Static Pressure
2. Defining Stillness in Fluid Dynamics
Let’s start with static pressure. The word ‘static’ gives us a big clue: it refers to something that isn’t moving. In the context of fluid dynamics, static pressure is the pressure exerted by a fluid when it’s at rest, or when we’re measuring the pressure without disturbing the fluid’s motion. Think of it as the “background” pressure, always there, regardless of what the fluid is doing.
Consider a glass of water sitting on your table. The water molecules are jiggling around, but overall, the water isn’t flowing. The pressure at the bottom of the glass, caused by the weight of the water above it, is static pressure. It’s a constant force exerted on the bottom of the glass, and it depends solely on the depth of the water and the density of the fluid (in this case, water).
Static pressure acts equally in all directions. It doesn’t matter if you’re measuring the pressure on the side of the glass or at the bottom; at the same depth, the static pressure will be the same. This is because the pressure is due to the weight of the fluid pushing equally in every direction. If it didn’t, the water would start flowing in the direction of lower pressure, which, in a static situation, simply doesn’t happen.
To measure static pressure, we typically use a device that doesn’t interfere with the flow. A simple pressure gauge connected to a hole in the side of a pipe, for example, will give you a reading of the static pressure inside the pipe, as long as the hole is perpendicular to the flow and doesn’t create any turbulence. Its like asking the water, Hey, whats the pressure around here? without poking it too much.