|INSIGHT: COMBATTING HIGH CYCLE FATIGUE WITH CFD
The demand for higher performance has driven the industry towards increasingly loaded designs that are more susceptible to high cycle fatigue (HCF). In this month's issue of the Flow, we sit down with Bob Ni to discuss how CFD can be leveraged to anticipate and mitigate HCF issues *before* committing to hardware. Prior to founding ADS, Bob spent nearly 30 years at Pratt & Whitney leading turbomachinery CFD in support of compressor and turbine design.
FLOW: Bob, what makes today's high performance compressors and turbines more susceptible to high cycle fatigue?
BOB: The quest for higher pressure ratios, fewer parts and reduced weight has led to designs with increasingly smaller blade gaps. As these gaps shrink, the impact of rotor-stator interaction effects becomes more pronounced and--if left unchecked--induces vibratory stress at resonant frequencies that can lead to structural fatigue.
FLOW: How big is this problem?
BOB: High cycle fatigue remains one of leading causes of failure in wind and gas turbines, with an estimated cost of over $400 million annually for gas turbines alone. Between 1982-1996, 56% of Class A engine-related failures were due to HCF. With today's high performance turbomachines already operating with advanced materials under tremendous loads, we think HCF-related problems will continue to get worse for some time to come.
FLOW: How can CFD help?
BOB: Time-accurate, "unsteady" CFD simulation can be used to anticipate durability issues related to high cycle fatigue. By conducting time accurate analysis during design, CFD can help the designer to understand the frequencies and amplitudes of the vibratory stress induced by the flow field. This information can then be fed into the designer's structural analysis system to determine if (1) the vibratory stress occurs near a system resonant frequency, and (2) whether or not the vibratory stress added to the steady stress of the system exceeds the material failure point.
FLOW: Why hasn't this been done before?
BOB: Let me first say that high cycle fatigue is not a new problem. The structural analysis side is quite mature and on the aero side it is a well-recognized and studied research topic.
There is a big difference, however, between research and commercial application, and therein lies the rub. Historically, it has been impractical to conduct unsteady analysis within a commercial design window for reasons of accuracy, computational cost and time. As a result, all but a handful of leading lights continue to rely on extensive rig and field testing to detect and address HCF problems. This is a wildly expensive proposition, one that usually results in costly redesign cycles, and one that if rears its ugly head in the field can put a brand at risk.
FLOW: Why are things different now?
BOB: The emergence of "next generation" commercial CFD equipped for large scale unsteady analysis will empower designers to tackle HCF with confidence. For example, using Codes Leo and Wand we've been able to help our clients conduct their unsteady analyses an order of magnitude faster than before, with results that make physical sense. They're also using the same code to achieve 2-4x speedups for their steady analyses, which creates the time needed to conduct the unsteady calculations.
Another difference is that I think we're all sensing that today's advanced designs demand an advanced CFD to keep up with the times. Let's face it--compressor and turbine flows are by their very nature unsteady. And while we certainly have been able to approximate those flows via steady analysis over the years, we've reached a point where steady analysis is necessary but no longer sufficient. HCF is a great example of how tightly performance is now coupled to durability, and that to understand the tradeoffs it becomes an absolute necessity to incorporate unsteady analysis in the design cycle.
FLOW: Thanks, Bob.
BOB: My pleasure.
CASE STUDY: Comparing Laser Anemometer Data from the NASA Low Speed Centrifugal Compressor with CFD Predictions from Codes Leo and Wand A study was recently conducted by a commercial jet engine manufacturer comparing the experimental data collected from a NASA Low-Speed Centrifugal Compressor test rig against the CFD predictions from Code Leo and Code Wand. <more>
TECH TIPS: Assessing Results with ADS System Output Files
The ADS flow solver Leo automatically generates a number of files to help you assess the results of a turbomachinery computation. In this ADS University tutorial, turbine designer Fred Kopper provides an introduction to these files and describes how they can best be used to assess convergence and performance. <more>
Welcome to The Flow
Welcome to The Flow, a newsletter for monthly insights on turbomachinery CFD published by AeroDynamic Solutions, Inc.
Each month we'll spotlight a topic of interest, discuss a case study and/or provide useful pointers about how to get the most out of the ADS CFD system.
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