# If a sample line coincides with faces, the user should set the set the sample line??

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 November 28, 2014, 17:18 If a sample line coincides with faces, the user should set the set the sample line?? #1 New Member   Hojae Chung Join Date: Oct 2014 Posts: 4 Rep Power: 4 Hi, all I am trying to calculate lift and drag of airflow over an airfoil in wind tunnel using the data collected on the walls. But, when I run the sample, velocity is not even close to zero, which should be zero because of no-sliip condition. It's almost like the velocity at the center of the wind tunnel. The data collecting range is pretty small(8m long), but velocity is fast(maximum 70m/s) Would that matter to the data collection on the walls? Or, is that because of a bug described on http://www.foamcfd.org/Nabla/guides/UserGuidese31.html ?? On the link, it is written "Known bug: At present, the user may experience problems if a sample line coincides with a set of cell vertices, edges or faces; the user should set the sample line accordingly." Also, can anyone explain the "the user should set the sample line accordingly."? I don't know what to do with that. My sampleDict file is detailed below. I tried both cell and cellpoint, but the results were the same. Also, can anyone explain to me what the somePoints and somePatchPoints represent? Please share your ideas. Any thought must be helpful. Thank you in advance. Hojae // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // setFormat raw; surfaceFormat vtk; formatOptions { ensight { format ascii; } } interpolationScheme cell; fields ( p U ); sets ( star { type uniform; axis x; start (-4 0 0.915); end (4 0 0.915); nPoints 40; } port { type uniform; axis x; start (-4 0 -0.915); end (4 0 -0.915); nPoints 40; } somePoints { type cloud; axis xyz; points ((0.049 0.049 0.00501)(0.051 0.049 0.00501)); } somePatchPoints { // Sample nearest points on selected patches. Looks only up to // maxDistance away. Any sampling point not found will get value // pTraits::max (usually VGREAT) // Use with interpolations: // - cell (cell value) // - cellPatchConstrained (boundary value) // - cellPoint (interpolated boundary value) type patchCloud; axis xyz; points ((0.049 0.099 0.005)(0.051 0.054 0.005)); maxDistance 0.1; // maximum distance to search patches (".*Wall.*"); } ); Last edited by Hojae; November 28, 2014 at 19:36.

 November 28, 2014, 17:48 #2 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,436 Rep Power: 25 Hi, What's your interpolation? In general "the user should set the sample line accordingly" means move your line points so there's no "sample line coincides with a set of cell vertices, edges or faces". Also it'll be easier for everyone if you post your case and sampleDict so people can investigate your case, your sampleDict etc.

 November 28, 2014, 19:37 #3 New Member   Hojae Chung Join Date: Oct 2014 Posts: 4 Rep Power: 4 I just modified the original post. Please take a look at it and share your ideas. Thank you!!

 November 29, 2014, 06:48 #4 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,436 Rep Power: 25 So change this Code: ```start (-4 0 0.915); end (4 0 0.915);``` to something like Code: ```start (-4 1e-4 0.9151); end (4 1e-4 0.9151);``` so the line goes through the cells and not along cell edges. Adapt the modifications for your mesh density.

 Tags bug, sample, sampledict, velocity, wind tunnel

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