# Video of “Compressible vs incompressible flow and conservation of mass”

Below are GIFs of the compressible and incompressible versions of the “Compressible vs incompressible flow and conservation of mass” demonstration. The full videos are linked from the GIF headings.

Compressible flow (air)

Incompressible flow (water)

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Compressible vs incompressible flow and conservation of mass

This is a really simple demonstration of how conservation of volume can be used for incompressible fluids but not for compressible fluids. The demonstration was suggested by Dr. Baburaj of IIT Madras. I teach in a civil engineering department where practically everything is incomopressible and we mostly talk about conservation of volume. The demonstration below is so simple yet so clear.

Equipment

You will need:

1. Two identical syringes,
2. A few feet of clear tubing that fits tightly over the end of each syringe,
3. Some water, and
4. Food dye (optional)

Demonstration

Compressible flow

1. Have one syringe (A) with the plunger fully pushed in and the second plunger (B) fully pulled out.
2. Connect each end of the tube to the syringes
3. Slowly press the plunger on syringe (B)

Assuming that the syringe plunger’s are a little stiff you should be able to push the plunger on (B) all the way in before the plunger on (A) is pushed all the way out. Mass is conserved because there are no leaks but volume is not conserved as the plungers move different distances on identical syringes. This works better with stiffer syringe plungers.

Incompressible flow

1. Have one syringe (A) with the plunger fully pushed in and the second plunger (B) fully pulled out and the syringe full of water.
2. Fill the tube with water (food dye can help with visualization) and connect the tubes in the same way as for the previous version. This is tricky as you want to ensure that there are no air bubbles in the lines.
3. Slowly push in the plunger on syringe (B). The plunger in syringe (A) should move out at exactly the same speed. you can show this clearly by having the syringes pointing away from each other with the plunger ends next to each other. As you push one in the other should move right next to it.

Analysis

There is no analysis for this demonstration. The gas is compressible so volume is not conserved whereas the liquid is incompressible so volume is conserved. Analysis of the change in pressure in the compressible case and resulting motion of the plungers is complex as you need to know about the friction in the syringe.

Thanks again to  Dr. Baburaj for suggesting the demonstration.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Video of “Compressibility and incompressibility demonstrated with soda bottles and ketchup”

Here is a video of the “Compressibility and incompressibility demonstrated with soda bottles and ketchup” demonstration. The full video is here.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Compressibility and incompressibility demonstrated with soda bottles and ketchup

Introduction

I teach in a civil engineering department so we pretty much only deal with incompressible flows. However, this is a really simple demonstration to illustrate the compressibility of gasses and the relative incompressibility of liquids that uses stuff you can pick up at a fast food restaurant and recycling bin. I found it in a number of different books on science experiments for kids.

Equipment

1. 2 liter soda bottle with cap
2. a small ketchup (or other condiment) packet that floats (test this before you wedge it in to the bottle).
3. water

Demonstration

1. Stick the ketchup packet into the soda bottle and then fill the bottle with water until there is only a small volume of air below the top of the bottle.
2. Tightly screw on the cap so that the bottle is sealed. The ketchup packet should be floating.
3. Squeeze the bottle firmly with your hand and the ketchup packet should sink.
4. release the bottle and the ketchup packet will float back up to the surface.

Discussion

This is effectively a cheap way to make a Cartesian diver. The demonstration relies on the water being effectively incompressible and the air being compressible. When you squeeze the bottle it is the air pocket at the top of the bottle that is compressed by the change in volume. This increases the pressure in the water but does not compress it so the water density stays the same. However, the ketchup packet has a small air bubble in it which also compresses. This reduces the volume of the bubble enough that the net density of the packet changes from being less than that of water to greater than that of water so it sinks. This process is reversed when you stop squeezing the bottle.

You can also get the packet to sink just by leaving the bottle out in the sun. In this case the water and air both heat up. however, given the finite volume, as the water expands slightly from heating, the air is compressed and the packet sinks.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.