Articles de revista
http://hdl.handle.net/2117/3966
2024-03-28T14:15:15ZBehavior of NiTi wires for dampers and actuators in extreme conditions
http://hdl.handle.net/2117/82959
Behavior of NiTi wires for dampers and actuators in extreme conditions
Isalgué Buxeda, Antonio; Auguet Sangrá, Carlota E.; Grau, R.; Torra Ferré, Vicenç; Cinca i Luis, Núria; Fernández, J.
Shape memory alloys are considered smart materials because of their singular thermo-mechanical properties, due to a thermoelastic martensitic transformation, enabling possible uses as actuators (because of mechanical recovery induced from temperature changes) and as dampers (because of hysteresis). NiTi wires for dampers in Civil Engineering had been characterized and tested in facilities. Guaranteed performance needs to know behavior during fatigue life and knowledge of effects in the event of extreme conditions, as eventual overstraining. In this work, we check the possibilities to absorb mechanical energy on the fatigue life depending on stress level and explore the consequences of overstraining the material during installation, the possibilities of partial healing by moderate heating, and some effects of over-stressing the wires. The mechanical energy absorbed by the unit weight of damper wire might be very high during its lifetime if maximum stresses remain relatively low allowing high fatigue life. We show also some results on NiTi wire working as an actuator. The lifetime mechanical work performed by an actuator wire can be very high if applied stresses are limited. The overstraining produces relevant
2016-02-15T15:38:38ZIsalgué Buxeda, AntonioAuguet Sangrá, Carlota E.Grau, R.Torra Ferré, VicençCinca i Luis, NúriaFernández, J.Shape memory alloys are considered smart materials because of their singular thermo-mechanical properties, due to a thermoelastic martensitic transformation, enabling possible uses as actuators (because of mechanical recovery induced from temperature changes) and as dampers (because of hysteresis). NiTi wires for dampers in Civil Engineering had been characterized and tested in facilities. Guaranteed performance needs to know behavior during fatigue life and knowledge of effects in the event of extreme conditions, as eventual overstraining. In this work, we check the possibilities to absorb mechanical energy on the fatigue life depending on stress level and explore the consequences of overstraining the material during installation, the possibilities of partial healing by moderate heating, and some effects of over-stressing the wires. The mechanical energy absorbed by the unit weight of damper wire might be very high during its lifetime if maximum stresses remain relatively low allowing high fatigue life. We show also some results on NiTi wire working as an actuator. The lifetime mechanical work performed by an actuator wire can be very high if applied stresses are limited. The overstraining produces relevantAvailability of crop by-products in Spain: new raw materials for natural thermal insulation
http://hdl.handle.net/2117/78514
Availability of crop by-products in Spain: new raw materials for natural thermal insulation
Palumbo Fernández, Mariana; Avellaneda Diaz-Grande, Jaime; Lacasta Palacio, Ana María
Vegetal materials were one of the first construction materials used by humans, but the development of petrol-based synthetic materials in the last century replaced them in mainstream construction techniques. However, since environmental impact and resource depletion are increasingly becoming a central issue, vegetal materials are revisited. Natural thermal insulations are mainly developed from woody materials and industrial fibres, but these raw materials are not always locally available. Thus, the use of crop by-products is proposed here. The availability of crop by-products to be used as raw materials for building thermal insulations in Spain is evaluated. It is then compared to demand forecasts based on two different scenarios. Results vary greatly from one scenario to another, but they indicate that the amount of crop by-products is sufficient to meet estimated demand. Barley and wheat straw are by far the most abundant by-products, followed by corn, rice and sunflowers. Available corn by-products would be sufficient to insulate between 250,000 and 450,000 dwellings yearly
2015-10-29T15:20:45ZPalumbo Fernández, MarianaAvellaneda Diaz-Grande, JaimeLacasta Palacio, Ana MaríaVegetal materials were one of the first construction materials used by humans, but the development of petrol-based synthetic materials in the last century replaced them in mainstream construction techniques. However, since environmental impact and resource depletion are increasingly becoming a central issue, vegetal materials are revisited. Natural thermal insulations are mainly developed from woody materials and industrial fibres, but these raw materials are not always locally available. Thus, the use of crop by-products is proposed here. The availability of crop by-products to be used as raw materials for building thermal insulations in Spain is evaluated. It is then compared to demand forecasts based on two different scenarios. Results vary greatly from one scenario to another, but they indicate that the amount of crop by-products is sufficient to meet estimated demand. Barley and wheat straw are by far the most abundant by-products, followed by corn, rice and sunflowers. Available corn by-products would be sufficient to insulate between 250,000 and 450,000 dwellings yearlyDA white dwarfs from the LSS-GAC survey DR1: the preliminary luminosity and mass functions and formation rate
http://hdl.handle.net/2117/78424
DA white dwarfs from the LSS-GAC survey DR1: the preliminary luminosity and mass functions and formation rate
Rebassa Mansergas, Alberto; Cojocaru, Elena Ruxandra; Torres, S.; García-Berro Montilla, Enrique; Liu, X.; Yuan, H-B.; Xiang, M.-X.; Huang, Y.; Koester, D.; Hou, Y.; Li, G.; Zhang, Y.
Modern large-scale surveys have allowed the identification of large numbers of white dwarfs. However, these surveys are subject to complicated target selection algorithms, which make it almost impossible to quantify to what extent the observational biases affect the observed populations. The LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) Spectroscopic Survey of the Galactic anticentre (LSS-GAC) follows a well-defined set of criteria for selecting targets for observations. This advantage over previous surveys has been fully exploited here to identify a small yet well-characterized magnitude-limited sample of hydrogen-rich (DA) white dwarfs. We derive preliminary LSS-GAC DA white dwarf luminosity and mass functions. The space density and average formation rate of DA white dwarfs we derive are 0.83 +/- 0.16 x 10(-3) pc(-3) and 5.42 +/- 0.08 x 10(-13) pc(-3) yr(-1), respectively. Additionally, using an existing Monte Carlo population synthesis code we simulate the population of single DA white dwarfs in the Galactic anticentre, under various assumptions. The synthetic populations are passed through the LSS-GAC selection criteria, taking into account all possible observational biases. This allows us to perform a meaningful comparison of the observed and simulated distributions. We find that the LSS-GAC set of criteria is highly efficient in selecting white dwarfs for spectroscopic observations (80-85 per cent) and that, overall, our simulations reproduce well the observed luminosity function. However, they fail at reproducing an excess of massive white dwarfs present in the observed mass function. A plausible explanation for this is that a sizable fraction of massive white dwarfs in the Galaxy are the product of white dwarf-white dwarf mergers
2015-10-28T13:26:12ZRebassa Mansergas, AlbertoCojocaru, Elena RuxandraTorres, S.García-Berro Montilla, EnriqueLiu, X.Yuan, H-B.Xiang, M.-X.Huang, Y.Koester, D.Hou, Y.Li, G.Zhang, Y.Modern large-scale surveys have allowed the identification of large numbers of white dwarfs. However, these surveys are subject to complicated target selection algorithms, which make it almost impossible to quantify to what extent the observational biases affect the observed populations. The LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) Spectroscopic Survey of the Galactic anticentre (LSS-GAC) follows a well-defined set of criteria for selecting targets for observations. This advantage over previous surveys has been fully exploited here to identify a small yet well-characterized magnitude-limited sample of hydrogen-rich (DA) white dwarfs. We derive preliminary LSS-GAC DA white dwarf luminosity and mass functions. The space density and average formation rate of DA white dwarfs we derive are 0.83 +/- 0.16 x 10(-3) pc(-3) and 5.42 +/- 0.08 x 10(-13) pc(-3) yr(-1), respectively. Additionally, using an existing Monte Carlo population synthesis code we simulate the population of single DA white dwarfs in the Galactic anticentre, under various assumptions. The synthetic populations are passed through the LSS-GAC selection criteria, taking into account all possible observational biases. This allows us to perform a meaningful comparison of the observed and simulated distributions. We find that the LSS-GAC set of criteria is highly efficient in selecting white dwarfs for spectroscopic observations (80-85 per cent) and that, overall, our simulations reproduce well the observed luminosity function. However, they fail at reproducing an excess of massive white dwarfs present in the observed mass function. A plausible explanation for this is that a sizable fraction of massive white dwarfs in the Galaxy are the product of white dwarf-white dwarf mergersAftershock sequences of three seismic crises at southern California, USA, simulated by a cellular automata model based on self-organized criticality
http://hdl.handle.net/2117/78319
Aftershock sequences of three seismic crises at southern California, USA, simulated by a cellular automata model based on self-organized criticality
Monterrubio Velasco, Marisol; Lana Pons, Francisco Javier; Martínez Santafé, Maria Dolors
Several properties of aftershock series related to the main shocks of Landers, Northridge and Hector Mine (southern California, USA) are reproduced by the Dynamic Fiber Bundle Model, DFBM. Optimum values for the three parameters governing DFBM are determined by searching for the best agreement of real aftershock series properties and those of synthetic sequences generated by this model. The analysis of the model parameter values provides details on the underlying physical mechanism of the aftershock sequence generation. First, the ratio of seismic energy radiated as seismic waves and transferred as stress-strain to adjacent faults; second, the degree of stress heterogeneity reproducing the complex behavior of real aftershock series. Additionally, the results of simulations support the coexistence of two types of relaxation processes. One of them is associated with the well-known modified Omori's (MO) law, which involves elapsed times between consecutive aftershocks monotonically increasing; the other is manifested by episodes of sudden stress release, with inter-event times much shorter than those predicted by MO law. These episodes are assumed to be a consequence of the complex distribution of tectonic stresses and fault geometry. The first process is associated to events designed as leading aftershocks, LA. The second process generates series of events which are designed as cascades, CA. It is worth of mention that several properties concerning CAs can be reasonably related to critical changes on stress field along the simulation process.
2015-10-27T11:12:24ZMonterrubio Velasco, MarisolLana Pons, Francisco JavierMartínez Santafé, Maria DolorsSeveral properties of aftershock series related to the main shocks of Landers, Northridge and Hector Mine (southern California, USA) are reproduced by the Dynamic Fiber Bundle Model, DFBM. Optimum values for the three parameters governing DFBM are determined by searching for the best agreement of real aftershock series properties and those of synthetic sequences generated by this model. The analysis of the model parameter values provides details on the underlying physical mechanism of the aftershock sequence generation. First, the ratio of seismic energy radiated as seismic waves and transferred as stress-strain to adjacent faults; second, the degree of stress heterogeneity reproducing the complex behavior of real aftershock series. Additionally, the results of simulations support the coexistence of two types of relaxation processes. One of them is associated with the well-known modified Omori's (MO) law, which involves elapsed times between consecutive aftershocks monotonically increasing; the other is manifested by episodes of sudden stress release, with inter-event times much shorter than those predicted by MO law. These episodes are assumed to be a consequence of the complex distribution of tectonic stresses and fault geometry. The first process is associated to events designed as leading aftershocks, LA. The second process generates series of events which are designed as cascades, CA. It is worth of mention that several properties concerning CAs can be reasonably related to critical changes on stress field along the simulation process.Montaje experimental para el estudio de la mezcla generada por penachos turbulentos
http://hdl.handle.net/2117/78186
Montaje experimental para el estudio de la mezcla generada por penachos turbulentos
Lopez Gonza, Pilar; Cano Marchante, Jose Leandro; Redondo Apraiz, José Manuel; Tijera, Manuel
Se presenta un sencillo modelo de laboratorio para generar mezcla turbulenta entre dos fluidos miscibles que están en una situación inicial inestable en un campo gravitatorio. El proceso de mezcla se genera por medio de la evolución de un conjunto discreto de penachos turbulentos forzados. Se describe el modelo experimental, los instrumentos de laboratorio y la forma de proceder. También se describen algunas características de los instrumentos que pueden clasificarse utensilios para la generación de los flujos fluidos, instrumentos eléctricos y aparatos para el registro en video y la digitalización. Este modelo de laboratorio permite medir los perfiles de densidad y las alturas de las capas fluidas por medio de la técnica shadowgraph y la visualización directa de los flujos fluidos.
2015-10-23T10:53:51ZLopez Gonza, PilarCano Marchante, Jose LeandroRedondo Apraiz, José ManuelTijera, ManuelSe presenta un sencillo modelo de laboratorio para generar mezcla turbulenta entre dos fluidos miscibles que están en una situación inicial inestable en un campo gravitatorio. El proceso de mezcla se genera por medio de la evolución de un conjunto discreto de penachos turbulentos forzados. Se describe el modelo experimental, los instrumentos de laboratorio y la forma de proceder. También se describen algunas características de los instrumentos que pueden clasificarse utensilios para la generación de los flujos fluidos, instrumentos eléctricos y aparatos para el registro en video y la digitalización. Este modelo de laboratorio permite medir los perfiles de densidad y las alturas de las capas fluidas por medio de la técnica shadowgraph y la visualización directa de los flujos fluidos.Amorphous carbon modified nano-sized tungsten carbide as a gas diffusion electrode catalyst for the oxygen reduction reaction
http://hdl.handle.net/2117/77679
Amorphous carbon modified nano-sized tungsten carbide as a gas diffusion electrode catalyst for the oxygen reduction reaction
Liu, Zhiwei; Li, Ping; Zhai, Fuqiang; Wan, Qi; Volinsky, Alex A.; Qu, Xuanhui
Nanostructured tungsten carbide with high surface area and containing amorphous carbon is prepared by a
low temperature combustion synthesis method. The nanostructured tungsten carbide is used as the
catalyst in a gas diffusion electrode. The obtained WC with different carbon content was investigated by
XRD, FESEM, TG-DTA, BET and XPS. When the molar ratio of C/W is 19/3, the WC particles with different
carbon content are obtained after a carbonizing process between 900 and 1400 C, and the content of
free carbon in WC also increases gradually with temperature. The carbon content is tested by the high
frequency combustion-infrared absorption method in a carbon sulfur analyzer. The electrochemical
properties of the tungsten carbide for the oxygen reduction reaction are characterized by its polarization
curves and electrochemical impedance. The results show that the presence of the appropriate
amorphous carbon is beneficial for improving the conductivity and dispersibility of the tungsten carbide
catalyst, and more active centers can be provided by the catalyst
2015-10-14T11:21:45ZLiu, ZhiweiLi, PingZhai, FuqiangWan, QiVolinsky, Alex A.Qu, XuanhuiNanostructured tungsten carbide with high surface area and containing amorphous carbon is prepared by a
low temperature combustion synthesis method. The nanostructured tungsten carbide is used as the
catalyst in a gas diffusion electrode. The obtained WC with different carbon content was investigated by
XRD, FESEM, TG-DTA, BET and XPS. When the molar ratio of C/W is 19/3, the WC particles with different
carbon content are obtained after a carbonizing process between 900 and 1400 C, and the content of
free carbon in WC also increases gradually with temperature. The carbon content is tested by the high
frequency combustion-infrared absorption method in a carbon sulfur analyzer. The electrochemical
properties of the tungsten carbide for the oxygen reduction reaction are characterized by its polarization
curves and electrochemical impedance. The results show that the presence of the appropriate
amorphous carbon is beneficial for improving the conductivity and dispersibility of the tungsten carbide
catalyst, and more active centers can be provided by the catalystLifted temperature minimum during the atmospheric evening transition
http://hdl.handle.net/2117/77669
Lifted temperature minimum during the atmospheric evening transition
Blay-Carreras, Estel; Pardyjak, Eric; Pino González, David; Hoch, Sebastian; Cuxart Rodamillans, Joan; Martínez, Daniel; Reuder, Joachim
Observations of lifted temperature minimum (LTM) profiles in the nocturnal boundary layer were first reported in 1932. It was defined by the existence of a temperature minimum some centimetres above the ground. During the following decades, several research studies analysed this phenomenon verifying its existence and postulating different hypotheses about its origin.
The aim of this work is to study the existence and characteristics of LTM during the evening transition by using observations obtained during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign. Data obtained from two masts instrumented with thermocouples and wind sensors at different heights close to the ground and a mast with radiometers are used to study the role of mechanical turbulence and radiation in LTM development.
The study shows that LTM can be detected under calm conditions during the day–night transition, several hours earlier than reported in previous work. These conditions are fulfilled under weak synoptic forcing when the local flow shifts associated with a mountain–plain circulation in relatively complex orography. Under these special conditions, turbulence becomes a crucial parameter in determining the ideal conditions for observing LTM. Additionally, LTM observed profiles are also related to a change in the atmospheric radiative characteristics under calm conditions.
2015-10-14T09:52:48ZBlay-Carreras, EstelPardyjak, EricPino González, DavidHoch, SebastianCuxart Rodamillans, JoanMartínez, DanielReuder, JoachimObservations of lifted temperature minimum (LTM) profiles in the nocturnal boundary layer were first reported in 1932. It was defined by the existence of a temperature minimum some centimetres above the ground. During the following decades, several research studies analysed this phenomenon verifying its existence and postulating different hypotheses about its origin.
The aim of this work is to study the existence and characteristics of LTM during the evening transition by using observations obtained during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign. Data obtained from two masts instrumented with thermocouples and wind sensors at different heights close to the ground and a mast with radiometers are used to study the role of mechanical turbulence and radiation in LTM development.
The study shows that LTM can be detected under calm conditions during the day–night transition, several hours earlier than reported in previous work. These conditions are fulfilled under weak synoptic forcing when the local flow shifts associated with a mountain–plain circulation in relatively complex orography. Under these special conditions, turbulence becomes a crucial parameter in determining the ideal conditions for observing LTM. Additionally, LTM observed profiles are also related to a change in the atmospheric radiative characteristics under calm conditions.Turbulence vertical structure of the boundary layer during the afternoon transition
http://hdl.handle.net/2117/77577
Turbulence vertical structure of the boundary layer during the afternoon transition
Darbieu, Clara; Lohou, Fabienne; Lothon, Marie; Vilà Guerau de Arellano, Jordi; Couvreux, Fleur; Durand, Pierre; Pino González, David; Patton, Edward; Nilsson, Erik; Blay-Carreras, Estel; Gioli, Beniamino
We investigate the decay of planetary boundary
layer (PBL) turbulence in the afternoon, from the time the
surface buoyancy flux starts to decrease until sunset. Dense
observations of mean and turbulent parameters were acquired
during the Boundary Layer Late Afternoon and Sunset Tur-
bulence (BLLAST) field experiment by several meteorolog-
ical surface stations, sounding balloons, radars, lidars and
two aircraft during the afternoon transition. We analysed a
case study based on some of these observations and large-
eddy simulation (LES) data focusing on the turbulent vertical
structure throughout the afternoon transition.
The decay of turbulence is quantified through the temporal
and vertical evolution of (1) the turbulence kinetic energy
(TKE), (2) the characteristic length scales of turbulence and
(3) the shape of the turbulence spectra. A spectral analysis of
LES data, airborne and surface measurements is performed
in order to characterize the variation in the turbulent decay
with height and study the distribution of turbulence over eddy
size.
This study highlights the LES ability to reproduce the tur-
bulence evolution throughout the afternoon. LESs and obser-
vations agree that the afternoon transition can be divided in
two phases: (1) a first phase during which the TKE decays
at a low rate, with no significant change in turbulence char-
acteristics, and (2) a second phase characterized by a larger
TKE decay rate and a change in spectral shape, implying an
evolution of eddy size distribution and energy cascade from
low to high wave number.
The changes observed either in TKE decay (during the first
phase) or in the vertical wind spectra shape (during the sec-
ond phase of the afternoon transition) occur first in the upper
region of the PBL. The higher within the PBL, the stronger
the spectra shape changes
2015-10-13T09:13:50ZDarbieu, ClaraLohou, FabienneLothon, MarieVilà Guerau de Arellano, JordiCouvreux, FleurDurand, PierrePino González, DavidPatton, EdwardNilsson, ErikBlay-Carreras, EstelGioli, BeniaminoWe investigate the decay of planetary boundary
layer (PBL) turbulence in the afternoon, from the time the
surface buoyancy flux starts to decrease until sunset. Dense
observations of mean and turbulent parameters were acquired
during the Boundary Layer Late Afternoon and Sunset Tur-
bulence (BLLAST) field experiment by several meteorolog-
ical surface stations, sounding balloons, radars, lidars and
two aircraft during the afternoon transition. We analysed a
case study based on some of these observations and large-
eddy simulation (LES) data focusing on the turbulent vertical
structure throughout the afternoon transition.
The decay of turbulence is quantified through the temporal
and vertical evolution of (1) the turbulence kinetic energy
(TKE), (2) the characteristic length scales of turbulence and
(3) the shape of the turbulence spectra. A spectral analysis of
LES data, airborne and surface measurements is performed
in order to characterize the variation in the turbulent decay
with height and study the distribution of turbulence over eddy
size.
This study highlights the LES ability to reproduce the tur-
bulence evolution throughout the afternoon. LESs and obser-
vations agree that the afternoon transition can be divided in
two phases: (1) a first phase during which the TKE decays
at a low rate, with no significant change in turbulence char-
acteristics, and (2) a second phase characterized by a larger
TKE decay rate and a change in spectral shape, implying an
evolution of eddy size distribution and energy cascade from
low to high wave number.
The changes observed either in TKE decay (during the first
phase) or in the vertical wind spectra shape (during the sec-
ond phase of the afternoon transition) occur first in the upper
region of the PBL. The higher within the PBL, the stronger
the spectra shape changesExperimental study of heat transport in fractured network
http://hdl.handle.net/2117/77497
Experimental study of heat transport in fractured network
Pastore, Nicola; Cherubini, claudia; Gassi, C.I.; Allegretti, N.M.; Redondo Apraiz, José Manuel; Tarquis Alfonso, Ana Maria
Fractured rocks play an important role in transport of natural resources through subsurface systems. In recent years, interest has
grown in investigating heat transport by means of tracer tests, driven by the important current development of geothermal
applications. Many field and laboratory tracer tests in fractured media show that fracture - matrix exchange is more significant
for heat than mass tracers, thus thermal breakthrough curves are strongly controlled by matrix thermal diffusivity. In this study,
the behaviour of heat transport in a fractured network, at bench laboratory scale, has been investigated.
2015-10-08T11:46:52ZPastore, NicolaCherubini, claudiaGassi, C.I.Allegretti, N.M.Redondo Apraiz, José ManuelTarquis Alfonso, Ana MariaFractured rocks play an important role in transport of natural resources through subsurface systems. In recent years, interest has
grown in investigating heat transport by means of tracer tests, driven by the important current development of geothermal
applications. Many field and laboratory tracer tests in fractured media show that fracture - matrix exchange is more significant
for heat than mass tracers, thus thermal breakthrough curves are strongly controlled by matrix thermal diffusivity. In this study,
the behaviour of heat transport in a fractured network, at bench laboratory scale, has been investigated.Continuation of bifurcations of periodic orbits for large-scale systems
http://hdl.handle.net/2117/76843
Continuation of bifurcations of periodic orbits for large-scale systems
Net Marcé, Marta; Sánchez Umbría, Juan
A methodology to track bifurcations of periodic orbits in large-scale dissipative systems depending on two parameters is presented. It is based on the application of iterative Newton-Krylov techniques to extended systems. To evaluate the action of the Jacobian it is necessary to integrate variational equations up to second order. It is shown that this is possible by integrating systems of dimension at most four times that of the original equations. In order to check the robustness of the method, the thermal convection of a mixture of two fluids in a rectangular domain has been used as a test problem. Several curves of codimension-one bifurcations, and the boundaries of an Arnold's tongue of rotation number 1/8, have been computed.
2015-09-16T09:56:41ZNet Marcé, MartaSánchez Umbría, JuanA methodology to track bifurcations of periodic orbits in large-scale dissipative systems depending on two parameters is presented. It is based on the application of iterative Newton-Krylov techniques to extended systems. To evaluate the action of the Jacobian it is necessary to integrate variational equations up to second order. It is shown that this is possible by integrating systems of dimension at most four times that of the original equations. In order to check the robustness of the method, the thermal convection of a mixture of two fluids in a rectangular domain has been used as a test problem. Several curves of codimension-one bifurcations, and the boundaries of an Arnold's tongue of rotation number 1/8, have been computed.