wind pressure calculate

Discussion in 'Hydrodynamics and Aerodynamics' started by kalvens, Jun 4, 2013.

  1. kalvens
    Joined: Mar 2010
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    Location: china

    kalvens Junior Member

    i just now calculate the wind pressure accrodance with abs rules(american bureau of shipping)
    equation : P = f Vk^2 Ch Cs N/m2 (kgf/m2, lbf/ft2)
    where
    f = 0.611 (0.0623, 0.00338)
    Vk = wind velocity in m/s (m/s, kn)
    Ch = height coefficient is 1.5
    Cs = shape coefficient is 1.3

    my question: wind pressure P unit is N/m^2
    wind velocity Vk unit is m/s
    i want to know what conversion unit?
     
  2. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    It is not clear what conversion unit you are looking for. The pressure in the ABS formula is given in N/m2, the windspeed is in m/s. f=0.611 kg/m3 is the air density. Hence all units are SI and coherent, there is no need for conversion factors.

    By the way, the coefficient Ch serves only if you are calculating the wind pressure for structures high above the ground level. At ground level and up to 15.3 m, Ch=1.
    Ch=1.5 is used for structures situated at a height over 110 m. Is that your case?

    Cheers
     
  3. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    To be quite correct, the value given for "f" is (air density)/2.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Oooops, right. :)
     
  5. Petros
    Joined: Oct 2007
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    Petros Senior Member

    that is correct, the basic force equation is half the mass times the velocity squared.

    or: [M/2]xV^2

    the other elements of the equations are adjustments for exposure or shape. In English units you get pounds per square ft, so the metric equivalent should be newtons per square meter.
     
  6. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Errrrr... My turn now to be pedantic. :p
    The equation you have written gives the kinetik energy, not the force.
    In order to get the pressure, a density (mass per unit volume) is required instead of mass M. ;)

    Cheers
     
  7. johneck
    Joined: Nov 2011
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    johneck Senior Member

    This is an intermediate calculation. q is dynamic pressure (1/2*rho*v^2), then we need a drag coefficient to turn this into a pressure, then an area to turn it into a force. The Ch and Cs may be a way to sort of turn this into a pressure, but often they roll a lot of empirical stuff into these calcs, so you should be careful about using it for something other than for what it was intended.
     
  8. DCockey
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    DCockey Senior Member

    Since folks are being accurate in terminology, dynamic pressure multiplied by drag coefficient also has units of pressure, but the result does not represent the pressure at any particular location on the body.

    dynamic pressure * drag coefficient = aerodynamic/hydrodynamic drag / reference area

    Depending on the application and definition of drag coefficient used the reference area could be the frontal area, the planview area, the side view area, the wetted surface area, or some other area.
     
  9. Petros
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    Petros Senior Member

    that is true to both issues, I was trying to avoid the complication of dynamic pressure, q. The basis for the equation is the half the mass times the velocity squared, the pressure is derived from the density (which includes the mass) to get a "mass rate" that gives us pressure. Essentially the pressure comes from the mass of the air flowing over the surface. So you need: 1) mass, 2) velocity, 3) surface area. And there are different coefficients for both lift and for drag.

    You are almost at the equivalent "flat plate" pressure with just dynamic pressure, q.
     

  10. kalvens
    Joined: Mar 2010
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    kalvens Junior Member


    thanks for your reply be suddenly enlightened for me , it is my neligence,
    the coefficient of Ch & Cs is upside down, the height coefficient is 1.3
    the shape coefficient is 1.5

    Values of Cs
    isolated structural shapes(cranes, angles, channels, beams, etc.) 1.5
    Values of Cv
    height(Meters) Ch
    46.0-61.0 1.3
     
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