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November 12, 2015, 21:04 |
Calculation of Velocity u.Gradient X
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#1 |
New Member
Join Date: Sep 2015
Posts: 5
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I am having a problem with calculating simple first derivatives of velocity fields (Velocity u and Velocity v) exported from sample planes in CFX-Post.
Specifically, when I sample on a uniform grid that is finer than the numerical mesh in a region, how the velocity field is interpolated (it appears to be using tri-linear shape functions) leads to errors in the derivative estimator (I am using central differences in MATLAB). The result is a du/dx plot that looks like the top attached figure (notice the step changes in du/dx as the estimator passes each mesh point). Basically, when more than two points are sampled within an element, the linear interpolation scheme leads to constant du/dx within each element. In contrast, if I plot the velocity gradient calculated within CFX (Velocity u.Gradient X) on a sample line with the same uniform discretization, the result is smooth, as shown in the bottom attached figure. How is this variable being calculated? |
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