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Old 09-03-2011, 12:47 PM
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GeoffW1 (Geoff)
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Quote:
Originally Posted by that_guy View Post
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
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

Last edited by GeoffW1; 16-03-2011 at 01:58 AM. Reason: accuracy
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