When the engines operate at high thrust, during take-off, the fan suction is much stronger and disrupts the airframe boundary layer flow. This produces an extremely unsteady turbulent flow alongside fan-induced flow distortion. The interaction between fan blades and the distorted air flow results in fan haystacking, where the large span of the rotating blades further slices the unsteady flow.
“These two hidden sound signatures make future embedded aircraft engines feel perceptually irritating, not just loud,” explained Feroz Ahmed, who was involved with the work while at the University of Bristol in a press release.