Comparison of Eigenmode Extraction Techniques for Separated Nozzle Flows

Paper in proceeding, 2014

Results of a previously published Arnoldi eigenmode analysis on a separated flow inside a convergent-divergent nozzle are compared with results obtained using a Dynamic Mode Decomposition (DMD) algorithm. The Arnoldi analysis employs a linearized flow solver and as a result, does not consider nonlinearity and turbulence. The DMD method is a snapshot- based approach, which approximates the Koopman modes of the nonlinear flow. In the present study the DMD algorithm has been applied to a data set from a two-dimensional URANS simulation of the separated nozzle flow. As such, it can take into account the full information of the nonlinear flow, including turbulence. The objective of this study is to investigate the effects of turbulence on the linear analysis. The results show that the Arnoldi and the DMD algorithms do in certain cases produce almost identical modes in terms of frequency, damping and structure. This indicates that even though the Arnoldi method needs an explicit linearization of the flow dynamics and excludes turbulence, it does reveal modes with discrete frequency that could be excited in the nonlinear flow with modeled turbulence.