电磁助推翼段加速地面效应及稳定性分析
Ground Effects and Stability Analysis of Airfoil Accelerated by Electromagnetic Propulsion
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摘要: 电磁发射空天飞行器是未来可重复使用天地往返运输系统的重要发展方向之一, 近地助推加速过程受电磁悬浮力影响, 面临着复杂的地面效应与弹性稳定性问题。以NACA0012二维翼段为研究对象, 建立了电磁力与气动力耦合的动力学模型, 对翼段近地Ma=0~1.5加速过程中的流场特征、运行姿态和气动特性进行了数值模拟。结果表明, 翼段加速诱导地面效应可分为4阶段。第1阶段, 上下翼面为亚声速流动, 翼段姿态和气动载荷基本无振荡。第2阶段, 上翼面开始出现跨声速流动, 下翼面流动以典型的变截面跨声速流动为主导, 并伴随壅塞-通流模态转换。第3阶段, 上翼面保持跨声速流动, 下翼面流动壅塞再现, 并呈现出完全膨胀的跨声速壅塞流动状态。在第2、3阶段, 翼段姿态和气动载荷低频大幅振荡。第4阶段, 上翼面发展为超声速流动, 下翼面保持完全膨胀壅塞流动, 翼段姿态和气动载荷高频小幅振荡。在此基础上, 探究了悬浮高度、悬浮刚度、磁体间距对系统稳定性的影响。发现增加悬浮高度, 有利于在一定程度上提高系统稳定性; 适当增加悬浮刚度或悬浮磁体间距, 同时限定电磁助推目标速度小于系统振荡发散临界Mach数, 有利于明显提高系统稳定性。Abstract: Aerospace vehicle launched by electromagnetic propulsion is a potential option for future reusable space transportation systems. Complex ground effects and stability issues are induced generally due to the introduction of electromagnetic levitation force. A dynamic model coupled with electromagnetic force and aerodynamic force was established for the two-dimensional wing (NACA0012). Numerical simulations were conducted on the flow characteristics, operational attitude, and aerodynamic characteristics of the wing during the Ma=0~1.5 acceleration process. It indicates that ground effects can be divided into four stages. In the first stage, subsonic flows are presented on both the upper and lower wing surfaces, and there is basically no oscillation for the attitude and aerodynamic loads of the wing. In the second stage, transonic flow emerges on the upper wing surface, while the flow on the lower wing surface is dominated by a typical variable cross-section transonic flow accompanied by the transition from the choked flow mode to the unchoked one. In the third stage, the upper wing surface maintains transonic flows, while the lower wing surface undergoes choked flows which are fully expanded. In both the second and third stages, the wing attitude and aerodynamic loads oscillate significantly at low frequencies. In the fourth stage, the upper wing surface undergoes supersonic flows, while the lower wing surface maintains choked flows. The wing attitude and aerodynamic loads oscillate slightly at high frequencies. On this basis, the effects of suspension height, suspension stiffness and spacing between suspension magnets on the system stability were explored. It is found that increa-sing the suspension height is beneficial for improving system stability. Increasing the suspension stiffness or spacing between suspension magnets appropriately, while limiting the target speed of electromagnetic propulsion to be less than the critical Mach number of system oscillation divergence, is beneficial for significantly improving system stability.