Bernoulli's principle says a faster-flowing fluid has lower pressure. Drag the sliders below to change the inlet and throat diameters, the inlet flow speed and the inlet pressure of this Venturi pipe, and watch the throat flow accelerate and its pressure drop in real time.
A single parcel of water slides into the wide inlet at whatever inlet flow speed v1 you dial in, and for a moment nothing dramatic happens. Then the pipe pinches down to the throat, and that parcel has nowhere to go but faster. Because the same volume must cross every section each second, continuity forces the throat speed up in step with the squeeze: v2 = v1·(d1/d2)². Shrink the throat diameter d2 against the inlet diameter d1 and the readout for throat speed v2 climbs sharply, since the ratio is squared.
Riding along with the parcel, watch what its pressure does. As it accelerates it buys speed with pressure energy, and the throat pressure readout P2 = P1 + ½·ρ·(v1² - v2²) falls below the inlet pressure P1 you set. The pressure drop ΔP widens exactly as the throat narrows. Here is the trap the sim is built to expose: the faster fluid carries the lower pressure, not the higher one. Squeezing does not pile pressure up in the constriction.
It balances because P + ½ρv² holds steady along the horizontal streamline in this ideal, incompressible flow. That single trade-off runs the Venturi meter, the carburettor, the aspirator and the scent atomiser — each speeds a fluid through a constriction so its pressure drops and it draws another fluid in. Put numbers to the same energy split with the Bernoulli equation calculator, or trace another flowing system over at our full collection of physics simulations.
Along a streamline in a steady, ideal flow, faster-moving fluid has lower pressure: P + ½ρv² stays constant when there is no height change. Where a pipe narrows and the flow speeds up, the pressure drops.
Because of continuity: the same volume of fluid must pass every cross-section each second, so A1·v1 = A2·v2. A narrower throat has a smaller area, which forces a higher speed, v2 = v1·(d1/d2)².
No — the opposite. Faster fluid has lower pressure, so the throat, where the flow is fastest, has the lowest pressure. The throat pressure is P2 = P1 + ½ρ(v1²−v2²), below the inlet pressure.
In Venturi meters, carburettors, aspirators and scent atomisers — each speeds a fluid through a constriction so its pressure drops and it can draw in another fluid. (Aeroplane wing lift is related but more complex — it is not a simple Venturi throat.)