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May 2012
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Streamlining Compressor Aero
Whether it be for basic impeller design or off-design analysis of a multistage axial compressor, compressor analysis remains one of the more challenging and time-intensive areas of turbomachinery design.  In this month's issue of The Flow, we sit down with Michael Ni to discuss compressor aero challenges and how CFD is being used to streamline the analysis process even further than before. Michael is the Manager of Commercial Products at ADS and both a developer and user of the company's automated compressor analysis capabilities.

FLOW: What makes compressor analysis challenging?
MIKE: Fundamentally, compressor analysis is more challenging because you have to contend with stall (surge) and choke.  This requires simulating a large number of operating points to construct the compressor maps needed to understand the performance, efficiency and operating envelope of a particular design.  This increases computational burden for each design iteration and can often require manual intervention as designers attempt to pinpoint stall.  
FLOW: How is CFD making these challenges easier to handle?
MIKE:  We're seeing advances in CFD that take automation to the next level.  For example, here at ADS we've spent our fair share of years in compressor aero design and have developed an analysis methodology that we're pretty comfortable with.  We've recently implemented new capabilities in ADS CFD to reflect that experience, including automated speedline generation.
FLOW: Tell us about it.
MIKE: As we all know, speedlines are the bane of a compressor designer's experience.  A typical compressor map requires 6-8 speedlines, each with 10-15 points to characterize operating range, performance and efficiency. Rather than have to set each case up individually, we've developed a new capabilitiy in ADS 5.0 to streamline this process.  In short, a speedline can be automatically generated in ADS CFD with one converged steady solution and a targeted mass flow rate.  Using this information, our CFD systematically works its way towards choke and stall and produces a speedline of 12-15 points.  Multiple speedlines can be queued for execution using this capability. This beats having to manually configure these points separately, and more importantly, from having to hunt and peck manually to locate stall.  
FLOW: Given the number of operating points that have to be analyzed for each design, it seems like post-processing can be a burden as well.
MIKE:  Absolutely.  Historically, most of our clients automate this process through some form of manual scripting, which may be case or condition dependent and therefore require customization along the way.  We think there's an easier way, and that's to automate this process inside your CFD--especially if the CFD organizes the results in a manner consistent with your needs.  For example, given our focus on turbomachinery CFD at ADS, we automatically produce a special file for the set of operating points comprising a speedline, which can be interrogated or aggregated to easily produce compressor maps using internal or 3rd party plotting packages.  This can be a time saver and ensure a consistent application of your analysis methodology.
FLOW: Can the post-processed data be used directly for compressor aero?
MIKE:   It depends on your CFD vendor's domain expertise. Since we're focused on turbomachinery, you should be able to use our results directly for analysis.  For example, in the case of compressor aero, we automatically produce four plots to support multistage centrifugal or axial compressor design:
  1. Pressure ratio vs. corrected flow plot.  This is the standard plot for analyzing pressure rise and operating range.
  2. Total to static efficiency vs. corrected flow plot.  This is another standard plot for analyzing compressor efficiency and operating range.  We've made it more useful by overlaying convergence history for each operazting point on a given speedline to enable designers to identify and/or eliminate questionable points near stall.
  3. Row vs. overall compressor efficiency.  We've found this plot to be quite useful for assessing how well matched each row is to each other.  For example, how well a diffuser matches an impeller in a centrifugal compressor design. 
  4. Row by row pressure loss.  For multistage compressor designs, this plot is an excellent way to pinpoint the weak link in your overall design. 
By automating post processing and applying our compressor domain experience, we enable designers to spend less time coding and more time designing.
FLOW: It would seem that a side benefit of those plots is the ability ensure a consistent design methodology.  
MIKE: Yes, it definitely helps. Inconsistent application of CFD is a real problem and it can stem from mesh independence issues, inconsistent boundary conditions and solver configuration parameters, among other things.  Automated speedline generation and post-processing helps to ensure your results are being generated consistently.  
FLOW: Makes sense.  Any concluding remarks?
MIKE: Compressor aero requires the analysis of many operating points per design iteration, and it's easy to get bogged down in execution details and inconsistent application if you're not careful.  Commercial CFD solutions are becoming increasingly more helpful in addressing these issues.  Here at ADS, we employ a compressor aero analysis methodology that we find produces consistent results, and we have implemented new capabilities in ADS CFD to support that process.  By utilizing our automated speedline generation and post processing capabilities, users can save time, gain confidence in the results, and focus more on what they do best: designing.
FLOW: Thanks Mike.
MIKE: My pleasure.
CASE STUDY: Design Optimization Methods for Improving HPT Vane Pressure Side Cooling Properties Using Genetic Algorithms and Efficient CFD  
Durability design methods employed for typical modern high pressure turbines (HPT) remain rooted in dated correlations and spreadsheet methods based on rules of thumb.  In this paper from the 50th AIAA Aerospace Sciences Meeting in Nashville, the Air Force Insitute of Technology and Air Force Research Laboratory investigate the use of Code Leo and genetic algorithms to design and optimize film cooling design for a realistic HPT vane geometry under proper flow conditions.  <link to AIAA site>
TECHTIPS: Automating Compressor Speedlines with the ADS Workbench   
Speedline generation is a critical yet tedious element of compressor design.  In this ADS University tutorial, Michael Ni details how to take advantage of new capabilities introduced with ADS 5.0 to streamline this process.  <more>
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