Monday, 22 August 2016

Static Pressure and Dynamic Pressure

When explaining lift and various other aerodynamic and meteorologic phenomena it is vital to understand the difference between dynamic pressure and static pressure.

We feel dynamic pressure all the time, when we feel it, we call it wind. If you blow on your hand right now, you will feel dynamic pressure.

To get technical, dynamic pressure is equal to one half of density times velocity squared.
  • q = Dynamic Pressure
  • ρ = Density
  • V = Velocity
Dynamic pressure acts in only one direction, the direction of the velocity.

At sea level, static pressure is usually many times the strength of dynamic pressure but we rarely feel it. That's because the only things we feel are forces, and forces are caused by differences in pressure. And usually we are at the same pressure as the environment, which is about 14.7psi at sea level and about 12.2psi at 5,000 feet. If you are at sea level it's not just the air that's at 14.7psi, it's every cell in your body, it's every molecule that you are composed of that is at this pressure.

The immense power of static pressure, which unlike dynamic pressure acts in ALL directions, is only apparent when it interacts with a much higher or much lower pressure, such as the near zero pressure of space or the very low pressure experienced by aircraft cruising at high altitude. Below is a truly disgusting scene from the movie Alien (no seriously, don't watch it if heinous disembowelment grosses you out) where the immense forces created by static pressure acting against a vacuum suck the alien out of the spaceship. There are similar scenes in Star Wars and Star Trek but I couldn't find clips of them on YouTube.
Now it isn't only in gruesome space movies where effects of static pressure are demonstrated. If you've ever left toothpaste or contact solution or some other bottle of liquid in your car while traveling up to high elevation you've probably noticed the pressure escaping when you opened that bottle. Additionally if you have a sinus blockage or inner ear infection while travelling by car or aircraft you'll have felt that pressure in your sinuses or ears (which can be quite painful). Additionally if you've ever swam to the bottom of a pool you have probably felt the extra static pressure added by the water on your ears.
One last way to try to understand static pressure. Sea level pressure is 14.7psi, the average human adult has about 800 square inches on one side of their body, that means that if one side of your body had normal sea level pressure on it but the other side was somehow exposed to the vacuum of space that there would be a force of 11,760 pounds blowing you into space. You get 11,760 pounds by applying 14.7 pounds of force to each square inch of this average adult body. 

So there you have it. Static pressure and dynamic pressure, post questions and corrections below.

No comments:

Post a comment