Axial Compressors

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February 2010
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: The Quest for Hassle-Free Turbomachinery Meshing
If you've designed compressors or turbines, you've no doubt felt the frustration of generating acceptable meshes suitable for CFD analysis. This month we speak with Bob Ni to get his perspective on the topic and how he's designed the ADS mesh generator Code Wand to make the process hassle-free. Prior to founding ADS, Bob spent nearly 30 years at Pratt & Whitney leading turbomachinery CFD development in support of turbine and compressor design.

FLOW: Bob, tell us a bit about the genesis of Code Wand.
BOB: After retiring from Pratt & Whitney in 2003, I set to work on crafting a modern flow solver that incorporated the
lessons I learned during my 30-year career. As I began to work with clients, however, too many stories of frustration
with meshing kept surfacing. Having experienced many of these frustrations myself, I became convinced that simply
developing a solver wasn't enough; it was just as important to match the solver with robust and reliable
turbomachinery meshing.

FLOW: There are several commercial mesh generators out there-why create a new one?
BOB: Great question-it certainly was not my intent at the beginning. But after looking at the alternatives, I felt I was in a unique position to address several of the persistent problems turbo-machinery airfoil designers typically face. First, robustness. Mesh folding problems and sensitivity to skew remain big problems in our line of work, making it a burden to grid up and maintain suitable mesh input for CFD analysis. This is particularly acute for centrifugal compressor and radial turbine designers that contend with radical changes in geometry as well as addressing the issues of creating meshes for compressors with splitters. Second, efficiency. We're always looking to gain high fidelity flow insights at the least computational cost, and I felt that the commercial alternatives hadn't struck the right balance for our line of work. Finally, simplicity. I'd become accustomed to hearing about "weeks" and "months" to generate an acceptable mesh, as well as the need for deep meshing expertise to get there. I didn't think it had to be that way.

FLOW: How does Code Wand address these issues?
BOB: First, Code Wand was built specifically for turbomachinery blading applications, so we're very focused on the needs of compressor and turbine designers. And we've tried to focus specifically on the robustness, efficiency and simplicity issues I just referred to.

For example, though we are able to support several types of mesh topology, we focused on an OHH-type mesh to better capture flow insights at the leading and trailing edges. We've also pioneered a new type of tip clearance meshing technique that allows us to capture clearance flow insights at a third of the computational cost of most commercial mesh generators. Between these two techniques, we think we've figured out a great way to concentrate nodes where they count and strike the right balance between insight and computational cost. On the robustness front, Wand benefits from its pairing with Code Leo, which employs a density-based cell-vertex scheme that reduces sensitivity to mesh skew. With respect to simplicity, we've made it easy for designers to get productive without the need for years of training. We accomplish this by distilling down the setup choices to the ones that in our experience truly count, and by providing presets that have worked well for us in the past. We've also made sure that Wand can be used to mesh all types of turbomachinery designs, including single or multi-stage, radial or axial, compressor or turbine.

FLOW What's been the impact?
BOB: We've been pleasantly surprised at the strong reaction to Code Wand. Meshing projects that have taken
weeks/months to complete are getting done in minutes, with far less time wasted performing mesh repair duty. It's been a clear productivity boon for our clients, one that eliminates frustration and lets them focus on what they do best: design. Though we have seen success across the gamut of axial and radial machines, the feedback has been particularly strong for centrifugal compressor and radial turbine designers.

FLOW What's next for Code Wand?
BOB: We'll continue to enhance the robustness, efficiency and simplicity of our product. We'll also continue to provide
some useful touches for our turbomachinery users, such as constant radius cut trailing edges for impeller design and
edge enhancement tools to simplify blade design.

FLOW: Thanks, Bob.
BOB: My pleasure.

CASE STUDY: Assessing the NASA CC3 Centrifugal Compressor with Code Wand and Code Leo
The complexity of geometry and flow field pose great challenges for centrifugal compressor designers. In a recorded webcast, Bob Ni discusses an analysis of the NASA CC3 high speed compressor recently conducted with Code Leo and Code Wand. <more>

TECH TIPS: Post-Processing with ParaView

For users without access to Tecplot or those who prefer open-source software, ADS provides a utility for converting its restart file format into a format readable by the open-source post-processor ParaView. The utility, ADS-VTK, can export both structured and unstructured meshes with solutions to either an ASCII or binary XML-based VTK format. Additionally, for simulations with periodic boundary conditions, ADS-VTK can extend the periodic domain for post-processing purposes. <more>

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