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http://hdl.handle.net/2117/1087
Fri, 18 Apr 2014 01:54:15 GMT
20140418T01:54:15Z
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Universitat Politècnica de Catalunya. Servei de Biblioteques i Documentació
no

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.
http://hdl.handle.net/2117/21331
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
no
Highdimensional dynamical systems, Numerical bifurcation analysis, Transitions in fluid flows
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
Mon, 31 Mar 2014 16:27:40 GMT
http://hdl.handle.net/2117/22461
20140331T16:27:40Z
Matulka, Anna Magdalena; Lopez GonzalezNieto, Pilar; Redondo Apraiz, José Manuel; Tarquis Alfonso, Ana Maria
no
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

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.
Mon, 31 Mar 2014 11:59:46 GMT
http://hdl.handle.net/2117/22447
20140331T11:59:46Z
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.
no
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.

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.
Fri, 28 Mar 2014 13:41:16 GMT
http://hdl.handle.net/2117/22429
20140328T13:41:16Z
Mercader Calvo, María Isabel; Sánchez Casals, Odalys de la Caridad; Batiste Boleda, Oriol
no
LOW PRANDTL NUMBER, NATURALCONVECTION, MOLTEN GALLIUM, HADLEY CIRCULATION, ASPECTRATIO, STABILITY, INSTABILITIES, ENCLOSURES, EQUATIONS, CAVITIES
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.

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.
Thu, 27 Mar 2014 16:17:45 GMT
http://hdl.handle.net/2117/22419
20140327T16:17:45Z
López Moscat, Juan Manuel; Marqués Truyol, Francisco
no
Oscillating sidewall
Shear waves
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.

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.
Wed, 26 Mar 2014 18:46:26 GMT
http://hdl.handle.net/2117/22400
20140326T18:46:26Z
Curbelo Hernández, Jezabel; López Moscat, Juan Manuel; Mancho Sánchez, Ana María; Marqués Truyol, Francisco
no
Large Prandtl number
Centrifugal effects
Thermal convection
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.

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.
Wed, 19 Mar 2014 12:33:29 GMT
http://hdl.handle.net/2117/22280
20140319T12:33:29Z
Mazón Bueso, Jordi; Pino González, David; Barriendos, Mariano
no
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.

Highthroughput microcapillary pump with efficient integrated low aspect ratio micropillars
http://hdl.handle.net/2117/21942
Title: Highthroughput microcapillary pump with efficient integrated low aspect ratio micropillars
Authors: Madadi, Hojjat; Casals Terré, Jasmina; Castilla López, Roberto; Sureda Anfres, Miquel
Abstract: Prediction and reduction of pressure drop and resistance flow in micropillar arrays are important for the design of microfluidic circuits used in different labonachip and biomedical applications. In this work, a diamond microchannelintegrated micropillar pump (dMIMP) with a resistance flow 35.5 % lower than a circularbased micropillar pump (cMIMP) has been developed via the optimization of the fluid dynamic behavior of different pillar shapes in a low aspect ratio (H/D ranged from 0.06 to 0.2) integrated pillar microchannel. The effect of different geometrical parameters (such as pillar shape and its distribution) has been considered to minimize the microchannel resistance flow. Sixmicrometerdepth polidimetilsiloxane (PDMS) channels have been fabricated using a modified soft lithography process, which prevents the PDMS deformation under highpressure operation. Flow through the fabricated samples has been numerically solved and experimentally measured, with an agreement higher than 90 %. The results have been used to validate the derived analytical formulation to determine the flow resistance in this type of channels, a fast approach to obtain the resistance flow in the design stage of microdevices. The analysis of the results indicates that, although porosity can be a determinant parameter to predict the resistance flow of MIMP, other geometrical parameters such as side distance between pillars and pillar shape play a major role in this scenario. Finally, a highthroughput optimized diamond MIMP pump has been designed, tested and validated as a capillary pump, showing that it can provide a flow rate 73 % higher than a circular MIMP pump.
Fri, 07 Mar 2014 15:45:30 GMT
http://hdl.handle.net/2117/21942
20140307T15:45:30Z
Madadi, Hojjat; Casals Terré, Jasmina; Castilla López, Roberto; Sureda Anfres, Miquel
no
Microchannelintegrated micropillars (MIMP), Microcapillar pump, Resistance flow, Pressure drop
Prediction and reduction of pressure drop and resistance flow in micropillar arrays are important for the design of microfluidic circuits used in different labonachip and biomedical applications. In this work, a diamond microchannelintegrated micropillar pump (dMIMP) with a resistance flow 35.5 % lower than a circularbased micropillar pump (cMIMP) has been developed via the optimization of the fluid dynamic behavior of different pillar shapes in a low aspect ratio (H/D ranged from 0.06 to 0.2) integrated pillar microchannel. The effect of different geometrical parameters (such as pillar shape and its distribution) has been considered to minimize the microchannel resistance flow. Sixmicrometerdepth polidimetilsiloxane (PDMS) channels have been fabricated using a modified soft lithography process, which prevents the PDMS deformation under highpressure operation. Flow through the fabricated samples has been numerically solved and experimentally measured, with an agreement higher than 90 %. The results have been used to validate the derived analytical formulation to determine the flow resistance in this type of channels, a fast approach to obtain the resistance flow in the design stage of microdevices. The analysis of the results indicates that, although porosity can be a determinant parameter to predict the resistance flow of MIMP, other geometrical parameters such as side distance between pillars and pillar shape play a major role in this scenario. Finally, a highthroughput optimized diamond MIMP pump has been designed, tested and validated as a capillary pump, showing that it can provide a flow rate 73 % higher than a circular MIMP pump.

The mechanism why colliders could create quasistable black holes
http://hdl.handle.net/2117/21838
Title: The mechanism why colliders could create quasistable black holes
Authors: Torres Herrera, Ramon; Fayos Vallés, Francisco; Lorente Espín, Oscar
Abstract: It has been postulated that black holes could be created in particle collisions within the range of the available energies for nowadays colliders (LHC). In this paper we analyze the evaporation of a type of black holes that are candidates for this specific behaviour, namely, small black holes on a brane in a world with large extradimensions. We examine their evolution under the assumption that energy conservation is satisfied during the process and compare it with the standard evaporation approach. We claim that, rather than undergoing a quick total evaporation, black holes become quasistable. We comment on the (absence of) implications for safety of this result. We also discuss how the presence of black holes together with the correctness of the energy conservation approach might be experimentally verified.
Tue, 04 Mar 2014 08:32:47 GMT
http://hdl.handle.net/2117/21838
20140304T08:32:47Z
Torres Herrera, Ramon; Fayos Vallés, Francisco; Lorente Espín, Oscar
no
Black holes, Colliders, LHC, Hawking radiation, Large extradimensions
It has been postulated that black holes could be created in particle collisions within the range of the available energies for nowadays colliders (LHC). In this paper we analyze the evaporation of a type of black holes that are candidates for this specific behaviour, namely, small black holes on a brane in a world with large extradimensions. We examine their evolution under the assumption that energy conservation is satisfied during the process and compare it with the standard evaporation approach. We claim that, rather than undergoing a quick total evaporation, black holes become quasistable. We comment on the (absence of) implications for safety of this result. We also discuss how the presence of black holes together with the correctness of the energy conservation approach might be experimentally verified.

On how energy conservation might prevent black hole explosions and violations of the third law
http://hdl.handle.net/2117/21837
Title: On how energy conservation might prevent black hole explosions and violations of the third law
Authors: Torres Herrera, Ramon; Fayos Vallés, Francisco
Abstract: We claim that imposing energy conservation to the emission of Hawking radiation and to the modeling of black hole (BH) evaporation might prevent BH explosions as well as violations of the third law of BH thermodynamics. This is specifically shown for the general class of spherically symmetric quantum BHs described by an effective quantum vacuum invariant under boosts in the radial direction. No assumptions are made with regard to the specific framework from which the quantum BHs are derived.
Tue, 04 Mar 2014 08:25:56 GMT
http://hdl.handle.net/2117/21837
20140304T08:25:56Z
Torres Herrera, Ramon; Fayos Vallés, Francisco
no
Black Holes, Hawking radiation, Black Hole Thermodynamics, Third Law
We claim that imposing energy conservation to the emission of Hawking radiation and to the modeling of black hole (BH) evaporation might prevent BH explosions as well as violations of the third law of BH thermodynamics. This is specifically shown for the general class of spherically symmetric quantum BHs described by an effective quantum vacuum invariant under boosts in the radial direction. No assumptions are made with regard to the specific framework from which the quantum BHs are derived.

A comparison of highorder time integrators for highly supercritical thermal convection in rotating spherical shells
http://hdl.handle.net/2117/21534
Title: A comparison of highorder time integrators for highly supercritical thermal convection in rotating spherical shells
Authors: Garcia Gonzalez, Ferran; Net Marcé, Marta; Sánchez Umbría, Juan
Abstract: The efficiency of implicit and semiimplicit time integration codes based on backward differentiation and extrapolation formulas for the solution of the threedimensional Boussinesq thermal convection equations in rotating spherical shells was studied in [5] at weakly supercritical Rayleigh numbers R, moderate (10−3) and low (10−4) Ekman numbers, E, and Prandtl number = 1. The results presented here extend the previous study and focus on the effect of and R by analyzing the efficiency of the methods for obtaining solutions at E = 10−4, = 0.1 and low and high supercritical R. In the first case (quasiperiodic solutions) the decrease of one order of magnitude does not change the results significantly. In the second case (spatiotemporal chaotic solutions) the differences in the behavior of the semiimplicit codes due to the different treatment of the Coriolis term disappear because the integration is dominated by the nonlinear terms. As in [5], high order methods, either with or without time step and order control, increase the efficiency of the time integrators and allow to obtain more accurate solutions.
Tue, 11 Feb 2014 18:15:31 GMT
http://hdl.handle.net/2117/21534
20140211T18:15:31Z
Garcia Gonzalez, Ferran; Net Marcé, Marta; Sánchez Umbría, Juan
no
Thermal convection
Rotating Spherical Shells
The efficiency of implicit and semiimplicit time integration codes based on backward differentiation and extrapolation formulas for the solution of the threedimensional Boussinesq thermal convection equations in rotating spherical shells was studied in [5] at weakly supercritical Rayleigh numbers R, moderate (10−3) and low (10−4) Ekman numbers, E, and Prandtl number = 1. The results presented here extend the previous study and focus on the effect of and R by analyzing the efficiency of the methods for obtaining solutions at E = 10−4, = 0.1 and low and high supercritical R. In the first case (quasiperiodic solutions) the decrease of one order of magnitude does not change the results significantly. In the second case (spatiotemporal chaotic solutions) the differences in the behavior of the semiimplicit codes due to the different treatment of the Coriolis term disappear because the integration is dominated by the nonlinear terms. As in [5], high order methods, either with or without time step and order control, increase the efficiency of the time integrators and allow to obtain more accurate solutions.

Exponential versus IMEX highorder time integrators for thermal convection in rotating spherical shells
http://hdl.handle.net/2117/21530
Title: Exponential versus IMEX highorder time integrators for thermal convection in rotating spherical shells
Authors: Garcia Gonzalez, Ferran; Bonaventura, Luca; Net Marcé, Marta; Sánchez Umbría, Juan
Abstract: We assess the accuracy and efficiency of several exponential time integration methods coupled to a spectral discretization of the threedimensional Boussinesq thermal convection equations in rotating spherical shells. Exponential methods are compared to implicit–explicit (IMEX) multistep methods already studied previously in [1]. The results of a wide range of numerical simulations highlight the superior accuracy of exponential methods for a given time step, especially when employed with large time steps and at low Ekman number. However, presently available implementations of exponential methods appear to be in general computationally more expensive than those of IMEX methods and further research is needed to reduce their computational cost per time step. A physically justified extrapolation argument suggests that some exponential methods could be the most efficient option for integrating flows near Earthʼs outer core conditions.
Tue, 11 Feb 2014 17:37:05 GMT
http://hdl.handle.net/2117/21530
20140211T17:37:05Z
Garcia Gonzalez, Ferran; Bonaventura, Luca; Net Marcé, Marta; Sánchez Umbría, Juan
no
Exponential integrators, Semiimplicit schemes, Geophysical modeling, Thermal convection, Low Ekman number flows
We assess the accuracy and efficiency of several exponential time integration methods coupled to a spectral discretization of the threedimensional Boussinesq thermal convection equations in rotating spherical shells. Exponential methods are compared to implicit–explicit (IMEX) multistep methods already studied previously in [1]. The results of a wide range of numerical simulations highlight the superior accuracy of exponential methods for a given time step, especially when employed with large time steps and at low Ekman number. However, presently available implementations of exponential methods appear to be in general computationally more expensive than those of IMEX methods and further research is needed to reduce their computational cost per time step. A physically justified extrapolation argument suggests that some exponential methods could be the most efficient option for integrating flows near Earthʼs outer core conditions.

Instability of plumes driven by localized heating
http://hdl.handle.net/2117/21254
Title: Instability of plumes driven by localized heating
Authors: López Moscat, Juan Manuel; Marqués Truyol, Francisco
Abstract: Plumes due to localized buoyancy sources are of wide interest owing to their prevalence in many situations. This study investigates the transition from laminar to turbulent dynamics. Several experiments have reported that this transition is sensitive to external perturbations. As such, a wellcontrolled setup has been chosen for our numerical study, consisting of a localized heat source at the bottom of an enclosed cylinder whose walls are all maintained at a fixed uniform temperature, except for the localized heat source. At moderate Rayleigh numbers Ra, the flow consists of a steady, axisymmetric purely poloidal plume. On increasing Ra, the flow undergoes a supercritical Hopf bifurcation to an axisymmetric ‘puffing’ plume, where a vortex ring is periodically emitted from the localized heater. At higher Ra, this state becomes unstable to a sequence of symmetrybreaking bifurcations, going through a quasiperiodic ‘fluttering’ stage where the axisymmetric rings are tilted, and other states in which the sequence of tilted rings interact with each other. The sequence of symmetrybreaking bifurcations in the transition to turbulence culminates in a torus breakup event in which all the spatial and spatiotemporal symmetries of the system are broken.
Thu, 16 Jan 2014 14:55:27 GMT
http://hdl.handle.net/2117/21254
20140116T14:55:27Z
López Moscat, Juan Manuel; Marqués Truyol, Francisco
no
bifurcation, nonlinear instability, plumes/thermals
Plumes due to localized buoyancy sources are of wide interest owing to their prevalence in many situations. This study investigates the transition from laminar to turbulent dynamics. Several experiments have reported that this transition is sensitive to external perturbations. As such, a wellcontrolled setup has been chosen for our numerical study, consisting of a localized heat source at the bottom of an enclosed cylinder whose walls are all maintained at a fixed uniform temperature, except for the localized heat source. At moderate Rayleigh numbers Ra, the flow consists of a steady, axisymmetric purely poloidal plume. On increasing Ra, the flow undergoes a supercritical Hopf bifurcation to an axisymmetric ‘puffing’ plume, where a vortex ring is periodically emitted from the localized heater. At higher Ra, this state becomes unstable to a sequence of symmetrybreaking bifurcations, going through a quasiperiodic ‘fluttering’ stage where the axisymmetric rings are tilted, and other states in which the sequence of tilted rings interact with each other. The sequence of symmetrybreaking bifurcations in the transition to turbulence culminates in a torus breakup event in which all the spatial and spatiotemporal symmetries of the system are broken.

A novel fabrication technique to minimize poly(dimethylsiloxane)microchannels deformation under highpressure operation
http://hdl.handle.net/2117/20745
Title: A novel fabrication technique to minimize poly(dimethylsiloxane)microchannels deformation under highpressure operation
Authors: Madadi, Hojjat; Mohammadi, Mahdi; Casals Terré, Jasmina; Castilla López, Roberto
Abstract: PDMS is one of the most common materials used for the flow delivery in the microfluidics chips, since it is clear, inert, nontoxic, and nonflammable. Its inexpensiveness, straightforward fabrication, and biological compatibility have made it a favorite material in the exploratory stages of the biomicrofluidic devices. If small footprint assays want to be performed while keeping the throughput, high pressurerated channels should be used, but PDMS flexibility causes an important issue since it can generate a large variation of microchannel geometry. In this work, a novel fabrication technique based on the prevention of PDMS deformation is developed. A photosensible thiolene resin (Norland Optical Adhesive 63, NOA 63) is used to create a rigid coating layer over the stiff PDMS micropillar array, which significantly reduces the pressureinduced shape changes. This method uses the exact same soft lithography manufacturing equipment. The verification of the presented technique was investigated experimentally and numerically and the manufactured samples showed a deformation 70% lower than PDMS conventional samples.
Mon, 25 Nov 2013 15:33:22 GMT
http://hdl.handle.net/2117/20745
20131125T15:33:22Z
Madadi, Hojjat; Mohammadi, Mahdi; Casals Terré, Jasmina; Castilla López, Roberto
no
Microchannelintegrated micropillars, Microfabrication technique, PDMS deformation
PDMS is one of the most common materials used for the flow delivery in the microfluidics chips, since it is clear, inert, nontoxic, and nonflammable. Its inexpensiveness, straightforward fabrication, and biological compatibility have made it a favorite material in the exploratory stages of the biomicrofluidic devices. If small footprint assays want to be performed while keeping the throughput, high pressurerated channels should be used, but PDMS flexibility causes an important issue since it can generate a large variation of microchannel geometry. In this work, a novel fabrication technique based on the prevention of PDMS deformation is developed. A photosensible thiolene resin (Norland Optical Adhesive 63, NOA 63) is used to create a rigid coating layer over the stiff PDMS micropillar array, which significantly reduces the pressureinduced shape changes. This method uses the exact same soft lithography manufacturing equipment. The verification of the presented technique was investigated experimentally and numerically and the manufactured samples showed a deformation 70% lower than PDMS conventional samples.

Analysis of turbulent exchange and coherent structures in the stable atmospheric boundary layer based on tower observations
http://hdl.handle.net/2117/20740
Title: Analysis of turbulent exchange and coherent structures in the stable atmospheric boundary layer based on tower observations
Authors: Ferreres Soler, Enriqueta; Soler Duffour, Maria Rosa; Terradellas, Enric
Abstract: Using data collected at the Spanish low troposphere research centre CIBA
(Centro de Investigación de la Baja Atmósfera) and at the Cabauw Experimental Site for
Atmospheric Research (CESAR) in the Netherlands, we analysed the most significant
features of different coherent structures occurring in the stable atmospheric boundary layer.
In particular, we used both the Reynolds and wavelet methods to analyse a solitary wave, a
gravity wave, a density current and a lowlevel jet. For each of these structures, we found
that wavelet analysis had the capacity to distinguish the different scales involved in these
events due to the different timing and heights of the thermal instabilities and downdrafts
associated with the disturbances. In addition, the wavelet method highlights the different
roles of turbulence and coherent structures in the transfer of heat, moisture and CO2 in the
nocturnal boundary layer.
Mon, 25 Nov 2013 15:19:45 GMT
http://hdl.handle.net/2117/20740
20131125T15:19:45Z
Ferreres Soler, Enriqueta; Soler Duffour, Maria Rosa; Terradellas, Enric
no
Coherent structures, Intermittent turbulence, Nocturnal boundary layer, Wavelet transform
Using data collected at the Spanish low troposphere research centre CIBA
(Centro de Investigación de la Baja Atmósfera) and at the Cabauw Experimental Site for
Atmospheric Research (CESAR) in the Netherlands, we analysed the most significant
features of different coherent structures occurring in the stable atmospheric boundary layer.
In particular, we used both the Reynolds and wavelet methods to analyse a solitary wave, a
gravity wave, a density current and a lowlevel jet. For each of these structures, we found
that wavelet analysis had the capacity to distinguish the different scales involved in these
events due to the different timing and heights of the thermal instabilities and downdrafts
associated with the disturbances. In addition, the wavelet method highlights the different
roles of turbulence and coherent structures in the transfer of heat, moisture and CO2 in the
nocturnal boundary layer.