Notion of stability
Classes of instability
Properties of instabilities
The notion of stability is very important in Fluids Mechanics for viscous fluids flows. This notion is based on the amplification or not of perturbations introduced in the flow.
Effectively, if a perturbation is introduced in the viscous fluid flow we can have two cases :
 the perturbation
can grows less in time : the flow is said stable
 the perturbation
can develops in time : the flow is said unstable and instabilities
can appear which can
lead to turbulence sometimes
Principle of stability/unstability
We can inventory a lot of instabilities. We are going to explain you some of them most known in the next pages.
Instabilities can de classified in two classes :
 instabilities which
occur inside fluid : Internal Instabilities
 instabilities which
occur on particular surfaces marking boundaries of different flows : Surface
Instabilities
For each class, we are going to explain several instabilities. Instabilities noticed in this work are most famous instabilities we can found in intrustry or Nature. Some instabilities can be linked, for example Stroual instability and instability in a jet are special cases of KelvinHelmholtz instability.
All instabilities dealed with in this work are inventory
in the following table :




Internal Instabilities present two common points. Firstly, there is in general a break of equilibrum between stabilizing forces and unstabilizing forces. Lastly, this break of equilibrum appears after a threshold of an adimensional parameter caracteristic of the flow.
For Surface Instabilities, there is only one common
point : in fact, there is in this case, no notion of treshold. The only
presence of perturbations is sufficient to develop instabilities.