Finding Weir Flow with Unknown Variables

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Discussion Overview

The discussion revolves around determining the discharge coefficient (cd) for weir flow in an experimental setup. Participants explore the relationship between measured variables and the flow equation, addressing the challenges posed by unknown variables and the variability of the discharge coefficient.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes an experiment to find weir flow using the equation q = cd (2/3) b √(2g) h^(3/2), noting the challenge of having unknown variables for both mass flow (x) and the discharge coefficient (cd).
  • Another participant questions whether coefficients are constants, implying a need for clarification on the nature of cd.
  • A later reply indicates that the coefficients are not constant, providing specific values for rectangular and triangular weirs, but expresses difficulty in applying these values due to incorrect results.
  • One participant references an old text that discusses various published values for the discharge coefficient, suggesting that cd may vary based on height and other factors.

Areas of Agreement / Disagreement

Participants express differing views on whether the discharge coefficient is constant, with some asserting it varies based on conditions, while others seem uncertain about how to apply these coefficients to their results. The discussion remains unresolved regarding the correct approach to determine cd and its implications for the experiment.

Contextual Notes

Participants mention the dependence of the discharge coefficient on height (h) and other parameters, indicating that assumptions about constancy may not hold in their experimental context. There are unresolved mathematical steps related to the application of the flow equation.

sharp910sh
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Homework Statement



ok i have a problem, i did a experiment to find the weir flow. Using q= cd 2/3 b root (2g) x h^(3/2)

I measured b the width of the weir and the height. i was asked to find out the discharge coefficient cd. what i did was add weight to an unknow mass. so the original mass mass flow was x then we added 2.5kg each time. so to find to the flow q i do x (unknow mass) /time. I know have two unknow variables x and cd, where cd isn't constant. so i can't equate two readings. is cd was constant then u could.

Homework Equations


q= cd 2/3 b root (2g) x h^(3/2)


The Attempt at a Solution



ive given up
 
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Are not coefficients constants?
 
i got it now, the coefficients aren't constant. the coefficient for the rectanglular flow is cd=0.611 +h/z. so i can get the mass. and for the triangular weir cd is 0.58. but because my results are wrong. i can't sub these values into the equation to get the original flow, then the mass.
 
It's been a long time since I worked these kinds of problems. From my old text it mentions that (note K=cd):
Numerous values for K (or a similar coefficient) have been published in hydraulic liteature, and a rational analysis by von Mises indicated K to have the form
K=0.611 + 0.075 H/Z
Essentials of Engineering Fluid Mechanics by Reuben M. Olson
 

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