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1. Entropy-Conserving Scheme for Modeling Nonthermal Energies in Fluid Dynamics Simulations

   Semenov, Vadim A. ; Kravtsov, Andrey V ; Diemer, Benedikt ( August 2021 , The astrophysical journal)

   null (Ed.)

   We compare the performance of energy-based and entropy-conservative schemes for modeling nonthermal energy components, such as unresolved turbulence and cosmic rays, using idealized fluid dynamics tests and isolated galaxy simulations. While both methods are aimed to model advection and adiabatic compression or expansion of different energy components, the energy-based scheme numerically solves the non-conservative equation for the energy density evolution, while the entropy-conserving scheme uses a conservative equation for modified entropy. Using the standard shock tube and Zel'dovich pancake tests, we show that the energy-based scheme results in a spurious generation of nonthermal energy on shocks, while the entropy-conserving method evolves the energy adiabatically to machine precision. We also show that, in simulations of an isolated Lstar galaxy, switching between the schemes results in ~20-30% changes of the total star formation rate and a significant difference in morphology, particularly near the galaxy center. We also outline and test a simple method that can be used in conjunction with the entropy-conserving scheme to model the injection of nonthermal energies on shocks. Finally, we discuss how the entropy-conserving scheme can be used to capture the kinetic energy dissipated by numerical viscosity into the subgrid turbulent energy implicitly, without explicit source terms that require calibration and can be rather uncertain. Our results indicate that the entropy-conserving scheme is the preferred choice for modeling nonthermal energy components, a conclusion that is equally relevant for Eulerian and moving-mesh fluid dynamics codes. 

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2. A mass- and energy-conserved DG method for the Schrödinger-Poisson equation

   https://doi.org/10.1007/s11075-021-01139-0  

   Yi, Nianyu ; Liu, Hailiang ( February 2021 , Numerical Algorithms)

   null (Ed.)

   In this paper, we construct, analyze, and numerically validate conservative discontinuous Galerkin (DG) schemes for approximating the Schr\"{o}dinger-Poisson equation. The proposed schemes all satisfy both mass and energy conservation. For the semi-discrete DG scheme optimal $L^2$ error estimates are obtained. Efficient iterative solvers are also constructed to solve the second order implicit time discretization. A number of numerical tests are presented to demonstrate the method’s accuracy and robustness, confirming that both mass and energy are well preserved over long time simulations. 

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3. Detecting Preheating in Protoclusters with Lyα Forest Tomography

   https://doi.org/10.3847/1538-4357/ac4cb1  

   Kooistra, Robin ; Inoue, Shigeki ; Lee, Khee-Gan ; Cen, Renyue ; Yoshida, Naoki ( March 2022 , The Astrophysical Journal)

   Studies of low-redshift galaxy clusters suggest the intracluster medium (ICM) has experienced nongravitational heating during the formation phase of the clusters. Using simple phenomenological heating prescriptions, we simulate the effect of this preheating of the nascent ICM in galaxy protoclusters and examine its effect on Lyαforest tomographic maps. We analyze a series of cosmological zoom-in simulations of protoclusters within the framework of the Lyαtransmission−dark matter (DM) density distribution. We find that the more energy is injected into the proto-ICM atz= 3, the more the distribution at high DM density tilts toward higher Lyαtransmission. This effect has been confirmed in both low-resolution simulations adopting a preheating scheme based on entropy floors, as well as in higher-resolution simulations with another scheme based on energy floors. The evolution of the slope of this distribution is shown to vary with redshift. The methodology developed here can be applied to current and upcoming Lyαforest tomographic survey data to help constrain feedback models in galaxy protoclusters.

    

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4. Electron Injection via Modified Diffusive Shock Acceleration in High-Mach-number Collisionless Shocks

   https://doi.org/10.3847/2041-8213/ad0cf9  

   Grassi, A. ; Rinderknecht, H. G. ; Swadling, G. F. ; Higginson, D. P. ; Park, H.-S. ; Spitkovsky, A. ; Fiuza, F. ( November 2023 , The Astrophysical Journal Letters)

   The ability of collisionless shocks to efficiently accelerate nonthermal electrons via diffusive shock acceleration (DSA) is thought to require an injection mechanism capable of preaccelerating electrons to high enough energy where they can start crossing the shock front potential. We propose, and show via fully kinetic plasma simulations, that in high-Mach-number shocks electrons can be effectively injected by scattering in kinetic-scale magnetic turbulence produced near the shock transition by the ion Weibel, or current filamentation, instability. We describe this process as a modified DSA mechanism where initially thermal electrons experience the flow velocity gradient in the shock transition and are accelerated via a first-order Fermi process as they scatter back and forth. The electron energization rate, diffusion coefficient, and acceleration time obtained in the model are consistent with particle-in-cell simulations and with the results of recent laboratory experiments where nonthermal electron acceleration was observed. This injection model represents a natural extension of DSA and could account for electron injection in high-Mach-number astrophysical shocks, such as those associated with young supernova remnants and accretion shocks in galaxy clusters.

    

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5. Global Weak Solutions of a Hamiltonian Regularised Burgers Equation

   https://doi.org/10.1007/s10884-022-10171-0  

   Guelmame, Billel ; Junca, Stéphane ; Clamond, Didier ; Pego, Robert L. ( May 2022 , Journal of Dynamics and Differential Equations)

   A nondispersive, conservative regularisation of the inviscid Burgers equation is proposed and studied. Inspired by a related regularisation of the shallow water system recently introduced by Clamond and Dutykh, the new regularisation provides a family of Galilean-invariant interpolants between the inviscid Burgers equation and the Hunter-Saxton equation. It admits weakly singular regularised shocks and cusped traveling-wave weak solutions. The breakdown of local smooth solutions is demonstrated, and the existence of two types of global weak solutions, conserving or dissipating an H1 energy, is established. Dissipative solutions satisfy an Oleinik inequality like entropy solutions of the inviscid Burgers equation. As the regularisation scale parameter tends to zero or infinity, limits of dissipative solutions are shown to satisfy the inviscid Burgers or Hunter-Saxton equation respectively, forced by an unknown remaining term. 

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