Another "myth to kill"??.....what makes it go???

[Ford tractor]

Well "flow" sure makes the keyboards go!

I couldn't add any to the group mind, except for some levity!

 

[Egon]

How you doin' tonight Egon?

Been outside working on the camper all day.

That means that tonight I'm tired, the feet hurt and most of the major joints ach and pain!

 

[gpflepsen]

Well "flow" sure makes the keyboards go!

I couldn't add any to the group mind, except for some levity!

Well, I had some flow going last night, but that just gave me some pressure in the head this morning! :laughing:


For the formula units folks, I pulled out a reference for the resistance units in the SI system. Note the Hydraulics entry in Table 3.1 of the attachment*.

I think Newtons second law is probably more applicable here, since we're talking about "making it go", F=ma, or a=F/m.

Now think about the causality of what the laws of motion (equations) say in terms of cause and effect. Cause is the input and effect is the output. For a=F/m, the force is the input (or what we control) and the output is the resultant acceleration.

Now applying causality to the "makes it go" myth The two different "make it go" scenarios for simple hydraulic systems are as I stated earlier;

1) Positive displacement pumps: Pressure = Flow x Resistance. These pumps can only produce flow, thus flow is causing the pressure according to the causality of the defining relationship.

2) Centrifugal pumps: Flow = Pressure/Resistance. These pumps produce pressure (input or cause) and the pressure results in flow (output or effect) according to the causality of the defining relationship.

So in case 1, Flow causes pressure which converts to force which causes acceleration.

In case 2, pressure converts to force which causes acceleration.

This is just an exercise in thinking which does extend to real world applications. This is a process that allows logical steps between understanding, design and implementation. I think we can all agree that when the rubber hits the road (or the oil hits the piston face:laughing and you boil down the "myth", Newton says it best via the Second Law of Motion;

a=F/m

which says motion is the result of a force applied!



*Karnopp, Dean. (2006). System Dynamics: Modeling and Simulation of Mechatronic Systems. Hoboken New Jersey: John Wiley and Sons, Inc.

 

[SPYDERLK]

Riddle me this...

Say I have a reservoir of fluid which I can vary the height above ground. This reservoir is connected via hose to a hydraulic cylinder. This cylinder is connected to a load of some kind, say a block on the ground. To make the block move, I'll have to raise the tank to a sufficient height to produce the pressure to overcome the friction... then the block will move. I am supplying pressure which results in motion (or flow). Ergo, pressure makes it go. Say the Pressure-Flow interaction is a linear relationship with friction,

Flow=Pressure/Friction

Here Flow is a function of the pressure, that is the pressure dictates (is in charge of) the motion. The myth is dead!

Take the tank in the example above and replace it with a positive displacement pump (spinning at a constant speed) providing a constant flow. The relationship between the motion and the pressure then becomes

Pressure=Flow x Resistance.

In this case, the pressure is a function of the Flow, i.e. the Flow dictates the pressure. The myth is alive! The flow made it go!

The two examples above are different. One has an input of flow and the other has an input of pressure. The myth is... well it depends!

I dont dispute the "sensibleness" of your equations. The reason they dont work out right as AKKAMAAN shows is that the treatment does not accomodate interfunctionalities. For instance, in the P=FR, Resistance is a function of Flow -- true in the piping but not the actual load. In truth Pxarea=Resist. These are forces. Throw in Flow and suddenly your talking Power so youve got Pressure = Power in your equation. You cant be jumping around like this. These things will mix up without a rigorous treatment, and skew your conclusions.

I see the difficulty is grasping that Pressure makes it go because the movement from pressure is incrementally zero but done an infinite number of times. Each motion is just followed an infinitisimal time later by flow to maintain the motive force from the pressure and therefore motion. Its a good physical demo of the infinitely precise approximation resulting from integration in calculus.
larry

 

[AKKAMAAN]

Thank you for "sobering up" from your first initial posts....LOL
your take on this start making sense now....I have a few comments on your pump alternatives

1) Positive displacement pumps: Pressure = Flow x Resistance. These pumps can only produce flow, thus flow is causing the pressure according to the causality of the defining relationship.

2) Centrifugal pumps: Flow = Pressure/Resistance. These pumps produce pressure (input or cause) and the pressure results in flow (output or effect) according to the causality of the defining relationship.

Centrifugal pumps are creating pressure from the kinetic energy of the fluid. kinetic energy is the product of velocity and mass. A positive pump create pressure from the force behind displacing the fluid. I do not see a reason to draw any further parallels to to centrifugal pumps, since they are of zero interest for hydrostatic applications.
(However, here is a link to a introduction to centrifugal pumps)

Your pressure-flow-resistance discussion is a matter of how we want to vary the hydraulic power output.
We can sum up to two basic pump systems, applied into three different power systems.

A: a fixed displacement pump in a open center system. FLOW is CONSTANT, and PRESSURE will be VARIABLE with the resistance. >>> "Pressure = Flow x Resistance"

B: a variable displacement pump, pressure compensated, in a closed center system. PRESSURE will be CONSTANT and FLOW will be VARIABLE with the resistance. >>> "Flow = Pressure/Resistance"

(Note: A centrifugal pump application is basically a CONSTANT PRESSURE system, and "Flow = Pressure/Resistance" applies, so is a basic electric system, I=U/R )

C: a variable displacement pump, load pressure compensated, Load sensing system, LS, where both flow and out put pressure can be varied to regulate the power output

So in case 1, Flow causes pressure which converts to force which causes acceleration.

Sorry I can not "buy" this one, only force can cause pressure....force from kinetic energy, gravity, torque, pressure etc .....force unit includes "lbs", and there is no "lbs" in the flow unit.......

In case 2 ???, pressure converts to force which causes acceleration.

I think we can all agree that when the rubber hits the road (or the oil hits the piston face:laughing and you boil down the "myth", Newton says it best via the Second Law of Motion;

a=F/m

which says motion is the result of a force applied!

:thumbsup:

 

[gpflepsen]

Oh, I was sober at my first post :thumbsup:

Guys, remember the preface to my string of post here;

...In some world of dynamic modeling...

That world actually exist!

Sorry I can not "buy" this one, only force can cause pressure....force from kinetic energy, gravity, torque, pressure etc .....force unit includes "lbs", and there is no "lbs" in the flow unit.......

I'm talking in terms of causality in the functional relationships, cause and effect.

Thanks for the pumpintro pdf.

...velocity is converted to pressure...

Another way to say this is... velocity (flow) causes pressure. Just look at a pump curve! :laughing: Another example of the velocity/pressure interdependence is a jet pump. And look what Bernoulli figured out!

Just remember Newton's 2nd Law, a=F/m!

And Larry, perhaps I should have said

P=f(Q,R) Pressure is a function of flow and resistance,

and

Q=f(P,R).

I had stated, for this example and the sake of simplicity, that the relationship was linear. But I think most know the relationship is really a bit more complicated than that!


Here's a semi-related question for one to ponder; would you use Newtons laws or Bernoulli's principle to explain how an airplane produces lift?

 

[mmurphy]

I wish I was smart enough to add more to this, but I do have the popcorn popper going! Keep bringing more guys, It is a good debate.:thumbsup:

 

[J_J]

There are engineers that dream this stuff up, and there are technicians that make it work. The end user however, is the owner, and usually brings it back to the technician to get it fixed. Even the engineer goes back to the technician to make sure it works to get the job done. I am just saying.

 

[SPYDERLK]

Another example of the velocity/pressure interdependence is a jet pump. And look what Bernoulli figured out!

Just remember Newton's 2nd Law, a=F/m!

And Larry, perhaps I should have said

P=f(Q,R) Pressure is a function of flow and resistance,
[[[...Yes, but resistance can be a function of flow, so youre multiplying something by a function of itself.]]]
and

Q=f(P,R).

I had stated, for this example and the sake of simplicity, that the relationship was linear. But I think most know the relationship is really a bit more complicated than that!


[[[Here's a semi-related question for one to ponder; would you use Newtons laws or Bernoulli's principle to explain how an airplane produces lift? ]]]

I would say Newtons Laws because the air can be standing still and must suddenly do a semi orbit around the top surface of a wing. The acceleration it undergoes toward the center of the orbit is A = Vrel^2/r. To achieve the Force needed to accelerate its mass the air sucks on the wing. The Force of this sucking is F=MairA=MVrel^2/r.
larry

 

[Egon]

would you use Newtons laws or Bernoulli's principle to explain how an airplane produces lift?

First we should determine the mode the wing is working in should we not?:thumbsup:

I've always wondered how a wing produces lift yet a plane can fly upside down?:thumbsup: