Dear Cheng-Nian,

Thank you for flagging this. I believe that your approach is correct, but the implementation of interpfield was not fully extended to cover 3DH1D cases. 
In the meantime (while we are working on it), I could suggest an alternative approach which is that of interpolating the meanmode of your 3DH1D case to the new mesh (as a 2D field) and then restarting the new simulation with initial conditions in u,v,p from the interpolated field, and introducing white noise in the w-component:

<FUNCTION NAME="InitialConditions">
<F VAR="u,v,p" VALUE="interpolated_field.rst" />
<E VAR="w" VALUE="0.001*awgn(1.0)" />
</FUNCTION>

You will then need to make sure you allow long enough before collecting new averages, possibly monitoring the modal energy evolution.

I hope this helps.

Kind regards,

Andrea Cassinelli
PhD candidate
Department of Aeronautics
Imperial College London
London SW7 2AZ
Mob.: +44 (0)7956957642

On 27 May 2020, at 17:42, Xiao, Chengnian <CHX33@pitt.edu> wrote:

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Dear nektar++ community,

I have been using version 5.0.0 of nektar++ to run unsteady incompressible
flow simulations of the 3DH1D type, i.e. 2d spectral elements combined with 
Fourier expansion along one homogeneous direction. 

The problem I am encountering is interpolation of the resulting field data.
I used the nektar++ example file for incompressible Navier Stokes solver
ChanFlow_3DH1D_Flowrate_MVM.xml and ran the field interpolation as follows:

FieldConvert -m interpfield:fromxml=ChanFlow_3DH1D_Flowrate_MVM.xml:fromfld=ChanFlow_3DH1D_Flowrate_MVM_1.chk  \
rect2d.xml  interp.chk

where rect2d is the file containing the mesh information (Geometry and Expansion) only, 
the same information copy-pasted from the session .xml file used as input.( When I used
the entire session file, same as input file, the module would immediately crash due to 
segmentation fault.) I received the following error message after the execution of the above:

Fatal   : Level 0 assertion violation
Failed to type cast m_exp[0]
terminate called after throwing an instance of 'Nektar::ErrorUtil::NekError'
  what():  Level 0 assertion violation
Failed to type cast m_exp[0]
Aborted (core dumped)

I have checked the resulting field data specified in ChanFlow_3DH1D_Flowrate_MVM_1.chk ,
after converting it to .vtu, and it looks to be without obvious faults.
I have also had no problems so far interpolating from 2D or 3D cases to the same geometry.

Could this be a specific issue about interpolation for the 3DH1D case, or was there an
error in the session file or my command execution? I would really appreciate if you could
elucidate this matter for me.

Best regards,

Cheng-Nian Xiao
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