Stability Of Flow Around A Slender Body Of Revolution At High Angle Of Attack

Modern aircraft often endure large side force and yawing moment when performing maneuver flight at large angle of attack because of the emergency of asymmetric forebody vortex.The investigations of the formation mechanism and developing roles of the asymmetric forebody vortex flow have great importance in both academic field and engineering application areas.In this dissertation,these issues are explored by numerical simulation of the asymmetric vortex flow around a slender ogive-cylinder body and the global and spatial stability analysis of the flow field.This study aims at better understanding of the character and mechanism of the asymmetric flow field,to contribute to the control of forebody asymmetric vortex.A parallel direct numerical simulation program has been developed,based on a finite difference method in generalized curvilinear coordinate;a three-dimensional global stability analysis method has been developed using a finite difference method for compressible flow and implicitly restarted Arnoldi method;in addition,the analysis method to investigate the spatial evolution of perturbation based on analysis of disturbance field has been discussed.The numerical simulations in this dissertation mainly intend to investigate the asymmetric characters and the corresponding development of sectional side force along the body for different strength and circumferential angle of the disturbance.The results show that,at large angle of attack(typically 50°),the sectional side force varies continuously with the circumferential angle of the disturbance;this variation tends toward a square-wave form further downstream.With the increment of the strength of disturbance,the section where the square-wave form variation emerges gets closer to the tip.It is also found that at lower Reynolds number,the flow field is less sensitive to tip disturbance,and the asymmetry decreases;the variation of side force with the circumferential angle of disturbance turns from double periodic square-wave form to single periodic square-wave form,and at last the variation turns to be single periodic and continuous.The global stability analysis for the symmetric three-dimensional basic flow indicates that,the asymmetry does not caused by a global instability mechanism.At the angle of attack 40°-50°(when large side force was observed in experiments),for small Reynolds number,the symmetric flow field is globally stable;for larger Reynolds number, the symmetric flow field turns to be globally unstable,but the unstable eigenvectors won't lead to large asymmetry and side force.There are two kinds of dominant eigen- vectors found:the high-frequency ones show the result of shear-layer unsteadiness;the low-frequency ones represent the fluctuation of the main vortex cores.The effects of both kinds were found in experiments,and the unsteady numerical simulation results confirm to the global stability analysis well.The analysis of disturbance field shows that after an initial growth regime,there is an exponential growth regime of the disturbance energy along the body,followed by nonlinear growth regimes.The linear spatial growth rate can be obtained from the exponential growth regime.The growth rate is independent to the strength and circumferential angle of the disturbance,and is determined by the parameters of the basic flow:it gets larger with the angle of attack increases,and is insensitive to Reynolds number at the Reynolds number range under consideration.The nonlinear saturation of the disturbance energy is corresponding to the emergency of bistable phenomenon.The Van der pol equation under impulse excitation can be used to simulate the development of perturbation along the body,and it can also explain the exponential growth of the disturbance energy.