The word hysteresis is derived from a Greek word meaning "to lag 'behind". As specifically applied to fluid flow around bodies with transient angles of attack in and near the stall region, "aerodynamic hysteresis" is used to describe the effect of delay in boundary layer separation and reattachment upon the lift, drag and pitching moment. Experimental work done on airfoils by H. Studer showed that for increasing angles of attack flow "separation is delayed to an angle of attack appreciably greater than that for a stationary airfoil. On the return movement, re-establishment of a smooth flow is also delayed." * The result is that under transient conditions "more than one value of flow coefficient (and thus lift, drag and pitching moment) can be obtained for a single angle of attack. . ., depending upon the direction in which the particular angle of attack is approached." 
College and Department
BYU ScholarsArchive Citation
Peterson, Gerald Heber, "Investigation of Aerodynamic Hysteresis" (1964). Theses and Dissertations. 7171.
Aerodynamics, Hysteresis, Lift (Aerodynamics), Drag (Aerodynamics), Flutter (Aerodynamics)