Characteristics of Fluid Flow

Fluid Mechanics is the study of how working fluids flow and interact with each other and our environment.
The basis of any study into the wonderful world of fluid mechanics thus hinges around fluids and their properties.

What is a Fluid?

Substances in our universe exist in three basic phases

Liquid
Gas
Solid

Liquids and Gases are regarded as fluids.
How this is determined is by analyzing the substance reaction to shear stress.
Solids under shear will deform to a strain angle and will return to their original state if the shear stress is removed.

This is only true if the object remained in the elastic stress region.

Liquids and gases react differently to shear stress though. Fluids will deform continuously without reaching a specified strain angle.
This one distinction separates substances from being a fluid or not, at least scientifically.

Fluid Flow Types

Fluid mechanics largely deals with fluids in motion and their application to the engineering industry.
The study of fluid flow is used in the automotive, marine and civil engineering fields to name a few.
Fluid flows can be grouped as internal or external flows.
Internal flows deal with liquids and gases that flow within pipes or areas where they can be guided and controlled.
External flows typically occur in nature where their are no definite boundaries, such as the flow of air over an aircraft wing.
The internal characteristics of fluid flow are more interesting, this is the study of how the fluid behaves internally.
These are classified as flow types and three exist

Laminar Flow
Turbulent Flow
Transitional Flow

Every engineering fluid will be in one of these states at any given moment.

Laminar Fluid Flow

Laminar flow is extremely smooth and ordered.
Fluids that have a high viscosity generally will experience laminar flow.

Turbulent Fluid Flow

Turbulent flow is disorganized and is typically the condition of most fluids used in engineering.
Fluids with low viscosity will generally experience turbulent flow.

Transitional Fluid Flow

In between laminar and turbulent flow exists a transitional flow state.
How we determine flow states in fluid mechanics is by using Reynolds Number. This dimensionless number was developed by Osborne Reynolds, a British engineer and pioneer in fluid dynamics.
This number is calculated using the following flow conditions

Fluid Velocity
Flow Medium Dimensions
Viscosity

The dimensionless number will then fall into one of the three ranges.

Summary

Fluids are the cornerstone of Fluid Mechanics.
Even though distinguishing between fluids and solids is simple and intuitive the actual state of the fluid has more importance.
Most simple engineering problems though will have fluids in a turbulent state.