http://hdl.handle.net/2117/1087 Thu, 23 May 2013 16:40:35 GMT 2013-05-23T16:40:35Z webmaster.bupc@upc.edu Universitat Politècnica de Catalunya. Servei de Biblioteques i Documentació no http://hdl.handle.net/2117/19150 Title: Small-scale characteristics and turbulent statistics of the flow in an external gear pump by time-resolved PIV Authors: Ertürk, S.; Vernet, Anton; Pallarés, J.; Castilla López, Roberto; Raush Alviach, Gustavo Adolfo Thu, 09 May 2013 12:44:56 GMT http://hdl.handle.net/2117/19150 2013-05-09T12:44:56Z Ertürk, S.; Vernet, Anton; Pallarés, J.; Castilla López, Roberto; Raush Alviach, Gustavo Adolfo no PIV, Spatial resolution, Turbulent flow, External gear pump, Dissipation rate http://hdl.handle.net/2117/18497 Title: Nocturnal offshore precipitation near the Mediterranean coast of the Iberian Peninsula Authors: Mazón Bueso, Jordi; Pino González, David Abstract: While nocturnal offshore precipitation, which produces rain cells and bands, has been studied in tropical areas, few studies have analyzed the occurrence of this phenomenon at higher latitudes. Using radar reflectivity, nocturnal rainfall in the western Mediterranean area has been detected near the coast of the Iberian Peninsula and North Africa. More than 50 events have been recorded since 2009. MM5 mesoscale simulations of some of the recorded events allow us to establish that the most likely causes for these precipitation events are: (1) the interaction between cold air masses conducted by drainage and kata- batic winds, and (2) a wetter and warmer synoptic wind. Two different episodes are presented: one in the north- eastern Iberian Peninsula, caused by stratiform clouds, which occurred at the mouths of three rivers; the other case, produced by convective clouds, occurred at the southern Iberian Peninsula and was caused by the drainage winds flowing down from some mountain ranges located close to the coast. Fri, 22 Mar 2013 13:11:41 GMT http://hdl.handle.net/2117/18497 2013-03-22T13:11:41Z Mazón Bueso, Jordi; Pino González, David no While nocturnal offshore precipitation, which produces rain cells and bands, has been studied in tropical areas, few studies have analyzed the occurrence of this phenomenon at higher latitudes. Using radar reflectivity, nocturnal rainfall in the western Mediterranean area has been detected near the coast of the Iberian Peninsula and North Africa. More than 50 events have been recorded since 2009. MM5 mesoscale simulations of some of the recorded events allow us to establish that the most likely causes for these precipitation events are: (1) the interaction between cold air masses conducted by drainage and kata- batic winds, and (2) a wetter and warmer synoptic wind. Two different episodes are presented: one in the north- eastern Iberian Peninsula, caused by stratiform clouds, which occurred at the mouths of three rivers; the other case, produced by convective clouds, occurred at the southern Iberian Peninsula and was caused by the drainage winds flowing down from some mountain ranges located close to the coast. http://hdl.handle.net/2117/17977 Title: The role of sea-land air thermal difference, shape of the coastline and sea surface temperature in the nocturnal offshore convection Authors: Mazón Bueso, Jordi; Pino González, David Abstract: Nocturnal precipitation cells and lines occur near the coastline in the whole Mediterranean basin in all seasons. The precipitation events are mainly located in areas where coastal mountain ranges and rivers enhance convergence though the interaction of nocturnal mesoscale and local flows (land breeze, katabatic and drainages winds) with prevailing synoptic wind or with other mesoscale and local flows. The methodology used here to study this phenomenon consists of three stages. First, the Tropical Rainfall Measuring Mission (TRMM) radar satellite database is used to detect nocturnal precipitation near the coastline, from 18 to 09 UTC. An event is included in the study if the 3 hours accumulated precipitation detected by TRMM is stationary near the coast, or has moved slightly onshore or offshore, and has lasted no more than six consecutive hours. Second, the NCEP reanalysis database is used to describe the synoptic conditions and to discard precipitation associated with synoptic events (large low pressure areas, dynamic polar fronts, or troughs, for example). In the final step by using the version 3 of the Weather Research Forecast model, we simulate and analyse some of the selected events to determine the role of the land sea temperature differences, the curvature of the coastline and the sea surface temperature. The simulations confirm that the nocturnal precipitation studied in the Mediterranean basin near the coastline is formed from the interaction between relatively warm and wet sea-air with the cold air mass from drainage winds, as well as from the convergence of several drainage winds offshore. The mechanism is the same that is used to explain nocturnal precipitation in tropical areas. Tue, 26 Feb 2013 12:23:12 GMT http://hdl.handle.net/2117/17977 2013-02-26T12:23:12Z Mazón Bueso, Jordi; Pino González, David no Nocturnal precipitation cells and lines occur near the coastline in the whole Mediterranean basin in all seasons. The precipitation events are mainly located in areas where coastal mountain ranges and rivers enhance convergence though the interaction of nocturnal mesoscale and local flows (land breeze, katabatic and drainages winds) with prevailing synoptic wind or with other mesoscale and local flows. The methodology used here to study this phenomenon consists of three stages. First, the Tropical Rainfall Measuring Mission (TRMM) radar satellite database is used to detect nocturnal precipitation near the coastline, from 18 to 09 UTC. An event is included in the study if the 3 hours accumulated precipitation detected by TRMM is stationary near the coast, or has moved slightly onshore or offshore, and has lasted no more than six consecutive hours. Second, the NCEP reanalysis database is used to describe the synoptic conditions and to discard precipitation associated with synoptic events (large low pressure areas, dynamic polar fronts, or troughs, for example). In the final step by using the version 3 of the Weather Research Forecast model, we simulate and analyse some of the selected events to determine the role of the land sea temperature differences, the curvature of the coastline and the sea surface temperature. The simulations confirm that the nocturnal precipitation studied in the Mediterranean basin near the coastline is formed from the interaction between relatively warm and wet sea-air with the cold air mass from drainage winds, as well as from the convergence of several drainage winds offshore. The mechanism is the same that is used to explain nocturnal precipitation in tropical areas. http://hdl.handle.net/2117/17973 Title: A conceptual framework to quantify the influence of convective boundary layer development on carbon dioxide mixing ratios Authors: Pino González, David; Vilà-Guerau de Arellano, Jordi; Peters, Wouter; Schröter, Joel; van Heerwaarden, Chiel C.; Krol, Maarten Abstract: Interpretation of observed diurnal carbon dioxide (CO2) mixing ratios near the surface requires knowledge of the local dynamics of the planetary boundary layer. In this paper, we study the relationship between the boundary layer dynamics and the CO2 budget in convective conditions through a newly derived set of analytical equations. From these equations, we are able to quantify how uncertainties in boundary layer dynamical variables or in the morning CO2 distribution in the mixed-layer or in the free atmosphere (FA) influence the bulk CO2 mixing ratio. We find that the largest uncertainty incurred on the mid-day CO2 mixing ratio comes from the prescribed early morning CO2 mixing ratios in the stable boundary layer, and in the free atmosphere. Errors in these values influence CO2 mixing ratios inversely proportional to the boundary layer depth (h), just like uncertainties in the assumed initial boundary layer depth and surface CO2 flux. The influence of uncertainties in the boundary layer depth itself is one order of magnitude smaller. If we "invert" the problem and calculate CO2 surface exchange from observed or simulated CO2 mixing ratios, the sensitivities to errors in boundary layer dynamics also invert: they become linearly proportional to the boundary layer depth. We demonstrate these relations for a typical well characterized situation at the Cabauw site in The Netherlands, and conclude that knowledge of the temperature and carbon dioxide profiles of the atmosphere in the early morning are of vital importance to correctly interpret observed CO2 mixing ratios during midday. Tue, 26 Feb 2013 11:55:01 GMT http://hdl.handle.net/2117/17973 2013-02-26T11:55:01Z Pino González, David; Vilà-Guerau de Arellano, Jordi; Peters, Wouter; Schröter, Joel; van Heerwaarden, Chiel C.; Krol, Maarten no Interpretation of observed diurnal carbon dioxide (CO2) mixing ratios near the surface requires knowledge of the local dynamics of the planetary boundary layer. In this paper, we study the relationship between the boundary layer dynamics and the CO2 budget in convective conditions through a newly derived set of analytical equations. From these equations, we are able to quantify how uncertainties in boundary layer dynamical variables or in the morning CO2 distribution in the mixed-layer or in the free atmosphere (FA) influence the bulk CO2 mixing ratio. We find that the largest uncertainty incurred on the mid-day CO2 mixing ratio comes from the prescribed early morning CO2 mixing ratios in the stable boundary layer, and in the free atmosphere. Errors in these values influence CO2 mixing ratios inversely proportional to the boundary layer depth (h), just like uncertainties in the assumed initial boundary layer depth and surface CO2 flux. The influence of uncertainties in the boundary layer depth itself is one order of magnitude smaller. If we "invert" the problem and calculate CO2 surface exchange from observed or simulated CO2 mixing ratios, the sensitivities to errors in boundary layer dynamics also invert: they become linearly proportional to the boundary layer depth. We demonstrate these relations for a typical well characterized situation at the Cabauw site in The Netherlands, and conclude that knowledge of the temperature and carbon dioxide profiles of the atmosphere in the early morning are of vital importance to correctly interpret observed CO2 mixing ratios during midday. http://hdl.handle.net/2117/17969 Title: The role of nocturnal Low-Level-Jet in nocturnal convection and rainfalls in the west Mediterranean coast: the episode of 14 December 2010 in northeast of Iberian Peninsula Authors: Mazón Bueso, Jordi; Pino González, David Abstract: The night of 14 December 2010 radar images of the Spanish Weather Agency recorded a large rain band that moved o ff shore at the Northeast coast of the Iberian Peninsula. MM5 mesoscale model is used to study the atmospheric dynamics during that day. A Nocturnal Low Level Jet (NLLJ) generated by an inertial oscillation that brings cold air to the coast from inland has been simulated in the area. This cold air interacts with a warmer air mass some kilometers o ff shore. According to the MM5 mesoscale model simulation, the cold air enhances upward movements of the warm air producing condensation. Additionally, there is a return flow to the coastline at 600–900 m high. This warm air mass interacts again with the cold air moving downslope, also producing condensation inland. The simulation for the night before this episode shows large drainage winds with a NLLJ profile, but no condensation areas. The night after the 14th the simulation also shows drainage winds but without a NLLJ profile. However, an o ff shore convergence area was produced with a returned flow, but no condensation inland occurred. This fact is in agreement with radar observations which reported no precipitation for these two days. Consequently, NLLJ in combination with a synoptic wind over the sea could enhance condensation and eventually precipitation rates in the Mediterranean Iberian coast. Tue, 26 Feb 2013 11:22:36 GMT http://hdl.handle.net/2117/17969 2013-02-26T11:22:36Z Mazón Bueso, Jordi; Pino González, David no The night of 14 December 2010 radar images of the Spanish Weather Agency recorded a large rain band that moved o ff shore at the Northeast coast of the Iberian Peninsula. MM5 mesoscale model is used to study the atmospheric dynamics during that day. A Nocturnal Low Level Jet (NLLJ) generated by an inertial oscillation that brings cold air to the coast from inland has been simulated in the area. This cold air interacts with a warmer air mass some kilometers o ff shore. According to the MM5 mesoscale model simulation, the cold air enhances upward movements of the warm air producing condensation. Additionally, there is a return flow to the coastline at 600–900 m high. This warm air mass interacts again with the cold air moving downslope, also producing condensation inland. The simulation for the night before this episode shows large drainage winds with a NLLJ profile, but no condensation areas. The night after the 14th the simulation also shows drainage winds but without a NLLJ profile. However, an o ff shore convergence area was produced with a returned flow, but no condensation inland occurred. This fact is in agreement with radar observations which reported no precipitation for these two days. Consequently, NLLJ in combination with a synoptic wind over the sea could enhance condensation and eventually precipitation rates in the Mediterranean Iberian coast. http://hdl.handle.net/2117/17392 Title: Production of short-lived radioactive nuclei in super asymptotic giant branch star Authors: Doherty, Carolyn L.; Lugaro, M.; Lau, H.; Siess, Lionel; Lattanzio, John; Gil Pons, Pilar Thu, 17 Jan 2013 10:12:51 GMT http://hdl.handle.net/2117/17392 2013-01-17T10:12:51Z Doherty, Carolyn L.; Lugaro, M.; Lau, H.; Siess, Lionel; Lattanzio, John; Gil Pons, Pilar no http://hdl.handle.net/2117/17388 Title: The upper-mass limit for the formation of super-agb stars and the dredge-out phenomenon Authors: Gil Pons, Pilar; Doherty, Carolyn L. Abstract: We have computed the evolution of Super-AGB stars from the main sequence and up to a few hundred thermal pulses, with special attention to the low metallicity cases (Z = 10􀀀10; 10􀀀5; 10􀀀4 and 10􀀀3). Our computations have been performed using time– dependent mixing and new opacity tables that admit variations in the abundances of carbon and oxygen. By following the evolution along the main central burning stages and the early TP-SAGB, we resolve the upper mass limits for the formation of TP-SAGB stars and determine the mass range at which the dredge-out phenomenon occurs. This phenomenon involves the merger of a convective shell sustained by helium burning at the top of the degenerate core with the hydrogen–rich convective envelope and the occurrence of a hydrogen flash. The dredge–out allows elements synthesised through helium burning to be transported to the stellar surfaces and therefore it can a ect the initial composition of the TP-SAGB stars. Wed, 16 Jan 2013 13:01:41 GMT http://hdl.handle.net/2117/17388 2013-01-16T13:01:41Z Gil Pons, Pilar; Doherty, Carolyn L. no We have computed the evolution of Super-AGB stars from the main sequence and up to a few hundred thermal pulses, with special attention to the low metallicity cases (Z = 10􀀀10; 10􀀀5; 10􀀀4 and 10􀀀3). Our computations have been performed using time– dependent mixing and new opacity tables that admit variations in the abundances of carbon and oxygen. By following the evolution along the main central burning stages and the early TP-SAGB, we resolve the upper mass limits for the formation of TP-SAGB stars and determine the mass range at which the dredge-out phenomenon occurs. This phenomenon involves the merger of a convective shell sustained by helium burning at the top of the degenerate core with the hydrogen–rich convective envelope and the occurrence of a hydrogen flash. The dredge–out allows elements synthesised through helium burning to be transported to the stellar surfaces and therefore it can a ect the initial composition of the TP-SAGB stars. http://hdl.handle.net/2117/17386 Title: On the fate of extremely low metallicity stars Authors: Gil Pons, Pilar; Gutierrez Cabello, Jorge Luis; García-Berro Montilla, Enrique; Althaus, Leandro Gabriel Abstract: We have computed and analyzed the evolution of intermediate-mass stars of metallicity Z = 10−5 from the main sequence until the early stages of the TP-(S)AGB phase. In order to check the influence of mixing we have performed our calculations using two different evolutionary codes, EVOLVE — which does not include diffusion but allows for overshooting— and LPCODE, which includes diffusion and overshooting “à-la-Herwig”. Important differences appear during the TP– AGB phase, as the calculations done with EVOLVE lead to an almost negligible third dredge-up, whereas the sequences computed with LPCODE show important third dredge-up and, therefore, significant enrichment in metals of the envelope. This fact has consequences on the strength of the winds during the TP-AGB phase and, ultimately, on the final fate of the considered stars. Also, the models computed with LPCODE show strong double flashes produced by the ingestion of protons on the helium burning shell. Wed, 16 Jan 2013 11:52:32 GMT http://hdl.handle.net/2117/17386 2013-01-16T11:52:32Z Gil Pons, Pilar; Gutierrez Cabello, Jorge Luis; García-Berro Montilla, Enrique; Althaus, Leandro Gabriel no We have computed and analyzed the evolution of intermediate-mass stars of metallicity Z = 10−5 from the main sequence until the early stages of the TP-(S)AGB phase. In order to check the influence of mixing we have performed our calculations using two different evolutionary codes, EVOLVE — which does not include diffusion but allows for overshooting— and LPCODE, which includes diffusion and overshooting “à-la-Herwig”. Important differences appear during the TP– AGB phase, as the calculations done with EVOLVE lead to an almost negligible third dredge-up, whereas the sequences computed with LPCODE show important third dredge-up and, therefore, significant enrichment in metals of the envelope. This fact has consequences on the strength of the winds during the TP-AGB phase and, ultimately, on the final fate of the considered stars. Also, the models computed with LPCODE show strong double flashes produced by the ingestion of protons on the helium burning shell. http://hdl.handle.net/2117/16940 Title: Spiral vortices between concentric cylinders Authors: Sánchez Umbría, Juan; Crespo Artiaga, Daniel; Marqués Truyol, Francisco Abstract: Spiral vortices appearing in Couette- Taylor flows are studied by means of numerical simulation. Transition curves from Couctte to spiral vortices for different radius ratios and wavenumbers have been calculated in order to test our technique. Critical Reynolds numbers, angular velocities and slopes of the spirals at the onset of the instability agree with previous results [1]. Non-linear solutions obtained by a pseudospectral collocation method are studied, and they show a weak net axial ftow. In arder to counteract this effect, which is absent in the usual experimental set-up, an axial pressure gradient has been included. This procedure has proved to be sufficient to make the axial flow negligible. The onset of a quasiperiodic flow for larger Reynolds numbers, corresponding to a secondary bifurcation is also presented. Fri, 16 Nov 2012 17:50:05 GMT http://hdl.handle.net/2117/16940 2012-11-16T17:50:05Z Sánchez Umbría, Juan; Crespo Artiaga, Daniel; Marqués Truyol, Francisco no Spiral vortices appearing in Couette- Taylor flows are studied by means of numerical simulation. Transition curves from Couctte to spiral vortices for different radius ratios and wavenumbers have been calculated in order to test our technique. Critical Reynolds numbers, angular velocities and slopes of the spirals at the onset of the instability agree with previous results [1]. Non-linear solutions obtained by a pseudospectral collocation method are studied, and they show a weak net axial ftow. In arder to counteract this effect, which is absent in the usual experimental set-up, an axial pressure gradient has been included. This procedure has proved to be sufficient to make the axial flow negligible. The onset of a quasiperiodic flow for larger Reynolds numbers, corresponding to a secondary bifurcation is also presented. http://hdl.handle.net/2117/16712 Title: Numerical study of the onset of thermosolutal convection in rotating spherical shells Authors: Net Marcé, Marta; García González, Fernando; Sánchez Umbría, Juan Abstract: The influence of an externally enforced compositional gradient on the onset of convection of a mixture of two components in a rotating fluid spherical shell is studied for Ekman numbers E = 10−3 and E = 10−6, Prandtl numbers σ = 0.1, 0.001, Lewis numbers τ = 0.01, 0.1, 0.8, and radius ratio η = 0.35. The Boussinesq approximation of the governing equations is derived by taking the denser component of the mixture for the equation of the concentration. Differential and internal heating, an external compositional gradient, and the Soret and Dufour effects are included in the model. By neglecting these two last effects, and by considering only differential heating, it is found that the critical thermal Rayleigh number Rec depends strongly on the direction of the compositional gradient. The results are compared with those obtained previously for pure fluids of the same σ. The influence of the mixture becomes significant when the compositional Rayleigh number Rc is at least of the same order of magnitude as the known Rec computed without mixture. For positive and sufficiently large compositional gradients, Rec decreases and changes sign, indicating that the compositional convection becomes the main source of instability. Then the critical wave number mc decreases, and the drifting waves slow down drastically giving rise to an almost stationary pattern of convection. Negative gradients delay the onset of convection and determine a substantial increase of mc and ωc for Rc sufficiently high. Potential laws are obtained numerically from the dependence of Rec and of the critical frequency ωc on Rc, for the moderate and small Ekman numbers explored. Wed, 10 Oct 2012 17:44:36 GMT http://hdl.handle.net/2117/16712 2012-10-10T17:44:36Z Net Marcé, Marta; García González, Fernando; Sánchez Umbría, Juan no The influence of an externally enforced compositional gradient on the onset of convection of a mixture of two components in a rotating fluid spherical shell is studied for Ekman numbers E = 10−3 and E = 10−6, Prandtl numbers σ = 0.1, 0.001, Lewis numbers τ = 0.01, 0.1, 0.8, and radius ratio η = 0.35. The Boussinesq approximation of the governing equations is derived by taking the denser component of the mixture for the equation of the concentration. Differential and internal heating, an external compositional gradient, and the Soret and Dufour effects are included in the model. By neglecting these two last effects, and by considering only differential heating, it is found that the critical thermal Rayleigh number Rec depends strongly on the direction of the compositional gradient. The results are compared with those obtained previously for pure fluids of the same σ. The influence of the mixture becomes significant when the compositional Rayleigh number Rc is at least of the same order of magnitude as the known Rec computed without mixture. For positive and sufficiently large compositional gradients, Rec decreases and changes sign, indicating that the compositional convection becomes the main source of instability. Then the critical wave number mc decreases, and the drifting waves slow down drastically giving rise to an almost stationary pattern of convection. Negative gradients delay the onset of convection and determine a substantial increase of mc and ωc for Rc sufficiently high. Potential laws are obtained numerically from the dependence of Rec and of the critical frequency ωc on Rc, for the moderate and small Ekman numbers explored. http://hdl.handle.net/2117/16696 Title: Edge state in pipe flow experiments Authors: De Lozar, Alberto; Mellibovsky Elstein, Fernando; Avila, M.; Hof, Björn Abstract: Recent numerical studies suggest that in pipe and related shear flows, the region of phase space separating laminar from turbulent motion is organized by a chaotic attractor, called an edge state, which mediates the transition process. We here confirm the existence of the edge state in laboratory experiments. We observe that it governs the dynamics during the decay of turbulence underlining its potential relevance for turbulence control. In addition we unveil two unstable traveling wave solutions underlying the experimental flow fields. This observation corroborates earlier suggestions that unstable solutions organize turbulence and its stability border. Tue, 09 Oct 2012 17:24:51 GMT http://hdl.handle.net/2117/16696 2012-10-09T17:24:51Z De Lozar, Alberto; Mellibovsky Elstein, Fernando; Avila, M.; Hof, Björn no Chaotic attractors, Edge state, Flow experiments, In-pipe, Laboratory experiments, Numerical studies, Phase spaces, Transition process, Traveling wave solution, Turbulence controls, Turbulent motion, Unstable solutions Recent numerical studies suggest that in pipe and related shear flows, the region of phase space separating laminar from turbulent motion is organized by a chaotic attractor, called an edge state, which mediates the transition process. We here confirm the existence of the edge state in laboratory experiments. We observe that it governs the dynamics during the decay of turbulence underlining its potential relevance for turbulence control. In addition we unveil two unstable traveling wave solutions underlying the experimental flow fields. This observation corroborates earlier suggestions that unstable solutions organize turbulence and its stability border. http://hdl.handle.net/2117/16494 Title: On the invariant causal characterization of singularities in spherically symmetric spacetimes Authors: Fayos Vallés, Francisco; Torres Herrera, Ramon Abstract: The causal character of singularities is often studied in relation to the existence of naked singularities and the subsequent possible violation of the cosmic censorship conjecture. Generally, one constructs a model in the framework of general relativity described in some specific coordinates and finds an ad hoc procedure to analyze the character of the singularity. In this paper, we show that the causal character of the zero-areal-radius (R = 0) singularity in spherically symmetric models is related to some specific invariants. In this way, if some assumptions are satisfied, one can ascertain the causal character of the singularity algorithmically through the computation of these invariants and, therefore, independent of the coordinates used in the model. Fri, 14 Sep 2012 11:10:11 GMT http://hdl.handle.net/2117/16494 2012-09-14T11:10:11Z Fayos Vallés, Francisco; Torres Herrera, Ramon no The causal character of singularities is often studied in relation to the existence of naked singularities and the subsequent possible violation of the cosmic censorship conjecture. Generally, one constructs a model in the framework of general relativity described in some specific coordinates and finds an ad hoc procedure to analyze the character of the singularity. In this paper, we show that the causal character of the zero-areal-radius (R = 0) singularity in spherically symmetric models is related to some specific invariants. In this way, if some assumptions are satisfied, one can ascertain the causal character of the singularity algorithmically through the computation of these invariants and, therefore, independent of the coordinates used in the model. http://hdl.handle.net/2117/16338 Title: On the use of linear stability model to characterize the morphological behaviour of a double bar system. Application to Truc Vert beach (France) Authors: Brivois, Olivier; Idier, Déborah; Thiébot, Jérôme; Castelle, Bruno; Le Cozannet, Gonéri; Calvete Manrique, Daniel Abstract: Sandy barred beaches are often characterized by the presence of rhythmic patterns such as crescentic bars. In this article, a linear stability analysis (LSA) model is used to characterize the morphological behaviour of the double bar system of Truc Vert beach. Using a limited number of combinations of representative bathymetries, wave classes and water levels, the morphodynamic response of the system is analysed, focussing on the geometrical characteristics of 3D patterns generated with the model. These characteristics are described and then compared with available observations. The shapes and the wavelengths of the instabilities predicted by the model compare well with field observations. Thus, the use of the linear stability model, with representative hydrodynamic conditions and bathymetries of the considered site, allows a characterization of the global morphodynamic behaviour of a double-barred system. Tue, 24 Jul 2012 17:27:22 GMT http://hdl.handle.net/2117/16338 2012-07-24T17:27:22Z Brivois, Olivier; Idier, Déborah; Thiébot, Jérôme; Castelle, Bruno; Le Cozannet, Gonéri; Calvete Manrique, Daniel no Sandy barred beaches are often characterized by the presence of rhythmic patterns such as crescentic bars. In this article, a linear stability analysis (LSA) model is used to characterize the morphological behaviour of the double bar system of Truc Vert beach. Using a limited number of combinations of representative bathymetries, wave classes and water levels, the morphodynamic response of the system is analysed, focussing on the geometrical characteristics of 3D patterns generated with the model. These characteristics are described and then compared with available observations. The shapes and the wavelengths of the instabilities predicted by the model compare well with field observations. Thus, the use of the linear stability model, with representative hydrodynamic conditions and bathymetries of the considered site, allows a characterization of the global morphodynamic behaviour of a double-barred system. http://hdl.handle.net/2117/16280 Title: A field test of a linear stability model for crescentic bars Authors: Tiessen, M C H; Leeuwen, S M Van; Calvete Manrique, Daniel; Dodd, N Abstract: A morphodynamical linear stability analysis is used to predict the natural development of crescentic bed patterns and rip channels. The purpose is to investigate whether this technique, which is useful for understanding the physics of emerging bed-forms, can be used to make quantitative predictions in the field, which may then be of use for coastal engineers. To this end a morphodynamical linear stability model (Morfo60) is used to describe the development of crescentic bed patterns at the coast at the USACE Field Research Facility in Duck, North Carolina, USA. Wave, tide and alongshore-averaged bathymetry data recorded at Duck over a 2 month period in 1998 are used to model the development of these morphodynamical patterns at an open coast. The model predictions are compared with field observations by van Enckevort et al. (2004) made at Duck using the Argus imaging technique, over the same 2 month period. Field observations and model predictions show similar length scales of the crescentic bed patterns. Immediately after a storm, large length scales of around 500 to 800 m are predicted and observed, which in a couple of days decrease to around 150 to 400 m, until the next storm occurs. The model predictions show more fluctuations in the predicted length scale than those observed in the field. These fluctuations are due to variations in the wave conditions and tidal level and the lack of pre-existing bed patterns in a linear stability analysis. An algorithm is developed to identify the more physically significant model predictions based on large growth rates and consistency in length scales. The presented algorithm, referred to as a Physically Significant Development (PSD) algorithm, is able systematically to identify the more physically representative model results. These compare better with field observations, as shown by the good agreement between predicted and observed crescentic bed pattern length scales. Accordingly, the conclusion is that linear stability analysis in combination with an appropriate physically based significant development algorithm may be of use to coastal engineers. Tue, 17 Jul 2012 17:33:15 GMT http://hdl.handle.net/2117/16280 2012-07-17T17:33:15Z Tiessen, M C H; Leeuwen, S M Van; Calvete Manrique, Daniel; Dodd, N no A morphodynamical linear stability analysis is used to predict the natural development of crescentic bed patterns and rip channels. The purpose is to investigate whether this technique, which is useful for understanding the physics of emerging bed-forms, can be used to make quantitative predictions in the field, which may then be of use for coastal engineers. To this end a morphodynamical linear stability model (Morfo60) is used to describe the development of crescentic bed patterns at the coast at the USACE Field Research Facility in Duck, North Carolina, USA. Wave, tide and alongshore-averaged bathymetry data recorded at Duck over a 2 month period in 1998 are used to model the development of these morphodynamical patterns at an open coast. The model predictions are compared with field observations by van Enckevort et al. (2004) made at Duck using the Argus imaging technique, over the same 2 month period. Field observations and model predictions show similar length scales of the crescentic bed patterns. Immediately after a storm, large length scales of around 500 to 800 m are predicted and observed, which in a couple of days decrease to around 150 to 400 m, until the next storm occurs. The model predictions show more fluctuations in the predicted length scale than those observed in the field. These fluctuations are due to variations in the wave conditions and tidal level and the lack of pre-existing bed patterns in a linear stability analysis. An algorithm is developed to identify the more physically significant model predictions based on large growth rates and consistency in length scales. The presented algorithm, referred to as a Physically Significant Development (PSD) algorithm, is able systematically to identify the more physically representative model results. These compare better with field observations, as shown by the good agreement between predicted and observed crescentic bed pattern length scales. Accordingly, the conclusion is that linear stability analysis in combination with an appropriate physically based significant development algorithm may be of use to coastal engineers. http://hdl.handle.net/2117/16278 Title: Nonlinear modelling of shoreface-connected ridges; impact of grain sorting and interventions Authors: de Swart, Huib E.; Walgreen, M; Calvete Manrique, Daniel; Vis-Star, N C Abstract: The evolution of shoreface-connected sand ridges on micro-tidal inner shelves and the variations in the mean grain size over these ridges are investigated with process-based models. A review of previous studies is presented, as well as new results that concern the influence of grain sorting on the finite-amplitude behaviour of the ridges, the application of the model to La Barrosa beach and the role of wave-topography feedbacks. The ridges initially form due to morphodynamic self-organisation, in which the presence of waves and a storm-driven current are crucial. Predicted growth time scales, migration speeds, topography and spatial pattern of the mean grain size agree with field data collected on micro-tidal shelves in the case that both bedload and suspended load sediment transport are accounted for, together with spatially non-uniform wave orbital motion. The model can not successfully explain the presence of large-scale ridges observed on La Barrosa inner shelf, because strong and complex behaving tidal currents occur in that area. Nonlinear model simulations show that on the long term the height of the ridges evolves towards a finite, constant value, whilst their migration speed hardly changes during the evolution. In the saturated stage the ridges have asymmetrical profiles, with steep slopes on the downstream sides. The maximum variation in mean grain size also tends to a constant value and during the evolution the spatial lag between the patterns in the mean grain size and topography decreases. The processes that cause these changes are identified and explained. Model results can be obtained for transverse bottom slopes up to 50% of their observed values on micro-tidal shelves. Extrapolation of results to realistic values of the inner shelf slope yields, in case of Long Island shelf, a final height that agrees with observed ridge heights, but the modelled variation in mean grain size is small compared to field data. Finally, the response of ridges to large-scale interventions is considered. Experiments reveal that extraction of sand on the inner shelf causes a decrease of the sand volume stored in the surf zone. Tue, 17 Jul 2012 17:20:28 GMT http://hdl.handle.net/2117/16278 2012-07-17T17:20:28Z de Swart, Huib E.; Walgreen, M; Calvete Manrique, Daniel; Vis-Star, N C no The evolution of shoreface-connected sand ridges on micro-tidal inner shelves and the variations in the mean grain size over these ridges are investigated with process-based models. A review of previous studies is presented, as well as new results that concern the influence of grain sorting on the finite-amplitude behaviour of the ridges, the application of the model to La Barrosa beach and the role of wave-topography feedbacks. The ridges initially form due to morphodynamic self-organisation, in which the presence of waves and a storm-driven current are crucial. Predicted growth time scales, migration speeds, topography and spatial pattern of the mean grain size agree with field data collected on micro-tidal shelves in the case that both bedload and suspended load sediment transport are accounted for, together with spatially non-uniform wave orbital motion. The model can not successfully explain the presence of large-scale ridges observed on La Barrosa inner shelf, because strong and complex behaving tidal currents occur in that area. Nonlinear model simulations show that on the long term the height of the ridges evolves towards a finite, constant value, whilst their migration speed hardly changes during the evolution. In the saturated stage the ridges have asymmetrical profiles, with steep slopes on the downstream sides. The maximum variation in mean grain size also tends to a constant value and during the evolution the spatial lag between the patterns in the mean grain size and topography decreases. The processes that cause these changes are identified and explained. Model results can be obtained for transverse bottom slopes up to 50% of their observed values on micro-tidal shelves. Extrapolation of results to realistic values of the inner shelf slope yields, in case of Long Island shelf, a final height that agrees with observed ridge heights, but the modelled variation in mean grain size is small compared to field data. Finally, the response of ridges to large-scale interventions is considered. Experiments reveal that extraction of sand on the inner shelf causes a decrease of the sand volume stored in the surf zone.