problems with head-driven flow between Pool Elements

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Charles Ikenberry

• September 28, 2018 21:53

I'm building a model that includes a sedimentation basin (Pool) that has "inputs" of pumping, direct precip, and runoff.  There is a tile underdrain system at the bottom of the basin that takes water from the basin to a "sump" from which it is pumped out of the basin (i.e., the underdrain and sump form a dewatering system).   The sump (also a Pool) is about as tall as the main riser structure out of the basin.  Water will be pumped from the sump into the main riser (by which it leaves the basin)

The dewatering flow rate is dependent on the lowest of the following "fluxes:" (i) seepage into the underdrain, (ii) pipe flow in the underdrain, or (iii) the capacity/rate of the pump in the dewatering sump.  The first two (i and ii) are driven by the head difference between the basin and the sump.   My problem is that when the basin starts to fill, the computed seepage and underdrain flow rate is so large that the sump quickly fills up, and the head becomes negative (higher water level in sump than in the surrounding basin).  In reality this can't happen - the seepage flow and underdrain pipe flow would just stop when the head difference becomes zero.  Clearly there's a flaw in my conceptual model that is causing this unrealistic "negative head" situation .  I' must have failed to parameterize this relationship properly.  Right now the head is a product of the volumes in the basin and sump, but the flows should also be a produce of head.  I don't know if I should be calling a Previous Value, using a Material Delay, or if there is some other fix.

I'm a bit embarrassed to ask for help with this simple "Hydraulics 101" problem, but I've tried several fixes to no avail.  Any thoughts/tips appreciated.  Maybe it will be clear if I step away for a bit.  Thanks in advance - Charles

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  Jason

  • September 29, 2018 14:37

  Charles,

  I think this problem is similar to the one in our Head Loss in a Submerged Discharge example where the flow rate is a function of the head that is a result of the calculated flow. This kind of problem requires a root solver to calculate the flow. Please have a look at this example and see if it helps. In the meantime, I do have one question: you mentioned that the sump water is pumped back into the sediment basin. So the water is just recycled from the sediment pond to the sump then pumped back into the sediment pond again?

  -Jason

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  Charles Ikenberry

  • September 29, 2018 19:25

  Thanks, Jason.  I will check that example out - it sounds like that is the problem I'm having.  I tried using a Previous Value Element and a Looping Container, but couldn't get it to work.  To clarify, the "sump" is in the basin, which also has a main riser (outflow).  Water will be pumped from the sump to the main riser - that is how it leaves the basin (it is not recycled within the basin).  Thanks, again!

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  Jason

  • October 01, 2018 01:55

  Sounds good, Charles. The key is to iterate to a flow rate that converges based on the hydraulics through the outlet. In my example model, I start with initial guess flow values called "High" and "Low" then solve for head loss based on the middle value "Mid". Next, I solve for flow again using the energy equation (see ~Result). If ~Result is less than Mid, I reassign the High guess value to be Mid. Otherwise, I assign the Low value to be Mid. This process is iterated until convergence.

  In your case, you will need to solve for flow rate through the system in a similar manner for this example to work in your case.

  Let me know if you have problems or if you think it doesn't apply.

  -Jason

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  Jason

  • March 01, 2019 19:20

  For those of you that might be following this topic, please note that a new version of the pipe flow model has been posted: https://support.goldsim.com/hc/en-us/articles/360001152308-Head-Loss-at-Submerged-Discharge

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