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Fluid Dynamics interesting critical problem

  1. Feb 18, 2010 #1
    I am having a tank at the bottom of which a pump suction line is connected..

    there is another parralel line to that bottom line which is coming from another tank and open to atmospheric pressure.

    Please note that this second pipe is connected to the bottom pipe; not to the first tank...

    there is only flow inlet to the first tank.

    what will happen when the pump will start??

    will all the fluid in the second tank which was initially present will be sucked??

    or the first tank will compensate that fluid by passing some fluid to second tank to maintain the same level as in the first tank ???

    Please suggest...

    Please find the detail drawing for explaination.....as an attachment...


    Attached Files:

    Last edited: Feb 18, 2010
  2. jcsd
  3. Feb 18, 2010 #2

    Vanadium 50

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  4. Feb 18, 2010 #3
    Thanks bro...

    i m a new user....

    Please go thru my problem .. i have attached a drawing for reference....
  5. Feb 18, 2010 #4


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    i've never posted here before, but i can make a guess at this problem

    you want to know what happens when the pump starts?

    there is no net loss of fluid, so the amount of fluid in the pipes and tanks stays the same in total.
    assume the tanks are large, and Q isn't so massive as to drain the tanks in an instant.
    then the system will reach some sort of equilibrium, but the question is do the tank levels stay the same, or do they change?
    my guess is that they stay the same, since atmospheric pressure would keep the water levels of both tanks the same.

    good luck, hope i have helped a bit.
  6. Feb 19, 2010 #5
    Thanks 4 ur response...

    But i think, when the pump will start sucking fluid from both the tanks; how can the flow from 1st tank to 2nd tank is simultaneously possible when the fluuid from 2nd tank is also being sucked by pump...

    u have got my concern; what i care about is the level of fluid in the 2nd tank.....

    ha...i agree that because of atmospheric pressure the level should remain same...but we have to consider the presence of Pump at the bottom....

    have u gone through my sketch???

    Thanks and do reply....
  7. Feb 21, 2010 #6
    Theoretically both tanks will see an equal drop in water head. But in effect, tank 2 will have little or no change in head, depending on the length of the connection pipe.

    IF Qin = Qout
  8. Feb 22, 2010 #7
    r u sure about this??

    because what i feel; there is only 1 directional flow is possible in a pipe...

    so in the 2nd bypasss pipeline...if liquid strat coming out towards the pump...can simultaneously the liquid from 1st tank will go to the 2nd tank...

    refer figure attached....2 tank problem...
  9. Feb 22, 2010 #8


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    it is possible.. for example:

    --Raising tank 2 level:
    1.5Q of water flowing out of tank:
    0.5Q pushes up into tank 2, filling it up, 1.0Q goes down to the pump

    --Lowering tank 2 level:
    0.5Q from tank 1, 0.5Q from tank 2, combine for 1.0Q to the pump

    doesn't have to be "two directional flow" but just think that the direction of flow can change over time. tank 2 might go down slightly, then go up slightly, but with no visible change in water level
  10. Feb 22, 2010 #9
    your explaination sounds convincing...

    but the suction of the pump is too high and the 2nd tank is diameter is very small....

    so will this make any difference...

    weather it will be dependant on the suction of the pump??
  11. Feb 22, 2010 #10
    greater suction means higher mass flow, resulting in an increase in discharge Q...
  12. Feb 22, 2010 #11
    According to me:
    Initially water from both tanks will fall down equally(actually depnds upon pipe lenghts upto junction) but since you give continous input to tank 1, so pressure in it will never decrease on an average but still water was flowing from tank2 so pressure in it will fall down gradually. This will create pressure difference between tank 1 and tank 2 and water then should goto tank 2 but at the same time pump is doing work and it will also create a pressure difference, so water from tank 1 should be sucked. Since flow from pipe is possible only in one direction , so the pressure difference which is more, there will the water flow. Normally pressure difference created by the pump would be more than that created by tank 2 so tank 2 will become empty after some time.
  13. Feb 22, 2010 #12
    great answer...

    i was exactly worried about this....but after getting empty...will the 2nd tank will never get filled ???
  14. Feb 23, 2010 #13
    hmmnn... NO
  15. Feb 23, 2010 #14
    the levels will be the same, given that the supply pipe manifold is not a restriction of any sort
    pumps don't suck, air pressure pushes the fluid. heads are equal (same level), as will be atmospheric pressure. as the pump moves fluid out, it will be replaced by tank 1. if tank 2 were low (lower head), fluid would flow into it from tank 1 (higher head). tank 1's head could be slightly higher than the normal level with the water running in.

    that said, pipe dimensions, pump flow, line restrictions, cavitation, all would potentially change the answer
    more variables need defined.

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