Acoustic Control of Flow Instabilities
The lecture reviews the common origin of sound and instabilities in weak perturbations of a smooth flow. The degree to which sound and instabilities interact, or indeed the degree to which they are separable, depends on their semantic definition. At one level they are merely secondary unsteady effects dependent on the main flow while at the other they display distinctive characteristics of propagation and exponential growth. Even if they exist as separable entities in a pure flow they remain coupled by the inevitable inhomogeneities of realistic geometrical arrangements so that in practice flow instability and sound are actually inseparable.
Sound, being essentially linear, admits to the attenuation brought by superposition of its phase-inverted replica. Anti-sound cancels sound, a principle now being implemented in rapidly growing noise control technology. The same principle implies change in stability whenever its coupled sound field is modified. Flows that are traditionally unstable can be stabilized by active systems. Whether all flows can be stabilized or not might depend on the fundamental constraint that it is impossible to forecast the future. More probably the limit is set by technical complexity and by the need to keep all perturbations within the linear regime where superposition is valid. The scope of the idea will only become clear as definite experiments are completed.
Experiments have progressed from the control of a small combustion system with a single instability mode to the acoustic suppression of violent unsteadiness at aero-engine reheat conditions. A wing section flexibly mounted in a wind tunnel has been brought out of flutter oscillation by switching on a wall-mounted loudspeaker. The fundamental Strouhal frequency in the Karman street wake of a circular cylinder has been suppressed by acoustic feedback and both rotating stall and surge have been avoided at conditions which made those instabilities inevitable without active control. An ONR programme to examine the applicability of these methods to a real gas turbine engine has led to a power increase of some 10% and to active methods of restoring the stable condition following the onset of surge. The lecture will conclude by reviewing these developments and issues that have arisen in developing the control methods. In particular it will raise the prospect of aerofoil sections controlled to operate with radically superior characteristics and speculate on that being an element of the performance improvement observed on the test engine.
KeywordsCombustion Vortex Attenuation Coherence Compressibility
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