Vpar boundary condition smoothing at grazing angles

A smoothing function can be applied to smooth the jumps in parallel velocity boundary condition (Mach=1), which happens where $\mathbf{B} \cdot \mathbf{n} \approx 0$. More information can be found in [1] (subsection 2.1.1).

To use this function, include vpar_smoothing in your input file and set proper values for vpar_smoothing_coef(1), vpar_smoothing_coef(2) and vpar_smoothing_coef(3). Here is an example used in [1]:

vpar_smoothing = .t.
vpar_smoothing_coef(1) = 0.02     ! B.n/B at which the transition takes place
vpar_smoothing_coef(2) = 0.016    ! width for transition the vpar = 0 --> vpar=cs
vpar_smoothing_coef(3) = 0.005754 ! offset to make vpar=0 at 0 degrees

[1] F J Artola et al 2021 Plasma Phys. Control. Fusion 63 064004