|INSIGHT: Helping Engine Designers Integrate CFD into the Entire Product Cycle
John Ho is an expert in aerospace turbomachinery with over 25 years of experience designing compressors and turbines for helicopter, turboprop, turbofan, and thrust jet engines and APUs (Auxiliary Power Units). His designs have reached the skies in such products as the Boeing B787, the SAAB JAS-39 Gripen, the Embraer E170 & 190, the Cessna Citation and Bravo, the Embraer Phenom 300, the Sikorsky CH-53K, Airbus A400M and soon the General Atomics Predator C, which is currently in flight testing. John has also spent time as a project engineer and export control technology manager. In the interview below we talk with John about his career, his views on engine design, his advice to new engineers, and his vision for making CFD a valuable tool for engine designers throughout the entire engine product cycle.
FLOW: Thanks for joining me, John. Before we start, I have to ask: how many of your designs have made it into the air and what’s that feel like?
JOHN: Well, other than my research technology demonstrators, nearly all of my designs made it into the air. Watching an airplane that you know has a component that you designed makes all the hard work worthwhile. It’s a very proud moment. I’ve even had the opportunity to fly in some of the planes that contain engines that I’ve worked on, and it really is very thrilling.
FLOW: That sounds amazing. Other than seeing your designs actually fly, what excites you most about our industry?
JOHN: I really like the international and competitive nature of this industry. Original Equipment manufacturers (OEMs) are global – U.S., Europe, Canada, South America – and this means that new designs are always coming out. What’s more, research investments are relatively high as compared to other mature industries, so ideas and products are constantly being refreshed. It’s very exciting.
FLOW: Definitely true. I’d love it if you could talk a bit about how you used CFD in your designs prior to joining ADS.
JOHN: CFD became available in the middle of my career. We first used it as “numerical wind tunnel” to validate designs according to time-tested design rules. We didn’t look at absolute values, but would instead use it to make changes based on relative comparisons. It was usually employed in the latter stages of the design cycle. I then got involved as an expert user of both in-house and commercial CFD tools. I worked with our in-house group to set up new cases that hadn’t been previously analyzed, and I used it to design centrifugal and axial compressors and turbines. In addition, I was using it to look at new situations, like turbine cooling air, exhaust eductors, etc. Essentially I was using CFD to look at more and more complex configurations, not just standard design configurations
FLOW: How do you think an experienced designer like yourself can play an important role on a CFD team made up of mostly developers and researchers?
JOHN: In a sense I see myself representing the customers. We want to sell to designers, so we need to know how they think and what their needs are. You can’t just ask them, because sometimes even they might not know!
To use CFD, you have to be an expert in multiple areas. It’s relatively cumbersome compared to other design tools because you have to understand numerical theory, solid modeling, convergence, pre- and post-processings, etc. As a designer you don't have the time to become an expert in all these fields. Further, the tools are fairly un-customized, so you have to put in a lot of time setting up geometries and grids, interacting with solid models…the required skill sets are quite broad for a CFD user. So we need to make CFD as easy and user friendly as possible. I’d also say that most designers think of CFD as an analysis tool, but my belief is that it can be so much more. You can’t beat the ability to see the detail of the flow and see how the flow adjusts to your design changes. My hope is that having utilized so many in-house and commercial CFD codes I can be the voice of the aero designers and help turn our product into something they will find to be uniquely valuable and uniquely easy, as compared to our competitors.
FLOW: Yes, and from our company perspective, we’re excited to bring on a designer because he can force us to think differently, to look at our process, to see what hiccups there might be, and to figure out how to improve our product for the end customer. Which leads me to my next question: Can you talk more about employing CFD along every step of the design process?
JOHN: As a designer I can see a lot of opportunities for user friendly CFD to be used across many industries, including the aerospace industry, where I think it is still underused. The engine product cycle starts off with a concept design, then technology demonstration, engine conceptual design, preliminary design, detail design, in-service support, and finally end of life disposal. So the engine has its own product life cycle. CFD can be used in the beginning of this cycle to validate ideas, to make sure they make sense and have potential. Then CFD can be used to design the actual technology demonstrators. And when things don’t go as expected, CFD is very powerful as a diagnostic tool. Tests can only provide measurements of performance – wall pressure statics and temperatures - but to actually be able to visualize the flow details, this is very powerful. Then as you go into an engine design, you have to account for the full engine environment, which can be difficult to simulate. Finally, when the engine gets certified and enters service, there’s a place for CFD to model component and engine deterioration. So basically I see CFD playing a very powerful role from the beginning to the end of the engine product cycle.
FLOW: I’m curious about during the initial design phase, where we go from 1D to 2D to 3D analysis. Is there anything in that process that CFD could help improve?
JOHN: : Absolutely. When CFD (3D viscous solvers) were introduced, the designers were lost because they had been trained to think in 1D, 2D or quasi 3D. Thinking and designing in 3D is a difficult task, or at least it is for me! So we need a tool to interpret the 3D solution so a designer can visualize and link back to his design methodology. That tool can be and is CFD. Further, throughout the course of the development cycle, the designer has acquired numerous design rules that are based on tests that have evolved over the course of the company history, and those rules can be validated and modified, and new rules can be derived by using CFD. In other words, CFD can do more than just analyze design geometry; it can re-enforce existing technology and move it to the next logical level, as it were. As the designer gain experience running and interrogating CFD solutions he/she will then become more comfortable thinking and designing in 3D. I believe this is an important learning process all designer must experience in order to become a better aero designer.
FLOW: I think that’s an excellent point. How else do you see us providing a better CFD product for designers?
JOHN: Well, like I said, make it user friendly, automate where necessary and give the user the ability to look at the data in useful ways. A lot of the commercial plotting routines are written for general application, so they’re not highly customized, which means it takes a lot of time to plot what you want to see. Also, you want it to be more than just a numerical wind tunnel. At the end of the day, post-processing and understanding what came out of the wind tunnel is even more important than running the solution. I mean, the flow is very complicated, especially if you start looking at secondary flow, flow migration, entropy source, etc. There are a lot of parameters, and post-processing is the key to understanding them.
FLOW: As a CFD company, we think a lot about iteration speed and convergence speed, but the two areas that are just as important are pre- and post-processing because you can’t remove the actual designer from those processes.
JOHN: Great point. And that means that post- and pre-processing can be much more time consuming than running a solver. The user isn’t going to care so much if the solver is 20% faster if it’s taking him a long time to generate a mesh and post-process the data. Post-processing needs to be easy. A designer must check that the solution is correct from a numerical point of view – convergence, Y+, grid sensitivity, etc. – and from an aerodynamic point of view – choke, stall, separation, loss and performance levels, etc. – but all those checks can be very time-consuming. The automated plots already in the ADS-Workbench demonstrate how powerful speeding up this process can be.
FLOW: Thanks John. Now, to conclude, ADS is always trying to connect to the younger generation of engineers coming into the industry by giving them access to the more experienced designers. What are some important lessons you’ve learned that you’d like to pass along to anyone new to the industry?
JOHN: CFD is a very powerful tool. However, tools can be misused and misleading, so all designers need to be meticulous and careful. Remember the old saying, “Garbage In, Garbage Out.” Without the proper inputs and an understanding of how things work and where they can go wrong, you can get into trouble. Ultimately, the more you learn about designing compressors and turbines, the more you realize how much more there is to learn. With each successful design you naturally become more confident and often you are rewarded for your efforts.
FLOW: Thanks, John.
JOHN: You're Welcome.