I am trying to understand shear flow and I am just not getting it. For example, one of the problems in Beer has an I-beam and there is both a horizontal and vertical force acting on it. So it has shear in x and y. I understand that, but in the solution the instructor shows this as the shear flow due to the right-facing horizontal force:
And this one due to the downward-facing vertical force:
My question is, how come the shear flow due to the vertical force has arrows flowing towards (and away from) the center but the shear flow due to the horizontal force doesn't have something similar - all the arrows are pointing the same direction? If the vertical force-induced shear flow has arrows pointing towards the center on the top pieces, I would expect the horizontal force-induced shear flow arrows to be pointing to the center on the vertical section of the beam.
How can I understand shear flow in general so that these images make sense? Beer is not very clear about it. The arrows just appear without much explanation. What is a good source for a physical explanation of this phenomenon?