The Experts below are selected from a list of 29880 Experts worldwide ranked by ideXlab platform
Yang Liu - One of the best experts on this subject based on the ideXlab platform.
-
an experimental investigation on the dynamic glaze ice Accretion Process over a wind turbine airfoil surface
International Journal of Heat and Mass Transfer, 2020Co-Authors: Linyue Gao, Yang LiuAbstract:Abstract Icing events, particularly under precipitation-icing conditions in which high-liquidity glaze ice tends to form, could pose significant threats to the safe and effective operations of wind turbines in cold and wet environments. During the glaze ice Accretion Process, the wind-driven unfrozen water was coupled with the growth of ice structures and difficult to be quantified and characterized. In the present study, we introduced a Digital Image Projection (DIP) technique to quantitatively measure the unsteady water runback behaviors and dynamic ice Accretion Process under typical glaze icing conditions over a highly-cambered wind turbine airfoil surface, i.e., the pressure-side surface of DU91-W2-250 airfoil, in the Icing Research Tunnel at Iowa State University (ISU-IRT). DIP measurement results were found to be able to successfully capture the time-resolved three-dimensional information of the water transport behaviors over the ice accreting surface of the airfoil model during the glaze icing Processes. The stumbling motions of the rivulet flows were observed during the icing Processes, coupled with an increasing fluctuations induced by the underneath ice roughness. The forces acting on the rivulet flows were analyzed, and a theoretical model based on the force balance was built to predict the rivulet flows. The effects of the incoming airflow velocity on the glaze icing Process over the airfoil surface were also studied, and it was found that, as the incoming flow velocity increased, the runback rivulet flows would move farther downstream and became thinner and narrower due to the increased aerodynamic stress acting on them. The rivulet bulged shape was found to have an inverse relationship with the inflow velocity squared. In addition, the ice accreted over the airfoil surface during an icing Process was quantitatively decoupled from the unfrozen wind-driven water, and the quantitative results can be used to validate and optimize current ice Accretion models.
-
dynamic ice Accretion Process and its effects on the aerodynamic drag characteristics of a power transmission cable model
Cold Regions Science and Technology, 2020Co-Authors: Ramsankar Veerakumar, Linyue Gao, Yang LiuAbstract:Abstract An experimental study was conducted to examine the dynamic ice Accretion Process over the surface of a high-voltage power transmission cable model and characterize the effects of the ice Accretion on the aerodynamic forces acting on the test model. The experimental study was carried out by leveraging the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT) to generate typical wet glaze and dry rime icing conditions experienced by power transmission cables. A cylindrical power cable model, which has the same diameter as that of typical power transmission cables, was mounted in ISU-IRT for the ice Accretion experiments. In addition to using a high-speed digital imaging system to record the dynamic ice Accretion Process, a novel digital image projection (DIP) based technique was utilized to quantify the 3D shapes of the ice structures accreted on the surface of the power cable model as a function of the ice Accretion time. The time variations of the aerodynamic drag force acting on the test model during the dynamic ice Accretion Process were also measured quantitatively by using high-sensitive force/moment traducers mounted at two ends of the test model. The ice structures accreted over the surface of the power cable model were found to change significantly under different icing conditions (i.e., rime icing vs. glaze icing). The characteristics of the aerodynamic drag acting on the test model was found to vary significantly during the dynamic ice Accretion Process depending on what types of ice structures were accreted on the test model. The acquired snapshots of the ice Accretion images and the measured 3D shapes of the accreted ice structures on the test model are correlated with the aerodynamic force measurement results to elucidate the underlying physics.
-
an experimental study on dynamic ice Accretion Process over the surfaces of rotating aero engine spinners
Experimental Thermal and Fluid Science, 2019Co-Authors: Yang LiuAbstract:Abstract An experimental study was conducted to investigate the dynamic ice Accretion Process over the surfaces of rotating aero-engine spinners and to examine the detriment effects of the ice Accretion on the airflow to be inhaled by aero-engines. Three scaled spinner-fan models with different spinner shapes (i.e., conical-shaped, coniptical-shaped, and elliptical-shaped spinners) were manufactured and exposed under typical rime and glaze icing conditions for a comparative study. During the experiments, while a high-speed imaging system was used to record the dynamic ice Accretion Process over the rotating spinner models, a high-resolution particle image velocimetry (PIV) system was utilized to examine the trajectories of super-cooled water droplets as they approach to the surfaces of the spinner models. It was found that, under typical rime conditions, while accreted ice layers were found to conform well with the original shapes of the spinner models in general, the total amount of the ice mass accreted over the spinner surfaces were found to be a strong dependent on the spinner shapes. While the conical-shaped spinner was found to have the largest amount of ice Accretion (i.e., 60–80% more than those over the other two spinner models) over almost entire spinner surface, ice Accretion was found to take place mainly in the front portion of the coniptical-shaped and elliptical-shaped spinners. Under the glaze icing condition, in addition to forming ice layers over the spinner surfaces, very complicated, needle-shaped icicles were also found to grow rapidly out from the spinner surfaces and extrude into the incoming airflow, due to the effects of the centrifugal forces associated with the rotation motion. The complex glaze ice structures accreted over the spinner surface were found to induce significant disturbances/distortions and even cause large-scale flow separations for the airflow near the iced spinner surfaces, which would significantly degrade the quality of the inlet airflow to be inhaled by aero-engines, thereby, adversely affecting the performance of aero-engines.
-
an experimental study on the aerodynamic performance degradation of a uas propeller model induced by ice Accretion Process
Experimental Thermal and Fluid Science, 2019Co-Authors: Yang Liu, Wei Tian, Wenli ChenAbstract:Abstract An experimental study was performed to investigate the effects of ice Accretion on the aerodynamic performances and wake characteristics of a propeller model used for Unmanned-Aerial-System (UAS) under different icing conditions (i.e., rime vs. glaze). The experimental study was conducted in the unique Icing Research Tunnel available at Iowa State University (ISU-IRT). In addition to acquiring the important features of ice Accretion on the rotating propeller blade using a “phase-locked” imaging technique, the wake characteristics of the rotating UAS propeller under the different icing conditions were also resolved by using the Particle Imaging Velocimetry (PIV) technique along with the time-resolved measurements of aerodynamic forces and power consumption of the UAS propeller model. Both “free-run” and “phase-locked” PIV measurements were performed on the propeller model at the different stages of the icing experiments (i.e., before, during and after the dynamic icing Processes) to provide both the instantaneous flow characteristics and the ensemble-averaged flow statistics (e.g., mean velocity, vorticity, and turbulence kinetic energy) in the wake of the rotating propeller model. To the best knowledge of the authors, this is the first work of its kind to provide detailed, temporally-resolved wake flow field measurements of UAS propeller under real icing conditions. It is found that while the rime ice Accretion could closely follow the original profiles of the propeller blades, the glaze ice usually forms into very irregular structures that can significantly disturb the wake flow field of the rotating propeller model, generating the much larger and more complex vortices. Such complex large-scale vortices are found to enhance the turbulent mixing in the propeller wake and produce an evident velocity deficit channel around the outer board of the propeller blades, providing direct evidences in elucidating the dramatic decrease in thrust generation and the significant increase in power consumption of the rotating propeller model in icing conditions. The findings derived from this study are believed to be essential and very helpful to elucidate the underlying mechanisms of the aerodynamic performance degradation of ice accreting UAS propellers.
-
an experimental investigation of dynamic ice Accretion Process on a wind turbine airfoil model considering various icing conditions
International Journal of Heat and Mass Transfer, 2019Co-Authors: Linyue Gao, Yang LiuAbstract:Abstract In the present study, the dynamic ice Accretion Process over a typical wind turbine airfoil model (i.e., DU96-W-180 airfoil) was experimentally investigated under various icing conditions. The experimental study was conducted in the Icing Research Tunnel of Iowa State University (i.e., ISU-IRT). Different icing conditions (i.e., rime, mixed and glaze) that wind turbine may experience in winter were reproduced by manipulating the airflow temperature, velocity, and liquid water content (LWC) in ISU-IRT. While a high-speed imaging system was used to reveal the dynamic ice Accretion Process over the surface of the test model, an infrared (IR) thermal imaging system was used to map the corresponding temperature distributions over the ice accreting airfoil surface. Time variations of the ice thickness accreted along the leading edge (LE) of the test model were extracted based on the acquired high-resolution images of the ice Accretion Process under different test conditions. It was found that, due to the obvious runback of the impacted water (i.e., formation of water film and rivulets) over the airfoil surface, the growth rate of the ice layer accreted along the airfoil leading edge was much slower under the glaze icing condition, in comparison with those under the rime and mixed icing conditions. Such surface water transport behavior was also found to expand the ice influencing region. From the temperature evolutions during the dynamic icing Processes, the transient Processes of droplet impingement, water film/rivulets formation, and ice roughness growth were temporally resolved, providing comprehensive details of the unsteady heat transfer during the dynamic icing Process. While the surface temperature increment due to the direct droplet impingement was found to decrease monotonously along the chord in rime case, a stream-wise ‘plateau’ region was observed in the glaze and mixed icing cases due to the complex multiphase mass/heat transfer associated with the surface water transport behaviors.
Cosimo Bambi - One of the best experts on this subject based on the ideXlab platform.
-
Evolution of the spin parameter of accreting compact objects with non-Kerr quadrupole moment
Journal of Cosmology and Astroparticle Physics, 2011Co-Authors: Cosimo BambiAbstract:There is robust observational evidence supporting the existence of 5?20 M? compact bodies in X-ray binary systems and of 105?109 M? bodies at the center of many galaxies. All these objects are commonly interpreted as black holes, even is there is no direct evidence that they have an event horizon. A fundamental limit for a black hole in 4-dimensional general relativity is the Kerr bound |a*| ? 1, where a* is the spin parameter. This is just the condition for the existence of the event horizon. The Accretion Process can spin a black hole up to a* ? 0.998 and some super-massive objects in galactic nuclei could be rapidly rotating black holes with spin parameter close to this limit. However, if these super-massive objects are not black holes, the Kerr bound does not hold and the Accretion Process can spin them up to a* > 1. In this paper, I consider compact bodies with non-Kerr quadrupole moment. I study the evolution of the spin parameter due to Accretion and I find its equilibrium value. Future experiments like the gravitational wave detector LISA will be able to test if the super-massive objects at the center of galaxies are the black holes predicted by general relativity. If they are not black holes, some of them may be super-spinning objects with a* > 1.
-
evolution of the spin parameter of accreting compact objects with non kerr quadrupole moment
arXiv: General Relativity and Quantum Cosmology, 2011Co-Authors: Cosimo BambiAbstract:There is robust observational evidence supporting the existence of $5 - 20$ $M_\odot$ compact bodies in X-ray binary systems and of $10^5 - 10^9$ $M_\odot$ bodies at the center of many galaxies. All these objects are commonly interpreted as black holes, even is there is no direct evidence that they have an event horizon. A fundamental limit for a black hole in 4-dimensional general relativity is the Kerr bound $|a_*| \le 1$, where $a_*$ is the spin parameter. This is just the condition for the existence of the event horizon. The Accretion Process can spin a black hole up to $a_* \approx 0.998$ and some super-massive objects in galactic nuclei could be rapidly rotating black holes with spin parameter close to this limit. However, if these super-massive objects are not black holes, the Kerr bound does not hold and the Accretion Process can spin them up to $a_* > 1$. In this paper, I consider compact bodies with non-Kerr quadrupole moment. I study the evolution of the spin parameter due to Accretion and I find its equilibrium value. Future experiments like the gravitational wave detector LISA will be able to test if the super-massive objects at the center of galaxies are the black holes predicted by general relativity. If they are not black holes, some of them may be super-spinning objects with $a_* > 1$.
-
Three-dimensional simulations of the Accretion Process in Kerr space-time with arbitrary value of the spin parameter
Physical Review D, 2010Co-Authors: Cosimo Bambi, Naoki YoshidaAbstract:We present the results of three-dimensional general relativistic hydrodynamic simulations of adiabatic and spherically symmetric Accretion in Kerr space-time. We consider compact objects with spin parameter $|{a}_{*}|\ensuremath{\le}1$ (black holes) and with $|{a}_{*}|g1$ (superspinars). Our full three-dimensional simulations confirm the formation of equatorial outflows for high values of $|{a}_{*}|$, as found in our previous work in 2.5 dimensions. We show that the critical value of $|{a}_{*}|$ determining the onset of powerful outflows depends mainly on the radius of the compact object. The phenomenon of equatorial outflows can hardly occur around a black hole and may thus be used to test the bound $|{a}_{*}|\ensuremath{\le}1$ for astrophysical black hole candidates.
-
numerical simulations of the Accretion Process in kerr space times with arbitrary value of the kerr parameter
arXiv: General Relativity and Quantum Cosmology, 2009Co-Authors: Cosimo BambiAbstract:According to the Cosmic Censorship Conjecture, all the singularities produced by the collapsing matter must be hidden behind an event horizon. In 4D general relativity, this implies that the final product of the collapse is a Kerr-Newman black hole. Here I consider the possibility that the Cosmic Censorship Conjecture can be violated. I present the results of some numerical simulations of the Accretion Process onto Kerr black holes (objects with event horizon) and Kerr super-spinars (fast-rotating objects without event horizon). This is a preliminary study to investigate how the Cosmic Censorship Conjecture can be tested by astrophysical observations.
-
Accretion Process onto super spinning objects
Physical Review D, 2009Co-Authors: Cosimo Bambi, Katherine Freese, Tomohiro Harada, Rohta Takahashi, Naoki YoshidaAbstract:The Accretion Process onto spinning objects in Kerr spacetimes is studied with numerical simulations. Our results show that Accretion onto compact objects with Kerr parameter (characterizing the spin) |a| M is very different. In the superspinning case, for |a| moderately larger than M, the Accretion onto the central object is extremely suppressed due to a repulsive force at short distance. The accreting matter cannot reach the central object, but instead is accumulated around it, forming a high density cloud that continues to grow. The radiation emitted in the Accretion Process will be harder and more intense than the one coming from standard black holes; e.g. {gamma}-rays could be produced as seen in some observations. Gravitational collapse of this cloud might even give rise to violent bursts. As |a| increases, a larger amount of accreting matter reaches the central object and the growth of the cloud becomes less efficient. Our simulations find that a quasisteady state of the Accretion Process exists for |a|/M > or approx. 1.4, independently of the mass Accretion rate at large radii. For such high values of the Kerr parameter, the accreting matter forms a thin disk at very small radii. We provide some analytical argumentsmore » to strengthen the numerical results; in particular, we estimate the radius where the gravitational force changes from attractive to repulsive and the critical value |a|/M{approx_equal}1.4 separating the two qualitatively different regimes of Accretion. We briefly discuss the observational signatures which could be used to look for such exotic objects in the Galaxy and/or in the Universe.« less
J Bouvier - One of the best experts on this subject based on the ideXlab platform.
-
investigating the magnetospheric Accretion Process in the young pre transitional disk system doar 44 v2062 oph a multiwavelength interferometric spectropolarimetric and photometric observing campaign
arXiv: Solar and Stellar Astrophysics, 2020Co-Authors: J Bouvier, E Alecian, S H P Alencar, A P Sousa, J F Donati, K Perraut, A Bayo, L M Rebull, C Dougados, G DuvertAbstract:Young stars interact with their Accretion disk through their strong magnetosphere. We investigate the magnetospheric Accretion Process in the young stellar system DoAr 44. We monitored the system over several rotational cycles, combining high-resolution optical and near-IR spectropolarimetry with long-baseline near-IR interferometry and multicolor photometry. DoAr 44 is a young 1.2 solar mass star, moderately accreting from its disk, and seen at a low inclination. We derive a rotational period of 2.96 d from the system's light curve. Several optical and near-IR line profiles probing the Accretion funnel flows and the Accretion shock are modulated at the stellar rotation period. The most variable line profile, HeI 1083 nm, exhibits modulated redshifted wings a signature of Accretion funnel flows, as well as deep blueshifted absorptions indicative of transient outflows. The Zeeman-Doppler analysis suggests the star hosts a mainly dipolar magnetic field, inclined by about 20 deg. onto the spin axis, with an intensity reaching about 800 G at the photosphere, and up to 2 +/- 0.8 kG close to the Accretion shock. The magnetic field appears strong enough to disrupt the inner disk close to the corotation radius, at a distance of about 4.6 stellar radii (0.043 au). This supports the upper limit of 5 stellar radii (0.047 au) we derived for the size of the magnetosphere from long baseline interferometry. DoAr 44 is a pre-transitional disk system, exhibiting a 25-30 au gap in its circumstellar disk, with the inner and outer disks being misaligned. On a scale of 0.1 au or less, our results indicate that the system steadily accretes from its inner disk through its tilted dipolar magnetosphere. We conclude that in spite of a highly structured outer disk, perhaps the signature of ongoing planetary formation, the magnetospheric Accretion Process proceeds unimpeded at the star-disk interaction level.
-
Magnetospheric Accretion in the intermediate-mass T Tauri star HQ Tauri
Astronomy and Astrophysics - A&A, 2020Co-Authors: K. Pouilly, J Bouvier, E Alecian, S H P Alencar, J F Donati, A.-m. Cody, K. Grankin, G. A. J. Hussain, L. Rebull, C. P. FolsomAbstract:Context. Classical T Tauri stars are pre-main sequence stars surrounded by an Accretion disk. They host a strong magnetic field, and both magnetospheric Accretion and ejection Processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution.Aims. The goal of this study is to investigate the Accretion Process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M⊙).Methods. The time variability of the system is investigated both photometrically, using Kepler-K2 and complementary light curves, and from a high-resolution spectropolarimetric time series obtained with ESPaDOnS at CFHT.Results. The quasi-sinusoidal Kepler-K2 light curve exhibits a period of 2.424 d, which we ascribe to the rotational period of the star. The radial velocity of the system shows the same periodicity, as expected from the modulation of the photospheric line profiles by surface spots. A similar period is found in the red wing of several emission lines (e.g., HI, CaII, NaI), due to the appearance of inverse P Cygni components, indicative of Accretion funnel flows. Signatures of outflows are also seen in the line profiles, some being periodic, others transient. The polarimetric analysis indicates a complex, moderately strong magnetic field which is possibly sufficient to truncate the inner disk close to the corotation radius, rcor ∼ 3.5 R⋆. Additionally, we report HQ Tau to be a spectroscopic binary candidate whose orbit remains to be determined.Conclusions. The results of this study expand upon those previously reported for low-mass T Tauri stars, as they indicate that the magnetospheric Accretion Process may still operate in intermediate-mass pre-main sequence stars, such as HQ Tauri.
-
csi 2264 Accretion Process in classical t tauri stars in the young cluster ngc 2264
Astronomy and Astrophysics, 2016Co-Authors: Alana Sousa, Silvia H P Alencar, J Bouvier, John R Stauffer, Laura Venuti, Lynne A Hillenbrand, Ann Marie Cody, Paula S TeixeiraAbstract:Context. NGC 2264 is a young stellar cluster (~3 Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the Accretion Process taking place in the pre-main sequence. Aims. Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical Processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. Methods. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several Accretion diagnostics and disk parameters. Results. The morphology of the CoRoT light curve reflects the evolution of the Accretion Process and of the inner disk region. Accretion burst stars present high mass-Accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-Accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-Accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed in the 2008 and 2011 CoRoT runs changed their light-curve morphology. Transitions from AA Tau-like and spot-like to aperiodic light curves and vice versa were common. The analysis of the Hα emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the Hα line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical Processes. Classical T Tauri stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric Accretion and outflow models, including variations from stable to unstable Accretion regimes on timescales of a few years.
-
csi 2264 Accretion Process in classical t tauri stars in the young cluster ngc 2264
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: Alana Sousa, Silvia H P Alencar, J Bouvier, John R Stauffer, Laura Venuti, Lynne A Hillenbrand, Ann Marie Cody, Paula S TeixeiraAbstract:Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars, of the star-forming cluster NGC 2264, to the physical Processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several Accretion diagnostics and disk parameters. The morphology of the CoRoT light curve reflects the evolution of the Accretion Process and of the inner disk region. Accretion burst stars present high mass-Accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-Accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-Accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed in the 2008 and 2011 CoRoT runs changed their light-curve morphology. Transitions from AA Tau-like and spot-like to aperiodic light curves and vice versa were common. The analysis of the $H\alpha$ emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the $H\alpha$ line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical Processes. Accreting stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric Accretion and outflow models, including variations from stable to unstable Accretion regimes on timescales of a few years
L I Haibo - One of the best experts on this subject based on the ideXlab platform.
-
numerical simulation of airfoil ice Accretion Process on horizontal axis wind turbine blade
Energy technology, 2010Co-Authors: L I HaiboAbstract:The objective of this paper is to model the rime-ice Accretion Process on a horizontal-axis wind turbine blade(HAWT)operating under icing conditions with the help of Fluent,a commercial CFD code.A 4-order Runge-Kutta method is used to solve the droplet trajectory equation.Ice shape is determined with the assumption that all droplets freeze at their points of impact and that the ice grows in the direction normal to the surface.The flow field around the airfoil with software Fluent was simulated,to compare the correspondent aerodynamic performances variation before and after icing.Results indicate that the ice Accretion leads to the loss of maximum lift coefficient by about 27%,the decrease of stall angel by about 4°and the drag coefficient increase by nearly 38% under the weather conditions of the simulation.Stall is the main reason for all these aerodynamic performances worsen.
Wenwu Zhou - One of the best experts on this subject based on the ideXlab platform.
-
an experimental study on the aerodynamic performance degradation of a wind turbine blade model induced by ice Accretion Process
Renewable Energy, 2019Co-Authors: Wenwu Zhou, Hui HuAbstract:Abstract An experimental study was conducted to characterize aerodynamic performance degradation of wind turbine blades induced by dynamic ice Accretion Process. The experimental study was performed in an Icing Research Tunnel with a turbine blade model under a typical glaze icing condition. Ice structures were found to accrete rapidly over both the upper and lower surfaces of the blade model after starting the ice Accretion experiment. Irregular-shaped ice structures were found to disturb the airflow around the blade model greatly, resulting in large-scale flow separations and shedding of unsteady vortex structures from the ice accreting surface. The aerodynamic performance of the blade model was found to degrade significantly. The performance degradation induced by the ice Accretion was found to be a strong function of the angle of attack of the blade model with more significant degradations at lower angles of attack. For the test case at the angle of attack of 5.0°, while the lift decreases to only ∼12% of its original value after 600 s of the ice Accretion experiment, the drag was found to increase 4.5 times correspondingly. The detailed flow field measurements were correlated with the aerodynamic force data to elucidate the underlying physics.