fluid compressibility: a question
manish prabhu
(Please bear with me for this rather elementary question.)
Hi,
I recently has a discussion with a friend over the idea of compressibi-
lity of liquids v/s gases. We reached a point where he claimed that liquids were
incompressible where as my belief was that even liquids can be compressed. My
understanding of liquid compressiblity was based on the notion of hydraulic
devices which use compressed oil to perform mechanical work. So the questions
that you could help me figure out are:
+ are liquids compressible (what's the standard definition of compressibility)
+ if they aren't why is the oil compressor in hydraulic devices called a
compressor at all?
Thanks in advance for your responses.
Manish
r
very small in many cases. Compressibility is the fractional decrease in
volume of a certain mass of the liquid when subjected to a specific higher
pressure. For example, the compressibility of water is about 49 x 10^-6 per
atmosphere at 13 atmos. This decreases to about 9 x 10^-6 at 12000 atmos,
as you might expect since the molecules are then much closer together.
Organic liquids tend to have higher compressibilities, that for ethyl
alcohol being about 100 x 10^-6 per atmos at 23 atmos. Even mercury is
compressible to the extent of about 4 x 10^-6 at around 300 atmos.
In answer to your last question, any device which increases the pressure of
a liquid should be called a pump, not a compressor. I have not come across
a hydraulic pump being called a compressor.
Todd R. Rose
> + are liquids compressible (what's the standard definition of compressibility)
YES!!!!
Look in a thermo book for the definition. Liquids are only approximated
as incompressible in reguimes were this is not an important factor. How
is sound propogated in liquids if they are not compressible? This is a
good question to ask your friend.
--
Todd R. Rose http://www.cae.wisc.edu/~rose
NEEP Dept., ERC, and RRC, The University of Wisconsin
1500 Engineering Drive, Madison, WI 53706
ro...@cae.wisc.edu (608)265-4498 Rm. 538 ERB
trr...@students.wisc.edu (608)233-7340 Home
Gerhard Bosch
manish prabhu (pra...@paris.cis.ohio-state.edu) wrote:
: (Please bear with me for this rather elementary question.)
: Hi,
: I recently has a discussion with a friend over the idea of compressibi-
: lity of liquids v/s gases. We reached a point where he claimed that liquids
: were
: incompressible where as my belief was that even liquids can be compressed. My
: understanding of liquid compressiblity was based on the notion of hydraulic
: devices which use compressed oil to perform mechanical work. So the questions
: that you could help me figure out are:
: + are liquids compressible (what's the standard definition of compressibility
: + if they aren't why is the oil compressor in hydraulic devices called a
: compressor at all?
Hi,
I guess the use of certain words is not very well-defined. You can
put pressure on all fluids. This is done f.e. in a compresssor.
Look at the Diesel injection pumps, they generate up to 1000 bar
today and the volume of the liquid (Diesel) does not change.
You should discuss the change of volume as compressibility
in this aspect. As will be mentioned in a different follow-up
it also depends if it a small or big change of volume. small
changes are responsible for the transport of acustics.
BTW: Diesel = Gazoline
// EOJ Mit freundlichen Gruessen, Gerhard Bosch
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Dana Swift
On 5 Oct 1996, manish prabhu wrote:
> (Please bear with me for this rather elementary question.)
>
> Hi,
> I recently has a discussion with a friend over the idea of compressibi-
> lity of liquids v/s gases. We reached a point where he claimed that liquids were
> incompressible where as my belief was that even liquids can be compressed. My
> understanding of liquid compressiblity was based on the notion of hydraulic
> devices which use compressed oil to perform mechanical work. So the questions
> that you could help me figure out are:
>
> + are liquids compressible (what's the standard definition of compressibility)
>
> + if they aren't why is the oil compressor in hydraulic devices called a
> compressor at all?
Yes, liquids are compressible as are solids. However, they are often
referred to as `incompressible' because liquids' compressibility tends to be
negligible when compared to that of gasses. Also, in many mathematical
treatments of practical problems, the compressibility of the liquid can be
ignored. For other problems, the compressibility is an essential element of
the physics. For example, propagation of sound in liquids or solids depends
fundamentally on the compressibility of the liquid or solid.
The compressibility is defined as the fractional rate of change of volume
with respect to pressure (with a minus sign):
1 dV
compressibilty = - - --
V dP
where V is a the parcel's volume, and the ratio dV/dP measures the small
change in volume due to a small change in pressure.
regards,
-dds
=========================================================================
= Dana D. Swift Office: =
= School of Oceanography o__ ____ Fax: =
= University of Washington _,>/'_ ----- sw...@ocean.washington.edu =
= Box 357940 (_) \(_) ------ ORB 106 =
= Seattle, WA 98195 =
= http://www.ocean.washington.edu/people/staff/swift/Swift.html =
=========================================================================
kenneth paul collins
Todd R. Rose wrote:
>
> > + are liquids compressible (what's the standard definition of compressibility)
>
>
> Look in a thermo book for the definition. Liquids are only approximated
> as incompressible in reguimes were this is not an important factor. How
> is sound propogated in liquids if they are not compressible? This is a
> good question to ask your friend.
...wave transmission can also occur via displacement within an imcompressible
medium if there are "state" transformation dynamics inherent in that medium... in
this view, water's freezing constitutes a sloooowly-transmitted wave :-) ken
collins
Todd R. Rose
>Look at the Diesel injection pumps, they generate up to 1000 bar
>today and the volume of the liquid (Diesel) does not change.
Yes, it does change. I worked for Cummins Engine Company modeling fuel
injection systems. All fluids and solids are compressible. It is a
matter of order of magnitude.
Alastair Martin
r wrote:
>
> In article <5371rh...@paris.cis.ohio-state.edu>, pra...@paris.cis.ohio-state.edu (manish prabhu) says:
> >
> >
> >Hi,
> > I recently has a discussion with a friend over the idea of compressibi-
> >lity of liquids v/s gases. We reached a point where he claimed that liquids were
> >incompressible where as my belief was that even liquids can be compressed. My
> >understanding of liquid compressiblity was based on the notion of hydraulic
> >devices which use compressed oil to perform mechanical work.
> >
This post is not meant to be of much use to the original questioner, but maybe
someone can clarify things for me...
All fluids are compressible, right, so what we refer to as an `incompressible'
fluid is merely one which changes volume by only a very small amount (not zero)
when compressed. This means that all fluids can store energy as pressure, because
you do work on them when you put them under pressure, however nearly
incompressible they may be.
Bernoulli's equation:
Pressure head + velocity head + gravity head = a constant on a streamline
This explains why the pressure in a fluid drops when it speeds up (e.g. goes
through a constriction). The standard explanation is that as the fluid elements
flow (steadily) along their streamlines, they swap energy between these three
terms. The total amount of energy (in the absence of dissipation) stays the same,
and the three "head" terms are energy terms.
However, B.S. Massey (Mechanics of Fluids, 6th ed., 1989) says (p.79)...
"The quantity p/rho is sometimes misleadingly termed `pressure energy'. It has,
however, nothing to do with the elastic energy given to a fluid when it is
compressed - even when it is easily compressible. The fluid does not even
_possess_ the `pressure energy' (as it possesses kinetic energy, for example). A
transmission belt transmits energy between two pulleys simply because it is under
stress; the transmission of energy is in fact in the _opposite_ direction to the
movement of the belt [picture] and so it is clearly absurd to regard the energy as
being carried along the belt. Likewise a fluid under stress (pressure) can
transmit energy without necessarily possessing it. The terms in Bernoulli's
equation, then, do not represent energy _stored_ in unit mass of fluid but rather
the total mechanical energy transmitted by this amount of fluid. The equation may
be likened to the cash-book of an honest treasurer keeping account of the
mechanical energy transactions of his small society, Unit Mass of Fluid, during
its steady, frictionless travel along a streamline without change of density."
My question is this: When looking at the pressure term in Bernoulli's equation, is
it wrong to think of the fluid as a compressed spring, storing a certain amount of
energy related to the force which was required to compress it? Massey seems to say
so. If it is wrong to talk of `pressure energy', then what is happening to the
energy that the fluid possesses as it travels along a streamline?
If anyone can point out what I've missed here, I'd be grateful!
Alastair
kenneth paul collins
r wrote:
[snip]
> All liquids are compressible to some extent, even though the amount may be
> very small in many cases. Compressibility is the fractional decrease in
> volume of a certain mass of the liquid when subjected to a specific higher
> pressure. For example, the compressibility of water is about 49 x 10^-6 per
> atmosphere at 13 atmos. This decreases to about 9 x 10^-6 at 12000 atmos,
> as you might expect since the molecules are then much closer together.
> Organic liquids tend to have higher compressibilities, that for ethyl
> alcohol being about 100 x 10^-6 per atmos at 23 atmos. Even mercury is
> compressible to the extent of about 4 x 10^-6 at around 300 atmos.
in the experiemnts which give these results, how is the deformation of the
apparatus kept separate from the deformation of the fluid...? diamond anvil...?
ken
[snip]
_____________________________________________________
People hate because they fear, and they fear because
they do not understand, and they do not understand
because hating is less work than understanding.
kenneth paul collins
r wrote:
[snip]
> All liquids are compressible to some extent, even though the amount may be
> very small in many cases.
FWIW, I've developed an alternative view on Physical Reality which
incorporates an incompressible "extreme fluid"... an incompressible, but
flowing, "aether"... "non-particulate energy"...
...I'd greatly appreciate any criticisms of this view from you experts in
Fluid Mechanics...
...the theory is called "Tapered Harmony", in it, what have been referred to
as "atoms" are seen as being Spherical Standing Waves (SSWs) that are in
"compression"-"expansion" harmonic interaction with the Universal Energy
Supply (UES; SSW<->UES harmonics)...
...the SSWs exchange energy with the UES, but the quantity of energy
"trapped" in the harmonic remains "invariant" (not quite it, but ok for
now)... as energy is exchanged it undergoes the energy-matter
(matter-energy) phase transition...
...it was this to which I was referring in a prior post in this thread...
waves can travel in an incompressible fluid if there's a phase change...
...anyway, what's analogous to "compressibility" in this view is "degree of
motion" within the pure-energy "extreme-fluid" (the non-particulate energy
of the Universe)...
...I've been able to discover no problems with this view... and would
appreciate any applications of the Fluid Mechanics "hammer" that folks might
apply to the view...
...I introduced the concept of energy as constituting an "extreme-fluid"...
the extreme fluid is like a "superfluid", except that it has no particles,
and that it's incompressible =because= it lacks particles... in this view,
therefore, what's referred to as "compressibility" reduces to a "tightening"
of the SSW<->UES harmonics... a decreasing of energy's freedom to move...
...it requires more discussion, but in Tapered Harmony's view everything
within Physical Reality, including the 4 conventional "forces", reduces
directly to what's described by 2nd Thermo (WDB2T)... so it's really worth
the "struggle"... in this view, "entropy" is the ever-increasing degree of
energy's freedom to move as the Universe "expands"... not that stuff like
radioactive decay are built right in because, as the universe expands,
energy's freedom to move increases, and this allows an atom here & there to
disintegrate into its decay products... which releases energy which
decreases the non-SSW energy's freedom to move, so the decay "stops"...
everything has this sort of explanation in Tapered Harmony... the theory is
a Fluid Mechanic's "playground" :-)
...what I'd like to receive are insights with respect to the
incompressibility of the "extreme fluid"...
Terry Frangakis
manish prabhu <pra...@paris.cis.ohio-state.edu> wrote in article
<5371rh...@paris.cis.ohio-state.edu>...
> (Please bear with me for this rather elementary question.)
>
> Hi,
> I recently has a discussion with a friend over the idea of compressibi-
> lity of liquids v/s gases. We reached a point where he claimed that
liquids were
> incompressible where as my belief was that even liquids can be
compressed. My
> understanding of liquid compressiblity was based on the notion of
hydraulic
> devices which use compressed oil to perform mechanical work. So the
questions
> that you could help me figure out are:
>
> + are liquids compressible (what's the standard definition of
compressibility)
>
> + if they aren't why is the oil compressor in hydraulic devices called
a
> compressor at all?
>
> Manish
Hi Manish
Yes, fluids are compressible! At low pressures it is assumed to be
incompressible because the effect is negligible. In my MSc I came across
the Tait equation, originally derived in experiments of underwater
explosions. The equation predicts the density of the fluid as a function of
pressure as:
(P+B)/(rho^n) = C where P is liquid pressure, B,n and C and constant and
rho density of fluid. For water the values of B and n are of the order of
3075 bar and 7.14
I imagine that for different fluids different values of B and n would be
available.
Have a look at "Underwater Explosions" by Cole R H 1948 or 1965 if you can
lay your hands on it.
You can also compare the equation with data for compressed water by
Bridgeman P "Physics of High Pressure". I found that for water at 50degC a
fitted curve to the Bridgemand data resulted in B as 3750 bar and n as
6.09756. There are a few texts around with different constants for
different fluids. Have a look around!
Hope this helps
Terry Frangakis
CSIR Mining Technology
tfra...@csir.co.za
Julian Scarfe
Alastair Martin wrote:
> All fluids are compressible, right, so what we refer to as an `incompressible'
> fluid is merely one which changes volume by only a very small amount (not zero)
> when compressed. This means that all fluids can store energy as pressure, because
> you do work on them when you put them under pressure, however nearly
> incompressible they may be.
...
> My question is this: When looking at the pressure term in Bernoulli's equation, is
> it wrong to think of the fluid as a compressed spring, storing a certain amount of
> energy related to the force which was required to compress it? Massey seems to say
> so. If it is wrong to talk of `pressure energy', then what is happening to the
> energy that the fluid possesses as it travels along a streamline?
In all elastic systems, the energy that you can store in the system for
a certain force reduces with the stiffness of the system.
In a spring
where F = k x,
energy stored = 1/2 F^2/k
In a capacitor with reactance X (capacitance 1/X)
where V = X Q
energy stored = 1/2 V^2/X
In a fluid with compressibility b,
where (pressure change) = b * (fraction volume change),
energy stored per unit vol = 1/2 (pressure change)^2/b
So in a fluid that is almost incompressible, there is almost no elastic
energy stored at attainable pressures (or at least it is insignificant
compared to other energy terms). As Massey says, the pressure energy in
Bernoulli's equation is not the elastic energy.
--
Julian Scarfe ja...@scigen.co.uk
Scientific Generics http://www.generics.co.uk/
Cambridge CB2 5NH Tel: +44-1223-875200
UK Fax: +44-1223-875201
charliew
In article <5371rh...@paris.cis.ohio-state.edu>,
pra...@paris.cis.ohio-state.edu (manish prabhu) wrote:
> (Please bear with me for this rather elementary
question.)
>
>Hi,
> I recently has a discussion with a friend over the
idea of compressibi-
>lity of liquids v/s gases. We reached a point where he
claimed that liquids were
>incompressible where as my belief was that even liquids can
be compressed. My
>understanding of liquid compressiblity was based on the
notion of hydraulic
>devices which use compressed oil to perform mechanical work.
So the questions
>that you could help me figure out are:
>
> + are liquids compressible (what's the standard definition
of compressibility)
>
> + if they aren't why is the oil compressor in hydraulic
devices called a
> compressor at all?
>
>
> Manish
You may be getting into an argument in semantics here.
Liquids are "relatively" incompressible. So are solids.
However, if you put 100,000 atmospheres of pressure on
anything (e.g., in a diamond anvil cell for instance), you
will positively see evidence of compression. Before
continuing your argument, specify an upper pressure limit,
and an upper limit for "compressibility" vs. applied
pressure. In other words, you may want to define an
incompressible substance as one which "loses" less than 1% of
its volume when 1000 atmospheres of pressure is applied to it
(or some other measure that you choose). Have fun.
============================================================================
For some *very* interesting alternate viewpoints, look at
http://www.hamblin.com/mf.main/articles.html
Anilkumar R Shenoy
charliew wrote:
>
> In article <5371rh...@paris.cis.ohio-state.edu>,
> pra...@paris.cis.ohio-state.edu (manish prabhu) wrote:
> > (Please bear with me for this rather elementary
> question.)
> >
> >Hi,
> > I recently has a discussion with a friend over the
> idea of compressibi-
> >lity of liquids v/s gases. We reached a point where he
> claimed that liquids were
> >incompressible where as my belief was that even liquids can
> be compressed. My
> >understanding of liquid compressiblity was based on the
> notion of hydraulic
> >devices which use compressed oil to perform mechanical work.
> So the questions
> >that you could help me figure out are:
> >
> > + are liquids compressible (what's the standard definition
> of compressibility)
> >
> > + if they aren't why is the oil compressor in hydraulic
> devices called a
> > compressor at all?
> >
> >
> >Thanks in advance for your responses.
> > Manish
>
I would like to add a few comments about the term P/rho that appears in
the Bernoulli equation.
The Bernoulli equation states that
For Incompressible Fluid:
P/rho + V^2/2 + gz = constant ( along a streamline )
This constant is same for all the streamlines if the flow is
irrotational.
The question is , What is P/rho ??
Is it Pressure Energy ?????
The answer is No. It is the pressure work. It is the work done by
pressure to push the fluid. This is similar to the work done to push a
solid object. It cannot be called energy because it is not a property of
the state. It is a phenomena that brings about a change in the state.
So In simple words it is the work that the pressure force do to displace
the fluid.It grings about a change in the kinetic and potential energy
term, but is not energy itself.
For Compressible Fluids ,we have
Integral( P/rho) + V^2/2 + gz in the Bernoulli equation:
In this case the pressure is not only displacing the fluid but also
compressing or expanding it. We should know what process the fluid
follows eg: isothermal, isentropic etc .To evaluate the integral as it
is path dependent. This tells us that it cannot be energy which is a
function of state only.
Again here the pressure work brings about a change in the kinetic and
potential energy. But now it will influence the internal energy also.
It may seem that the pressure integral term only affects the internal
energy and not the fluid flow. But from continuity equation:
rho*v*area = constant
So pressure changes density, which changes velocity.
So the main point being that p/rho is not energy, but WORK.
I hope this helps.
Bye
*************************************************************
Anil Kumar R. Shenoy
Graduate Research Assistant
School Of Aerospace and Mechanical Engineering
University Of Oklahoma
Norman , OK
Residence :
212 D , Wadsack Drive
Norman, OK 73072
Telephone: 405-325-8053
email :
nasa...@mailhost.ecn.uoknor.edu
ash...@mailhost.ecn.uoknor.edu
ash...@suman.cs.uoknor.edu
************** GREAT MEN ARE MEN OF FEW WORDS
**************************
r
Whoa there! I think Boeing would disagree with you there on the insignificant
energy stored in a compressed liquid. When a 747 comes in to land, the
oleo struts that support the wheels compress a liquid, not a gas, and 400
tons of earthward bound 747 come to rest in seconds. That is quite a lot of
energy stored.
Ralph.
Marco Mulas
Alastair Martin wrote:
>
> text deleted
>
> My question is this: When looking at the pressure term in Bernoulli's equation, is
> it wrong to think of the fluid as a compressed spring, storing a certain amount of
> energy related to the force which was required to compress it? Massey seems to say
> so. If it is wrong to talk of `pressure energy', then what is happening to the
> energy that the fluid possesses as it travels along a streamline?
>
>
> Alastair
first of all: the Total Energy E, which is the quantity conserved according
to the 1st law of thermodynamics, is made of Internal Energy e and Kinetic
Energy k.
E = e + k
there is NO other type of energy stored but e and k. So "pressure energy"
doesn't mean anything unless you specify more precisely.
Then: the PDE for E can be seen as the sum of a PDE for e and another PDE
for K.
The "compressibility" term p*dV (V is volume) doesn't appear in the PDE for E,
but it does appear in both the e and the K PDEs, with opposite sign, meaning
that it represents an exchange of energy between internal and kinetic. So that
it shouldn't be taken into acount when playing with the Total Energy alone
(which does not change).
More precisely the "compressibility" term is written in the PDEs as
P * div (U)
which can be called for instance, pressure dilatation work.
div(U) is in fact a measure of the state of fluid compression (expansion) and
as well as a measure of the volume (density) change. In fact from the continuity
equation you have:
div (U) = - 1/rho * D(rho)/Dt
Now, if P * div (U) > 0 this means that the fluid is expanding and the
term represents a tranformation of internal energy into kinetic energy
(e decreases, k increases and E as usual remains the same).
if p * div(U) < 0 the fluid is slowing down and k is being transformed
back into internal energy e.
The transformation is reversible and energy can be transformed back and forth
between e and k.
If the fluid is assumed incompressible (liquids for instance), then
div(U) is extremely small and can be neglected.
The meaning is that in an incompressible fluid the pressure cannot perform
work on the fluid particle (better: the pressure cannot change the internal
energy of the fluid particle).
If the fluid is also inviscid-adiabatic, then e and k
do not comunicate any longer.
More precisely e remains constant and k follows Bernoulli eq.
k + p/rho = const
(rho = const as well, and gravity is neglected)
if the fluid is viscous-heat-conducting, then e can be changed by
viscous dissipation and heat conduction, and k and e do have a
"one way" comunication line: k is always dissipated into e.
i.e. this is irreversible energy exchange between e and k
One final remark: in case the fluid is compressible, Bernoulli eq.
has to be written as:
k + int (dp/rho) = const
since rho is no longer constant and cannot be taken out of the integral.
All the above can easily be read from the PDEs for E, e and k, if properly
written.
Marco Mulas
M.S.Cramer
Alastair Martin wrote:
[snip]
>
> However, B.S. Massey (Mechanics of Fluids, 6th ed., 1989) says (p.79)...
>
> "The quantity p/rho is sometimes misleadingly termed `pressure energy'. It has,
> however, nothing to do with the elastic energy given to a fluid when it is
> compressed - even when it is easily compressible. The fluid does not even
> _possess_ the `pressure energy' (as it possesses kinetic energy, for example). A
> transmission belt transmits energy between two pulleys simply because it is under
> stress; the transmission of energy is in fact in the _opposite_ direction to the
> movement of the belt [picture] and so it is clearly absurd to regard the energy as
> being carried along the belt. Likewise a fluid under stress (pressure) can
> transmit energy without necessarily possessing it. The terms in Bernoulli's
> equation, then, do not represent energy _stored_ in unit mass of fluid but rather
> the total mechanical energy transmitted by this amount of fluid. The equation may
> be likened to the cash-book of an honest treasurer keeping account of the
> mechanical energy transactions of his small society, Unit Mass of Fluid, during
> its steady, frictionless travel along a streamline without change of density."
>
> My question is this: When looking at the pressure term in Bernoulli's equation, is
> it wrong to think of the fluid as a compressed spring,
storing a certain amount of
> energy related to the force which was required to compress it? Massey seems to say
> so. If it is wrong to talk of `pressure energy', then what is happening to the
> energy that the fluid possesses as it travels along a streamline?
A:
This is not really a compressibility question.
You don't need fluid compressibility to interpret
the pressure term in Bernoulli's eqn. It is not
neccessarily an elastic energy. It is the POTENTIAL
energy associated w/ the work done by the NET pressure
force acting on a blob. When the fluid is incompressible,
then the pressure force is conservative & there is an
associated potential energy. In the simplest cases considered
in engineering fluids texts the work done by "gravity" &
pressure forces is what changes the kinetic energy of a fluid
blob.
When compressiblity is non-negligible the reversibility,
i.e.,path dependence,must be considered.The existence of a
potential will then depend on whether the flow is isentropic or not.
In an isentropic compressible flow the blob energy is exchanged
among kinetic,work due to the pressure ---- both due to moving
the fluid blob around & changing its volume (<---this is your
"elastic" energy)---- and "internal" energy ----- this latter
mode results in temperature changes in ideal gases.
Massey is undoubtedly thinking in terms of control volumes
& therefore is attempting to interpret "flow work". You have
asked a question about the physics & the best way to think about
physics is always through the Lagrangian view (the point a view of a
particle).
Hope this is of some use-----good luck
Mark Cramer
Engr.Sci.& Mech.
VPI&SU(aka Virginia Tech)
Julian Scarfe
I wrote:
> >So in a fluid that is almost incompressible, there is almost no elastic
> >energy stored at attainable pressures (or at least it is insignificant
> >compared to other energy terms). As Massey says, the pressure energy in
> >Bernoulli's equation is not the elastic energy.
Ralph (rto...@oanet.com) wrote:
> Whoa there! I think Boeing would disagree with you there on the insignificant
> energy stored in a compressed liquid. When a 747 comes in to land, the
> oleo struts that support the wheels compress a liquid, not a gas, and 400
> tons of earthward bound 747 come to rest in seconds. That is quite a lot of
> energy stored.
It's also quite a lot of pressure! The stiffer (the more
incompressible) the material used, the higher the decleration forces
will be. Mooney wrecks my landings every time by using rubber discs --
I'll take the oleo any day. The oleos are designed to dissipate the
energy rather than store it. Or have you been flying QANTAS again? ;-)
The point is, you can make any material with a non-zero compressibility
store an arbitrary energy, but more incompressible the material, the
higher the pressure you need to do it. At "everyday" pressures, water
doesn't hold much elastic energy.
--
Guy Deraspe
ralf wrote:
>
> Whoa there! I think Boeing would disagree with you there on the insignificant
> energy stored in a compressed liquid. When a 747 comes in to land, the
> oleo struts that support the wheels compress a liquid, not a gas, and 400
> tons of earthward bound 747 come to rest in seconds. That is quite a lot of
> energy stored.
>
I am not an expert in landing gear design, but I would thing that for
the B-747 must of the energy is transferd to compressed gas.
I have heard about shock absorber with no gas at all, oil only, for
aircraft which land on Aircraft Carrier, but I do not know the
reliabilty of my source on that mather. Anyboby could comment on that?
Guy Deraspe