Study On The Mechanism And Passive/Active Control Methods Of The Dynamic Stability Of The Reentry Capsules

Reentry capsules typically have blunt conical noses, which are designed to achieve the aerodynamic drag and decrease the aerodynamic heating. Unfortunately these blunt bodies tend to experience dynamic instability and this can lead to angular motions that frequently develop into limit cycles. The dynamic instability of the reentry capsules has been studied for long time, but many problems still remain unresolved. In order to study the mechanism of dynamic instability, it has to de done at first to simulate the transonic steady/unsteady flowfield of reentry capsules with high order accuracy schemes developed in this thesis. The passive control methods are developed on the basis of study of the mechanism of dynamic instability. With computational fluid dynamic, the intercourse of flight mechanics and control techniques, active control methods are developed elementarily to weaken or eliminate the dynamic instability of reentry capsules.Firstly, based on the demands for schemes in simulating the unsteady flowfield numerically, the NND schemes and WNND(Weighted NND) schemes are updated with the power limiter. The second-order accurate flux- or conservational variable- PNND (Power NND) schemes and the third-order accurate flux- or conservational variable-PWNND(Power WNND) schemes are designed. According to different characteristics of physical problems, different explicit or implicit time discrete schemes are constructed. Some typical numerical tests, including the linear-wave equation, 1D Euler equations and 3D Navier-Stokes equations, are presented to demonstrate the excellent capability of the PNND schemes and the PWNND schemes in reducing smearing near discontinuities and getting good resolution in smooth regions.Secondly, with the high order accurate conservational variable- PWNND schemes, the steady flowfield, the forced oscillation flowfield and the free oscillation flowfield are simulated to analyze the mechanism on the dynamic instability of reentry capsules.①The numerical results of steady flowfield show: The proportion between base aerodynamic and the total aerodynamic in transonic speeds is larger than that in supersonic speeds. The ring vortex distorts with the incident angle increasing, the horseshoe vortex becomes dominant behind the capsule.②The numerical results of forced/free oscillation flowfield show: The hysteresis loops of pitch moment coefficient become in the style of double "8" in C_m～θplane. The dynamic instability is caused by the hysteresis of the base pitch moment coefficient.③The characteristic length and velocity are analyzed for the dynamic instability of reentry capsules. According to the energy relation between the capsules and the flowfield, the passive control methods of modifying the base shape are proposed to improve the dynamic instability of reentry capsules.Thirdly, based on the analysis of the mechanism on the dynamic instability, the control effects are evaluated for several passive control methods including base shape modified, mass center moved and the combination methods.①The numerical results of free oscillation flowfield with six passive control conditions of base shapes modified show: Five shapes improve the dynamic stability(the swing of the limit cycle decreases) and one fails with the energy increasing from the flowfield.②The numerical results of free oscillation flowfield with three passive control conditions of mass center moved show: The work conducted by the front body of the reentry capsules to the flowfield increases with the mass center moving to the top point of the reentry capsules, and the dynamic stability turns better. The dynamic stability turns worse with the mass center moving to the base of the reentry capsules.③The numerical results of free oscillation flowfield with three combination passive control methods show: To improve the dynamic stability of the reentry capsules, the combination passive control methods should be considered.Lastly, with computational fluid dynamic, the intercourse of flight mechanics and control techniques, active control methods are developed elementarily to weaken or eliminate the dynamic instability of reentry capsules. The unsteady coupling flowfield with cross jet are solved in simplified attitude control system by the high order accurate PWNND schemes. The results are analyzed to evaluate the control effects.①The numerical results of forced oscillation flowfield of a space vehicle with cross jet show: During the forced oscillation, the normal force amplification factor changes with the pitching angle periodically. The oscillation of the space vehicle makes unsteady effects on the jet flowfield and the aerodynamic performance shows great difference with the steady jet flowfield.②The first classic active control method can break the limit cycle oscillation with the damp ratio increasing. The control moment turns weak when the angular velocity of the reentry capsule converges to zero. The reentry capsules can't keep the balance place and the error of the pitch angle and the balance place is a bit large.③The second classic active control method considers the contribution of the pitch angle. This control method can also break the limit cycle oscillation with the damp ratio increasing. With the angular frequency increasing, the reentry capsules can get the steady state in shorter time. The reentry capsules can get closer to the balance place. A control strategy with subsection angular frequency is introduced to the second classic active control method. The pitch angle gets more close to the balance place and the control effect gets better.④In the fuzzy control methods, the exact oscillation mathematic model of the reentry capsule isn't needed, which is indispensability for the above two classic active control methods. The numerical results show: The first classic fuzzy control method has a bug that the adjust dead region appears during the control process. The other four fuzzy control examples all display effective control to the dynamic instability of the reentry capsule without the exact oscillation mathematic model.