Drage Forces on Flags
Drage Forces on Flags
(OP)
Does anyone know of any published data on the drag force cooefficients for different types and sizes of flags. The Australian wind code does not give any guidance.
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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RE: Drage Forces on Flags
RE: Drage Forces on Flags
One of my engineers has found a reference book with some published data on the drag resistance of flags. It comes from "Fluid Dynamic Drag" by Hoerner 1965 pp 3-25. This refernece quotes research done by Faithorne "Drag of Flags", ARC RM 1345 (1931). I would like to know if anyone has done any further research on it.
The reference gives two graphs, one where drag is expressed as a function of the material weight (tested on an aspect ratio=1.0 and wind speed of 30ft/sec) and the second shows drag expressed as a function of aspect ratio (tested for a height of 4ft and wind speed of 80ft/sec). As an example of the second graph, an aspect ratio of 1.0 gives a Cd=0.06 wheras an aspect ratio of 4.0 gives Cd=0.2.
If anyone would like to read the full test of this article (1 page) I will galdly email a scanned image.
RE: Drage Forces on Flags
RE: Drage Forces on Flags
getting an approximate pressure coefficient
on a flag.
this can be used to design the flagpole supporting
it.
RE: Drage Forces on Flags
Force exerted by flag: F = 0.0003*A*V^1.9
F = pounds
A = flag area in ft2
V = wind velocity in mph.
Another formula developed by Sighard Hoerner in a book called Fluid Dynamic Drag is this:
F = 0.000218*A*V^2 --- Where the same definitions apply, except that it was tested for flags having an L/W ratio of 1.5.
Compared to some other formulas available, these seem to give comparable results for smaller flags (say, 4x6), but become very conservative for larger ones or high wind velocity.
Hope this will help. Don't forget to add the wind on the flagpole itself. By the way, if a flag is mounted on a very tall building (or a pole exceeding 30 ft), then one must add a height and gust factor to the equation. See ASCE 7-95 for more on this.
wyf (The Polecat)
SESI
RE: Drage Forces on Flags
RE: Drage Forces on Flags
You didn't state the flag material, but the code provides values for nylon and cotton flags, and for polyester flags.
For nylon and cotton: Wf = .0010 x V^2 x Af^.5 x Ch
For polyester: Wf = .0014 x V^2 x Af^.5 x Ch
where V = design wind velocity in mph.
Af = flag area in ft^2
Ch = 2.01 x (z/900)^(2/9.5) where z = height of the
pole in feet.
It's probably a good idea to obtain this code, or if you have the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminares and Traffic Signals, 1994, there are mostly the same procedures except the AASHTO Standard doesn't have the flag load formulas.
RE: Drage Forces on Flags
The best bet with flags and banners is to specify the size of the halyard to be used on the flag. When the drag exceeds the capacity of the rope, it breaks and the flag blows away (hopefully long before the structure falls down).
It appears to me that the general public is quite forgiving when structures blow over. They talk of freak wind gusts and rogue waves, not appreciating that we think we know what we are doing. So loosing the flag will not reflect on your expertise.
Russell Keays