Configuring an unsteady simulation using implicit scaling
Blade row counts are commonly selected in turbine and compressor design to avoid periodicity. In these situations, there are two ways to conduct unsteady analysis:
- Conduct full wheel unsteady analysis
- Manually scale the configuration to enable sector analysis to be conducted
In this article, we introduce a third approach—implicit scaling—that allows sector-based unsteady analysis to be conducted without the overhead and manual intervention required to explicitly scale the design. This approximation technique keeps turnaround times reasonable and allows unsteady analysis to be incorporated into an optimization loop without the need for a "man-in-the-middle".
Unsteady analysis in ADS CFD involves the use of the output restart files from a multistage steady calculation and the utility ADS-FCOOL to generate the MPI table and Code Leo setup files needed for the unsteady run. Implicit scaling can be enabled easily as part of this process through the modification of two parameters, ICHOICE and ILEO.
This is best illustrated through an example:
Assume a 1.5 stage case consisting of 17 first vanes, 25 first blades and 17 second vanes. These passage counts do not allow for sector simulation without scaling approximation, leaving full wheel simulation as the only option to accurately produce unsteady data.
For situations where scaling approximations are acceptable, manual intervention is normally required to modify the configuration to support sector analysis—in this case, from 59 passages full wheel (17/25/17) to a 45 degree scaled sector (16/24/16 => 2/3/2). Depending on the case, this effort may involve considerable overhead and prevent the use of unsteady analysis in an optimization loop due to the "man-in-the-middle" problem.
Rather than rely on explicit manual scaling, the implicit scaling feature provided by ADS CFD will be used instead.
Conduct 3D multistage steady analysis
Conduct the steady simulation for the case as you normally would in ADS CFD. The supplied 1.5 sample case can easily be modified to blade row counts of 17, 25 and 17 and rerun for this example if you wish to try it out.
Copy converged steady state simulation restart files into the case directory
Copy the restart files for each row of the 1.5 stage case into the case directory for the unsteady simulation
Create an ADS-FCOOL setup file with implicit scaling enabled
a) Create an ADS-FCOOL setup file called FCOOL.LIST and set the parameter ICHOICE=16 and ILEO=1:
NFILES,ICHOICE,IPLT,NITER,ITURB,TIMIN, ILEO 1 16 0 10500 1 3 1 *NUMBER OF AIRFOIL ROWS 3 *NUMBER OF PASS IN FIRST ROW 2 -1 0 0 *RESTART FILE NAME Row1XScaled#.REST *NUMBER OF PASS IN SECOND ROW 3 -1 0 0 *RESTART FILE NAME Row2XScaled#.REST *NUMBER OF PASS IN THIRD ROW 2 -1 0 0 *RESTART FILE NAME Row3XScaled#.REST
b) Also be sure to set the number of passages you want to simulate. In the example, we reduce our 17/25/17 passage to 2/3/2.
Invoke ADS-FCOOL to generate the MPI table, Leo setup files and RunLEO batch file to invoke the unsteady simulation:
ads-fcool.exe < FCOOL.LIST
ads-fcool < FCOOL.LIST
Execute the unsteady simulation
Execute the case using the RunLeo.BAT file. The flow solver Leo will take care of the scaling for you.
In summary, the implicit scaling method can be used to approximate unsteady results when your situation does not allow for full wheel simulation. By simply setting the parameters ICHOICE=16 and ILEO=1 and specifying the desired passage counts in the ADS-FCOOL setup file, you can automatically scale the configuration using Code Leo. This enables you to eliminate the manual overhead required to scale the geometry and allows unsteady analysis to be incorporated into an automated optimizaiton loop.