The Experts below are selected from a list of 10896 Experts worldwide ranked by ideXlab platform
Taiping Liu - One of the best experts on this subject based on the ideXlab platform.
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Supersonic flow onto a solid wedge
Communications on Pure and Applied Mathematics, 2008Co-Authors: Volker Elling, Taiping LiuAbstract:We consider the problem of two-dimensional Supersonicflow onto a solid wedge, or equivalently in a concave corner formed by two solid walls. For mild corners, there are two possible steady state solutions, one with a strong and one with a weak shock emanating from the corner. The weak shock is observed in Supersonic flights. A longstanding natural conjecture is that the strong shock is unstable in some sense. We resolve this issue by showing that a sharp wedge will eventually produce weak shocks at the tip when accelerated to a Supersonic Speed. More precisely, we prove that for upstream state as initial data in the entire domain, the timedependent solution is self-similar, with a weak shock at the tip of the wedge. We construct analytic solutions for self-similar potential flow, both isothermal and isentropic with arbitrary� � 1. In the process of constructing the self-similar solution, we develop a large number of theoretical tools for these elliptic regions. These tools allow us to establish large-data results rather than a small perturbation. We show that the wave pattern persists as long as the weak shock is Supersonic-Supersonic; when this is no longer true, numerics show a physical change of behavior. In addition, we obtain rather detailed information about the elliptic region, including analyticity as well as bounds for velocity components and shock tangents. c � 2007 Wiley Periodicals, Inc.
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Supersonic flow onto a solid wedge
arXiv: Mathematical Physics, 2007Co-Authors: Volker Elling, Taiping LiuAbstract:We consider the problem of 2D Supersonic flow onto a solid wedge, or equivalently in a concave corner formed by two solid walls. For mild corners, there are two possible steady state solutions, one with a strong and one with a weak shock emanating from the corner. The weak shock is observed in Supersonic flights. A long-standing natural conjecture is that the strong shock is unstable in some sense. We resolve this issue by showing that a sharp wedge will eventually produce weak shocks at the tip when accelerated to a Supersonic Speed. More precisely we prove that for upstream state as initial data in the entire domain, the time-dependent solution is self-similar, with a weak shock at the tip of the wedge. We construct analytic solutions for self-similar potential flow, both isothermal and isentropic with arbitrary $\gamma\geq 1$. In the process of constructing the self-similar solution, we develop a large number of theoretical tools for these elliptic regions. These tools allow us to establish large-data results rather than a small perturbation. We show that the wave pattern persists as long as the weak shock is Supersonic-Supersonic; when this is no longer true, numerics show a physical change of behaviour. In addition we obtain rather detailed information about the elliptic region, including analyticity as well as bounds for velocity components and shock tangents.
Douglas N C Lin - One of the best experts on this subject based on the ideXlab platform.
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globular cluster formation from colliding substructure
The Astrophysical Journal, 2020Co-Authors: Piero Madau, Alessandro Lupi, Juerg Diemand, Andreas Burkert, Douglas N C LinAbstract:We investigate a scenario where the formation of Globular Clusters (GCs) is triggered by high-Speed collisions between infalling atomic-cooling subhalos during the assembly of the main galaxy host, a special dynamical mode of star formation that operates at high gas pressures and is intimately tied to LCDM hierarchical galaxy assembly. The proposed mechanism would give origin to "naked" globulars, as colliding dark matter subhalos and their stars will simply pass through one another while the warm gas within them clashes at highly Supersonic Speed and decouples from the collisionless component, in a process reminiscent of the Bullet galaxy cluster. We find that the resulting shock-compressed layer cools on a timescale that is typically shorter than the crossing time, first by atomic line emission and then via fine-structure metal-line emission, and is subject to gravitational instability and fragmentation. Through a combination of kinetic theory approximation and high-resolution $N$-body simulations, we show that this model may produce: (a) a GC number-halo mass relation that is linear down to dwarf galaxy scales and agrees with the trend observed over five orders of magnitude in galaxy mass; (b) a population of old globulars with a median age of 12 Gyr and an age spread similar to that observed; (c) a spatial distribution that is biased relative to the overall mass profile of the host; and (d) a bimodal metallicity distribution with a spread similar to that observed in massive galaxies.
A G Kosovichev - One of the best experts on this subject based on the ideXlab platform.
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the cause of photospheric and helioseismic responses to solar flares high energy electrons or protons
The Astrophysical Journal, 2007Co-Authors: A G KosovichevAbstract:Analysis of the hydrodynamic and helioseismic effects in the photosphere during the solar flare of 2002 July 23, observed by MDI/SOHO, and high-energy images from RHESSI show that these effects are closely associated with sources of the hard X-ray emission but that no such effects existed in the centroid region of the flare's gamma-ray emission. These results demonstrate that, contrary to expectations, these hydrodynamic and helioseismic responses ("sunquakes") are more likely to be caused by accelerated electrons than by high-energy protons. A series of multiple impulses of high-energy electrons form a hydrodynamic source that is moving in the photosphere at Supersonic Speed. This moving source plays a critical role in the formation of the anisotropic wave front of sunquakes.
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the cause of photospheric and helioseismic responses to solar flares high energy electrons or protons
arXiv: Astrophysics, 2007Co-Authors: A G KosovichevAbstract:Analysis of the hydrodynamic and helioseismic effects in the photosphere during the solar flare of July 23, 2002, observed by Michelson Doppler Imager (MDI) on SOHO, and high-energy images from RHESSI shows that these effects are closely associated with sources of the hard X-ray emission, and that there are no such effects in the centroid region of the flare gamma-ray emission. These results demonstrate that contrary to expectations the hydrodynamic and helioseismic responses (''sunquakes") are more likely to be caused by accelerated electrons than by high-energy protons. A series of multiple impulses of high-energy electrons forms a hydrodynamic source moving in the photosphere with a Supersonic Speed. The moving source plays a critical role in the formation of the anisotropic wave front of sunquakes.
Vierinen Juha - One of the best experts on this subject based on the ideXlab platform.
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Ionospheric Bow Waves and Perturbations Induced by the 21 August 2017 Solar Eclipse
'Wiley', 2018Co-Authors: Vierinen Juha, Erickson, Philip J., Goncharenko Larisa, Zhang Shun-rong, Coster, Anthea J, Rideout, William CAbstract:During solar eclipses, the Moon's shadow causes a large reduction in atmospheric energy input, including not only the stratosphere but also the thermosphere and ionosphere. The eclipse shadow has a Supersonic motion which is theoretically expected to generate atmospheric bow waves, similar to a fast-moving river boat, with waves starting in the lower atmosphere and propagating into the ionosphere. However, previous geographically limited observations have had difficulty detecting these weak waves within the natural background atmospheric variability, and the existence of eclipse-induced ionospheric waves and their evolution in a complex coupling system remain controversial. During the 21 August 2017 eclipse, high fidelity and wide coverage ionospheric observations provided for the first time an oversampled set of eclipse data, using a dense network of Global Navigation Satellite System receivers at ∼2,000 sites in North America. We show the first unambiguous evidence of ionospheric bow waves as electron content disturbances over central/eastern United States, with ∼1 h duration, 300–400 km waveleng th and 280 m/s phase Speed emanating from and tailing the totality region. We also identify large ionospheric perturbations moving at the Supersonic Speed of the maximum solar obscuration which are too fast to be associated with known gravity wave or large-scale traveling ionospheric disturbance processes. This study reveals complex interconnections between the Sun, Moon, and Earth's neutral atmosphere and ionosphere and demonstrates persistent coupling processes between different components of the Earth's atmosphere, a topic of significant community interest.United States. National Aeronautics and Space Administration (Grant NNX17AH71G)United States. National Aeronautics and Space Administration (Grant NNX15AB83G)United States. Office of Naval Research. Multidisciplinary University Research Initiative (Grant ONR15-FOA-0011)United States. Office of Naval Research. Multidisciplinary University Research Initiative (Grant ONR N00014-17-1-2186
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Ionospheric bow waves and perturbations induced by the 21 August 2017 solar eclipse
'Wiley', 2017Co-Authors: Zhang Shun-rong, Erickson, Philip J., Coster Anthea, Rideout William, Goncharenko Larisa, Vierinen JuhaAbstract:During solar eclipses, the Moon's shadow causes a large reduction in atmospheric energy input, including not only the stratosphere but also the thermosphere and ionosphere. The eclipse shadow has a Supersonic motion which is theoretically expected to generate atmospheric bow waves, similar to a fast‐moving river boat, with waves starting in the lower atmosphere and propagating into the ionosphere. However, previous geographically limited observations have had difficulty detecting these weak waves within the natural background atmospheric variability, and the existence of eclipse‐induced ionospheric waves and their evolution in a complex coupling system remain controversial. During the 21 August 2017 eclipse, high fidelity and wide coverage ionospheric observations provided for the first time an oversampled set of eclipse data, using a dense network of Global Navigation Satellite System receivers at ∼2,000 sites in North America. We show the first unambiguous evidence of ionospheric bow waves as electron content disturbances over central/eastern United States, with ∼1 h duration, 300–400 km wavelength and 280 m/s phase Speed emanating from and tailing the totality region. We also identify large ionospheric perturbations moving at the Supersonic Speed of the maximum solar obscuration which are too fast to be associated with known gravity wave or large‐scale traveling ionospheric disturbance processes. This study reveals complex interconnections between the Sun, Moon, and Earth's neutral atmosphere and ionosphere and demonstrates persistent coupling processes between different components of the Earth's atmosphere, a topic of significant community interest
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Ionospheric bow waves and perturbations induced by the 21 August 2017 solar eclipse
'Wiley', 2017Co-Authors: Zhang Shun-rong, Erickson, Philip J., Coster Anthea, Rideout William, Goncharenko Larisa, Vierinen JuhaAbstract:An edited version of this paper was published by AGU. Copyright 2017 American Geophysical Union. Zhang, S., Erickson, P.J., Goncharenko, L., Coster, A., Rideout, W. & Vierinen, J. (2017). Ionospheric bow waves and perturbations induced by the 21 August 2017 solar eclipse. Geophysical Research Letters, 44(24), 12067-12073. https://doi.org/10.1002/2017GL076054. To view the published article, go to https://doi.org/10.1002/2017GL076054.During solar eclipses, the Moon's shadow causes a large reduction in atmospheric energy input, including not only the stratosphere but also the thermosphere and ionosphere. The eclipse shadow has a Supersonic motion which is theoretically expected to generate atmospheric bow waves, similar to a fast‐moving river boat, with waves starting in the lower atmosphere and propagating into the ionosphere. However, previous geographically limited observations have had difficulty detecting these weak waves within the natural background atmospheric variability, and the existence of eclipse‐induced ionospheric waves and their evolution in a complex coupling system remain controversial. During the 21 August 2017 eclipse, high fidelity and wide coverage ionospheric observations provided for the first time an oversampled set of eclipse data, using a dense network of Global Navigation Satellite System receivers at ∼2,000 sites in North America. We show the first unambiguous evidence of ionospheric bow waves as electron content disturbances over central/eastern United States, with ∼1 h duration, 300–400 km wavelength and 280 m/s phase Speed emanating from and tailing the totality region. We also identify large ionospheric perturbations moving at the Supersonic Speed of the maximum solar obscuration which are too fast to be associated with known gravity wave or large‐scale traveling ionospheric disturbance processes. This study reveals complex interconnections between the Sun, Moon, and Earth's neutral atmosphere and ionosphere and demonstrates persistent coupling processes between different components of the Earth's atmosphere, a topic of significant community interest.Plain Language Summary: During solar eclipses, the Moon's shadow causes a large reduction in atmospheric energy input, including the stratosphere and both the thermosphere and ionosphere (∼100–1,000 km altitudes). Theoretical studies since the 1960s have predicted that the Moon's Supersonic shadow should generate atmospheric bow waves, similar to a fast‐moving river boat. However, observations were geographically limited for these weak and complicated waves. In 2017, high fidelity and wide coverage ionospheric observations were made using a North American Global Navigation Satellite System (GNSS) ∼2,000 receiver network. Eclipse passage generated clear ionospheric bow waves in electron content disturbances emanating from totality primarily over central/eastern United States. Study of wave characteristics reveals complex interconnections between the Sun, Moon, and Earth's neutral atmosphere and ionosphere
Peter H. Dahl - One of the best experts on this subject based on the ideXlab platform.
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on the underwater sound field from impact pile driving arrival structure precursor arrivals and energy streamlines
Journal of the Acoustical Society of America, 2017Co-Authors: Peter H. Dahl, David R DallostoAbstract:Underwater noise from impact pile driving is studied through measurements using a vertical line array (VLA) placed at range 120 m from the pile source (water depth 7.5 m) over which bathymetry varied gradually increasing to depth 12.5 m at the VLA. The data were modeled assuming the pile impact produces a radial expansion that acts as sound source and propagates along the pile at Supersonic Speed. This leads to the conceptualization of the pile as a discrete, vertical line source for which frequency- and source-depth-dependent complex phasing is applied. Dominant features of the pressure time series versus measurement depth are reproduced in modeled counterparts that are linearly related. These observations include precursor arrivals for which arrival timing depends on hydrophone depth and influence of a sediment sound Speed gradient on precursor amplitude. Spatial gradients of model results are taken to obtain estimates of acoustic particle velocity and vector intensity for which active intensity is stud...
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on the underwater sound field from impact pile driving arrival structure precursor arrivals and energy streamlines
Journal of the Acoustical Society of America, 2017Co-Authors: Peter H. Dahl, David R DallostoAbstract:Underwater noise from impact pile driving is studied through measurements using a vertical line array (VLA) placed at range 120 m from the pile source (water depth 7.5 m) over which bathymetry varied gradually increasing to depth 12.5 m at the VLA. The data were modeled assuming the pile impact produces a radial expansion that acts as sound source and propagates along the pile at Supersonic Speed. This leads to the conceptualization of the pile as a discrete, vertical line source for which frequency- and source-depth-dependent complex phasing is applied. Dominant features of the pressure time series versus measurement depth are reproduced in modeled counterparts that are linearly related. These observations include precursor arrivals for which arrival timing depends on hydrophone depth and influence of a sediment sound Speed gradient on precursor amplitude. Spatial gradients of model results are taken to obtain estimates of acoustic particle velocity and vector intensity for which active intensity is studied in the time domain. Evaluation of energy streamlines based on time-integrated active intensity, and energy path lines based on instantaneous (or very-short-time integrated) active intensity reveal interesting structure in the acoustic field, including an inference as to the source depth of the precursor.
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beam forming of the underwater sound field from impact pile driving
Journal of the Acoustical Society of America, 2013Co-Authors: Peter H. Dahl, Per G. ReinhallAbstract:Observations of underwater noise from impact pile driving were made with a vertical line array. Previous studies [Reinhall and Dahl, J. Acoust. Soc. Am. 130, 1209–1216 (2011)] show that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile at Supersonic Speed after impact. Here precise estimates of the vertical arrival angles associated with the down- and up-going Mach wave are made via beam forming, and the energy budget of the arrival structure is quantified.
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Underwater Mach wave radiation from impact pile driving: Theory and observation
The Journal of the Acoustical Society of America, 2011Co-Authors: Per G. Reinhall, Peter H. DahlAbstract:The underwater noise from impact pile driving is studied using a finite element model for the sound generation and parabolic equation model for propagation. Results are compared with measurements using a vertical line array deployed at a marine construction site in Puget Sound. It is shown that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile after impact at Supersonic Speed. The predictions of vertical arrival angle associated with the Mach cone, peak pressure level as function of depth, and dominant features of the pressure timeseries compare well with corresponding field observations.
