The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Jonathan B Freund - One of the best experts on this subject based on the ideXlab platform.
-
the near Field Pressure radiated by planar high speed free shear flow turbulence
Journal of Fluid Mechanics, 2017Co-Authors: David Buchta, Jonathan B FreundAbstract:Jets with Mach numbers are well known to emit an intense, fricative, so-called crackle sound, having steep compressions interspersed with weaker expansions that together yield a positive Pressure skewness . Its shock-like features are obvious hallmarks of nonlinearity, although a full explanation of the skewness is lacking, and wave steepening alone is understood to be insufficient to describe its genesis. Direct numerical simulations of high-speed free-shear flows for Mach numbers , , and in the Reynolds number range are used to examine the mechanisms leading to such Pressure signals, especially the Pressure skewness. For and , the Pressure immediately adjacent the turbulence already has the large associated with jet crackle. It also has a surprisingly complex three-dimensional structure, with locally high Pressures at compression-wave intersections. This structure is transient, and it simplifies as radiating waves subsequently merge through nonlinear mechanisms to form the relatively distinct and approximately two-dimensional Mach-like waves deduced from laboratory visualizations. A transport equation for is analysed to quantify factors affecting its development. The viscous dissipation that decreases is balanced by a particular nonlinear flux, which is (of course) absent in linear acoustic propagation and confirmed to be independent of the simulated Reynolds numbers. Together these effects maintain an approximately constant in the near acoustic Field.
-
near Field shocks radiated by high speed free shear flow turbulence
AIAA CEAS Aeroacoustics Conference, 2014Co-Authors: David Buchta, Aaron T Anderson, Jonathan B FreundAbstract:Temporally developing turbulent planar free shear layers with Mach numbers M = 1.5, 2.5, and 3.5 are directly simulated to provide a geometrically simplified model of the generation of a peculiar sound, known as ‘crackle’. Sound-Field Pressure skewness for M = 2.5 and 3.5 cases exceed Sk(p ′) > 0.4, which has been correlated with perception of crackle. Statistics related to Mach wave angle and wave density indicate nonlinear interactions in the very near acoustic Field in these same cases. Results show both merging of multiple waves and flattening of non-planar waves into approximately planar weak shocks.
-
turbulence and sound Field pod analysis of a turbulent jet
International Journal of Aeroacoustics, 2009Co-Authors: Jonathan B Freund, Tim ColoniusAbstract:A Proper Orthogonal Decomposition (POD) is constructed for a Mach 0.9 turbulent jet using a well-validated direct numerical simulation database. Norms are defined based on near-Field volume integrals of Pressure, turbulence kinetic energy, streamwise velocity, and total enthalpy, two-dimensional integrals of streamswise velocity (to match experimental measurements), and far-Field integrals of Pressure over a sphere. We find substantially different POD modes for the different norms, and their efficiency at representing the full data is strongly dependent upon the norm and specifically which data we attempt to represent. To reproduce near-Field turbulence statistics requires relatively few modes computed by a kinetic energy or Pressure norm. However, a large number of the POD modes computed using a near-Field norm are required to represent the sound Field. The dominant near-Field POD modes computed with either the near-Field Pressure norm or the sound Field norm have the structure of wave packets.
David Buchta - One of the best experts on this subject based on the ideXlab platform.
-
the near Field Pressure radiated by planar high speed free shear flow turbulence
Journal of Fluid Mechanics, 2017Co-Authors: David Buchta, Jonathan B FreundAbstract:Jets with Mach numbers are well known to emit an intense, fricative, so-called crackle sound, having steep compressions interspersed with weaker expansions that together yield a positive Pressure skewness . Its shock-like features are obvious hallmarks of nonlinearity, although a full explanation of the skewness is lacking, and wave steepening alone is understood to be insufficient to describe its genesis. Direct numerical simulations of high-speed free-shear flows for Mach numbers , , and in the Reynolds number range are used to examine the mechanisms leading to such Pressure signals, especially the Pressure skewness. For and , the Pressure immediately adjacent the turbulence already has the large associated with jet crackle. It also has a surprisingly complex three-dimensional structure, with locally high Pressures at compression-wave intersections. This structure is transient, and it simplifies as radiating waves subsequently merge through nonlinear mechanisms to form the relatively distinct and approximately two-dimensional Mach-like waves deduced from laboratory visualizations. A transport equation for is analysed to quantify factors affecting its development. The viscous dissipation that decreases is balanced by a particular nonlinear flux, which is (of course) absent in linear acoustic propagation and confirmed to be independent of the simulated Reynolds numbers. Together these effects maintain an approximately constant in the near acoustic Field.
-
near Field shocks radiated by high speed free shear flow turbulence
AIAA CEAS Aeroacoustics Conference, 2014Co-Authors: David Buchta, Aaron T Anderson, Jonathan B FreundAbstract:Temporally developing turbulent planar free shear layers with Mach numbers M = 1.5, 2.5, and 3.5 are directly simulated to provide a geometrically simplified model of the generation of a peculiar sound, known as ‘crackle’. Sound-Field Pressure skewness for M = 2.5 and 3.5 cases exceed Sk(p ′) > 0.4, which has been correlated with perception of crackle. Statistics related to Mach wave angle and wave density indicate nonlinear interactions in the very near acoustic Field in these same cases. Results show both merging of multiple waves and flattening of non-planar waves into approximately planar weak shocks.
Y Tokura - One of the best experts on this subject based on the ideXlab platform.
-
electrical magnetochiral effect induced by chiral spin fluctuations
Nature Communications, 2017Co-Authors: Tomoyuki Yokouchi, Naoya Kanazawa, Akiko Kikkawa, D Morikawa, K Shibata, T Arima, Y Taguchi, Fumitaka Kagawa, Y TokuraAbstract:Chirality of matter can produce unique responses in optics, electricity and magnetism. In particular, magnetic crystals transmit their handedness to the magnetism via antisymmetric exchange interaction of relativistic origin, producing helical spin orders as well as their fluctuations. Here we report for a chiral magnet MnSi that chiral spin fluctuations manifest themselves in the electrical magnetochiral effect, i.e. the nonreciprocal and nonlinear response characterized by the electrical resistance depending on inner product of current and magnetic Field. Prominent electrical magnetochiral signals emerge at specific temperature-magnetic Field-Pressure regions: in the paramagnetic phase just above the helical ordering temperature and in the partially-ordered topological spin state at low temperatures and high Pressures, where thermal and quantum spin fluctuations are conspicuous in proximity of classical and quantum phase transitions, respectively. The finding of the asymmetric electron scattering by chiral spin fluctuations may explore new electromagnetic functionality in chiral magnets. The magnetism-induced chirality in electron transportation is of fundamental importantance in condensed matter physics but the origin is still unclear. Here the authors demonstrate that the asymmetric electron scattering by chiral spin fluctuations can be the key to the electrical magnetochiral effect in MnSi.
-
electrical magnetochiral effect induced by chiral spin fluctuations
arXiv: Strongly Correlated Electrons, 2017Co-Authors: Tomoyuki Yokouchi, Naoya Kanazawa, Akiko Kikkawa, D Morikawa, K Shibata, T Arima, Y Taguchi, Fumitaka Kagawa, Y TokuraAbstract:Chirality of matter can produce unique responses in optics, electricity and magnetism. In particular, magnetic crystals transmit their handedness to the magnetism via antisymmetric exchange interaction of relativistic origin, producing helical spin orders as well as their fluctuations. Here we report for a chiral magnet MnSi that chiral spin fluctuations manifest themselves in the electrical magnetochiral effect (eMChE), i.e. the nonreciprocal and nonlinear response characterized by the electrical conductance depending on inner product of electric and magnetic Fields $\boldsymbol{E} \cdot \boldsymbol{B}$. Prominent eMChE signals emerge at specific temperature-magnetic Field-Pressure regions: in the paramagnetic phase just above the helical ordering temperature and in the partially-ordered topological spin state at low temperatures and high Pressures, where thermal and quantum spin fluctuations are conspicuous in proximity of classical and quantum phase transitions, respectively. The finding of the asymmetric electron scattering by chiral spin fluctuations may explore new electromagnetic functionality in chiral magnets.
T Arima - One of the best experts on this subject based on the ideXlab platform.
-
electrical magnetochiral effect induced by chiral spin fluctuations
Nature Communications, 2017Co-Authors: Tomoyuki Yokouchi, Naoya Kanazawa, Akiko Kikkawa, D Morikawa, K Shibata, T Arima, Y Taguchi, Fumitaka Kagawa, Y TokuraAbstract:Chirality of matter can produce unique responses in optics, electricity and magnetism. In particular, magnetic crystals transmit their handedness to the magnetism via antisymmetric exchange interaction of relativistic origin, producing helical spin orders as well as their fluctuations. Here we report for a chiral magnet MnSi that chiral spin fluctuations manifest themselves in the electrical magnetochiral effect, i.e. the nonreciprocal and nonlinear response characterized by the electrical resistance depending on inner product of current and magnetic Field. Prominent electrical magnetochiral signals emerge at specific temperature-magnetic Field-Pressure regions: in the paramagnetic phase just above the helical ordering temperature and in the partially-ordered topological spin state at low temperatures and high Pressures, where thermal and quantum spin fluctuations are conspicuous in proximity of classical and quantum phase transitions, respectively. The finding of the asymmetric electron scattering by chiral spin fluctuations may explore new electromagnetic functionality in chiral magnets. The magnetism-induced chirality in electron transportation is of fundamental importantance in condensed matter physics but the origin is still unclear. Here the authors demonstrate that the asymmetric electron scattering by chiral spin fluctuations can be the key to the electrical magnetochiral effect in MnSi.
-
electrical magnetochiral effect induced by chiral spin fluctuations
arXiv: Strongly Correlated Electrons, 2017Co-Authors: Tomoyuki Yokouchi, Naoya Kanazawa, Akiko Kikkawa, D Morikawa, K Shibata, T Arima, Y Taguchi, Fumitaka Kagawa, Y TokuraAbstract:Chirality of matter can produce unique responses in optics, electricity and magnetism. In particular, magnetic crystals transmit their handedness to the magnetism via antisymmetric exchange interaction of relativistic origin, producing helical spin orders as well as their fluctuations. Here we report for a chiral magnet MnSi that chiral spin fluctuations manifest themselves in the electrical magnetochiral effect (eMChE), i.e. the nonreciprocal and nonlinear response characterized by the electrical conductance depending on inner product of electric and magnetic Fields $\boldsymbol{E} \cdot \boldsymbol{B}$. Prominent eMChE signals emerge at specific temperature-magnetic Field-Pressure regions: in the paramagnetic phase just above the helical ordering temperature and in the partially-ordered topological spin state at low temperatures and high Pressures, where thermal and quantum spin fluctuations are conspicuous in proximity of classical and quantum phase transitions, respectively. The finding of the asymmetric electron scattering by chiral spin fluctuations may explore new electromagnetic functionality in chiral magnets.
Mo Samimy - One of the best experts on this subject based on the ideXlab platform.
-
near Field and acoustic far Field response of a high speed jet to excitation
AIAA Journal, 2015Co-Authors: Michael Crawley, Aniruddha Sinha, Mo SamimyAbstract:The near-Field and acoustic far-Field response of an unheated Mach 0.9 jet with Reynolds number 6.2×105 was investigated. The study included both the baseline and controlled jets utilizing plasma actuators. Simultaneous acquisition of the near- and far-Field signals with the actuation phase enabled the use of phase averaging to reconstruct the signature of the large-scale coherent structures generated by excitation of instabilities in the shear layer of the jet. Decomposition of the near-Field Pressure into its constitutive hydrodynamic and acoustic Fields is accomplished via the application of a filter in the frequency/wave-number domain. The results showed that both the hydrodynamic and acoustic response to excitation for Strouhal numbers StDF less than 0.50 could be well predicted by a simple linear superposition of the impulse response of the jet. The results appear to indicate that the dominant acoustic radiation reaching the far-Field aft angles is being generated over an extended region of the jet ...
-
near Field Pressure and far Field acoustic response of forced high speed jets
52nd Aerospace Sciences Meeting, 2014Co-Authors: Michael Crawley, Aniruddha Sinha, Mo SamimyAbstract:The near-Field Pressure of an unheated, Mach 0.9 jet with a ReD of 6.2x10 5 excited by plasma actuators has been investigated in order to evaluate the hydrodynamic and acoustic response of the jet, and the link between the two. Simultaneous acquisition of the far-Field acoustic, the near-Field Pressure, and the actuation phase enables the use of phase-averaging of the Pressure and acoustic signals and space-time correlations between the near Field and the far Field. By applying a filter in the frequency-wavenumber space, the nearField Pressure is decomposed into its constitutive hydrodynamic and acoustic components. Finally, wavelet analysis is utilized to assess the decomposed Fields in the time domain. Both the hydrodynamic and acoustic response to forcing for StDF < 0.50 are found to follow a quasi-linear interaction model, in which the response to periodic forcing can be well predicted by a simple linear superposition of the impulsive response. Measurements of the Pressure fluctuations and correlations to the far-Field aft angle display an upstream shift in the structure saturation point as well as the dominant acoustic source region with increasing forcing frequency. Preliminary investigation in the time-domain found significantly lower temporal coherency of the acoustic response versus the hydrodynamic response; this difference was reduced by periodic forcing.
-
correlation of irrotational near Field Pressure and far Field acoustic in forced high speed jets
AIAA CEAS Aeroacoustics Conference, 2013Co-Authors: Michael Crawley, Aniruddha Sinha, Hind Alkandry, Mo SamimyAbstract:The near-Field Pressure of a Mach 0.9 jet with Reynolds number of 6.2x10 5 has been investigated in order to characterize the hydrodynamic and acoustic response to low-frequency forcing with localized arc filament plasma actuators. Simultaneous acquisition of the far-Field acoustic, the near-Field Pressure, and the actuation phase enables the use of phase-averaging of the Pressure and acoustic signals and space-time correlations between the near Field and the far Field. Results show that forcing at very low frequencies results in structures that evolve independently as they advect through the shear layer. As the forcing frequency is increased, the structures begin to interact quasi-linearly. Autocorrelations of the phase-averaged near Field signals show coherent acoustic radiation, indicating that the coherent large-scale structures generate coherent acoustic radiation. The interaction and potential disintegration of the large-scale structures as they pass through the end of the potential core has been suggested as a source mechanism for downstream acoustic radiation; the results of this study also point towards this mechanism as being the dominant noise source towards the downstream polar angles in this jet.
-
the impulse response of a high speed jet forced with localized arc filament plasma actuators
Physics of Fluids, 2012Co-Authors: Aniruddha Sinha, Mo Samimy, Hind Alkandry, Martin Kearneyfischer, Tim ColoniusAbstract:We present experimental and theoretical analyses of the response of high-speed, high-Reynolds-number, round jets to impulsive forcing with arc-filament-plasma actuators. The impulse response is obtained with forcing Strouhal numbers, based on the nozzle exit diameter and exit center line velocity, less than 0.1. The resulting phase-averaged near-Field Pressure signature displays a compact wave with a positive peak preceding a negative one, indicative of a large scale structure in the shear layer of the jet. Scaling laws derived by operating the jet at four subsonic Mach numbers are used to distinguish this hydrodynamic component of the phase-averaged jet response from the direct actuator noise. As the forcing frequency increases, the compact waves in the near-Field Pressure signal overlap each other, indicating interaction of the growing seeded structures. For this regime, the phase-averaged response is approximately replicated by linear superposition of the impulse response, thereby demonstrating the quasi-linearity of structure interaction. A novel application of linear parabolized stability theory yields a successful model of the impulse response.