Investigation On The Mechanism And Reynolds Number Effect On Flow Asymmetry At High Incidence

Slender body is a widely used fore-body configuration for modern advanced fighters and tactic missiles. At high incidence, asymmetric vortices appear usually on the lee side of the body, and large lateral force and yawing moment would occur, leading to severe effects on the operational performance and flight safety of vehicle. With the improving of modern advanced flight vehicle, more and more attentions have been paid to the asymmetric flow involved over slender bodies at high incidence. But we haven't grasped the cause of this phenomena from mechanism by so far because of the extremely difficulty while the research. And the research on flow asymmetry over slender fore-body at high incidence is behind the development of modern vehicle performance, so that we can just estimate the aerodynamic forces in traditional wind tunnels at high incidence. We cannot obtain the aerodynamic characteristics exactly only by experiments in wind tunnel. Manny modern vehicles, such as F-15, have encountered side departure in flight at high incidence, which was not detected in traditional wind tunnels. Such departure not only behaves very suddenly but also differs much over different vehicles of the same configuration. The difference of Reynolds number caused by the scale difference between wind tunnel models and true vehicles makes us difficult in obtaining the aerodynamic characteristics in wind tunnel. So it is very necessary to research the effect and simulation technology of Reynolds number effect on flow asymmetry over slender fore-body at high incidence.We first studied the unsteadiness of flow asymmetry by dynamic force measurements and analyzed the relationship between flow structures and characteristics at high incidence. Reynolds number effect and scale effect on flow asymmetry were taken emphasis on in sub-critical Reynolds number range with different scale models. Technology of fixed transition was applied to obtain flow characteristic of higher Reynolds number. And the phenomenon of flow asymmetry was observed to predict the development of flow asymmetry in higher Reynolds number range.Present study on flow characteristics at high incidence shows that: energy in the flow field over slender bodies at high incidence is centralized to low frequency. And the effect of energy components in high frequency range is much weak. The flow asymmetry is dominated by the behavior of the main vortices. Though the development of asymmetric flow along axes is an unsteady process, the explicit behavior of flow asymmetry behaves unsteadily. We can study this phenomenon by traditional experiment technology in wind tunnel.In sub-critical Reynolds number range, the circulation of the vortex line from the boundary layer near the separation is increased by the increasing of velocity. And this will enforce the intensity of the vortices, which expedites the taking off of the vortices. When the strong vortex takes off, its interference with the lower vortex decreases. Then the domination range of the lower vortex will enlarge, which will make flow asymmetry more severe.In critical Reynolds number range, separation position of the flow was suspended from the azimuth angle about 100 deg to 140 deg. And flow asymmetry was induced by the asymmetric boundary layer on both sides of the cross section, which resulting in lower side force.In super critical Reynolds number range, flow asymmetry is induced by both the asymmetric vortices and asymmetric boundary layer state. The vortices, boundary layer state and side force are all insensitive to the Reynolds number.