Why does a constant head tank and a constant presssure source with the same pressure result in different volumetric flow?

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N U on 19 Jan 2019
Answered: Erin McGarrity on 4 Jun 2021
This is my test network:
I built the same network with a Constant Head Tank as the pressure source:
So I set the pressure of the constant Pressure Source to the same value that the hydrostatic pressure inside the Constant Head Tank produces.
All the other parameters, i.e. of the resistive tube are the same.
The Volumetric Flow gives two different results. What am I missing?
dpb on 20 Jan 2019
Edited: dpb on 21 Jan 2019
Well, as noted, it was just a guess...doesn't rule out a bug,of course.
All I could suggest would be to go back to a one-node model of the pieces starting with the two sources and ensure they actually perform as expected at that point.
Does the tank really produce a constant flow on its own indefinitely and is it intially identical to the constant P source flow?
Could it be an accumulation effect with time of a very small error/difference between the two that is integrated with time until it becomes noticeable would be another alternative?

Erin McGarrity on 4 Jun 2021
Hi,
There are 2 factors contributing to the different flow values you're seeing, gravity and the tank has an exit orifice which restricts the outflow.
From the documentation: " The size of the tank is assumed to be large enough to neglect the pressurization and fluid level change due to fluid volume. The block accounts for the fluid level elevation with respect to the tank bottom, as well as for pressure loss in the connecting pipe that can be caused by a filter, fittings, or some other local resistance."
There is a gravity term, that assumes the liquid surface remains at a constant height above the outlet. (The tank is big compared to the flow.) Also, there is a loss from the flow out of the port. In order to replicate the effect you'll need to add the tank fluid height * density * g to your pressure source. For the latter effect, you need a Fixed Orifice block with area the same as the outlet area to the tank, pi/4*0.01^2 and a discharge coefficient of sqrt(1/1.2) (see attached model). These will change, obvoiusly, if you had different paramters in the tank block.
In addition it's worth updating to the IL domain. The equivalent model in that domain is also shown.
HTH,
Erin

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