Venturi Effect: Speed Up, Pressure Down
1 · Predict
Water flows through a pipe that narrows partway through, like a garden hose with your thumb over the end. What happens to the water's pressure in the narrow section?
- Pressure increases in the narrow section because the water is more compressed.
- Pressure decreases in the narrow section because the water speeds up.
- Pressure stays the same throughout — only speed changes.
2 · Set Up
- Open the fluid-venturi preset and press Reset. Water flows steadily through a pipe with a wide section (A₁) and a narrow throat (A₂).
- Enable the throat-speed and pressure-drop readouts.
- Set the wide-section area, throat area, and inlet speed for each trial, and record the throat speed and pressure-drop readings.
3 · Collect Data
| Wide-section area A₁ (m²) | Throat area A₂ (m²) | Inlet speed v₁ (m/s) | Throat speed v₂ (m/s) | Pressure drop ΔP (kPa) |
|---|---|---|---|---|
| 0.0004 | 0.0001 | 2 | ||
| 0.0004 | 0.0001 | 3 | ||
| 0.0004 | 0.0002 | 4 |
Plot the pressure drop ΔP (y-axis) against the throat speed v₂ (x-axis) for your three trials.
4 · Analyze
- For one trial, compute v₂ = A₁v₁/A₂ from continuity, then ΔP = ½ρ(v₂² − v₁²) from Bernoulli's equation. Compare both to the readings.
- Explain, in terms of energy conservation, why a fluid's pressure must drop where its speed increases in a horizontal pipe.
5 · Extend
- Carburetors and perfume atomizers use the Venturi effect to draw in a second fluid at the narrow throat. Using your pressure-drop data, explain why a low-pressure region there can pull liquid upward through a side tube.
- The fluid-wing experiment uses this same continuity + Bernoulli model with air instead of water. Would you expect the pressure difference to be larger or smaller for the same speeds and areas? Why?
The Physics Behind This Experiment
Continuity Equation
For an incompressible fluid in a pipe, the flow rate (area × speed) must be the same everywhere along the pipe: A₁v₁ = A₂v₂. Narrowing the pipe forces the fluid to speed up.
Bernoulli's Equation
Along a horizontal streamline, a fluid's pressure plus its kinetic-energy-per-volume stays constant: P₁ + ½ρv₁² = P₂ + ½ρv₂². Where speed increases, pressure must decrease to compensate.