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http://hdl.handle.net/2117/1085
20140917T07:57:03Z

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.

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 Charles
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 Charles
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

How kilometric sandy shoreline undulations correlate with wave and morphology characteristics: preliminary analysis on the Atlantic coast of Africa
http://hdl.handle.net/2117/22793
Title: How kilometric sandy shoreline undulations correlate with wave and morphology characteristics: preliminary analysis on the Atlantic coast of Africa
Authors: Idier, Deborah; Falqués Serra, Albert
Abstract: Sandy coasts are characterized by a number of rhythmic patterns like, amongst others, shoreline undulations or sandwaves at a kilometric scale. One hypothesis for their formation is that high angle waves (large incidence angle with respect to shore normal) could induce an instability of the shoreline (Ashton et al., 2001). More recently, a scaling for their wavelength has also been proposed (van den Berg et al., 2014). The existing studies rely mainly on modelling but quantitative field tests are lacking. We aim at investigating how both the formation hypothesis of these shoreline undulations and the theoretical scaling do fit with nature at a global scale. The first step, which is the goal of this paper, is to set up the methodology by analyzing the Atlantic African coast as test site. First, based on global databases, shoreline wavelength LS, wave characteristics (obliquity ¿W and wavelength ¿W) and mean shoreface slope ß are determined. Then the wave obliquity is confronted with the presence of shoreline undulations. Finally the values of the ratio ßLS / ¿W are estimated and discussed in comparison with the estimate of van den Berg et al. (2014). It is found that the correlation between shoreline sandwave occurrence and wave obliquity is very good, allowing the identification of 5 new potential unstable shoreline stretches, whereas the results on the scaling are not conclusive and deserve further investigations.
20140430T15:18:33Z

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

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