Aerodynamic Heating

Aerodynamic heating is the heating of a solid body produced by the passage of fluid (such as air) over a body such as a meteor, missile, or airplane. It is a form of forced convection in that the flow field is created by forces beyond those associated with the thermal processes. The heat transfer essentially occurs at the vehicle surface where aerodynamic viscous forces ensures that the flow is at zero speed relative to the body for a very thin layer of molecules at the surface.

When fluid flow slows down its kinetic energy is converted to heat; in high speed flows, tremendous energy is represented by the mean motion of the flow. As the flow is slowed to near zero speed, its temperature increases, the gradient in the speed in a direction normal to the surface allows small scale mass transport effects to dissipate the temperature in the outward direction and thus the temperature at the surface is less than the stagnation temperature; the actual temperature is referred to as the recovery temperature. These viscous dissipative effects to neighboring sub-layers make the boundary layer slow down via a non-isentropic process. Heat then conducts into the surface material from the higher temperature air. The result is an increase in the temperature of the material and a loss of energy from the flow. The forced convection ensures that other material replenishes the gases that have cooled to continue the process.

The stagnation and the recovery temperature of a flow increases with the speed of the flow and are greater at high speeds. The total thermal loading of the structure is a function of both the recovery temperature and the mass flow rate of the flow. Aerodynamic heating is greatest at high speed and in the lower atmosphere where the density is greater. In addition to the convective process described above, there is also radiative heat transfer from the flow to the body and vice versa with the net direction set by the relative temperature of each.

Aerodynamic heating increases with the speed of the vehicle and is continuous from zero speed. It produces much less heating at subsonic speeds but becomes more important at supersonic speeds. At these speeds it can induce temperatures that begin to weaken the materials that compose the object. The heating effects are greatest at leading edges. Aerodynamic heating is dealt with by the use of high temperature alloys for metals, the addition of insulation of the exterior of the vehicle, or the use of ablative material.