If the pressure of the fluid, , is expressed only by the density, ,
the number of dependent variables is reduced to two and the above equations are sufficient
to describe the dynamics of the fluid.
Occasionally, the presure is assumed proportional to the power of as
where is a constant.
This assumption is called polytropic relation.
The fact should be reminded that the validity of the assumption comes from the physical condition of the system.
In the case that the temperature of the gas is kept constant owing to the cooling and heating process,
the gas pressure is proportional to the density
(: Boltzmann constant, average molecular weight, and
is the proton mass) represents the isothermal sound speed and is constant.
Another example is the isentropic fluid, in which the entropy is kept constant.
In this case the pressure is proportional to , as
where is the specific heat ratio
represents the adiabatic sound speed.
In these cases the polytropic replations of equations (A.19) and
(A.20) plays a role as the third equation of basic equations of hydrodynamics.