Help me calculate venturi flow rate

[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

 

[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)

 

[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 .

 

[tiny-tim]

Is the formula always true , even in the scenario where its just an open venturi (narrowing) pipe facing into the tidal flow .

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