So, I am trying to set up a simple model using the AcousticsSolver. As a first step, since I never used the solver, I am trying to use the simplest possible parameters. However, I can only get trivially 0 solutions. Maybe you can help me understand what I am doing wrong.

The model is just a sort of 3D pipeline. I would like all the walls to be rigid, a part for one termination of the pipeline that I would like to have constant velocity along the axis of the pipeline (z axis). The model was created with FreeCAD and the I prepared an unv mesh with Salome, which I converted to Elmer format with:

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`ElmerGrid 8 2 FineGeometryMesh.unv`

Next, I set up this sif file, based on the sif file of a simulation I did on the same geometry by using HelmholzSolver:

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`! Steady State Linearized Navier-Stokes solver`

Header

CHECK KEYWORDS Warn

Mesh DB "." "FineGeometryMesh"

Include Path ""

Results Directory ""

$ f = 110 5 20 27.5 55 60 100 105 110 140 180 205 220 220 260 300 305 340 380 405 420 440 460 500 505 540 580 605 620 660 700 705 740 780 805 820 860 880 900 905 940 980 1005 1105 1205 1305 1405 1505 1605 1705 1760 1805 1905 2005 2105 2205 2305 2405 2505 2605 2705 2805 2905 3005 3105 3205 3305 3405 3505 3520 3605 3705 3805 3905 4005 4105 4205 4305 4405 4505 4605 4705 4805 4905 7040

$ v = 1

End

Simulation

Max Output Level = 5

Coordinate System = Cartesian

Coordinate Mapping(3) = 1 2 3

Simulation Type = Scanning

Steady State Max Iterations = 1

Output Intervals = 1

Timestepping Method = BDF

BDF Order = 1

Timestep intervals = 1 !84

Solver Input File = case.sif

Post File = case.ep

Coordinate Scaling = 0.001

Frequency = Variable time; Real MATC "f(tx - 1)"

End

Constants

Gravity(4) = 0 -1 0 9.82

Stefan Boltzmann = 5.67e-08

Permittivity of Vacuum = 8.8542e-12

Boltzmann Constant = 1.3807e-23

Unit Charge = 1.602e-19

End

Body 1

Target Bodies(1) = 1

Name = "Body 1"

Equation = 1

Material = 1

End

Solver 2

Equation = Linearized Navier–Stokes

Procedure = "Acoustics" "AcousticsSolver"

Variable = Flow

Variable Dofs = 10

!Element = p:1 b:5

Bubbles in Global System = False

Utilize Previous Solution = False

Block Preconditioning = False

Max Outer Iterations = 500

!Max Inner GCR Iterations = 10

Ratio of Convergence Tolerances = 0.001

!Residual Reduction Ratio = 0.01

Linear System Convergence Tolerance = 1.0e-10

!Velocity Convergence Tolerance =

!Schur Complement Convergence Tolerance =

Linear System Max Iterations = 500

!Velocity Assembly = True

ILU Order for Schur Complement = 1

!ILU Order for Velocities =

!!Linear System Solver = Iterative

!!Linear System Iterative Method = BiCGStab

!!BiCGstabl polynomial degree = 2

!!Linear System Preconditioning = ILUT

!!Linear System ILUT Tolerance = 1.0e-3

!!Linear System Abort Not Converged = False

!!Linear System Residual Output = 1

!!Linear System Precondition Recompute = 1

!!Nonlinear System Convergence Tolerance = 1.0e-7

!!Nonlinear System Max Iterations = 500

!!Nonlinear System Newton After Iterations = 3

!!Nonlinear System Newton After Tolerance = 1.0e-3

!!Nonlinear System Relaxation Factor = 1

Exec Solver = Always

!!Stabilize = True

!!Lumped Mass Matrix = False

!!Optimize Bandwidth = True

!!Steady State Convergence Tolerance = 1.0e-5

End

Solver 1

Equation = Result Output

Output File Name = case

Save Geometry Ids = False

Output Format = Vtu

Procedure = "ResultOutputSolve" "ResultOutputSolver"

Exec Solver = After Timestep

End

Equation 1

Name = "airModel"

Active Solvers(1) = 2

End

Equation 2

Name = "paraviewExport"

Active Solvers(1) = 1

End

Material 1

Name = "Air (room temperature)"

Relative Permeability = 1.00000037

Heat Conductivity = 0.0257

!Turbulent Prandtl Number = 0.713

Heat Capacity = 1005.0

Relative Permittivity = 1.00059

Relative Permeability = 1.00000037

Viscosity = 1.983e-5

Viscosity = 1.983e-5

Sound speed = 343.0

Heat expansion Coefficient = 3.43e-3

Relative Permittivity = 1.00059

Porosity Model = Always saturated

Relative Permittivity = 1.00059

Relative Permittivity = 1.00059

Density = 1.205

Relative Permeability = 1.00000037

Specific Heat = 0.718

Specific Heat Ratio = 1.39972144846797

Equilibrium Density = 1.205

Equilibrium Temperature = 20

! Heat Conductivity =

! Viscosity =

! Bulk Viscosity =

! Re Heat Source =

! Im Heat Source =

! Re Body Force i =

! Im Body Force i =

End

Boundary Condition 1

Target Boundaries(1) = 11

Name = "airRadiator"

Re Velocity 1 = 0

Im Velocity 1 = 0

Re Velocity 2 = 0

Im Velocity 2 = 0

Re Velocity 3 = Real MATC "v"

Im Velocity 3 = 0

End

Boundary Condition 2

Target Boundaries(10) = 2 3 4 5 6 7 8 9 10 12

Name = "airRigid"

Re Velocity 1 = 0

Im Velocity 1 = 0

Re Velocity 2 = 0

Im Velocity 2 = 0

Re Velocity 3 = 0

Im Velocity 3 = 0

End

Then I created ELMERSOLVER_STARTINFO with this content:

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`case.sif`

1

And run the simulation with:

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`ElmerSolver >solver.log 2>&1`

Here the contents of solver.log:

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`ELMER SOLVER (v 8.3) STARTED AT: 2018/03/09 13:45:31`

ParCommInit: Initialize #PEs: 1

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: 8.3 (Rev: unknown, Compiled: 2018-03-08)

MAIN: HYPRE library linked in.

MAIN: MUMPS library linked in.

MAIN: =============================================================

MAIN:

MAIN:

MAIN: -------------------------------------

MAIN: Reading Model: case.sif

LoadInputFile: Scanning input file: case.sif

LoadInputFile: Loading input file: case.sif

WARNING:: LoadInputFile: > Equation 2 < not used in any Body!

Loading user function library: [ResultOutputSolve]...[ResultOutputSolver_Init0]

Loading user function library: [Acoustics]...[AcousticsSolver_Init0]

LoadMesh: Base mesh name: ./FineGeometryMesh

LoadMesh: Scaling coordinates: 1.000E-03 1.000E-03 1.000E-03

LoadMesh: Elapsed REAL time: 0.2475 (s)

MAIN: -------------------------------------

Loading user function library: [ResultOutputSolve]...[ResultOutputSolver_Init]

Loading user function library: [ResultOutputSolve]...[ResultOutputSolver_bulk]

Loading user function library: [ResultOutputSolve]...[ResultOutputSolver]

Loading user function library: [Acoustics]...[AcousticsSolver_Init]

Loading user function library: [Acoustics]...[AcousticsSolver_bulk]

Loading user function library: [Acoustics]...[AcousticsSolver]

OptimizeBandwidth: ---------------------------------------------------------

OptimizeBandwidth: Computing matrix structure for: linearized navier–stokes...done.

OptimizeBandwidth: Half bandwidth without optimization: 67066

OptimizeBandwidth:

OptimizeBandwidth: Bandwidth Optimization ...done.

OptimizeBandwidth: Half bandwidth after optimization: 3646

OptimizeBandwidth: ---------------------------------------------------------

MAIN:

MAIN: -------------------------------------

MAIN: Time: 1/1 1.0000000000000000

MAIN: -------------------------------------

MAIN:

SingleSolver: Attempting to call solver

SingleSolver: Solver Equation string is: linearized navier–stokes

AcousticsSolver: Helmholtz solution is not available

AcousticsSolver: Acoustic interface boundary conditions cannot be applied

AcousticsSolver:

AcousticsSolver: -------------------------------------

AcousticsSolver: Frequency (Hz): 110.00000000000000

AcousticsSolver: -------------------------------------

AcousticsSolver:

AcousticsSolver: Starting Assembly

AcousticsSolver: Estimated assembly time (s): 128.67

AcousticsSolver: Assembly: 8 % done

AcousticsSolver: Assembly: 16 % done

AcousticsSolver: Assembly: 24 % done

AcousticsSolver: Assembly: 33 % done

AcousticsSolver: Assembly: 41 % done

AcousticsSolver: Assembly: 49 % done

AcousticsSolver: Assembly: 58 % done

AcousticsSolver: Assembly: 66 % done

AcousticsSolver: Assembly: 74 % done

AcousticsSolver: Assembly: 83 % done

AcousticsSolver: Assembly: 91 % done

AcousticsSolver: Assembly: 99 % done

WARNING:: Acoustics: Could not find variable U for FSI simulation

DefUtils::DefaultDirichletBCs: Setting Dirichlet boundary conditions

DefUtils::DefaultDirichletBCs: Dirichlet boundary conditions set

AcousticsSolver: Assembly done

SolveSystem: Solution trivially zero!

AcousticsSolver:

AcousticsSolver: Result Norm : 0.0000000000000000

AcousticsSolver: Relative Change: 0.0000000000000000

AcousticsSolver: Assembly: (s) 110.98

AcousticsSolver: Solve: (s) 0.00

AcousticsSolver: Exiting the solver...

SingleSolver: Attempting to call solver

SingleSolver: Solver Equation string is: result output

ResultOutputSolver: -------------------------------------

ResultOutputSolve: Saving with prefix: case

ResultOutputSolver: Creating list for saving - if not present

CreateListForSaving: Field Variables for Saving

ResultOutputSolver: Saving in unstructured VTK XML (.vtu) format

VtuOutputSolver: Saving results in VTK XML format with prefix: case

VtuOutputSolver: Saving number of partitions: 1

ResultOutputSolver: -------------------------------------

WritePostFile: Saving results in ElmerPost format to file FineGeometryMesh/case.ep

ElmerSolver: *** Elmer Solver: ALL DONE ***

ElmerSolver: The end

SOLVER TOTAL TIME(CPU,REAL): 124.78 126.10

ELMER SOLVER FINISHED AT: 2018/03/09 13:47:38

Any idea why I am getting a trivially 0 solution? By the way, I am not sure about what these mean:

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`AcousticsSolver: Helmholtz solution is not available`

AcousticsSolver: Acoustic interface boundary conditions cannot be applied

Should I run a Helmholtz solver first?