It is primarily used for unconfined aquifers, since the elastic storage component, S_s, is relatively small and usually has an insignificant contribution. Specific yield can be close to effective porosity, but there are several subtle things which make this value more complicated than it seems. Some water always remains in the formation, even after drainage; it clings to the grains of sand and clay in the formation. Also, the value of specific yield may not be fully realized for a very long time, due to complications caused by unsaturated flow. Problems related to unsaturated flow are simulated using the numerical solution of Richards Equation, which requires estimation of the specific yield, or the numerical solution of the Soil Moisture Velocity Equation, which does not require estimation of the specific yield.
Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:
Note that the units used affect the specific speed value in the above equation and consistent units should be used for comparisons. Pump specific speed can be calculated using British gallons or using Metric units (m 3 /s or L/s and metres head), changing the values listed above.
Low-specific speed radial flow impellers develop hydraulic head principally through centrifugal force. Pumps of higher specific speeds develop head partly by centrifugal force and partly by axial force. An axial flow or propeller pump with a specific speed of 10,000 or greater generates its head exclusively through axial forces. Radial impellers are generally low flow/high head designs whereas axial flow impellers are high flow/low head designs. In theory, the discharge of a "purely" centrifugal machine (pump, turbine, fan, etc.)is tangential to the rotation of the impeller whereas a "purely" axial-flow machine's discharge will be parallel to the axis of rotation. There are also machines that exhibit a combination of both properties and are specifically referred to as "mixed-flow" machines.
As the specific speed increases, the ratio of the impeller outlet diameter to the inlet or eye diameter decreases. This ratio becomes 1.0 for a true axial flow impeller.
The specific speed value for a turbine is the speed of a geometrically similar turbine which would produce unit power (one kilowatt) under unit head (one meter). The specific speed of a turbine is given by the manufacturer (along with other ratings) and will always refer to the point of maximum efficiency. This allows accurate calculations to be made of the turbine's performance for a range of heads.
Centrifugal pump impellers have specific speed values ranging from 500 to 10,000 (English units), with radial flow pumps at 500-4000, mixed flow at 2000-8000 and axial flow pumps at 7000-20,000. Values of specific speed less than 500 are associated with positive displacement pumps.
The following equation gives a dimensionless specific speed.
Note: The "pound of fuel" refers to some particular mass of fuel measured in an arbitrary gravitational field (for example, Earth's); the "pound of force" refers to the force exerted by that pound-mass pressing down in the same arbitrary gravitational field; the particular acceleration of gravity is unimportant because it merely relates the two units, and thus specific impulse is not tied to gravity in any way—it is measured the same on any planet or in space.
Mnemonics and Qualitative Reasoning: In many instances, specific properties are more intuitive or are easier to remember than the original properties in SI or English units. For instance, it is easier to conceptualize an acceleration of 2 g's than an acceleration of 19.6 meters per second squared. It is hard to remember that the specific gravity of water is 1.0 and that something with a higher specific gravity will sink in water. But if we understand it, it is very easy.
The use of specific stiffness in tension applications is straightforward. Both stiffness in tension and total mass for a given length are directly proportional to cross-sectional area. Thus performance of a beam in tension will depend on Young's modulus divided by density.
The net suction specific speed is defined as:
Unlike density, specific weight is not absolute. It depends upon the value of the gravitational acceleration, which varies with location. Pressure may also affect values, depending upon the bulk modulus of the material, but generally, at moderate pressures, has a less significant effect than the other factors.
There is very little strong evidence for specific appetite in humans. However, it has been demonstrated that humans have the ability to taste calcium, and indirect evidence supports the idea that patients on kidney dialysis who develop hypocalcemia prefer cheese with greater amounts of calcium added. Exercise also increases the preference for salt. Some diseases, including Gitelman syndrome and the salt-wasting variant of Congenital adrenal hyperplasia, impair the kidney's ability to retain sodium in the body and cause a specific craving for sodium. Extreme sodium depletion in human volunteers has been demonstrated to increase the desire for high-salt foods.
A modern civil turbofan has a low specific thrust (~30 lbf/(lb/s)) to keep the jet noise at an acceptable level, and to achieve low fuel consumption, because a low specific thrust helps to improve specific fuel consumption (SFC). This low specific thrust is usually achieved with a high bypass ratio. Additionally a low specific thrust implies that the engine is relatively large in diameter, for the net thrust it generates. Consequently, such aircraft engines are normally located externally, in a separate nacelle or pod, attached to the wing, or the rear fuselage.
In other words, given a particular engine and a pound of a particular fuel on the surface of the earth, specific impulse measures how long of a time that engine can exert a continuous pound of force (thrust) until fully burning through that pound of propellant. A given mass of a more energy-dense fuel can burn for a longer duration than some less energy-dense fuel made to exert the same force while burning in an engine. [note] Different engine designs burning the same fuel may not be equally efficient at directing their fuel's energy into effective thrust. In the same manner, some car engines are better built than others to maximize the miles-per-gallon of the gasoline they burn.
The net suction specific speed is mainly used to see if there will be problems with cavitation during the pump's operation on the suction side. It is defined by centrifugal and axial pumps' inherent physical characteristics and operating point. The net suction specific speed of a pump will define the range of operation in which a pump will experience stable operation . The higher the net suction specific speed, then the smaller the range of stable operation, up to the point of cavitation at 8500 (unitless). The envelope of stable operation is defined in terms of the best efficiency point of the pump.
By contrast, military turbofans often feature fairly high specific thrust (45-110 lbf/(lb/s)), which keeps the cross-sectional area of the engine low to more easily accommodate a narrow fuselage, which minimizes drag. A high specific thrust usually results in higher noise levels, which is not an important consideration for most military applications.
Ranking, Classifying, and Comparing: Specific properties are useful for making comparisons about one attribute while cancelling out the effect of variations in another attribute. For instance, steel alloys are typically stronger than aluminum alloys but are also much denser. Greater strength allows less metal to be used, which makes the choice between the two metals less than obvious. To simplify the comparison, one would compare the specific strength (strength to weight ratio) of the two metals. A more everyday example is grocery shopping. The two kilogram package sells for a higher price than the one kilogram package, but what matters is the "specific price", commonly called the unit cost (cost per kilogram).
For all vehicles, specific impulse (impulse per unit weight-on-Earth of propellant) in seconds can be defined by the following equation: