Antonio Crespo - Academia.edu (original) (raw)

Papers by Antonio Crespo

Wind turbine wakes are an interesting topic of study. Velocity defects and the increase of turbul... more Wind turbine wakes are an interesting topic of study. Velocity defects and the increase of turbulence generated by the turbines in a wind farm usually cause a reduction in power generation and unsteady loads over other machines. In wind farms where, because of shortage of space, high cost of civil works, etc., the wind turbines are placed too close ones to the others, turbulence intensity may increase sufficiently to cause measurable damage due to fatigue and dynamic loads on some of them. ÍNDICE xv 4.1.2.1. Condiciones de contorno en la entrada y salida del dominio……………………………………………….... 53 4.1.2.2. Condición de contorno en los bordes inferior y superior del recinto……………………………………. 56 4.1.2.3. Condiciones de contorno en las caras laterales del dominio…………………………………………………. 59 4.2.

Energies

The application of wind-generated energy is increasing at a great rate, about 11% per year, with ... more The application of wind-generated energy is increasing at a great rate, about 11% per year, with an installed capacity of 837 GW in 2021, and it is the primary non-hydro renewable technology; in many countries, it is the main source of electric energy [...]

Journal of Applied Fluid Mechanics, 2022

An optimal Kriging surrogate model based on a 5-fold cross-validation method and improved artific... more An optimal Kriging surrogate model based on a 5-fold cross-validation method and improved artificial fish swarm optimization is developed for improving the aerodynamic optimization efficiency of a high-speed train running in the open air. The developed optimal Kriging model is compared with the original Kriging model in two test sample points, and the prediction errors are all reduced to within 5%. Thus, the optimal Kriging model is selected for use in each iteration to approximate the CFD simulation model of a high-speed train in subsequent optimization. After that, the strong Pareto evolutionary algorithm II (SPEA2) is adopted to obtain a series of Pareto-optimal solutions. Based on the above work, a multi-objective aerodynamic optimization design for the head shape of a high-speed train is performed using a free-form deformation (FFD) parameterization approach. After optimization, the aerodynamic drag coefficient of the head car and the aerodynamic lift coefficient of the tail car are reduced by 5.2% and 32.6%, respectively. The results demonstrate that the optimization framework developed in this paper can effectively improve optimization efficiency.

Journal of Applied Fluid Mechanics, 2015

The adjoint method is used in this paper for the aerodynamic optimization of the nose shape of a ... more The adjoint method is used in this paper for the aerodynamic optimization of the nose shape of a train. This method has been extensively applied in aircraft or ground vehicle aerodynamic optimization, but is still in progress in train aerodynamics. Here we consider this innovative optimization method and present its application to reduce the aerodynamic drag when the train is subjected to front wind. The objective of this paper is to demonstrate the effectiveness of the method, highlighting the requirements, limitations and capabilities of it. Furthermore, a significant reduction of the aerodynamic drag in a short number of solver calls is aimed as well. The independence of the computational cost with respect to the number of design variables that define the optimal candidate is stressed as the most interesting characteristic of the adjoint method. This behavior permits a more complete modification of the shape of the train nose because the number of design variables is not a constraint anymore. The information obtained from the sensitivity field permits determining the regions of the geometry where a small modification of the nose shape might introduce a larger improvement of the train performance. A good agreement between this information and the successive geometry modifications is observed here.

PloS one, 2017

Minimizing the aerodynamic drag and the lift of the train coach remains a key issue for high-spee... more Minimizing the aerodynamic drag and the lift of the train coach remains a key issue for high-speed trains. With the development of computing technology and computational fluid dynamics (CFD) in the engineering field, CFD has been successfully applied to the design process of high-speed trains. However, developing a new streamlined shape for high-speed trains with excellent aerodynamic performance requires huge computational costs. Furthermore, relationships between multiple design variables and the aerodynamic loads are seldom obtained. In the present study, the Kriging surrogate model is used to perform a multi-objective optimization of the streamlined shape of high-speed trains, where the drag and the lift of the train coach are the optimization objectives. To improve the prediction accuracy of the Kriging model, the cross-validation method is used to construct the optimal Kriging model. The optimization results show that the two objectives are efficiently optimized, indicating th...

Numerical study of the influence of synthetic turbulent inflow conditions on the aerodynamics of a train

Journal of Fluids and Structures, 2015

ABSTRACT The necessity of a more complete definition of the turbulent wind acting on a train is s... more ABSTRACT The necessity of a more complete definition of the turbulent wind acting on a train is studied in this paper using computational fluid dynamics (CFD). A stochastic approach for the modeling of turbulent winds is proposed here. Synthetic winds are defined based on two different spectral models, namely the Kaimal spectrum and the Kraichnan spectrum. These are generated using and software, respectively. To complete the comparison, a third oncoming wind definition is considered, corresponding to a uniform (low-turbulence) wind. Large-Eddy Simulation (LES) and Scale-Adaptive Simulation (SAS) turbulence models have been used for the numerical simulation. Comparison is made of the average, standard deviations and extreme values of the loads calculated with the different methods. The corresponding flow fields are also studied and compared. The transient behavior is analyzed using the spectra of the velocity and loads, and the aerodynamic admittance curves. The results obtained for the last inlet condition are in good agreement with previous studies, while the importance of the spectral model choice is evidenced in the analysis of the velocity and force spectra, as well as in the aerodynamic admittance curves.

Prediction of wake effects on wind farm power production using a RANS approach. Offshore case studies from the UPWIND project

A semi-parabolized wake model for big offshore wind farms based on the open source CFD solver OpenFOAM

A hybrid Eulerian-Lagrangian approach is employed to simulate heavy particle dispersion in turbul... more A hybrid Eulerian-Lagrangian approach is employed to simulate heavy particle dispersion in turbulent pipe flow. The mean flow is provided by the Eulerian simulations developed by mean of JetCode, whereas the fluid fluctuations seen by particles are prescribed by a stochastic differential equation based on normalized Langevin. The statistics of particle velocity are compared to LES data which contain detailed statistics of velocity for particles with diameter equal to 20.4 µm. The model is in good agreement with the LES data for axial mean velocity whereas rms of axial and radial velocities should be adjusted.

ITM Web of Conferences, 2014

Wake effect represents one of the main sources of energy loss and uncertainty when designing offs... more Wake effect represents one of the main sources of energy loss and uncertainty when designing offshore wind farms. Traditionally analytical models have been used to optimize and estimate power deficits. However these models have shown to underestimate wake effect and consequently overestimate output power [1, 2]. This means that analytical models can be very helpful at optimizing preliminary layouts but not as accurate as needed for an ultimate fine design. Different techniques can be found in the literature to study wind turbine wakes that include simplified kinematic models and more advanced field models, that solve flow equations with different turbulence closure schemes. See the review papers of Crespo et al. [3], Vermeer et al. [4], and Sanderse et al. [5]. Purely elliptic Computational Fluid Dynamics (CFD) models based on the actuator disk technique have been developed during the last years [6-8]. They consider wind turbine rotor as a disk where a distribution of axial forces act over the incoming air. It is a fair approach but it can still be computationally expensive for big wind farms in an operative mode. With this technique still active, an alternative approach inspired on the parabolic wake models [9, 10] is proposed. Wind turbine rotors continue to be represented as actuator disks but now the domain is split into subdomains containing one or more wind turbines. The output of each subdomain is mapped onto the input boundary of the next one until the end of the domain is reached, getting a considerable decrease on computational time, by a factor of order 10. As the model is based on the open source CFD solver OpenFOAM, it can be parallelized to speed-up convergence. The near wake is calculated so no initial wind speed deficit profiles have to be supposed as in totally parabolic models and alternative turbulence models, such as the anisotropic Reynolds Stress Model (RSM) can be used. Traditional problems of elliptic models related to the estimation of the reference wind speed at each rotor position are mitigated due to the semi-parabolic algorithm. The model has been validated at the ECN test farm and at the offshore Horns Rev wind farm with significant results and also have been compared to other wake models. 1. Description of the semi-parabolic model The semi parabolic wake model is an in-house code based on the open source CFD solver OpenFOAM [26] and specifically designed for offshore wind farms. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Wind Energy, 2011

An elliptic computational fluid dynamics wake model based on the actuator disk concept is used to... more An elliptic computational fluid dynamics wake model based on the actuator disk concept is used to simulate a wind turbine, approximated by a disk upon which a distribution of forces, defined as axial momentum sources, is applied on an incoming non-uniform shear flow. The rotor is supposed to be uniformly loaded with the exerted forces estimated as a function of the incident wind speed, thrust coefficient and rotor diameter. The model is assessed in terms of wind speed deficit and added turbulence intensity for different turbulence models and is validated from experimental measurements of the Sexbierum wind turbine experiment. The elliptic model is also compared with other wake models in the literature on the basis of parabolic and large eddy simulation approximations.

Wind Energy, 2011

During recent years, wind energy has moved from an emerging technology to a nearly competitive te... more During recent years, wind energy has moved from an emerging technology to a nearly competitive technology. This fact, coupled with an increasing global focus on environmental concern and a political desire of a certain level of diversification in the energy supply, ensures wind energy an important role in the future electricity market. For this challenge to be met in a cost-efficient way, a substantial part of new wind turbine installations is foreseen to be erected in big onshore or offshore wind farms. This fact makes the production, loading and reliability of turbines operating under such conditions of particular interest. The key in understanding the above-raised issues is to identify and model the wind farm wind climate, which is known to deviate significantly from the wind climate faced by solitary turbines. This is due to the creation of wind turbine wakes and their mutual downstream interaction. This special issue of Wind Energy (WE) contains selected papers from Euromech Colloquium 508 on 'Wind Turbine Wakes' held at the Universidad Politécnica de Madrid on 20-22 October 2009. The colloquium was organized under the auspices of the EU TOPFARM project, chaired by Antonio Crespo, co-chaired by Gunner Chr. Larsen and coordinated by Emilio Migoya. The colloquium was organized in 10 sessions addressing a variety of wake aspects, and a total of 47 papers were presented. The first session was about wake models. Different existing models were analysed and compared, and possible improvements were discussed. The second session dealt with turbulence closure models associated with both elliptic and parabolic computational fluid dynamics (CFD) approaches. The third session was dedicated to CFD work on turbine aerodynamics and the near wake, including actuator surface, actuator line and actuator disc approaches. The theme of the fourth session was the influence of topography and atmospheric characteristics on wakes and the modification of the atmospheric boundary layer caused by very large arrays of wind farms. Sessions 5 and 6 were about experimental work including analysis and interpretation of full-scale and wind tunnel measurements. Sessions 7 and 8 dealt with wake meandering. This is an important topic that highlights the need for non-stationary modelling of wind farm wind fields when wind turbine loads are to be assessed. Session 9 was about large offshore wind farms. Both the flow fields within and behind large wind farms were addressed, including the interference of an upstream wind farm on the power output of a downstream wind farm. Finally, session 10 included contributions on wind farm and wind turbine control strategies, wind turbine fatigue loading within wind farms and wind farm layout optimization. The selected eight papers included in this WE special issue cover a breadth of topics, ranging from theory and numerical modelling to measurements and interpretation of such. Troldborg et al. 1 analysed wake interaction between two wind turbines using the actuator line technique in combination with full unsteady Navier-Stokes computations. Various ambient turbulence intensity levéis, turbine inter-spacings and types of wake interaction are considered, and main deterministic and stochastic flow field characteristics are extracted and discussed. España et al. studied the wake meandering phenomenon using physical modelling in a boundary layer wind tunnel. Based on particle image velocimetry, quantitative information on the meandering process is achieved including the instantaneous wake width and wake meandering envelope as a function of downstream distance. It is concluded that the mean wake width is primarily dictated by the meandering process. Cabezón et al. 3 introduced an elliptic actuator disc CFD model and assessed the model in terms of wind speed déficit and added turbulence intensity. Different turbulence models are considered, and the performance is validated through comparison with full-scale experimental results and results from alternative CFD-based wake models. Sanderse et al. 4 reviewed the state of the art of the numerical calculation of wind-turbine wake aerodynamics. Various CFD techniques for modelling the rotor and the wake are discussed. S0rensen and van Kuik analysed the 'free vortex' model of Joukowsky. In its classical formulation, this model relies on an approximate momentum consideration, which can be justified for high tip speed ratios only. Taking into account

La tecnología eólica es compleja y costosa, por lo que, desde el punto de vista de generación elé... more La tecnología eólica es compleja y costosa, por lo que, desde el punto de vista de generación eléctrica comercial, solo suele ser rentable la agrupación de aeroturbinas en un parque. Debido al tamaño, las máquinas interfieren entre si generando estelas que condicionan el diseño del parque produciendo: •Disminución de producción de energía por la reducción de la velocidad incidente en las máquinas aguas abajo, ya que el viento es frenado por la máquina de delante •Incremento de cargas dinámicas sobre los aerogeneradores por el aumento del nivel de turbulencia incidente en cada una de ellas, por lo que la vida útil de la máquina se acortará respecto a si estuviera en una corriente sin perturbar por la estela.

Aerodynamic Optimization of High-Speed Trains Nose using a Genetic Algorithm and Artificial Neural Network

Modelization of a large wind farm, considering the modification of the atmospheric boundary layer

Modeling of turbulence dissipation for gas-particle flows

ABSTRACT A model proposed by García & Crespo (1997 and 2000) to study the modification of... more ABSTRACT A model proposed by García & Crespo (1997 and 2000) to study the modification of turbulence characteristics by dispersed particles is applied to several configurations appearing in the literature. The model is valid for dilute flows and small Stokes number. The classical k-e method is used, modified with additional terms in the k and epsilon equations, that take into account the effect of particles on the carrier phase. The additional dissipation term included in the equation for k is proportional to the mass centration of particles and to e, and has a proportionality constant equal to 3/2 of Kolmogorov constant, C_o. The additional source term for the e equation is taken as proportional to e/k, using a C_e3 coefficient. The model applies for particles small enough so that they increase the dissipation of turbulent kinetic energy. Previously, the model was applied to jets and mixing layers, and in this work is applied to homogeneous turbulence and channel flows, in order to further check its validity, and find the most appropriate values of C_o and C_e3. First, it is considered the case of decay of k and e in homogeneous and isotropic turbulence; the results of the direct numerical simulation of Elghobashi & Truesdell (1993) and the experiments of Schreck & Kleis (1993) are used for comparison. Comparison is also made with the results of Squires & Eaton (1994), that reproduce isotropic and homogeneous turbulence under forced conditions and steady state. Channel flow is also studied and a comparison is made with the measurements of Kulick et al. (1994). From the comparisons is deduced that C_e3 has to be very similar to the C_e2 coefficient of the k-e method, although with different values in each case. The trends are correctly predicted although quite different values of C_o are obtained.

A Theoretical Model for the Combustión of Droplets in Supercritical Conditions and Gas Pockets. S... more A Theoretical Model for the Combustión of Droplets in Supercritical Conditions and Gas Pockets. Supercritical combustión of droplets is studied by means of a physical model which assumes spherical symmetry, laminar conditions, constant pressure and a zero-thickness ñame. Boundary conditions at the infinity state that temperature and composition of the mixture are given and constant, Initial distributions of temperature and mass fractions of the species are given, as well as the initial conditions at the droplet surface. As combustión proceeds, droplet surface is not considered to exist as a physical boundary allowing unrestricted diffusion of species through it. With some additional simplifications for the density and transport coefficients, a numetical solution of the problem is obtained. An analytical solution of the problem is also obtained by means of an asymptotic analysis. This solution applies when the initial temperature of the droplet is small as compared with the temperature of the sourounding atmosphere. It is shown that this is the most important case from the technological point of view. For this case results show that an apparent droplet exists throughout most of the process, in which its surface is characterized by an abrupt change in temperature and composition of the chemical species. Results show burning rates, combustión times, fíame radius and temperature at the droplet center as function of the principal variables of the process. In particular, the square of the apparent droplet radius is a linear function of time as occurs in subcritical combustión. It is shown that combustión times are faster in supercritical conditions than in subcritical conditions with the mínimum valué existing at critical conditions. A numerical application is carried out for the case of oxygen droplets burning in hydrogen and a comparison is carried out between the theoretical results obtained numerically and analytically as well as with those expenmentally obtamed.

Wind turbine wakes are an interesting topic of study. Velocity defects and the increase of turbul... more Wind turbine wakes are an interesting topic of study. Velocity defects and the increase of turbulence generated by the turbines in a wind farm usually cause a reduction in power generation and unsteady loads over other machines. In wind farms where, because of shortage of space, high cost of civil works, etc., the wind turbines are placed too close ones to the others, turbulence intensity may increase sufficiently to cause measurable damage due to fatigue and dynamic loads on some of them. ÍNDICE xv 4.1.2.1. Condiciones de contorno en la entrada y salida del dominio……………………………………………….... 53 4.1.2.2. Condición de contorno en los bordes inferior y superior del recinto……………………………………. 56 4.1.2.3. Condiciones de contorno en las caras laterales del dominio…………………………………………………. 59 4.2.

Energies

The application of wind-generated energy is increasing at a great rate, about 11% per year, with ... more The application of wind-generated energy is increasing at a great rate, about 11% per year, with an installed capacity of 837 GW in 2021, and it is the primary non-hydro renewable technology; in many countries, it is the main source of electric energy [...]

Journal of Applied Fluid Mechanics, 2022

An optimal Kriging surrogate model based on a 5-fold cross-validation method and improved artific... more An optimal Kriging surrogate model based on a 5-fold cross-validation method and improved artificial fish swarm optimization is developed for improving the aerodynamic optimization efficiency of a high-speed train running in the open air. The developed optimal Kriging model is compared with the original Kriging model in two test sample points, and the prediction errors are all reduced to within 5%. Thus, the optimal Kriging model is selected for use in each iteration to approximate the CFD simulation model of a high-speed train in subsequent optimization. After that, the strong Pareto evolutionary algorithm II (SPEA2) is adopted to obtain a series of Pareto-optimal solutions. Based on the above work, a multi-objective aerodynamic optimization design for the head shape of a high-speed train is performed using a free-form deformation (FFD) parameterization approach. After optimization, the aerodynamic drag coefficient of the head car and the aerodynamic lift coefficient of the tail car are reduced by 5.2% and 32.6%, respectively. The results demonstrate that the optimization framework developed in this paper can effectively improve optimization efficiency.

Journal of Applied Fluid Mechanics, 2015

The adjoint method is used in this paper for the aerodynamic optimization of the nose shape of a ... more The adjoint method is used in this paper for the aerodynamic optimization of the nose shape of a train. This method has been extensively applied in aircraft or ground vehicle aerodynamic optimization, but is still in progress in train aerodynamics. Here we consider this innovative optimization method and present its application to reduce the aerodynamic drag when the train is subjected to front wind. The objective of this paper is to demonstrate the effectiveness of the method, highlighting the requirements, limitations and capabilities of it. Furthermore, a significant reduction of the aerodynamic drag in a short number of solver calls is aimed as well. The independence of the computational cost with respect to the number of design variables that define the optimal candidate is stressed as the most interesting characteristic of the adjoint method. This behavior permits a more complete modification of the shape of the train nose because the number of design variables is not a constraint anymore. The information obtained from the sensitivity field permits determining the regions of the geometry where a small modification of the nose shape might introduce a larger improvement of the train performance. A good agreement between this information and the successive geometry modifications is observed here.

PloS one, 2017

Minimizing the aerodynamic drag and the lift of the train coach remains a key issue for high-spee... more Minimizing the aerodynamic drag and the lift of the train coach remains a key issue for high-speed trains. With the development of computing technology and computational fluid dynamics (CFD) in the engineering field, CFD has been successfully applied to the design process of high-speed trains. However, developing a new streamlined shape for high-speed trains with excellent aerodynamic performance requires huge computational costs. Furthermore, relationships between multiple design variables and the aerodynamic loads are seldom obtained. In the present study, the Kriging surrogate model is used to perform a multi-objective optimization of the streamlined shape of high-speed trains, where the drag and the lift of the train coach are the optimization objectives. To improve the prediction accuracy of the Kriging model, the cross-validation method is used to construct the optimal Kriging model. The optimization results show that the two objectives are efficiently optimized, indicating th...

Numerical study of the influence of synthetic turbulent inflow conditions on the aerodynamics of a train

Journal of Fluids and Structures, 2015

ABSTRACT The necessity of a more complete definition of the turbulent wind acting on a train is s... more ABSTRACT The necessity of a more complete definition of the turbulent wind acting on a train is studied in this paper using computational fluid dynamics (CFD). A stochastic approach for the modeling of turbulent winds is proposed here. Synthetic winds are defined based on two different spectral models, namely the Kaimal spectrum and the Kraichnan spectrum. These are generated using and software, respectively. To complete the comparison, a third oncoming wind definition is considered, corresponding to a uniform (low-turbulence) wind. Large-Eddy Simulation (LES) and Scale-Adaptive Simulation (SAS) turbulence models have been used for the numerical simulation. Comparison is made of the average, standard deviations and extreme values of the loads calculated with the different methods. The corresponding flow fields are also studied and compared. The transient behavior is analyzed using the spectra of the velocity and loads, and the aerodynamic admittance curves. The results obtained for the last inlet condition are in good agreement with previous studies, while the importance of the spectral model choice is evidenced in the analysis of the velocity and force spectra, as well as in the aerodynamic admittance curves.

Prediction of wake effects on wind farm power production using a RANS approach. Offshore case studies from the UPWIND project

A semi-parabolized wake model for big offshore wind farms based on the open source CFD solver OpenFOAM

A hybrid Eulerian-Lagrangian approach is employed to simulate heavy particle dispersion in turbul... more A hybrid Eulerian-Lagrangian approach is employed to simulate heavy particle dispersion in turbulent pipe flow. The mean flow is provided by the Eulerian simulations developed by mean of JetCode, whereas the fluid fluctuations seen by particles are prescribed by a stochastic differential equation based on normalized Langevin. The statistics of particle velocity are compared to LES data which contain detailed statistics of velocity for particles with diameter equal to 20.4 µm. The model is in good agreement with the LES data for axial mean velocity whereas rms of axial and radial velocities should be adjusted.

ITM Web of Conferences, 2014

Wake effect represents one of the main sources of energy loss and uncertainty when designing offs... more Wake effect represents one of the main sources of energy loss and uncertainty when designing offshore wind farms. Traditionally analytical models have been used to optimize and estimate power deficits. However these models have shown to underestimate wake effect and consequently overestimate output power [1, 2]. This means that analytical models can be very helpful at optimizing preliminary layouts but not as accurate as needed for an ultimate fine design. Different techniques can be found in the literature to study wind turbine wakes that include simplified kinematic models and more advanced field models, that solve flow equations with different turbulence closure schemes. See the review papers of Crespo et al. [3], Vermeer et al. [4], and Sanderse et al. [5]. Purely elliptic Computational Fluid Dynamics (CFD) models based on the actuator disk technique have been developed during the last years [6-8]. They consider wind turbine rotor as a disk where a distribution of axial forces act over the incoming air. It is a fair approach but it can still be computationally expensive for big wind farms in an operative mode. With this technique still active, an alternative approach inspired on the parabolic wake models [9, 10] is proposed. Wind turbine rotors continue to be represented as actuator disks but now the domain is split into subdomains containing one or more wind turbines. The output of each subdomain is mapped onto the input boundary of the next one until the end of the domain is reached, getting a considerable decrease on computational time, by a factor of order 10. As the model is based on the open source CFD solver OpenFOAM, it can be parallelized to speed-up convergence. The near wake is calculated so no initial wind speed deficit profiles have to be supposed as in totally parabolic models and alternative turbulence models, such as the anisotropic Reynolds Stress Model (RSM) can be used. Traditional problems of elliptic models related to the estimation of the reference wind speed at each rotor position are mitigated due to the semi-parabolic algorithm. The model has been validated at the ECN test farm and at the offshore Horns Rev wind farm with significant results and also have been compared to other wake models. 1. Description of the semi-parabolic model The semi parabolic wake model is an in-house code based on the open source CFD solver OpenFOAM [26] and specifically designed for offshore wind farms. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Wind Energy, 2011

An elliptic computational fluid dynamics wake model based on the actuator disk concept is used to... more An elliptic computational fluid dynamics wake model based on the actuator disk concept is used to simulate a wind turbine, approximated by a disk upon which a distribution of forces, defined as axial momentum sources, is applied on an incoming non-uniform shear flow. The rotor is supposed to be uniformly loaded with the exerted forces estimated as a function of the incident wind speed, thrust coefficient and rotor diameter. The model is assessed in terms of wind speed deficit and added turbulence intensity for different turbulence models and is validated from experimental measurements of the Sexbierum wind turbine experiment. The elliptic model is also compared with other wake models in the literature on the basis of parabolic and large eddy simulation approximations.

Wind Energy, 2011

During recent years, wind energy has moved from an emerging technology to a nearly competitive te... more During recent years, wind energy has moved from an emerging technology to a nearly competitive technology. This fact, coupled with an increasing global focus on environmental concern and a political desire of a certain level of diversification in the energy supply, ensures wind energy an important role in the future electricity market. For this challenge to be met in a cost-efficient way, a substantial part of new wind turbine installations is foreseen to be erected in big onshore or offshore wind farms. This fact makes the production, loading and reliability of turbines operating under such conditions of particular interest. The key in understanding the above-raised issues is to identify and model the wind farm wind climate, which is known to deviate significantly from the wind climate faced by solitary turbines. This is due to the creation of wind turbine wakes and their mutual downstream interaction. This special issue of Wind Energy (WE) contains selected papers from Euromech Colloquium 508 on 'Wind Turbine Wakes' held at the Universidad Politécnica de Madrid on 20-22 October 2009. The colloquium was organized under the auspices of the EU TOPFARM project, chaired by Antonio Crespo, co-chaired by Gunner Chr. Larsen and coordinated by Emilio Migoya. The colloquium was organized in 10 sessions addressing a variety of wake aspects, and a total of 47 papers were presented. The first session was about wake models. Different existing models were analysed and compared, and possible improvements were discussed. The second session dealt with turbulence closure models associated with both elliptic and parabolic computational fluid dynamics (CFD) approaches. The third session was dedicated to CFD work on turbine aerodynamics and the near wake, including actuator surface, actuator line and actuator disc approaches. The theme of the fourth session was the influence of topography and atmospheric characteristics on wakes and the modification of the atmospheric boundary layer caused by very large arrays of wind farms. Sessions 5 and 6 were about experimental work including analysis and interpretation of full-scale and wind tunnel measurements. Sessions 7 and 8 dealt with wake meandering. This is an important topic that highlights the need for non-stationary modelling of wind farm wind fields when wind turbine loads are to be assessed. Session 9 was about large offshore wind farms. Both the flow fields within and behind large wind farms were addressed, including the interference of an upstream wind farm on the power output of a downstream wind farm. Finally, session 10 included contributions on wind farm and wind turbine control strategies, wind turbine fatigue loading within wind farms and wind farm layout optimization. The selected eight papers included in this WE special issue cover a breadth of topics, ranging from theory and numerical modelling to measurements and interpretation of such. Troldborg et al. 1 analysed wake interaction between two wind turbines using the actuator line technique in combination with full unsteady Navier-Stokes computations. Various ambient turbulence intensity levéis, turbine inter-spacings and types of wake interaction are considered, and main deterministic and stochastic flow field characteristics are extracted and discussed. España et al. studied the wake meandering phenomenon using physical modelling in a boundary layer wind tunnel. Based on particle image velocimetry, quantitative information on the meandering process is achieved including the instantaneous wake width and wake meandering envelope as a function of downstream distance. It is concluded that the mean wake width is primarily dictated by the meandering process. Cabezón et al. 3 introduced an elliptic actuator disc CFD model and assessed the model in terms of wind speed déficit and added turbulence intensity. Different turbulence models are considered, and the performance is validated through comparison with full-scale experimental results and results from alternative CFD-based wake models. Sanderse et al. 4 reviewed the state of the art of the numerical calculation of wind-turbine wake aerodynamics. Various CFD techniques for modelling the rotor and the wake are discussed. S0rensen and van Kuik analysed the 'free vortex' model of Joukowsky. In its classical formulation, this model relies on an approximate momentum consideration, which can be justified for high tip speed ratios only. Taking into account

La tecnología eólica es compleja y costosa, por lo que, desde el punto de vista de generación elé... more La tecnología eólica es compleja y costosa, por lo que, desde el punto de vista de generación eléctrica comercial, solo suele ser rentable la agrupación de aeroturbinas en un parque. Debido al tamaño, las máquinas interfieren entre si generando estelas que condicionan el diseño del parque produciendo: •Disminución de producción de energía por la reducción de la velocidad incidente en las máquinas aguas abajo, ya que el viento es frenado por la máquina de delante •Incremento de cargas dinámicas sobre los aerogeneradores por el aumento del nivel de turbulencia incidente en cada una de ellas, por lo que la vida útil de la máquina se acortará respecto a si estuviera en una corriente sin perturbar por la estela.

Aerodynamic Optimization of High-Speed Trains Nose using a Genetic Algorithm and Artificial Neural Network

Modelization of a large wind farm, considering the modification of the atmospheric boundary layer

Modeling of turbulence dissipation for gas-particle flows

ABSTRACT A model proposed by García & Crespo (1997 and 2000) to study the modification of... more ABSTRACT A model proposed by García & Crespo (1997 and 2000) to study the modification of turbulence characteristics by dispersed particles is applied to several configurations appearing in the literature. The model is valid for dilute flows and small Stokes number. The classical k-e method is used, modified with additional terms in the k and epsilon equations, that take into account the effect of particles on the carrier phase. The additional dissipation term included in the equation for k is proportional to the mass centration of particles and to e, and has a proportionality constant equal to 3/2 of Kolmogorov constant, C_o. The additional source term for the e equation is taken as proportional to e/k, using a C_e3 coefficient. The model applies for particles small enough so that they increase the dissipation of turbulent kinetic energy. Previously, the model was applied to jets and mixing layers, and in this work is applied to homogeneous turbulence and channel flows, in order to further check its validity, and find the most appropriate values of C_o and C_e3. First, it is considered the case of decay of k and e in homogeneous and isotropic turbulence; the results of the direct numerical simulation of Elghobashi & Truesdell (1993) and the experiments of Schreck & Kleis (1993) are used for comparison. Comparison is also made with the results of Squires & Eaton (1994), that reproduce isotropic and homogeneous turbulence under forced conditions and steady state. Channel flow is also studied and a comparison is made with the measurements of Kulick et al. (1994). From the comparisons is deduced that C_e3 has to be very similar to the C_e2 coefficient of the k-e method, although with different values in each case. The trends are correctly predicted although quite different values of C_o are obtained.

A Theoretical Model for the Combustión of Droplets in Supercritical Conditions and Gas Pockets. S... more A Theoretical Model for the Combustión of Droplets in Supercritical Conditions and Gas Pockets. Supercritical combustión of droplets is studied by means of a physical model which assumes spherical symmetry, laminar conditions, constant pressure and a zero-thickness ñame. Boundary conditions at the infinity state that temperature and composition of the mixture are given and constant, Initial distributions of temperature and mass fractions of the species are given, as well as the initial conditions at the droplet surface. As combustión proceeds, droplet surface is not considered to exist as a physical boundary allowing unrestricted diffusion of species through it. With some additional simplifications for the density and transport coefficients, a numetical solution of the problem is obtained. An analytical solution of the problem is also obtained by means of an asymptotic analysis. This solution applies when the initial temperature of the droplet is small as compared with the temperature of the sourounding atmosphere. It is shown that this is the most important case from the technological point of view. For this case results show that an apparent droplet exists throughout most of the process, in which its surface is characterized by an abrupt change in temperature and composition of the chemical species. Results show burning rates, combustión times, fíame radius and temperature at the droplet center as function of the principal variables of the process. In particular, the square of the apparent droplet radius is a linear function of time as occurs in subcritical combustión. It is shown that combustión times are faster in supercritical conditions than in subcritical conditions with the mínimum valué existing at critical conditions. A numerical application is carried out for the case of oxygen droplets burning in hydrogen and a comparison is carried out between the theoretical results obtained numerically and analytically as well as with those expenmentally obtamed.