Characterization and understanding of unsteady airfoil behavior using modal analysis*

Characterizing unsteady airfoil behavior including dynamic stall in a methodical way has been a challenge due to lack of an objective means of classifying these complex flows.  Using modal analysis techniques with data from experiments and computations promises to make the process more objective.  Modal techniques have recently been applied to experimental data taken for a range of airfoils under different conditions to investigate their potential.  Modal analysis has been applied to both surface pressure data as well as flow-field data in an attempt to develop characterization approaches.  Analysis of phase-averaged data has led to classification strategies for different dynamic stall types based on the coupling of the shape of spatial modes with the time dependence of the coefficients multiplying the modes.  Similarly, modal analysis of the unsteady data has led to a means of identifying and understanding cycle-to-cycle variations in these flows.  In addition to developing better classification schemes, these modal analysis methods promise to aid in better understanding of these complex flows.