Finished notes for 1st video

Introduction to fluid dynamics.md

@@ -11,5 +11,31 @@ 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
+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.