Numerical investigation of aeroacoustic sound generation by wave packets in turbulent jets

• Applicant: Dr.-Ing. Oliver Schmidt
• Subject Area: Fluid Mechanics
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 261668027

The reduction of jet-noise is one of the key challenges of modern aviation. During the takeoff phase of commercial jet aircraft, the turbulent free jet generated by the jet engines is the dominant source of noise. There exists experimental as well as numerical evidence that the relevant acoustic emissions stem from long-scale coherent structures in the acoustic near-field. The physical mechanisms that lead to the generation of such structures or wave packets are not well understood up to now. However, an in-depth understanding of the underlying physical processes is required for efficient development of noise prevention strategies.This is the starting point of the proposed research project. The origin and the spatio-temporal evolution of long-scale wave packets is to be analyzed in a technically relevant Mach and Reynolds number regime. For this purpose, a turbulent free jet is computed by means of large eddy simulation. The necessary turbulent inflow boundary condition is based on the superposition of solutions to the local linear stability problem. Coherent structures are extracted from the data via spectral and orthogonal decomposition methods. The flow parameters and the geometry of the numerical setup are taken from a published experimental study, and are provided by the host institution. The experimental data also serves for the validation of the simulation. The wave packets to be investigated are an inherent feature of the turbulent free jet. In order to understand the generation process despite that fact, two novel approaches are introduced to extract natural and artificially triggered wave structures.In both cases, natural and artificially forced, coherent structures are extracted from the difference between a reference and the perturbed calculation, and are tested for relevance for sound generation. The aim of the study is to establish a physical understanding of the underlying aeroacoustic sound generation processes that can be utilized to reduce turbulent jet noise emissions, i.e. for the efficient realization of active or passive means of control.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Tim Colonius, Ph.D.