A Navier-Stokes/Kirchhoff method for noise radiation from ducted fans

complicated aeroacoustic phenomena we must simultaneously solve the fluid dynamics and the aeroacoustics Nonlinear acoustic effects associated with high speed due to the coupling between them. Noise radiation from flow, engine liners, and sound propagation through (poshigh bypass ratio advanced ducted fans forms one of sibly turbulent) boundary layers make the prediction of the most complicated aeroacoustic problems. Nonlinear noise radiation from advanced ducted fans a very difacoustic effects associated with high speed flow, engine ficult problem. This paper discusses a numerical alliners, and sound propagation through (possibly turgorithm being developed to solve this highly complibulent) boundary layers make the prediction of noise cated problem on a massively parallel Connection Maradiation from ducted fans a very difficult task. chine computer. These computers currently have a peak Noise radiated forward from a high bypass ratio enspeed of 131 billion operations per second and in the gine is mostly generated by the fan. Due to high tip near future will be capable of over a trillion operaspeeds of the blades of the fan, there is usually a shock tions per second, which will allow us to predict aero&wave pattern that radiates into the duct of the encaustics more accurately than previously possible. The gine inlet [I] in addition to the rotor-stator interaction scheme that has been implemented is a hybrid scheme tones (21. Considering that there is also a high speed that solves the 3-D Navier-Stokes equations in the nearsheared mean flow in an engine inlet duct, the comfield and then uses a moving Kirchhoffsurface to predict plexity of the problem becomes apparent. Analysis of the far-field. The Navier-Stokes equations are solved in sound radiation from ducted fans has been mostly lima time-accurate manner using a finite volume scheme ited to linear theory [l], [3]. However, a more comand Runge-Kutta time marching. The resulting sohplete theory is required to analyze sound propagation tion from this is then passed to the Kirchhoff surface and radiation in high bypass ratio ducted fans due to routine, based on the theory of Farassat and Myers [SI. nonlinearity involved. This research is not concerned The Kirchhoff method is shown to work very well with with sound generating mechanisms at the fan, but is a moving monopole test case. Nonreflecting boundary concerned with sound propagation through nonuniform conditions are important in computational aeroacousengine inlets and radiation to free space. Therefore, the tics, and these conditions of the Navier-Stokes algaNavier-Stokes equations will be solved throughout the rithm are also shown to work well. The coupling of engine inlet and in the close surroundings with a given the Navier-Stokes and Kirchhoff methods is described sound source or a simple model at the fan stage. and some test results are presented. Preliminary reln computational aeroacoustic (CAA) calculations sults show that the schrmes are able to effectively use that use standard finite difference or finite volume meththe massively parallel computers. ods in 3-D space, one may not be able to use a computational domain that extends to the point of interest. This is due to both computational speed and memory limitations and possible phase and amplitude errors that can accumulate, Therefore, one usually truncates the comComputational fluid dynamics techniques and computational aeroacoustics techniques have become more and tions on the artificial boundary. One can then use the more similar over the last few years. This is due to two Kirchhoff method that has recently been used l.4 to effects: l) faster computers a'1ow us to more predict far-field sound radiation from moving surfaces. of the physics of the flow and 2, as we investigate more 'phis method can be effectively used provided that the time-history of the acoustic pressure on such a surface -