Is the Reverse of Bernoulli's Principle True in Venturi Outflow Pressure?

In summary, the reverse of Bernoulli's principle states that if you go from a smaller diameter pipe to a larger one, the pressure will increase. This applies to Venturi tubes as well, where adding an inlet that adds mass due to suction will require a larger diameter pipe to return to the original pressure. However, there may be some energy loss in this process. Additionally, a pressure recovery zone is needed to decrease speed and increase pressure, as seen in devices like siphons for aquariums.
  • #1
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?
  • #3
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.
  • #4
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.


1. What is Venturi outflow pressure?

Venturi outflow pressure is a phenomenon in fluid mechanics where the pressure of a fluid decreases as it flows through a constricted area, such as a Venturi tube. This decrease in pressure is due to the increase in velocity of the fluid as it passes through the constriction.

2. How does Venturi outflow pressure affect flow rate?

Venturi outflow pressure has a direct impact on flow rate. As the pressure decreases, the fluid velocity increases, resulting in a higher flow rate. This is known as the Bernoulli effect, where the total energy of a fluid remains constant as it flows through different parts of a system.

3. What factors can affect Venturi outflow pressure?

Several factors can affect Venturi outflow pressure, including the shape and size of the constriction, the viscosity of the fluid, and the velocity of the fluid before and after the constriction. Changes in any of these factors can alter the pressure and flow rate in a Venturi system.

4. What are the practical applications of Venturi outflow pressure?

Venturi outflow pressure has many practical applications, including in carburetors for automobiles, where it helps mix air and fuel for efficient combustion. It is also used in medical devices, such as oxygen therapy equipment, and in industrial processes, such as controlling the flow of fluids in pipelines.

5. How is Venturi outflow pressure measured?

Venturi outflow pressure can be measured using a pressure gauge or a manometer, which measures the difference in pressure between two points in a system. The pressure difference between the inlet and outlet of a Venturi tube can then be used to calculate the flow rate of the fluid passing through the system.

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