Venturi effect
From Free net encyclopedia
The Venturi effect is a special case of Bernoulli's principle, in the case of fluid or air flow through a tube or pipe with a constriction in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum via the Bernoulli effect. It is named after the Italian physicist Giovanni Battista Venturi.
The reduction in pressure in the constriction is a result of conservation of energy: the fluid (or gas) gains kinetic energy as it enters the constriction, and that energy is supplied by a pressure gradient force from behind. The pressure gradient reduces the pressure in the constriction, in reaction to the acceleration. Likewise, as the fluid leaves the constriction, it is slowed by a pressure gradient force that raises the pressure back to the ambient level.
The Venturi effect is visible in the capillaries of the human circulatory system or in large cities where wind is forced between buildings. It is used in gas jets that mix air and flammable gas in barbecues, gas stoves, and Bunsen burners; in water aspirators that produce partial vacuum from a water spigot; in atomizers that disperse perfume; and in carburetors that use the effect to suck gasoline into an engine's intake air stream. It is also traditionally used as an explanation as to how a wing generates lift to allow an airplane to fly. This simple theory of lift is now regarded as incorrect.
The limiting case of the venturi effect is choked flow, in which a constriction in a pipe or channel limits the total flow rate through the channel, because the pressure cannot drop below zero in the constriction. Choked flow is used to control the delivery rate of water and other fluids through spigots and other valves.
Practical uses
A simple way to demonstrate the Venturi effect is to squeeze and release a flexible hose that is carrying water. If the flow is strong enough, the constriction will remain even if the hose would normally spring back to its normal shape: the partial vacuum produced in the constriction is sufficient to keep the hose collapsed.
Buchner funnels use the Venturi effect to provide the suction on force against the contents of the funnel.