2.3 LAMINAR & TURBULENT FLOW & REYNOLDS NUMBER

Question 1

Laminar & turbulent flow

Answer 1

Reynolds experimented by changing e.g. fluid velocity u, or viscosity, fluid density, or diameter of pipe.

Change from laminar flow where fluid particles travel in regular straight lines along planes parallel to base/boundary of flow, to irregular turbulent flow.

Question 2

When does the flow become turbulent?

Answer 2

When Re > 2000

Question 3

How does the fluid move in LAMINAR flow?

Answer 3

• in linear paths generally parallel to the boundary of the flow
• at constant velocity
• fluid motion is predictable
• may be analysed by mathematical theory

Question 4

How does the fluid move in TURBULENT flow?

Answer 4

• in irregular, sinuous paths
• with velocity changing (subject to accelerations/decelerations)
• fluid motion is unpredictable
• cannot be analysed by mathematical theory - requires empirical/experimental study

Question 5

What is the turbulence syndrome?

Answer 5

• disorder (irreproducable in detail)
• efficient mixing
• vorticity (irregularity distributed in 3 dimensions)
  ^ eddies on various scales are a key part of turbulence

Question 6

What happens to eddies in turbulence?

Answer 6

They progressively decay into smaller eddies, until “smeared out” by viscosity.

Question 7

What can Re also be known as?

Answer 7

A ratio of the inertial/viscous force

Question 8

What is the term for systems with the same Reynolds number?

Answer 8

Dynamically similar

Question 9

What forces are dominant in the high/low Re number worlds?

Answer 9

High Reynolds number world: inertial forces dominant

Low Reynolds number world: viscous forces dominant

Most marine orgs adapt to only one of these worlds; dictated by size and speed.
A few inhabit both, like copepods.