Expert: Mark Janus - 10/23/2007

Question
i'd like to know that is water a compressible fluid? if not then how the water jet machining is done since it uses a pressurised water to cut the metal?

Answer
Yes for all practical purposes, water is incompressible...  
Fluid compressibility has to do with the ability to "squeeze" the fluid into a smaller space...  in engineering we use the fluid's "mass density" as a metric (or measure).... mass density refers to how much mass of a fluid occupies a certain amount of volume....   If no more mass of a fluid can be "squeezed" into a volume, the fluid said to be incompressible.  So the misunderstanding here, I believe, is that compressibility has something to do with a fluids pressure when in reality it has to do with its density.  An incompressible fluid can and is under differing pressures.....very high pressures I might add for a water knife...  The real reason a water knife works as well as it does is do to the abrasive that is added to the water.

Thanks for the good question,
mj