1. Introduction to Fluid Mechanics
Chapter 2
Fundamental Concepts

2. Main Topics Fluid as a Continuum Velocity Field Stress Field Viscosity
Surface Tension
Description and Classification of Fluid Motions

3. Fluid as a Continuum
This molecular structure is one in which the mass is not continuously distributed in space, but is concentrated in molecules that are separated by relatively large regions of empty space. For very small volumes, the density varies greatly, but above a certain volume, the density becomes stable, the volume now encloses a huge number of molecules.

4. Velocity Field Velocity is a vector
it indicates the velocity of a fluid particle that is passing through the point x , y , z at time instant t
it can be written in terms of its three scalar components

5. One-, Two-, and Three-Dimensional Flows
A flow is classified as one-, two-, or three-dimensional depending on the number of space coordinates required to specify the velocity field
1-D flow
2-D flow

6. Timelines, Pathlines, Streaklines, and Streamlines
Timelines - lines formed by adjacent fluid particles at a given instant
Pathlines - path or trajectory traced out by a moving fluid particle
Streaklines - lines joining different articles passing through a point
Streamlines - lines that at a given instant are tangent to the direction of flow at every point in the flow field
In a steady flow, pathlines, streaklines, and streamlines are identical lines

7. Stress Field What kinds of forces act on fluid particles?
surface forces (pressure, friction) and body forces (gravity and electromagnetic)

8. Viscosity Shear stresses arise due to viscous flow Newtonian Fluids
Most of the common fluids (water, air, oil, etc.)
“Linear” fluids

9. Viscosity
Fluids in which shear stress is not directly proportional to deformation rate
Non-Newtonian Fluids
Special fluids (e.g., most biological fluids, toothpaste, some paints, etc.)
“Non-linear” fluids

10. Viscosity
Non-Newtonian Fluids

11. Surface Tension
Whenever a liquid is in contact with other liquids or gases an interface develops that acts like a stretched elastic membrane, creating surface tension.

12. Description and Classification of Fluid Motions