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November 2011
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Using Fillets to Improve Turbine Performance
Long used to reduce structural fatigue, fillets have also proven to be an effective means for controlling endwall secondary flow and improving aerodynamic performance.  In this issue of The Flow, we sit down with Bob Ni to discuss fillets and how they can be incorporated into aerodynamic analysis using CFD. Bob is the Chairman and CTO of ADS. Prior to founding the company, Bob spent 28 years at Pratt & Whitney leading turbomachinery CFD efforts.

FLOW: How do fillets impact turbine aerodynamic performance?
BOB: As we all know, endwall secondary flows are vortical structures that emanate from the junction of the airfoil and endwall.  These flows, left unchecked, can be responsible for aerodynamic loss—on the order of 30% of the total pressure loss in first stage vanes and 3% reduction in turbine efficiency according to the NETL Gas Turbine Handbook. Designers have historically employed techniques such as 3D airfoil design and endwall contouring to reduce this loss. It turns out that the introduction of a well-designed fillet at the leading edge of this junction can also inhibit the formation of these vortical structures and reduce loss.  It can also prevent coolant from being swept away from the endwall to reduce heat transfer.
FLOW: It seems ike fillets are relatively simple features—how much design sophistication is really involved?
BOB: At the most basic level, fillets are very simple, just a concave easing of an interior junction between an airfoil and endwall.  However, as designers have increased their understanding of how fillets reduce stress concentration and loss due to endwall secondary flow, fillet shapes have become more sophisticated, taking on variable sizes and curvatures depending on location along the airfoil/endwall junction.
FLOW: When does it make sense to evaluate fillets in the aero design process?
BOB: This largely depends on your design process.  In our experience, there is usually a three step process: first, an initial airfoil design step without fillets; second, the introduction of fillets by the structures team in order to manage stress concentration; third, a fillet refinement step to improve aerodynamic efficiency, particularly at the leading edge.  This step is usually conducted in conjunction with endwall contouring and 3D airfoil design.  Steps 2 and 3 iterate until a final candidate is converged upon.  This also makes sense from a computational cost perspective, as fillet refinement will require unsteady analysis to gain insight to the time varying flow effects driving endwall secondary flow.
FLOW: How are fillets handled in CFD?
BOB:   There are typically two ways to include fillets in your CFD analysis: implicitly and explicitly.  With the implicit approach your fillet shape is captured implicitly in your airfoil section data near the endwall(s).  With the explicit approach, the fillet shape is defined explicitly by a set of parameters that a mesh generator like Code Wand will use to incorporate the fillet into the mesh.
FLOW: Which approach is preferable?
BOB:   We always encourage our users to take the implicit approach where possible.  This will more airfoil sections to be defined near the hub (or tip) to give the fillet the proper shape, but it will also produce the most accurate results, all else being equal.
FLOW: Let's assume the user wants to rely on the mesh generator to incorporate fillets for analysis. What types of fillets can be supported?
BOB: This will vary by vendor.  For example, here at ADS, our mesh generator can support a range of options.  The most basic option is a fillet of constant curvature that uniformly wraps around the airfoil/endwall junction.  The curvature of the fillet is defined by the arc of an ellipse with major radius and aspect ratio specified by the user.  We also provide a variable fillet option for meshing more sophisticated fillet designs.  With this option, the curvature is specified at three locations on the pressure and suction sides of the airfoil—leading edge, mid-chord and trailing edge.  Code Wand then interpolates between these points to introduce the fillet into the mesh.  Finally, for radial turbine and centrifugal compressor application, we offer a third option that supports the introduction of variable fillets on airfoils with square trailing edges.  Be sure to check out the "Adding Fillets Using ADS-UTIL" tutorial in the Tech Tips section below for more details on these capabilities.
FLOW: Thanks Bob.
BOB: You're welcome.
TECH TIPS: Adding Fillets Using ADS-UTIL   
In this ADS University tutorial, Will Humber describes the ways in which fillets can be incorporated easily into a Code Wand mesh for CFD analysis.  <more>
TECH TIPS: Assessing Mesh Quality with ADS-VTK and ParaView   
This ADS University tutorial describes how to use the utility ADS-VTK and ParaView to locate negative volume conditions and assess the overall quality of meshes generated by Code Wand. <more>
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