Fluid Mechanics Lesson 1A: Introduction

Instructor

• John Zimbala

Overview

• Definition: Fluid Mechanics is the study of fluids (liquids and gases) and their behavior under various forces.

Key Concepts

Fluid

• Common Definition: A fluid can be a liquid or a gas.
• States of Matter: Solid, liquid, gas.
• Behavior: Liquids and gases can often be treated similarly under certain conditions (negligible compressibility effects and no free surfaces).
  • Exceptions:
    • Liquids with free surfaces (e.g., waves generated by a boat)
    • Gases in high-speed flow (e.g., rockets)
• Formal Definition: A fluid is a substance that deforms continuously under the application of a shear stress.

Mechanics

• Application of the laws of force and motion.
• Two branches: Statics and Dynamics.

Stress in Fluids

Definition of Stress

• Stress: Force per unit area acting on a surface.
• Types of Stress:
  • Normal Stress: 90 degrees to the surface.
  • Shear Stress: Tangential to the surface.

Normal Stress

• Solids and liquids at rest can resist normal stress.
• Pressure: A common example of normal stress.
  • Acts inward and normal to surfaces, whether real or imaginary (like a fluid particle).

Shear Stress

• Solids: Can resist shear stress, deforms but remains in new configuration.
• Liquids: Cannot resist shear stress; they deform and flow.
• Fluids at Rest: Cannot resist shear stress; they deform and flow.
• Fluids in Motion: Can have both shear stress and normal stress.

Free Body Diagrams

• Used to analyze stresses.
• Solid Element: Has shear and normal stresses.
• Liquid Element: At rest has only normal stresses; in motion has both.
• Equations:
  • Statics: ( \Sigma F = 0 )
  • Dynamics: ( \Sigma F = ma )

Application

Fluid Statics

• Also known as hydrostatics.
• Study of fluids at rest.

Fluid Dynamics

• Study of fluids in motion.
• Fluids in motion involve acceleration.

Conclusion

• Fluid Mechanics: Application of force and motion laws on fluids (substances that deform under shear stress).
• Branches: Fluid statics and fluid dynamics.

This lesson sets the groundwork for understanding the behavior of fluids, necessary for more advanced topics in fluid mechanics.