Internal Flow and External Flow in Fluid Mechanics

Fluid mechanics is a very important field of engineering that lots of kinds of engineering systems are produced according to the fluid mechanic’s laws. Especially in mechanical engineering, fundamentals and essentials of fluid mechanics are commonly used in lots of machinery. 

To understand the fluid mechanics, you must understand the general classification of fluids. One of these classifications is internal and external fluid flow. 

Check: Difference Between Inviscid and Viscous Flow 

What is the External Flow? 

Source: Source: White Frank M., Fluid Mechanics, McGraw-Hill Education, 7th edition, February, 2010, ISBN: 978-0077422417

Fluid flow is classified according to its flowing medium. In this medium, if the fluid flow is not confined to boundaries, this kind of fluid flow is called external flow. There are lots of kinds of engineering applications where external flow characteristics must be applied. 

One of the most important applications of external flow is engineering-level refrigeration. Refrigeration is a very wide area that includes different applications also. Radiators of automobiles are a very good example of them. In the radiator panels, refrigeration fluid(generally water) is circulated. On these panels, airflow is forced to pass as the external flow that takes out the heat of this refrigeration water. 

The aerodynamical design of airplanes and automobiles is another important application of external flow. The external flow must be well designed to minimize the drag coefficient of the vehicle. 

Check: The Best Fluid Mechanics Books

What is the Internal Flow in Fluid Mechanics? 

Unlike the external flow, if the fluid flow is confined inside the boundaries, this type of flow is called fluid flow. Internal flows are affected by the viscosities near the boundaries in a harsh way. 

There are also lots of applications of internal flows in engineering. We gave the fluid refrigerations of automobile engines with water circulation. This water circulation takes place by circulating water through pipes. So, the flow of water through the pipes in these refrigeration systems is a very good example of internal flow. All the flows which are confined inside pipes or ducts, these flows are classified as internal flows. 

There is another sub-classification flow which is also called an open-channel flow. In open-channel flow, one side of the flow is confined such as water flow in rivers. 


As you understand, the difference between external and internal flow is very simple in the fluid mechanics field. 

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