PI: Loren E. Held

Project dp377, which started in April of 2025, is devoted to exploring the (magneto)-hydrodynamics of distorted accretion disks. Simple models of accretion disks (i.e. disks of gas or dust around young stars or compact objects such as black holes) often treat the disk as being flat (planar). However, many disks in nature are found to be warped. This could be due to a misalignment in the system, such as a circumstellar disk around a tilted spinning black hole, a circumbinary disk in which the disk plane is tilted with respect to the binary orbital plane, or a circumstellar disk with an external binary (or planetary) companion on a tilted orbit. Warped disks exhibit various dynamics and processes not found in their planar counterparts such as precession, vertical oscillations (bouncing), and a hydrodynamic instability (known as the parametric instability). A particularly interesting process in strongly warped discs which has received a great deal of attention over the last decade is that of disc breaking, in which the disc is unable to support itself and breaks into distinct rings. In this project we carried out hydrodynamic simulations of disk breaking using local (shearing box) simulations of the disk, which enabled us to reach much higher resolutions than are typically accessible in global simulations. These simulations have helped inform ongoing work on the dynamics of distorted disks in the presence of magnetic fields. This work was published in MNRAS (Held & Ogilvie (2026): Disk breaking and parametric instability in warped accretion disks). The simulations were run on the CSD3 cluster in Cambridge.

We presented the first local simulations of disk breaking/tearing in a warped accretion disc. In this project we isolated the mechanism of disk breaking in high-resolution, quasi-2D, local (shearing box), hydrodynamic simulations of a Keplerian disc. We considered the evolution of a free (unforced) warp in the wavelike (alpha < H/r) regime. At large warp amplitudes (psi > 1) the disk breaks into four rings on time-scales of around 20 orbits which are separated by gaps of around 10 scale-heights. The warp exhibits a rich tapestry of small-scale dynamics, including horizontal sloshing motions, vertical oscillations or bouncing, warp steepening, and shocks. The shocks act as a source of enhanced dissipation which facilitates gap opening and thus disk breaking. At smaller warp amplitudes (psi < 1), for which we also developed a quasi-linear theory, the disk does not break, but instead exhibits hydrodynamic parametric instability. We also investigated the effect of explicit viscosity (alpha ~ 0.03)): at small warp amplitudes the parametric instability is damped and the warp propagates as a pure bending wave, while at large warp amplitudes the emerging gaps are partially filled by viscous diffusion.