Over the years, a recurring theme in Dr Day’s research has been the stability of aero-engine compressors. In an engine, the low flow-rate end of the operating range is limited by stall or surge – both being aerodynamic disturbances which can cause damage to the fabric of the engine. A study of these instabilities is thus essential for the design of reliable compressors - especially as the instabilities themselves are not amenable to numerical modelling. Dr Day’s work has been mostly experimental concentrating on the disturbances themselves and then on finding ways of suppressing them. His studies have cover stall inception, stall recovery, stall warning, casing treatment, over-tip leakage, active control, passive control and blade tip clearance effects.
Other fields of interest include the study of rain ingestion by aero-engine compressors and the ensuing problem of rain ejection from the compressor core. Water spray cooling for hot-day operation of industrial power turbines has also been a field of interest. Furthermore, in relation to the use of gas turbines for power generation, Dr Day has worked on combustion instabilities caused by unsteady fuel injection. He holds a number of patents on this and other topics.