Eigenmode decomposition of turbulent velocity fields behind a swept, backward-facing step

The transitional flow over a swept, backward-facing step is analysed using empirical eigenmode decomposition in order to identify dominant flow structures. Databases of three-dimensional velocity fields were generated by direct numerical simulation for sweep angles α between 0° and 60°. The Reynolds number, based on the step height and the free stream component normal to the step, is kept constant at Re = U0H/ν = 3200. The eigenvalue spectra of the decompositions exhibit a similar decay for all sweep angles. The energy distribution of the dominant eigenmodes in the step-normal direction reveals that each mode contributes energy only to a bounded region. The most energetic eigenmode is found to resolve energy in the region of the unstable free shear layer emanating from the step edge. The peak value of the kinetic energy captured locally by this mode is at least 10%, depending on the sweep angle. It represents roller vortices evolving from instability waves. This interpretation is corroborated by flow visualizations and a comparison with predictions from linear stability theory. The axes of the roller vortices are approximately normal to the direction of the incoming flow. This is consistent with the orientation of the mean vorticity of the skewed shear layer, which is dominantly spanwise with respect to the direction of the incoming flow. Besides the change in orientation relative to the step, no qualitative change in the dominant shear layer structures due to the introduction of sweep was found.     «

The transitional flow over a swept, backward-facing step is analysed using empirical eigenmode decomposition in order to identify dominant flow structures. Databases of three-dimensional velocity fields were generated by direct numerical simulation for sweep angles α between 0° and 60°. The Reynolds number, based on the step height and the free stream component normal to the step, is kept constant at Re = U0H/ν = 3200. The eigenvalue spectra of the decompositions exhibit a similar decay for all...     »