Initialisation plus a bit of text already

.gitignore

@@ -0,0 +1 @@
+*.pdf

Introduction to fluid dynamics.md

@@ -0,0 +1,15 @@
+# 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