Subgrid flux model for scalars

Currently, for Large Eddy Simulations, subgrid flux of scalars are obtained using a constant subgrid Prandtl number (default value is 1). It would be interesting to have more options as scalar fluctuations are not always similar to velocity fluctuations.

(transferred from Mantis bug tracker issue 215, opened by Mathtrack).

该 issue 涉及 Large Eddy Simulations 中的标量亚网格通量，但没有指出源文件、测试或入口点。首先定位现有的 constant subgrid-Prandtl 实现及相关的验证案例；完成应意味着支持额外的标量通量选项，并通过适当的模拟结果进行验证。

由索引模型根据 Issue 内容生成。