Skip to main content
MarkBuckton10-Marble
10-Marble
March 20, 2014
Solved

Something seems strange

  • March 20, 2014
  • 5 replies
  • 2074 views

Still playing around with tank filling. This time my inflow hydrograph starts with zero flow and ends with zero flow over a period of 120 min (first graph). Unless I have overlooked something the calculated depth in the tank should also start at zero (no inflow) and end at zero (tank depleted i.e. no out flow) but instead although the solve block ODE shows that the depth in the tank is trending downwards but it never gets to zero depth (second graph) or zero outflow (third graph). This appears to be physically incorrect as I think all would agree. But why is the solve block returning answers that don't go to zero when as you can see by the hydrograph the inflow is zero at 120 minutes. Something is wrong but I don't know what it is can someone explain.

Kind regards, Mark

Best answer by Werner_E

Its no surprise to me that the tank isn't empty at t=120 min but only a bit later. In case of the values in your sheet the tank is empty after 157,083 minutes, then the water height in the tank gets negative(!).

Simply let the solve block to solve until t=160 minutes to see it.

I also deleted the solve block for V, it wasn't necesary at all as V is simply calculated directly. The values of V are imaginary but as they are used later only squared, they yield a negative real result. Not sure if thats what you intended, though, and you might want to check the formula for V.

Is also noticeable that the water height in the tank is negative for the first five minutes (see attached sheet) which clearly isn't realistic, so there seems to be still something wrong with the model.Maybe it has something to do with the negaitive V^2?

5 replies

MarkBuckton10-MarbleAuthor
10-Marble
March 20, 2014

I think I have worked it out i.e. the outflow orifice was too small. That said given enough time, in my view, the tank should still drain. However even if you increase the time period over which the solve block calculates you still don't see the tank fully emptying which to me is strange but this must be some deep math reason that I don't understand as yet. But if someone knows why I am still interested.

Kind regards, Mark

Werner_E
Werner_E25-Diamond IAnswer
25-Diamond I
March 21, 2014

Its no surprise to me that the tank isn't empty at t=120 min but only a bit later. In case of the values in your sheet the tank is empty after 157,083 minutes, then the water height in the tank gets negative(!).

Simply let the solve block to solve until t=160 minutes to see it.

I also deleted the solve block for V, it wasn't necesary at all as V is simply calculated directly. The values of V are imaginary but as they are used later only squared, they yield a negative real result. Not sure if thats what you intended, though, and you might want to check the formula for V.

Is also noticeable that the water height in the tank is negative for the first five minutes (see attached sheet) which clearly isn't realistic, so there seems to be still something wrong with the model.Maybe it has something to do with the negaitive V^2?

MarkBuckton10-MarbleAuthor
10-Marble
March 21, 2014

Werner;

What can I say. If only my brain could work like yours your mathematical intuition astounds me. If you don't mind me asking what is your academic background?

Kind regards, Mark

Terms & ConditionsAccessibility statement