ASTR 320: Physical Processes in Astronomy

Prof. Frank C. van den Bosch

Table of Contents

Chapter 1

Introduction

Fluid Examples

Chapter 2

Dynamical Treatments of Fluids

Dynamical Theory:

1. Way to describe state of system
2. a (set of) equation(s) to describe how state variables change over time

Ehrenfest’s Theorem: a system of N quantum particles can be treated as a system of N classical particles if the characteristic separation is large compared to the de Broglie wavelength.

$$ \lambda = \frac{h}{p} \simeq \frac{h}{\sqrt{mk_BT}} $$

Levels of Fluid Dynamics

Macroscopic Continuum Approach

→ Knudsen number: $K_n = \frac{\lambda}{l_{scale}}$

• Fluids typically have $K_n \ll 1$, if not, then continuum approach is not justified (use level 2 approach instead)

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Fluid elements CANNOT be defined for a collisionless fluid (which has $\infty$ mfp)

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