Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor
    Gold Member

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook