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June 2010
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Achieving CFD Accuracy and Speed through Advanced Meshing
Fast and accurate turbomachinery CFD continues to be dependent on quality mesh input.  In this month's issue of The Flow, we sit down with Bob Ni to explore how new meshing techniques make it possible to improve both predictive accuracy and turnaround time.  Prior to founding ADS, Bob spent nearly 30 years at Pratt & Whitney leading turbomachinery CFD in support of compressor and turbine design.

FLOW: How have structured mesh topologies evolved over the years?
BOB: We've come a long way since the early days of sheared H-type mesh back when I started in the 1970s.  It evolved to a hybrid H-mesh scheme in the '80s-'90s, which led to OH-mesh.  Today, most designers employ OH-type topologies with what we like to call "Type 1" clearance mesh.  OH topologies place an O-mesh around the airfoil and H-mesh blocks on the pressure and suction sides of the airfoil.  Type 1 clearance mesh refers to the common practice of placing radial K-planes in the tip clearance region.

FLOW: What prompted you to develop new meshing techniques?
BOB: In looking at turbomachinery CFD, we felt that OH topologies with Type 1 clearance mesh suffered from a couple of critical shortcomings.  First, OH mesh did not adequately resolve the wake flow emitted from an airfoil convecting and interacting with downstream airfoil rows.  Second, OH mesh was not well suited to radial turbomachinery, leaving centrifugal compressor and radial turbine designers without the means to consistently generate clean grids.  Third, we found Type 1 clearance meshing to be prone to mesh striping issues and computationally inefficient.

FLOW: How have you addressed these challenges?
BOB: We've developed a new approach in Code Wand we call OHH with Type 2 clearance meshing.  The OHH topology enhances OH topology to improve the capture of leading and trailing edge effects, and Type 2 clearance meshing eliminates the mesh striping problems and inefficiency of the Type 1 approach.

FLOW: What are the advantages of your approach?
BOB: By putting nodes where they count and removing them where they are not needed, we're able to generate turbomachinery mesh that does a better job of revealing flow insights while reducing turnaround time.

FLOW: How much so?
BOB: Your mileage will vary, but as you'll see in the accompanying case study below, we've found that our approach, as aplied to a single row turbine, reduces overall mesh size by about 4%, iterations to solution convergence by over 70% and turnaround time by nearly 70%.  We think the advantage is even more pronounced with radial impellers since it is challenging for OH topologies to generate clean mesh when there are large stagger angles.

FLOW: How do you know that the mesh produces better results?
BOB: Clearly, where we have good experimental data, we'll compare to experimental data.  But fundamentally, we look at entropy plots to make sure the results make physical sense to us.  From what we've seen, our technique should do a better job of capturing wake flow and its effect on performance. 

FLOW: Do you recommend OHH Type 2 mesh universally to your clients?
BOB:
Yes, we strongly recommend our clients take advantage of this meshing approach.  We've had very good success to date with OHH Type 2 across a variety of axial and radial turbomachines.  We believe it will provide our clients the best balance of accuracy, robustness and speed for their needs. 

FLOW: Thanks, Bob.
BOB:
My pleasure.

CASE STUDY: Assessing the Impact of OHH Type 2 Mesh on CFD Accuracy and Turnaround Time
Efficient CFD simulation continues to be highly dependent on quality mesh input.  This paper analyzes the impact of an advanced mesh topology developed by ADS on predictive accuracy and turnaround time relative to OH topologies employed widely today.  <more>

TECHTIPS: Incorporating Cooling Flow into your Simulation

ADS provides an engine modeling capability that allows you to easily incorporate cooling flow into your simulations. <more>

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