JW Lecture 3 - Motion of Particles in a Fluid

Question 1

What does particle weight cause and what happens when the fluid drag is equal to the gravity force?

Answer 1

Particle weight causes an acceleration which is restricted by the fluid drag force. When the fluid drag is equal to the gravity force, the motion will be uniform. The particle will no longer accelerate and will attain its terminal settling velocity.

Question 2

What is drag force and what are its two components?

Answer 2

Refers to the forces acting opposite to the relative motion of any object moving with respect to a surrounding fluid. Drag force consists of two components:

1. Form drag (or pressure) drag, which arises because of the shape of the object. Generally, thinner, more ‘streamlined’ bodies/particles have a lower form drag
2. Skin drag which arises from the interactions between the fluid and the skin of the body/particle

Question 3

What is terminal falling velocity?

Answer 3

If a spherical particle is allowed to settle in a fluid under gravity, its velocity will increase until the accelerating force is exactly balanced by the resistance force. This state is approached exponentially, the effective acceleration period is generally of short duration for very small particles.

Question 4

Terminal settling velocity assumptions

Answer 4

• The settling velocity is not affected by the presence of other particles in the fluid, this is known as ‘free settling’. When there is interference of other particles, the process is known as hindered settling.
• The walls of the containing vessel do not affect the settling
• The fluid is considered as a continuous medium and the particle size is large compared with the mean free path of the molecules of the fluid