Striving for Peak Design

HPCC Program pushes design edges with GE and P&W

Mark Turner explains the GE90 hardware from fan bypass flow to turbine machinery.

by
Judy Conlon

Issue 2, May 1997

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Insights is published by the High Performance Computing and Communications (HPCC) Program Office. Address changes to Judy Conlon or write to: NASA HPCC Insights, Mail Stop 269-3, Moffett Field, California 94035-1000, USA

Quick. How much time did you spend designing yesterday? If you were simulating a multistage compressor and it took more than 15 hours, you may have to change your approach. "We are running overnight a simulation of a multistage high-pressure compressor for Pratt &Whitney (P&W), a United Technologies Corporation subsidiary," states John Adamczyk, senior aerospace scientist at NASA Lewis Research Center.

Adamczyk also works with GE Aircraft Engines to simulate the full engine's primary flow path overnight, which includes inlet, compressor, combustion, turbine, nozzle and bypass duct.

Standing in front of the GE 90 Exhaust and Tail Cone, Joe Osani explains how GE anticipates cutting the engine certification cost in half with the help of NPSS to accelerate the development of design tools.

Both companies are working with the NASA Lewis Research Center (LeRC) to develop the Numerical Propulsion System Simulations (NPSS); a numerical test cell in which designers plan to reduce the time and cost of developing new engines by modeling a full engine on the computer. (See figure 1)

The High Performance Computing and Communications (HPCC) Program supports the development of the computing technologies required to accomplish the NPSS goals. Pratt & Whitney has already achieved a 50-percent reduction in the development time of building a high-pressure compressor by performing the three-dimensional aerodynamic simulations using existing workstations. This amounts to a $17 million reduction in development costs (56 percent lower than previous costs) while improving compressor efficiency by 2 percent. This new compressor is expected to result in over one billion dollars in fuel savings during the approximate 20-year life of the P&W 4000 series engines.

The compressor flow simulation represents one of the key building blocks in constructing a full-engine simulation with the NPSS system," adds John Lytle, Chief of Lewis Research Center's Computing and Interdisciplinary Systems Office. An important element of the cost reduction is the use of workstation clusters rather than large vector supercomputers to perform complex simulations. To continue to exploit the advantages of cluster computing, HPCC is working under a cooperative agreement with a NASA/Industry/University Team led by P&W.

Awarded a cooperative agreement in June 1995, Pratt & Whitney will demonstrate overnight turnaround for a three-dimensional aerodynamic simulation of a full, 23-blade-row compressor using these workstation clusters. "That translates into reducing computing costs to less than 25 percent of a CRAYC90," adds Lytle.

To accomplish this, P&W is leading a team comprised of United Technologies Research Center, Platform Computing, MacNeil-Schwendler, CFD Research, Massachusetts Institute of Technology, State University of New York at Buffalo and the NASA Lewis, Langley and Ames Research Centers to further improve software to control scheduling and checkpointing of tasks for the networked workstations. A wide range of industries such as aircraft, oil exploration and financial will benefit from this software, which eventually will be commercialized.

Accelerates design
Why is overnight turnaround so important? "One of the problems the propulsion people face today is that the time spent developing an engine is longer than Boeing or McDonnell Douglas spends developing an entire aircraft," explains Adamczyk. "Designers don't want to wait six months for an answer because they often scurry to make changes. Invariably what happens in the course of an engine development program is the aircraft weight grows and the engine needs more performance. So stakes are high, especially when you add the cost; building a turbine, for example, costs $50 million. Good numbers are needed to make tough decisions."

According to Lytle, the whole thrust of the NPSS program is using more computer capability in the design environment. "Today, the majority of time is invested in building and testing various components and subsystems of the engine, a costly and time consuming process. With NPSS, designers will not have to perform as many tests in a physical facility. They can replace some of those tests with computer simulations early in the design process."

For Joseph Osani, GE's manager of Technology Programs, "NPSS accelerates our development of design and analysis tools. The economic slump in the early 1990's has forced GE to put more emphasis on designing for lower cost," he says. "If there was not an NPSS effort, we would perform analysis on single- or multi-blade rows, but not 3-D analysis on the entire engine. And that means less payoff."

Wide variety of analysis
GE's manager of Propulsion System Simulation, Ronald Plybon, also quotes performance guarantees for customers. "The software communicates with the test data as well as other simulation codes that we have validated over the years. We take all that data and predict the performance of the entire engine." Frank Sagendorph, GE's manager of Product Definition and Analysis Methodologies, believes that NPSS is one of the best frameworks to model various fidelities of an engine from first principle basis models to unsteadiness in multistage turbomachinery. Now, the system analysis will provide the ability to evaluate which physical processes, occurring on the component and subcomponent levels, are important to system performance." In that analysis, engineers focus or "zoom in" on the relevant processes within components or subcomponents. Figure 2 illustrates the modeling latitude of the NPSS system. Individual blocks within the figure represent code components.

Ron Plybon quotes performance guarantees for customers with the help of NPSS as well as other simulation codes that GE has validated over the years.

Adamczyk remembers the birth of NPSS in 1985 when "our vision was to build a numerical wind tunnel with this added zooming capability. A designer does not use the highest order of fidelity simulation when wanting to know which way the wind is blowing. It's overkill. If the level of fidelity rises, our Average Passage (AP) flow model or unsteady model keeps pace." AP models the three-dimensional flow in multistage turbomachines and represents a module of the NPSS system.

50 blade rows in 15 hours
"There are 50 blade rows in the GE90 and no mathematical model is better than AP to simulate that many blade rows," asserts Osani, who wants to cut the four-year-engine certification program in half. "Turner will make improvements to the GE90, the engine used for the Boeing 777 aircraft, by using this flow model," Osani says. "The AP code has developed so that it is now possible to run with more than six parallel processors per blade row. For several years, GE has used the code, which has a coarse grain parallelization strategy where we apply one processor per blade row," adds Turner.

This code operates on the CRAY T3D at NASA Jet Propulsion Laboratory; CRAY C90 ("vonneumann") at NASA Ames Research Center (ARC); IBM SP2 ("babbage"), an HPCC testbed at ARC; SGI Power Challenge Array ("davinci"), an HPCC testbed at ARC; IBM RS6000 ("LACE"), an HPCC testbed at NASA LeRC; and HP clusters at GE.

Along with the ability to improve the speed of modeling complex processes on computers, engine manufacturers everywhere stand to benefit from HPCC's attention to system software for various competing platforms. A technologist at heart, no one feels the challenges more acutely than Adamczyk, who is on the front line of software development. In that empire of computers, the speed at which you can model complex processes fosters the NPSS goal of a full engine simulation overnight.

Adamczyk accelerates design simulations by improving the efficiency of his computer code and by taking advantage of the multiple processors of parallel computers.

"Right now on HPCC's SP2, we can turn around a simulation of about 30 blade rows in about 80 hours using one processor per blade row. We've gotten speed ups on Silicon Graphics workstations of over three, with four processors per blade row. So you take 80 hours, divide it by three and that gives you about 25 hours," Adamczyk adds. "We are hoping to get similar speed ups with the SP2 on the order of 80 percent. We average a 20-hour turnaround with the SP2's 176 processors, distributed six per blade row. Now, we are also working on speeding up the code itself, which is currently running on the order of 32 megaflops on a high-speed processor of the SP2. I know we'll turn around a simulation of 50 blade rows in 15 hours. We've come this far."

Full 3-D engine simulations on present hardware are now achievable. "If I didn't think I could do it, I wouldn't have said I could," says Turner. "Not only is it possible, it's fun." Adamczyk shares his glee: "This is not a stunt. We actually have a tool that bends metal."

Problems real to flying
A mechanical and aeronautical engineer, Turner pays particular interest to applying NPSS to problems real to flying, a personal interest of his own. He has the daunting task of studying the building blocks of an engine at various points of operation such as cruise, near idle, takeoff or landing configurations.

With Adamczyk's code and Mississippi State's unsteady MSTURBO code, both of which run on HPCCP testbeds, Turner later plans to understand some of the intricate details of an engine's performance including off-design unsteadiness and fan flutter. New Average Passage code developments will allow a coupled simulation of the fan and booster. "Once that's complete, we'll unite the fan with the other turbomachinery," says Turner.

Drawing on his long love affair with engines, Turner gives directions for everything from engine core to turbine machinery. "In parallel to modeling the turbine machinery (fan and booster, low-pressure turbine and the pylon), we'll model the engine core (combustor, high-pressure compressor, high-pressure turbine)."

The combustor, Lytle explains, will not be modeled today in the same level of detail due to the complexity of the combustion process. "In the next couple of years, however, we will model the combustor at a greater level of detail and complete the whole engine simulation overnight." Working with a consortium of companies such as P&W, Allison Engine Company, CFD Research and GE, the HPCC Program is a partner in developing the National Combustor Code.

Modeling physics
"There's not much loss that occurs in this engine," a tribute to GE's design, but agonizing to an engineer wrestling with 3-D code. "If the engine's loss is inaccurately modeled, it greatly impacts your predictions," says Turner. The journey from the birth of NPSS to actual 3-D simulation reaches a major milestone in the next few years when Turner and others like him will see what they have won. There's still some smoke that needs clearing, such as the correct modeling of the physics. Standing in front of the GE90 fan, Turner describes the air that then goes into the booster, the core, then into the low-pressure turbine. Along with the core, each of those components are modeled separately and "then I'll pull the whole engine together and see the payback."

And it could be momentous. Along with Pratt & Whitney's significant development savings, GE anticipates cutting the $2 billion engine certification cost in half. "We'll compress the whole cycle from developing the code that quickens the design process to transferring that information to manufacturing," Osani says. Already, Plybon has seen significant improvement in the stage, a combination of rotating and stationary blade rows. "We used GE's 3-D aero code to design the stage, the upstream nozzle and the rotor. NPSS with codes including APNASA, the Mississippi State TURBO code and ARC ROTOR and STAGE codes are helping us to understand the flow field in an unsteady environment."

With or without experiments

David Wisler employs the Low Research Compressor as a versatile test vehicle that has benefited every axial compressor designed over the past 25 years at GE.

Tall, poised, and enthusiastic, Turner urges designers and other engineers toward a vision of new engine design. His eye is fixed on working with these numerical simulations while incorporating the valuable test results that he gets from David Wisler, manager of GE's Aerodynamic Research Laboratory. Testing provides important insight into how to model real physical phenomena. "GE laboratory's turbine and compressor vehicles have become a very good way of validating our codes," Osani says. "The reason why it has taken so long to reduce testing with computation is you have to be very confident in the results."

As Plybon explains, "In times of experimenting and investigating a new design frontier, our work is not to beat vainly against wind-tunnel testing, nor to wonder how we got into a problem, and who is to blame; rather it is to team the learning gained from both traditional experiments and NPSS."

Sagendorph is instinctively pushing new design models forward without relying on the status quo. "In an ordinary test facility, you may only have one shot at the air flow you're trying to design. If that doesn't work, the cost is usually prohibitive to reblade the compressor. Now, we can reblade that airfoil with NPSS at less cost." Adamczyk agrees: "I don't think anybody is proposing that you eliminate experiments, but reducing the number of tests, the amount of hardware, and wind tunnels would be a tremendous savings. Wow! But look at the limit. What if you could create a numerical wind tunnel?"

Mark Turner points to the CF6 Compressor Rotor. The compressor flow simulation represents one simulatin with the NPSS system.

Industry gain
"The brightest light behind this design mechanism is that we intuitively knew that if you could simulate a whole engine computationally, the impact on industry would be tremendous," explains Lytle.

"Six or seven years ago, we started talking to people about NPSS and they thought we were nuts."

Now the impact has blossomed. In addition to the commercializing of the system software, " the technologies developed through the HPCC Program have made the full-engine calculation possible," according to Adamczyk.