We present results from Direct Numerical Simulations (DNS) of Rayleigh–Taylor instability at Atwood numbers up to 0.9. After the layer width had developed substantially, additional branched simulations have been run under reversed and zero gravity conditions. We focus on the modifications of the mixing layer structure and turbulence in response to the acceleration change. After the gravity reversal, the flow undergoes a complex transient process in which the vertical mass flux changes sign multiple times and, consequently, the buoyancy term in the turbulent kinetic energy transport equation changes its role back and forth from production to destruction. This behavior is examined in detail using the turbulent kinetic energy and mass flux transport equations and time instances when the vertical mass at the centerline crosses zero and reaches local minima and maxima. While the transient process significantly affects the flow anisotropy at all scales, other turbulence characteristics, like the alignment between the vorticity and eigenvectors of the strain rate tensor, retain their fully developed turbulence behavior in the interior of the layer. In addition, after the gravity reversal, the edges of the layer also exhibit characteristics closer to those of the turbulent interior, even as the fluids become more mixed. None of these changes affects the mean density profile, which still collapses among various cases. Such significant changes in some turbulence quantities and not others are difficult to capture with existing turbulence models.

我们介绍了从瑞利-泰勒不稳定性的直接数值模拟（DNS）得出的结果，阿特伍德数高达0.9。层宽基本发展之后，在反向重力和零重力条件下进行了其他分支模拟。我们专注于响应加速度变化对混合层结构和湍流的修改。在重力逆转之后，流体经历了一个复杂的瞬变过程，在该过程中，垂直质量通量发生了多次符号变化，因此，湍流动能传输方程中的浮力项从生产到破坏来回变化。当中心线处的垂直质量越过零并达到局部最小值和最大值时，将使用湍动能和质量通量传输方程式以及时间实例详细研究这种行为。尽管瞬变过程会在所有尺度上显着影响流动各向异性，但其他湍流特性（如应变率张量的涡度和特征向量之间的对齐）会在层内部保留其充分发展的湍流特性。另外，在重力反转之后，即使流体变得更加混合，该层的边缘也表现出更接近湍流内部的特征。这些变化都不会影响平均密度曲线，该平均密度曲线在各种情况下仍然崩溃。