07-14T12:05:35Z