A stiff breeze may blow at a few meters per second, but the average speed of an air molecule at room temperature is around 500 m/s (=1100 mi/hr), and is known as the "thermal velocity". Even in "still" air, air molecules are constantly bouncing around and colliding with each other at high speed. To understand how this happens, we must first recognize that molecules are generally moving much faster than the typical flow speed of a fluid. The short answer is that molecules in the constriction are more likely to be traveling in the direction of flow, rather than bouncing up and down, and therefore they hit the walls of the tube less frequently, and with less force. the microscopic viewĪt the microscopic level, how do we understand that molecules speed up as they enter the constriction of a Venturi tube and produce lower pressure on the walls of the tube? High pressure before the constriction accelerates molecules into the low pressure region of the constriction, and high pressure after the constriction slows them down again as they exit.
Indeed, in order for the molecules to speed up as they enter the constriction, and then slow down again as they leave, there must be a pressure difference at the entrance and exit of the constriction. Since the molecules are flowing faster in the constriction, Bernoulli's principle indicates that the pressure in the constriction should be lower than it is outside. Since the cross section is smaller in the constriction, the molecules must move faster in order for enough molecules to get through in the specified time. However many molecules enter the tube in a given time must be the same as the number of molecules going through the constriction and coming out the other end.
The molecules must speed up in the constricted region in order for the total flow rate to remain the same. It also offers a particularly clear example of the Bernoulli principle.Īs the fluid flows through the constriction, the fluid molecules speed up, as indicated by the animation in figure 2. The Venturi tube provides a handy method for mixing fluids or gases, and is popular in carburetors and atomizers, which use the low pressure region generated at the constriction to pull the liquid into the gas flow. The Venturi effect, published in 1797 by Giovanni Venturi, applies Bernoulli's principle to a fluid that flows through a tube with a constriction in it, such as in figure 2. As the fluid goes through the constriction, it speeds up, and the pressure drops. High pressure regions are dark blue low pressure regions are white. 2: The flow of material through a Venturi tube. Kinetic energy is increased at the expense of pressure energy, while the total energy remains constant.įig. $$\rho v_2^2 + \rho g h_2$$įor two regions at the same height ($h_1 = h_2$), an increase in flow velocity in one region must necessarily correspond to a decrease in pressure in order to keep the equation balanced.
1 To see Bernoulli's approach, write the energy density for a flowing fluid as: Bernoulli derived his principle from the conservation of energy, though it can also be derived directly from Newton's second law. It is named after Daniel Bernoulli, a Dutch-Swiss scientist who published the principle in his book Hydrodynamica in 1738. The Bernoulli principle states that a region of fast flowing fluid exerts lower pressure on its surroundings than a region of slow flowing fluid. The water in the U-bend at the bottom of the picture is pushed up towards the left by the pressure difference. 1: Venturi tube showing that the narrow portion of the tube at the left has lower pressure than the wider portion of the tube at the right.
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