I am a PhD student studying astrophysical fluid dynamics at the University of Cambridge, in the Department of Applied Mathematics and Theoretical Physics. I work with Dr. Henrik Latter and Professor Gordon Ogilvie to explore the dynamics of astrophysical accretion disks.

My research

Accretion disks are both ubiquitous in astrophysics, and essential to
many of the phenomenon observed in our universe. Appearing around young
stars, compact objects like black holes, and even Saturn, astrophysical
disks provide a host of interesting problems for theoreticians to consider.
In my PhD I have focused in particular on the magnetohydrodynamic aspects
of accretion disk oscillations and
instabilities.

`High-frequency quasi-periodic oscillations' (HFQPOs) observed in the
emission from black hole binary systems present an important but unresolved
problem in astrophysics. One explanation involves the amplification of
`diskoseismic' trapped inertial waves, confined to the inner
regions of a black hole accretion disk by relativistic effects.

Color-plot showing the pressure perturbation (in radius and height
above/below the disk) due to a trapped inertial wave in the
inner regions of an accretion disk around a Kerr black hole.

The un-magnetized, hydrodynamic theory of r-modes fits well with observations.
However, the waves' properties are altered by magnetic fields, which provide
restoring forces through magnetic tension and pressure. In pursuing my PhD, I have
examined the effects of magnetic fields with different geometries
on trapped inertial waves. While the tension force from a
vertically aligned magnetic field can modify the r-mode trapping cavity, I have found that a
`toroidal' magnetic field aligned with the circulation of the flow reduces
this effect by altering the acoustic properties of the oscillations.

An exagerated illustration of a hydromagnetic r-mode's distortion of a
vertically aligned magnetic field.

Trapped inertial waves grow in amplitude when they interact with large-scale deformations
(warps and eccentricities) in the accretion disc. I have been running non-linear, magnetohydrodynamic
simulations to explore this excitation mechanism in relativistic disks with eccentric (non-circular) streamlines.

This video of azimuthally averaged mass flux shows the excitation of a
trapped inertial wave via coupling with accretion disk eccentricity.

Wave excitation in eccentric discs

I am also interested in the evolution of eccentric disks with non-circular
streamlines in more general (non-relativistic) contexts. One of my current projects is related to the excitation of
oscillations on both large and small scales in eccentric, purely Newtonian disks.

This video shows vertical averages of the fluid variables in a simulation of
a 3D eccentric disk. The flow precesses due to pressure effects, while the non-circular
streamlines drive the growth of small-scale inertial oscillations and subsonic turbulence.

Magnetorotational instability

The magnetorotational instability (MRI) provides a widely accepted
explanation for turbulent accretion in astrophysical disks. Although studied
extensively in local simulations, numerical capabilities
have only recently permitted explorations of the dynamics of MRI turbulence
on a truly global scale. I am interested in how the MRI might drive, damp or
otherwise interact with other waves and instabilities in astrophysical accretion flows.

A slice showing the radial magnetic field perturbation associated
with a large-scale MRI mode in the linear stage of growth.

Publications

Dewberry J, Latter H, Ogilvie G, Fromang S, 2019. HFQPOs and discoseismic
mode excitation in eccentric, relativistic discs. I. Hydrodynamical simulations.
MNRAS, submitted.