Theoretical question- fluid flow from high to low energy

[DanielWW]

I know that fluid flows from areas of high energy to low energy, and not necessarily from high to low pressure. If you look at flow through a horizontal pipe that expands to a wider cross-section, some of the velocity head is converted into pressure head, which causes the pressure to be higher downstream. What confuses me is when Bernoulli's principle is stated as "In a flow of constant energy, when fluid experiences a decrease in velocity, it experiences an increase in pressure. If there is constant energy at all points in the flow, what exactly is driving the flow?

I know that clearly there must be a pump or something in the system, but the statements 1) fluid flows from areas of high energy to low energy and 2) Bernoulli's principle applies to flows of constant energy, seem to be contradictory to me.

Thanks

 

[desertfox]

Hi

Look at this link

http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Section3/bernoulli.htm

 

[DanielWW]

Thanks for the link.

What confuses me is that in section 2 on Bernoulli's principle, the total energy upstream is equal to the total energy downstream. What then, is meant when it is said that "fluid flows from regions of high energy to regions of low energy". How can there be flow if there is no difference in energy?

 

[desertfox]

hi Daniel

In practice their are losses, the basis for no loss of energy is an assumption, just like when you release a ball from height the energy changes from potential energy to kinetic energy, if you assume no energy is lost the ball just keeps bouncing but in real life that doesn't happen, the ball comes to rest.
So using the Bernoulli equation just gives an estimate of velocity, pressure at a particular point in a system, the formula can be modified to include losses and obtain a better estimate.

 

[Compositepro]

Bernoulli's equations assume total energy is constant and just converts from one form to another. This is the principle of conservation of energy and is the basis for all of physics. "Fluid flows from high to low energy energy" is not a basic principle and is wrong. The relevant concept here is that entropy of the universe never decreases.

 

[BigInch]

When the fluid is static (kinetic energy component = 0), there can only be a difference in potential energy, so that is obviously what is available to drive flow. Potential energy is converted into kinetic energy as the velocity of the fluid picks up. This can be seen by plotting the 1) pressure head, the 2) velocity head and 3) the hydraulic grade line, the sum of potential energy head + velocity head. Pick two points, one just inside a tank, where velocity = 0, and another point a few feet, or meters, down a discharge pipe, where velocity has increased. The change from static head to velocity head at the nozzle is described by the Cd coefficient of the nozzle. Also seen at an orifice, look for the HGL plot across the orifice on this page,

http://www.google.co.uk/url?sa=t&rc...=aQ1OLon28lJS6zT0qqeuxQ&bvm=bv.61535280,d.d2k

I hate Windowz 8!!!!

 

[DanielWW]

Thanks for the responses! I think I have a better understanding now