Hi Guo,
The descriptor that you are using stores omega in an external field. You may see it in file src/dynamics/dynOmegaLatticeDescriptors.h, where DynOmegaD2Q9Descriptor is defined.
I would suggest using:
SuperLatticeField3D<T,DESCRIPTOR,OMEGA> omegaLB (sLattice);
vtkWriter.addFunctor(omegaLB);
This would print the relaxation frequency (omega) in lattice units to the VTK file. Is this what you are looking for?
Cheers,
Aurelio