MATLAB Central Newsreader - SimHydraulics: "Hydraulic Fluid" question

SimHydraulics: "Hydraulic Fluid" question

Clemens Winkler — Tue, 12 Aug 2008 15:05:02 -0400

Hello,

the SimHydraulics toolbox provides the "Hydraulic Fluid" block, as
described here:http://www.mathworks.com/access/helpdesk/help/toolbox/
physmod/hydro/ref/hydraulicfluid.html

The mentioned formula calculates a new bulk modulus against the
"Relative gas content at atmospheric pressure" (alpha).

But let us assume, that we have pure liquid, so alpha = 0. Now there's
no dependency of the pressure at all, because all terms with pressure
vanish, due to alpha = 0.

What I do not understand is: shouldn't there still be a dependency,
even if there's no gas in my liquid? The formula provides only a
pressure-dependency for the unsolved gas in the liquid. What about the
liquid itself?

Can anyone help me to understand that?
Thanks a lot!
Clemens

Re: SimHydraulics: "Hydraulic Fluid" question

Simo Kauth — Tue, 12 Aug 2008 15:44:33 -0400

The bulk modulus already tells you, how the fluid reacts on a change in
pressure:
http://en.wikipedia.org/wiki/Bulk_modulus
So: Yes, there is still a dependency on the pressure

Your formula only adapts the bulk modulus so that it copes with
dissolved gasses.

I hope this helped...

Clemens Winkler schrieb:
> Hello,
>
> the SimHydraulics toolbox provides the "Hydraulic Fluid" block, as
> described here:http://www.mathworks.com/access/helpdesk/help/toolbox/
> physmod/hydro/ref/hydraulicfluid.html
>
> The mentioned formula calculates a new bulk modulus against the
> "Relative gas content at atmospheric pressure" (alpha).
>
> But let us assume, that we have pure liquid, so alpha = 0. Now there's
> no dependency of the pressure at all, because all terms with pressure
> vanish, due to alpha = 0.
>
> What I do not understand is: shouldn't there still be a dependency,
> even if there's no gas in my liquid? The formula provides only a
> pressure-dependency for the unsolved gas in the liquid. What about the
> liquid itself?
>
> Can anyone help me to understand that?
> Thanks a lot!
> Clemens

Re: SimHydraulics: "Hydraulic Fluid" question

Clemens Winkler — Tue, 12 Aug 2008 16:00:14 -0400

On 12 Aug., 17:44, Simo Kauth <ka...@hrz.tu-chemnitz.de> wrote:
> The bulk modulus already tells you, how the fluid reacts on a change in
> pressure:http://en.wikipedia.org/wiki/Bulk_modulus
> So: Yes, there is still a dependency on the pressure

But I do have a book, which gives for a mineral oil the dependency of
the bulk modulus against the pressure. So the bulk modulus must change
with the pressure. I see that you are from the "TU", so you surely
know "Dubbel". The chapter about "Fluidische Antriebe" gives a plot
for the oil "HLP46": bulk modulus over pressure (H - Fluidische
Antriebe, page H22 Bild 3, Dubbel (2007)).

But as this is not the oil I need the data for, I'm looking for a
formula which gives me the correct dependency.

Clemens.

Re: SimHydraulics: "Hydraulic Fluid" question

Clemens Winkler — Tue, 12 Aug 2008 16:14:56 -0400

"Der Kompressionsmodul B beschreibt die Volumen=E4nderung bei
Druck=E4nderung (und damit nat=FCrlich auch die Dichte=E4nderung mit dem
Druck), er nimmt mit steigender Temperatur ab und steigendem Druck zu.
F=FCr ein Mineral=F6l wurde bei 20 =B0C B zu 2,1 * 10^4 bar bei
Atmosph=E4rendruck und 2,3 * 10^4 bar bei 250 bar Druck und bei 80 =B0C zu
1,4 bzw. 1,6 * 10^4 bar gemessen (s. Anh. H1 Bild 3). Der B
entsprechende E-Modul von Stahl ist 2,1 * 10^6 bar." (Dubbel)

(So to all the englishspeaking readers...! The text says, that the
bulk modulus describes the volume change at pressure change, and it
decreases with the temperature and increases with the pressure. Source
is a famous mechanical engineering book in german.).

Clemens

Re: SimHydraulics: "Hydraulic Fluid" question

Simo Kauth — Tue, 12 Aug 2008 16:49:33 -0400

Clemens Winkler schrieb:
> On 12 Aug., 17:44, Simo Kauth <ka...@hrz.tu-chemnitz.de> wrote:
>> The bulk modulus already tells you, how the fluid reacts on a change in
>> pressure:http://en.wikipedia.org/wiki/Bulk_modulus
>> So: Yes, there is still a dependency on the pressure
>
> But I do have a book, which gives for a mineral oil the dependency of
> the bulk modulus against the pressure. So the bulk modulus must change
> with the pressure. I see that you are from the "TU", so you surely
> know "Dubbel". The chapter about "Fluidische Antriebe" gives a plot
> for the oil "HLP46": bulk modulus over pressure (H - Fluidische
> Antriebe, page H22 Bild 3, Dubbel (2007)).
>
> But as this is not the oil I need the data for, I'm looking for a
> formula which gives me the correct dependency.
>
> Clemens.

Unfortunately I don't have the Dubbel here right now, nor would I call
me an expert on fluid dynamics. But I would guess that the mentioned
formula is rather an approximation, which might come close to reality in
most cases. Or under common operation pressures. In a real hydraulic
system, there will always be some gas dissolved in the fluid, which is
then probably dominating the scene.

Re: SimHydraulics: "Hydraulic Fluid" question

Clemens Winkler — Tue, 12 Aug 2008 17:03:40 -0400

On 12 Aug., 18:49, Simo Kauth <ka...@hrz.tu-chemnitz.de> wrote:

> Unfortunately I don't have the Dubbel here right now, nor would I call
> me an expert on fluid dynamics. But I would guess that the mentioned
> formula is rather an approximation, which might come close to reality in
> most cases. Or under common operation pressures. In a real hydraulic
> system, there will always be some gas dissolved in the fluid, which is
> then probably dominating the scene.

Thanks for your commments.

I would have thought that, too. But the strange thing is, that I
calculated the change in the bulk modulus with the SimHydraulics
formula with alpha = 0.01 (default value in the block is 0.005) for a
given fluid and get changes for the bulk modulus at about 0.002 * 10^9
Pa. for a pressure of >200 * 10^5 Pa. The book "Dubbel" talks about
changes of 0.x * 10^9 Pa over a pressure range from 0...250 * 10^5 Pa.
So from this point it is vice versa, the gas doesn't have much
influence over the change in pressure.

Maybe someone else have more information about this?
Thanks! Clemens