MMath&Phys Mathematics and Physics

Many physical systems can become unstable in the sense that small disturbances superimposed on their basic state can amplify and significantly alter their initial state. In this course we introduce the basic theoretical and physical methodology required to understand and predict instability in a variety of situations with focus on hydrodynamic instabilities and on instabilities in reaction-diffusion systems.

The aim of this course unit is to introduce students to the basic concepts and techniques of modern stability theory, through case studies in fluid mechanics and transport phenomena.

Assuming general mechanics and fluid mechanics in particular (viscous/inviscid), as well as some aspects of dynamical systems as prerequisites for course.

1. Introduction to stability

Nonlinear dynamics. Linear instability versus nonlinear instability. Outline of the basic procedure involved in a linear stability analysis: dispersion relation, marginal stability curve. Role of weakly nonlinear theory, e.g. normal form for pitchfork bifurcation.

2. Linear stability analysis: a case study of Rayleigh-Benard convection

Introduction to physical system, Boussinesq equations, dimensional analysis, Basic state, linear theory, normal modes, marginal stability curve: Analytical approach for idealised boundary conditions.

3. Interfacial instabilities

Examples: Rayleigh-Taylor and capillary instabilities.

4. Shear flow instabilities

Inviscid/viscous, Squire's theorem. Rayleigh's equation, Rayleigh's inflexion point criterion, Howard's semi-circle theorem, Orr-Sommerfeld equation. Examples: plane Couette flow, plane Poiseuille flow, pipe flow, Taylor-Couette flow.

5. Stability in reaction diffusion systems.

Stability of propagating fronts.

6. Bifurcation theory

Local bifurcations, normal forms.

7. Nonlinear stability theory

Weakly nonlinear theory, derivation of Stuart-Landau equation, Ginzburg-Landau equation.

8. Introduction to pattern formation (if time allows)

Stripes, squares and hexagons, three-wave interactions, role of symmetry, long-wave instabilities of patterns: Eckhaus.

• Mid-semester coursework: 20%
• End of semester examination: weighting 80%

• P.G. Drazin, Introduction to hydrodynamic stability. Cambridge University Press (2002)
• F. Charru, Hydrodynamic Instabilities. Cambridge University Press (2011)
• P. Manneville, Instabilities, chaos and turbulence. Imperial College Press (2004)