  Classical eddy viscosity models add a viscosity term with turbulent viscosity
coefficient whose specification varies from model to model. Turbulent viscosity
coefficient approximations of unknown accuracy are typically constructed by
solving associated systems of nonlinear evolution equations or by data driven
approaches such as deep neural networks. Often eddy viscosity models
over-diffuse, so additional fixes are added. This process increases model
complexity and decreases model comprehensibility, leading to the following two
questions: Is an eddy viscosity model needed? Does the eddy viscosity model
fail? This report derives a posteriori computable conditions that answer these
two questions.
