# Introduction to fluid dynamics

## 3 main laws
- Momentum balance
- Mass balance
- Energy balance

## The continuum assumption

Let lambda be the mean free path, L a characteristic dimension of the problem

Kn = lambda/L

Hypothesis: 
If Kn<<1, it is possible to use a model based on the continuum hypothesis

That hypothesis is often valid, but not always. Within this course's scope, this hypothesis is always valid.

## Types of flows

Three ways flows can be be split:

### (Un)Steady flows
Steady: flow variables do not depend on time.

d/dt = 0

### (In)compressible flows
A flox can be considered incompressible is the Mach number is low enough:

Let v be the flow velocity, c be the celerity of sound

Approx. M = v/c <= 0.3

### Turbulent or laminar flows

Turbulent flows : flow variable are stochastic and vary with space and time. This happens when the Reynolds number is high enough :

Re = rho*v*D/mu > 2500 (for a tube)

Turbulent flows happen often but we'll find ways get around it.