Research On Three-dimensional Compressible Unsteady Flow Separation And Nonlinear Dynamic Stability For The Coupling Between The Aircraft’s Motion And Fluid Flow

Key technologies of near space hypersonic glide maneuver vehicle are currently the important issues of vehicle. Its maneuver flights are characterized by three main aspects. Firstly, maneuverable motions of vehicle, such as the rolling, sideslip and pitching, cause the unsteady flow around vehicle. Therefore, vehicle’s movement and fluid flow are strongly coupled. Aerodynamic force, aerodynamic heating, fluid calculations and experiments of the kinds of nonlinear problems are involved in dynamic and coupling. Secondly, unsteady separation appears at leeside and local turning surfaces when the vehicle flying with angle of attack. What’s the definition of unsteady flow separation? What’s the difference of separated features between steady and unsteady flows? Unsteady flow separation will bring what kind of influence to aerodynamic force, aerodynamic heating and vehicle’s movement? These are new issues to be studied. Thirdly, the requirements of movement of the vehicle flight arc stable and controllable, these result in the need to solve problems of nonlinear coupling of stability and control. Consequently, new computational fluid dynamics(CFD) methods, new experimental methods and measurement techniques are essential to develop the maneuvers flight, meanwhile, two important theoretical issues needed to be addressed. The first one is how to define unsteady flow separation? The other is how to establish the method for analyzing the nonlinear dynamic stability of vehicle? In this paper, these two theoretical issues are studied.The main contents of unsteady separation studies are as follow:when and where flow separation and attachment occur, how to describe the separation line and attachment line, and what’s the growth of the development of spatial flow state? Unsteady flow separation can be divided into two categories. The one is unsteady flow separation on the object with uniform motion. The other is on the object with non-uniform motion. For moving objects in the inertial coordinate system, unsteady flow separation occurs outside the object surface, which produces the difficulty to analyze flow separation phenomena. By means of the coordinate system fixed at the object, the former category can be converted to fixed wall flow separation, the government equation of flow is still NS equations. For the second category, although the velocity of object surface is zero in the fixed coordinate system, i.e., the surface does not move, but the flow must be described by NS equations with additional inertia term. To our knowledge, until now, the above problems have not been analyzed thoroughly.Research on dynamic stability of hypersonic vehicle carried out very late. There is extensive literature on the dynamic stability of airplane and some of their findings are useful to understand the stability problems of hypersonic vehicle, but there are significant distinctions between them. Firstly, the moment of inertia, which can be ignored at low altitudes, must be taken into account for hypersonic vehicle flying at high altitudes, where atmospheric density is low. Secondly, in the area of the dynamic stability of vehicle, dynamic torque can be expressed by dynamic derivative, which could be formulated through a large number of ground experiments, flight experiments and theoretical analyses. But it’s difficult to obtain dynamic derivative values through these methods for hypersonic vehicle. In addition, the linear theory is often adopted for coupling stability analysis of airplane, which is a decoupling method. However, it’s hard to decouple the flight mechanics equations of hypersonic problem due to inertia coupling. Although faced with these difficulties, dynamic factors and dynamic derivative of hypersonic vehicle can be obtained by numerical simulations with rapidly developed computer technologies and CFD methods. This provides the possibility for coupling analysis of dynamic stability for hypersonic vehicle.The method of combining qualitative analyses with numerical simulations is used to analyze the two non-linear problems of unsteady flow separation and dynamic stability to near space hypersonic glide maneuver vehicle. The following researches were performed:1. The theoretical framework for analysis of fixed wall unsteady flow separation established by Academician ZHANG HanXin was implemented and improved. The form of the limit line around the separation line, the distribution pattern of cross section flow, the rule of vortex section flow pattern and its evolution along the axial direction were analyzed.2. For moving wall cases, the unsteady flow separation was studied in noninertial frame, which is fixed on the object. Separation conditions for moving wall were obtained. The regular pattern of pressure and flux variation with time at attachment line was analyzed.3. The separation conditions and theories of fixed/moving wall were verified by several numerical simulations.4. Through the three-axis angular motion analysis, the approximate three dimensional flight mechanics equations for pitching, yawing and rolling movements were established, which contained moment of inertia and aerodynamic torque obtained by numerical calculations. Using static and dynamic derivatives obtained by the numerical method and the nonlinear theory, the stability analysis of coupled rolling, pitching and sideslip were conducted, which can be divided into two cases, the regular rolling and the deflected rolling. The conclusion is as follow:For plane-symmetric vehicles, when the rolling moment is zero at the symmetric plane(γ=0), if the static derivatives (mzα、mxγ、myβ)and dynamic derivatives (mxωx、 myωy、mzωz) of pitching, rolling and sideslip motions are less than zero, and if the temporal derivatives of rolling (γ) and pitching (θ) angle satisfies the following relations, The rolling, pitching and sideslip motions are dynamically stable. Where Ix, Iy and Iz denote the momentum of inertia around x, y and z axes respectively. ωx is angular velocities of X-axis. If these conditions are not met, the movement of the vehicle will be unstable or appear divergence rolling.For plane-symmetric vehicles, if the rolling moment(mx)0 is not zero at the symmetric plane, the stationary state of the vehicle is now a deflected one, with a deflected rolling angle: If static derivatives and damping coefficient are less than zero, and the above stability criterion is satisfied at the position γi, the movement of the vehicle is stable. Otherwise, the movement is dynamic unstable divergence. Where subscript "0" indicates the symmetrical plane, and αx=(Iy-Iz)/Ix.5. Numerical simulation, dynamic wind tunnel experimental results show a good agreement with the stability criterion presented.