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arXiv 1505.04423physics.flu-dyn
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可压缩湍流混合:施密特数的影响

Compressible turbulent mixing: Effects of Schmidt number

Qionglin Ni

摘要:

研究了Schmidt数对受迫可压缩湍流中被动标量输运的影响。在惯性对流范围内,标量谱遵循k^{-5/3}幂律。对于Sc >> 1,在粘性对流范围内出现k^{-1}幂律,而对于Sc << 1,在惯性扩散范围内发现k^{-17/3}幂律。速度-标量增量混合三阶结构函数的标度常数在Sc上增大,压缩性的影响使其小于经典的4/3值。在小振幅下,标量波动的PDF崩溃高斯分布,而在大振幅下,它衰减得比高斯更快。在大尺度下,标量增量的概率密度函数表现为标量涨落的概率密度函数,而在小尺度下,则表现为标量梯度的概率密度函数。由于分子扩散系数较低,当Sc >> 1时,标量场卷起并充分混合。然而,当Sc << 1时,标量场由于高分子扩散率而失去了小尺度结构,只保留了大尺度的云状结构。在Sc >> 1和Sc << 1两种流动中,标量平流和耗散的谱密度都遵循k^{-5/3}标度。这表明,在可压缩湍流中,除标量-膨胀耦合外,对流和耗散过程都服从Kolmogorov图象。结果表明,在高波数时,Sc >> 1和Sc << 1流的谱相干性的幅度衰减得比不可压缩流的理论预测值k^{-2/3}快。最后,与不可压湍流结果的比较表明,Sc=1的可压缩湍流中标量在其谱中没有明显的凸点结构,但在耗散范围内更加断续。

英文摘要:

We investigated the effects of Schmidt number on passive scalar transport in forced compressible turbulence. In the inertial-convective range the scalar spectrum followed the k^{-5/3} power law. For Sc >> 1, there appeared a k^{-1} power law in the viscous-convective range, while for Sc << 1, a k^{-17/3} power law was identified in the inertial-diffusive range. The scaling constant for the mixed third-order structure function of velocity-scalar increment grew over Sc, and the effect of compressibility made it smaller than the classical 4/3 value. At small amplitudes, the PDF of scalar fluctuations collapsed to the Gaussian distribution, whereas at large amplitudes it decayed more quickly than Gaussian. At large scales, the PDF of scalar increment behaved similarly to that of scalar fluctuation, while at small scales it resembled the PDF of scalar gradient. The scalar dissipation occurring at large magnitudes was found to grow with Sc. Due to low molecular diffusivity, for Sc >> 1, the scalar field rolled up and got mixed sufficiently. However, for Sc << 1, the scalar field lost the small-scale structures by high molecular diffusivity, and retained only the large-scale, cloudlike structures. The spectral densities of scalar advection and dissipation in both Sc >> 1 and Sc << 1 flows followed the k^{-5/3} scaling. This indicated that in compressible turbulence the processes of advection and dissipation except that of scalar-dilatation coupling might defer to the Kolmogorov picture. It then showed that at high wavenumbers, the magnitudes of spectral coherency in both Sc >> 1 and Sc << 1 flows decayed faster than the theoretical prediction of k^{-2/3} for incompressible flows. Finally, the comparison with incompressible results showed that the scalar in compressible turbulence with Sc=1 lacked a conspicuous bump structure in its spectrum, but was more intermittent in the dissipative range.

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