scholarly journals An efficient method for unfolding kinetic pressure driven VISAR data

2015 ◽  
Vol 3 ◽  
Author(s):  
M. Hess ◽  
K. Peterson ◽  
A. Harvey-Thompson
Keyword(s):  
Precise Measurement ◽  
Magnetic Pressure ◽  
Pulsed Power ◽  
Time Dependent ◽  
Surface Velocity ◽  
Critical Component ◽  
Shock Physics ◽  
Reliable Prediction ◽  
Kinetic Pressure ◽  
Real Challenge

Velocity Interferometer System for Any Reflector (VISAR) [Barker and Hollenbach, J. Appl. Phys. 43, 4669 (1972)] is a well-known diagnostic that is employed on many shock physics and pulsed-power experiments. With the VISAR diagnostic, the velocity on the surface of any metal flyer can be found. For most experiments employing VISAR, either a kinetic pressure [Grady, Mech. Mater. 29, 181 (1998)] or a magnetic pressure [Lemke et al., Intl J. Impact Eng. 38, 480 (2011)] drives the motion of the flyer. Moreover, reliable prediction of the time-dependent pressure is often a critical component to understanding the physics of these experiments. Although VISAR can provide a precise measurement of a flyer’s surface velocity, the real challenge of this diagnostic implementation is using this velocity to unfold the time-dependent pressure. The purpose of this paper is to elucidate a new method for quickly and reliably unfolding VISAR data.

scholarly journals 3-D Hydro dynamic Simulation of Accretion Disk Formation in LMC-X4

1997 ◽  
Vol 163 ◽  
pp. 779-779 ◽  
Author(s):  
Michael P. Owen ◽  
John M. Blondin
Keyword(s):  
Accretion Disk ◽  
Three Dimensional ◽  
Orbital Plane ◽  
Time Dependent ◽  
Surface Velocity ◽  
General Interest ◽  
Hydrodynamic Simulation ◽  
Disk Formation ◽  
Tidal Stream ◽  
Natural Result

We present preliminary results of a time-dependent, three-dimensional hydrodynamic simulation of LMC-X4, an HMXB known to be undergoing RLOF. The simulation is initialized with the collapsed companion embedded in the undisturbed primary wind. The primary is in contact with the Roche surface, although no tidal stream or accretion disk is initialed; they are allowed to form independently.Several features of general interest to disk-fed HMXBs are apparent in the simulation. First, the primary immediately develops a compressed-wind disk in the orbital plane. This may be a natural result in most disk-fed HMXBs. Fora circularized system in an orbit close enough for RLOF to take place, we may expect the primary to be in corotation. The surface velocity may then be a significant fraction of the breakup velocity, leading to a compressed-wind disk.

scholarly journals Infrared variability due to magnetic pressure-driven jets, dust ejection and quasi-puffed-up inner rims

2020 ◽  
Vol 493 (3) ◽  
pp. 4022-4038 ◽  
Author(s):  
Kurt Liffman ◽  
Geoffrey Bryan ◽  
Mark Hutchison ◽  
Sarah T Maddison
Keyword(s):  
Accretion Rate ◽  
Jet Flow ◽  
Flow Speed ◽  
Magnetic Pressure ◽  
Scale Height ◽  
Time Dependent ◽  
Gas Flows ◽  
Centrifugal Acceleration ◽  
The Face ◽  
Disc Region

ABSTRACT The interaction between a YSO stellar magnetic field and its protostellar disc can result in stellar accretional flows and outflows from the inner disc rim. Gas flows with a velocity component perpendicular to disc mid-plane subject particles to centrifugal acceleration away from the protostar, resulting in particles being catapulted across the face of the disc. The ejected material can produce a ‘dust fan’, which may be dense enough to mimic the appearance of a ‘puffed-up’ inner disc rim. We derive analytical equations for the time-dependent disc toroidal field, the disc magnetic twist, the size of the stable toroidal disc region, the jet speed, and the disc region of maximal jet flow speed. We show how the observed infrared variability of the pre-transition disc system LRLL 31 can be modelled by a dust ejecta fan from the inner-most regions of the disc whose height is partially dependent on the jet flow speed. The greater the jet flow speed, the higher is the potential dust fan scale height. An increase in mass accretion on to the star tends to increase the height and optical depth of the dust ejection fan, increasing the amount of 1–8 µm radiation. The subsequent shadow reduces the amount of light falling on the outer disc and decreases the 8–40 µm radiation. A decrease in the accretion rate reverses this scenario, thereby producing the observed ‘see-saw’ infrared variability.

Seasonal to multi-annual speedup and slowdown of Greenland outlet glaciers inferred from time-dependent remote sensing observations

2020 ◽  
Author(s):  
Lizz Ultee ◽  
Bryan Riel ◽  
Brent Minchew
Keyword(s):  
Time Series ◽  
Flow Velocity ◽  
Ice Sheet ◽  
Surface Air Temperature ◽  
Greenland Ice Sheet ◽  
Time Dependent ◽  
Surface Velocity ◽  
Short Term ◽  
Ice Flow

<p>The rate of ice flux from the Greenland Ice Sheet to the ocean depends on the ice flow velocity through outlet glaciers. Ice flow velocity, in turn, evolves in response to multiple geographic and environmental forcings at different timescales. For example, velocity may vary daily in response to ocean tides, seasonally in response to surface air temperature, and multi-annually in response to long-term trends in climate. The satellite observations processed as part of the NASA MEaSUREs Greenland Ice Sheet Velocity Map allow us to analyse variations in ice surface velocity at multiple timescales. Here, we decompose short-term and long-term signals in time-dependent velocity fields for Greenland outlet glaciers based on the methods of Riel et al. (2018). Patterns found in short-term signals can constrain basal sliding relations and ice rheology, while the longer-term signals hint at decadal in/stability of outlet glaciers. We present example velocity time series for outlets including Sermeq Kujalleq (Jakobshavn Isbrae) and Helheim Glacier, and we highlight features indicative of dynamic drawdown or advective restabilization. Finally, we comment on the capabilities of a time series analysis software under development for glaciological applications.</p>

scholarly journals Data-Driven Inference of the Mechanics of Slip Along Glacier Beds Using Physics-Informed Neural Networks

2021 ◽  
Author(s):  
Bryan Riel ◽  
Brent Minchew ◽  
Tobias Bischoff
Keyword(s):  
Neural Networks ◽  
Remote Sensing Data ◽  
Time Dependent ◽  
Surface Velocity ◽  
Time Varying ◽  
Ice Flow ◽  
Physical Constraints ◽  
Training Strategy ◽  
Proper Form ◽  
Ice Surface

<p>Reliable projections of sea level rise depend on accurate representations of how fast-flowing glaciers slip along their beds. Specifically, ice sheet models require a quantitative sliding law that relates basal drag to sliding velocity and glacier geometry, yet the proper form of the law remains uncertain. Here, we present a novel deep learning-based framework for learning the time evolution of basal drag from time-dependent ice surface velocity and elevation observations. We train a pair of probabilistic neural networks through a combination of time-dependent surface observations, governing equations for ice flow, and known physical constraints. Neural network outputs are stochastic predictions of time-varying basal drag that do not require any prior assumptions on the form of the sliding law. This training strategy is well-suited to large volumes of remote sensing data while providing a natural way to integrate our existing understanding of the physics of ice flow into the learning process.</p><p>We test this framework on 1D and 2D ice flow simulations and demonstrate that, under certain stress conditions, recovery of the underlying sliding law parameters and their uncertainties can be derived from the stochastic predictions of time-varying basal drag. We also apply these methods to Rutford Ice Stream and Pine Island Glacier in Antarctica to investigate subglacial hydrological effects for the former and evidence for regularized Coulomb sliding for the latter.</p>

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