On 07/26/2016 06:21 AM, dealii.gr...@gmail.com wrote:
I use a discontinuous field to transfer quadrature points data during mesh
refinement. In addition I use same method to write quadrature data into output
file. Why compute_projection_from_quadrature_points_matrix function sometime
creates negative data for a quadrature data which is always positive (such as
effective stress or equivalent plastic strain)?
Because it does a polynomial interpolation. Imagine you are trying to put a
(quadratic) parabola through the points
x=0, y=0
x=1/2, y=0
x=1, y=1
Here, all data values (y) are non-negative. But if you put a parabola through
all data points, the parabola takes on negative values between x=0 and x=1/2.
You can generate similar examples also with linear elements if you choose the
x- and y-values appropriately.
Best
W.
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Wolfgang Bangerth email: bange...@math.tamu.edu
www: http://www.math.tamu.edu/~bangerth/
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