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Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum.[1][2] It is usually measured as a water surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer. In an aquifer, it can be calculated from the depth to water in a piezometric well (a specialized water well), and given information of the piezometer's elevation and screen depth. Hydraulic head can similarly be measured in a column of water using a standpipe piezometer by measuring the height of the water surface in the tube relative to a common datum. The hydraulic head can be used to determine a hydraulic gradient between two or more points.
Contents
1 "Head" in fluid dynamics
2 Components of hydraulic head
2.1 Fresh water head
3 Hydraulic gradient
4 Hydraulic head in groundwater
4.1 Atmospheric pressure
5 Head Loss
6 Analogs to other fields
7 See also
8 Notes
9 References
//
"Head" in fluid dynamics
In fluid dynamics, head is a concept that relates the energy in an incompressible fluid to the height of an equivalent static column of that fluid. From Bernoulli's Principle, the total energy at a given point in a fluid is the energy associated with the movement of the fluid, plus energy from pressure in the fluid, plus energy from the height of the fluid relative to an arbitrary datum. Head is expressed in units of height such as meters and feet.
The static head of a pump is the maximum height (pressure) it can deliver. The capability of the pump can be read from its Q-H curve (flow vs. height).
Head is equal to the fluid's energy per unit weight. Head is useful in specifying centrifugal pumps because their pumping characteristics tend to be independent of the fluid's density.
There are four types of head used to calculate the total head in and out of a pump:
Velocity head is due to the bulk motion of a fluid (kinetic energy).
Elevation head is due to the fluid's weight, the gravitational force acting on a column of fluid.
Pressure head is due to the static pressure, the internal molecular motion of a fluid that exerts a force on its container.
Resistance head (or friction head or Head Loss) is due to the frictional forces acting against a fluid's motion by the container.
Components of hydraulic head
A mass free falling from an elevation (in a vacuum) will reach a speed
when arriving at elevation z=0, or when we rearrange it as a head:
where
g is the acceleration due to gravity
The term is called the velocity head, expressed as a length measurement. In a flowing fluid, it represents the energy of the fluid due to its bulk motion.
The velocity head, or total hydraulic head of a fluid is composed of pressure head and elevation head.[1][2] The pressure head is the equivalent gauge pressure of a column of water at the base of the piezometer, and the elevation head is the relative potential energy in terms of an elevation. The Head Equation, a simplified form of the Bernoulli Principle for incompressible fluids, can be expressed as:
where
h is the hydraulic head (Length in m or ft), also known as the velocity head.
? is the pressure head, in terms of the elevation difference of the water column relative to the piezometer bottom (Length in m or ft), and
z is the elevation at the piezometer bottom (Length in m or ft)
In an example with a 400m deep piezometer, with an elevation of 1000m, and a depth to water of 100m: z=600m, ?=300m, and h=900m.
The pressure head can be expressed as:
where
P is the gauge pressure (Force per unit area, often Pa or psi),
? is the unit weight of water (Force per unit volume, typically N?3 or lbf/ft3),
? is the density of the water (Mass per unit volume, frequently kg?3), and
g is the gravitational acceleration (velocity change per unit time, often m?2)
Fresh water head
The pressure head is dependent on the density of water, which can vary depending on both the temperature and chemical composition (salinity, in particular). This means that the hydraulic head calculation is dependent on the density of the water within the piezometer. If one or more hydraulic head measurements are to be compared, they need to be standardized, usually to their fresh water head, which can be calculated as:
where
is the fresh water head (Length, measured in m or ft), and
is the density of fresh water (Mass per unit volume, typically in kg?3)
Hydraulic gradient
The hydraulic gradient is a vector gradient between two or more hydraulic head measurements over the length of the flow path. It is also called the Darcy slope, since it determines the quantity of a Darcy flux, or discharge. A...(and so on)
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