用一种新的拉格朗日方法模拟巨碰撞中的增强混合
Enhanced mixing in giant impact simulations with a new Lagrangian method
摘要：
巨型撞击（GI）在行星形成的后期阶段很常见。光滑粒子流体动力学（SPH）方法被广泛用于模拟这种剧烈碰撞的结果，一个突出的例子是月球的形成。然而，在计算天体物理学的各个领域的十年的数值研究表明，SPH的标准制定遭受几个缺点，如人工表面张力和它的倾向，迅速阻尼湍流运动的尺度远远大于物理耗散尺度，都导致混合的抑制。为了量化这些限制是多么严重的建模时，我们进行了模拟与相同的初始条件进行了比较与标准SPH以及与新的拉格朗日无网格有限质量（MFM）方法在GIZMO代码。我们确认缺乏的冲击器和目标之间的混合时，SPH是采用，而MFM是能够驱动有力的亚音速湍流，并导致显着的两个机构之间的混合。具有人工传导性的现代SPH变体，不同的水力配方或降低的人工粘度，不会显著改善混合。在这两种方法中，角动量同样守恒，但MFM不会受到人为粘性引起的虚假运输的影响，从而导致原月球盘的角动量略高。此外，SPH初始条件在核幔边界处表现出非物理的密度不连续性，这在MFM中很容易消除。
英文摘要：
Giant impacts (GIs) are common in the late stage of planet formation. The Smoothed Particle Hydrodynamics (SPH) method is widely used for simulating the outcome of such violent collisions, one prominent example being the formation of the Moon. However, a decade of numerical studies in various areas of computational astrophysics has shown that the standard formulation of SPH suffers from several shortcomings such as artificial surface tension and its tendency to promptly damp turbulent motions on scales much larger than the physical dissipation scale, both resulting in the suppression of mixing. In order to quantify how severe these limitations are when modeling GIs we carried out a comparison of simulations with identical initial conditions performed with the standard SPH as well as with the novel Lagrangian Meshless Finite Mass (MFM) method in the GIZMO code. We confirm the lack of mixing between the impactor and target when SPH is employed, while MFM is capable of driving vigorous sub-sonic turbulence and leads to significant mixing between the two bodies. Modern SPH variants with artificial conductivity, a different formulation of the hydro force or reduced artificial viscosity, do not improve mixing as significantly. Angular momentum is conserved similarly well in both methods, but MFM does not suffer from spurious transport induced by artificial viscosity, resulting in a slightly higher angular momentum of the proto-lunar disk. Furthermore, SPH initial conditions exhibit an unphysical density discontinuity at the core-mantle boundary which is easily removed in MFM.
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