Drag Force determination 3

[TowerEngineer]

I am a civil/structural engineer that specializes in the design of non-building structures to resist wind and seismic forces. My question is in regard to wind drag coefficients for different shape and size objects that are typically used in the aeronautical engineering community.

Can anyone refer me to a concise and authoritative source if information with regard to drag coefficients commonly used in aeronautic design? My goal is to use this information to reverse a recent trend in civil and structural engineering toward drag coefficients that I believe are unrealistically high. I am interested in Cd for wind velocities in the range of 40 to 200 MPH.

 

well, in aero, we drag = cd*1/2*density*V^2*reference wing area

Also drag is divided into parasite and induced components. Parasite drag is due to the shape of the aircraft, and remains pretty much constant almost always. Induced drag is due to the lifting bodies i.e wing and dependson many factors such as the vortex flow.

If you are really interested in aerodynamic drag then try the following book :

THRUST AND DRAG:it's prediction and varification.

Donno who wrote it but if you put this name in Amazon book search, it'll come up.

 

[vtl]

simple estimation is this.
if you know or have the estimate value of the wind speed.
find the Reynolds number.
then look up graph of Reynolds number vs Dragcoeff for Cd.

 

[vtl]

oops
then use the formula given by Aero.

Fd=1/2rho*(Area_subjected_to_wind)*V^2*Cd

there U go.

 

[TowerEngineer]

AeroStudent and vtl:

We engineers have known of the basic force equation F = 1/2(d/g)V^2 since the time of Sir Isaac Newton (we all stand on his shoulders).

Since the 18th century, we have also known that this basic equation must be modified by a drag coefficient. The primary difficulty has been and continues to be determining exactly what numerical value to apply to different shapes and sizes of bodies subject to air flow. We in the structural engineering field of building and non-building design have certain values of Cd which are prescribed in the wind design codes such as ASCE-7.

In recent years (and especially the last 18 years) the lateral wind force applied to building structures has increased rather significantly.

My particular area of specialty is the design of guyed and self-supporting tower structures. As stated in the last sentence of original inquiry, what I am interested in is a concise and authoritative reference (such as a text book or transactions report) for drag force coefficients for obects of various shapes and sizes that are typically applied in the range of 40 to 200 MPH.

My question then relates to values of Cd typically used in aerodynamic and mechanical engineering by those engineering practioners outside the field of civil or structural engineering (for instance, mechanical or aeronautical engineering).

What I am looking for is something similar to Table 11-1 of 'Engineering Fluid Mechanics' by Roberson and Crowe, Houghton-Mifflin Co., 1975, p. 343.
This table is based on the work of Brevoort, Lindsey, Morrison, Roberson, Rouse, and Scher. Their work pre-dates 1975. Table 11-1 applies to Reynolds numbers, Re > 10^4.

I am also interested in the influences of solidity ratio and shielding effects of the object on drag coefficient.

It would be greatly appreciated if someone could provide similar references for experimental works / textbooks published subsequent to 1975.

 

[Miper]

Hoerner's book "Fluid Dynamic Drag" remains the most complete source of general drag coefficient data. It is old, but air is still the same. It takes several readings to get the hang of it.

Perhaps the wind force increases you mentioned are due to higher assumed velocities, caused by a desire to make structures more resistant to extreme weather.

Hoerner's book can be bought at
HOERNER FLUID DYNAMICS
P.O. Box 21992
Bakersfield, CA 93390
Phone/Fax: (661)665-1065

See

http://members.aol.com/hfdy/drag.htm

 

[WKTaylor]

TowerEngineer...

have a peek at:

ISO 4302 Wind-load Assessment ~ cranes [and similar constructed towers]

ISO 4354 Wind action on structures Regards, Wil Taylor

 

[poetix99]

Tower Engineer, I think that you make an important distinction when you mentioned "influence of solidity ratio and effects of shielding".

It is not sufficient to simply look up a Cd for the appropriate cross-sectional shape as your "beams", as others seem to have suggested. WKTaylor's references appear to be the right places to look, but perhaps they are too close to the industry whose particular values you wish to challenge.

But why are you questioning these values? Are you familiar with the history of the evolution of these standards? If standards bodies in civil engineering are like ASME (with which I am more familiar), there are many intelligent, experienced engineers who are on these committees. Did I mention CONSERVATIVE? Yes, without a doubt, and for understandable economic reasons (legal liability), but predictably with some technical basis.

If you look in enough places, you might find values that you are "more happy" with. But would you design a structure that is not in compliance with the relevant standards of your industry?

Do not re-invent the wheel, nor "tilt at windmills".

 

[vtl]

His question was on drag force, drag coeffiecient. It got nothing to do with foundation work.

 

[poetix99]

Dear vtl,

Huh?

I'm not sure that any of the posts are alluding to "foundation work"; I know that mine was not (regardless of whether it was otherwise on the mark).

My point (one of them) was that if several structural elements are sufficiently near each other that the "downstream" elements are in the wake of the upstream elements, the aggregate drag is different from (less than) what would be predicted from calculating one element in an otherwise undisturbed free stream and multiplying by the number of similar elements. The whole is different than the sum of the parts, if you will.

 

[vtl]

Sorry to mention but you are talking somthing a bout the rigidity of the structure. Anyway the formula given was just an approximation, and for a certain that you would get a higher drag than you would expected, but then that is when a safety factor comes into play. That is when guyed wire or other means of supporting structure comes in handy.

As You can see if you complicate the problem too much you need to perform very complicate buckling analysis as well.

No text book will give you the exact value of Cd for a realworld shape, since wind change it loads and directions at different times. As you can see vibration now comes into play since we have dinamics loading.

So I recommend first go with the fundamental approximation to have a better saftety factor (this ignore the fact that costing inlvolved), then approximate by superposition method, the next to do is compare that result with the computer finite element method.

Good Luck

 

[sciguyjim]

TowerEngineer,
I'm not sure, but I found some data that sounds like it might be what you want. This was in a library book and I have a hard time believing you couldn't find better info in reference books, but here goes. There are over 30 shapes listed here so I'll just list a selection for now and you tell me if youu want more.

Porous parabolic dish of diameter D; Frontal area=[Pi/4]*D^2
porosity=open area/total area
porosity = 0.0 0.2 0.5
wind toward front >) Cd=1.42 1.20 0.82
wind toward back )< Cd=0.95 0.90 0.80


Average person standing CdA=9ft^2
sitting CdA=6ft^2
Crouching CdA=2.5ft^2


Tree Frontal Area Velocity=10m/s Cd=0.43
&quot; &quot; &quot; Velocity=20m/s 0.26
&quot; &quot; &quot; Velocity=30m/s 0.20


Cube, length of side =D, wind toward flat face, A=D^2
Cd=1.05, Re>10^4


Upright rider on bike, A=5.5ft^2, Cd=1.1


Well, does any of this stuff look helpful? It gives Cd values (and some Re values) for 2 & 3 dimensional shapes and some common things seen in a city (building, fluttering flag, passenger train, trucks, birds). Let me know.
Jim.

 

[JohnCT]

I have been looking for some references on the wind drag on flags. So, I would be interested to know what the title of the library book that Jim (sciguyjim)was refering to.
John

 

[TowerEngineer]

Hello sciguyjim:

Sorry I didn't see your post earlier. Received a message from Eng-Tips that they have had trouble forwarding mail through my e-mail account.

I have been looking for more textual references that list drag coefficients for a variety of objects with different shapes.

Much the same as JohnCT, I am extremely interested in knowing the title of the library book from which you extracted the listed drag coefficients.

That is exactly the type of data that I would like to see.

I have Hoerner and Milne-Thomson books on fluid dynamic drag, as well as Roberson-Crowe text and some others.

Thanks, TowerEngineer

 

[sciguyjim]

TowerEngineer & JohnCT,

I'll try to find the library book and post the info here if and when I get it.

 

[sciguyjim]

TowerEngineer & JohnCT,

I'm sorry to report that I haven't been able to locate the book I had back in August. I was hoping the library had some record on their computer of items previously checked out but they don't. I tried searching the card catalog but couldn't find it. I won't forget, if I ever do find it, I'll post it here.

 

[radomir]

As far as I know a lot of testing (wind tunnel testing) has been done lately on so called “blunt bodies” (where “lately” = 25 years at least). Therefore experimental data on all kind of strange non-meant-to-fly-objects’ Cds must exist somewhere in some “Blunt Bodies’ Aerodynamics”.

The guilty object is probably that (in)famous flatter destroyed bridge but that is even more complicated matter than purely a Cd. Suddenly there is a Cl too, causing troubles like flying roofs and some other UFOs and in the extreme cases twisting and bending bridges like pieces of a (toilet) paper.

Ladies and gentlemen keep in mind that slight difference in pressure (if the surface is large enough) keeps those metal birds flying. In other words forces could be tremendous and if misfortunately oscilating-pulsing resonantely (maybe this word does not exist but you should forgive me I am not of an Anglo-Saxon origin) with a natural frequency of the body that could result in a spectacular phenomenon even if the force is not so large.

And at the end to quote a gentleman from some of these forums:
“The more I know the less I am certain of.”

 

[sciguyjim]

I found a database of drag coefficients for 6 catagories of objects. Maybe this will help someone. Some of the data looks similar to what I saw in the library book I haven't been able to find:

Description

Table 1 reports some typical values for widely studied bluff bodies.

Table 2 is a summary of typical drag coefficients of streamlined bodies.

Table 3 is a summary of drag data for airborne systems and road vehicles.

Table 4 reports a summary of drag data for a human being performing some well known physical activities.

Table 5 reports drag coefficients and total drag forces for some automobiles.

Table 6 reports data for other systems, including towers and tall buildings.

http://www.aerodyn.org/Drag/tables.html

 

[sciguyjim]

Here is some related stuff linked to the previous article:

Drag Data

Viscous Drag of Aerodynamic Systems
Subsonic/Supersonic Drag with LFC
Drag Coefficients of Bluff Bodies
Drag Coefficients of Streamlined Bodies
Drag Coefficients of Aerodynamic Systems
Drag Coefficients of Human
Drag Coefficients of Automobiles
Drag Coefficients of Other Systems
Drag Levels of Fixed/Rotary Wing Aircraft
Drag Levels of Road Vehicles

Lift-to-Drag Ratios

L/D of Miscellaneous Systems
L/D of Jet Aircraft
L/D of Birds

http://www.aerodyn.org/Resources/database.html

 

[pblanco]

I came across this thread like a few others who wanted more information on the wind drag of flags and streamers - and there is not a whole lot out there. I am pleased to report that I have found a good reference on this subject: an article by J. M. Roehm in the Jan-Feb 1986 issue of Building Standards Magazine, page 10. (You can obtain a fax copy of this article by calling the nice folks at

http://www.icbo.org,

who run the magazine).

The article presents drag force results from towing flags behind an airplane and they come up with the following formulae which are DIFFERENT from the Hoerner and previous estimates, and generally give lower drag force values:

V = wind speed, Area = one-sided area of flag

Nylon: Drag(pounds) = 0.001 x (V,mph)^2 x Sqrt(Area, sq. ft)

Polyester: Drag(pounds) = 0.0014 x (V,mph)^2 x Sqrt(Area, sq. ft)

For those of you who prefer MKS units, like myself, this converts to:

Drag(Newtons) = 0.073 x (V,m/s)^2 x Sqrt(Area, m^2) for nylon, with polyester flags giving a force 1.4 times greater.

Note that unlike traditional formulae for aerodynamic drag, here the force is proportional to the _square-root_ of area, not area directly, so a &quot;drag coefficient&quot; is not really appropriate. This makes some kind of sense to me.

There are few papers on this subject and independent confirmation would be nice, as would experiments for a wider variety of flags, pennants, and streamers, so anyone out there with wind-tunnel access - here's a project for you!