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Title: As a Matter of Force—Systematic Biases in Idealized Turbulence Simulations

Abstract

Many astrophysical systems encompass very large dynamical ranges in space and time, which are not accessible by direct numerical simulations. Thus, idealized subvolumes are often used to study small-scale effects including the dynamics of turbulence. These turbulent boxes require an artificial driving in order to mimic energy injection from large-scale processes. In this paper, we show and quantify how the autocorrelation time of the driving and its normalization systematically change the properties of an isothermal compressible magnetohydrodynamic flow in the sub- and supersonic regime and affect astrophysical observations such as Faraday rotation. For example, we find that δ-in-time forcing with a constant energy injection leads to a steeper slope in kinetic energy spectrum and less-efficient small-scale dynamo action. In general, we show that shorter autocorrelation times require more power in the acceleration field, which results in more power in compressive modes that weaken the anticorrelation between density and magnetic field strength. Thus, derived observables, such as the line-of-sight (LOS) magnetic field from rotation measures, are systematically biased by the driving mechanism. Finally, we argue that δ-in-time forcing is unrealistic and numerically unresolved, and conclude that special care needs to be taken in interpreting observational results based on the use ofmore » idealized simulations.« less

Authors:
ORCiD logo [1]; ORCiD logo [2];  [3]
  1. Michigan State Univ., East Lansing, MI (United States). Dept. of Physics and Astronomy
  2. Michigan State Univ., East Lansing, MI (United States). Dept. of Physics and Astronomy. Dept. of Computational Mathematics, Science and Engineering. National Superconducting Cyclotron Lab.
  3. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Michigan State Univ., East Lansing, MI (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA); National Aeronautic and Space Administration (NASA); National Science Foundation (NSF)
OSTI Identifier:
1472269
Alternate Identifier(s):
OSTI ID: 1496999; OSTI ID: 1497000
Report Number(s):
SAND2018-4724J; SAND-2018-2657J; SAND-2018-1399J
Journal ID: ISSN 2041-8213; 667358
Grant/Contract Number:  
NA0003525; NNX15AP39G; 1514700; ACI-1548562; AC04-94AL85000
Resource Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal. Letters
Additional Journal Information:
Journal Volume: 858; Journal Issue: 2; Journal ID: ISSN 2041-8213
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; magnetohydrodynamics (MHD); numerical methods; turbulence

Citation Formats

Grete, Philipp, O’Shea, Brian W., and Beckwith, Kris. As a Matter of Force—Systematic Biases in Idealized Turbulence Simulations. United States: N. p., 2018. Web. doi:10.3847/2041-8213/aac0f5.
Grete, Philipp, O’Shea, Brian W., & Beckwith, Kris. As a Matter of Force—Systematic Biases in Idealized Turbulence Simulations. United States. doi:10.3847/2041-8213/aac0f5.
Grete, Philipp, O’Shea, Brian W., and Beckwith, Kris. Thu . "As a Matter of Force—Systematic Biases in Idealized Turbulence Simulations". United States. doi:10.3847/2041-8213/aac0f5. https://www.osti.gov/servlets/purl/1472269.
@article{osti_1472269,
title = {As a Matter of Force—Systematic Biases in Idealized Turbulence Simulations},
author = {Grete, Philipp and O’Shea, Brian W. and Beckwith, Kris},
abstractNote = {Many astrophysical systems encompass very large dynamical ranges in space and time, which are not accessible by direct numerical simulations. Thus, idealized subvolumes are often used to study small-scale effects including the dynamics of turbulence. These turbulent boxes require an artificial driving in order to mimic energy injection from large-scale processes. In this paper, we show and quantify how the autocorrelation time of the driving and its normalization systematically change the properties of an isothermal compressible magnetohydrodynamic flow in the sub- and supersonic regime and affect astrophysical observations such as Faraday rotation. For example, we find that δ-in-time forcing with a constant energy injection leads to a steeper slope in kinetic energy spectrum and less-efficient small-scale dynamo action. In general, we show that shorter autocorrelation times require more power in the acceleration field, which results in more power in compressive modes that weaken the anticorrelation between density and magnetic field strength. Thus, derived observables, such as the line-of-sight (LOS) magnetic field from rotation measures, are systematically biased by the driving mechanism. Finally, we argue that δ-in-time forcing is unrealistic and numerically unresolved, and conclude that special care needs to be taken in interpreting observational results based on the use of idealized simulations.},
doi = {10.3847/2041-8213/aac0f5},
journal = {The Astrophysical Journal. Letters},
number = 2,
volume = 858,
place = {United States},
year = {2018},
month = {5}
}

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Figures / Tables:

Figure 1 Figure 1: Temporal evolution of the spatial root mean square (RMS) sonic Mach number (a) and Alfvénic Mach number (b). The gray area between 2.5T ≤ t ≤ 5T indicates the temporal range we use as stationary regime throughout the paper.

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Figures/Tables have been extracted from DOE-funded journal article accepted manuscripts.