Experimental Investigation of Unsteady Flow Separation on a Maneuvering Axisymmetric Body

摘要

An experimental study on maneuvering-undersea-vehicle geometry was conducted to give detailed information on unsteady turbulent flow separation and to investigate different time-lag models for approximating the unsteady separation locations. Both unsteady and steady results are presented for the axisymmetric configuration of the model, which can also be thought of as a missile or aircraft-fuselage geometry. Steady and unsteady skin-friction magnitudes were measured using hot-film sensors. The minimum of the circumferential skin-friction distribution at each axial-measurement station was used to detect the separation locations. The linear pitch-up maneuver studied is comparable to the dynamic motion of full-scale vehicles of interest, in terms of the nondimensional time. Significant time lags observed between unsteady and steady crossflow separation locations indicate the increase in attached circulation over the maneuvering model, which may explain why unsteady forces measured on similar geometries lead the steady values. A first-order differential time-lag model approximates the unsteady separation locations reasonably well. This model can be considered as a method of correlating the unsteady measurements to the quasi-steady data with a time lag. Using this model, the prediction of the unsteady flow characteristics from the steady data can be possible with the development of methods for estimating the time lags that vary based on the geometry and the maneuvers performed. The results of the unsteady experiments presented in this paper show that the time required to displace the volume of leeside inviscid fluid during a maneuver with vortical separated flow may be the main factor creating the difference in the time-lag magnitudes and their spatial variation observed for different axisymmetric geometries and points of rotation.