
Meeting program
Agenda topics
Topics are given in the agenda file: agenda.txt
Talk abstracts
 Axel Brandenburg (Nordita, Stockholm):
Detection of negative effective magnetic pressure instability in
turbulence simulations
We present the first numerical demonstration of the negative effective
magnetic pressure instability in direct numerical simulations of
stablystratified, externallyforced, isothermal hydromagnetic
turbulence. By the action of this instability, initially uniform
horizontal magnetic field forms flux concentrations whose scale is large
compared to the turbulent scale. We further show that the magnetic
energy of these largescale structures is only weakly dependent on the
magnetic Reynolds number. Our results supports earlier meanfield
calculations and analytic work which identified this instability.
Applications to the formation of active regions in the Sun are
discussed.
[PDF of the talk]
 Chikwan Chan (Nordita, Stockolm):
Making the Pencil Code Run on GPUs: The Rolling Cache Algorithm
Abstract: The architecture of GPUs is significantly different from
CPUs. The pencil code needs to be rewritten from a very fundamental
level in order to take advantage of these massive parallel processors.
In this talk, I will report the status of rewriting the pencil code
from scratch using CUDA C. I will give a basic introduction to GPU
programming and describe the (very important) rolling cache algorithm.
I will then show some benchmark of the new code.
[PDF of the talk]
 Michiel Lambrechts (Lund Observatory,
Lund): Pebble Interaction with Gas Giant Cores
Abstract: The core accretion scenario is the most successful theoretical model
for gasgiant formation. However, the initial growth of the core
depends on arbitrary assumptions on planetesimal sizes. Growing the
solid core with largerthankmsized planetesimals, before the gas in
the protoplanetary disc is accreted or photoevaporated away, is
problematic due to the long timescale for runaway accretion,
especially in the outer distant regions of the disc.
We have studied the dynamics of gascoupled cmsized pebbles,
gravitationally interacting with larger than kmsized embryo cores.
The presence of these pebbles in the gaseous disc influences the
growth rate of the gasgiant core. Under favourable conditions, i.e.
unity midplane dusttogas ratio and particle growth to mm and cm
sizes, pebble accretion turns out to be significantly faster than
runaway accretion of planetesimals. The Pencil Code is used to
correctly model the gas drag hydrodynamics.
[PDF of the talk]
 Karsten Dittrich (MPIA, Heidelberg): Gravoturbulent Planetesimal Formation with P3M in the Pencil Code
Abstract: A very attractive way to form the kilometer sized planetesimals
in the early solar system is the 'Gravoturbulent Fragmentation' of a
cloud of relatively small icy and dusty objects. Using the Pencil
Code, Anders Johansen et al. (see papers 2007 (Nature) and 2010
(A&A)) did not reach convergence in the size of the planetesimals.
In trying to overcome this resolution problem, we intend to implement
another selfgravity solution into the Pencil Code. It should solve the
shortrange forces in a direct particleparticle calculation.
[PDF of the talk, movie1.mpg and movie2.avi]
 Julien Morin (Institut für Astrophysik,
University of Göttingen): Starinabox simulations of
fullyconvective stars
Abstract: Important observational progress has been made on fully
convective stars  either main sequence M dwarfs or young T Tauri
stars  during the past decade. Further theoretical work is
required to model these observational results, in particular
differential rotation and dynamo action. I will present starinabox
numerical simulations of fullyconvective stars carried out with
the Pencil Code. The setup and the main differences with the work
by Dobler, Stix & Brandenburg (2006) are first detailed. Then the
first results obtained on differential rotation and dynamo action
will be described.
[PDF of the talk]
 Alexander Hubbard (MPIA, Heidelberg): An investigation into turbulence induced dust collisional velocities
Abstract: Turbulence induced dust collisional velocities are a key piece to
understanding dust grain growth in protoplanetary disks. I have inserted
inertial dust grains into an artificial cascade, and found an interesting
collisional velocity probability distribution.
[PDF of the talk]
 Boris Dintrans (IRAP, Toulouse):
Thermal excitation of sound in stars
Abstract: Pulsations are usually excited in stars through
two main mechanisms: (i) the kappamechanism based on opacity bumps;
(ii) the smallscale fluctuations induced by turbulent convection.
I will present a new original excitation mechanism based on the
interplay between gravity and thermal diffusion at the star surface.
The associated instability strips have been computed by the means
of a linear stability analysis and growning rates as well as the
longterm nonlinear saturation have been confirmed by 1D and 2D
direct simulations done with the Pencil Code.
[PDF of the talk]
 Chunlin Tian (University of Helsinki): Numerical simulations of turbulent magnetoconvection in the Star
Abstract: Based on recently developed gas kinetic BGKMHD scheme,
the interaction between vertical magnetic fields and stellartype
of turbulent convection is parametrically studied. Besides reproduced
numerically three types of magnetoconvection: ie., intermittent
field, flux separation and small scale convection, which have been
reported by the other authors, we find the intermediate cases between
flux separation and small scale convection are very interesting and
can be used to interpret the umbral dots.
[PDF of the talk]
 Wladimir Lyra (American Museum of Natural
History, NewYork): The gasrich phase: dynamics of the turbulent
Solar Nebula
Abstract: During the first million years of evolution, the forming
solar system was embedded in a dense diskshaped cloud of gas, the
Solar Nebula. Due to the presence of magnetization, the disk is
susceptible to the magnetorotational instability (MRI), which
brings about turbulence and enables accretion of the gas onto the
protosun. Turbulence also has a positive effect on the small solids,
concentrating them into transient pressure maxima for long enough
to achieve gravitational collapse into kmsized bodies. Vortices
may exist in the resistive dead zone, which are even more prone to
collecting solid material, producing the first cores of giant
planets. The architecture of the planetary system formed comes about
through migration in the gas disk, Nbody interactions, and random
walks owing to torques from the turbulent gas, scattering small
bodies in their wake. In this talk, I will review the current state
of the art on the dynamics of small bodies in the Solar Nebula,
including yet unpublished work.
[PDF of the talk]
 Colin McNally (American Museum of Natural
History, NewYork): Getting Pedantic about KelvinHelmholtz
Instability
Abstract: We used the Pencil Code to generate a verifiable reference
solution to a well posed KelvinHelmholtz test problem. As the flow is
smooth and subsonic, the Pencil Code is well suited to producing a high
accuracy result. We preformed a convergence study from 128^2 to 4096^2,
while configuring Pencil Code to so that outputs were made at exact time
intervals and so that the grids at successive resolutions overlap on
every second point. This allows us to compute a Richardson extrapolation
based Grid Convergence Index, and hence rigorously state an uncertainty
on the highest resolution result. This result lets us demonstrate the
convergence behaviour of other codes to the correct solution to the test
problem.
[PDF of the talk]
 Nils Erland L. Haugen (SINTEF Energy Research,
Trondheim): Development of an incompressible module for the
Pencil Code
Abstract: current state of the incompressible module.
[PDF of the talk]
