Windmill aerodynamics following a fan blade off event

The windmill condition occurs after the in-flight shutdown of a turbofan, when the fan continues to spin with reduced rotational speed due to the ram pressure of the oncoming air, causing high loss and drag. This often follows damage to the fan blades, which leads to a further reduction of the rotational speed as well as out-of-balance loads and potentially harmful vibrations of the aircraft. Knowledge of the fan windmill rotational speed and drag at early stages of the engine design is crucial, as the aircraft needs to be designed to withstand the high drag and out-of-balance loads. In this paper, the windmill condition is studied for cases where sectors of the fan are damaged following a fan blade-off event. Experimental measurements in a low speed rig as well as numerical simulations are used to investigate the bladeto- blade flow features, such that differences for blades that are damaged can be observed. Cases with realistic damage are related to idealised damage cases with a similar amount of periodic damage at the blade tip. The rotor operating point for a realistic damage case is shown to depend on the rotor exit relative flow angle and axial velocity. Overall, damage towards the rotor tip is found to have a much higher impact on rotational speed than damage extending towards the hub and a case representing damage following a fan blade-off event causes a decrease in rotational speed of only 5.9%. The aerodynamic losses in the fan rotor are shown to be directly related to the flow blockage, which decreases with increasing damage area. Fan blades adjacent to damaged regions are also found to have a significant impact on the rotor work through the presence of the casing pressure side vortex.