Hi Zach,
AC stands for the method of artificial compressibility. Instead of
relying on a Poisson based projection, the system is driven towards a
divergence free state by introducing artificial pressure waves through
the continuity equation. The formulation preserves the hyperbolic nature
of the system, but destroys the time accuracy, which is then recovered
with dual time stepping. For the ac formulation you can refer to
http://www.sciencedirect.com/science/article/pii/S0021999116001686
The artificial compressibility factor ac-zeta is the coefficient of the
fluxes in the continuity equation. This results in characteristics
V + c, V, V - c,
where c = sqrt(V^2 + ac-zeta) is the pseudo speed of sound. Thus, in the
current implementation ac-zeta is the free parameter that is used to
downscale the speed of the pseudo-waves to globally reduce the pseudo
system stiffness. The parameter is something that one can experiment
with, typical values varying from 1.25 - 10 times the freestream
velocity. Currently, I am looking into making ac-zeta and pseudo-dt
spatially and temporally varying.
The source terms specify a sponge region near the domain edges (|y|>5,
x<-5, x>25) to damp the initial pressure wave that is generated when the
simulation is started from scratch. Please note that the sponge turns
off at t=5 because of the (1 - tanh(1.5*(t - 5.0)))*0.5 coefficient. You
can see how the sponge works if you write the solution files before t=5.
The plugin [soln-plugin-pseudostats] is used to output the residual of
the pseudo time problem to monitor the divergence. The
[soln-plugin-residual] on the other hand computes the "residual" of two
consecutive real time steps. The [soln-plugin-fluidforce] plugin can be
used with the ac systems.
Coarsening the mesh and increasing the order is something that would be
beneficial, especially when using a polynomial multigrid for
accelerating the pseudo time problem. P-multigrid should be added in the
next release.
Thanks,
Niki
On 14/03/17 23:37, Zach Davis wrote:
Tuesday, 14 March 2017
Peter & Freddie,
I believe the mesh export issue from Pointwise using the PyFR exporter
has been resolved in PyFR 1.6. I still seem to have issues running
using the OpenCL backend which persistently complains about an invalid
workgroup size. It use to work at one point, but something has
changed in the intervening releases which is causing problems for me
at least. I’ve tried adjusting the values per Freddie’s guidance to
no avail. He also suggested a tool that might be helpful in
determining the appropriate workgroup size needed for my card.
Unfortunately that tool seems to be NVIDIA card specific requiring
installation of NVIDIA software that won’t run on my machine. I’m
using an AMD card instead, and the tool won’t compile due to missing
dependencies. It’s not a pressing matter, but just something that I
thought you both might want to be aware of.
Nikki,
I’ve looked over your incompressible 2d-cylinder case, and I was
wondering if you could elaborate a bit, or point me to some reference,
about how you came up with the source terms you’re using in the input
file. It also appears that the [soln-plugin-pseudostats] is used in
place of the [soln-plugin-residual] namelist for incompressible
cases—is that right? Does the [soln-plugin-fluidforce] namelist still
work for the ac-navier-stokes solver? Another question if you don’t
mind—what is this artificial compressibility factor, ac-zeta and why
is value of 6.0 used? Oh, and what does the ac prefix stand for? Thanks!
I think it would be interesting to see how coarse of a higher-order
mesh could be made for this case while increasing the polynomial
solution basis such that you essentially recover the linear mesh
spacing in each element, and see if you could capture one or more
vortices within a single element with any noticeable diffusion over time.
Best Regards,
Pointwise, Inc.
Zach Davis
Pointwise®, Inc.
Sr. Engineer, Sales & Marketing
213 South Jennings Avenue
Fort Worth, TX 76104-1107
*E*: [email protected] <mailto:[email protected]>
*P*: (817) 377-2807 x1202
*F*: (817) 377-2799
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Niki Andreas Loppi MSc
Postgraduate Researcher
Department of Aeronautics
Imperial College London
South Kensington
London
SW7 2AZ
UK
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