  The dynamic model is one of the most successful inventions in subgrid-scale
(SGS) modeling as it alleviates many drawbacks of the static coefficient SGS
stress models. The model coefficient is often calculated dynamically through
the minimization of the Germano-identity error (GIE). However, the driving
mechanism behind the dynamic model's success is still not well understood. In
wall-bounded flows, we postulate that the principal directions of the resolved
rate-of-strain tensor play an important role in the dynamic models.
Specifically, we find that minimization of the GIE along only the three
principal directions (or less), in lieu of its nine components in its original
formulation, produces equally comparable results as the original model when
examined in canonical turbulent channel flows, a three-dimensional turbulent
boundary layer, and a separating flow over periodic hills. This suggests that
not all components of the Germano identity are equally important for the
success of the dynamic model, and that there might be dynamically more
important directions for modeling the subgrid dynamics.
