What is a Fluid?

Fluid mechanics is the study of liquids and gases that are either at rest or in motion. It is present in our everyday lives either directly or indirectly, and the first thing that you need ask yourself is what a fluid is? Scientifically speaking a fluid is based off of how a substance's molecules behave. A fluid's molecules are allowed to move freely, instead of being held together like they would be for a solid. Basically what this means is any substance that will readily deform under any magnitude of shear stress is considered a fluid.

There are two different states that a fluid can take. These states are a liquid or a gas. Both will act like a fluid, however there are couple of key differences between a liquid and a gas. First, a gas will expand to fill the volume of its container, a liquid will not. Finally, a gas compresses easily, while a liquid will not, and in most cases we can assume that liquids are incompressible due to the magnitude of pressure required to compress them.

Finally, there are substances that have characteristics of a fluid and a solid. An example would be tooth paste. Notice that tooth paste will hold its shape, but when a shear stress is applied to it, it will deform. The name of the field that studies these types of substances is rheology.

Density, Specific Volume, & Specific Weight

All objects have density, whether it's a solid, liquid, or gas. However, having an understanding of what density is, is very important for fluid mechanics, since density is used in a majority of fluid mechanics calculations. So what is density. Density is the mass of a substance over a certain unit volume. Density is represented by the greek letter rho. Refer to equation 1.

(1)

Specific volume on the other hand is the reciprocal of density. Refer to equation 2.

(2)

Finally, another aspect that is important to know for fluid mechanics is a fluid's specific weight. Specific weight relates the fluid's density and the effects of the gravitational constant. It is represented by the greek letter gamma. Refer to equation 3 to calculate specific weight.

(3)

Pressure

Pressure is another important aspect of fluid mechanics, and is one of the elements normally calculated in a fluids problem. Pressure can be expressed in two different ways. It can be expressed as gage pressure, and absolute pressure. Gage pressure is a pressure measurement that doesn't take in consideration of atmospheric pressure, while absolute pressure includes atmospheric pressure. Refer to equation 4.

(4)

Atmospheric pressure at sea level is 101 kpa in SI units and 14.7psi in English units.

Flow Rate

Flow rate is a product of the fluids velocity and a pipes cross-sectional area. Refer to equation 5.

(5)

Knowing the flow rate of a fluid will make it possible to determine how much volume of fluid has flowed through a pipe after a given amount of time.

Viscosity

Viscosity can be thought of as the stickiness of a fluid, or the fluids friction. Viscosity will cause the fluid's flow to be slower as it nears the wall, as can be seen in the figure below.

Different fluids have different viscosities. Some are more viscous then others, which means that they don't flow as easily. Also, a fluid viscosity will change as temperature changes. This is why engine oil has two numbers such as 5W-30. 5W represents the viscosity when the oil is cold, and 30 represents the viscosity when the oil is at operating temperature. For climates that have cold winters it is sometimes recommended to put a different W number in the car due to the colder climate.

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