EASA/GCAA Part-66 · Module 2 — Physics · Fluid Dynamics

Bernoulli & the Venturi

Objective: force a fluid through a narrowing (a venturi). By continuity the fluid speeds up in the throat, and by Bernoulli its static pressure falls — the manometer standpipes make the pressure drop visible. This is how a carburettor venturi draws in fuel.

Venturi tube — flow, pressure field & manometers

Velocity & static pressure along the tube

Inlet velocity V₁ 3.0 m/s

Throat area ratio A₂/A₁ 0.35

Fluid density ρ 1.23 kg/m³

Inlet velocity V₁

0m/s

Throat velocity V₂

0m/s

Inlet static P₁

0kPa

Throat static P₂

0kPa

Continuity: A₁V₁ = A₂V₂ · Bernoulli: P + ½ρV² = constant

By continuity, the same volume must pass every cross-section each second, so where the tube narrows the fluid must speed up (A·V is constant). By Bernoulli, the total of static + dynamic pressure is constant along a streamline, so where the velocity is high the static pressure is low. The standpipes are manometers reading static pressure: watch the throat column fall as the flow accelerates. A carburettor exploits exactly this low-pressure throat to draw fuel into the airstream.

Try this: Raise the inlet velocity — see V₂ climb (continuity) and the throat manometer drop (Bernoulli).
Try this: Make the throat narrower (smaller area ratio). What happens to V₂ and the pressure drop?
Try this: Increase density — for the same speeds the dynamic pressure ½ρV² rises, deepening the drop.
Try this: Switch to Carburettor and watch fuel drawn up into the low-pressure throat.