The combustion chamber in a rocket engine contains several hundred fuel injectors. Each injector consists of a jet of liquid oxygen surrounded by a faster stream of hydrogen. By trial and error, engineers have found that combustion efficiency improves if the oxygen tube is recessed inside the hydrogen injector. This is due to improved mixing.
A local instability analysis shows that confinement causes some shear flows to transition from convective instability to absolute instability. The rocket injector is typical of such a flow. Absolute instability can cause a global mode which, in this case, flaps the jet from side to side. This flapping motion aids atomization and mixing downstream.
The local instability analysis also shows that, when confinement is combined with swirl, many shear flows become absolutely unstable. This is a configuration often used in aeroplane engines.
At the moment, this is just a theory. We are currently planning a series of experiments with two particular goals: (a) carefully to test the theory and (b) to explore parameters which are currently inaccessible to the theory. This work will lead to the development of useful rules for injector design.
The concept of an experimental rig has been finalised. The aim is to determine when a confined shear flow transitions from a globally stable mode to a globally unstable mode. Globally unstable modes beat at their own frequency and are insensitive to small forcing. Globally stable modes, if locally convectively unstable, amplify the forcing signal. More details are in the research opportunities section.