Drag force calculations and equations. Drag coefficients. Drag force is caused by fluid flow (liquid or gas) on an object.

Drag force on spheres, cylinders, screens, square objects, flat plates, tilted plates, beams, hemispheres, disks, people, bicycles, and other objects. Drag force is caused by a fluid (such as water or air; or any liquid or gas) impinging upon an object. The drag force is a function of the fluid velocity and density along with the object's reference area and drag coefficient. The drag coefficient may further be a function of the Reynolds number. Reynolds number depends on the fluid density, viscosity, and velocity as well as the object's characteristic length.

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Owner is Ken Edwards, Ph.D., P.E. Contact me at LMNO@LMNOeng.com or (740) 592-1890. 



The following table shows drag coefficients for various objects (photographs taken by Ken Edwards). More will be added.

Tree (C values from Munson et al., 1998)
A=Tree frontal area
C=0.43 if V=10 m/s (36.0 km/h, 22.4 mph)
C=0.26 if V=20 m/s (72.0 km/h, 44.7 mph)
C=0.20 if V=30 m/s (108 km/h, 67.1 mph)
Tree
 Flag (C values from Munson et al., 1998)
A=DL
C=0.07 if L/D=1
C=0.12 if L/D=2
C=0.15 if L/D=3
Flag
 Thin Rectangular Plate (C from Blevins, 2003)
Flow coming at the sign
A=DL
C=1.05 if L/D=1.0
C=1.10 if L/D=2.0
C=1.12 if L/D=4.0
C=1.20 if L/D=8.0
C=1.22 if L/D=10.0
C=1.22 if L/D=12.0
C=1.33 if L/D=17.8
C=1.90 if L/D=infinity
Thin rectangular plate
      

Solid hemisphere. Flow toward flat end.
A=πD2/4
C=1.17 (Blevins, 2003)

Solid hemisphere. Flow toward flat end
 Solid hemisphere. Flow toward round end.
A=πD2/4
C=0.42 (Blevins, 2003)
Solid hemisphere. Flow toward round end
 Hollow hemisphere. Flow into cup.
A=πD2/4
C=1.42 (Blevins, 2003)
Hollow hemisphere. Flow into cup

Hollow hemisphere. Flow toward round end.
A=πD2/4
C=0.38 (Blevins, 2003)

Hollow hemisphere. Flow toward round end
    



Drag force is computed as:   F = 0.5 C ρ A V2

The units in the drag force equation can be any consistent set of units, such as SI (International System) which uses meter for distance, second for time, kilogram for mass, and Newton for force or BG (British Gravitational) which uses feet for distance, second for time, slug for mass, and pound (force) for force.

Some drag coefficients are valid for a particular range of Reynolds numbers. Reynolds number is computed as: Re = ρVD/μ

We will have a Java applet here for computation of force with unit conversions.

Notation (ft=feet, N=Newton, lb=Pound (force), m=meter, s=second)
A = Reference area as (see figures above), m2 or ft2.
C = Drag coefficient (see figures above), unitless.
D = Characteristic length (see figures above), m or ft.
F = Drag force, N or lb
Re = Reynolds number, unitless.
V = Velocity, m/s or ft/s.
ρ = Density of fluid (liquid or gas), kg/m3 or slug/ft3. (note 1 slug = 1 lb/32.174 ft/s2)
μ = Dyanamic viscosity of fluid (liquid or gas), N-s/m2 or lb-s/ft2.


Visit LMNO Engineering's Fluid Flow Calculations website (www.LMNOeng.com) for pressure pipe, open channel, tank volume, runoff, and groundwater calculations.

References
Blevins, Robert D. 2003. Applied Fluid Dynamics Handbook. Krieger Publishing Co.

Munson, Bruce R., Donald F. Young, and Theodore H. Okiishi. 1998. Fundamentals of Fluid Mechanics. John Wiley and Sons, Inc. 3ed.


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www.FluidDrag.com site launched October 2007