Impact pressure

In compressible fluid dynamics, impact pressure (dynamic pressure) is the difference between total pressure (also known as pitot pressure or stagnation pressure) and static pressure.[1][2] In aerodynamics notation, this quantity is denoted as ${\displaystyle q_{c}}$ or ${\displaystyle Q_{c}}$.

When input to an airspeed indicator, impact pressure is used to provide a calibrated airspeed reading. An air data computer with inputs of pitot and static pressures is able to provide a Mach number and, if static temperature is known, true airspeed.

Some authors in the field of compressible flows use the term dynamic pressure or compressible dynamic pressure instead of impact pressure.[3][4]

Isentropic flow

In isentropic flow the ratio of total pressure to static pressure is given by:[3]

${\displaystyle {\frac {P_{t}}{P}}=\left(1+{\frac {\gamma -1}{2}}M^{2}\right)^{\tfrac {\gamma }{\gamma -1}}}$

where:

${\displaystyle P_{t}}$ is total pressure

${\displaystyle P}$ is static pressure

${\displaystyle \gamma \;}$ is the ratio of specific heats

${\displaystyle M\;}$ is the freestream Mach number

Taking ${\displaystyle \gamma \;}$ to be 1.4, and since ${\displaystyle \;P_{t}=P+q_{c}}$

${\displaystyle \;q_{c}=P\left[\left(1+0.2M^{2}\right)^{\tfrac {7}{2}}-1\right]}$

Expressing the incompressible dynamic pressure as ${\displaystyle \;{\tfrac {1}{2}}\gamma PM^{2}}$ and expanding by the binomial series gives:

${\displaystyle \;q_{c}=q\left(1+{\frac {M^{2}}{4}}+{\frac {M^{4}}{40}}+{\frac {M^{6}}{1600}}...\right)\;}$

where:

${\displaystyle \;q}$ is dynamic pressure

See also

• Dynamic pressure
• Pitot-static system
• Pressure
• Static pressure

References

1. DoD and NATO definition of impact pressure Retrieved on 2008-10-01
2. The Free Dictionary Retrieved on 2008-10-01
3. L. J. Clancy (1975) Aerodynamics, Section 3.12 and 3.13
4. "the dynamic pressure is equal to half rho vee squared only in incompressible flow."
   Houghton, E.L. and Carpenter, P.W. (1993), Aerodynamics for Engineering Students, Section 2.3.1