Hydrostatic Theory

[Gen1GT]

I'm sure this is a rarely spoken about subject here on the forums, but is anyone familiar with hydrostatic fluid theory, essentially the theory of hydraulics? An electrical engineer will understand the principles, as hydraulics are similar to electrics.

 

There is a myth in the hydraulic business that "flow makes it go." The idea of this fallacy is that flow (i.e., gallons per minute, or litres per minute) is what makes things move, and pressure is only created when there is resistance to this flow.

 

I have a question: Is there any way to create motion other than application of force?

[caKus]

 

I have a question: Is there any way to create motion other than application of force?

 

According to Wikipedia, he definition of a force is the cause of a change of velocity or of a deformation of a flexible object. So, no force => no change of velocity ; it is just the definition of a force.

 

IMO, Hydrostatic has been studied by Archimedes and Pascal. What is exactly your question ?

[Gen1GT]

 

 

Well, I was looking for help in creating a hydraulic law that does not currently

exist (although the principles of the law exist, just in different forms), and

which needs to replace a commonly used, and utterly false, jingle.

 

The jingle is, as I mentioned, “Flow makes it

go.” A correlated jingle is, “Pressure is resistance to flow,” and is just as

ridiculous. You’d be dumbfounded to hear how many highly educated and highly

experienced gentlemen in the fluid power industry belief these two terms.

 

I wish to create Cosford’s Law, which states,

“Force makes it go, and flow is the rate in which you can create pressure.” I’m

not sure how one creates a scientific law, but I’m hoping persons here will

help me ensure I’m not missing something. I’m not a scientist, but just a

person in the hydraulic industry that knows a little about physics.

 

The two components of hydraulic power are pressure

and flow, just like the components of mechanical power are force and velocity, and

just like the components of electrical power are volts and amperes. If you

apply force to a hydraulic cylinder via pressure, the rate in which you are

able to apply the force is dictated by the rate of flow into that cylinder. You

can have force in a cylinder with no movement, such as when the work pressure

is equalized with the load induced pressure.

 

My theory is that until you start packing in

more oil molecules, there is no net force differential to be able to create

movement. My analogy is that hydraulic oil can be, for intents and purposes, be

considered a fluid rod. The larger the fluid rod or the faster you push it on,

the faster you can create a force differential to create movement.

 

What are your thoughts?

 

 

 

[caKus]

 

The two components of hydraulic power are pressure

and flow

 

Yes. I think it can be established this way :

Let suppose that a surface S moves of a distance dl under the action of a pressure P.

The variation of energy is

[math] dW = F dl = P S dl[/math]

If dV is the variation of volume for a surface S and a displacement dl,

[math] dV = S dl[/math]

it comes [math] dW= P dV[/math]

The volumetric flow of fluid is defined as [math]Q = dV / dt[/math]

Then, [math]dW = P Q dt[/math]

 

So, Power = [math] dW / dt = P Q[/math]

The power is the product of the pressure by the volumetric flow.

 

 

“Pressure is resistance to flow,”

Perhaps it can be stated this way, in more physic terms :

If a pressure is applied to a surface S and that this surface don't move, it is because the material opposes a force, the reaction that compensates the action of the pressure. This reaction has an intensity equal to the one due to the pressure but in opposite direction.

Perhaps some people make a confusion between the force due to the pressure and the reaction of the materials submitted to this pressure ?

 

I hope it can help.

 

[Gen1GT]

caKas, that's great, thanks. Also a resistance to volumetric flow does not create pressure, but only results in pressure; and that is the mistake persons in my industry tend to make. The increase in pressure is an example of Newton's Third Law.

 

I always that if the resistance created the pressure then the resistance creates its own energy, which clearly defies the second law of thermodynamics. Energy comes from the pump, not the resistance (be it a restriction or load).

[alpha2cen]

In the case of incompressible fluid and the same height.

[latex]\frac{\Delta P}{\rho } +\frac{\alpha \Delta v^{2}}{2} + f=0[/latex]

When there is no fraction loss, the fluid velocity difference between two points appeares to their pressure difference.

And, pressure difference also appeares to their velocity difference.

Edited January 26, 2013 by alpha2cen

[Gen1GT]

In the case of incompressible fluid and the same height.

 

[latex]\frac{\Delta P}{\rho } +\frac{\alpha \Delta v^{2}}{2} + f=0[/latex]

 

When there is no fraction loss, the fluid velocity difference between two points appeares to their pressure difference.

And, pressure difference also appeares to their velocity difference.

 

That's a great formula for "Force makes it go." Pressure is merely force over a defined area. Thanks alpha, I'll just this formula in my examples.

[caKus]

In the case of incompressible fluid and the same height.

 

[latex]\frac{\Delta P}{\rho } +\frac{\alpha \Delta v^{2}}{2} + f=0[/latex]

 

When there is no fraction loss, the fluid velocity difference between two points appeares to their pressure difference.

And, pressure difference also appeares to their velocity difference.

 

Alpha2Cen, could you please be kind enough and explicit the meaning of variables ?