point source paradox?
point source paradox?
(OP)
Forgive me for showing my ignorance, but I'm a structural ...
If pressure drops with velocity, then it would seem that the pressure surrounding a point source in an infinite medium must be less than the pressure at an infinite distance from the source, regardless of the sign of that source. That would seem to imply that if one wishes to inject fluid into a reservoir, he needs a suction pump! What am I missing here?
DBruceG
If pressure drops with velocity, then it would seem that the pressure surrounding a point source in an infinite medium must be less than the pressure at an infinite distance from the source, regardless of the sign of that source. That would seem to imply that if one wishes to inject fluid into a reservoir, he needs a suction pump! What am I missing here?
DBruceG





RE: point source paradox?
RE: point source paradox?
PUMPDESIGNER
RE: point source paradox?
Bernoulli is the chap who did the hard yards.
Ignoring friction, the total energy (static + velocity) remains the same.
This concept drove me nuts at first; it was my understanding of static pressure that caused me the grief.
Static pressure and velocity(pressure) are interconvertible, but in such a way that there is no change in total energy.
Whatever that means............
You will get it; aint that hard.
Cheers
Steve
RE: point source paradox?
Must be the Altzheimer's.
DBruceG
RE: point source paradox?
Conversely, if the mass is moving slower, the energy was lost, likewise, if energy was lost, the mass moves slower.
TTFN
RE: point source paradox?
RE: point source paradox?
pump bhp = (gpm x FT TDH)/(3960 x eff)
and fan bhp
fan bhp = (CFM x Inches wg)/ ((6344 x eff)
where eff is fan or pump efficiency in decimal. A value of 0.65 is good for initial estimate that should be refined with actual catalog data.
The above formulas are important because with them you can easily estimate motor size required.
RE: point source paradox?
DBruceG
RE: point source paradox?
TTFN
RE: point source paradox?
DBruceG
RE: point source paradox?
I have ignored friction; davefitz covers this.
When allowing for friction, fluid flows from a higher total energy(pressure) point to a lower energy point; the energy gradient slopes downhill in the direction of flow. The less friction, the flatter the gradient. To increase total energy we need a pump.
It is not true to say flow always travels from higher to lower static pressure. It is true to say flow always travels from higher to lower total pressure, ignoring pumps and a few "special" cases.
I think The understanding problem comes about because in many real-world cases (e.g. pumps and pipes), the static pressure is far higher than the velocity pressure, so we can often ignore the latter without causing a serious error.
Your example appears to have the static pressure gradient going the wrong way. There is nothing wrong with this provided the total energy gradient is either zero (ideal frictionless case) or slopes downwards in the direction of flow. This effect is common to divergent flow and is used to good effect in a venturi pump.
Cheers
Steve
RE: point source paradox?
DBruceG
RE: point source paradox?
YOU tell me that whatever that form of energy density is, it appears as a pressure. I have to buy that, because if I don't, I arrive at the paradox - you need a suction pump to inject fluid. The necessary conclusion is that since both the kinetic energy and the mysterious (potential?) energy densities appear as pressures, then the pressure must be constant throughout the system.
Thanks!
DBruceG
RE: point source paradox?
Pressure is just mechanical energy per unit volume.
However when dealing with compressibles, things get a bit more tricky because of heating and cooling effects (gases cool when they expand), so conservation of thermal energy also needs to be considered. If your density changes are moderate - say 10% of the absolute pressure, then you probably dont need to go there.
Cheers
Steve