# Fluid Dynamics

Fluid dynamics is a study that deals with flow of fluids. Its sub-disciplines include aerodynamics (study of gases in motion) and hydrodynamics (study of liquids in motion).

### Viscosity

Viscosity is defined as an internal frictional force present between each layer of molecules as they slide over one another. The higher the viscosity the more the fluid will act in a slow manner. It is an important property in engine and hydraulic system oils. The fluids with low viscosity may be prone to leaks and may not provide friction protection.

The viscosity depends on the temperature, as the molecules tend to expand with rise in temperature. The change in viscosity with temperature is called as viscosity index. The higher the temperature the lower the viscosity and vice-versa.

The viscosity of liquids is higher than that of gases, due to better molecular freedom in gases.

### Fluid Flow

Fluid flow is measured in terms of cubic metres per second. The rate of flow (Q) is defined as the volume passing by per second and the representation is shown below.

Q = av m3/s

a = cross-sectional area (m2)

v = velocity (m/s)

The two main forms of fluid flow are described below.

Laminar flow: It is a flow in which all the molecules travel in parallel layers, without any interchange between the layers. Each layer has a drag effect over the adjacent layers so that a velocity gradient is produced across the flow. This flow type is more efficient and is preferred in both aerodynamics and hydrodynamics.

Turbulent flow: In this form, the fluid flow is swirling with a complete interchange between the layers. It creates a complete random motion, in which the particle movement is unpredictable.

### Compressibility

A measure of the relative volume change of a fluid or solid as a response to pressure change is known as compressibility. In liquids, it is obvious only under very high pressures. So, liquids are usually considered incompressible whereas gases are easily compressed. When a body moves through air at low speeds, the compression level is very minimum. But as the sound speed (762mph at sea level) is approached, the effect of compressibility gains more importance and must be considered.