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The investigations of forebody vortex flow and its flow control have great importance in both academic field and engineering application areas. A large number of papers and many review papers have been published. However in this research field of forebody asymmetric vortices, three problems such as tip perturbation effect, Reynolds number effect and flow instability are less studied and thus not understood completely. So many researches are still working on the issues in recent years. The present paper attempts to provide a review of recent research progress on first two problems. The first problem is mainly concerned with how the vortex flow evolves after tip perturbation; how to solve the problem of repeatability and reproducibility of wind tunnel testing data; how to develop a conception of active flow control technique with tip perturbation based on the study of vortex flow response to tip perturbation. For the second problem one is mainly concerned that how the asymmetric vortices are developed with the increase of Reynolds number; how to classify the vortex flow patterns in different Reynolds number regimes; how to develop an appropriate boundary layer transition technique to simulate flows at high Reynolds number in the convention wind tunnels. Finally, some important questions that deserve answers are proposed in the concluding remarks.

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A wind-tunnel experiment was conducted to study the effects of forced asymmetric transition on the asymmetric vortex system of a slender body by adding a grit strip on either side of the body. The results showed that the effect is very different depending on whether transition of the boundary layer occurs first on the higher vortex side or on the lower vortex side. If the transition first occurs on the higher vortex side, the transition will strongly influence the asymmetric vortex behavior and the side forces induced. The asymmetric vortex system will abruptly reverse its orientation when transition occurs and then gradually recover to its original position with increasing Reynolds number. However, if the transition first occurs on the lower vortex side, the trend of the asymmetric vortex system with increasing Reynolds number will be very similar to the case of natural transition. In addition, the asymmetric transition of the boundary layers on the forebody seems to have a dominant effect on the behavior of the asymmetric vortex system. The associated separation angles of the boundary layers appear to play a pronounced role in the development of the asymmetric vortex system. Nomenclature = angle of attack C p = surface pressure coefficient, P P 1 = 1 2 V 2 1 C y = sectional side-force coefficients calculated by the integral of surface pressure D = cylinder diameter Re = Reynolds number based on the cylinder diameter, UD= X = axial coordinates = roll angles, deg = azimuth angles, deg s = separation angle (i.e., the minimum included angle between the separation point and the windward symmetry plane of a model), deg

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