Quote:
Originally Posted by that_guy
heyy all, i need some help with my physics assignement,
I need a formula for finding density of water using temperature and salinity. Also, i need a fomula for drag underwater
cheers
tony
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Hi Tony,
The formula for water density is hard to find because it is empirical, that is, derived from observations, referenced to a density of 1000 kg/cu metre at 4 deg C.
First get density 'rho' at a certain temperature T:
rho = 1000(1 - (T+288.9414)/(508929.2*(T+68.12963))*(T-3.9863)^2)
then get saline density 'rhos' at a salinity S:
rhos = rho + AS + BS^(3/2) + CS^2
where:
rho and rhos in kg/cu metre
T in deg C
S in g/kg dissolved (seawater is about 35 g/kg)
A, B, C are coefficients
A = 8.24493E-1 - 4.0899E-3*T + 7.6438E-5*T^2 -8.2467E-7*T^3 + 5.3675E-9*T^4
B = -5.724E-3 + 1.0227E-4*T - 1.6546E-6*T^2
C = 4.8314E-4
Here is the reference you will need:
http://www.earthwardconsulting.com/density.xls
The formula for drag in water (or any fluid including air) is well known, due to Lord Rayleigh:
http://en.wikipedia.org/wiki/Drag_equation
Typical values for drag in water would be:
density rho at 20 deg C : 998 kg/cu metre
'reference area' is approximately cross sectional area, so anything you like
'drag coefficient' Cd depends on shape. For:
sphere 0.47
cylinder 0.82
flat plate at 90 degrees 1.17
dolphin 0.0036
modern submarine 0.0015
modern surface ship 0.2 to 1.2
inefficient ship hull 3.8
Hope that helps
Cheers