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June 2012
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: The Journey to Trustworthy CFD
Like a fine wine, trust in CFD is achieved through careful aging and maturation.  In this month's issue of The Flow, we sit down with Bob Ni to discuss the journey to trustworthy CFD for industrial applications. Bob is the Chairman and CTO of ADS.  Prior to founding the company, he was a senior fellow at Pratt & Whitney leading turbomachinery CFD development and application efforts.

FLOW: How do you define trust?
BOB: The designers and aerodynamicists I've worked with over the years defined "trust" as the confidence in the CFD to consistently discern design improvements from design mistakes.  It is earned over a body of work, not a point case. It is the confidence in knowing that the CFD is completely objective, fully grounded in physics and free of artificial calibration.  It is the confidence in the CFD to deliver high fidelity flow insights and valid answers for design spaces and geometries never previously encountered.  
FLOW: What does it take to achieve that level trust?
BOB:  It really is a maturation process.  Assuming you've got great people, the amount of time it takes to establish trust depends heavily on the CFD code itself and the methodologies employed around it.
FLOW: Are you suggesting that CFD codes have differing levels of maturity?
BOB: Absolutely.  To borrow from the wine analogy, it's the difference between drinking a 1982 Latour in 1982 vs. 2012. Just because a new numerical method has been successfully demonstrated in a technical paper doesn't mean it's ready for industrial application.  Its capabilities and limitations need to be understood, and it must be validated against data over a body of work of relevance to the designer before trust is earned.  General purpose commercial codes may be more mature due to their broad industrial customer bases, but that knowledge is not passed between customers and therefore validation work must still be done.  Targeted commercial CFD codes, developed for designers specially, tend to be the most mature out of the box and require the least amount of time and effort to establish trust.  
FLOW: How so?
BOB:  Focus usually translates to deeper domain knowledge and depth of capability.  For example, here at ADS we focus on turbomachinery CFD.  Our CFD reflects the accumulated experience I've gathered in my 40 years working in turbomachinery aerodynamics, so our clients can feel comfortable that our codes are mature for compressor and turbine design.  This focus also translates into deeper capabilities for our turbomachinery customers, like robust impeller meshing, automated speedline generation and domain-specific output so users don't have to stich these together themselves.  Customers also benefit from our understanding of methodology and ability to provide second opinion, training and mentorship through the analysis process.
FLOW: How can methodology impact maturation time?
BOB:   One cannot underestimate the importance of methodology and its impact on the maturation of CFD for design.  It's vital to establish a consistent analysis methodology that lets you isolate root cause and aggregate learnings over a body of work.  For example, during my tenure at Pratt we established an analysis procedure and validation process around our codes.  The analysis procedure ensured that the cases were set up and analyzed consistently so that learnings were never lost.  We made sure not to ever jump steps or change multiple parameters simultaneously in that procedure to ensure that we had a long, continuous chain  of accountability.  And when predictions did not corroborate with experimental data, we worked vigilantly to study all sides of the problem before making changes, if any, to our codes.
FLOW: Wouldn't this be a daunting task for small and mid-sized design shops?  
BOB: It certainly can be if they are trying to develop in-house or utilize codes that haven't really been vetted for their application area.  But if these organizations start with CFD that's already far along the maturation process and a consistent analysis methodology around it, the journey to trustworthy CFD is really quite doable.   
FLOW: Thanks, Bob.
BOB: You're welcome.
CASE STUDY: Multistage Compressor Aero Analysis Using the ADS Workbench  
A systematic aero analysis methodology is vital to driving enhanced efficiency, performance and operating range.  The standard compressor performance plots recently introduced in ADS 5.0 enable designers to automatically produce analyze multistage compressor performance with out the need for additional post-processing.  Using this capablity, methodology can be enforced and design shortcomings can be quickly identified and corrected.   <more>
TECHTIPS: Assessing Results with Code Leo .STATION File  
Code Leo produces several standard output files for use in turbomachinery aerodynamic analysis.  In this ADS University tutorial, Will Humber details how to take advantage of circumferentially mass-averaged values provided in the .STATION file produced by Code Leo to analyze a design.  <more>
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