# Venturi outflow pressure

• I
Can anyone tell me if the reverse of Bernoulli's principal is true, in other words if I go from a smaller diameter pipe to a large pipe does the pressure increase? Applying this to a Venturi tube if I go from a large pipe to a smaller pipe and add an inlet that adds mass due to the suction (pressure differential), then I want to return to the pressure that I originally had do I just need a large diameter pipe than I began with? If this is the case where did the extra energy come from, or is there an energy loss I am missing?

Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

Gold Member
What makes you think this is the "reverse" of Bernoulli's principle? Bernoulli's equation works both ways, so yes, if you move from a smaller pipe to a larger pipe (for inviscid, incompressible flow), the pressure goes up.

rcgldr
Homework Helper
Consider power = pressure x volume flow. The power required to draw in the mass via the inlet pipe ideally equals the input pressure x volume flow minus the output pressure times volume flow. In a real world situation, there will be some losses in the process. Assuming density isn't significantly changed, then volume flow in (source + inlet) equals volume flow out, so pressure decreases a bit more than ideal.

There also needs to be a pressure recovery zone where the mass flow decreases in speed in increases in pressure (what the original post calls reverse Bernoulli). Example image of such a device used to start a siphon, usually for aquariums. The bottom piece can be turned so instead of being used for pressure recovery, it seals off the exit, so water can be put back into an aquarium.