The Experts below are selected from a list of 306 Experts worldwide ranked by ideXlab platform
Babak Seradjeh - One of the best experts on this subject based on the ideXlab platform.
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Midgap spectrum of the fermion Vortex System
Nuclear Physics, 2008Co-Authors: Babak SeradjehAbstract:Abstract I study the midgap spectrum of the fermion–Vortex System in two spatial dimensions. The existence of bound states, in addition to the zero modes found by Jackiw and Rossi, is established. For a singly quantized Vortex, I present complete analytical solutions in terms of generalized Laguerre polynomials in the opposite limits of vanishing and large Vortex core size. There is an infinite number of such bound states, with a spectrum that is, when squared, given by, respectively, the Coulomb potential and the isotropic harmonic oscillator. Possible experimental signatures of this spectrum in condensed-matter realizations of the System are pointed out.
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Midgap spectrum of the fermion–Vortex System
Nuclear Physics B, 2008Co-Authors: Babak SeradjehAbstract:I study the midgap spectrum of the fermion-Vortex System in two spatial dimensions. The existence of bound states, in addition to the zero modes found by Jackiw and Rossi, is established. For a singly quantized Vortex, I present complete analytical solutions in terms of generalized Laguerre polynomials in the opposite limits of vanishing and large Vortex core size. There is an infinite number of such bound states, with a spectrum that is, when squared, given by, respectively, the Coulomb potential and the isotropic harmonic oscillator. Possible experimental signatures of this spectrum in condensed-matter realizations of the System are pointed out.Comment: 10 pages, no figure
Raffel Markus - One of the best experts on this subject based on the ideXlab platform.
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Blade Tip Vortex System of a Rotor with Cyclic Pitch
'American Institute of Aeronautics and Astronautics (AIAA)', 2020Co-Authors: Braukmann, Johannes N., Wolf Christian, Goerttler Andreas, Raffel MarkusAbstract:To characterize the Vortex System of a helicopter rotor with cyclic pitch, an experimental investigation is carried out at the rotor test facility of the DLR, German Aerospace Center in Göttingen. A test case with attached flow conditions is chosen to study the blade tip Vortex aerodynamics on a four-bladed rotor. The resulting azimuth-dependent Vortex System of the rotor is analyzed. In addition to velocity fields acquired by a particle image velocimetry System, the current investigation also uses a background-oriented schlieren setup to capture the density gradients caused by the blade tip vortices. These optical measurement techniques enable the reconstruction of the azimuth-dependent Vortex System for Vortex ages up to 70 deg. The pitch variation, and thus the change in lift, is found to be the main influence on the characteristics of the tip Vortex System. The topology of the Vortex, the interconnected shear layer, and the position of the Vortex with respect to the blade tip appear to be highly pitch dependent. An analysis of the blade tip vortices is carried out, focusing on the swirl velocity, core radius, and circulation distribution. The results are compared to previous investigations at the same test facility and a numerical simulation
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Blade Tip-Vortices of a Four-Bladed Rotor with Axial Inflow
'American Helicopter Society', 2020Co-Authors: Goerttler Andreas, Braukmann, Johannes N., Wolf C. Christian, Gardner, Anthony D., Raffel MarkusAbstract:The Vortex System of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using German Aerospace Center (DLR)’s finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry carried out at the rotor test facility (RTG) in Göttingen. Algorithms deriving the Vortex position, swirl velocity, circulation, and core radius were implemented. Hover-like conditions with a fixed blade pitch were analyzed giving further physical insights of the static Vortex System. These results are used to understand the Vortex development for the unsteady pitching conditions, which can be described as a superpositioning of static Vortex states. The use of a zonal detached-eddy simulations approach improved physical modeling of the Vortex development by resolving finer scales than URANS. Trimmed cases agree well with differences less than 0.5% in the circulation and swirl velocity
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The Tip Vortex System of a Four-Bladed Rotor in Dynamic Stall Conditions
'American Helicopter Society', 2019Co-Authors: Wolf C. Christian, Braukmann, Johannes N., Stauber Wolfgang, Schwermer Till, Raffel MarkusAbstract:The tip Vortex System downstream of a four-bladed instrumented rotor was investigated experimentally through the application of stereoscopic particle image velocimetry (PIV). A dynamic stall test case was facilitated by a high cyclic pitch setting of the swashplate, with additional attached-flow and constant-pitch test cases for comparison reasons. The phase-locked PIV System and a rotation of the swashplate assembly allowed for an acquisition of the tip Vortex System over the entire dynamic stall cycle and Vortex ages up to at least 235°. The Vortex structure and its relation to the blade shear layers were studied by means of both phase-averaged flow fields and the identification of Vortex properties such as circulation and swirl velocity distributions. When approaching dynamic stall, a breakdown of the Vortex structure started at high Vortex ages, accompanied by the entrainment of turbulent structures from the passing blade shear layers into the tip vortices. After the flow over the blade is fully separated and during large parts of the downstroke, the wake of the rotor tips appears as a highly turbulent area with no individual tip vortices traceable, before reestablishing an ordered tip Vortex structure shortly before the minimum blade pitch angle
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Blade Tip-Vortices of a Four-Bladed Rotor with Axial Inflow
2019Co-Authors: Goerttler Andreas, Braukmann, Johannes N., Wolf C. Christian, Gardner, Anthony D., Raffel MarkusAbstract:The Vortex System of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using DLR's finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry (PIV) carried out at the rotor test facility in Gottingen (RTG). Algorithms deriving the Vortex position, swirl velocity, circulation and core radius were implemented. Hover-like conditions with a fixed blade pitch were analyzed giving a good picture of the static Vortex System. These results are used to understand the Vortex development for the unsteady pitching conditions, which can be described as a superpositioning of static Vortex states. The use of a zonal DES approach reduced numerical dissipation and improved physical modelling of the Vortex development
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The Tip Vortex System of a Four-Bladed Rotor in Dynamic Stall Conditions
2018Co-Authors: Wolf C. Christian, Braukmann, Johannes N., Stauber Wolfgang, Schwermer Till, Raffel MarkusAbstract:The tip Vortex-System downstream of a four-bladed instrumented rotor was investigated experimentally through the application of stereoscopic particle image velocimetry (PIV). A dynamic stall test case was facilitated by a high cyclic pitch setting of the swashplate, with additional attached-flow and constant-pitch test cases for comparison reasons. The phase-locked PIV System and a rotation of the swashplate assembly allowed for an acquisition of the tip Vortex System over the entire dynamic stall cycle and Vortex ages up to at least 235 degree. The Vortex structure and its relation to the blade shear layers were studied by means of both phase-averaged flow fields and the identification of Vortex properties such as circulation and swirl velocity distributions. When approaching dynamic stall, a break-down of the Vortex structure started at high Vortex ages, accompanied by the entrainment of turbulent structures from the passing blade shear layers into the tip vortices. After the flow over the blade is fully separated and during large parts of the downstroke, the wake of the rotor tips appeared as a highly turbulent area with no individual tip vortices traceable, before reestablishing an ordered tip Vortex structure shortly before the minimum blade pitch angle
Chang Lin - One of the best experts on this subject based on the ideXlab platform.
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Characteristics of Steady Horseshoe Vortex System near Junction of Square Cylinder and Base Plate
Journal of Engineering Mechanics, 2008Co-Authors: Chang Lin, Subhasish DeyAbstract:This paper presents an experimental investigation on the characteristics of a horseshoe Vortex System near the juncture of a square cylinder and a horizontal base plate, using particle image velocimetry and flow visualization technique. Experiments were conducted for Reynolds numbers (based on the free stream velocity and the width of square cylinder) ranging from 2.0× 102 to 6.0× 103 . The flow patterns are first classified into four major regimes: Steady horseshoe Vortex System, periodic oscillation Vortex System with small displacement, periodic breakaway Vortex System, and irregular Vortex System. The classifications can be demonstrated as a figure of Reynolds number versus the ratio of the height of square cylinder to undisturbed boundary layer thickness. The study then mainly focused on the characteristics of steady horseshoe Vortex System (corresponding to Reynolds numbers ranging from 2.0× 102 to 2.5× 103 ). The nondimensional characteristics, including the horizontal and vertical distances from t...
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Simultaneous Particle Image Velocimetry and Laser Doppler Velocimetry Measurements of Periodical Oscillatory Horseshoe Vortex System near Square Cylinder-Base Plate Juncture
Journal of Engineering Mechanics, 2003Co-Authors: Chang Lin, Wen-jiun Lai, Kuang-an ChangAbstract:This paper presents simultaneous measurements using particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) techniques on the study of a horseshoe Vortex System. The horseshoe Vortex System is generated near the juncture of a vertical square cylinder and a horizontal base plate. The combination of PIV and LDV not only gives the spatial distribution and time history of velocity near the juncture for spatial and time domain analyses, it also allows phase averaging the PIV velocity data to reduce noise and, in a turbulent flow, result in turbulence statistics. A flow visualization technique displaying particle streaklines has also been used to help the classification of the Vortex System and visualize the flow motion and Vortex evolution. The classification of the horseshoe Vortex was briefly categorized as steady, periodical oscillatory, and turbulence-like chaotic Vortex Systems through the use of the flow visualization technique and time-domain spectral analysis. Phase-averaged flow characteristics of the periodical oscillatory Vortex System with a Reynolds number of 2,250 are presented in detail through the use of PIV and LDV as well as the flow visualization technique.
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Characteristics of horseshoe Vortex System near a vertical plate–base plate juncture
Experimental Thermal and Fluid Science, 2002Co-Authors: Chang Lin, Peng-hao Chiu, Shyh-jiunn ShiehAbstract:Abstract In-depth study of horseshoe Vortex flows, near the juncture of a vertical plate and a base plate, is performed using flow visualization technique, fiber laser Doppler velocimetry and particle image velocimetry Systems. The flow patterns are classified into four major categories while the Reynolds number, based on the free stream velocity and the width of vertical plate, varies from 400 to 11 000, and the height-to-width ratio changes from 0.5 to 4.0. These four major categories are defined as (1) steady Vortex System, (2) periodic oscillation Vortex System with small displacement, (3) periodic breakaway Vortex System, and (4) turbulent-like Vortex System. In the categories of periodic oscillation Vortex System with small displacement and periodic breakaway Vortex System, the Strouhal number increases with increasing Reynolds number if the width of the vertical plate is employed in the non-dimensional parameters. However, when the uniform flow speed and the height of vertical plate are maintained the same, the frequency of periodic oscillation and periodic breakaway Vortex Systems do not increase with the increasing width. This indicates that the width of vertical plate is not a characteristic length that affects the frequency of the periodic motion of the horseshoe Vortex System. Conversely, the Strouhal numbers, based on the boundary layer thickness, change from 0.0611 to 0.0896 and are found to be nearly independent of the Reynolds numbers. This clearly reveals that the boundary layer thickness is an important length scale that controls the frequency of the horseshoe Vortex System.
Louis Taillefer - One of the best experts on this subject based on the ideXlab platform.
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onset of plasticity and hardening of the hysteretic response in the Vortex System of yba2cu3o7 delta
Physical Review Letters, 1999Co-Authors: S Kokkaliaris, P A J De Groot, S N Gordeev, A A Zhukov, R Gagnon, Louis TailleferAbstract:A method based on partial magnetization loops has been used to study memory effects in detwinned YBa2Cu3O(7-delta) single crystals. These measurements have revealed the transition from a dislocation-free Bragg glass to a disordered Vortex phase. We have mapped this boundary in the B-T phase diagram and have found it to be in proximity to the onset of the second magnetization peak. For fields above the transition line, metastable topological disorder invades the Vortex System leading to a pronounced dependence of the critical current on the formation history of the Vortex lattice.
D J Jang - One of the best experts on this subject based on the ideXlab platform.
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collapse of the peak effect due to ac induced flux creep in an isotropic Vortex System of mgcni3 single crystals
Physical Review Letters, 2009Co-Authors: D J JangAbstract:: In an isotropic Vortex System of MgCNi_{3} single crystal, we first observed the collapse of the peak effect (PE), which is a sudden increase in the critical current (I_{c}) near the end of superconductivity. By moving magnetic vortices with dc and ac driving currents, we investigated the Vortex dynamics related to the PE. For the dc driving, a sharp peak in I_{c} was observed. As the driving frequency increases, the PE was collapsed and observable flux creep was developed in contrast to the result obtained from the well-studied anisotropic System of NbSe_{2}. Because the Vortex matter in our experimental situation is isotropic and has large correlation volume without severe deformation by pinning, the mobility of the Vortex matter is intrinsically high. From the detailed observation, we are convinced that the PE is definitely a dynamic phenomenon.
