Compressible Flow
Many authors define dynamic pressure only for incompressible flows. (For compressible flows, these authors use the concept of impact pressure.) However, some British authors extend their definition of dynamic pressure to include compressible flows.
If the fluid in question can be considered an ideal gas (which is generally the case for air), the dynamic pressure can be expressed as a function of fluid pressure and Mach number.
By applying the ideal gas law:
the definition of speed of sound and of Mach number :
- and
dynamic pressure can be rewritten as:
where (using SI units):
-
= static pressure in Pascals, = density in kg/m3 = specific gas constant (287.05 J/(kg·K) for air), = absolute temperature in Kelvin (K), = Mach number (non-dimensional), = ratio of specific heats (non-dimensional) (1.4 for air at sea level conditions), = fluid velocity in m/s, = speed of sound in m/s
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