# For a fluid travelling in a tube, if the pressure is decreased at a constant temperature, how does it affect the velocity and fluid density?

I am interested in understanding the relationship between pressure and velocity, as well as pressure and fluid density.

I know that the principle of Bernoulli states that there exists a relationship between the terms. But I'm confused as to wether reducing the pressure of a fluid flowing in a closed tube, would increase the velocity at which the fluid is travelling.

Does this relate to the Drag Force Equation?

• Apparently you are assuming that the fluid is compressible. Is the tube of constant cross sectional area? Is the flow steady (not changing with time)? Aug 7, 2018 at 11:55
• Normally, one worries about the pressure drop through a pipe or tube. You want to decrease the pressure of the fluid in your tube. What do you want to do with the pressure drop ($\Delta P$) in your tube? Aug 7, 2018 at 15:15
• Hi @ChesterMiller, thanks for responding. In the question above I was assuming that the fluid is ideal (non-compressible), the cross sectional area and the flow rate are constant. Aug 8, 2018 at 13:14
• Hi @DavidWhite, thanks for posting a comment. I want to know if by dropping the pressure the fluid will flow faster through the tube. Aug 8, 2018 at 13:20
• @AidanDeaves, there are standard methods for calculating flow. Flow rate depends on the pressure difference across the tube. For liquid flows, which are incompressible, I would expect the flow rate to remain constant for a constant pressure drop, even as the total pressure was decreasing. For a gas or vapor flow, the fluid density goes down as the pressure goes down. This means that pressure drop increases for a given mass flow rate. I suspect that the pressure drop would decrease slightly for a constant volumetric flow rate, but I would have to run a calculation to verify this. Aug 8, 2018 at 17:30