Hello,
Turbulent kinetic energy is defined as ­Øæÿ=0.5((­ØæóÔÇ▓ ­ØæóÔÇ▓ ) +(­ØæúÔÇ▓ ­Øæú' ) + (­ØæñÔÇ▓ ­ØæñÔÇ▓ ) ). The terms inside the brackets are referred to as the normal stresses. Based on the Boussinesq approximation, Reynolds stress can be written as a combination of the Strain rate tensor (made up of the mean flow velocity gradients) and the turbulent kinetic energy. Because the velocity fluctuations (uÔÇÖ,vÔÇÖ,wÔÇÖ) and the normal stresses are generally very small compared to the mean flow velocity gradients, the turbulent kinetic energy part of the Reynolds stress formulation can be ignored.
Hope this helps.
Thanks Kalyan

Hello,
Fluctuations are always calculated. Each solver may have its own way of calculating time varying fluctuations, such as tracking a running average of a quantity and subtracting it from the instantaneous value.
The user doesn't have to do anything. Everything is taken care off by the solver. For e.g.,in Fluent, when using the Spalart-Allmaras Turbulence Model, the Reynolds stresses are calculated without using the TKE term. However, for all the other turbulence models where an evolution equation for TKE is used (like k-e, k-w etc.), the effect of TKE is included in calculating the Reynolds stresses.
Kalyan