Fluid pressure
From Freepedia
Fluid pressure is the pressure on an object submerged in a fluid, such as water. The pressure can be provided from a number of sources:
- the shear weight of the fluid, such as in scuba diving, when the diver goes deeper into the water, the water pressure increases; or in the earth's atmosphere, as a plane goes higher, the air pressure decreases;
- a pump, such as when water "pumped" into a water tower; or
- a compressor, such as in a small water supply system in a rural well for a house connected to an air compressor.
Fluid pressure occurs in one of two situations: (1) an open condition, such as the ocean, or a swimming pool, or (2) a closed condition, such as a water line or a gas line.
Open conditions are considered to be "static" or not moving (even in the ocean where there are waves and currents) because the fluid is essentially "at rest." The pressure in open conditions conform with principles of fluid statics.
Closed bodies of fluid are either "static," when the fluid is not moving, or "dynamic," when the fluid is moving, like through a pipe. The pressure in closed conditions conform with the principles of fluid dynamics.
The concepts of fluid pressure are predominately attributed to the discoveries of Blaise Pascal and Daniel Bournoulli. These concepts are given in greater detail in the remainder of this article.
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Concepts of fluid pressure
There are two components to pressure: force and area. The more force, the larger the pressure; the more area, the smaller the pressure. As such, the force on an object in contact with fluid depends on three factors (as summarized in Bernoulli's equation):
- Is the fluid at rest or moving? If moving, how fast and in what direction in relation to the object?
- What is the vertical distance from the surface of the fluid (if a liquid) to the object?
- Are there any sources of pressure, such as from a pump or compressor?
Hydrostatic pressure
In the case where the fluid is at rest, called fluid statics or hydrostatics (from hydro meaning "water" and static meaning "at rest"), the force acting on the object is the sheer weight of the fluid above, up to the water's surface—such as from a water tower. The resulting hydrostatic pressure is isotropic: the pressure acts in all directions equally, according to Pascal's law:
In the following example. Assume a column of 1 meter high, the pressure on the floor of the column will be as follow:
- <math>p = \rho g h = 1000 \cdot 10 \cdot 1 = 10,000</math> Pa.
The pressure increases linear with the water depth.
Fluid dynamic pressure
Next, when the fluid is moving in a pipe, called fluid dynamics or hydrodynamics (dynamics meaning "moving" or "changing"), the interaction of the fluid with the walls of the pipe creates friction and energy loss. In fluid dynamics, the fluid pressure is anisotropic (not isotropic), i.e. the pressure is not the same in all directions. The anisotrophy is mainly caused by the friction losses, so the further an object is away from the surface of the fluid (assuming the fluid is moving from the fluid surface to the object), the less pressure there will be on that object in the fluid. (Note: There are other reasons for the anisotrophy that are beyond the scope of this article.) These concept are mathematically explained by Bernoulli's equation, and is applicable to all fluids.
Consider this simplified example: assume a 10 m high water tower with a large reservoir, that would give us a static pressure of 100,000 Pa (100 kPa or 100,000 N/m2 at ground level. Now connect a 100-meter long horizontal pipe to the bottom of the column. From experiment it is known that the friction loss in this typical pipe, depending on material and diameter, is 100 Pa per meter of pipe length. So for the 100 meter pipe, that would be 10 kPa of pressure loss. So the pressure at the end of our pipe would be 100 kPa - 10 kPa = 90 kPa.
Pressure source
Finally, when a pressure source (such as an air pressure tank, a pump, or the human heart) is attached to the system, that pressure adds to the static pressure. In very small systems, such as in the human body or in a domestic water supply system, all of the pressure is derived from the pressure source (the heart or an air compressor), and none (or very little) is due to fluid static force.
