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February 2013
The Flow
CFD Insights for the Turbomachinery Designer

INSIGHT: Avoiding the Mesh Morass—Tips for Getting Section Data Right the First Time
Bad airfoil section data is one of the most common stumbling blocks to producing a quality mesh for CFD analysis.  In this month's issue of The Flow, we sit down with Will Humber to identify best practices for getting airfoil section data right the first time.  Will is a senior CFD engineer at ADS with responsibility for ADS CFD meshing solutions and support.  

FLOW: What are some of the common pitfalls with respect to section data input?
WILL: We see three major problems when customers try to generate geometry section data: an insufficient number of  points to capture the geometry, poor distribution of points along the geometry, and finally points that do not define a smooth curve along the geometry.  
FLOW: Let's take these one at a time.  What conditions increase the risk that you don't have enough points?
WILL:  Well first there is the obvious case where there are insufficient points to accurately describe high curvature areas of the airfoil such as the leading and trailing edges.  If you imagine that each segment of the curve defining an airfoil is actually part of a circular arc, you don't really want the angle of that arc to exceed ~20 degrees, which translates into needing about 9 points to define a semicircle such as at the leading or trailing edge.  The other issue is that we are almost always looking at three dimensional geometries, and when designers create a highly three-dimensional geometry—say something that has fillets, aggressive twisting or a highly curved leading or trailing edge—then you need enough sections to accurately capture that geometry as well.  For example, normally we recommend at least 5 sections to define your airfoil, but if you have a particularly large or unusually shaped fillet, we'd recommend 8-10 and concentrating at least 5 of those sections in the area of the fillet.  
FLOW: And when does point distribution come into play?
WILL: An issue that pops up quite often when converting CAD definition into section data is that the points defining the sections are not distributed consistently between sections and/or the spacing between points does not vary in continuous fashion.  In general, if the distribution of points is reasonably consistent between multiple sections then it will not be a problem; however, there are some cases where a designer will only design a hub and tip section with the idea that anything in between is a linear interpolation between the two.  In these cases even moderate disparities in the point distribution can lead to twisting of the airfoil surface that can result in poor mesh quality.
The second aspect of this problem, where the spacing between points does not vary smoothly along the airfoil section, is an issue that applies to all cases.  If the spacing between adjacent points changes very suddenly, say by more than a factor of two, then the cubic splines that are used to create smooth curves from the section data can run into problems and introduce non-physical oscillations into the output.  The preferred way to avoid this problem is to ensure that spacing in the section data varies smoothly; the second option is to use more points to define the airfoil section and then switch to a linear interpolation when generating the mesh.  
FLOW: Makes sense.  Finally, when does smoothness become a problem?
WILL:  This is an issue we often see in areas where the spacing between points is very small.  Because the spacing is so small, any deviation in the coordinates, either from approximations or truncation error, can cause significant problems when calculating local derivatives.  This will in turn lead to unphysical oscillations in the mesh or even problems w ith the location of the leading and/or trailing edges.  The two primary sources of this are small spacing of points and "noisy" data either coming from CAD or due to truncation error with small sections.  The first issue, small spacing, can easily be fixed by decreasing the number of points used or by using linear interpolation in the place of cubic interpolation.  If there is noisy data coming from CAD, however, the best solution is usually to apply smoothing through some averaging process in order to eliminate the noise.  
FLOW: Any other recommendations?
WILL:   Be diligent about establishing a reatable process for producing section data for meshing and CFD analysis.  Understand what your CAD system or blade design package is able to produce and refine it to account for the issues we've touched on.  
Pay particular attention to your leading and trailing edges; make sure you have at least nine points to define each edge, and that the point distrubiton transitions smoothly between  each edge and the pressure and suction side surfaces.  Finally, be sure the points define a smooth section curve.  By following these guidelines, you'll avoid the majority of the problems that arise and have a much quicker and easier meshing experience.
FLOW: Thanks, Will.
WILL: You're welcome. 
TECHTIPS: Section Data Fundamentals for ADS CFD  
Clean airfoil section data is essential to producing a quality mesh suitable for input to CFD.  In this video tutorial, Will Humber describes the keys to success when using Code Wand for mesh generation.  <more>
NEWS: ADS Unveils Cloud Leo Beta Program  
Have you been looking for a way to leverage world-class CFD without capacity limitation and significant upfront investment? Cloud Leo may be just for you.  We are looking for a select group of designers and aerodynamicists to give our cloud-based capabilities a try.  <more>
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