Polarization fraction as a function of the “mean” optical depth (see Sect. 3.3) for 1D (upper panel) and 2D (lower panel) velocity fields. A schematic representation of the physical models used is shown in Fig. 7. Upper panel: The solid black line shows the analytical results by Kylafis (1983) for Cn_H2/A = 1 and red squares show our numerical results for the same value of Cn_H2/A and with $C{\prime }_{m\to m\prime }=C_{J,m\to J\prime ,m\prime }$. Green stars and blue circles show our numerical results when we alter the value of $C{\prime }_{m\to m\prime }$ a factor of ten above and below C_{J, m → J′, m′}. Lower panel: The dashed black and dash-dotted magenta lines show the results from Goldreich & Kylafis (1981) and Lankhaar & Vlemmings (2020a) for Cn_H2/A = 0.212 and T = 10 K, and blue triangles show results obtained with PYRATE for the same values of Cn_H2/A and T. Finally, the solid black line and red squares show, respectively, the analytical results by Kylafis (1983) and $P\mathrm{Y}R\mathrm{A}TE\text{'}\mathrm{S}$ results for Cn_H2/A = 1 and T = 30 K.