If the turbulence is homogeneous, the equations for the TKE and dissipation can be reduced to The closure coefficient C_ϵ2 is already known. Therefore, C_ϵ1 can be estimated from experimental data. According to experimental data [3], a self similar state develops with k ⁄ ϵ ~ const and ϵ ⁄ P ~ 0.68. It follows that or Keeping the value C_ϵ2 = 1.77 determined in the previous section, it follows that C_ϵ1 = 1.52 to satisfy ϵ ⁄ P = 0.68. The reader probably realized that this process of determining the closure coefficients is questionable. The turbulence model will only be calibrated for some given flows. If the flow changes, so should the closure coefficients. Turbulence models are typically calibrated for some particular applications. For example, the Spalart-Allmaras model has been tuned specifically for simple aerospace flows (boundary layer growth over a wing, etc). Typically, some compromises are made to ensure that the model performs reasonably well in a wide range of flows. The standard k − ϵ model has C_ϵ1 = 1.44 and C_ϵ2 = 1.92, which suggests n ~ 1.1 instead of 1.3 for decaying isotropic turbulence, and ϵ ⁄ P ~ 0.67 instead of 0.68 for homogeneous shear flows. These coefficients could be modified to capture more accurately these two flows. However, the model accuracy would suffer for some other situations.