In Chap. 5 we introduced the RANS equation set to attain the ability to seek solutions of the set of primary variables of engineering interest: \(\left\langle \underline{V_i}\right\rangle \) and \(\left\langle \underline{p}\right\rangle \) . If we could attain this ability, this would have paved the way for finding the expected values of the velocity and the pressure fields without relying on any type of averaging. However, the RANS equation set ( 5.10 ) turned out to be mathematically unclosed due to the appearance of the Reynolds stress tensor in the mean momentum equation. Many turbulence closure models have been proposed to achieve this mathematical closure. However, these closure models are approximate by their very nature, and they invariably add uncertainty to the solution of the mean velocity and the mean pressure fields that emerge using these closure models.