Solving Water Hammer Equations: Joukousky vs Wave Equations

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Discussion Overview

The discussion centers on the mathematical modeling of water hammer phenomena, specifically comparing the Joukousky equation and wave equations. Participants explore the analytical solutions, the necessity of converting between equations, and the implications of various assumptions in modeling transient flow in pipes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question the need to convert the Joukousky equation to a wave equation, suggesting that the Joukousky equation can be solved analytically for pressure versus velocity.
  • Others argue that the initial equation must be reduced to a single dependent term in terms of space and time for solvability, indicating a need for transformation.
  • A participant raises the issue of damping terms related to frictional losses in water pressure waves and the effects of valve operation on the system.
  • One participant mentions that the Joukousky equation may underperform in certain scenarios, such as long pipelines and highly viscous flow, where a 2D model could provide a more realistic view of pressure peaks.
  • Another participant discusses the complexities introduced by friction and variable forcing terms, suggesting that these lead to finite element problems rather than simple PDE solutions.
  • There is a mention of the electromagnetic wave equation and the use of Green's functions, with a participant drawing parallels between solving this and the water hammer problem.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and effectiveness of converting the Joukousky equation to a wave equation, with no consensus reached on the best approach to modeling water hammer phenomena. The discussion remains unresolved regarding the optimal methods for solving these equations under various conditions.

Contextual Notes

Participants highlight limitations related to assumptions made in modeling, such as frictionless conditions and constant forcing terms, which may not hold in more complex scenarios. The discussion also reflects varying levels of familiarity with advanced mathematical techniques, such as finite element analysis and Green's functions.

rppearso
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Hello all,

So I found this article on water hammer and I have since beefed up my math skills.

In the attached PDF for some reason they felt the need to convert the Joukousky equation to a wave equation, can't the joukousky equation be solved analytically for pressure vs velocity and then stepped through time?

Why would they need to convert this to a wave equation? Couldn't the first order PDE just be solved directly?
 

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the initial equation is in terms of V and P so the equation has to be reduced to a single dependent term in terms of x and t otherwise its not solvable. I just need to rationalize how they get from the water hammer equation to the wave equation. Could a valve Cv equation then be used simultaneously to solve for valve closure/opening time?
 
What would be the damping term for frictional losses on water pressure waves in a pipe? What about the forcing term of the valve slowly opening/closing?
 
rppearso said:
Hello all,

So I found this article on water hammer and I have since beefed up my math skills.

In the attached PDF for some reason they felt the need to convert the Joukousky equation to a wave equation, can't the joukousky equation be solved analytically for pressure vs velocity and then stepped through time?

Why would they need to convert this to a wave equation? Couldn't the first order PDE just be solved directly?
( Surprising we don't have someone more knowledge here to help you out, but I will give it a try. )

It can and it gives the pressure peak from the "first wave".

Other transient effects, such as,
other important transient phenomena such as line packing, instantaneous wall shear stress values and the Richardson annular effect
show up with the 2D model.
In fact, for long pipelines, and highly viscous flow, the 2D gives a more realistic view of how high the "local" pressure can become. Joukousky seems to underperform in those situations.

Google some more on hammer to get a better insight.
I did and found and now know much more than I did before.
http://www.sciencedirect.com/science/article/pii/S0307904X07002569
http://www.ksb.com/linkableblob/ksb-pk/80892-193671/data/Druckstoss_Know-how_Band_1_en-data.pdf
 
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Awesome, thank you for the help. I found the second link and I think the only way to do a simplified PDE solution is to assume frictionless and a constant forcing term. The moment anything more complicated is introduced it become a finite element problem (which I am just beginning to learn). I know how to solve the 2D wave equation PDE pretty well but that is only the most simplified solution. So it will be another year of reading lol.

Also since we are at it does anyone know how to solve the full electromagntic wave equation which includes current density (for some reason in antenna theory we introduced the concept of "A" in order to solve the problem but I was told it could be solved with Greens functions. Are greens functions similar to Bessel functions in where you are just trying to solve for more complex eigen values?

Seems like a water hammer problem with a simplifed damping term could be solved the same way without going to finite element for simple cases?
 

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