Stability analysis of time-averaged jet flows: fundamentals and application

We report on experimental and theoretical investigations of shear flow instabilities in jet flows. Linear stability analysis is applied to the time-averaged flow taken from experiments, contrasting the ’classic’ stability approach that is based on a stationary base flow. To some extend, mean flow stability eigenmodes may deal as a model for instability waves at their nonlinearly saturated state, which is typically encountered in experiments. The capability of mean flow stability models is first demonstrated on laminar oscillating jets where the primary interaction takes place between the mean flow and the instability wave. We then focus on turbulent swirling jets where additional interactions occur between the fine-scale turbulence and the instability waves. Swirling flows are widely used in combustion applications where the associated high turbulence levels and internal recirculation zones (vortex breakdown bubble) are exploited for flame stabilization. We demonstrate the application of mean flow stability analysis on the flow field of a industryrelevant swirl-stabilized flame. We show that the flame response to acoutstic perturbations is closely linked to the flow receptivity predicted from linear stability analysis, which suggests that the adopted theoretical framework is very useful for thermoacoustic modeling.