Fluid dynamics of unsteady separated flow. Part I. Bodies of revolution

Abstract

Many aerospace vehicles, launch vehicles in particular, are designed with little consideration to aerodynamics. As a consequence, the aerodynamic characteristics are often dominated by the effects of separated flow. The vehicle designer must, therefore, be able to assess what these effects will be especially in regard to the vehicle dynamics, as those effects are often adverse and always of large magnitude relative to the attached flow characteristics. In spite of rapid development of computational means, purely theoretical methods for prediction of the effect of separated flow on rigid and elastic body dynamics are not presently available and will not be for some time. To compound the problem experimental dynamic data by which one can determine the effect of flow separation on full scale vehicle dynamics are usually not available either. The present paper shows a practical solution to this dilemma. An analytic method is described that uses static experimental results to predict the separated flow effect on rigid and elastic vehicle dynamics. A key parameter in the analytic relationship between steady and non-steady aerodynamics is the time lag occurring before a change of flow conditions can affect the separation-induced aerodynamic loads. The time lag causes a statically stabilizing load to produce negative aerodynamic damping, and vice versa. This time-lagged quasi-steady theory provides predictions that are in good agreement whith dynamic experimental results for a great variety of vehicle geometrics and associated types of flow separation.