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. "
"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!