In
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, the term total pressure may indicate two different quantities, both having the dimensions of a
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
:
For compressible flow the
isentropic relations can be used (also valid for incompressible flow):
:
At
standard conditions
Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union o ...
the scientific community usually takes
as the threshold to compressibility. Flows with a higher Mach number
cannot approximate the total pressure using the incompressible formula given above. At non-standard conditions the flow may be considered compressible at lower Mach numbers. An example would be a flow through a long insulated pipe where wall friction is not neglected. Then the flow's temperature increases farther down the pipe and must be considered compressible after some length. The higher the pressure and temperature are, the less good the incompressible formula becomes. At standard conditions and low Mach numbers it is very good and widely used.
Pressure
Fluid dynamics
Gases