Decaying turbulence in salt-stratified fluid with Schmidt number $700$ is investigated by direct numerical simulation. In the final period of decay, and after the Ozmidov scale becomes smaller than the Kolmogorov scale, potential-energy distribution due to salinity fluctuations shows large-scale clouds composed of structures smaller than the Kolmogorov scale. When these clouds appear, potential energy has a flat spectrum in the viscous-convective subrange, rather than a $k^{-1}$ spectrum observed initially before the stratification effect becomes significant. This transition occurs since the potential energy near the Kolmogorov scale or the primitive scale of stratified turbulence defined by $\sqrt{\unicode[STIX]{x1D708}^{\ast }/N^{\ast }}$ , where $\unicode[STIX]{x1D708}^{\ast }$ is the kinematic viscosity and $N^{\ast }$ the Brunt–Vaisala frequency, decreases significantly due to the persistent conversion of potential energy into kinetic energy by the counter-gradient density flux.

中文翻译：

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盐层流体湍流的直接数值模拟

通过直接数值模拟研究了施密特数为$700$的盐层流体中的衰减湍流。在衰减的最后阶段，Ozmidov 尺度变得小于 Kolmogorov 尺度后，由于盐度波动引起的势能分布显示出由小于 Kolmogorov 尺度的结构组成的大尺度云。当这些云出现时，势能在粘性对流子范围内具有平坦的频谱，而不是在分层效应变得显着之前最初观察到的 $k^{-1}$ 频谱。这种转变发生是因为接近 Kolmogorov 尺度或由 $\sqrt{\unicode[STIX]{x1D708}^{\ast }/N^{\ast }}$ 定义的分层湍流的原始尺度附近的势能，