DSpace Collection:
http://hdl.handle.net/2117/1087
20141221T16:52:02Z

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.

Pressure 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 TimeResolved 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 twodimensional 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 twodimensional 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.
20141219T16:57:53Z

Pattern 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 threedimensional 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 spatiotemporal 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 timedependent 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.
20141030T14:06:50Z

Observational 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.
20141006T08:16:38Z

Threedimensional instabilities in a discretely heated annular flow: onset of spatiotemporal complexity via defect dynamics
http://hdl.handle.net/2117/24190
Title: Threedimensional instabilities in a discretely heated annular flow: onset of spatiotemporal complexity via defect dynamics
Authors: Marqués Truyol, Francisco; Lopez, Juan M
Abstract: The transition to threedimensional 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 threedimensional 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 threedimensional states, solution branches with localized defects break parity and result in spatiotemporal dynamics. We have identified two such time dependent states. One is a limit cycle that while breaking spatial parity, retains spatiotemporal parity. The other branch corresponds to quasiperiodic states that have globally broken parity. (C) 2014 AIP Publishing LLC.
20140930T17:35:55Z

Evolution and CNO yields of Z = 105 stars and possible effects on carbonenhanced metalpoor production
http://hdl.handle.net/2117/24107
Title: Evolution and CNO yields of Z = 105 stars and possible effects on carbonenhanced metalpoor 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 intermediatemass, extremely metalpoor (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 = 105 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 dredgeup process, the dredgeout phenomenon, or the third dredgeup 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 = 105 stars. However, we find that the reduction of only one order of magnitude in the massloss 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 electroncapture 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 carbonenhanced EMP star problem.
20140918T17:16:10Z

La 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; RuizBellet, 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 again
20140918T16:11:31Z

Streamwiselocalized solutions at the onset of turbulence in pipe flow
http://hdl.handle.net/2117/24091
Title: Streamwiselocalized 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.
20140917T16:53:27Z

Subcritical equilibria in TaylorCouette flow
http://hdl.handle.net/2117/23990
Title: Subcritical equilibria in TaylorCouette 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 TaylorCouette 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.
20140905T15:22:03Z

Role of the residual layer and largescale subsidence on the development and evolution of the convective boundary layer
http://hdl.handle.net/2117/23501
Title: Role of the residual layer and largescale 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, mixedlayer theory and the Dutch LargeEddy 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 largescale 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 boundarylayer 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.; Largescale 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.
20140714T12:05:35Z

Effects of sea level rise on the formation and drowning of shorefaceconnected sand ridges, a model study
http://hdl.handle.net/2117/23397
Title: Effects of sea level rise on the formation and drowning of shorefaceconnected sand ridges, a model study
Authors: Nnafie, Abdel; Swart, Huib E. de; Calvete Manrique, Daniel; Garnier, Roland
Abstract: Shorefaceconnected sand ridges occur on many stormdominated inner shelves. These rhythmic features have an alongshelf spacing of 210 km, a height of 112 m, they evolve on timescales of centuries and they migrate several meters per year. An idealized model is used to study the impact of sea level rise on the characteristics of the sand ridges during their initial and longterm evolution. Different scenarios (rates of sea level rise, geometry of inner shelf) are examined. Results show that with increasing sea level the height of sand ridges increases and their migration decreases until they eventually drown. This latter occurs when the nearbed wave orbital velocity drops below the critical velocity for erosion of sediment. In contrast, in the absence of sea level rise, the model simulates shorefaceconnected sand ridges with constant heights and migration rates. Model results furthermore indicate that sand ridges do not form if the rate of sea level rise is too high, or if the initial depth of the inner shelf is too small. A larger transverse bottom slope enhances growth and height of sand ridges and they drown quicker. When shoreface retreat due to sea level rise is considered, new ridges form in the landward part of the inner shelf, while ridges on the antecedent part of the shelf become less active and ultimately drown. Only if sea level rise is accounted for, merging of ridges is reduced such that multiple ridges occur in the end state, thereby yielding a better agreement with observations. The physical mechanisms responsible for these findings are also explained. (C) 2014 Elsevier Ltd. All rights reserved.
20140704T07:11:37Z

Application of multifractal analysis to the study of SAR features and oil spills on the ocean surface
http://hdl.handle.net/2117/23257
Title: Application of multifractal analysis to the study of SAR features and oil spills on the ocean surface
Authors: Tarquis Alfonso, Ana Maria; Platonov, A.; Matulka, Anna Magdalena; Grau, J.; Sekula, Emil; Diez, M.; Redondo Apraiz, José Manuel
Abstract: The use of synthetic aperture radar (SAR) to investigate the ocean surface provides a wealth of useful information that is very seldom used to its full potential. Here we will discuss the application of multifractal techniques to detect oil spills and the dynamic state of the sea regarding turbulent diffusion. We present different techniques in order to relate the shape of the multifractal spectral functions and the maximum fractal dimension to the behaviour of the ocean surface. We compare eddy and sheared dominated flows with convective driven flows and discuss the different features and observation methods. We also compare the scaling of different oil spills detected by means of SAR images. Recent spills and weathered ones are selected and compared to investigate their behaviour in different spatial and temporal ranges. We calculate the partition function based on the grey intensity value of each SAR pixel deriving several types of multifractal spectra as a function of spill residence time estimated for each image. Image manipulations are seen to reduce the speckle noise and thus distinguish much better the texture of the oil spill images. The results are used to discuss how eddy diffusivity may be estimated and used in a description of the ocean surface using a simple turbulence kinematic simulation model to predict the shape of oil spills. Differences in the multifractal spectrum among SAR images may detect the slicks due to plankton and also provide information on the age of the oil spills, on the Lagrangian turbulent structure and on ocean surface diffusivity.
20140618T10:49:17Z

Modeling the response of shorefaceconnected sand ridges to sand extraction on an inner shelf
http://hdl.handle.net/2117/23249
Title: Modeling the response of shorefaceconnected sand ridges to sand extraction on an inner shelf
Authors: Nnafie, abdel; Swart, Huib E. de; Calvete Manrique, Daniel; Garnier, Roland
Abstract: Shorefaceconnected sand ridges are rhythmic bedforms that occur on many stormdominated inner shelves. The ridges span several kilometers, are a few meters high, and they evolve on a timescale of centuries. A processbased model is used to gain a fundamental insight into the response of these ridges to extraction of sand. Different scenarios of sand extraction (depth, location, and geometry of the extraction area; multiple sand extractions) are imposed. For each scenario, the response timescale as well as the characteristics of the new equilibrium state are determined. Results show that ridges partially restore after extraction, i.e., the disturbed bathymetry recovers on decadal timescales. However, in the end, the ridge original sand volume is not recovered. Initially, most sand that accomplishes the infill of the pit originates from the area upstream of the extraction, as well as from the areas surrounding the pit. The contribution of the latter strongly decreases in the subsequent time period. Depending on the location of the pit, additional sand sources contribute: First, if the pit is located close to the downstream trough, the pit gains sand by reduction of sand transport from the ridge to this trough. Second, if the pit is located close to the adjacent outer shelf, the ridge recovery is stronger due to an import of sand from that area. Furthermore, pits that are located close to the nearshore zone have a weak recovery, deeper pits have longer recovery timescales, wide and shallow pits recover most sand, while multiple sand pits slow down the recovery process.
20140617T13:41:00Z

Singularity free gravitational collapse in an effective dynamical quantum spacetime
http://hdl.handle.net/2117/23149
Title: Singularity free gravitational collapse in an effective dynamical quantum spacetime
Authors: Torres Herrera, Ramon; Fayos Vallés, Francisco
Abstract: We model the gravitational collapse of heavy massive shells including its main quantum corrections. Among these corrections, quantum improvements coming from Quantum Einstein Gravity are taken into account, which provides us with an effective quantum spacetime. Likewise, we consider dynamical Hawking radiation by modeling its backreaction once the horizons have been generated. Our results point towards a picture of gravitational collapse in which the collapsing shell reaches a minimum nonzero radius (whose value depends on the shell initial conditions) with its mass only slightly reduced. Then, there is always a rebound after which most (or all) of the mass evaporates in the form of Hawking radiation. Since the mass never concentrates in a single point, no singularity appears.
20140604T07:25:43Z

Numerical simulations of thermal convection in rotating spherical shells under laboratory conditions
http://hdl.handle.net/2117/23064
Title: Numerical simulations of thermal convection in rotating spherical shells under laboratory conditions
Authors: García González, Fernando; Sánchez Umbría, Juan; Net Marcé, Marta
Abstract: An exhaustive study, based on numerical threedimensional simulations, of the Boussinesq thermal convection of a fluid confined in a rotating spherical shell is presented. A moderately low Prandtl number fluid (ro = 0.1) bounded by differentiallyheated solid spherical shells is mainly considered. Asymptotic power laws for the mean physical properties of the flows are obtained in the limit of low Rossby number and compared with laboratory experiments and with previous numerical results computed by taking either stressfree boundary conditions or quasigeostrophic restrictions, and with geodynamo models. Finally, using parameters as close as possible to those of the Earth's outer core, some estimations of the characteristic time and length scales of convection are given. © 2014 Elsevier B.V.
20140527T17:22:19Z