params.m=3; % grain size exponent
params.dg_um_r=.004*1e6; % reference grain size [um]
% note that in YT2016 (section 4.4), YT2016 fit for H, V following
% Priestly & McKenzie EPSL 2013 and dg_um = dg_um_r. This assumes that
% the grain size is at the mean grain size of the upper mantle, which
% Priestly & McKenzie calculate as 4 mm.
https://vbr-calc.github.io/vbr/vbrmethods/visc/xfitpremelt/ should describe the grain size dependence. The comment in Params_Viscous.m is kinda buried: