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Determination of density at a depth in a compressible liquid.
Hi all,
Let us consider a tank containing some compressible liquid with bulk modulus K and having density at its surface s. Consider some part of liquid with volume V, a depth h below the surface. Due to liquid above it there will be some extra pressure dP, acting on the volume V of the liquid considered. This pressure or volumetric stress will cause some volumetric strain(negative) dV in liquid volume V.
The bulk modulus is related with the pressure and volume changes as K= -dPV/dV or
1/K= -dV/dPV. ...(1)
We also know density = mass/volume...
For the sake of analysis we can assume Liquid volume V at the surface(where the liquid is negligibly compressed). If its density is s(as specified already), then we can say:
s=m/V. ...(2)
where m is the mass of liquid with volume V at surface.
To form a similar equation for density at a depth h, let us imagine that the liquid of volume,V is now dipped below a depth h. At this depth the volume is this liquid changes by dV(a negative value) the mass of this liquid will however remains same. This accounts for a change in density say s* being the density at depth h. The equation will then be:
s*= m/(V+dV). ...(3)
from equation (2), m=sV, putting this value in 3 we get
s*= sV/(V+dV). or
V+dV= sV/s*. or
dV= sV/s* -V. or
dV= V(s/s*-1). ...(4)
Putting this value in equation 1,we get
1/K= -V(s/s*-1)/VdP. or
1/K= (1-s/s*)/dP...(5)
Equation (5), will give the density s* at a depth h, provided we know the pressure differenece at depth h.
This is where I am finding difficulty. At one look, one may simply say the pressure at depth h will be hsg. But as a matter of fact the liquid is compressible its density is not s throught. The formula hsg will work only if the density were constant and liquid is incompressible.
I have tried a lot of integration tools but in vain.
Any help will be highily appreciated.
Thanks a bunch.
Hi all,
Let us consider a tank containing some compressible liquid with bulk modulus K and having density at its surface s. Consider some part of liquid with volume V, a depth h below the surface. Due to liquid above it there will be some extra pressure dP, acting on the volume V of the liquid considered. This pressure or volumetric stress will cause some volumetric strain(negative) dV in liquid volume V.
The bulk modulus is related with the pressure and volume changes as K= -dPV/dV or
1/K= -dV/dPV. ...(1)
We also know density = mass/volume...
For the sake of analysis we can assume Liquid volume V at the surface(where the liquid is negligibly compressed). If its density is s(as specified already), then we can say:
s=m/V. ...(2)
where m is the mass of liquid with volume V at surface.
To form a similar equation for density at a depth h, let us imagine that the liquid of volume,V is now dipped below a depth h. At this depth the volume is this liquid changes by dV(a negative value) the mass of this liquid will however remains same. This accounts for a change in density say s* being the density at depth h. The equation will then be:
s*= m/(V+dV). ...(3)
from equation (2), m=sV, putting this value in 3 we get
s*= sV/(V+dV). or
V+dV= sV/s*. or
dV= sV/s* -V. or
dV= V(s/s*-1). ...(4)
Putting this value in equation 1,we get
1/K= -V(s/s*-1)/VdP. or
1/K= (1-s/s*)/dP...(5)
Equation (5), will give the density s* at a depth h, provided we know the pressure differenece at depth h.
This is where I am finding difficulty. At one look, one may simply say the pressure at depth h will be hsg. But as a matter of fact the liquid is compressible its density is not s throught. The formula hsg will work only if the density were constant and liquid is incompressible.
I have tried a lot of integration tools but in vain.
Any help will be highily appreciated.
Thanks a bunch.
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