Doctorat en Física Computacional i Aplicada
http://hdl.handle.net/2117/184576
2024-03-28T19:43:33ZFlash sintering of potassium-sodium niobate-based piezoceramics: refining (micro)structure for properties enhancement
http://hdl.handle.net/2117/405192
Flash sintering of potassium-sodium niobate-based piezoceramics: refining (micro)structure for properties enhancement
López Blanco, Samuel; Rubio Marcos, Fernando; Barrón Portela, Andrea; Ochoa Guerrero, Diego A.; García García, José Eduardo
Low-consumption ceramics processing routes are expected to replace conventional ones due to environmental concerns. In this context, flash sintering is garnering interest because it allows dense ceramics to be obtained in just a few minutes and at relatively low temperatures. This is particularly interesting for sintering alkaline-based compounds due to the easy volatilization of these elements. In this work, current-controlled flash sintering is used to obtain potassium-sodium niobate (KNN)-based piezoceramics with refined microstructure and suitable stoichiometry, leading to improved functional properties. KNN-based materials are currently outstanding lead-free piezoceramics, with their properties highly sensitive to the proper construction of the polymorphic phase boundary, as in the case of the first promising composition (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3. Results of this work show that sintering parameters may determine the polymorphic behavior of this system, thereby evincing flash sintering allows polymorphic phase boundary to be fine-tuned. It is demonstrated that the convergence of microstructure refinement and compositional control holds the potential for enhancing properties through a proper electric current control during flash sintering.
2024-03-22T15:43:55ZLópez Blanco, SamuelRubio Marcos, FernandoBarrón Portela, AndreaOchoa Guerrero, Diego A.García García, José EduardoLow-consumption ceramics processing routes are expected to replace conventional ones due to environmental concerns. In this context, flash sintering is garnering interest because it allows dense ceramics to be obtained in just a few minutes and at relatively low temperatures. This is particularly interesting for sintering alkaline-based compounds due to the easy volatilization of these elements. In this work, current-controlled flash sintering is used to obtain potassium-sodium niobate (KNN)-based piezoceramics with refined microstructure and suitable stoichiometry, leading to improved functional properties. KNN-based materials are currently outstanding lead-free piezoceramics, with their properties highly sensitive to the proper construction of the polymorphic phase boundary, as in the case of the first promising composition (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3. Results of this work show that sintering parameters may determine the polymorphic behavior of this system, thereby evincing flash sintering allows polymorphic phase boundary to be fine-tuned. It is demonstrated that the convergence of microstructure refinement and compositional control holds the potential for enhancing properties through a proper electric current control during flash sintering.Creep deformation in metallic glasses: a global approach with strain as an indicator within transition state theory
http://hdl.handle.net/2117/405188
Creep deformation in metallic glasses: a global approach with strain as an indicator within transition state theory
Zhang, LangTing; Wang, Yun-Jiang; Nabahat, Mehran; Pineda Soler, Eloi; Yang, Yong; Pelletier, Jean Marc; Crespo Artiaga, Daniel; Qiao, Jichao
Within the framework of transition state theory, the isothermal creep behavior of metallic glasses is elucidated through a unique global approach, where the topological state is exclusively linked to measured strain. Our methodology allows the computation of the average activation volume and activation energy of deformation units as a function of strain. Experimental data from four representative metallic glasses (La30Ce30Ni10Al10Co20, La65Ni15Al25, La56.16Ce14.04Ni19.8Al10, and Cu46Zr46Al8) reveal two distinct characteristics. Below the glass transition temperature, the mechanical response is primarily influenced by secondary relaxation processes and excess configuration entropy, with activation volume increasing with strain. Upon reaching the glass transition temperature, the activation volume becomes notably larger and strain-independent. Additionally, the activation energy exhibits an increase with strain, and deformation units of varying sizes are progressively activated, from smaller to larger units. The decoupling and competition among relaxation events are correlated with the increase in the activation volume of deformation units. These findings provide valuable insights into the dynamic behavior of metallic glasses and their mechanical response across different states.
2024-03-22T14:55:07ZZhang, LangTingWang, Yun-JiangNabahat, MehranPineda Soler, EloiYang, YongPelletier, Jean MarcCrespo Artiaga, DanielQiao, JichaoWithin the framework of transition state theory, the isothermal creep behavior of metallic glasses is elucidated through a unique global approach, where the topological state is exclusively linked to measured strain. Our methodology allows the computation of the average activation volume and activation energy of deformation units as a function of strain. Experimental data from four representative metallic glasses (La30Ce30Ni10Al10Co20, La65Ni15Al25, La56.16Ce14.04Ni19.8Al10, and Cu46Zr46Al8) reveal two distinct characteristics. Below the glass transition temperature, the mechanical response is primarily influenced by secondary relaxation processes and excess configuration entropy, with activation volume increasing with strain. Upon reaching the glass transition temperature, the activation volume becomes notably larger and strain-independent. Additionally, the activation energy exhibits an increase with strain, and deformation units of varying sizes are progressively activated, from smaller to larger units. The decoupling and competition among relaxation events are correlated with the increase in the activation volume of deformation units. These findings provide valuable insights into the dynamic behavior of metallic glasses and their mechanical response across different states.Multisystem inflammatory syndrome in children and SARS-CoV-2 variants: a two-year ambispective multicentric cohort study in Catalonia, Spain
http://hdl.handle.net/2117/405167
Multisystem inflammatory syndrome in children and SARS-CoV-2 variants: a two-year ambispective multicentric cohort study in Catalonia, Spain
Pino Ramirez, Rosa Maria; Antoñanzas, Jesús M.; Paredes-Carmona, Fernando; Perramon Malavez, Aida; Rivière, Jacques G.; Martínez Mejías, Abel; Gatell Carbó, Anna; Soler-Palacín, Pere; Fina Avilés, Francesc; Prats Soler, Clara; Soriano-Arandes, Antoni
Multisystem inflammatory syndrome in children (MIS-C) is a rare but severe disease temporarily related to SARS-CoV-2. We aimed to describe the epidemiological, clinical, and laboratory findings of all MIS-C cases diagnosed in children < 18 years old in Catalonia (Spain) to study their trend throughout the pandemic. This was a multicenter ambispective observational cohort study (April 2020– April 2022). Data were obtained from the COVID-19 Catalan surveillance system and from all hospitals in Catalonia. We analyzed MIS-C cases regarding SARS-CoV-2 variants for demographics, symptoms, severity, monthly MIS-C incidence, ratio between MIS-C and accumulated COVID-19 cases, and associated rate ratios (RR). Among 555,848 SARS-CoV-2 infections, 152 children were diagnosed with MIS-C. The monthly MIS-C incidence was 4.1 (95% CI: 3.4–4.8) per 1,000,000 people, and 273 (95% CI: 230–316) per 1,000,000 SARS-CoV-2 infections (i.e., one case per 3,700 SARS-CoV-2 infections). During the Omicron period, the MIS-C RR was 8.2 (95% CI: 5.7–11.7) per 1,000,000 SARS-CoV-2 infections, which was significantly lower (p < 0.001) than that for previous variant periods in all age groups. The median [IQR] age of MIS-C was 8 [4–11] years, 62.5% male, and 80.2% without comorbidities. Common symptoms were gastrointestinal findings (88.2%) and fever > 39 °C (81.6%); nearly 40% had an abnormal echocardiography, and 7% had coronary aneurysm. Clinical manifestations and laboratory data were not different throughout the variant periods (p > 0.05). Conclusion: The RR between MIS-C cases and SARS-CoV-2 infections was significantly lower in the Omicron period for all age groups, including those not vaccinated, suggesting that the variant could be the main factor for this shift in the MISC trend. Regardless of variant type, the patients had similar phenotypes and severity throughout the pandemic.
2024-03-22T12:37:53ZPino Ramirez, Rosa MariaAntoñanzas, Jesús M.Paredes-Carmona, FernandoPerramon Malavez, AidaRivière, Jacques G.Martínez Mejías, AbelGatell Carbó, AnnaSoler-Palacín, PereFina Avilés, FrancescPrats Soler, ClaraSoriano-Arandes, AntoniMultisystem inflammatory syndrome in children (MIS-C) is a rare but severe disease temporarily related to SARS-CoV-2. We aimed to describe the epidemiological, clinical, and laboratory findings of all MIS-C cases diagnosed in children < 18 years old in Catalonia (Spain) to study their trend throughout the pandemic. This was a multicenter ambispective observational cohort study (April 2020– April 2022). Data were obtained from the COVID-19 Catalan surveillance system and from all hospitals in Catalonia. We analyzed MIS-C cases regarding SARS-CoV-2 variants for demographics, symptoms, severity, monthly MIS-C incidence, ratio between MIS-C and accumulated COVID-19 cases, and associated rate ratios (RR). Among 555,848 SARS-CoV-2 infections, 152 children were diagnosed with MIS-C. The monthly MIS-C incidence was 4.1 (95% CI: 3.4–4.8) per 1,000,000 people, and 273 (95% CI: 230–316) per 1,000,000 SARS-CoV-2 infections (i.e., one case per 3,700 SARS-CoV-2 infections). During the Omicron period, the MIS-C RR was 8.2 (95% CI: 5.7–11.7) per 1,000,000 SARS-CoV-2 infections, which was significantly lower (p < 0.001) than that for previous variant periods in all age groups. The median [IQR] age of MIS-C was 8 [4–11] years, 62.5% male, and 80.2% without comorbidities. Common symptoms were gastrointestinal findings (88.2%) and fever > 39 °C (81.6%); nearly 40% had an abnormal echocardiography, and 7% had coronary aneurysm. Clinical manifestations and laboratory data were not different throughout the variant periods (p > 0.05). Conclusion: The RR between MIS-C cases and SARS-CoV-2 infections was significantly lower in the Omicron period for all age groups, including those not vaccinated, suggesting that the variant could be the main factor for this shift in the MISC trend. Regardless of variant type, the patients had similar phenotypes and severity throughout the pandemic.Mode-cleaning in antisymmetrically modulated non-Hermitian waveguides
http://hdl.handle.net/2117/404158
Mode-cleaning in antisymmetrically modulated non-Hermitian waveguides
Akhter, Mohammad Nayeem; Botey Cumella, Muriel; Herrero Simon, Ramon; Staliunas, Kestutis
We demonstrate all-optical spatial mode-cleaning in non-Hermitian waveguides. The effect is accounted by a unidirectional coupling among the modes resulting from a simultaneous modulation of the refractive index and the gain/loss along graded index multimodal waveguides. Depending on the spatial delay between the real and imaginary part of the potential modulation, higher or lower order modes are favored, which in latter case eventually leads to an nearly-monomode propagation. In this way, for any arbitrary initial field distribution an antisymmetric non-Hermitian modulation results in an effective mode-cleaning. The effect is demonstrated analytically, based on coupled mode theory in 1D waveguides, and numerically proven by solving the wave propagation equation with the antisymmetric non-Hermitian potential. The proposal is also generalized to the more involved case of 2D waveguides, leading to a significant reduction of the beam quality factor and improvement of beam spatial quality.
2024-03-11T16:23:51ZAkhter, Mohammad NayeemBotey Cumella, MurielHerrero Simon, RamonStaliunas, KestutisWe demonstrate all-optical spatial mode-cleaning in non-Hermitian waveguides. The effect is accounted by a unidirectional coupling among the modes resulting from a simultaneous modulation of the refractive index and the gain/loss along graded index multimodal waveguides. Depending on the spatial delay between the real and imaginary part of the potential modulation, higher or lower order modes are favored, which in latter case eventually leads to an nearly-monomode propagation. In this way, for any arbitrary initial field distribution an antisymmetric non-Hermitian modulation results in an effective mode-cleaning. The effect is demonstrated analytically, based on coupled mode theory in 1D waveguides, and numerically proven by solving the wave propagation equation with the antisymmetric non-Hermitian potential. The proposal is also generalized to the more involved case of 2D waveguides, leading to a significant reduction of the beam quality factor and improvement of beam spatial quality.On the dynamics of the turbulent flow past a three-element wing
http://hdl.handle.net/2117/403992
On the dynamics of the turbulent flow past a three-element wing
Montalà Sales, Ricard; Lehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María
A comprehensive analysis of the unsteady flow dynamics past the 30P30N three-element high lift wing is performed by means of large eddy simulations at different angles of attack (a¿=¿5°, 9°, and 23°) and at a Reynolds number of (based on the nested chord). Results are compared with experimental and numerical investigations, showing a quantitatively good agreement and, thus, proving the reliability and accuracy of the present simulations. Within the slat and main coves, large recirculation bubbles are bounded by shear layers, where the onset of turbulence is triggered by Kelvin–Helmholtz instabilities. In the energy spectrum of the velocity fluctuations, the footprint of these instabilities is detected as a broadband peak; its frequency being moved toward lower values as the angle of attack increases. Kelvin–Helmholtz vortices roll-up and break down into small scales that eventually impinge into the slat and main coves lower surfaces. The slat impingement shows to be more prominent, and hence, larger velocity and pressure fluctuations are observed. The impingement strength diminishes with the angle of attack in both coves, while higher fluctuations are originated on the slat and main respective suction sides, leading to larger boundary layers. This is associated with the displacement of the stagnation point with the angle of attack. Another salient feature observed is the laminar-to-turbulent flow transition in the main and flap leading edges although the average location of this transition seems to not be affected by the angle of attack. Tollmien–Schlichting instabilities precede this transition, with the disturbances amplified by the inviscid mode at low angles of attack, while at ¿, the local Reynolds number on the main suction side is incremented and the viscous mode becomes important. The analysis shows that the turbulent wake formed at the trailing edge of all elements dominates the dynamics downstream. This is especially true at the higher angle of attack, where a large region of velocity deficit above the flap is observed, thus indicating the onset of stall conditions.
2024-03-08T10:57:25ZMontalà Sales, RicardLehmkuhl Barba, OriolRodríguez Pérez, Ivette MaríaA comprehensive analysis of the unsteady flow dynamics past the 30P30N three-element high lift wing is performed by means of large eddy simulations at different angles of attack (a¿=¿5°, 9°, and 23°) and at a Reynolds number of (based on the nested chord). Results are compared with experimental and numerical investigations, showing a quantitatively good agreement and, thus, proving the reliability and accuracy of the present simulations. Within the slat and main coves, large recirculation bubbles are bounded by shear layers, where the onset of turbulence is triggered by Kelvin–Helmholtz instabilities. In the energy spectrum of the velocity fluctuations, the footprint of these instabilities is detected as a broadband peak; its frequency being moved toward lower values as the angle of attack increases. Kelvin–Helmholtz vortices roll-up and break down into small scales that eventually impinge into the slat and main coves lower surfaces. The slat impingement shows to be more prominent, and hence, larger velocity and pressure fluctuations are observed. The impingement strength diminishes with the angle of attack in both coves, while higher fluctuations are originated on the slat and main respective suction sides, leading to larger boundary layers. This is associated with the displacement of the stagnation point with the angle of attack. Another salient feature observed is the laminar-to-turbulent flow transition in the main and flap leading edges although the average location of this transition seems to not be affected by the angle of attack. Tollmien–Schlichting instabilities precede this transition, with the disturbances amplified by the inviscid mode at low angles of attack, while at ¿, the local Reynolds number on the main suction side is incremented and the viscous mode becomes important. The analysis shows that the turbulent wake formed at the trailing edge of all elements dominates the dynamics downstream. This is especially true at the higher angle of attack, where a large region of velocity deficit above the flap is observed, thus indicating the onset of stall conditions.Stabilization of microlasers by non-Hermitian potentials
http://hdl.handle.net/2117/403892
Stabilization of microlasers by non-Hermitian potentials
Herrero Simon, Ramon; Botey Cumella, Muriel; Benadouda Ivars, Salim; Akhter, Mohammad Nayeem; Staliunas, Kestutis
Vertical-cavity semiconductor lasers as well as single units or arrays of Edge Emitting Lasers suffer from dynamical spatiotemporal instabilities leading to temporally unstable and low spatial beam quality. We propose a feasible stabilization mechanism for microlasers based on periodic non-Hermitian potentials, i.e. simultaneous modulations of refractive index and gain-loss. The proposed spatiotemporal modulations can be introduced by a potential directly acting on the field or by carrier modulations. The stabilization effect is based either on the suppression of the modulation instability or on asymmetric couplings in the transverse direction to localize and stabilize the field.
Copyright (2023) Society of Photo‑Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this publication for a fee or for commercial purposes, and modification of the contents of the publication are prohibited.
2024-03-06T15:53:48ZHerrero Simon, RamonBotey Cumella, MurielBenadouda Ivars, SalimAkhter, Mohammad NayeemStaliunas, KestutisVertical-cavity semiconductor lasers as well as single units or arrays of Edge Emitting Lasers suffer from dynamical spatiotemporal instabilities leading to temporally unstable and low spatial beam quality. We propose a feasible stabilization mechanism for microlasers based on periodic non-Hermitian potentials, i.e. simultaneous modulations of refractive index and gain-loss. The proposed spatiotemporal modulations can be introduced by a potential directly acting on the field or by carrier modulations. The stabilization effect is based either on the suppression of the modulation instability or on asymmetric couplings in the transverse direction to localize and stabilize the field.Dielectric spectroscopy studies of conformational relaxation dynamics in molecular glass-forming liquids
http://hdl.handle.net/2117/403413
Dielectric spectroscopy studies of conformational relaxation dynamics in molecular glass-forming liquids
Romanini, Michela; Macovez, Roberto; Valenti, Sofia; Noor, Wahi; Tamarit Mur, José Luis
We review experimental results obtained with broadband dielectric spectroscopy concerning the relaxation times and activation energies of intramolecular conformational relaxation processes in small-molecule glass-formers. Such processes are due to the interconversion between different conformers of relatively flexible molecules, and generally involve conformational changes of flexible chain or ring moieties, or else the rigid rotation of planar groups, such as conjugated phenyl rings. Comparative analysis of molecules possessing the same (type of) functional group is carried out in order to test the possibility of assigning the dynamic conformational isomerism of given families of organic compounds to the motion of specific molecular subunits. These range from terminal halomethyl and acetyl/acetoxy groups to both rigid and flexible ring structures, such as the planar halobenzene cycles or the buckled saccharide and diazepine rings. A short section on polyesters provides a generalisation of these findings to synthetic macromolecules.
2024-02-28T18:02:07ZRomanini, MichelaMacovez, RobertoValenti, SofiaNoor, WahiTamarit Mur, José LuisWe review experimental results obtained with broadband dielectric spectroscopy concerning the relaxation times and activation energies of intramolecular conformational relaxation processes in small-molecule glass-formers. Such processes are due to the interconversion between different conformers of relatively flexible molecules, and generally involve conformational changes of flexible chain or ring moieties, or else the rigid rotation of planar groups, such as conjugated phenyl rings. Comparative analysis of molecules possessing the same (type of) functional group is carried out in order to test the possibility of assigning the dynamic conformational isomerism of given families of organic compounds to the motion of specific molecular subunits. These range from terminal halomethyl and acetyl/acetoxy groups to both rigid and flexible ring structures, such as the planar halobenzene cycles or the buckled saccharide and diazepine rings. A short section on polyesters provides a generalisation of these findings to synthetic macromolecules.In silico drug derivatives for KRAS-G12D: free-energy surfaces in aqueous solution by well-tempered metadynamics simulations
http://hdl.handle.net/2117/403331
In silico drug derivatives for KRAS-G12D: free-energy surfaces in aqueous solution by well-tempered metadynamics simulations
Hu, Zheyao; Martí Rabassa, Jordi
KRAS oncogenes are the largest family of mutated RAS isoforms, participating in about 30% of all cancers. Due to their paramount medical importance, enormous effort is being devoted to the development of inhibitors using clinical tests, wet-lab experiments and drug design, being this a preliminary step in the process of creating new drugs, prior to synthesis and clinical testing. One central aspect in the development of new drugs is the characterisation of all species that can be used for treatment. In this aim, we propose a computational framework based on combined all-atom molecular dynamics and metadynamics simulations in order to accurately access the most stable conformational variants for several derivatives of a recently proposed small-molecule, called DBD15-21-22. Free energy calculations are essential to unveil mechanisms at the atomic scale like binding affinities or dynamics of stable states. Considering specific atom-atom distances and torsional angles as reliable reaction coordinates we have obtained free-energy landscapes by well-tempered metadynamics simulations, revealing local and global minima of the free-energy hypersurface. We have observed that a variety of stable states together with transition states are clearly detected depending on the particular species, leading to predictions on the behaviour of such compounds in ionic aqueous solution.
2024-02-27T19:51:56ZHu, ZheyaoMartí Rabassa, JordiKRAS oncogenes are the largest family of mutated RAS isoforms, participating in about 30% of all cancers. Due to their paramount medical importance, enormous effort is being devoted to the development of inhibitors using clinical tests, wet-lab experiments and drug design, being this a preliminary step in the process of creating new drugs, prior to synthesis and clinical testing. One central aspect in the development of new drugs is the characterisation of all species that can be used for treatment. In this aim, we propose a computational framework based on combined all-atom molecular dynamics and metadynamics simulations in order to accurately access the most stable conformational variants for several derivatives of a recently proposed small-molecule, called DBD15-21-22. Free energy calculations are essential to unveil mechanisms at the atomic scale like binding affinities or dynamics of stable states. Considering specific atom-atom distances and torsional angles as reliable reaction coordinates we have obtained free-energy landscapes by well-tempered metadynamics simulations, revealing local and global minima of the free-energy hypersurface. We have observed that a variety of stable states together with transition states are clearly detected depending on the particular species, leading to predictions on the behaviour of such compounds in ionic aqueous solution.Connection between mechanical relaxation and equilibration kinetics in a high-entropy metallic glass
http://hdl.handle.net/2117/402314
Connection between mechanical relaxation and equilibration kinetics in a high-entropy metallic glass
Duan, Yajuan; Nabahat, Mehran; Tong, Yu; Ortiz Membrado, Laia; Jiménez Piqué, Emilio; Zhao, Kun; Wang, Yun-Jiang; Yang, Yong; Wada, Takeshi; Kato, Hidemi; Pelletier, J. M.; Qiao, Jichao; Pineda Soler, Eloi
The slow transition from an out-of-equilibrium glass towards a supercooled liquid is a complex relaxation phenomenon. In this Letter, we study the correlation between mechanical relaxation and equilibration kinetics in a Pd20Pt20Cu20Ni20P20 high-entropy metallic glass. The evolution of stress relaxation with aging time was obtained with an unprecedented detail, allowing us to pinpoint new interesting features. The long structural relaxation towards equilibrium contains a wide distribution of activation energies, instead of being just associated to the ß relaxation as commonly accepted. The stress relaxation time can be correlated with the equilibration rate and we observe a decrease of microstructural heterogeneity which contrasts with an increase of dynamic heterogeneity. These results significantly enhance our insight of the interplay between relaxation dynamics and thermodynamics in metallic glasses.
2024-02-20T15:19:14ZDuan, YajuanNabahat, MehranTong, YuOrtiz Membrado, LaiaJiménez Piqué, EmilioZhao, KunWang, Yun-JiangYang, YongWada, TakeshiKato, HidemiPelletier, J. M.Qiao, JichaoPineda Soler, EloiThe slow transition from an out-of-equilibrium glass towards a supercooled liquid is a complex relaxation phenomenon. In this Letter, we study the correlation between mechanical relaxation and equilibration kinetics in a Pd20Pt20Cu20Ni20P20 high-entropy metallic glass. The evolution of stress relaxation with aging time was obtained with an unprecedented detail, allowing us to pinpoint new interesting features. The long structural relaxation towards equilibrium contains a wide distribution of activation energies, instead of being just associated to the ß relaxation as commonly accepted. The stress relaxation time can be correlated with the equilibration rate and we observe a decrease of microstructural heterogeneity which contrasts with an increase of dynamic heterogeneity. These results significantly enhance our insight of the interplay between relaxation dynamics and thermodynamics in metallic glasses.On the dynamics of the turbulent flow past a three-element wing
http://hdl.handle.net/2117/402088
On the dynamics of the turbulent flow past a three-element wing
Montalà Sales, Ricard; Lehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María
A comprehensive analysis of the unsteady flow dynamics past the 30P30N three-element high lift wing is performed by means of large eddy simulations at different angles of attack (AoA=5°, 9°, and 23°) and at a Reynolds number of Rec=750.000 (based on the nested chord). Results are compared with experimental and numerical investigations, showing a quantitatively good agreement and, thus, proving the reliability and accuracy of the present simulations. Within the slat and main coves, large recirculation bubbles are bounded by shear layers, where the onset of turbulence is triggered by Kelvin–Helmholtz instabilities. In the energy spectrum of the velocity fluctuations, the footprint of these instabilities is detected as a broadband peak; its frequency being moved toward lower values as the angle of attack increases. Kelvin–Helmholtz vortices roll-up and break down into small scales that eventually impinge into the slat and main coves lower surfaces. The slat impingement shows to be more prominent, and hence, larger velocity and pressure fluctuations are observed. The impingement strength diminishes with the angle of attack in both coves, while higher fluctuations are originated on the slat and main respective suction sides, leading to larger boundary layers. This is associated with the displacement of the stagnation point with the angle of attack. Another salient feature observed is the laminar-to-turbulent flow transition in the main and flap leading edges although the average location of this transition seems to not be affected by the angle of attack. Tollmien–Schlichting instabilities precede this transition, with the disturbances amplified by the inviscid mode at low angles of attack, while at AoA=23°, the local Reynolds number on the main suction side is incremented and the viscous mode becomes important. The analysis shows that the turbulent wake formed at the trailing edge of all elements dominates the dynamics downstream. This is especially true at the higher angle of attack, where a large region of velocity deficit above the flap is observed, thus indicating the onset of stall conditions.
2024-02-16T10:38:02ZMontalà Sales, RicardLehmkuhl Barba, OriolRodríguez Pérez, Ivette MaríaA comprehensive analysis of the unsteady flow dynamics past the 30P30N three-element high lift wing is performed by means of large eddy simulations at different angles of attack (AoA=5°, 9°, and 23°) and at a Reynolds number of Rec=750.000 (based on the nested chord). Results are compared with experimental and numerical investigations, showing a quantitatively good agreement and, thus, proving the reliability and accuracy of the present simulations. Within the slat and main coves, large recirculation bubbles are bounded by shear layers, where the onset of turbulence is triggered by Kelvin–Helmholtz instabilities. In the energy spectrum of the velocity fluctuations, the footprint of these instabilities is detected as a broadband peak; its frequency being moved toward lower values as the angle of attack increases. Kelvin–Helmholtz vortices roll-up and break down into small scales that eventually impinge into the slat and main coves lower surfaces. The slat impingement shows to be more prominent, and hence, larger velocity and pressure fluctuations are observed. The impingement strength diminishes with the angle of attack in both coves, while higher fluctuations are originated on the slat and main respective suction sides, leading to larger boundary layers. This is associated with the displacement of the stagnation point with the angle of attack. Another salient feature observed is the laminar-to-turbulent flow transition in the main and flap leading edges although the average location of this transition seems to not be affected by the angle of attack. Tollmien–Schlichting instabilities precede this transition, with the disturbances amplified by the inviscid mode at low angles of attack, while at AoA=23°, the local Reynolds number on the main suction side is incremented and the viscous mode becomes important. The analysis shows that the turbulent wake formed at the trailing edge of all elements dominates the dynamics downstream. This is especially true at the higher angle of attack, where a large region of velocity deficit above the flap is observed, thus indicating the onset of stall conditions.