Towards an Airframe Noise Prediction Methodology: Survey of Current Approaches

In this paper, we present a critical survey of the current airframe noise (AFN) prediction methodologies. Four methodologies are recognized. These are the fully analytic method, CFD combined with the acoustic analogy, the semi-empirical method and fully numerical method. It is argued that for the immediate need of the aircraft industry, the semi-empirical method based on recent high quality acoustic database is the best available method. The method based on CFD and the Ffowcs William- Hawkings (FW-H) equation with penetrable data surface (FW-Hpds ) has advanced considerably and much experience has been gained in its use. However, more research is needed in the near future particularly in the area of turbulence simulation. The fully numerical method will take longer to reach maturity. Based on the current trends, it is predicted that this method will eventually develop into the method of choice. Both the turbulence simulation and propagation methods need to develop more for this method to become useful. Nonetheless, the authors propose that the method based on a combination of numerical and analytical techniques, e.g., CFD combined with FW-H equation, should also be worked on. In this effort, the current symbolic algebra software will allow more analytical approaches to be incorporated into AFN prediction methods

Use of sound for the interpretation of impact-echo signals

Presented at the 4th International Conference on Auditory Display (ICAD), Palo Alto, California, November 2-5, 1997.This paper describes the development and evaluation of techniques for using sound to aid in the interpretation of signals obtained from the nondestructive testing of concrete using the impact-echo method. The impact-echo method and the significance of using sound for the field engineer are introduced. The auditory representation scheme developed and the software used are described. Psychological experiments that evaluate the effectiveness of the representation scheme are discussed. Results indicate the success of using sound to enhance signal interpretation in real-time and also suggest ways of using sound to train field engineers in the proper use of the impact-echo method

Direct Harmonic Linear Navier-Stokes Methods For Efficient Simulation Of Wave Packets

Wave packets produced by localized disturbances play an important role in transition in three-dimensional boundary layers, such as that on a swept wing. Starting with the receptivity process, we show the effects of wave-space energy distribution on the development of packets and other three-dimensional disturbance patterns. Nonlinearity in the receptivity process is specifically addressed, including demonstration of an effect which can enhance receptivity of traveling crossflow disturbances. An efficient spatial numerical simulation method is demonstrated for the computation of these flows, allowing most of the simulations presented to be carried out on a workstation

Numerical Simulation of a Flap-Edge Flowfield

In this paper we develop an approximate computational framework for simulation of the fluctuating flowfield associated with the complex vortex system seen at the side edge of a flap in a multielement high-lift airfoil system. The eventual goal of these simulations is to provide an estimate of the spectral content of these fluctuations, in order that the spectrum of the noise generated by such flowfields may be estimated. Results from simulations utilizing this computational framework are shown. Introduction Sound from an aircraft induced purely by airflow not related to the engine is known as airframe noise. During approach its levels rival that of the engine, causing a threat to the successful certification of future subsonic aircraft. NASA's Noise Reduction Program began a recent effort to study Airframe Noise in 1995, partnering with United States major airframe industries. NASA Langley's role is to determine fundamental noise source mechanisms by relating sound generation to fund..

Numerical Simulation Of Fluctuations Leading To Noise In A Flap-Edge Flowfield

In this paper we develop an approximate computational framework for simulation of the fluctuating flowfield associated with the complex vortex system seen at the side edge of a flap in a multielement high-lift airfoil system. The eventual goal of these simulations is to provide an estimate of the spectral content of these fluctuations, in order that the spectrum of the noise generated by such flowfields may be estimated. Results from simulations utilizing this computational framework are shown

Fundamental Investigations Of Airframe Noise

An extensive numerical and experimental study of airframe noise mechanisms associated with a subsonic high-lift system has been performed at NASA Langley Research Center (LaRC). Investigations involving both steady and unsteady computations and experiments on a small-scale, part-span flap model are presented. Both surface (steady and unsteady pressure measurements, hot films, oil flows, pressure sensitive paint) and offsurface (5 hole-probe, particle-imaged velocimetry, laser velocimetry, laser light sheet measurements) were taken in the LaRC Quiet Flow Facility (QFF) and several hard-wall tunnels up to flight Reynolds number. Successful microphone array measurements were also taken providing both acoustic source maps on the model, and quantitative spectra. Critical directivity measurements were obtained in the QFF. NASA Langley unstructured and structured ReynoldsAveraged Navier-Stokes codes modeled the flap geometries excellent comparisons with surface and offsurface experimental dat..