Computational Study of the Effect of Installation Geometry on the Jet Noise

The jet flow from a nozzle with a conical center plug is computed using large eddy simulations. A grid refinement study is conducted for the baseline nozzle to establish the appropriate grid requirement in the shear layer of the nozzle. The mean flow along the centerline of the nozzle is compared with the PIV results for the baseline nozzle with the pylon. The effect of adding the dual stream flow of the fan to simulate forward flight conditions, resolving the center plug wake and using the ambient conditions for the experiment on the centerline velocity distribution are studied. The results show that significant improvement in the nozzle core length is obtained with the addition of the fan flow. For validation purposes, it is found that even small differences in the conditions used in the computations compared to experiments have significant effect. Far field noise spectra are computed using Ffowcs-Williams Hawkings (FWH) model from the pressure and velocity fluctuations computed on a near field surface. The effect of the pylon is studied by comparing the results of the baseline nozzle with and without the pylon. It is found that the presence of the pylon eliminates screech tones in the far field noise at 90° observation angle and in the peak jet noise direction. The turbulent kinetic energy distribution extends farther into the wake on the non-pylon side compared to the pylon side, a result qualitatively similar to that observed in the experiments. I. Introduction here is a growing need to reduce significantly the noise generated by high performance, supersonic military aircraft. The noise generated during take-off and landing on aircraft carriers has direct impact on shipboard health and safety issues. Noise complaints are increasing as communities move closer to military bases. There is a significant amount of literature dealing with noise reduction in civilian, subsonic aircrafts. Some of the techniques found effective there could possibly be applied for noise reduction in supersonic military jets. Computational Fluid Dynamics simulations can, in principle, play a significant role in the test and evaluation of various noise reduction concepts. However, for the results of these simulations to be credible, they need to be compared and evaluated against relevant experimental data. This should include geometries and flow conditions that are representative of realistic engine configurations and operating conditions. In this paper, we discuss the results of the monotonically integrated large eddy simulation (MILES) approach for simulating the flow field and the near field acoustics of imperfectly expanded jets. A finite-element flow solver described by Lohner 1 using unstructured grids allows us to model the nozzle geometry accurately and the LES approach directly computes the large-scale turbulent structures. This computational methodology has been previously employed by Liu et al. 2 and successfully validated against experimental studies for a C-D nozzle for both under expanded and over expanded jets. Although those simulations were of realistic nozzles, they did not include installation fixtures such as pylons. To understand better the acoustics associated with the installed jet engines, detailed analytical and computational studies and experimental measurements of the flowfield and its effect on the acoustics are needed. Henderson and Wernet 3 have conducted