How can the pressure be more in liquid flowing with less speed?

[Frigus]

Before I have read Bernoulli's theorem I believed that pressure is more in liquid flowing with more speed but today I have read Bernoulli's theorem where this theorem says that the pressure is more in liquid flowing with lesser velocity please tell me how can this explained,with equation I understood that if if velocity is more so as to conserve energy pressure should be more but how can be this explained practically.

 

[Dale]

I understood that if if velocity is more so as to conserve energy pressure should be more but how can be this explained practically

I am not sure what would count as a practical explanation if conservation of energy does not.

 

[Frigus]

I am not sure what would count as a practical explanation if conservation of energy does not.

Sir but I can't understand how pressure can be energy.

 

[russ_watters]

Sir but I can't understand how pressure can be energy.

Pressure isn't energy. Please post the units of pressure and the units of energy and compare them.

 

[Frigus]

Pressure isn't energy. Please post the units of pressure and the units of energy and compare them.

Sir units of pressure are N/m^2 and units of energy are Nm,so why we are conserving pressure

 

[Delta2]

well pressure has same units as Energy/volume that is same units as energy density...

 

[Frigus]

well pressure has same units as Energy/volume that is same units as energy density...

Sir I want to know why we need to conserve pressure

 

[Delta2]

Sir I want to know why we need to conserve pressure

because pressure relates to the microscopic kinetic energy of the molecules of the fluid. Also it has same units as energy per unit volume

 

[Frigus]

because pressure relates to the microscopic kinetic energy of the molecules of the fluid. Also it has same units as energy per unit volume

And why we write 1/2 rho v^2 if we are adding microscopic kinetic energy

 

[Delta2]

The term $\frac{1}{2}\rho v^2$ is for the mAcroscopic kinetic energy. So we are adding microscopic kinetic energy(the pressure term) plus macroscopic kinetic energy (the$\frac{1}{2}\rho v^2$ term) plus potential energy (the $\rho gh$ term) and this sum is conserved. That's Bernoulli's principle abit oversimplified perhaps.

 

[Frigus]

The term $\frac{1}{2}\rho v^2$ is for the mAcroscopic kinetic energy. So we are adding microscopic kinetic energy(the pressure term) plus macroscopic kinetic energy (the$\frac{1}{2}\rho v^2$ term) plus potential energy (the $\rho gh$ term) and this sum is conserved. That's Bernoulli's principle abit oversimplified perhaps.

Thanks sir

 

[sophiecentaur]

how can be this explained practically.

Here's a good arm waving explanation. If the fluid is flowing faster in one place than another, there must have been a force (differential) to accelerate it. That implies the higher pressure is in the region where the fluid came from . So its speed increases and its pressure decreases.
Of course there are conservation arguments which are more erudite and complete but the above is certainly very "practical".

 

[Vanadium 50]

Hemant, you are replying immediately to the messages you get, I think you will have a better outcome if you think about what people say before responding - it can take a few moments.

 

[Frigus]

Here's a good arm waving explanation. If the fluid is flowing faster in one place than another, there must have been a force (differential) to accelerate it. That implies the higher pressure is in the region where the fluid came from . So its speed increases and its pressure decreases.
Of course there are conservation arguments which are more erudite and complete but the above is certainly very "practical".

Thanks sir,it is what I was founding.

 

[Frigus]

Hemant, you are replying immediately to the messages you get, I think you will have a better outcome if you think about what people say before responding - it can take a few moments.

Thanks sir,physics forum is a place where I don't get answers to only questions but people like you help me to develop myself.

 

[russ_watters]

Sir units of pressure are N/m^2 and units of energy are Nm,so why we are conserving pressure

well pressure has same units as Energy/volume that is same units as energy density...

This may be water under the bridge by now, but what I was after was a deconstruction of Bernoulli's equation (but I was on my way to bed...). It isn't readily apparent how pressure relates to energy, but it becomes obvious if you back out of the derivation.

If you multiply the standard form of Bernoulli's equation through by volume, then P becomes PV, which should be recognizable as energy, like when you push down on a piston-pump. 1/2ρV² becomes 1/2mv², which is kinetic energy. So that's how you see they are part of a conservation of energy statement, and dividing through by volume gives you the standard form, with the funny sounding units of "energy per unit volume".

 

[Dale]

So that's how you see they are part of a conservation of energy statement, and dividing through by volume gives you the standard form, with the funny sounding units of "energy per unit volume".

Which is also why it only works for incompressible flow.

 

[sophiecentaur]

You have to admit my noddy explanation works every time.

 

[russ_watters]

Which is also why it only works for incompressible flow.

Sure, though it can be modified to work for compressible flow. Depending on the specifics of the flow and the assumptions, you can have varying density and temperature, and it starts to look a lot more like thermodynamics than fluids.

 

[rcgldr]

One missing detail in this is the assumption that there is no external forces involved in the fluids transition from slower speed to higher speed. The output from a fan is both higher pressure and higher speed because the fan performs work on the air. A classic example of Bernoulli is flow in a pipe. The source of energy to maintain the flow is ignored, and only the flow's interaction with the pipe is considered. Assuming no friction losses, and an incompressible fluid, then the energy per unit volume remains constant. If the kinetic energy increases due to an increase in velocity, such as a narrower section of the pipe, the the pressure energy decreases so that total energy remains constant.