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Astrophysical Fluid Dynamics and MHD

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Fundamental SPH

Star Formation

Planet Formation

Turbulence

The Astrophysical Fluid Dynamics group studies the processes involved in star and planet formation. We run global simulations of these systems, solving the equations of hydrodynamics during gravitational collapse to study how these objects are formed. The primary numerical method we utilise is Smoothed  Particle  Hydrodynamics  (SPH), originally developed by Joe Monaghan.

We are always on the lookout for interested and motivated students to join us in pursuing research towards a PhD or as part of their honours year (contact either Daniel Price or Joe Monaghan if you are interested). The group often hosts long term visitors interested in applying SPH to their own problems  in  wide range of  research fields, and we have access to a range of national, state-level and university-level supercomputing facilities that we utilise regularly.

Members


Fundamental SPH

Researchers: Daniel Price, Joe Monaghan, Alireza Valizadeh, Terrence Tricco

Our group is one of the leading centres in the world developing the SPH method, a numerical method for simulating hydrodynamic fluid flow which is widely used in astrophysics. We have pioneered methods for simulating magnetic field and dust physics with SPH, but are also considered with other fundamentals  such  as  interpolation  methods. We have performed comparison studies of SPH and grid based methods, which highlight the differing strengths or agreements between these methods.


Star Formation

Researchers: Daniel Price, Christoph Federrath, James Wurster, Terrence Tricco

The formation of stars is one of the most fundamental processes in the universe without which galaxies and indeed, ourselves would not exist. However our theoretical understanding of star formation is relatively poor, primarily because of the difficulty in modelling the physical process which involves  gravity,  highly  turbulent  gas  dynamics  (at  supersonic velocities), magnetic fields, radiation, molecular and dust chemistry. Star formation also involves a truly enormous range in length and time scales which makes simulating the process difficult even with the fastest computers. One of the key aspects  of  our  work  here  at  Monash  is to understand the role of magnetic fields in the star formation process, using simulations that form whole star clusters rather than just isolated stars, which gives a much deeper, statistical, picture of star formation that can be compared to real molecular clouds. Much  of  this  work  involves  developing  accurate  numerical algorithms for simulating self-gravitating radiation-magnetohydrodynamics over the wide range of length and time scales involved in star formation (which we achieve using the Smoothed Particle Hydrodynamics method). The simulation methods thus developed  are  readily  applicable  to  other  areas  of astronomy (and also to many earth-bound problems for which SPH is increasingly being used), amongst which we have or are working on application to the merger of binary neutron star systems; to the nature and statistics of turbulence in molecular clouds; to the  role  of  magnetic  fields  in  galaxies  and to warp propagation in accretion discs.


Snapshot from a calculation of star cluster formation (see the movies)

Calculation of warp propagation in an astrophysical accretion disc.

Snapshot of stars forming along filamentary lines in Mach 10 supersonic turbulence

Planet Formation

Researchers: Daniel Price

We study the many steps involved in forming terrestrial and gas giant planets. These processes begin with micron-sized dust grains that originate in the interstellar medium, and evolve in the gas and dust discs that form about young stars. This evolution involves the collisional growth of the dust grains,  and  their  concentration  or  redistribution  through  gas and dust two-fluid instabilities. Once such grains agglomerate to reach metre scales, we must consider the impact of gas head winds upon their motion, and at kilometre scales we must consider the dynamics of these larger bodies, known as planetesimals,  as  they  interact  with  one  another.  It is believed that terrestrial planets, and gas giant cores, form through the coalescence of these large icy and rocky planetesimals. For gas giants there is a subsequent phase of gas accretion during which they accrete a vast majority of their final mass.

We develop SPH algorithms with which to simulate planet formation. Using complex numerical simulations, we treat many of the physical processes mentioned above, and are striving to couple the various processes together to give a more complete picture of planet formation. With these we must include the  relevant  physics,  such  as  the  transport  of energy by radiation, the gravitational pull of the gas and dust media upon themselves, known as self-gravity, and the interactions of the solid and gaseous states of matter. Smoothed Particle Hydrodynamics (SPH) is an ideal tool for tackling these problems,  allowing  us  to  simultaneously  model  regions  of material with huge contrasts in density and temperature, with a natural approach to modelling self-gravity, and the ability to readily trace the flow of matter. In concert with these numerical simulations, we also produce analytical models that are simpler,  but  can  be  used  to  pin  down the essential physics involved in the different processes.


Compi sort

Density figure

Turbulence

Researchers: Daniel Price, Joe Monaghan, Christoph Federrath, Alireza Valizadeh, Terrence Tricco

Leonardo da Vinci was one of the first to be fascinated by turbulent fluid flows (see his drawings of water flowing into a tank and creating vortices). Since then generations of scientists have tried to explain and to model turbulence. The most successful phenomenological model for incompressible turbulence  was  presented  by  Kolmogorov  in  1941.  However, turbulence in astrophysical systems such as the interstellar medium or primordial gas halos in the early Universe is highly compressible and characterized by supersonic random motions. To gain insight into this multi-scale, highly non-linear regime of compressible  turbulence,  we  use  large-scale  computer  simulations  here at MoCA. Such simulations require massively parallelized supercomputers with thousands of processors working together in parallel to solve the gas and plasma equations governing turbulent gas flows. To enable simulations of highly compressible  turbulence,  learn  about  its  statistics  and  role for astrophysical systems, we develop efficient parallel codes and state-of-the-art supercomputing techniques here at MoCA.

One of the most important applications of our simulations is to study the role of turbulence for the formation of stars in magnetized gas clouds. We think that the role of turbulence is threefold. First, turbulence carries sufficient energy to stabilize molecular clouds against global collapse, which  would  be  a  quick  and  fatal  end for such clouds, thankfully not observed. Secondly, interstellar turbulence is supersonically fast, producing local gas compressions in filaments, which are the seeds for stellar birth. Finally, turbulence is the only process capable of amplifying a magnetic field exponentially  fast.  This  process  is  called  'turbulent  dynamo', and indeed it acts like a dynamo, efficiently converting turbulent energy into magnetic energy on extremely short timescales. Thus even the first stars that formed in our Universe probably did so in a highly magnetized plasma, a possibility often ignored.  Here  at  MoCA  we  aim  to  understand fundamental properties of turbulent dynamo amplification and its role for star formation in the present-day and early Universe.


Drawing by Leonardo Da Vinci of pouring water creating turbulent flow

Snapshot from a calculation of supersonic turbulence in star forming molecular clouds

Comparison of turbulence driven by solenoidal and compressive forcings