Comparison of compressible and incompressible CFD methods for the acoustic analysis of flow induced noise in confined flows

Obtrusions such as bends, constrictions and junctions occurring in confined flows give rise to turbulent flow structures and separation zones. Vortices and their interaction with the solid walls constitute acoustic sources and generate acoustic waves at various frequencies, which propagate through the medium, pipe wall and into the surrounding environment. The acoustic analysis of the flow-induced noise may be accomplished using two different approaches. The first is the highest modeling approach wherein the full set of compressible Navier Stokes equations is solved. Difficulties lie in the amplitude disparities between the acoustic and flow pressures, with differences of up to seven orders of magnitude. More favored is the hybrid approach. Based on an initial flow calculation comprising of the incompressible Navier Stokes formulations, acoustic source terms may be computed. With use of acoustic analogies, such as the Lighthill acoustic analogy or the perturbation ansatz, a subsequent acoustic analysis may be performed. The purpose of this research is to compare results of the acoustic analysis obtained by compressible and incompressible fluid simulations with focus on the identification and quantification of acoustically relevant flow structures.