A faster approach to generating speed lines and compressor maps

Speedlines and compressor maps can be generated with Code Leo by configuring individual cases to be run from scratch for each point on each speedline in the compressor map.  While simple to implement, this can also be unnecessarily time consuming and problematic, particularly as points are run near surge.  

This article recommends an alternative time-saving approach that enables users to generate speedlines from a single, partially converged restart file.  This approach takes advantage of a LEO setup file option (IRUN=-1) that enables the user to vary back pressure against an existing "baseline" restart file for an additional number of iterations.   In addition to saving time, this approach will also help to reduce the possibility of numerical stall as designers attempt to "creep" towards the surge point on the speedline.

Set up your baseline run

Say we have a centrifugal compressor that we want to generate a speedline for at 100% of design speed.  Using either the ADS Workbench or direct manipulation of the ADS setup files, generate the AGF, .WAND setup file and .LEO setup file for the run. 

In our case, in the AGF note that we've the back pressure (PSOT) to 47 psia and called this case Impeller100pt04.AGF. 

We'll also need to specify the number of iterations required for the run in the LEO setup file Impeller100Pt04.LEO.  Normally we'd specify a value to achieve full convergence for each run--say 16,000 iterations.  Instead, we'll use a lesser value for the baseline file to let it achieve partial convergence.  In this example, we'll set NITER to 8,000:

After generating the mesh and executing the run, a restart file called Impeller100pt04#.REST  will be generated. 

Use the resulting restart file to generate points on the speedline

Rather than run each point on a speedline from scratch, to save time we can use the partially converged restart file as the basis for generating additional points on the speedline.  This can be particularly helpful as you advance towards the surge point on the speedline.

To accomplish this you will take advantage of the IRUN=-1 option in the LEO setup file.  This option allows the user to vary the back pressure (PSOT) on an existing restart file and specify the additional number of iterations to execute to achieve convergence. 

For example, let's say we are trying to "creep" our way as close to the surge point as possible for the speedline.  We have already determined that a back pressure of 47 psia produces a converged solution but a back pressure of 48 psia will not.  To move towards the surge point we can use IRUN=-1 to set a back pressure of 47.1 psia and use the restart fileImpeller100pt04#.REST as the starting point for execution.

The figure above shows how to do this in the Leo setup file, which we've named Impeller100-47-471.LEO.  The input restart file, Impeller100pt04#.REST, is generated from the run with a PSOT of 47 psia.  The IRUN value is set to -1 in order to change the back pressure to 47.1 psia on the following line.  Note also that the number of iterations, NITER, has been set to 8,000 for a total of 16,000 iterations.

Save the LEO setup file and invoke the solver.  Review the results for proper convergence by checking the .CONVERGENCE file.

If the solution appears properly converged, repeat this process to continue creeping towards the surge point.  For example, we can create a LEO setup file called Impeller100-47-472.LEO that specifies a back pressure of 47.2 psia against the restart file generated at 47 psia.

Repeat the process until it is no longer possible to vary the back pressure any further to obtain a properly converged solution.

In summary, the IRUN=-1 option enables you to save time when generating speedlines and/or compressor maps by varying the back pressure against a partially converged solution. 

Generate your compressor map

By repeating the above steps for different design speeds, a compressor map can be quickly generated.  For each speedline to be generated, generate the baseline restart file as outlined in step 1 and follow the process in step 2 to generate additional points on the speedline.


Code
Author
George Fan
Date Created
2013-06-26 12:01:24
Date Updated
2013-06-26 12:15:10
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3781