Ideal Fluid

[priya.bork7031]

I'm having a hard trouble grasping this concept:

Suppose that a stream of fluid flows steadily through a horizontal pip of varying cross-sectional diameter. Neglecting viscosity, where is the fluid pressure greatest?
b) Answer: At the point of maximum diameter


Now the place I'm having trouble with is P=F/A. If area increases, shouldn't pressure decrease? Does the continuity equation even apply here, there's no external pressure. Pllllllease someone explain this to me.

[shielkpate7323]

Hey ill give it a shot....

So from the continuity equation the greater the diameter, the smaller the velocity.

Now if we look at the Bernoulli equation:

P1 + 1/2pv^2 + pgy = P2 + 1/2pv^2 +pgy

Now since this is a horizontal pipe, the height is not changing for cancel out the pgy's from the equation.

P1 + 1/2pv^2 = P2 + 1/2pv^2

Now since the area increases, Velocity will decrease on the right side of the equation. However both sides of the equation have to be equal. So to compensate for this, P2 must be larger than P1

hope this helps

[gaddy2948]

You must keep in mind also that a resting fluid and a moving fluid have separate properties.

A resting fluid has a pressure of P=F/A on an object surrounded by that fluid and P=(density)gy for a depth of fluid.

For a fluid in motion, Bernoulli's concepts should be used.