423b2e
423b2e

8.31.2011

the Bernoulli / Venturi question


Daniel Bernoulli

In my post on southern vernacular architecture, one of my resources cited the use of the Bernoulli principle in dog trot architecture. Soubriquet, one of my esteemed readers, took umbrage at this statement and responded thusly:

"However, an intensely argumentative donkey, the person quoted who says the dogtrot .."was cooled naturally by the Bernoulli Effect", is, as we say, talking out of his arse.
Bernoulli's Principle describes a reduction in pressure, proportional to the velocity of flow within a fluid.
Any cooling in a dogtrot or breezeway is down to heat energy being used to promote a phase change of moisture from liquid to vapour. Humans and animals will cool in a breeze, but dry materials will not change in temperature.
The reduction in pressure will have a tiny contributory effect, but barely measurable.
Bernoulli's NOT a cooling mechanism. "

Bulletholes, another esteemed reader, responded to Soubriquet's comment:

"Souby, I think what they intended was that even if the breeze was blowing in a direction perpendicular to the breezeway, the resulting drop in pressure (Bernoulli's?) at the ends of the openng would still create a draft not otherwise available. but I'm way over my head here, and relying on an answer I got wrong on a question when I was in the sixth grade, 1968. It still haunts me. "

After extreme googling, I'm going on the record to say both readers are somewhat right and somewhat wrong in their assumptions. This is what I've learned about the Bernoulli / Venturi question as it applies to architecture.

"The Bernoulli Effect, also known as Bernoulli’s principle, describes the relationship between the flow speed and pressure of a fluid; most notably, an increase in speed occurs with a decrease in pressure. Published in 1738 by Dutch-Swiss mathematician Daniel Bernoulli, the principle can be applied to both hydrodynamic systems (i.e. fluids move faster through a constricted pipe) and aerodynamic systems (i.e. air pressure helps an airplane achieve lift). In architecture, the Bernoulli Effect is the basis of many ventilation systems, especially passive ventilation systems that utilize no mechanical energy. Designers often employ Bernoulli’s principle for
passive cooling and passive ventilation to generate suction or control speed of air flow with windcatchers and deliberately sized windows and openings

An important concept in understanding how wind-generated pressure differentials produce air movement is "Venturi action," which is based on the Bernoulli effect. From Bernoulli's theorem, the pressure of a moving fluid decreases as its velocity increases. Figure 34 shows a funnel-shaped tube that opens to a side tube. When air is channeled into the larger end of the funnel, it accelerates as it passes through, owing to the reduced open area through which the same volume of air must pass in the same period. This increased airspeed lowers the pressure in the airstream at A with respect to the atmospheric pressure at B in the lower part of the side tube. Thus air is drawn up the side tube by the pressure difference which is proportional to the square of the velocity. This concept can be used in a variety of ways to provide steady streams of air through buildings.

Air movement by pressure difference: air moves from an area of high air pressure to an area of low pressure. Like heat, which moves from a hot zone to a cooler one, air seeks an equilibrium. On a large scale, the development of high and low air pressure areas creates wind. But as wind moves around obstructions, it creates mini-zones of relatively higher and lower air pressures. These can be tapped to bring airflow through buildings.

  • Wind blowing towards a surface – the windward side – exerts a positive pressure on it. As it hits the surface, the air stream splits and moves around the sides of the obstruction. The air stream then melds together again the far side of the obstruction, re-establishing its flow some little distance behind it. Immediately behind the obstruction – the leeward side – the air pressure is negative. Air is pulling away from the obstruction, which results in some suction. Think of sheltering behind a wall in a high wind: although the wind is not hitting you in the face, your hair and clothes flap behind you, pulled by the suction of the negative air pressure.
    In a light breeze, the negative air pressure generated in the lee of obstructions like a building is mild, not enough to ruffle clothes. But it is enough to suck air out of any openings in the building. This is one source of suction a passive solar design can exploit this.

  • The Venturi effect: when air is channeled into a constricted opening, its speed increases. Than, according to Bernoulli's principle, if a stream of air speeds up, its pressure drops. A cunning designer can use these principles to speed up air flows and/or generate suction. For example, when wind blows towards an open window, some of the air stream enters the opening. If the window is small, the Venturi effect means the air will force through under some pressure and so speed up. On the other hand, if the window is large, the pressure of the air flowing through it will drop, lessening its speed. Obviously, a window or an opening will not create the desired air movement in a room unless an air outlet of some sort is also provided. Experience has shown that air movement is faster and steadier when the area of the openings on the leeward side of a structure is larger than the inlets on the windward side.

From RDG: So.... from what I'm understanding, when wind blows around an obstruction, mini-areas of low pressure and high pressure are formed creating a 'suction' action. By channeling this air through a constricted opening, air speed increases, especially if the openings on the leeward side (negative pressure side) of the structure are larger than the windward (positive pressure side). If I build the leeward opening of my dog trot larger than I build the windward opening, I am encouraging an increase of air speed through my dog trot. More breeze and I feel cooler, evaporation rates non-withstanding.

Class is dismissed!
xxx

12 comments:

gz said...

:D

Bullets said...

Very good work Red. High marks!

goatman said...

Wow, its obvious you are a teacher! Could fill the hour with this rapp.
My physics book describes it thus: Two ships going the same direction at the same speed , say 100 yards apart, will create between them a flow of water faster in the opposite direction than that accounted for by the speed of the ships or any net flow in the ocean. Wheee

soubriquet said...

Good try, Mule.
Pity you quote a text by a guy who'd fail kindergarten level physics. His teachers would roll their eyes and bang their heads against the wall, "Yeah, I remember that kid, he had less sense than a lump of lard..."

I repeat. Bernoulli's principle has nothing whatsoever to do with why you would feel cooler sitting in a breeze, whether that breeze is in a dogtrot or a breezeway.

Your quoted guru's section on the venturi effect shows just how hopelessly confused he is.
"If the window is small, the Venturi effect means the air will force through under some pressure and so speed up. On the other hand, if the window is large, the pressure of the air flowing through it will drop, lessening its speed."

Here's a clue: 'increased velocity = reduced pressure'.

For 'pulled by suction', you might want to reflect that there is no mystery force called "suction", there's simply greater pressure and lesser pressure.
A fluid will flow from areas of higher pressure toward areas of lower pressure. That's what a breeze is.

Oh my.
"If I build the leeward opening of my dog trot larger than I build the windward opening, I am encouraging an increase of air speed through my dog trot."
No. all you are doing is making the air slow down as it heads toward the leeward opening. If your dogtrot narrowed in the middle, like a wasp-waisted pinch-point, then at that pinch point, the air velocity would be greatest, and the effect of evaporative cooling and conductive cooling would be greatest.
A pressure gauge at that point would show a lower reading than it would either at the entry or exit of the breezeway. A thermometer would show no difference.
If the dogway or breezetrot maintained a constant cross section, with equal entry and exit area,the pressure gauge would show higher at the entry than at the exit, but roughly similar all the way through. Cross-sectional flow would give higher velocity, lower pressure down the centre of the passage, lower at the edges and corners.

Sheesh! I realise that the 'Red Dirt Mule's Book of Facts' is written and edited "on the hoof", with a red crayola, but.... sheesh.

Carburettors is another venturi altogether. Bernoulli becomes important there, just not in southern vernaculated dogtrots.

soubriquet said...

Sheesh

red dirt girl said...

Oh my, what a can of worms. I will be the bigger person here and say, "Yes, I am no physics guru nor ever will be." I can't say I remember you ever majoring in physics either. Yes, you understand Bernoulli as it applies to your expertise in flow of fluids. You are a hopeless airplane designer and apparently have not looked at the literature on air flow and the use of the Bernoulli principle and Venturi effect in the design of architecture. It is not just southern vernacular architecture that applies its principles, but architecture around the world (look at hot arid climates for example). Apparently, Bernoulli is a routinely taught principle in architecture school. Go ahead. Type 'G O O G L E' into your search engine and look it up. I cut and pasted from a large range of sources, trying to keep this as user friendly as possible. Obstructions in the landscape create mini-zones of high and low pressure which can be manipulated by clever building design to create breezes / suction / whatever you want to call it. I will be happy to take this to a wind-tunnel test to prove the point.

I rest my red crayon on this subject.

As for my other faithful readers, I thank you for your kind comments and for your patience. Oh my....


xxx

red dirt girl said...

Better yet, Mr. Soubriquet, bring your test kit of pressure gauges, thermometers, balls of string and ear wax on your next trip to Texas, and we will GO AND SIT IN A DOGTROT and measure pressure, temperature and perception of coolness to your hearts content. Vindication is yours just an airplane ride away .... yes, it's an invitation.

Me? I'll bring my hoof and red crayon.

xxx

goatman said...

This by a guy who has never been hot!

soubriquet said...

Goatman: How dare you sir!

I have at times been extremely tepid, I'll have you know!
I demand satisfaction, Bernoullis at dawn.
Bring your second.

goatman said...

I'll be there, less it rains.

soubriquet said...

Bernoullis and brollys then.
Five a.m. in the meadow south of the mill.

goatman said...

Agreed,
and clean white antimacassars on each for identification.