论文标题

旋转各向异性湍流中的平均流量产生:太阳近表面剪切层的情况

Generation of mean flows in rotating anisotropic turbulence: The case of solar near-surface shear layer

论文作者

Barekat, A., Käpylä, M. J., Käpylä, P. J., Gilson, E. P., Ji, H.

论文摘要

太阳的近表面剪切层(NSSL)中旋转速率的径向梯度独立于纬度和半径。理论平均模型已成功地解释了太阳能NSSL的这一属性,而全球直接对流模型则未成功。我们通过测量NSSL条件下的平均流量,雷诺应力和湍流转运系数来研究这种差异的原因。模拟的成分最少。这些成分是由于边界,各向异性湍流和旋转而导致的不均匀性。选择模拟的参数,以使其与弱旋转约束的NSSL匹配。模拟在给定的深度和纬度上探测了恒星的当地笛卡尔斑块。斑块的深度通过改变旋转速率而变化,使得生成的科里奥利数<1。我们测量与雷诺强调的非扩散和扩散部分相关的湍流传输系数,并将其与当前均值场理论的预测进行比较。平均流量的负径向梯度类似于太阳NSSL,仅在不存在子午流的赤道上产生。在其他纬度上,子午流与对应于差分旋转的平均流量相当。我们还发现,雷诺压力的子午成分不能忽略。此外,我们发现湍流粘度通过从表面到NSSL的底部旋转约50 \%淬灭。我们的本地模拟未验证从平均场理论中产生NSSL的解释,在该理论中,子午应力被忽略了。但是,我们的模拟中湍流粘度的旋转依赖性与理论预测非常吻合。我们的结果与全球对流模拟的定性协议达到了定性协议,因为NSSL在赤道附近获得。

The radial gradient of the rotation rate in the near-surface shear layer (NSSL) of the Sun is independent of latitude and radius. Theoretical mean-field models have been successful in explaining this property of the solar NSSL, while global direct convection models have been unsuccessful. We investigate reason for this discrepancy by measuring the mean flows, Reynolds stress, and turbulent transport coefficients under NSSL conditions. Simulations have minimal ingredients. These ingredients are inhomogeneity due to boundaries, anisotropic turbulence, and rotation. Parameters of the simulations are chosen such they match the weakly rotationally constrained NSSL. The simulations probe locally Cartesian patches of the star at a given depth and latitude. The depth of the patch is varied by changing the rotation rate such that the resulting Coriolis numbers<1. We measure the turbulent transport coefficient relevant for the non-diffusive and diffusive parts of the Reynolds stress and compare them with predictions of current mean-field theories. A negative radial gradient of mean flow similar to the solar NSSL is generated only at the equator where meridional flows are absent. At other latitudes the meridional flow is comparable to the mean flow corresponding to differential rotation. We also find that meridional components of the Reynolds stress cannot be ignored. Additionally, we find that the turbulent viscosity is quenched by rotation by about 50\% from the surface to the bottom of the NSSL. Our local simulations do not validate the explanation for the generation of the NSSL from mean-field theory where meridional stresses are neglected. However, the rotational dependence of turbulent viscosity in our simulations is in good agreement with theoretical prediction. Our results are in qualitative agreement with global convection simulations in that a NSSL obtained near the equator.

扫码加入交流群

加入微信交流群

微信交流群二维码

扫码加入学术交流群,获取更多资源