Predictions and measurements of coherent structures in a transversely-forced multi-nozzle swirl combustor
This work compares predictions of hydrodynamic receptivity from linear stability analysis with experimental measurements from an array of three premixed swirling flames with imposed 390 Hz transverse acoustics. The stability analysis is performed about the Favre-averaged experimental flow field obtained from simultaneous time-resolved stereoscopic particle image velocimetry and OH Planar Laser-Induced Fluorescence measurements of an unforced single-nozzle test case. The global hydrodynamic modes are constructed using weakly-global theory based on independent parallel-flow solutions of the linearized compressible Navier-Stokes equations in the low Mach number limit. These local solutions are obtained using a Chebyshev spectral collocation method and combined into a global mode a posteriori using a WKBJ approach. At 390 Hz, the stability analysis predicts the highest receptivity to singly-periodic helical disturbances which wind in the direction of swirl and peak in amplitude along the inner shear layer. Axisymmetric and other helical disturbances show similar spatial structure but are predicted to be less receptive to forcing at this frequency. These predictions are in excellent agreement with the observed fluctuating field from the forced test cases, which suggests the validity of this reduced-order approach. This work also points to the relevance of the shape and not just the frequency of imposed acoustics on the resultant hydrodynamic response.
