0:00

So, in this video, we're going to be focusing

on the flow, that's coming out of the pump.

Now, you might say, well, doesn't the flow, flow continuous?

Well, actually, it's not and we refer to that by flow ripple.

And the definition of of a flow ripple, is just the variation

in the flow rate, coming out of the pump, over a given cycle.

Or over, you know, a, a single displacement of a piston, or, a gear.

So, what I'm showing here, is the pressure variation of, a, of an axial piston pump.

With time, and what you're seeing is that,

we get these little variations in the pressure.

Well, the reason for that, is the flow coming

out of the pump actually varies with time, and

what this causes, is the, the pressure variations like

this, but then it causes noise in the system.

We call that fluid borne, borne noise.

It also causes vibrations and, and other, you know,

things that we really would not prefer to have.

So, let's discuss where these flow ripples are coming from and why they're there.

Now, before I get any further, let me say that, what we are going to be discussing

here, is purely kinematic flow ripple, meaning it is

just due to the geometry of the pump itself.

We are neglecting any compressibility of

fluids, which adds additional flow ripple to

the, to the actual fluid flow coming out at a, at a given pressure.

So, first of all, let's start with a gear pump.

Now, I mentioned before that gear pumps are

quite noisy, and a large reason for that

is a fluid borne, borne noise of the, the varying flow rate, coming out of the pump.

Now, this pump right here, happens to have seven teeth on the

gears, which is a fairly low number compared to most gear pumps.

And because of that, we get a very large flow ripple.

So you can see the, the flow here, is a

non-dimensional flow rate coming out of this gear, gear flow pump.

And if you're curious about, where does, where does this all come from?

Well, you can imagine, i'm basically moving a constant volume of fluid, around

the outside of the gear teeth as i'm rotating but the volume that is

being occupied as I'm meshing these gear teeth, is really changing as the

teeth come into mesh and then start

moving through there, through the meshing process.

And because of that, I'm changing the amount of volume on the outlet side.

And creating this, this flow ripple.

Now, flow ripple on a gear pump, the higher the

number of the gear teeth are, the lower the flow

ripple will be and the higher the pressure angle is

of these gear teeth, the lower the flow ripple will be.

So, there are ways that we can minimize it but,

in general, it's somewhere to 25 to 35% of the,

the mean flow rate and again with this seven piston

pump, we get a large amount of, of flow ripple.

If you're curious about how do I create these plots?

Well, some of the, the readings that I've posted on the

course site, will, will give you some more background into that.

2:31

So, what about a piston pump?

Now, I'm saying piston pump generically because

most piston pumps, move in a sinusoidal motion.

Where the pistons are, moving back

and forth approximately sinusoidally or exactly sinusoidally.

And because of that I can, kind of, talk generically about piston pumps.

So, let me think about maybe, just a one cylinder piston pump.

What's going to happen is I'm going to get a flow ripple of

that half sine wave and then the second half of the sine

wave, I'm going to be, filling that cylinder, so I'm going to get

a little blip and then no flow and then a little blip.

When I go to, two pistons, I'm going to get a

flow ripple that looks like this, two humps, where I go

from zero flow to the maximum flow, back to zero,

and so on and so forth, continuously, over and over again.

And I'm plotting this across an entire cycle, so

the x-axis here, is an entire revolution of the pump.

3:36

Three to four actually get more detrimental pump performance

because I have, more flow ripple in four piston

pump than I do in a three and also

the magnitude of the, of these flow ripples is larger.

How about adding one more piston?

Let's go to five and now our, again, our

flow ripple magnitude decreases and the number of ripples increases.

Well, what's going on here, is the way we're adding up these sine waves,

it turns out that if we have an odd number of cylinders we're going to get

two times the number of cylinders as far as two, the number of ripples

it will have will be two times the number of cylinders multiplied by the frequency.

And if we have an even number, we're going to get that number of, of, ripples.

So, with a four piston pump, we get four ripples for every revolution.

With a five piston pump, we get 10.

And so, this is why a piston pump is typically always

an odd number of pistons 7, 9, 11, something like that.

Because we're trying to minimize the flow ripple coming out of the pump.

So, this flow ripple, again, can be important for noise considerations.

Vibrations in our systems but, you know, really as a, as a

defining characteristic and this kinematic flow ripples only a small piece of it.

so, as we talk about this throughout the

class, remember, there is this flow ripple going on.

And because we're accelerating flow coming out of the pump, will result in a,

in a pressure ripple which then we have to, to deal with later on.

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