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Hydraulic head or piezometric head is a specific measurement of liquid pressure above a geodetic datum.〔, 410 pages. See pp. 43–44.〕〔, 650 pages. See p. 22.〕 It is usually measured as a liquid 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. =="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 or feet. The ''static head'' of a pump is the maximum height (pressure) it can deliver. The capability of the pump at a certain RPM 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hydraulic head」の詳細全文を読む スポンサード リンク
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