Dynamic pressure

If a fluid is not moving, the measured pressure is the same in all directions static pressure. But if the fluid is moving, the measured pressure depends both on the Static pressure and on the pressure caused by the energy of its motion, in the direction of that motion. The added pressure from the fluid's motion is called dynamic pressure. In incompressible fluid dynamics dynamic pressure (indicated with q, or Q, and sometimes called velocity pressure) is the quantity defined by:

where (using SI units):

Dynamic pressure is the kinetic energy per unit volume of a fluid particle. Dynamic pressure is in fact one of the terms of Bernoulli's equation, which can be derived from the conservation of energy for a fluid in motion. In simplified cases, the dynamic pressure is equal to the difference between the stagnation pressure and the static pressure.

Another important aspect of dynamic pressure is that, as dimensional analysis shows, the aerodynamic stress (i.e. stress within a structure subject to aerodynamic forces) experienced by an aircraft travelling at speed ${\displaystyle v}$ is proportional to the air density and square of ${\displaystyle v}$, i.e. proportional to ${\displaystyle q}$. Therefore, by looking at the variation of ${\displaystyle q}$ during flight, it is possible to determine how the stress will vary and in particular when it will reach its maximum value. The point of maximum aerodynamic load is often referred to as max Q and it is a critical parameter in many applications, such as during spacecraft launch.

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