底凹结构减阻效应数值分析

为研究弹丸底凹结构的减阻机理，使用三维定常CFD方法对M910弹丸的流场特性进行了数值模拟。给出了零升阻力系数随马赫数的变化规律，所得结果与实验数据符合很好。在此基础上，为M910弹丸引入底凹结构并进行数值模拟。对比了不同弹底结构的底部流场特性，对底凹结构减阻效应的产生机理进行了分析。结果表明:亚声速下，底凹结构在底凹腔体内引入了高压“死水区”，并以“屈从”的流体边界代替了原固体底面，从而改变了尾部涡街的形成位置、形状和强度，最终增大底部压力，减小弹丸阻力；跨声速下，由于尾部涡街远离弹丸底面，固体底面与流体边界面的作用相同，使得底凹不再具有减阻效果；超声速下，底凹结构的减阻机理与底排弹丸减阻机理类似，即底凹结构中的流体为弹丸底部回流区添加质量从而达到减阻作用。 

Numerical analysis on drag reduction effect of base cavity

In order to investigate the drag reduction mechanism of the base-cavity projectile, the flow field characteristics of M910 projectile are numerically simulated through the 3-dimensional steady CFD method. The zero-lift drag coefficient variation with Mach number are presented. The computational results have a good agreement with the experimental data. On this basis, a base cavity is introduced for M910 projectile that is named M910BC in this paper and then numerically simulated. The base flow field characteristics of the projectile with different base structures are compared and the drag reduction mechanism of the base cavity is analyzed. The results show that at subsonic speed, the drag reduction of the base-cavity projectile is found to be mainly due to the introduction of the high-pressure "dead zone" in base cavity and the displacement of the solid base with the compliant fluid boundary of the cavity base. Because of that, the forming location, shape and strength of the wake vortex are slightly changed. At transonic speed, the drag reduction effect of base cavity is vanished since the wake vortex is further from the base of projectile and the effects of the solid base and fluid boundary are the same. At supersonic speed, the drag reduction mechanism of the base cavity is that the mass of the recirculation region is increased by the flow of the base cavity, which is similar to the drag reduction mechanism of the base bleed projectile.