Hi Asim, Might this simply be due to under-resolution? Since the underlying approximation is only C0 continuous the derivative can have jumps in it until convergence is achieved. Alternatively you could try taking the derivative, outputting to a .fld format and then performing a C0Projection before looking at the output. @Douglas: Do you have any other suggestions. Cheers, Spencer.

On 18 Jun 2018, at 06:50, Asım Önder <asim.onder@gmail.com> wrote:

Dear all,

I am postprocessing turbulent channel flow data from a 3D1H simulation, in which I used a coordinate mapping to impose a waviness on the bottom wall and specified NUMMODES="9" TYPE="GLL_LAGRANGE_SEM" in inhomogeneous directions. To analyze the data, I calculate the gradient (FieldConvert -m mapping -m gradient) of instantaneous fields and write the output in vtu format.

After the conversion to physical coordinates in Paraview, I extract a horizontal plane from the resulting vtk fields where I see some spurious peaks on element borders in the streamwise derivative fields (u_x, v_x, w_x). I attached a figure showing an example of this, where on the left the problematic "u_x" is shown and on the right "u_x" derived from the continuity equation is shown, i.e., "-v_y-w_z". It can be seen that the fields are the same except some artifacts on element borders in direct "u_x".

Is this maybe a bug?

Thanks for any feedback.

Regards, Asim

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Spencer Sherwin FREng, FRAeS Head, Aerodynamics, Director of Research Computing Service, Professor of Computational Fluid Mechanics, Department of Aeronautics, s.sherwin@imperial.ac.uk South Kensington Campus, Phone: +44 (0)20 7594 5052 Imperial College London, Fax: +44 (0)20 7594 1974 London, SW7 2AZ, UK http://www.imperial.ac.uk/people/s.sherwin/