Helmholtz equation error

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Gary R
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Helmholtz equation error

Post by Gary R »

I am having trouble getting my acoustic waveguide simulation to run. Attached is the cif file and a .unv file if anyone wants to load things from scratch (Just run the .unv file through ElmerGrid 8 2 to get an Elmer mesh file). I keep getting the error shown below and don't know how to fix it. There also seems to be an error in the time stepping setup. I suspect that I have messed up my Driver BC.

Code: Select all

ELMER SOLVER (v 8.4) STARTED AT: 2021/03/22 20:33:48
ParCommInit:  Initialize #PEs:            1
MAIN: OMP_NUM_THREADS not set. Using only 1 thread per task.
MAIN: 
MAIN: =============================================================
MAIN: ElmerSolver finite element software, Welcome!
MAIN: This program is free software licensed under (L)GPL
MAIN: Copyright 1st April 1995 - , CSC - IT Center for Science Ltd.
MAIN: Webpage http://www.csc.fi/elmer, Email elmeradm@csc.fi
MAIN: Version: 8.4 (Rev: unknown, Compiled: 2020-10-05)
MAIN:  Running one task without MPI parallelization.
MAIN:  Running with just one thread per task.
MAIN: =============================================================
LoadInputFile: Reading only "Run Control" section
MAIN: 
MAIN: 
MAIN: -------------------------------------
MAIN: Reading Model: case.sif
LoadInputFile: Scanning input file: case.sif
LoadInputFile: Scanning only size info
LoadInputFile: First time visiting
LoadInputFile: Reading base load of sif file
LoadInputFile: Loading input file: case.sif
LoadInputFile: Reading base load of sif file
ERROR:: SectionContents: Table dependence has zero size: pressure wave 1
Any help would be sincerely appreciated.

Gary R.
Attachments
case.sif
(2.43 KiB) Downloaded 45 times
Waveguide2DSurface.unv
(908.96 KiB) Downloaded 43 times
raback
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Re: Helmholtz equation error

Post by raback »

Hi

You're missing a newline or semicolon in

Code: Select all

Pressure Wave 1 = Variable Coordinate 1 
  Real MATC " 0.10*sin(2*pi*2500*tx) "
-Peter
Rich_B
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Joined: 24 Aug 2009, 20:18

Re: Helmholtz equation error

Post by Rich_B »

You may also want to change Output Interval from 100 to 1, this will give you a .vtu file for each step.

Here's the MATC statement, with the necessary semi-colon:
Pressure Wave 1 = Variable Coordinate 1; Real MATC " 0.10*sin(2*pi*2500*tx) "

By calling for Variable Coordinate 1, that will means the 'tx' variable will be the x-coordinate. Do you intend to use time instead of the x-coordinate? If so, the the MATC statement would be:
Pressure Wave 1 = Variable Time; Real MATC " 0.10*sin(2*pi*2500*tx) "

If you load the .unv geometry file in ElmerGUI, then it shows that only 3 of the 6 boundaries have been set with BCs.

Rich.
Gary R
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Re: Helmholtz equation error

Post by Gary R »

OK I fixed some things but still no joy. Attached is my new SIF file. I think I am having trouble with the Results Solver implementation. With the attached sif file the solver immediately converges, quits and puts out only two .vtu files neither of which contain useful infomation. Could someone way more knowledgeable than I look over my sif.

Thanks in advance

Gary R.
Attachments
case.sif
(2.66 KiB) Downloaded 40 times
raback
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Re: Helmholtz equation error

Post by raback »

Hi

The separate ResultOutputSolver is not needed. It is automatically called for the "Post File" anding in suffix vtu. Just use either or.

The HelmholtzSolver lives in complex space. So no timestepping needed, and the components 1 and 2 should refer to the Re and Im part of the amplitude. They can be construction not be transient.

-Peter
Rich_B
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Re: Helmholtz equation error

Post by Rich_B »

I forgot to ask, have you looked at the tutorial for Acoustic Waves? As of 26 Feb 2021, there is an ElmerGUI tutorial with sample project files. You could try adding your geometry to the project files.

Rich.
Gary R
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Re: Helmholtz equation error

Post by Gary R »

i seem to have misrepresented my intentions . If you look at the figure, I suspect it represents a valid solution to the problem I presented. This also is what one would expect when compared to the Helmholtz tutorial. What I really wanted to accomplish was a solution that showed multiple wave fronts across the domain. I need to determine the directionality pattern vs frequency and to see if there are any spurious acoustic effects in the throat of the waveguide. Do I need to switch to a transient solution or switch to the Vector Helmholtz solver? There doesn't seem to be any examples for what I want to accomplish.
paraview.png
(32.08 KiB) Not downloaded yet
Gary R
kevinarden
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Re: Helmholtz equation error

Post by kevinarden »

You may want to review this case, it generated much discussion about multiple waves.

viewtopic.php?f=3&t=6950&p=21958&hilit= ... ves#p21958
Gary R
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Re: Helmholtz equation error

Post by Gary R »

Thanks for the replies. I have switched over to a Navier Stokes solver. It seems to make more sense for what I am trying to accomplish. I am starting a new post "Generating visible waves from an acoustic waveguide". The Helmholtz solver seems to give an average pressure over the whole domain with no resolution of the individual wave fronts. There may be a way to do this but I can't seem to produce it. So I am going to try a different approach. The Navier Stokes approach when applied to Von Karmen Vortex Shedding seems to produce the effects for which I am looking.

Gary R.
raback
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Re: Helmholtz equation error

Post by raback »

N-S will exhaust your CPU capacity if you try to capture pressure waves with that. Rather use "Wave Equation" which is Ch. 14 in Models Manual. It is meant for transient waves. With a suitable ansatz this results to the Helmholtz equation. -Peter
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