Aerodynamic heating

Aerodynamic heating is the heating of a solid body produced by its high-speed passage through air (or by the passage of air past a test object in a wind tunnel), whereby its kinetic energy is converted to heat by skin friction on the surface of the object at a rate that depends on the viscosity and speed of the air. In science and engineering, it is most frequently a concern regarding meteors, reentry vehicles, and the design of high-speed aircraft.

At high speeds through the air, the object's kinetic energy is converted to heat through compression and friction. At lower speed, the object will lose heat to the air through which it is passing, if the air is cooler. The combined temperature effect of heat from the air and from passage through it is called the stagnation temperature; the actual temperature is called 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 object 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 Thermal radiation 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. Its effects are minimal at subsonic speeds but at supersonic speeds beyond about M2.2 it dictates the design/materials of the vehicle structure and internal systems. The heating effects are greatest at leading edges but the whole vehicle heats up to a stabilized temperature if it remains at speed. 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.

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