Xie Yufei

Physical analysis and numerical simulation for the dynamic behaviour of vehicles in pitching oscillations or rocking motions

Analytical methods of nonlinear dynamics and numerical simulations for the coupling equations of Navier-Stokes and flight mechanics are used to study the dynamic behaviour of pitching motions of reentry capsules with the variation of Mach number, and rocking motions of swept wings with the variation of angle of attack. Conditions under which the dynamic instability, Hopf bifurcation and saddle-node bifurcation occur are obtained. The node-saddle-node topological structure in the phase portrait, i.e. the state of bi-attractors (attracting basins) is described. The evolving process of dynamic behaviour and flow fields are given. The theories are compared with some numerical simulations conducted by the authors. Besides, some verifiable experimental results are cited. The agreement between them is very well.

Construction of third-order WNND scheme and its application in complex flow

According to the Lius weighted idea, a space third-order WNND (weighted non-oscillatory, containing no free parameters, and dissipative scheme) scheme was constructed based on the stencil of second-order NND (non-oscillatory, containing no free parameters, and dissipative scheme) scheme. It was applied in solving linear-wave equation, 1D Euler equations and 3D Navier-Stokes equations. The numerical results indicate that the WNND scheme which does not increase interpolated point (compared to NND scheme) has more advantages in simulating discontinues and convergence than NND scheme. Appling WNND scheme to simulating the hypersonic flow around lift-body shows: With the AoA (angle of attack) increasing from 0° to 50°, the structure of limiting streamline of leeward surface changes from unseparating, open-separating to separating, which occurs from the combined-point (which consists of saddle and node points). The separating area of upper wing surface is increasing with the AoAs increasing. The topological structures of hypersonic flowfield based on the sectional flow patterns perpendicular to the body axis agree well with Zhang Hanxins theory. Additionally, the unstable-structure phenomenon which is showed by two saddles connection along leeward symmetry line occurs at some sections when the AoA is bigger than 20°.

Analysis of kinematic characteristics of square cross section vehicle at multi-trim angle of attack

The velocity range of vehicle is quite considerable from launch phase to cruise phase, and the stability would be significantly altered with the change of Mach number, even worse, the number of trim angle of attack may be altered in some cases. Variety of kinematic characteristics is studied using the numerical simulation method on a square cross section vehicle which was changed from stable to instable, from one trim angle of attack to three trim angles of attack with the increase of Mach number. The simulation results show the existence of the critical Mach number Mcr in the aspect of stability. The vehicle is stable nearby the unique equilibrium point where Ma is smaller than Mcr ; otherwise, the number of equilibrium points would increase to three, the pre-equilibrium point is instable and the two new equilibrium points are stable. The saddle-node bifurcation phenomenon will take place in the pitching motion of the vehicle in case of having three equilibrium points, and the final convergent position is extremely sensitive to the variety of initial condition. The pitching motion could be convergent to one trim angle of attack under certain conditions, but the convergence may alter to the other trim angle of attack with little change in the initial conditions.