Dynamic Pressure in Compressible Flow

When a fluid’s speed approaches a significant fraction of the speed of sound, the assumption of constant density (incompressibility) breaks down. Changes in pressure cause significant changes in density, and 热力学 effects become important. This is the realm of compressible flow. The simple formula [latex]q = \frac{1}{2} \rho u^2[/latex] is still used as a formal definition, but its relationship to pressure changes is more complex. The formula [latex]q = \frac{1}{2} \gamma p M^2[/latex] provides a direct link between dynamic pressure and the key parameters of compressible flow: static pressure ([latex]p[/latex]), the ratio of specific heats ([latex]\gamma[/latex], which is a property of the gas, approximately 1.4 for air), and the Mach number ([latex]M = u/a[/latex], where [latex]a[/latex] is the local speed of sound).

This equation is derived from the definition of Mach number and the ideal gas equation of state. It is fundamental in high-speed aerodynamics. For instance, the pressure measured at a stagnation point ([latex]p_0[/latex]) in supersonic flow is not given by the simple Bernoulli equation. Instead, it is related to the static pressure by the isentropic flow relations or, if a shock wave is present, by the Rankine-Hugoniot relations. In these calculations, the term [latex]\frac{1}{2} \gamma p M^2[/latex] frequently appears, representing the kinetic energy component of the flow in a thermodynamically consistent way. This is crucial for accurately predicting the extreme pressures and temperatures experienced by supersonic aircraft, re-entry capsules, and meteorites entering the atmosphere. The concept is also sometimes referred to as “impact pressure” in this context, emphasizing the pressure rise due to the fluid’s momentum being brought to rest.