自然超空泡与尾喷流相互作用的数值模拟

认识带尾喷流和自然超空泡的水下高速航行体流体动力特性并发展其预报与控制方法仍是水动力学领域极具挑战性的课题．本研究采用CFD方法对尾喷流和自然超空泡之间的相互作用进行了数值研究．针对发动机欠膨胀超音速喷流，采用现有实验结果验证了基于两方程湍流模型的二维轴对称流动数值模型的可靠性．尾喷流在空气和蒸汽环境中流动的数值计算结果表明，由于蒸汽环境中背压较低，欠膨胀尾喷流无法及时形成压缩波，使得蒸汽环境中尾喷流的过膨胀区和气相扩散区的体积比空气中大；尾喷流很难形成规则的激波格栅，波系结构相对简单．针对携尾喷流的平头航行体超空泡流状态的数值模拟结果表明，尾喷流注入超空泡后迅速充满航行体周围的空腔区域；尾喷流与超空泡尾迹区域形成的回射流相互作用最终导致超空泡断裂，断裂过程中伴随着燃气泡的下泄现象；受空泡壁面约束，尾喷流难以在狭窄的超空泡空腔内完全膨胀，尾喷流的激波波系结构有显著的变化：在喷嘴附近受到压缩，在远离喷嘴区域受到超空泡水汽掺混的破坏；空泡内压强基本维持在饱和蒸汽压附近，没有显著增加．

Numerical Simulation of Interaction Between Natural Supercavitation and Base Jet

It is a critical issue to understand the effects of interaction of supercavitation with the base jet on the motion of projectile and to develop the prediction and control method. In this paper, the interaction between base jet flow and supercavitating flow is studied numerically. Compared with the experimental results, the two-equation turbulence model of axisymmetric flows has good feasibility in the simulation of underexpanded supersonic jet. The projectile used here moves horizontally with a natural supercavitation. The same gas jet in air and vapor, respectively, is also studied for comparison. The numerical results show that the volume of over-expanded zone and gas phase diffusion zone in vapor environment are larger than in air, because the vapor pressure is too low to form a compression wave in time. As a result, the jet in vapor can hardly form a regular shock diamond and the shock wave structure is relatively simple. Furthermore, the initial velocity and the cooling process of the jet in vapor are faster than in air but the energy dissipation and deceleration process are slower. The evolution of the cavity surface is also presented for the case of a supercavitating flow around a cylinder with a gas jet. Gas fills the supercavitation rapidly after injection. A water jet pointing to the tail of the projectile emerges at the wake region of the supercavitation. The interaction of the two jets eventually leads to the collapse of the supercavitation. The base jet cannot expand completely in the narrow space of supercavitation. Compared to the vapor environment, the shock wave structure of the base jet in the supercavitation changes significantly, that is to say, the structure is compressed in the vicinity of the nozzle, and disappears at the region far away from the nozzle. In addition, the pressure in the supercavitation is not significantly increased.