Bulletin of the American Physical Society
APS April Meeting 2021
Volume 66, Number 5
Saturday–Tuesday, April 17–20, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S08: Numerical Relativity: Algorithms and Code DevelopmentLive

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Sponsoring Units: DCOMP Chair: Joshua Dolence, LANL 
Monday, April 19, 2021 1:30PM  1:42PM Live 
S08.00001: Spritz code: Improving the robustness of GR Magnetohydrodynamics Jay Vijay Kalinani, Riccardo Ciolfi, Wolfgang Kastaun, Federico Cipolletta, Bruno Giacomazzo General relativistic magnetohydrodynamic (GRMHD) simulations are an indispensable tool to obtain a detailed physical understanding of binary neutron star (BNS) mergers. Fluxpreserving GRMHD codes numerically evolve a set of conservative equations based on conserved variables which then need to be converted back to the primitive physical variables. The corresponding conservativetoprimitive (c2p) recovery procedure is a crucial aspect at the core of any GRMHD code. In this talk, I will discuss a completely new c2p recovery scheme for ideal MHD. Using both standalone tests and by performing demanding threedimensional GRMHD tests, we have demonstrated robustness, accuracy, and efficiency over the whole relevant parameter space. This includes the critical case of very strong magnetizations. I will also talk about technical aspects of implementing the scheme in our new GRMHD code Spritz, such as the requirements for the EOS framework. [Preview Abstract] 
Monday, April 19, 2021 1:42PM  1:54PM Live 
S08.00002: Spritz: General Relativistic Magnetohydrodynamics with Neutrinos Federico Cipolletta I will present our new GRMHD code aimed at the study of compact binary mergers with finitetemperature equations of state and neutrino physics that is already available for public download. Numerical modeling of NSNS and NSBH binary mergers requires a fully general relativistic treatment taking into account accurate magnetic field's evolution and microphysics effects, to extract the most complete physical information from gravitational waves and electromagnetic signals observed. I will summarize the main features of our code, namely: the evolution of a staggered vector potential that automatically satisfies the magnetic field's divergencefree condition; the general treatment for the NS Equation Of State allowing for the use of either analytical or tabulated one; a neutrino leakage scheme that provides a useful tool for the study of the postmerger phase. I will present the tests that we performed, including TOV taking into account electron fraction evolution, temperature effects, neutrino leakage, and magnetic field. I will also show preliminary results obtained by the Spritz code in simulating BNS mergers with tabulated EOS within the collaboration funded by the NASA TCAN 80NSSC18K1488 grant. [Preview Abstract] 
Monday, April 19, 2021 1:54PM  2:06PM Live 
S08.00003: Evolving neutron stars with the Nmesh code Wolfgang Tichy, Ananya Adhikari, Liwei Ji We present an overview of the new Nmesh code, which is intended to efficiently run on large supercomputers to solve challenging relativistic astrophysics problems such as binary neutron star or black hole mergers. The principal spatial discretization used in Nmesh is based on a discontinuous Galerkin (DG) method. We have implemented the evolution equations for general relativistic hydrodynamics, as well as the evolution equations for gravity using the generalized harmonic formulation. We present first results from tests with neutron stars. In these tests we evolve stationary and boosted stars both in Cowling approximation and in full General Relativity. We discuss which methods we use to achieve stable evolutions. We also present a hybrid scheme where we use a finite volume method in part of the numerical domain, e.g. around the star surface. [Preview Abstract] 
Monday, April 19, 2021 2:06PM  2:18PM Live 
S08.00004: Scalar wave and shock propagation tests with the Nmesh code Ananya Adhikari, Wolfgang Tichy We present several tests that we have performed with the new Nmesh code. They include the propagation of simple scalar waves, as well as blast wave tests in relativistic hydrodynamics. We apply an exponential filter to the propagation of the scalar wave when using the discontinuous Galerkin (DG) method. The aim was to perform a preliminary test for stable wave propagation using DG method, with the help of the filter. For the hydrodynamics tests, we propagate a relativistic blast wave using both DG method and finite volume (FV) methods. In the DG case we apply two types of limiters (the minmodB limiter and the MRS limiter) to the propagation and study the efficiency of these limiters on the propagation of the blast wave. In the FV case we use a WENO scheme. We present and compare results from both DG and FV cases. The aim of these tests is to perform stable and accurate propagation of shock waves in relativistic hydrodynamics. [Preview Abstract] 
Monday, April 19, 2021 2:18PM  2:30PM Live 
S08.00005: Toward Moving Puncture Simulations with the GHG System Liwei Ji, Ananya Adhikari, Wolfgang Tichy We present a new conformal rescaling of the generalized harmonic system. With this rescaling it should be possible to evolve black holes without excising the black hole interior, by using a puncture method similar to the one used for standard moving puncture simulations with the BSSNOK system. The rescaled system is first order in both time and space, and thus amenable for use with a discontinuous Galerkin method. We present first results for single black hole evolutions using the Nmesh code. [Preview Abstract] 
Monday, April 19, 2021 2:30PM  2:42PM Live 
S08.00006: Testing the Accuracy of Gravitational Waveforms Computed with SpECTRE Sierra Thomas Nextgeneration gravitationalwave detectors will require modelled gravitational waveforms with substantially higher accuracy than current numerical or analytic models provide. The Simulating eXtreme Spacetimes collaboration is developing an opensource numericalrelativity code, called SpECTRE, that uses the Discontinuous Galerkin method and taskbased parallelism to achieve greater speed and accuracy by scaling to many more compute cores. In this talk, I will discuss simulations for a perturbed, ringing black hole in SpECTRE. The simulations run a Cauchy evolution of perturbedblackhole initial data in parallel with a Cauchy characteristic evolution that calculates the emitted gravitational waves at null infinity. I will present results assessing the computational efficiency of the simulations across several nodes and the accuracy of the simulated gravitational waveforms. [Preview Abstract] 
Monday, April 19, 2021 2:42PM  2:54PM Live 
S08.00007: SpECTRE's C++ tensor expression interface Alexandra Macedo, Nils Deppe, Geoffrey Lovelace SpECTRE is an opensource code for multiscale, multiphysics problems in astrophysics and gravitational physics. To tackle these problems, SpECTRE implements a tensor class in C++ that tracks the dimensionality and valence of each index, as well as whether the index is spatial or spacetime. Unfortunately, all tensor operations are currently written as for loops that perform elementwise operations. Not only is the same loop structure needed repeatedly throughout the code, but writing these loops is tedious and error prone. These issues are addressed by a newly developed tensor expression interface. This interface enables writing and evaluating tensor expressions with C++ syntax that closely resembles tensor index notation found in physics equations. Ultimately, the tensor expressions in SpECTRE make implementing new equations easier and less error prone, and make the code significantly easier to read. SpECTRE's tensor expressions implement addition, subtraction, contraction, and products. In this talk, I will discuss implementation details, outline what makes this interface different from existing tensor libraries and useful for physics, and present example tensor expressions using this interface in SpECTRE. [Preview Abstract] 
Monday, April 19, 2021 2:54PM  3:06PM Live 
S08.00008: New, userfriendly codes to study critical collapse Leonardo Werneck In this talk we present two welldocumented, opensourced, and userfriendly codes to study critical collapse in general relativity. Our goal was to produce a simple infrastructure that new users, particularly students, could quickly learn and use. The first of them is {\sc SFcollapse1D}, a small code written in {\tt C++} to study collapse problems in 1+1 dimensions, using sphericallike coordinates. It uses the ADM formalism to numerically solve the EinsteinKleinGordon (EKG) equations and does not require adaptive mesh refinement algorithms, instead adopting a nonuniform radial sampling. The second code, {\sc NRPy+Collapse}, is a collection of {\sc Jupyter} notebooks which implement the EKG equations using the BSSN formalism in full 3+1 dimensions and in a variety of singular curvilinear coordinate systems. {\sc NRPy+} uses {\sc SymPy} to generate highly optimized {\tt C} code from {\sc Python} expressions written in Einsteinlike notation. The notebooks allow the user to learn by example while providing high quality \LaTeX{} documentation, directly linking to the relevant papers on the arXiv. Both of these codes have been designed with efficiency in mind, allowing the study of many interesting problems in critical collapse using consumergrade desktop computers. [Preview Abstract] 
Monday, April 19, 2021 3:06PM  3:18PM Live 
S08.00009: Scalar fields on fluctuating hyperbolic geometries Muhammad Asaduzzaman, Simon Catterall We present results on the behavior of the boundaryboundary correlation function of scalar fields propagating on discrete 2D random triangulations with the topology of a disk. We use a gravitational action that includes a curvature squared operator which favors a regular tessellation of hyperbolic space for large values of its coupling. An ensemble of such geometries is generated for different values of the coupling using Monte Carlo simulation. We show that the conformal behavior expected for a uniform hyperbolic space survives as this coupling is decreased implying that holographic predictions survive at least weak quantum gravity corrections. We investigated the dependency of the scaling exponent of the correlators on the mass as we vary the coupling of the curvaturesquaredoperator. We discuss extensions of the model to allow for the inclusion of matter field interactions and backreaction on the geometry. [Preview Abstract] 
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