The Venturi effect occurs when a fluid speeds up as it moves through a narrowed section of pipe, causing the static pressure there to drop. This effect is important in engineering because it connects pipe shape, flow speed, and pressure in a predictable way. It helps engineers measure flow rate, mix fluids, create suction, and control motion without using many moving parts.
A Venturi tube makes the effect easy to see because it has a wide inlet, a narrow throat, and a widening outlet.
The mechanism comes from conservation of mass and conservation of energy. For an incompressible fluid, the continuity equation says that a smaller cross-sectional area must have a larger fluid speed. Bernoulli's equation then shows that when speed increases, static pressure decreases if height changes and losses are small.
Engineers use this pressure drop in devices such as flow meters, carburetors, aspirators, and ejectors.
Key Facts
- Continuity equation for incompressible flow: A1v1 = A2v2
- Bernoulli equation along a streamline: P + 1/2 rho v^2 + rho gh = constant
- At the Venturi throat, area is smallest, velocity is greatest, and static pressure is lowest.
- For a horizontal Venturi: P1 - P2 = 1/2 rho (v2^2 - v1^2)
- Volume flow rate is Q = Av, where Q is in m^3/s, A is in m^2, and v is in m/s.
- A real Venturi has energy losses, so practical flow meters use a discharge coefficient: Qactual = Cd Qideal.
Vocabulary
- Venturi tube
- A pipe section that narrows to a throat and then widens again to create a predictable change in fluid speed and pressure.
- Throat
- The narrowest part of a Venturi tube where the fluid speed is highest and the static pressure is lowest.
- Static pressure
- The pressure a fluid exerts due to its random molecular motion, not including pressure associated with bulk flow speed.
- Continuity equation
- A conservation of mass relation stating that steady incompressible flow has the same volume flow rate through every cross section.
- Bernoulli's principle
- A statement that, for ideal steady flow along a streamline, higher fluid speed corresponds to lower static pressure when height is unchanged.
Common Mistakes to Avoid
- Thinking pressure is highest at the throat, which is wrong because the fluid's speed is highest there and Bernoulli's equation predicts a drop in static pressure.
- Using diameter instead of cross-sectional area in A1v1 = A2v2, which is wrong because area is proportional to diameter squared, not diameter.
- Ignoring units when using density and pressure, which is wrong because Bernoulli calculations require consistent SI units such as kg/m^3, m/s, and pascals.
- Assuming Bernoulli's equation is exact for every real pipe, which is wrong because viscosity, turbulence, and fittings cause energy losses that reduce the ideal pressure recovery.
Practice Questions
- 1 Water flows through a horizontal Venturi tube. The inlet area is 0.012 m^2 and the throat area is 0.004 m^2. If the inlet speed is 2.0 m/s, what is the speed at the throat?
- 2 Air with density 1.2 kg/m^3 flows through a horizontal Venturi. The speed increases from 10 m/s at the inlet to 30 m/s at the throat. What is the static pressure drop from inlet to throat?
- 3 A Venturi carburetor draws fuel into an air stream at the throat. Explain why the fuel inlet is placed near the throat instead of in the wide inlet section.