Would holes in flat surface increase net pressure? 1

[soiset]

I'm in structures, so bear with me.
We often deal in wind pressures on vertical surfaces, and obviously have to design systems to resist them. I am looking at a small job: an 18' high wall, over 100' long, with the bottom close to or on the ground. It is a temporary plywood wall meant to conceal construction while providing a sign for the coming store.

The sign will be canvas attached to a plywood face. My boss has suggested cutting holes, maybe 6" diameter, at an even spacing, and cutting the cloth cover at the bottoms of the holes so they could flap back and forth and allow the wind through.

It is inuitive that such holes would reduce the wind pressures on the wall, but hydraulics often bypasses intuition, I realize. Could such holes actually increase the lateral force on the wall?

 

[40818]

No, your intuition is right. Putting holes in a wall will not increase the force applied to it.

 

[Paul6]

It is tempting to rely on intuition, but before you do, you might want to look at previous uses of this hole cutting technique. In WWII US dive bombers used dive flaps to slow their descent toward target ships, providing more time to correct their trajectory before releasing their bombs. The dive flaps were optimized to provide maximum drag for a given weight and area. These flaps were perforated with dozens of large, circular holes.
There is a specific theory behind this, as I recall having to do with the toroidal vortices generated behind the holes. I believe it may be covered in Hoerner's bible on drag, called (surprisingly enough), DRAG.

Paul6

 

[Paul6]

Correction: The actual title of the book is Fluid Dynamic Drag by Dr. Sighard F. Hoerner. Very readable and nearly comprehensive.

 

[40818]

Hi Paul6, not sure if you have a different book by hoerner, i have 2, Aerodynamic drag (1951), and fluid-dynamic lift (1975). Let me know if he has another please.

However, when hoerner talks about dive brakes and their influence on the drag of a flat plate, he doesn't mean that by adding perforations to a flat plate you develop a greater load on it due to the effect of an induced irregular turbulent flow.
The reason for dive brakes having holes is to allow a passage of air to reinvigorate the turbulant air created by the flat plate being stuck out into the airflow, the same principle as high pressure bleed air ducts carrying out the same job on aircraft which employ them. A simple flat plate creates all sorts of control problems and vibrations (and depending on the tailplane set-up can dramatically reduce "good" air over the horizontal stabilizer.
The hole alleviated this problem whilst allowing sufficient drag to help the plane slow down and pilot take aim.

Wind pressures exerted on a structure depend on the speed of the wind as well as the interaction between the air flow and the structure. Since wind is air in motion the pressures it can exert are related to its kinetic energy. If the full kinetic energy is transformed into pressure then the resulting increase is given by the expression

q = 1/2 rho V^2

where rho is the mass density and V the velocity of the air. This is called the "stagnation pressure" and is the maximum positive increase over ambient pressure that can be exerted on a building surface by wind of any given speed. It is the basic pressure to which all other pressures over the structure are referred.

The wind speed to be used in computing the design pressure depends on the particular component of the building being designed. For structural purposes the maximum value is required and will vary with the geographical location. Meteorological records of wind speed are analysed to yield the most probable maximum that will be equalled or exceeded, on the average, once during a given period of time comparable to the life of a structure.

The distribution of pressures and suctions over a building depends largely on how it disturbs the air flow. In this discussion the datum from which all pressures and suctions are measured is the ambient pressure in the undisturbed air flow.

When wind strikes a simple structure such as a free standing wall, the streamlines in line with the wall are forced, to diverge and pass around the edges. The direction and magnitude of the original wind velocity are therefore altered by the encounter and cause changes in pressure.

Stagnation pressure is produced near the centre of the wall, but there is an increasingly steep pressure gradient towards the edges where the flow, diverted by the wall, regains its velocity in a direction parallel instead of perpendicular to it as before.

Behind the wall a different situation prevails. The streamlines of flow are unable to come together immediately because of the inertia of the air and a wake is left where they are separated from the wall. Air from the wake region is "entrained" by the fast-moving flow lines, thus reducing the pressure below the ambient pressure of the undisturbed flow and creating suction.

Pressure is not usually constant over a wall surface; but to simplify design procedures an average coefficient is specified for a given surface; when multiplied by the area and the basic pressure it gives the total force on the surface. The net force on the free standing wall would of course be the result of both the pressure on the windward side and the suction on the leeward side.

As the pressure differential is usually low compared to the forces due to wind velocity, it is usually ignored for quick sizing calculations.

Hope this helps.

 

[volpe2]

A better analogy would be putting holes in a parachute. Dive bomber air flaps are completly different problem.

 

[soiset]

Thank you for the informative answers, particularly those of 40818. 40818 mentions the increase in pressure against a wall near the boundaries; this phenomenon is addressed in ASCE 7-05, which governs structural loads, by labeling "end," "interior," and "corner" zones. These zones are defined in proportion to the structure's dimensions.

It was specifically the existence of these zones of higher pressures at the boundaries that suggested to me that it was possible that holes in the wall might create higher net pressures, because they would seem to create, potentially, "corner" zones around every hole. It seems this would be a matter of scale and proportion, though. If a wall were 60' high and 3000 feet long, it seems more likely that a hole, say, 45 feet in diameter, might cause the creation of a higher pressure band around the hole. Of course, this higher pressure would be more than offset, in terms of the total structure, by the complete absence of pressure in the area of the hole itself.

Conversely, very small holes in a wall could easily have no effect at all. I think that on the scale of 6" holes in a 18' high by 180' long wall, the holes could reduce stresses all over the structure.

 

[crysta1c1ear]

If the holes are at regular intervals can a wind then blow along the wall and create a musical instrument that vibrates at particular frequencies and just once in a blue moon does something you don't expect, like bridges that get blown down by not-so-strong winds, etc?

If my son winds down a rear window and I hit just the right speed, then I get a horrendous pulsing as air is sucked in and out of the car, even though I am driving at a steady speed.

Replace my car window with a hole the same size in a plywood wall, imagine a wind that moves at a speed similar to my driving, place the plywood wall at a distance from a building equal to the internal width of my car, etc ...
is that sufficient to create some uncomfortable conditions inside of the holed plywood wall?

At the right road speed, conditions in the car are unsupportable! (Earache.)

 

[GregLocock]

crysta1clear You've built a Helmholtz resonator. Detune the mode by opening the diagonally opposite window a crack.

Cheers

Greg Locock

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