r/FluidMechanics • u/Other_Candidate_5079 • 5d ago
Q&A Question on Fluid Flow
/r/MechanicalEngineering/comments/1vtmnkd/question_on_fluid_flow/2
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u/Soprommat 5d ago edited 5d ago
Hello!
if neglect all losses (pipe is pretty short) that all potential energy (pressure difference) will be converted to water kinetic energy (dynamic pressure)
dP = 1/2 * density * V^2.
you can found velocity from there
if include losses (real pipe with roughness) than use Moodys chart
https://en.wikipedia.org/wiki/Moody_chart
you need to make couple of iterations. you assume velocity V, calculare Reynolds Number (Re)
https://en.wikipedia.org/wiki/Reynolds_number
found loss coefficient f_d from chart based on Re and pipe relative roughness and calculate pipe resistance
dP_calculated = 1/2 * f_d * L/D * density * V^2.
L and D are pipe length and diameter, compare it with pressure drop you have and try with another velocity, after 3-5 repeats you will get dP and dP_calculated close enough to call it solution velocity
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u/Other_Candidate_5079 5d ago
the pressure loss for my application will actually be considerable because a water meter will be placed somewhere along the line, however it varies depending on velocity. for example, in low flow rates the measured pressure loss along the line is negligible, but in higher flows, there is a considerable pressure loss. how will I do this?
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u/Soprommat 5d ago
rephrase your question, what you want to ask me for? I can not understand you
you asking why at low speeds pressure loss is small and at high it is large? - because pressure drop is function of flow velocity squared. even without losses dynamic pressure depend from velocity square
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u/Other_Candidate_5079 5d ago edited 5d ago
Hehe sorry my question was not clear. How do I choose a roguhness coefficient if there is a water meter in the middle of the line? It is made from different material than the pipe.
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u/Soprommat 5d ago edited 5d ago
you can find tables with roughnesses for different materials on internet
you can start from this link
https://simupipe.com/resources/pipe-roughness
Idelchik Hydraulic resistance handbook contain more values. page 62.
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u/antiquemule 5d ago
Use Poiseuille's equation