Articles de revista
http://hdl.handle.net/2117/3191
2016-02-10T07:09:00ZLevel-set simulations of buoyancy-driven motion of single and multiple bubbles
http://hdl.handle.net/2117/81668
Level-set simulations of buoyancy-driven motion of single and multiple bubbles
Balcázar Arciniega, Néstor; Lehmkuhl Barba, Oriol; Jofre Cruanyes, Lluís; Olivaa, Assensi
This paper presents a numerical study of buoyancy-driven motion of single and multiple bubbles by means of the conservative level-set method. First, an extensive study of the hydrodynamics of single bubbles rising in a quiescent liquid is performed, including its shape, terminal velocity, drag coefficients and wake patterns. These results are validated against experimental and numerical data well established in the scientific literature. Then, a further study on the interaction of two spherical and ellipsoidal bubbles is performed for different orientation angles. Finally, the interaction of multiple bubbles is explored in a periodic vertical channel. The results show that the conservative level-set approach can be used for accurate
modelling of bubble dynamics. Moreover, it is demonstrated that the present method is numerically stable for a wide range of Morton and Reynolds numbers.
2016-01-19T12:54:24ZBalcázar Arciniega, NéstorLehmkuhl Barba, OriolJofre Cruanyes, LluísOlivaa, AssensiThis paper presents a numerical study of buoyancy-driven motion of single and multiple bubbles by means of the conservative level-set method. First, an extensive study of the hydrodynamics of single bubbles rising in a quiescent liquid is performed, including its shape, terminal velocity, drag coefficients and wake patterns. These results are validated against experimental and numerical data well established in the scientific literature. Then, a further study on the interaction of two spherical and ellipsoidal bubbles is performed for different orientation angles. Finally, the interaction of multiple bubbles is explored in a periodic vertical channel. The results show that the conservative level-set approach can be used for accurate
modelling of bubble dynamics. Moreover, it is demonstrated that the present method is numerically stable for a wide range of Morton and Reynolds numbers.Parallel adaptive mesh refinement for large-eddy simulations of turbulent flows
http://hdl.handle.net/2117/81397
Parallel adaptive mesh refinement for large-eddy simulations of turbulent flows
Antepara Zambrano, Óscar; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Chiva Segura, Jorge; Oliva Llena, Asensio
In this paper a parallel adaptive mesh refinement (AMR) strategy for large eddy simulations (LES) of turbulent flows is presented. The underlying discretization of the Navier-Stokes equations is based on a finite-volume symmetry-preserving formulation, with the aim of preserving the symmetry properties of the continuous differential operators and ensure both, stability and conservation of kinetic-energy balance. The conservation properties are tested for the meshes resulting from the AMR process, which typically contain transitions between zones with different level of refinement. Our AMR scheme applies a cell-based refinement technique, with a physics-based refinement criteria based on the variational multi-scale (VMS) decomposition theory. The overall AMR process, from the selection of the cells to be refined/coarsened till the pre-processing of the resulting mesh, has been implemented in a parallel code, for which the parallel performance has been attested on an AMD Opteron based supercomputer. Finally, the robustness and accuracy of our methodology is shown on the numerical simulation of the turbulent flow around a square cylinder at Re = 22,000 and the turbulent flow around two side-by-side square cylinders at Re = 21,000.
2016-01-13T18:01:08ZAntepara Zambrano, ÓscarLehmkuhl Barba, OriolBorrell Pol, RicardChiva Segura, JorgeOliva Llena, AsensioIn this paper a parallel adaptive mesh refinement (AMR) strategy for large eddy simulations (LES) of turbulent flows is presented. The underlying discretization of the Navier-Stokes equations is based on a finite-volume symmetry-preserving formulation, with the aim of preserving the symmetry properties of the continuous differential operators and ensure both, stability and conservation of kinetic-energy balance. The conservation properties are tested for the meshes resulting from the AMR process, which typically contain transitions between zones with different level of refinement. Our AMR scheme applies a cell-based refinement technique, with a physics-based refinement criteria based on the variational multi-scale (VMS) decomposition theory. The overall AMR process, from the selection of the cells to be refined/coarsened till the pre-processing of the resulting mesh, has been implemented in a parallel code, for which the parallel performance has been attested on an AMD Opteron based supercomputer. Finally, the robustness and accuracy of our methodology is shown on the numerical simulation of the turbulent flow around a square cylinder at Re = 22,000 and the turbulent flow around two side-by-side square cylinders at Re = 21,000.Numerical analysis of the transpose diffusive term for viscoplastic-type non-Newtonian fluid flows using a collocated variable arrangement
http://hdl.handle.net/2117/81391
Numerical analysis of the transpose diffusive term for viscoplastic-type non-Newtonian fluid flows using a collocated variable arrangement
Carmona Muñoz, Ángel; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, Asensio
The aim of this work is to delve into the numerical analysis of viscoplastic-type non-Newtonian fluid flows. Specifically, improvements in the spatial discretization schemes and the temporal integration methods have been proposed to overcome the numerical problems introduced by the transpose diffusive term and associated with the velocity field discontinuity, the artificial viscous diffusion, and the transpose viscous coupling. The resulting knowledge may be useful, among many other reasons, to improve the corresponding numerical simulations and gain insight into the underlying physics of this class of non-Newtonian fluid flows.; The aim of this work is to delve into the numerical analysis of viscoplastic-type non-Newtonian fluid flows. Specifically, improvements in the spatial discretization schemes and the temporal integration methods have been proposed to overcome the numerical problems introduced by the transpose diffusive term and associated with the velocity field discontinuity, the artificial viscous diffusion, and the transpose viscous coupling. The resulting knowledge may be useful, among many other reasons, to improve the corresponding numerical simulations and gain insight into the underlying physics of this class of non-Newtonian fluid flows.
2016-01-13T17:16:17ZCarmona Muñoz, ÁngelLehmkuhl Barba, OriolPérez Segarra, Carlos DavidOliva Llena, AsensioThe aim of this work is to delve into the numerical analysis of viscoplastic-type non-Newtonian fluid flows. Specifically, improvements in the spatial discretization schemes and the temporal integration methods have been proposed to overcome the numerical problems introduced by the transpose diffusive term and associated with the velocity field discontinuity, the artificial viscous diffusion, and the transpose viscous coupling. The resulting knowledge may be useful, among many other reasons, to improve the corresponding numerical simulations and gain insight into the underlying physics of this class of non-Newtonian fluid flows.
The aim of this work is to delve into the numerical analysis of viscoplastic-type non-Newtonian fluid flows. Specifically, improvements in the spatial discretization schemes and the temporal integration methods have been proposed to overcome the numerical problems introduced by the transpose diffusive term and associated with the velocity field discontinuity, the artificial viscous diffusion, and the transpose viscous coupling. The resulting knowledge may be useful, among many other reasons, to improve the corresponding numerical simulations and gain insight into the underlying physics of this class of non-Newtonian fluid flows.Fixed-grid numerical modeling of melting and solidification using variable thermo-physical properties - Application to the melting of n-Octadecane inside a spherical capsule
http://hdl.handle.net/2117/80911
Fixed-grid numerical modeling of melting and solidification using variable thermo-physical properties - Application to the melting of n-Octadecane inside a spherical capsule
Galione Klot, Pedro Andrés; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Oliva Llena, Asensio
A fixed-grid enthalpy model for unstructured meshes and explicit time integration schemes (Galione et al., 2014) is here extended for taking into account the change in density and other thermo-physical properties with the temperature and phase. Thermal expansion and contraction associated to the phase change are taken into account in the conservation equations, and different strategies for the numerical treatment of the energy equation are discussed in detail. Further modifications to the original model are also presented.; The proposed model is used for simulating a case of melting of n-Octadecane inside a spherical capsule. Two and three-dimensional simulations are performed using constant and variable properties. The effect of adopting two different numerical schemes for the convective term of the energy equation is evaluated. A comprehensive examination of the thermo-physical properties is performed, and the different values and correlations used are here presented and criticized. Differences in the flow patterns are encountered between two and three-dimensional simulations. The effects of considering constant or variable properties are discussed. Two different thermal boundary conditions are tested and the results are compared against experimental data obtained from the literature.
2015-12-18T14:22:45ZGalione Klot, Pedro AndrésLehmkuhl Barba, OriolRigola Serrano, JoaquimOliva Llena, AsensioA fixed-grid enthalpy model for unstructured meshes and explicit time integration schemes (Galione et al., 2014) is here extended for taking into account the change in density and other thermo-physical properties with the temperature and phase. Thermal expansion and contraction associated to the phase change are taken into account in the conservation equations, and different strategies for the numerical treatment of the energy equation are discussed in detail. Further modifications to the original model are also presented.; The proposed model is used for simulating a case of melting of n-Octadecane inside a spherical capsule. Two and three-dimensional simulations are performed using constant and variable properties. The effect of adopting two different numerical schemes for the convective term of the energy equation is evaluated. A comprehensive examination of the thermo-physical properties is performed, and the different values and correlations used are here presented and criticized. Differences in the flow patterns are encountered between two and three-dimensional simulations. The effects of considering constant or variable properties are discussed. Two different thermal boundary conditions are tested and the results are compared against experimental data obtained from the literature.A multiple marker level-set method for simulation of deformable fluid particles
http://hdl.handle.net/2117/80876
A multiple marker level-set method for simulation of deformable fluid particles
Balcázar Arciniega, Néstor; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Oliva Llena, Asensio
A novel multiple marker level-set method is introduced for Direct Numerical Simulation of deformable fluid particles (bubbles and droplets), which is integrated in a finite-volume framework on collocated unstructured grids. Each fluid particle is described by a separate level-set function, thus, different interfaces can be solved in the same control volume, avoiding artificial and potentially unphysical coalescence of fluid particles. Therefore, bubbles or droplets are able to approach each other closely, within the size of one grid cell, and can even collide. The proposed algorithm is developed in the context of the conservative level-set method, whereas, surface tension is modeled by the continuous surface force approach. The pressure-velocity coupling is solved by the fractional-step projection method. For validation of the proposed numerical method, the gravity-driven impact of a droplet on a liquid-liquid interface is studied; then, the binary droplet collision with bouncing outcome is examined, and finally, it is applied on simulation of gravity-driven bubbly flow in a vertical column.
2015-12-17T16:26:02ZBalcázar Arciniega, NéstorLehmkuhl Barba, OriolRigola Serrano, JoaquimOliva Llena, AsensioA novel multiple marker level-set method is introduced for Direct Numerical Simulation of deformable fluid particles (bubbles and droplets), which is integrated in a finite-volume framework on collocated unstructured grids. Each fluid particle is described by a separate level-set function, thus, different interfaces can be solved in the same control volume, avoiding artificial and potentially unphysical coalescence of fluid particles. Therefore, bubbles or droplets are able to approach each other closely, within the size of one grid cell, and can even collide. The proposed algorithm is developed in the context of the conservative level-set method, whereas, surface tension is modeled by the continuous surface force approach. The pressure-velocity coupling is solved by the fractional-step projection method. For validation of the proposed numerical method, the gravity-driven impact of a droplet on a liquid-liquid interface is studied; then, the binary droplet collision with bouncing outcome is examined, and finally, it is applied on simulation of gravity-driven bubbly flow in a vertical column.A coupled volume-of-fluid/level-set method for simulation of two-phase flows in unstructured meshes
http://hdl.handle.net/2117/80871
A coupled volume-of-fluid/level-set method for simulation of two-phase flows in unstructured meshes
Balcázar Arciniega, Néstor; Lehmkuhl Barba, Oriol; Jofre Cruanyes, Lluís; Rigola Serrano, Joaquim; Oliva Llena, Asensio
This paper presents a methodology for simulation of two-phase flows with surface tension in the framework of unstructured meshes, which combines volume-of-fluid with level-set methods. While the volume-of-fluid transport relies on a robust and accurate polyhedral library for interface advection, surface tension force is calculated by using a level-set function reconstructed by means of a geometrical procedure. Moreover the solution of the fluid flow equations is performed through the fractional step method, using a finite-volume discretization on a collocated grid arrangement. The numerical method is validated against two- and three-dimensional test cases well established in the literature. Conservation properties of this method are shown to be excellent, while geometrical accuracy remains satisfactory even for the most complex flows.; This paper presents a methodology for simulation of two-phase flows with surface tension in the framework of unstructured meshes, which combines volume-of-fluid with level-set methods. While the volume-of-fluid transport relies on a robust and accurate polyhedral library for interface advection, surface tension force is calculated by using a level-set function reconstructed by means of a geometrical procedure. Moreover the solution of the fluid flow equations is performed through the fractional step method, using a finite-volume discretization on a collocated grid arrangement. The numerical method is validated against two- and three-dimensional test cases well established in the literature. Conservation properties of this method are shown to be excellent, while geometrical accuracy remains satisfactory even for the most complex flows.
2015-12-17T13:39:40ZBalcázar Arciniega, NéstorLehmkuhl Barba, OriolJofre Cruanyes, LluísRigola Serrano, JoaquimOliva Llena, AsensioThis paper presents a methodology for simulation of two-phase flows with surface tension in the framework of unstructured meshes, which combines volume-of-fluid with level-set methods. While the volume-of-fluid transport relies on a robust and accurate polyhedral library for interface advection, surface tension force is calculated by using a level-set function reconstructed by means of a geometrical procedure. Moreover the solution of the fluid flow equations is performed through the fractional step method, using a finite-volume discretization on a collocated grid arrangement. The numerical method is validated against two- and three-dimensional test cases well established in the literature. Conservation properties of this method are shown to be excellent, while geometrical accuracy remains satisfactory even for the most complex flows.
This paper presents a methodology for simulation of two-phase flows with surface tension in the framework of unstructured meshes, which combines volume-of-fluid with level-set methods. While the volume-of-fluid transport relies on a robust and accurate polyhedral library for interface advection, surface tension force is calculated by using a level-set function reconstructed by means of a geometrical procedure. Moreover the solution of the fluid flow equations is performed through the fractional step method, using a finite-volume discretization on a collocated grid arrangement. The numerical method is validated against two- and three-dimensional test cases well established in the literature. Conservation properties of this method are shown to be excellent, while geometrical accuracy remains satisfactory even for the most complex flows.Multi-layered solid-PCM thermocline thermal storage for CSP. Numerical evaluation of its application in a 50 MWe plant
http://hdl.handle.net/2117/76396
Multi-layered solid-PCM thermocline thermal storage for CSP. Numerical evaluation of its application in a 50 MWe plant
Galione Klot, Pedro Andrés; Pérez Segarra, Carlos David; Rodríguez Pérez, Ivette María; Torras Ortiz, Santiago; Rigola Serrano, Joaquim
Thermocline storage concept is considered as a possible solution to reduce the cost of thermal storage in concentrated solar power (CSP) plants. Recently, a multi-layered solid-PCM (MLSPCM) concept—consisting of a thermocline-like tank combining layers of solid and phase change filler materials—has been proposed. This approach was observed to result in lower thermocline degradation throughout charge/discharge cycles, due to the thermal buffering effect of the PCM layers located at both ends of the tank. MLSPCM prototypes designed for a pilot scale plant were numerically tested and compared against other designs of single-tank thermocline systems, such as: solid-filled thermocline, tanks filled with a single encapsulated PCM and cascaded-PCM configurations. Results showed promising results of the MLSPCM configurations for their potential use in CSP plants.
In this work, the MLSPCM concept is used for designing a thermal energy storage (TES) system for a CSP plant with the dimensions and operating conditions of a parabolic trough plant of 50 MWe, similar to Andasol 1 (Granada, Spain). The performance evaluation of each of the proposed prototypes is virtually tested by means of a numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. Two sets of cases are considered, one with the objective of testing the TES systems individually, by defining specific operating conditions and taking the systems to a periodic steady state; and another, aiming to evaluate their performance after several days of operation in a CSP plant, in which the weather variability and the thermal behavior of the tank walls and foundation are simulated. Thermal performance parameters, such as total energy and exergy stored/released and the efficiency in the use of the storage capacity, are calculated and compared with those obtained by other thermocline-like configurations (single-solid and single-PCM), and with a reference 2-tank molten-salt system. Obtained results allow to continue considering the MLSPCM concept as an interesting alternative for thermal storage in CSP facilities.
2015-07-30T10:14:45ZGalione Klot, Pedro AndrésPérez Segarra, Carlos DavidRodríguez Pérez, Ivette MaríaTorras Ortiz, SantiagoRigola Serrano, JoaquimThermocline storage concept is considered as a possible solution to reduce the cost of thermal storage in concentrated solar power (CSP) plants. Recently, a multi-layered solid-PCM (MLSPCM) concept—consisting of a thermocline-like tank combining layers of solid and phase change filler materials—has been proposed. This approach was observed to result in lower thermocline degradation throughout charge/discharge cycles, due to the thermal buffering effect of the PCM layers located at both ends of the tank. MLSPCM prototypes designed for a pilot scale plant were numerically tested and compared against other designs of single-tank thermocline systems, such as: solid-filled thermocline, tanks filled with a single encapsulated PCM and cascaded-PCM configurations. Results showed promising results of the MLSPCM configurations for their potential use in CSP plants.
In this work, the MLSPCM concept is used for designing a thermal energy storage (TES) system for a CSP plant with the dimensions and operating conditions of a parabolic trough plant of 50 MWe, similar to Andasol 1 (Granada, Spain). The performance evaluation of each of the proposed prototypes is virtually tested by means of a numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. Two sets of cases are considered, one with the objective of testing the TES systems individually, by defining specific operating conditions and taking the systems to a periodic steady state; and another, aiming to evaluate their performance after several days of operation in a CSP plant, in which the weather variability and the thermal behavior of the tank walls and foundation are simulated. Thermal performance parameters, such as total energy and exergy stored/released and the efficiency in the use of the storage capacity, are calculated and compared with those obtained by other thermocline-like configurations (single-solid and single-PCM), and with a reference 2-tank molten-salt system. Obtained results allow to continue considering the MLSPCM concept as an interesting alternative for thermal storage in CSP facilities.Multi-layered solid-PCM thermocline thermal storage concept for CSP plants. Numerical analysis and perspectives
http://hdl.handle.net/2117/76330
Multi-layered solid-PCM thermocline thermal storage concept for CSP plants. Numerical analysis and perspectives
Galione Klot, Pedro Andrés; Pérez Segarra, Carlos David; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio; Rigola Serrano, Joaquim
Thermocline storage concept has been considered for more than a decade as a possible solution to reduce the huge cost of the storage system in concentrated solar power (CSP) plants. However, one of the drawbacks of this concept is the decrease in its performance throughout the time. The objective of this paper is to present a new thermocline-like storage concept, which aims at circumventing this issue. The proposed concept consists of a storage tank filled with a combination of solid material and encapsulated PCMs, forming a multi-layered packed bed, with molten salt as the heat transfer fluid. The performance evaluation of each of the prototypes proposed is virtually tested by means of a detailed numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. The virtual tests carried out are designed so as to take into account several charging and discharging cycles until periodic state is achieved, i.e. when the same amount of energy is stored/released in consecutive charging/discharging cycles. As a result, the dependence of the storage capacity on the PCMs temperatures, the total energy and exergy stored/released, as well as the efficiencies of the storing process are compared for the different thermocline, single PCM, cascaded PCM and the proposed multi-layered solid-PCM (MLSPCM) configurations. The analysis shows that the multi-layered solid-PCM concept is a promising alternative for thermal storage in CSP plants.
2015-07-27T09:58:03ZGalione Klot, Pedro AndrésPérez Segarra, Carlos DavidRodríguez Pérez, Ivette MaríaOliva Llena, AsensioRigola Serrano, JoaquimThermocline storage concept has been considered for more than a decade as a possible solution to reduce the huge cost of the storage system in concentrated solar power (CSP) plants. However, one of the drawbacks of this concept is the decrease in its performance throughout the time. The objective of this paper is to present a new thermocline-like storage concept, which aims at circumventing this issue. The proposed concept consists of a storage tank filled with a combination of solid material and encapsulated PCMs, forming a multi-layered packed bed, with molten salt as the heat transfer fluid. The performance evaluation of each of the prototypes proposed is virtually tested by means of a detailed numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. The virtual tests carried out are designed so as to take into account several charging and discharging cycles until periodic state is achieved, i.e. when the same amount of energy is stored/released in consecutive charging/discharging cycles. As a result, the dependence of the storage capacity on the PCMs temperatures, the total energy and exergy stored/released, as well as the efficiencies of the storing process are compared for the different thermocline, single PCM, cascaded PCM and the proposed multi-layered solid-PCM (MLSPCM) configurations. The analysis shows that the multi-layered solid-PCM concept is a promising alternative for thermal storage in CSP plants.A finite-volume/level-set method for simulating two-phase flows on unstructured grids
http://hdl.handle.net/2117/26121
A finite-volume/level-set method for simulating two-phase flows on unstructured grids
Balcázar Arciniega, Néstor; Jofre Cruanyes, Lluís; Lehmkuhl Barba, Oriol; Castro González, Jesús; Rigola Serrano, Joaquim
The conservative level-set method for capturing the interface between two fluids is combined with a variable density projection scheme to simulate incompressible two-phase flows on unstructured meshes. All equations are discretized by using a conservative finite-volume approximation on a collocated grid arrangement. A high order scheme based on a flux limiter formulation, is adopted for approximating the convective terms, while the diffusive fluxes are centrally differenced. Gradients are computed by the least-squares approach. Physical properties are assumed to vary smoothly in a narrow band around the interface to avoid numerical instabilities. The numerical method is validated against classical advection test and two-phase flow examples including topology changes.
2015-01-27T15:18:32ZBalcázar Arciniega, NéstorJofre Cruanyes, LluísLehmkuhl Barba, OriolCastro González, JesúsRigola Serrano, JoaquimThe conservative level-set method for capturing the interface between two fluids is combined with a variable density projection scheme to simulate incompressible two-phase flows on unstructured meshes. All equations are discretized by using a conservative finite-volume approximation on a collocated grid arrangement. A high order scheme based on a flux limiter formulation, is adopted for approximating the convective terms, while the diffusive fluxes are centrally differenced. Gradients are computed by the least-squares approach. Physical properties are assumed to vary smoothly in a narrow band around the interface to avoid numerical instabilities. The numerical method is validated against classical advection test and two-phase flow examples including topology changes.Unsteady forces on a circular cylinder at critical Reynolds numbers
http://hdl.handle.net/2117/25143
Unsteady forces on a circular cylinder at critical Reynolds numbers
Lehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María; Borrell, Ricard; Chiva Segura, Jorge; Oliva Llena, Asensio
It is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence transition, reattachment of the flow, and further turbulent separation of the boundary layer. The transition to turbulence in the boundary layer causes the delaying of the separation point and an important reduction of the drag force on the cylinder surface known as the drag crisis. In the present work, large-eddy simulations of the flow past a cylinder at Reynolds numbers in the range 2.5 × 105-6.5 × 105 are performed. It is shown how the pressure distribution changes as the Reynolds number increases in an asymmetric manner, occurring first on one side of the cylinder and then on the other side to complete the drop in the drag up to 0.23 at Re = 6.5 × 105. These variations in the pressure profile are accompanied by the presence of a small recirculation bubble, observed as a small plateau in the pressure, and located around ¿ = 105° (measured from the stagnation point). This small recirculation bubble anticipated by the experimental measurements is here well captured by the present computations and its position and size measured at every Reynolds number. The changes in the wake configuration as the Reynolds number increases are also shown and their relation to the increase in the vortex shedding frequency is discussed. The power spectra for the velocity fluctuations are computed. The analysis of the resulting spectrum showed the footprint of Kelvin-Helmholtz instabilities in the whole range. It is found that the ratio of these instabilities frequency to the primary vortex shedding frequency matches quite well the scaling proposed by Prasad and Williamson [“The instability of the separated shear layer from a bluff body,” Phys. Fluids 8, 1347 (1996); “The instability of the shear layer separating from a bluff body,” J. Fluid Mech. 333, 375–492 (1997)] (f KH /fvs ¿ Re 0.67).
2014-12-29T09:40:53ZLehmkuhl Barba, OriolRodríguez Pérez, Ivette MaríaBorrell, RicardChiva Segura, JorgeOliva Llena, AsensioIt is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence transition, reattachment of the flow, and further turbulent separation of the boundary layer. The transition to turbulence in the boundary layer causes the delaying of the separation point and an important reduction of the drag force on the cylinder surface known as the drag crisis. In the present work, large-eddy simulations of the flow past a cylinder at Reynolds numbers in the range 2.5 × 105-6.5 × 105 are performed. It is shown how the pressure distribution changes as the Reynolds number increases in an asymmetric manner, occurring first on one side of the cylinder and then on the other side to complete the drop in the drag up to 0.23 at Re = 6.5 × 105. These variations in the pressure profile are accompanied by the presence of a small recirculation bubble, observed as a small plateau in the pressure, and located around ¿ = 105° (measured from the stagnation point). This small recirculation bubble anticipated by the experimental measurements is here well captured by the present computations and its position and size measured at every Reynolds number. The changes in the wake configuration as the Reynolds number increases are also shown and their relation to the increase in the vortex shedding frequency is discussed. The power spectra for the velocity fluctuations are computed. The analysis of the resulting spectrum showed the footprint of Kelvin-Helmholtz instabilities in the whole range. It is found that the ratio of these instabilities frequency to the primary vortex shedding frequency matches quite well the scaling proposed by Prasad and Williamson [“The instability of the separated shear layer from a bluff body,” Phys. Fluids 8, 1347 (1996); “The instability of the shear layer separating from a bluff body,” J. Fluid Mech. 333, 375–492 (1997)] (f KH /fvs ¿ Re 0.67).