Air from a compressed air supply is blown through a Venturi tube with a constriction in the middle. The diameter of the tube upstream from the constriction is the same as that downstream from the constriction. Three small tubes on the bottom of the Venturi tube lead from the Venturi tube at right angles to the air flow. A glass manometer containing colored water is connected to each of the small tubes coming from the Venturi tube so that one may observe the relative pressure at the constriction, upstream from the constriction, and downstream from the constriction in the Venturi tube. One observes first of all that the pressure downstream from the constriction is slightly lower than the pressure upstream from the constriction even though the diameters of the Venturi tube and hence the velocities of the air at these locations are nearly the same. This is due to the viscosity of the air. Second, one observes that the pressure is lower in the constriction (where the velocity of the air is larger) than either the upstream or downstream pressures. This is an example of Bernoulli\'s law and occurs whether the flowing fluid (air in theis case) has viscosity or not.







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