Fluid dynamics of unsteady separated flow. Part II. Lifting surfaces

In the quest for increased maneuverability advanced aircraft have to operate at high angles of attack where the aerodynamics are dominated by the effects of separated flow. Likewise, winged space vehicles, such as the space shuttle orbiter, also fly at high incidence during re-entry in order to restrict heating to the windward side, thereby minimizing heat shield requirements. Thus, the vehicle designer needs to be able to assess what effects the separated flow will have, especially on the vehicle dynamics, as these effects are often adverse and always of large magnitude relative to the attached flow aerodynamics. In spite of rapid development of computational means, purely theoretical methods for prediction of the effect of separated flow on rigid and elastic vehicle dynamics are not presently available, and will not be for some time. To compound the problem for the vehicle designer, dynamic simulation in an experiment requires the testing to be performed at full scale Reynolds number. The present paper shows a practical solution to this dilemma. An analytic method is described that uses static experimental data to predict the separated flow effect on rigid and elastic vehicle dynamics. Key parameters in the analytic relationship between steady and non-steady aerodynamics are the following: (1) The time lag occurring before a change of flow conditions can affect the separation-induced aerodynamic loads. (2) The accelerated flow effect, i.e. the pressure gradient lag relative to the static aerodynamic characteristics. (3) The moving wall effect, i.e. the effect of the non-steady boundary condition at the vehicle surface. Using the existing experimental data base an analytic theory is formulated that can predict the separation-induced unsteady aerodynamics if the static characteristics are known from theory or experiment.