Un Mixing
If I could turn back the clock! Will the spread dyes back together again?
What?
Demonstrate the special behavior of a laminar flow.
How?
Experiment Setup: As show in figure.
Set a solid acrylic cylinder inside a hollow acrylic cylinder and fill the gape with glycerine as the demonstration fluid. Eject dyes into the fluid with a needle. Rotate the inner cylinder and let the dyes spread out (record how many turns). Then turn the inner cylinder in the opposite direction (with the same number of turns), to see if the dyes will return to the original state.
×Experiments demonstration
post-producing
Why?
Turn the inner cylinder will make the dyes spread over the fluid, why the dyes will return to the original state by turning the inner cylinder backward?
×Think...
We use streamline to represent the motion of fluid particles. When the velocity is low, the streamlines will not crossover to each other (as shown in the figure below). This is so called the laminar flow.
Question
If we use water instead of glycerine, can we get the same result?
×Discussions
In the “Think…” we use speed as the index for the flow to be laminar or not. However, the real index shall be the Reynolds number of the flow.
Reynolds Number: The ratio of inertia force to the viscous force of the fluid flow, generally denoted as Re.For a pipe flow: 
The viscosity of glycerine is 1.5(Pa•s) and 0.001(Pa•s) for water. When everything else is equal, the higher viscosity gives lower Reynolds number. So, under the same conditions, the Reynolds number for the water shall be about a thousand times higher than that for the glycerine.
About the Experiment
The dye in the movie is a mixture of red ink and glycerine.
Reference
Producer
Dai, Bo-Cheng (戴伯誠), Miss Zeng (曾前助理)
Advisor
Ching-Chi Chu （朱慶琪）, Jhen-Yang Shih（施聖洋, 機械系教授）
Written by
Bo-Cheng Dai （戴伯誠）, Ching-Chi Chu （朱慶琪）
