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    Hi, I have a fluids problem.

    A design contest features a sumbarine that will travel at a steady speed of Vsub=1m/s om 15degC of water.
    The sub is powered by a water jet. This jet is created by drawing water from an inlet of diameter 25mm, passing this watter through a pump and then accelerating the water through a nozzle of diameter 5mm to a speed of Vjet.
    The hydrodynamice drag force can be calculated using F=CdAp(density*(Vsub^2)/2) where the Cd=0.3 and Ap=0.28m^2.
    Specify an acceptable value of Vjet.

    Please help.
    Thanks
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    (Original post by Hello1992)
    Hi, I have a fluids problem.

    A design contest features a sumbarine that will travel at a steady speed of Vsub=1m/s om 15degC of water.
    The sub is powered by a water jet. This jet is created by drawing water from an inlet of diameter 25mm, passing this watter through a pump and then accelerating the water through a nozzle of diameter 5mm to a speed of Vjet.
    The hydrodynamice drag force can be calculated using F=CdAp(density*(Vsub^2)/2) where the Cd=0.3 and Ap=0.28m^2.
    Specify an acceptable value of Vjet.

    Please help.
    Thanks

    Just balance forces. THrust generated by the jet = mass flow times change in velocity. THe inlet and outlet velocities are linked by mass flow in = mass flow out and with density constant the only unknown when you balance will be the exit velocity of the jet. Mass flow = density x area of inlet x inlet velocity. (ie replace inlet velocity in the rate of momentum equation at the start by exit velocity x exit area / inlet area.
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    (Original post by wdywuk)
    Just balance forces. THrust generated by the jet = mass flow times change in velocity. THe inlet and outlet velocities are linked by mass flow in = mass flow out and with density constant the only unknown when you balance will be the exit velocity of the jet. Mass flow = density x area of inlet x inlet velocity. (ie replace inlet velocity in the rate of momentum equation at the start by exit velocity x exit area / inlet area.
    Thank you.
    How about the hydrostatic equation?
    Also i do not know the velocity of the mass going into the jet.
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    (Original post by Hello1992)
    Thank you.
    How about the hydrostatic equation?
    Also i do not know the velocity of the mass going into the jet.
    Don't need the hydrostatic equation

    Anytime you see the inlet velocity in a formula replace it by exit velocity * (exit area) / (inlet area)
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    (Original post by wdywuk)
    Don't need the hydrostatic equation

    Anytime you see the inlet velocity in a formula replace it by exit velocity * (exit area) / (inlet area)
    Hi, sorry but i keep gettint the wrong answer.
    Can you show me how?
    Sorry for the poorness of my fluids knowledge
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    What answer did you get? without checking what I've done, I've got the jet velocity to be 47.2m/s.
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    I got 86.6.
    you got it right.
    when you say thrust = mass flow x change in velocity, what did you use as change in velocity?
    Sorry just tryong to get my head around it.
    Thanks
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    (Original post by Hello1992)
    I got 86.6.
    you got it right.
    when you say thrust = mass flow x change in velocity, what did you use as change in velocity?
    Sorry just tryong to get my head around it.
    Thanks
    Change in velocity = Vjet - Vinlet and again,
    replace Vinlet by Vjet*(A-outlet/A-inlet) (You can quickly get this by realising that the by continuity the mass flow in = mass flow out)

    As density of the water is constant then A1V1 = A2V2 (1- inlet, 2 - outlet)

    If I remember correctly the drag force on the sub worked out to be about 42N
    On the other side, the thrust worked out to be something like 0.0189 Vjet^2
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    (Original post by wdywuk)
    Change in velocity = Vjet - Vinlet and again,
    replace Vinlet by Vjet*(A-outlet/A-inlet) (You can quickly get this by realising that the by continuity the mass flow in = mass flow out)

    As density of the water is constant then A1V1 = A2V2 (1- inlet, 2 - outlet)

    If I remember correctly the drag force on the sub worked out to be about 42N
    On the other side, the thrust worked out to be something like 0.0189 Vjet^2
    I understand everything you say except how you got 0.0189Vjet^2 from A1V1=A2V2

    Sorry for my lack of knowledge,
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    (Original post by wdywuk)
    Change in velocity = Vjet - Vinlet and again,
    replace Vinlet by Vjet*(A-outlet/A-inlet) (You can quickly get this by realising that the by continuity the mass flow in = mass flow out)

    As density of the water is constant then A1V1 = A2V2 (1- inlet, 2 - outlet)

    If I remember correctly the drag force on the sub worked out to be about 42N
    On the other side, the thrust worked out to be something like 0.0189 Vjet^2
    Thank you for your help.
    I just got the answer. I was being stupid.
    Thanks again so much!
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    No worries, just glad I got it right and wasn't leading you down th wrong path!
 
 
 
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