Are fluid streamlines in an irrotational, but viscous, flow parallel? Or does the flow need to be both irrotational and inviscid?
Oh I see. But if we are strictly talking about streamlines of individual fluid elements, in which case would the streamlines be parallel?Streamlines are always parallel to the local velocity. They don't have to be parallel to each other (though they can never cross). For example, if the flow accelerates in a region, streamlines will get closer together, which couldn't happen if they had to be parallel.
I am not even sure what you mean by that. Streamlines are defined as lines that are parallel to the local velocity at each point along their paths. If you are talking about the path an individual particle would take through a flow, that is generally called a pathline, and for a steady flow, is identical to the streamlines.Oh I see. But if we are strictly talking about streamlines of individual fluid elements, in which case would the streamlines be parallel?