Background
The term instability predated the systematic study of flow instability, prior to which it had been widely used to study the dynamic systems of particles and rigid bodies described by Newton’s or Lagrange’s equations of motion. A system is stable when, as Maxwell put,
an infinitely small variation of the present state will alter only by an infinitely small quantity the state at some future time… But when an infinitely small variation in the present state may bring about a finite difference in the state of the system in a finite time, the condition of the system is said to be unstable.
Instability can also occur in flow systems, the study of which gave birth to the subject of hydrodynamic instability. Some classic problems of hydrodynamic stability were formulated in the nineteenth century, with notable contributions from Kelvin, Helmholtz, Reynolds etc. The onset of instability is often the prelude of flow transition from laminar to turbulent state, and hydrodynamic stability has therefore become a subject under extensive research since its emergence.
Current research
Flow instability is an extensive research field. There exist many types of instabilities, such as Kelvin-Helmholtz, Rayleigh-Bernard, Taylor-Couette, and Saffman-Taylor instabilities, to name a few. Here in our group we pay close attention to the Kelvin-Helmholtz type instability of high-speed jets. Not only because the instability waves can interact with the rigid surface in its close proximity leading to various important new flow features, but also because its role in generating jet noise remains inadequately understood. We are in the meantime also interested in the instability of other types of flows such as wakes and pipe flows of unusual geometry.