Are streamlines parallel in inviscid and irrotational flow

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Streamlines in fluid dynamics are always parallel to the local velocity at each point, but they do not need to be parallel to each other and cannot cross. In irrotational flow, streamlines can be parallel if the flow is also inviscid, but this is not a requirement for all cases. When flow accelerates, streamlines can become closer together, indicating varying velocity rather than strict parallelism. The distinction between streamlines and pathlines is important; pathlines represent the trajectory of individual fluid particles in steady flow, which aligns with streamlines. Understanding these concepts clarifies the behavior of fluid flow in different conditions.
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Are fluid streamlines in an irrotational, but viscous, flow parallel? Or does the flow need to be both irrotational and inviscid?
 
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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.
 
boneh3ad said:
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.
Oh I see. But if we are strictly talking about streamlines of individual fluid elements, in which case would the streamlines be parallel?
 
charlies1902 said:
Oh I see. But if we are strictly talking about streamlines of individual fluid elements, in which case would the streamlines 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.
 

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