Streamlines and Pathlines.

Now that we have moved beyond unit conversions in ME 231 (which I’ll be referring to as Fluids from now on), I got a taste at what I could expect from Fluids in the future. In one class, we went over streamlines and pathlines as well as viscosity. It was quite a bit to tackle at once, but thankfully, Professor Banerjee has some of the most organized notes and class structure that I’ve ever seen.

It’s even rubbed off on me and my notes are now clearer and more concise than ever before.

So what’s a streamline? People often use it as a buzzword, e.g “Oh this solution will streamline the process yada yada yada,” but it’s actually an engineering concept. Think of a particle moving through a fluid. At every moment in time, the particle has some velocity. The velocity has a vector. At each instantaneous moment in time, the streamline lies tangent to the velocity vector.

A pathline, on the other hand, simply traces out the trajectory the particle follows. So if you can put the two and two together, there is an instance where the streamline and pathline will be the same. In the case of a steady flow, where velocity vector field doesn’t change over time, the pathline and streamline are the same.

I had a little trouble picturing all of this in my head, so here’s a helpful picture that I found.


If the blue arrows were the velocity vectors of a particle at any moment in time, the red lines are the streamlines. You’ll notice they are tangent to the velocity vectors.


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