Title: Adjoint based optimization of free-shear-flow noise control

Speaker: Jonathan Freund, Theoretical and Applied Mechanics, University of Illinois Urbana-Champaign

Date/Time: Thursday, April 21, 2005, 9:00-10:00 am

Location: Building 980, Room 95 (Sandia NM)

Brief Abstract: Direct numerical solutions of the adjoint of the perturbed and linearized compressible flow equations are used to circumvent the complexity of an unsteady free shear flow and reduce its noise directly.  This affords a unique opportunity to contrast a baseline noisy flow and its slightly perturbed but much quieter counterpart.  We focus on a two-dimensional mixing layer as a model for the near-nozzle region of a turbulent jet.  Before and after application of the control, the flow is superficially unchanged.  The mean flow spreading, turbulence kinetic energy, and apparent evolution and pairing of the unsteady flow structures appear unaffected despite a factor of 10 reduction in the acoustic intensity.  However, a POD decomposition of the data illuminates a key change in the flow field: the normal basis modes come more closely in $\sin$/$\cos$-like pairs whose time coefficients trace more circular orbits in phase planes.  This is consistent with a smoother downstream advection of the flow's fluctuation energy which is not obvious in standard visualizations of the flow.  Smoother advection is less efficient at putting energy into radiation capable (supersonic) modes.  This is consistent with our observation that the randomly excited flow can be quieted to the same level as a harmonically excited highly regular flow.