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Corresponding Pressure gradient with flow velocity

  1. Jan 14, 2016 #1
    I have some questions concerning hydraulic engineering. I'm currently working an simulating laminar flow.
    This laminar flow is induced by a pressure gradient. The assumed length is 1 meter, therefore the pressure gradient is equal to the actual pressure in reference with zero.

    What are typical pressure gradient used in flow? I'm not certain but I think the imposed pressure gradient has units N/m². So typical values are around 1 N/m² to impose flow or am I wrong about this?
    On the other hand, I'm working on oscillating waves. These wave are characterised by a certain period T and velocity amplitude. The corresponding pressure gradient can be found using the velocity gradient.

    ∂P(osc)/∂x = -ρw*∂u(osc)/∂t

    The oscillating flow is described as with t, the time step and the density of water ;

    u(osc) = U(ampl)*cos(2*π*t/T)
    ∂P(osc)/∂x = ρw*U(ampl)*2*π/T

    If the amplitude of the velocity U(ampl) is equal to 1,5 m/s. What would my pressure gradient be equal to? I'm a little bit confused because I find a pressure gradient of 1308 kg/(m²s²) but I think I did something wrong.

    Many thanks
     
  2. jcsd
  3. Jan 14, 2016 #2

    boneh3ad

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    First, a N/m2 is a pascal. That is pressure. The units of pressure gradient are Pa/m in SI units. Second, pressure gradients can come in all sorts of magnitudes depending on the situation. It could range anywhere from 0 Pa/m, for example in a Blasius boundary layer, all the way up to several TPa/m, for example across a shock wave. What is your physical situation here? What sort of flow are you dealing with?
     
  4. Jan 14, 2016 #3
    I'm working with open channel flow. If I use pressure gradiënt of 103 Pa, I obtain speeds of 70m/s equivalent with 250 km/h which look a little bit unbelievable hehe
     
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