Author:

Keywords:

Precessing Instabilities, Spectral Proper Orthogonal Decomposition, Stereoscopic Particle Image Velocimetry, Sudden Expansion Flows, C3/19/015#55232502

Abstract:

Stereoscopic Particle Image Velocimetry (SPIV) measurements are performed to analyse the cold flow field in a sudden expansion dump combustor geometry, with an expansion ratio of 4.9. The Reynolds number is varied from 5200 till 63300. The time-averaged flow field is found to be axisymmetric and has a jet-like velocity profile near the nozzle with a spreading rate comparable to free round jets. Despite the axisymmetric nature of the mean flow field, it contains unsteady coherent structures, which are analysed using the Spectral Proper Orthogonal Decomposition (SPOD) method. It is found that the instantaneous flow field is asymmetric and it precesses around the central axis of the geometry. The structure of this precession is similar for all Reynolds numbers investigated and the Strouhal number increases with increasing Reynolds number till a value of 26000, after which it reaches a constant value of around 3×10⁻³. These precessing structures were previously only reported for swirling sudden expansion flows, where they are extensively studied, but this study shows they also exist in the non-swirling flow case.