Loo Kian Sing,
Before you try to apply the equations derived from the Bernoulli Principle, you need to review the underlying assumptions in the derivation of those equations:
[ul]
[li]Inviscid: No friction at all[/li]
[li]Incompressible: No change in density from one measurement point to the other (for gases, people often assume that 1-2% change in density is the maximum allowable, the Fanno-flow compressible flow equations support this magnitude of change)[/li]
[li]Irrotational[/li]
[li]Reversible: Again, losses to friction are not reversible[/li]
[li]Isothermal[/li]
[li]Isentropic:[/li]
[li]Adiabatic: No heat transfer from or to the environment[/li]
[li]Isenthalpic: No work done on or by the fluid[/li]
[/ul]
If friction is not zero then there is simply no way to derive the equations. If the fluid does as much work as turning a turbine meter, then the equations don't work.
The energy equation says that static pressure plus dynamic pressure plus hydrostatic pressure equals total pressure. This works everywhere. The (valuable) addition that the Bernoulli Principle makes is that in conditions where the assumptions are satisfied, the three kinds of pressure add up to a constant and the energy simply moves from one term to the others. That lets you calculate dynamic pressure and therefore velocity through an orifice, or velocity and pressure change through a change in duct diameter, or the lift on an airplane wing. Dang useful things to know. If there is friction then the total pressure is not constant and that is also useful to know, but it makes the relationship between static pressure an velocity unpredictable relative to each other.
Assuming that you can legitimately use the equations, then I have a hard time understanding the distinction between "pressure of the fluid" and "pressure at a particular location". The term "pressure" only has physical meaning relative to the fluid within the volume. Remove the fluid and pressure is zero.
Finally, what do you want to "apply the Bernoulli Principle" to? It sounds like you might have a solution in search of a problem. The pressure at the surface of the water in a tank is not "zero" it is "local atmospheric pressure" which tends to be somewhere between 11.2 and 15.1 psia depending on elevation. Most tank calculations that I've ever done have had a velocity term that was very very small so dynamic pressure was insignificant and the analysis becomes a fluid statics problem instead of a fluid dynamics problem that might utilize the Bernoulli equations.
[bold]David Simpson, PE[/bold]
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
