Explain the significance of the following cross-correlation terms:

\[\begin{aligned}& \overline{v_{x}^{\prime} v_{y}^{\prime}} \\& \overline{v_{x}^{\prime} T^{\prime}} \\& \overline{v_{x}^{\prime} C_{\mathrm{A}}^{\prime}} \\&\overline{C_{\mathrm{A}}^{\prime} C_{\mathrm{A}}^{\prime}} \\& \overline{T^{\prime} C_{\mathrm{A}}^{\prime}}\end{aligned}\]

Explain what is meant by the closure problem in turbulence modeling.