[Nek5000-users] Perturbation Mode

[nek5000-users at lists.mcs.anl.gov]

Hi Neks,

We have been running the perturbation mode in order to study the linear
evolution of disturbances in a couple of 2D boundary layer cases and are
now setting up a 3D boundary layer case.

Already for the 2D cases we experienced that the timestep needs to be
decreased significantly compared to a corresponding nonlinear simulation
in order to yield a numerically stable simulation. For the nonlinear
simulations we put the disturbance with a small amplitude on top of the
baseflow. Hence, running the perturbation mode for the 2D cases was more
expensive than running a nonlinear simulation but still reasonable.

However, the 3D case seems to become much more expensive. We first ran a
nonlinear simulation as described above and compared the disturbance
evolution to results of the parabolised stability equations which
matched perfectly.
Running then the perturbation mode for the same disturbance with the
same base flow did not yield a stable simulation even when the timestep
was reduced (divided by 5). The disturbance amplitude exploded very
early. We then took a higher box and increased the resolution which made
things a little better meaning that the simulation exploded much later.
However, it is still not stable although the resolution is higher and
the timestep is much smaller compared to the corresponding nonlinear
simulation which worked fine. Also, the number of pressure iterations is
around 20-30 times higher.

Is there any reason why the linear stability equations in nekton should
behave so much different than the full Navier-Stokes? Are there some
parameters (filtering etc.) that have to be used differently in this case?

Best regards,

David

-- 
David Tempelmann                                                             
Linné Flow Center, Mechanics KTH 
SE-100 44, Stockholm, Sweden 
Phone: +46 8 7907161
E-mail: david at mech.kth.se