Home  << 1 >> 
BenitezLlambay, P., Krapp, L., Ramos, X. S., & Kratter, K. M. (2023). RAM: Rapid Advection Algorithm on Arbitrary Meshes. Astron. J., 952(2), 106.
Abstract: The study of many astrophysical flows requires computational algorithms that can capture high Mach number flows, while resolving a large dynamic range in spatial and density scales. In this paper we present a novel method, RAM: Rapid Advection Algorithm on Arbitrary Meshes. RAM is a timeexplicit method to solve the advection equation in problems with large bulk velocity on arbitrary computational grids. In comparison with standard upwind algorithms, RAM enables advection with larger time steps and lower truncation errors. Our method is based on the operator splitting technique and conservative interpolation. Depending on the bulk velocity and resolution, RAM can decrease the numerical cost of hydrodynamics by more than one order of magnitude. To quantify the truncation errors and speedup with RAM, we perform one and twodimensional hydrodynamics tests. We find that the order of our method is given by the order of the conservative interpolation and that the effective speedup is in agreement with the relative increment in time step. RAM will be especially useful for numerical studies of disksatellite interaction, characterized by high bulk orbital velocities and nontrivial geometries. Our method dramatically lowers the computational cost of simulations that simultaneously resolve the global disk and potential well inside the Hill radius of the secondary companion.
Keywords: ORBITAL ADVECTION; MAGNETOHYDRODYNAMICS CODE; SCHEME; FLOWS; MHD; SIMULATIONS; FARGO; PPM

Krapp, L., GarridoDeutelmoser, J., BenítezLlambay, P., & Kratter, K. M. (2024). A Fast Secondorder Solver for Stiff Multifluid Dust and Gas Hydrodynamics. Astrophys. J. Suppl. Ser., 271(1), 7.
Abstract: We present MDIRK: a multifluid secondorder diagonally implicit RungeKutta method to study momentum transfer between gas and an arbitrary number (N) of dust species. The method integrates the equations of hydrodynamics with an implicitexplicit scheme and solves the stiff source term in the momentum equation with a diagonally implicit, asymptotically stable RungeKutta method (DIRK). In particular, DIRK admits a simple analytical solution that can be evaluated with O(N) operations, instead of standard matrix inversion, which is O(N)3 . Therefore, the analytical solution significantly reduces the computational cost of the multifluid method, making it suitable for studying the dynamics of systems with particlesize distributions. We demonstrate that the method conserves momentum to machine precision and converges to the correct equilibrium solution with constant external acceleration. To validate our numerical method we present a series of simple hydrodynamic tests, including damping of sound waves, dusty shocks, a multifluid dusty Jeans instability, and a steadystate gasdust drift calculation. The simplicity of MDIRK lays the groundwork to build fast highorder, asymptotically stable multifluid methods.

Lai, D., & Munoz, D. J. (2023). Circumbinary Accretion: From Binary Stars to Massive Binary Black Holes. Annu. Rev. Astron. Astrophys., 61, 517–560.
Abstract: We review recent works on the dynamics of circumbinary accretion, including time variability, angular momentum transfer between the disk and the binary, and the secular evolution of accreting binaries. These dynamics impact stellar binary formation/evolution, circumbinary planet formation/migration, and the evolution of (super)massive black hole binaries. We discuss the dynamics and evolution of inclined/warped circumbinary disks and connect with observations of protoplanetary disks. A special kind of circumbinary accretion involves binaries embedded in big disks, which may contribute to the mergers of stellarmass black holes in AGN disks. Highlights include the following:
Circumbinary accretion is highly variable, being modulated at Pb (the binary period) or similar to 5P(b), depending on the binary eccentricity e(b) and mass ratio q(b). The inner region of the circumbinary disk can develop coherent eccentric structure, which may modulate the accretion and affect the physical processes (e.g., planet migration) taking place in the disk. Over long timescales, circumbinary accretion steers binaries toward equal masses, and it does not always lead to binary orbital decay. The secular orbital evolution depends on the binary parameters (e(b) and q(b)) and on the thermodynamic properties of the accreting gas. A misaligned disk around a loweccentricity binary tends to evolve toward coplanarity due to viscous dissipation. But when e(b) is significant, the disk can evolve toward “polar alignment,” with the disk plane perpendicular to the binary plane. 
Siwek, M., Weinberger, R., Munoz, D. J., & Hernquist, L. (2023). Preferential accretion and circumbinary disc precession in eccentric binary systems. Mon. Not. Roy. Astron. Soc., 518(4), 5059–5071.
Abstract: We present a suite of highresolution hydrodynamic simulations of binaries immersed in circumbinary accretion discs (CBDs). For the first time, we investigate the preferential accretion rate as a function of both eccentricity e(b) and mass ratio q(b) in a densely sampled parameter space, finding that when compared with circular binaries, (i) mass ratios grow more efficiently in binaries on moderately eccentric orbits (0.0 less than or similar to e(b) less than or similar to 0.4), and (ii) high eccentricities (e(b) greater than or similar to 0.6) suppress mass ratio growth. We suggest that this nonmonotonic preferential accretion behaviour may produce an observable shift in the mass ratio distributions of stellar binaries and massive black hole binaries. We further find that the response of a CBD can be divided into three regimes, depending on eccentricity and mass ratio: (i) CBDs around circular binaries always precess freely, whereas CBDs around eccentric binaries either (ii) undergo forced precession or (iii) remain locked at an angle with respect to the binary periapsis. Forced precession in eccentric binaries is associated with strong modulation of individual accretion rates on the precession timescale, a potentially observable signature in accreting binaries with short orbital periods. We provide CBD locking angles and precession rates as a function of e(b) and q(b) for our simulation suite.
Keywords: hydrodynamics; accretion discs; binaries; accretion; transients
