Inspecting sound sources in an orifice-jet flow using Lagrangian coherent structures

A novel method is proposed to identify flow structures responsible for sound generation in confined flow past an inhibitor. Velocity fields obtained using Large Eddy Simulations (LES) are post-processed to compute the Finite Time Lyapunov Exponent (FTLE) field, the ridges of which in backward time represent an approximation to Lagrangian Coherent Structures (LCS), the structures that organize transport in the flow field. The flow-field is first decomposed using dynamic mode decomposition (DMD), and the organizing centers or vortices at the significant DMD frequencies are extracted. The results are then compared with the λ2 criterion. Features such as shear layer roll-up and development of secondary instabilities are more clearly visible in the FTLE field than with the λ2 criterion.