Fluid Drag on a wire inside of a pipe

  1. Hi Folks,

    How would one go about calculating the drag force that would be applied to a wire (or in our case a small (0.130") fiber optic cable, that is inside of a small diameter pipe (.56")? Ive tried several methods, but none seem to match what we actually measure when experimenting. We have determined viscous coupling to be a large component of the drag, as the fluid temp (Water) does drasticly change our results.

    Thanks for the great forum!
  2. jcsd
  3. stewartcs

    stewartcs 2,279
    Science Advisor

    A wire inside of a pipe would form an annulus. Are you trying to find the pressure loss due to the wire inside the pipe? If so, just use the effective hydraulic diameter created by the wire inside of the pipe.

    Here is an example:


  4. Thanks for the quick reply,

    We are not so concerned of the pressure drop across the anulus, what we need is to determine how hard we will have to pump in the annulus, to apply some force (f) to the wire inside.

    What we are trying to do, is lift a wire (without) direct mechanical contact, with fluid friction only, through means of some "velocity" string or such.

    I'll make a sketch and post it.
  5. im sure you did the age old: D = 1/2 * rho * v^2 * S * Cd

    also since the diameter of your fiber optic cable is nearly 20% of the diameter of the pipe, you may need to consider there is a large restriction in the pipe. there's probably some study on that....probably from a civil engineering site (something similar to thin vs. finite thickness airfoil theory, but probably less involved math)

    how are you measuring the drag? is the wire completely isolated from the pipe, or are you inadvertently measuring the drag along the inside the pipe as well? (see link above)

    also, part of the viscosity. of course viscosity will play a significant role, your velocity is quite low, therefore your Reynolds number is low, and by definition with low Reynolds number, viscous effects play a significant role. perhaps look at viscous drag or skin friction drag in fluid dynamics books. flat plates are the norm for that analysis, although there must be adaptation to circular cross sections

    just thought of this.....is the cable perpendicular to the pipe or parallel? if its parallel, i can see how the D = 1/2 * rho * v^2 * S * Cd wont help much
  6. Hi,

    The cable is parallel to the pipe, here is a sketch of whats going on. We have the ability to pump at very high pressures/rates (upwards of 10k PSI)

    Attached Files:

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