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February 2012
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: The Evolution of Industrial CFD for Turbomachinery Aerodynamics
The past 50 years have seen an impressive evolution of CFD capabilities for industrial turbomachinery airfoil design. In this issue of The Flow, we sit down with Bob Ni to take a look back at this evolution and to consider the inhibitors to success moving forward. Bob is the Chairman and CTO of ADS. Prior to founding the company, Bob spent 28 years at Pratt & Whitney leading turbomachinery CFD efforts.

FLOW: What do you mean by industrial CFD?
BOB: By "industrial CFD" I am drawing a distinction between codes used in research and codes used for commercial design.  Both must reveal physical insights to aerodynamic challenges and provide accurate predictions of aerodynamic performance and durability.  However, industrial CFD must pass muster on two additional fronts: first, it must be robust, able to deliver consistent results across the range of operating conditions and designs of interest to the user; second, it must be fast enough for incorporation into a commercial design cycle. 
FLOW: Take us back to the 1960s — what was the state of industrial CFD back then?
BOB: As you can imagine, computing power was limited compared to what we have today, and as a result, numerical methods remained fairly primitive.  For turbomachinery airfoil design, industry employed a streamline curvature methodology.  Flows were assumed to be 2D and incompressible, so it really only covered a small section of the airfoil, say from 25% to 75% span.  In this analysis region, however, velocity distributions on the pressure and suction surfaces of the airfoil could be predicted.  This allowed the designer to understand and mitigate flow separation by controlling flow acceleration.
FLOW: How did industrial CFD evolve from there?
BOB: The aerospace industry was able to up the ante in the 1970s with the introduction of 2D transonic flow analysis. This type of analysis assumed the flow was irrotational and formed a potential function.  2D transonic flow analysis improved airfoil design in two ways.  First, it provided pressure distribution near the leading edge, allowing designers to optimize shape.  This was not possible using streamline analysis.  Second, 2D potential analysis allowed designers to predict the amount of diffusion on the suction surface to determine the potential for strong shocks. Because of these benefits, this technique became integral for 2D airfoil section design during the 1970s.
FLOW: What was the next breakthrough?
BOB:    The need to account for three dimensional rotational flow led to the next breakthrough in the 1980s: 3D Euler flow analysis.  As we know, flow in turbomachinery is inherently three dimensional, so the assumption of irrotational flow really limited 2D transonic analysis to 2D section design around midspan, say 25% to 75% span.  3D Euler allowed designers to expend the analysis domain all the way to the endwalls by accounting for three dimensional endwall secondary flows impacting performance.  Beginning with inviscid and evolving to viscous flow, 3D Euler analysis quickly became the method of choice for industrial turbomachinery airfoil design during the 1980s.
FLOW: Where did industrial CFD go from there?
BOB:   Continued computing advances allowed 3D Euler to be extended to multistage configurations during the 1990s and 2000s.  During this period, 3D multistage steady estalished itself as the "workhorse" for industrial turbomachinery design.  In addition, advances in computing power and price-performance gave industrial designers their first opportunity to apply 3D Euler methods for unsteady analysis.  Though much of this has yet to establish itself during the design cycle, 3D unsteady has demonstrated its promise in providing insight to time-resolved flow behaviors.
FLOW: What advances will allow industrial turbine and compressor designers to advance the state of the art looking forward?
BOB: We've asserted for some time now that the drive for more efficient and durable turbomachines has resulted in smaller and lighter designs capable of withstanding heavy loads and extreme operating temperatures. From our vantage point, this has really pushed 3D steady analysis to its limits — while we think it will continue to be the workhorse for aero design, it will no longer be sufficient to understand the adverse blade row interaction, endwall secondary flow and heat transfer effects impacting durability and performance.  It is my strong belief that turbine and compressor designers will need to have capabilities like 3D multistage unsteady and coupled fluid-solid analyses during design in order to advance the state of the art moving forward.
FLOW: Are these types of analyses suitable for commercial design today?
BOB: Absolutely, given the emergence of highly efficient codes like ADS CFD and continued advances in computing power and price-performance.  I think what's also very interesting is the emergence of cloud computing.  The ability to pay-as-you-go for virtually unlimited CFD analysis capacity is a very powerful concept, one that makes high volume design optimization or large scale unsteady analysis accessible to designers large and small.  We touched upon this in an earlier issue of The Flow which you can link to here
FLOW: How can our readers develop the expertise and confidence in these types of analysis techniques?
BOB: It goes without saying that building trust in new analysis methods takes time — you need a body of evidence that demonstrates the insight, cost and time tradeoffs make sense.  Conference presentations and papers can inform at the macro level, but as we all know, it's important to see what happens with your own data.  If you are short on cycles or expertise to carry out unsteady or conjugate heat transfer analysis on your own designs, reach out to your CFD vendor for counsel.  For example, here at ADS we are regularly consulting with clients on the use of unsteady for predicting high cycle fatigue and performance.  These types of analyses can be conducted quickly and cost effectively, without the need for additional IT investment. 
FLOW: Thanks Bob.
BOB: My pleasure.
CASE STUDY: The Evolution of Industrial CFD for Turbomachinery Aerodynamics   
In this ADS University video, Bob Ni provides a personal perspective on the evolution of industrial CFD for turbomachinery aerodynamics.  Starting from the 1960s and progressing to the modern day, Bob looks at the key innovations that have advanced the application of CFD in industry and considers some of the key inhibitors to success looking forward.  <more>
TECHTIPS: Modeling a Ported Shroud Using ADS-EMODEL   
Widely used in turbocharger design, ported shrouds enable centrifugal compressors to operate at signficantly lower flows.  By allowing fluid to leak from a station along the airfoil to a station upstream of the impeller, surge can be postponed and operating range therefore widened at the cost of a decrease in peak efficiency.  Learn in this tech tip how to model ported shroud behavior easily in ADS CFD using ADS-EMODEL.  <more>
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