24.5.1. Static Pressure

Static pressure is the pressure exerted by a fluid at rest and can be measured using devices like piezometers.

Static pressure refers to the force exerted by a fluid when it is not moving. It is the pressure felt by an object submerged in that fluid. For instance, when measuring static pressure in a pipe, the fluid's weight and any external forces determine how much pressure is exerted at that point. Commonly, static pressure is measured using piezometers, which are small tubes that display fluid height relative to atmospheric pressure.

Imagine you’re at the bottom of a swimming pool. You can feel the water pushing down on you. This pressure you feel is static pressure. The deeper you go, the more pressure there is because of the weight of the water above you.

Dynamic pressure is the pressure due to the fluid's velocity. It combines with static pressure to form stagnation pressure, which occurs when fluid flows are brought to a complete stop.

Dynamic pressure depends on the movement of the fluid and can be calculated using the formula: $q = \frac{1}{2} \rho V^2$, where $\rho$ is the fluid density and $V$ is the velocity. When fluid encounters a barrier and comes to a standstill, the static pressure is increased by the dynamic pressure; this sum is known as stagnation pressure. Stagnation pressure is critical in various applications, including aircraft design, where it is utilized to determine airspeed.

Think of dynamic pressure as the speed of a car. When you’re driving fast, your car creates a lot of air pressure in front of it. If you drive into a wall (the barrier), all that speed (dynamic pressure) is converted into a buildup of pressure at the point of impact, similar to how stagnation pressure works.

To measure static and dynamic pressures simultaneously, a Pitot tube can be used. It helps determine both types of pressure in fluid flows.

A Pitot tube measures the stagnation pressure by capturing the fluid flow directly inside the tube. The pressure created by the fluid bringing it to a stop (stagnation pressure) is measured against ambient pressure. By knowing the static pressure (via another measurement, like from a piezometer), we can easily calculate the dynamic pressure and, consequently, the fluid's velocity using Bernoulli's equation.

Consider the way a straw works when you sip a drink. When you create a vacuum by sucking on the straw (akin to applying dynamic pressure), you can measure how much drink is pulled into the straw. Similarly, a Pitot tube measures how much of the fluid stops upon hitting it, allowing us to understand both the static and dynamic pressures.

In various engineering applications, static pressure plays a vital role in determining fluid behavior in systems, such as pipelines and open channels.

Static pressure is crucial in determining how fluids behave in various systems. For example, in pipelines, it influences how pressure changes occur along the pipe length. Understanding static pressure can help engineers design systems that effectively manage fluid distribution without causing damage or leaks. In open channels, it affects flow patterns and velocity distributions.

Think of a garden hose connected to a tap. If you open the tap slightly, static pressure in the hose will determine how much water flows out. If you block the end of the hose, static pressure will build up, showing you how critical it is to maintain the right pressure to ensure proper water flow.

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