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Frequency of unsteady pressure

  1. Jul 16, 2015 #1
    Hello everyone;

    What is the general frequency of unsteady pressure when we measure by the using transducer and diameter of pipe inside can be 2.2 mm?

    Can someone provide a proff also ? I mean scientific paper.

    Thank you
  2. jcsd
  3. Jul 16, 2015 #2


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    Staff: Mentor

    Unsteady pressure Doesn't necessarily have a frequency, but if you provide more detail about the problem you are facing we may be able to help better.
  4. Jul 16, 2015 #3
    During measurement of pressure differences by the using orifice and an electronic transducer, we can have some oscillation on diaphragm, I try to define this effect by the calculating pressure drop from vena contracta to transducer. I say that oscillation is 30 percent (maybe its too much, if someone has idea about it, could be nice). And I estimate frequency as 300 Hz.

    thickness of transducer 0.203 mm
    maximum volume deflection : 13.64 cubic milimeter
    maximum deflection in center:0.1798 mm
    surface area : 227.57
    after oscillation,
    volume deflection : 4.1 cubic milimeter (V1-V2)
    maximum deflection in center: 0.1798 mm
    Surface area : 227.54 mm2

    My problem is how I can define the velocity then?

    V(t) = (V1-V2) sin(wt) w= 2 pi f t I think according to formula t means time; f frequency;
    how i can define the time when frequency is 300 Hz?

    On the other hand does frequency different when Diapghram is in V1 position or V2 Position.

    After defining V here I can define volume flow rate Q =dV/dt m3/sn
    Q= A.c

    Here c shows velocity and I can calculate pressure loses then.

    I want to find realistic value instead of 300 hz please.

    2. Relevant equations

    3. The attempt at a solution
  5. Jul 16, 2015 #4


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    It still isn't entirely clear what you are trying to do. If your pressure fluctuations are on the order of 30%, that implies something is wrong with your wind tunnel. Then again, maybe not since we don't even know what you are measuring.

    In most cases, making pressure measurements will involve some fluctuation. The frequencies involved can come from a number of sources, like natural frequencies of the transducer diaphragm or the tunnel itself, blade-passing frequencies from the fan in the case of a closed-circuit tunnel, instabilities in the flow, turbulence in the flow. Those will all produce oscillations of varying frequency and amplitude. It's really not possible to say more based on what you've told us so far.
  6. Jul 17, 2015 #5
    It's fluid mechanic labarotary pipes. I just did assume. I wanna see what would be pressure loses under the worst condition so on. Please I need some help about it also
  7. Jul 17, 2015 #6


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    That still wasn't very descriptive but it sounds like you just have a very unsteady flow in your system.
  8. Jul 19, 2015 #7
    They all are assumptions, maybe not realistic. Is it a must to have turbulent flow when pressure is unsteady? I mean in transmission lines.
  9. Jul 19, 2015 #8

    jim hardy

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    i cannot follow your train of thought.

    if this is what you are up to


    i had the good fortune to wander around a power plant with a spectrum analyzer.

    The output of your DP transducer will contain several frequencies.
    1. Buffeting from turbulence in the pipe and particularly if your orifice is anywhere near an upstream elbow.
    2. Resonant frequency of the fluid in the sensing lines against the compliance of your DP transducer, simple spring-mass harmonic motion.
    3. Any mechanical motion of the sense lines in same direction they run, because the fluid is accelerated by that motion.
    4. Any motion of the dp sensor itself perpendicular to plane of its sensing diaphragm, same reason. We had to relocate a lot of sensors off non-rigid mounts and onto concrete floors or the ground. Speed of nearby rotating equipment will show up. We had 6,000 horsepower pumps whose 20 hz rotating speed showed up almost everywhere.
    5. Any control system actually affecting flow.

    When measuring water it is important to mount the transducer below the pipe so that air bubbles will work their way out of the sense lines. You must also maintain constant slope toward the pipe, same reason. Converse is so for a dry process.

    The sense lines and compliance of the sensor make a low pass filter , though not necessarily a low-Q one. So they'll attenuate turbulence at higher frequencies.
    Industrial DP sensors have a damping adjustment, sometimes hydraulic more often electronic..
    In extreme cases it is necessary to provide your own damping by throttling those local isolation valves immediately atop the DP sensor. In my experience that only was necessary with bellows type sensors which have large displacement.

    try searching on process noise analysis


    i have a friend in that business


    observe air lowers corner frequency and raises Q.
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