Mass Transport, Turbulent Mixing, and Inflow in Black Hole Accretion

Abstract We investigate mass transport, mixing, and disk evolution in nonradiative black hole accretion flows using Lagrangian tracer particles embedded in general-relativistic magnetohydrodynamics simulations. Our simulation suite spans magnetically arrested disk (MAD) and standard and normal evolution (SANE) states across a range of black hole spins. By tracking tracer trajectories, we directly measure both advective inflow and stochastic spreading of fluid elements. The tracer distributions are well described by a combination of coherent inward drift and Gaussian-like broadening, consistent with an advection-diffusion picture. MADs exhibit systematically faster inflow than SANEs, with retrograde flows showing the most rapid infall; the innermost stable circular orbit leaves little imprint in MADs but remains more visible in SANEs. Turbulent fluctuations drive strong radial dispersion in all cases, with a superdiffusive scaling of σ ∝ t 0.95 in MADs and σ ∝ t 0.75 in SANEs for high-spin prograde disks. Mixing times decrease toward the event horizon and are consistently shorter in MADs and retrograde configurations. Tracers also reveal how accretion sources shift over time: Turbulence draws inflow from a broad range of initial radii, with rapid torus depletion in MADs driving the mean source radius outward as r cyl ∝ t 2/3 , while SANEs evolve more gradually with r cyl ∝ t 1/2 . We show that the finite mass of the initial torus has a strong influence on late-time behavior, especially in MADs, where imprints of differently sized initial conditions may be accessible as early as t ≈ 10 4 GM / c 3 .