Janosz Dewberry

PhD candidate
University of Cambridge Department of Applied Maths and Theoretical Physics


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.

Learn more about my research here:

Diskoseismic oscillations

`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.

Contour-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 precisely quantified the effects of magnetic fields with different geometries on trapped inertial waves. In particular, I have found that while the tension force from a vertically aligned magnetic field can modify the r-mode trapping cavity, 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.

More recently, I have been taking a numerical approach to this problem, running non-linear, magnetohydrodynamic simulations to explore the growth of oscillations excited by non-linear coupling with highly eccentric streamlines in relativistic accretion disks.

The color-plots of azimuthally averaged mass flux in this video illustrate the growth of an r-mode due to a non-linear coupling with eccentric streamlines in a relativistic accretion disk simulation.

Parametric instability

To complement my work on relativistic r-mode excitation, I have also been exploring the development of a parametric instability in purely Newtonian disks. In this instability, small-scale inertial waves feed off of the free energy provided by eccentricity in the flow. The non-linear interaction results in turbulence which likely regulates the growth of eccentric disk deformations in many contexts, and which gives rise to complex phenomena on both large and small scales.

This video shows mid-plane density in a simulation of an eccentric accretion disk beset by the parametric instability. The non-linear eccentric mode precesses due to pressure effects, while driving the growth of small scale inertial oscillations.

Magnetorotational instability

The magnetorotational instability (MRI) provides a widely accepted explanation for turbulent accretion in astrophysical disks. However, although studied extensively in local simulations, numerical capabilities have only recently allowed for the dynamics of MRI turbulence to be explored on a global scale. I am interested in how the MRI might drive, damp or otherwise interact with other waves and instabilities theorized to occur on large scales 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.



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    DAMTP, Wilberforce Rd
    Cambridge, CB3 0WA, UK