Help Clarify: Wind Load or Wind Sheer?

[ReverseEntropy]

Trying to determine wind load on a smoke stack, I have read many a (fluid mechanics) text that only seem to address wind sheer. Some evaluate this wind sheer force as a drag force, and others use a similar approach - only without a drag coefficient in the (very similar) equation. So, which is correct? (Either way, they all address wind sheer only) But...

What about a normal force from the wind? Wouldn't any windward face of an object experience a normal force distributed longitudinally along said windward face? (Am I wrong? Am I misunderstanding something?) If I am correct, how does one equate wind speed to this normal force?

Any explanation or links to websites that may help clarify further would be appreciated.

Thanks in advance!

 

[JStephen]

Wind load on a smokestack would generally come from a building code. You might check ASCE 7, for example. There's also an ASME standard for stack design. It's not necessary to determine wind loading from fluid mechanics principles.

Flexible stacks can vibrate in the wind in connection with vortex shedding, and that should be considered in the design as well.

 

[ReverseEntropy]

JStephen, I appreciate this advice. However, I do not have access to ASCE# 7, although I have seen exerpts onlne. It is an excellent reference.

Also, I know that applying the fluid mechanics method is an oversimplified approach, geared for an academic exercise...not for professional practice. (Yet, I'd have to convince my boss.) As such, I am applying the most conservative Cd values (as I have found some conflicting data within the same text, for one).

In short, I did find an answer to my question. There is an overall drag coefficient (Cd) which is the sum of the friction drag coefficient (Cdf) + the pressure drag coefficient (Cdp). It seems, Cdf accounts for shear while Cdp accounts for what I was wondering about - that which is incident upon a windward surface (i.e., normal to the airstream).

Thanks again for your reply.

 

[MiketheEngineer]

You might want to post on the Structural Forum. And get your cheap boss to buy the latest ASCE manual

 

[prost]

In almost any college level fluid mechanics book there will be a discussion of the drag on a cylinder. You'll normally see a chart of drag coefficient as a function of the Reynolds number. In that drag coefficient is both the friction and pressure drag contributions, and normally they are not separated. The Reynolds number, you may recall, has two effects in it--the inertia forces (causing pressure drag) and the frictional forces (causing friction drag). Once you compute the Reynolds number, look up the Cd (drag coeff), you can compute the total drag force directly from this:
Drag=Cd*A*(rho*V*V/2); rho is the fluid's density, V is
the fluid's velocity, A is the cross sectional area of
the cylinder. Be careful of the actual definition of
Cd--there might be a different definition than the
one I've shown.

You're looking for something like Figure 2

http://www.princeton.edu/~asmits/Bicycle_web/blunt.html

or Figure 3 here

http://ctr.stanford.edu/ResBriefs01/wang.pdf

If you finally a really detailed discussion, you might find curves that take into account surface roughness.

 

[JStephen]

One of the issues you run into in trying to design from scratch is exactly what wind speed should be designed for. The building codes specify a design wind speed, but also specify how that wind speed is to be converted into a design pressure. For example, ASCE 7 uses wind loadings based on a 3-second gust. Shorter time periods will have higher peak speeds, so which do you use?. The pressure will vary with exposure, with height, etc. The wind speed itself is based on certain probabilities of occurance, and assuming different probabilities will change your pressure.

If at some point, you're asked to certify that the design meets a building code or the ASME stack standard, it would be advantageous to have it in hand.