Nearly everyone has tried sticking their arm out of the window of a moving car at least once, whether to feel the wind or simply out of boredom and be met with comforting coolness that gets colder as the vehicle goes faster. On warm days, this coldness can be perplexing when you recall what you learned from your physics tutor in Singapore.
After all, aerodynamic friction is now acting on your exposed arm, causing it to collide with many air molecules that should cause heating instead of cooling. This begs the question: why does your arm feel cool in these situations, whereas spaceships need heat shields to prevent damage from aerodynamic heating? Read on as we go over the basics of collision, friction, and heat transfer first before diving into this puzzling phenomenon.
Understanding Heat and Relative Motion
As you may recall, friction is the name of the force that resists motion, and it occurs whenever jagged surfaces interlock with each other. Pushing the two surfaces will lead to physical deformations at the contact points at the microscopic level and thus allow further movement to take place.
Friction is also prevalent in liquid flow, which involves many layers. The bottommost layer moves the slowest since it is in contact with the ground, while the topmost moves the quickest. This difference in speed between layers causes relative motion between them, which gives rise to the frictional force called viscosity. Similarly, there is also a viscous force that takes place when air and other gases flow, and its mechanism is identical to that of liquids.
Frictional Heating
Frictional force arises when two surfaces collide and move parallel relative to one another and results in some degree of kinetic energy transfer. Some of this energy then gets converted to heat energy, hence the increase in temperature where the two surfaces interface.
Now, when air strikes a surface, such as our extended arm outside a moving car, two things happen: the stagnation of the air molecules that hit the surface head-on due to its geometry and the glancing collisions at various angles between the air molecules and the surface.
In the first process, all the kinetic energy of those few air molecules gets lost, and so they remain at rest, causing pressure to increase in the affected region, which is called the stagnation point. In the second process, however, the air molecules collide at different angles and only make glancing contact and move away. Thus, these air molecules retain their high kinetic energy, and the region where this takes place is referred to as a low-pressure gradient region.
A Quick Look at Wind Chill
As a refresher, the human body has an average core temperature of around 36.1℃ – 37.2℃. When outside air comes into contact with the skin, heat transfer from the body to the air or from a higher temperature to a lower temperature takes place. This is followed by a thin layer of warm air forming near the skin’s surface, which functions as insulation to prevent further heat loss.
But when the skin is exposed to a steady flow of air, this warm layer constantly gets replaced by the incoming cool air. As you can see, this continuous cycle of heat loss causes that feeling of coldness or wind chill when you feel the breeze with your hands or pop your head out of the sunroof of a car.
Now that we know all the basics, you might be curious about what happens when frictional forces and wind chill oppose each other. In other words, what is the threshold beyond which aerodynamic heating overpowers wind chill?
Essentially, an increase in airspeed also increases the rate of convective heat loss, causing the body to turn colder. This heat loss continues at subsonic speeds, but once the supersonic threshold is reached, the heating effect becomes more dominant. This is why heat shields are a must when travelling at speeds twice the speed of sound or Mach 2.0 to protect against high temperatures that can reach up to 204℃.
Conclusion
Everyday life is full of curiosities that we never really get to ponder until we learn more about the natural world. In this case, upon seeing what a spacecraft experiences on re-entry to Earth and learning the physics behind it, one cannot help but think about how it differs from similar occurrences in our daily lives.
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