# FLUIDS

The matter exists in three different states according to the magnitude of the bonding bonds between their molecules these are: solid, liquid and gaseous.
The three parameters that define a fluid are density, pressure and temperature.

DENSITY AND COMPRESSIBILITY

Density is the mass per unit volume. ρ=m/v
In a fluid the density can vary from one point to another, then it does not make sense to speak of the density of a set of points.

Inside the fluids there are some in which the density varies a lot (compressible) and others in which it remains practically constant (incompressible). An example of Incompressible is water and compressible is air.
The compressibility of the air only rises from certain speeds ( better said  Mach Number)

The atmospheric density decreases exponentially :

TEMPERATURE

The molecules of the gases have a continuous random, due to that it has a kinetic energy which is revealed with an internal energy called temperature. It is messured on  kelvin
T (° K) = t (° C) +273.15

PRESSURE

It is the force per unit area. The pressure attached to a point is called a static pressure.

P=F/S  ( P= pressure, F = force, S = surface)
1 atmosphere of pressure = 1013.2mb = 760mmHG.

The atmospheric pressure decreases exponentially :

EQUATION OF PERFECT GASES

P * v = nRT

P-> Pressure , v-> Volume , n-> molecular mass ,  R -> is the universal constant of the perfect gases = 0.082 (atmosphere * liter) / (mol * ° K) , T -> temperature

For us the formula shown above is not very useful because we need to relate , pressure, temperature and density. So we transformed it in a way we like. As density( ρ)=m/v , and n is a mass  , we can modify the equation of perfect gases like this P = nRT/ v which is the same as P=ρRT . Now we have the relation among them.

We have already studied on the performance section that our start point to study the behaviour/aerodynamics of and aircraft, is the International Standard Atmosphere . So what we are really interested in is the variation of the fluid we have a normal day against the I.S.A fluid .

The equation of perfect gases realated to I.S.A is :

If we devide the both :

and then for not writing a lot we rename our variables like this :

The result is :

δ = σ * θ

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