In aerodynamics, unsteady phenomena occur from shock-boundary layer interaction or boundary layer separation, but also are present in the flows around rotating systems. They concern the external aerodynamics of all the vehicles: aircraft, helicopter, projectile and launcher. To answer the purpose of transport aircraft, the aims of studies are guided by the requirement of non separated flow in cruise and some constraints in the flight envelope: buffeting onset, flutter risk, limit cycle oscillation and level and spectral power of the loads on structures. Unsteadiness is an important parameter on the behaviour and the performances of flexible aircraft. For fighters at high angle of attack, separated flows are encountered in air intakes on forebody, wings and afterbody. Thus, hysteresis phenomena of lift coefficient is the characteristic of quick manoeuvres in aerial combat. The aircraft behaviour during flight in turbulence is also an important feature for penetration mission. Airflow around helicopter is of course unsteady due to the relative speed of local flow on the blades during the rotation of the rotor (advancing side and retreating side) and to the cyclic movements of articulated blades. The same kind of unsteady interaction is encountered between propfan and wing of commuters.
The buffeting observed at the base of launchers is created by strong separation of the external flow and interactions with exhaust plume. Internal separation in over-expanded nozzles at the take-off is unsteady and 3D and causes lateral forces beside the thrust. The behaviour in gust and during the activation of control surfaces is important data for the control of the vehicles. Unsteady flows are also encountered in turbomachinery due to the rotation and interactions between the stages.
(Source: ACARE Domain 102)