Temporal convergence criteria for time-accurate viscous simulations of separated flows

Airfoils and wings undergoing static and dynamic stall still elude accurate simulation by computational methods. While significant emphasis has been placed on the quantification of grid dependence, as well as influence of the turbulence method, many elements defining temporal convergence remain ad hoc. To address this, convergence and accuracy for two different turbulence methods were examined for both static and dynamic stall. New approaches to define numerical convergence that include an assessment of the physical accuracy have been developed and evaluated via a blind analysis at other stall conditions. A key finding is the need to ensure that the combination of time step and subiterations achieves a true second order accurate solution. It was also observed that accurate prediction of separation was controlled primarily by the turbulent transport terms, while the mean flow equations influenced reattachment. Temporal convergence of dynamic stall can be quantitatively assessed by an approach developed in this effort.

► True 2nd-order temporal accuracy must be applied to capture separated flow physics.
► The number of subiterations can be estimated to achieve convergence.
► Location of separation is controlled by convergence of turbulent subiterations.
► Location of reattachment depends on the convergence of the mean flow subiterations.
► A new method is demonstrated to predict convergence of dynamic stall.