Head Loss and Pressure Rise during Gradual Expansion

[mechanics11]

I would like to ask that while calculating the irreversible head loss in the question why do we use V1, the velocity of the narrow section instead of V2, velocity of the wide section.

Source: Fluid mechanics by Çengel/Cimbala

Thank you

 

[bimr]

The upstream section has more energy than the downstream sections.

 

[pocketengineer]

I remember there exists 2 different versions of formula. One is based on upstream velocity, and the other one is based on downstream velocity. Therefore, I am always care not to just reference to V1 or V2 as it depends where D1 or D2 is. Instead I use upstream or downstream velocity based on the particular formula definition.