Where the speed of a fluid is low, the pressure is high? Is this right?

Question

My teacher gave me a note and I dont feel it makes sence. Its about Bernoullis Principle. It says, Where the speed of a fluid is low, the pressure is high. Where the speed of the same fluid is high, the presure is low.

But I thought in high pressure, the speed is high. Like in a water hose, when u close half of the hole, putting more pressure, the speed of the water shoots out further.

Answer 1

Bernoulli's principle is a statement about the conservation of energy in a moving fluid. It says that the energy of a given volume of fluid consists of three parts:

kinetic energy = v^2/2*density

potential energy = g*h*density

pressure energy = P

When energy is conserved (meaning no losses to friction, viscosity or compression), the sum of these parts is constant although the individual parts may vary.

When you combine Bernoulli's principle with the continuity principle (which basically says the volume flow rate is constant throughout a flow channel despite changes in the flow area), the hose behavior becomes more understandable.

With the hose end open, the volume flow rate is determined by the faucet valve area and the large pressure drop across it. (Note there is an energy loss at the valve. The flow rate is the same on either side of the valve, velocities are about the same, but the pressure decreases to zero.) Water passes through the hose with some velocity and zero pressure, and exits with the same values.

When you apply your thumb, you change the system. You raise the pressure in the hose moderately compared to the supply pressure (say 10 psi compared to 50 psi) and the volume flow rate decreases, but only a little. Flow continuity means the velocity beyond your thumb increases. So it exits with higher velocity and zero pressure. This is roughly consistent with Bernoulli, because passage through your thumb's restriction reduces P (from 10 psi to zero) and increases v. The pressure energy is exchanged for velocity energy.

Mark's explanation is good but I have to comment on one minor point that is easily and often misunderstood: There is no tendency for air parcels to try to reunite with their old neighbors after wing passage. See the ref.

Answer 2

What you're thinking of is correct but that's not what your teacher is referring to.



Bernoulli's principle is what creates lift on an airplane's wing (among other things). Think of how an airplane wing is shaped. Sort of like a half-teardrop, with a flat bottom and rounded top. When the leading edge hits the atmosphere, the air has to "split" going above and going below. Now, the air wants to meet up with its friend molecule right next to each other on the other side. In order to do that, the air needs to move faster over the top of the wing relative to the bottom of the wing. This faster relative motion creates a low pressure zone above the wing, which is what "lift" is that allows the plane to fly (also being "pushed" from below with the higher pressure).



It's a totally different pressure-style than narrowing your garden hose into a tiny nozzle.

Answer 3

I quite seek for Honda Accord and it makes use of primary ATF indoors the flexibility training, quite like yet another automobiles do. The pump and hose shouldn't make the educational greater sturdy, yet much less complicated. So one danger is that as rapidly by using fact the previous pump wore out, it left some steel grit indoors the educational rack. 116,900 miles isn't a lot, and the educational pump would not have long previous undesirable in that element. touching directly to the only element which will kill one is working dry. I actual have by ability of no ability had to bleed the air out greater effective then one or 2 lock to lock turns. Sounds extraordinary. according to danger there's a kink indoors the return line or something?