Meaning
Turbulence models in computational fluid dynamics assume that turbulent stresses align directly with the mean strain rate of the flow. When this assumption fails in highly separating or swirling flows, eddy viscosity breakdown occurs and leads to inaccurate predictions of shear stress.
Turbulence Modelling
Standard linear models fail to resolve complex vortex structures because they oversimplify the transport of turbulent energy. Advanced closures are required to account for the anisotropy of the flow field. These higher-order models preserve physical accuracy by decoupling the turbulent viscosity from local strain rates.
Aerodynamic Consequence
Incorrect lift and drag predictions arise from the unphysical calculation of wake regions and separation points. The aerodynamic consequence of the failure is a significant underestimation of flow detachment, which can result in delayed stalls during simulation runs. Designers risk using optimistic performance figures when they rely on models that cannot capture this phenomenon.
In turbomachinery simulation, this failure hides flow blockages that would otherwise trigger premature blade stalling in physical prototypes.
Velocity Gradient
Extreme strain rates in the boundary layer cause the linear approximation of the stress tensor to diverge from physical reality. The velocity gradient determines the limit at which the linear model must be replaced by a non-linear variant. Beyond this boundary, the model produces unphysical negative turbulent kinetic energy.
