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In order to understand fluid dynamics, I felt it would be helpful to share how air behaves under different circumstances. This will be a multi-part series, where I describe air and why it flows the way it does. I will try to keep things simple and easy to understand, with little to no math. Any equations I present will be explained in simple terms that are intuitive.

Air is capable of creating incredible forces, it keeps planes up in the air, it can crush steel drums, it keeps water as a liquid, how is this possible? This is because air has mass, at sea level the pressure is nearly constant and there is a pressure of ~14.7 pounds over every square inch of area. This pressure is pressing on all objects from all sides, as above so below the saying goes. It is this fact that we use and exploit in order to create aerodynamic forces. Air wants to fill everything with itself, and whenever something moves through the air, the physical object removes air for a split second, and as a result air will try to fill that displacement. This is true of fluids in general.

You may have heard the term "nature abhors a vacuum" this is especially true of air. It is the motion of air attempting to fill that vacuum that creates aerodynamic forces. Then following this logic, air moves as a result of the difference in pressure, the default atmospheric pressure, or what is known as "static pressure" will move towards anything that is less than this standard pressure.

Where it starts to get complicated is the path the air takes to fill that difference in pressure. Logic would tell you that the most efficient path between two points is a straight line, so naturally you would think that air follows the path of least resistance, I.E. a straight line. However this is not the case, air rarely if ever flows in a smooth straight line except under the most carefully controlled conditions. By and large, air takes a curved path, that is its preferred path of motion, why does air do this?

The primary reason is that air has mass, and as a result it is subject to inertial and viscous forces. Thus we have the Reynolds number, which accounts for the inertial and viscous forces in a packet of air and serves as a rough estimate of when a flow becomes turbulent. The Reynolds number is estimated as the density of the fluid times the velocity, times the distance the fluid covers, over the viscosity of the fluid. The smaller the Reynolds number the more likely the flow is to be laminar, linear and smooth. Inversely, the greater the Reynolds number the more likely it is to be turbulent and chaotic.

This means that increasing the size of the air packet, or parcel, increasing the velocity of the flow, or its density will give you a greater reynolds number, in other words a flow more likely to be turbulent and chaotic. Inversely the greater the viscosity of air, the lower the reynolds number. Interestingly, the viscosity of air increases with temperature, and its density decreases. Thus temperature has a big influence in the tendency of air to become chaotic and turbulent, however this relationship is not linear.

Another reason why air likes to take curved paths is a result of the speed air moves at, this is because as mentioned previously air has mass, as a mass of air moves, it displaces air, and as a result lowers the static or default pressure of a given volume. Bernouli tried to capture this in his famous equation. As a result of moving or displaced air in a given volume the surrounding air will move to fill that slightly lower pressure from all sides. The easiest way for something to surround something from all sides is a sphere. However, if the air keeps moving, the leading part of the displaced air will have higher pressure as it butts or collides with the standard or static pressure, and a lower pressure on the trailing side. Thus, the tendency of air to fill that low pressure will be curved.

In the next part, I'll cover more advanced aspects of airflow, including why vortexes happen, thank you for your time.