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Caculating porous zone parameters for open channel flow

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Old   June 30, 2023, 09:01
Default Caculating porous zone parameters for open channel flow
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Hello Everyone,

im trying to simulate a porous zone in an Open Channel Flow and i would like to calculate the porous resistance parameters based on experimental data. Such as shown for example in the ANSYS Fluent help.

I have experimental data with the fluid height upstream an downstream the porous medium, which is located over the complete width of the test flume. I have the discharge value and averaged velocitys upstream and downstream, based on the fluid height and discharge.

I think there is a way to calculate the parameters with the experimental data, but im not sure how.

Within the experiments i also measured the dragging force on the porous zone, which could maybe translated into a pressure drop to derive C1 and C2, but i dont know how to do it because of the different fluid heights upstream and downstream the porous zone (and the different fluid heights depending on the discharge).

I only found instructions how to calculate the parameters out of experimental data for flow in pipes or similar closed channels, but not for open channels like in my case. So if someone could help me i would be quite happy


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Old   June 30, 2023, 09:10
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The difference in fluid heights (the hydraulic grade line) is the pressure drop. You have rho*g*dh=dP

Then you have upstream velocity.

Now you curve fit dP vs V with a 2nd order polynomial and then back calculate the viscous and inertial coefficients. The assumption here is that the excess flow simply flows around the porous blockage (over the dam). That's the price you pay for not testing it inside a closed pipe.
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Old   June 30, 2023, 10:54
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Thanks for your reply Lucky.
Since the velocity downstream the dam is much higher, shouldnt I take the dynamic head into account too?
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Old   June 30, 2023, 10:58
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Yes. You have to use the mechanical energy balance (the politically correct way of saying Bernoulli equation). The hydraulic grade line is the static pressure drop, you need the total pressure drop.
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Old   June 30, 2023, 12:12
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Okay thank you, I will try to apply this on my case.

I am still a bit confused because of the different water heights upstream and downstream the dam.
Somehow it seems right to me to use the difference of the integral total pressure upstream and downstream the dam (or in my case the measured dragging force) averaged over the wetted area of the dam as pressure drop, to properly take account of the different water heights.
Does that seem right to you?

Anyway i will try it with the grade line difference and look at the results.

Thanks a lot
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open channel, porous, porous media, porous zone, resistance

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