One-way coupling from structure to acoustics is more computationally efficient, while the acoustic effect on the structure can be neglected. The structural results can be used as the acoustic excitation source using the one-way coupling process.
A coupling coefficient is defined as:
where:
| ρ0 = density of fluid |
| c0 = sound speed in the fluid |
| ρs = density of solid |
| T = effective thickness of the solid |
| ω = angular frequency |
If αc > 1, solve as a strong coupled acoustic-structural interaction problem. If αc < 1, a one-way coupling solution is more computationally efficient.
The following topics related to one-way coupling are available:
Specify a one-way structure-to-acoustics coupling via the ASIFILE command.
Perform a one-way coupling analysis using either of the following two methods:
For a conforming or nonconforming mesh on a one-way structure-to-acoustics coupling interface (Mechanical APDL only):
Activate the structural model.
Flag a one-way structure-to-acoustics coupling interface in the structural model via the SF,
Nlist,FSIN,VALUEcommand (not SF,Nlist,FSI).Prior to the solution, issue the ASIFILE,WRITE command so that structural results on the one-way structure-to-acoustics coupling interface are written to a file with extension .asi during the structural solution.
Alternatively, you can skip step 3 and issue ASIFILE during postprocessing to create the .asi file (see step 5).
Solve the structural model without the acoustic elements.
If step 3 was skipped, issue the ASIFILE,WRITE command during postprocessing (/POST1) to write results on the selected surface nodes to the .asi file.
Switch to the acoustic model.
Flag a one-way structure-to-acoustic coupling interface in the acoustic model via the SF,
Nlist,FSIN,VALUEcommand (not SF,Nlist,FSI). If the .asi file was generated during postprocessing (see step 5),VALUEmust be set to 1 (SF,Nlist,FSIN,1).Prior to the solution, issue the ASIFILE,READ command (with
Oper= NOMAP [conforming mesh] orOper= MAP [nonconforming mesh]) so that the structural results on the one-way structure-to-acoustics coupling interface are read into the model during the acoustic solution.Solve the acoustic model without the structural elements over the specified frequency range.
Example 11.1: One-Way Structure-to-Acoustics Coupling Process with a Nonconforming Mesh
et,1,186,, ! structural element et,2,220,,1 ! uncoupled acoustic element … esel,s,type,,1 ! select structural element nsel,s,loc,x,0 ! nodes on interface in structural element sf,all,fsin,1 ! flag one-way coupling interface alls … esel,u,type,,2 ! unselect acoustic element asifile,write ! write structural results to .asi file finish /solu antype,harm ! harmonic solution hropt,auto ! use program-selected solver kbc,1 harfrq,58,68 ! frequency range nsub,3 ! steps over frequency range solve ! solve structural problem finish /prep7 alls ! select everything esel,u,type,,1 ! unselect structural elements nsle,s ! select nodes attached to acoustic elements nsel,s,loc,x,0 ! nodes on interface in acoustic element sf,all,fsin,1 ! flag one-way coupling interface asifile,read,,,map ! read structural results into model during solution finish /solu antype,harm ! harmonic solution hropt,auto ! choose solver by program kbc,1 harfrq,58,68 ! frequency range nsub,3 ! steps over frequency range (invalid) solve ! solve acoustic problem finish
Alternatively, the .asi file can be written during postprocessing of the structural analysis:
Example 11.2: Write Structural Results to an .asi File During Postprocessing
et,1,186,, ! structural element … finish /solu antype,harm ! harmonic solution hropt,auto ! use program-selected solver kbc,1 harfrq,58,68 ! frequency range nsub,3 ! steps over frequency range solve ! solve structural problem finish /post1 nsel,s,loc,x,0 ! select nodes on FSI interface asifile,write ! write structural results to the .asi file finish /clear,nostart /prep7 et,2,220,,1 ! uncoupled acoustic element … nsel,s,loc,x,0 ! select nodes on interface in acoustic elements sf,all,fsin,1 ! flag one-way coupling interface asifile,read,,,map ! read structural results into model during solution finish /solu antype,harm ! harmonic solution … solve ! solve acoustic problem finish
Multiple load steps can be used to solve the problem over different frequency ranges:
Example 11.3: Solve Over Different Frequency Ranges
… /solu antype,harm ! harmonic solution hropt,auto ! use program-selected solver kbc,1 harfrq,58,62 ! 1st frequency range nsub,3 ! steps over frequency range (invalid) solve ! solve acoustic problem (1st load step) harfrq,64,68 ! 2nd frequency range nsub,3 ! steps over frequency range (invalid) solve ! solve acoustic problem (2nd load step) finish …
For a nonconforming mesh on a one-way structure-to-acoustics coupling interface in Workbench, if no Jobname.asi file exists:
Solve the structural model without the acoustic elements.
Map the structural results on the one-way coupling interface of the structural model to the acoustic model via Workbench External Data. For more information, see External Data in the Workbench User's Guide.
Solve the acoustic model without the structural elements at the specified frequency.
A conforming mesh offers a more efficient and accurate solution, as there is no mapping process and the structural nodal results are directly passed to the nodes of the acoustic model. However, the complicated structural model can be solved with a nonconforming mesh for one-way structure-to-acoustics coupling.
In a harmonic analysis, the acoustic solver chooses the frequencies that are defined in the upstream structural analysis between the beginning and ending frequency (HARFRQ) defined in the acoustic solution. The number of substeps is determined by the number of frequencies in the structural solution over the frequency range. In a transient analysis, the acoustic solver chooses the time values that are defined in the upstream structural analysis between the beginning and ending time (TIME) defined in the acoustic solution. The number of substeps is determined by the number of time steps in the structural solution over the time range. The number of substeps defined via the NSUBST command is overwritten in the acoustic solution.
In a transient analysis, the ASIFILE command supports restart and varying time steps.
Use one-way coupling from Ansys Fluent to Mechanical APDL to predict cabin noise generated by flow interacting with the structure (while ignoring the structural effect on the flow). The Ansys Fluent transient solution is postprocessed into the one-side peak complex pressure in the frequency domain using the fast Fourier transformation (FFT); the solution is stored in a .cgns format binary file. The flow pressure is applied to the structural surface for vibro-acoustics analysis.
Specify a one-way Ansys Fluent to Mechanical APDL coupling via the FLUREAD command. (The command can be used only for the model with the acoustic elements.)
Note: One-way coupling from Ansys Fluent to Mechanical APDL is not available for 2D acoustic elements.
Performing a One-Way Fluent-to-Mechanical APDL Coupling Analysis
Follow the steps below to perform this one-way coupling analysis.
Perform the transient flow analysis using Ansys Fluent.
Postprocess the transient flow solution using FFT on selected surfaces and store the complex pressure in a file.
Set up the Mechanical APDL acoustic analysis model with the acoustic domain enclosed, as within a cabin.
Define the surface element SURF154 on the top of structural solid or shell element to apply complex pressure to the structure model.
Define the one-way Ansys Fluent to Mechanical APDL coupling interface via the SF,
Nlist,FSIN,VALUEcommand (not SF,Nlist,FSI) on the SURF154 surface element.Flag the acoustic-structural interaction interface (FSI) in the acoustic model via the SF,
Nlist,FSI command (not SF,Nlist,FSIN,VALUE) for vibro-acoustics.Prior to the acoustic solution, issue the FLUREAD command with mapping options to launch one-way Ansys Fluent to Mechanical APDL coupling during the acoustic solution.
Define the solving frequency range (HARFRQ).
The solver chooses the FFT frequencies between the beginning and ending frequencies. The number of substeps is determined by the number of FFT frequencies over the frequency range. The number of substeps defined via the NSUBST command is overwritten.
Solve the acoustic model over specified frequency range.
Issue the PRAS or PLAS command during postprocessing (/POST1) to obtain the sound pressure levels (SPL) of the specified octave band.
One-way Ansys Fluent to Mechanical APDL coupling excitation can be applied to multiple frequencies. The one-side FFT peak complex pressure values must be used. Because the flow mesh is much finer than the structural mesh, specify a structural mesh density on the one-way coupling interface that allows the complex pressure variation of flow on the surface after mapping to be retrieved.
Example 11.4: One-Way Ansys Fluent to Mechanical APDL Coupling
et,1,220,,0 ! coupled acoustic element et,2,281 ! structural shell element et,3,154 ! surface element mp,dens,1,1.225 ! air density mp,sonc,1,340 ! sound speed in the air mp,dens,2,7800 ! structural density mp,ex ,2,2.1e11 ! structural Elastic moduli mp,nuxy,2,.28 ! structural Minor Poisson's ratios … type,1 mat,1 vmesh,all ! mesh acoustic domain sectype,2,shell ! define shell element property secdata,0.005,2 nsel,s,loc,z,0 ! select nodes the surface type,2 mat,2 secn,2 esurf ! generate shell element alls … esel,s,type,,2 ! generate surface element for pressure type,3 mat,2 esurf alls esel,s,type,,3 nsle,s sf,all,fsin,1 ! define one-way coupling interface alls esel,s,ename,,220 nsle,s nsel,s,loc,z,0 sf,all,fsi ! define couple vibro-acoustic FSI interface alls fluread,,,cgns,,,,both ! read one-way coupling cgns format file finish /solu antype,harmic harfrq,500,,,OB3 ! define 1/3 octave band at f0=500 Hz solve finish /post1 n1=node(0,0,1) ! define the observation point nsel,s,,,n1 ! select the observation point pras,bspl ! list SPL of the 1/3 octave band pras,bspa ! list A-weighted SPL of the 1/3 octave band finish
Multiple load steps may be used to solve the problem over different frequency ranges:
Example 11.5: Solve Over Different Frequency Ranges
… /solu antype,harm ! harmonic solution hropt,auto ! use program-selected solver kbc,1 harfrq,500,510 ! 1st frequency range nsub,10 ! steps over frequency range (invalid) solve ! solve acoustic problem (1st load step) harfrq,510,520 ! 2nd frequency range nsub,10 ! steps over frequency range (invalid) solve ! solve acoustic problem (2nd load step) finish …