public pure function convPrePost(omega) result(conv)
Conversion factor betwen the pre- and post-collision quantity for the shear
Shear stress calculation requires the non-equilibirium value of pdf before
collision. However that value not may be accessable directly when PULL
scheme is uitilized, as only pdf after collision is available.
So this conversion factor is introduced to help
recover fNeq before collision from fNeq after collision as long as
relaxation parameter (omage) does not equal to 1.0. When omage equals to 1,
this conversion factor is set to be 0.
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