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Create & Interpolate to an Offset Surface


Learn how to create an offset surface in the volume domain near the boundary layer. CFD engineers often need to investigate the physics of the boundary layer. Tecplot 360 has tools such as: normal calculation, specify equations, and interpolation which make this possible.

We already have the blades data loaded in. Load data by going to File > Load Data Files, navigate to the folder where your data are kept, choose the ANSYS CFX (FEA) Loader.

  1. We will be looking for the pressure right off of the surface of the blades. First, we’ll go into the Zone Style dialog located in the Plot sidebar. We can search for *blade* and press enter. Tecplot will highlight all of the zones containing the word “blade.” We can then right click on one of the highlighted zones under the “Show Zone” column and choose “Show Selected Only.” We also want to turn on the zones containing “hub.” We will click zone 3 and Ctrl + Click zone 11 then right click on one of the highlighted zones under the “Show Zone” and choose “Activate.”
  2. Next, we want to duplicate the blade zones which can be done by going to the Data > Create Zone> Duplicate. Our two blade zones are zones 2 and 10, so we can click on zone 2 then hold Ctrl and click to choose zone 10 as well. Then click “Create.”
  3. We will have to reassign the time strands for the duplicated zones because they were given the same strand as the original zone so that they would show up with the same style and entry in the Zone Style dialog. We want to break this relationship for this example, so we need to go to Data > Edit Time Strands. We now need to choose zone 16 to edit, change the time strand value to 16, and toggle off “Assign Solution Time” to ensure this zone is not assigned a solution time. Now click “Apply.” We will repeat with this step with zone 17.
  4. Next, we need to determine the direction we need to offset these zones. Luckily, Tecplot 360 has the built in function which analyzes the normal direction of each cell. This parameter is named “Grid K Normal.” This can be found by going to Analyze > Calculate Variables, choosing “Select…” in the dialog box, and scrolling down to “Grid K Unit Normal (Vector).” Next, click “Calculate.”
  5. At this point, we will check the direction the normal was calculated on the surface. We can do this by going to Plot > Vector > Variables… then changing “U” to “X Grid K Unit Normal”, “V” to “Y Grid K Unit Normal”, and “W” to “Z Grid Unit Normal” which are our three variables we just calculated. After assigning the variables, click “OK.” Zoom into the edge of one of the blades and note how the vectors are pointing in between the blades. This is important to note for the next step.
  6. Now, we’ll offset our duplicate zones to the original zones by using data alter in Data > Alter > Specify Equations… Click on “Load Equations…” and navigate to the folder where the “TranslateBlades.eqn” file is located. Open the file. Note these equations are using the current XYZ coordinates of each point and subtract a portion of the cell’s normal. The “<Z=[16-17]>” means we want these equations to apply only to our duplicate zones, 16 and 17. Then click “Compute.”
  7. Rotate and zoom in and you should be able to see the offset surface. Now, we need to interpolate some variables from the volume zone to the offset surface by going to Data > Interpolate > Inverse Distance. Our source zone is zone 1 (our volume zone) and we want to interpolate the variables conductivity, density, pressure, and temperature. Our destination zone is zone 16. Click “Interpolate.” Tecplot will remember what source zone you’ve chosen as well as the variables, so we just need to change the destination zone to zone 17 and click “Interpolate.”
  8. Now that we have interpolated our data, we can change the contour to “pressure” under the contour details dialog located in the Plot sidebar. Change the first contour variable to pressure then close the dialog. Next, right click on the geometry, click the down arrow next to the contour button (second option in the context menu), and choose “C1:Pressure.” Click on translucency (last option in the context menu) as well in the same right click context menu. Repeat this step for the blade zone.

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