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Flow Duct Data for Validation of Acoustic Liner Codes for Impedance Education
Abstract and Figures
The objective of this study was to acquire acoustic and flow data with hard and lined duct wall sections for % situation of a liner prediction code being developed at NASA LaRC. Both the mean and acoustic flowfields were determined in a cross-plane of the rectangular duct. The test liner was of the locally-reacting type and w as made from a ceramic material. The material, consisting of a tubular structure, was provided by NASA LaRC. Flow measurements included pressure, temperature, and velocity profiles upstream of the liner section. The inflow sound pressure levels and phases were obtained with a microphone probe equipped with a nose cone in two cross planes upstream of the liner and in two cross plane downstream of the liner. In addition to the acoustic measurements at the cross planes, axial centerline acoustic data was acquired using an axially traversing microphone probe that was traversed from a Location upstream of the liner to some distance downstream of the liner. Much of the data was acquired for frequencies of 500, 1000, 1500, 2000 and 2500 Hz and for duct mean flow Mach numbers of 0.0, 0.1, 0.2, and 0.3.
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... Those materials have been widely experimentally and numerically studied by NASA/LaRC [42,63,2], as the theoretical mechanisms of the materials are well-known and because their impedance behaviour does not vary significantly in presence of high amplitude or flows. This last particularity may be of importance for further simulations with flow as well as for eduction techniques, that rely dramatically on the chosen condition for taking flow into account. ...
... Among others, a particular material referenced as the MA110602 will be subjected to a dual experimental study detailed in chapter 4. For those reasons, the mechanisms of such absorbing materials are explained in what follows. 2 The resistivity is directly linked to the plate resistance and is of best efficiency when R ρ0c0 = 1. Adding a wiremesh may help to reach this value without compromising the functionning of the material. ...
... Many experiments were conducted on NASA Langley Research Centre (NASA/LaRC) facilities to study different kinds of Ceramic Tubular (CT) absorbing materials under flow conditions. The tests concerning CT materials [2,63,42] have been very often re-used for numerical validations. It is then of interest here to describe NASA/LaRC benches and some of the main results obtained with the latter. ...
Theoretical and Numerical Investigation of Time-Domain Impedance Models for Computational AeroAcoustics
The reduction of acoustic emission induced by civil aircraft around major airports has become an important societal issue. To reduce the fan noise, induced by the engines, which has become preponderant over the past years with the advent of turbofan engines, manufacturers are led to generalize the employment of acoustic absorbing materials" (or \acoustic liners"). The present thesis is related to the numerical prediction of such absorbing materials, in the context of time-domain CAA (Computational AeroAcoustics) methods. Such modeling raises several key questions, which are related to various aspects such as the type of
ow involved (boundary layers e�ects, etc.), the sound levels considered (non-linear phenomena), the di�raction e�ects induced by ruptures of impedance, etc. The present study then consists in validating and improving the time-domain impedance boundary condition implemented in Oneras structured CAA solver
(named sAbrinA.v0 ). Theoretical developments are �rst devoted to the modeling of impedance in the time-domain, and lead to a discussion on the generalization of this modeling. The work then consists in CAA-simulating several canonical tests of noise absorption by acoustic liners. Outputs are compared against experimental and/or analytical results, delivering new insight in the way noise absorption materials can be accurately modeled and simulated using time-domain CAA-approaches.
Keywords: COMPUTATIONAL AEROACOUSTICS; ACOUSTIC LINERS; IMPEDANCE BOUNDARY CONDITION; TIME DOMAIN IMPEDANCE MODELS
A very ambitious study was initiated to obtain detailed acoustic and flow data with and without a liner in a duct containing a mean flow so that available theoretical models of duct liners can be validated. A unique flow-duct facility equipped with a sound source, liner box, flush-walled microphones, traversable microphones and traversable pressure and temperature probes was built. A unique set of instrumentation boxes equipped with computer controlled traverses were designed and built that allowed measurements of Mach number, temperature, SPLs and phases in two planes upstream of a liner section and two planes downstream at a large number of measurement points. Each pair of planes provided acoustic pressure gradients for use in estimating the particle velocities. Specially-built microphone probes were employed to make measurements in the presence of the flow. A microphone traverse was also designed to measure the distribution of SPLs and phases from the beginning of the liner to its end along the duct axis. All measurements were made with the help of cross-correlation techniques to reject flow noise and/or other obtrusive noise, if any. The facility was designed for future use at temperatures as high as 1500 F. In order to validate 2-D models in the presence of mean flow, the flow duct was equipped with a device to modify boundary layer flow on the smaller sides of a rectangular duct to simulate 2-D flow. A massive amount of data was acquired for use in validating duct liner models and will be provided to NASA in an electronic form. It was found that the sound in the plane-wave regime is well behaved within the duct and the results are repeatable from one run to another. At the higher frequencies corresponding to the higher-order modes, the SPLs within a duct are not repeatable from run to run. In fact, when two or more modes have the same frequency (i.e., for the degenerate modes), the SPLs in the duct varied between 2 dB to 12 dB from run to run. This made the calibration of the microphone probes extremely difficult at the higher frequencies.
this report is built on a generic test system (ref. 10) developed at the 14- by 22-Foot Subsonic Tunnel. This test system, the 2-Meter Rotor Test System (2MRTS), consists of a 29-hp rotor drive system with collective and cyclic blade pitch controls, a four-blade articulated hub mounted on a six-component strain-gauge balance, and a fuselage skin mounted on a separate similar balance. These two six-component strain-gauge balances provide independent measurements of the rotor and fuselage aerodynamic loads. Figure 4(a) shows the test configuration installed in the wind tunnel with the laser velocimeter shown in the background. The shape of the fuselage is designed to represent a wide range of helicopter fuselages without being specific to any particular one. The fuselage geometry can be described by a set of equations (refs. 6 and 11). Development of computer models is simplified by using a geometry derived from equations. Figure 4(b) shows the shape of the fuselage with example cross sections. The rotor hub used in this study is a four-bladed articulated hub (ref. 10). The hub articulation is set at 2.0 in. from the center of rotation with coincident flap and lag hinges. The blade grip component of the hub has a maximum diameter of 1.625 in. and extends to 7.5 in. from the center of rotation. The airfoil section of the blade is fully developed at 8.5 in. from the center of rotation. The center section of the hub has a maximum height of 2.125 in. from the plane of blade articulation. The pitch rod attaches to the blade grip at a radius of 2.0 in. from the center of rotation and is attached to the swash plate at a radius of 1.4 in. with a vertical offset of approximately 4 in. Figure 4(c) shows the exposed geometry of the hub. The rotor system tested consists of four recta...