# Homework Help: Help me calculate venturi flow rate

1. Jul 24, 2012

### declanka

Hi I have a challenge (Its over 30 years since I did physics). I have the following information and I want to calculate the flow rate change .

Opening diameter 2m , narrows smoothly to diameter 1m. Seawater, tidal flow of 2.5 m/s into the opening, what is the effective flow rate in the throat of the venturi

2. Jul 24, 2012

### tiny-tim

welcome to pf!

hi declanka! welcome to pf!

water is incompressible, so the flow rate (volume per second) must be constant, so the speed (metres per second) must be inversely proportional to cross-section area

(and in this case, the area is 1/4, so the speed must be x 4)

3. Jul 25, 2012

### declanka

Thanks for this response, it logically confirms the calculations I came up with. Is the formula always true , even in the scenario where its just an open venturi (narrowing) pipe facing into the tidal flow .

4. Jul 25, 2012

### tiny-tim

yes, it's conservation of mass …

the water can't bunch up anywhere, or thin out, so the flow into any part of the pipe must be the same as the flow out of it … ie the flow rate (volume per second) past any two cross-sections must be the same