Turbulent Boundary Layer Noise Research Articles

Recent advances in active noise and vibration control at NASA Langley Research Center

Over the past 15 years NASA has investigated the use of active control technology for aircraft interior noise. More recently this work has been supported through the Advanced Subsonic Technology Noise Reduction Program (1994–2001), High Speed Research Program (1994–1999), and through the Quiet Aircraft Technology Program (2000–present). The interior environment is recognized as an important element in flight safety, crew communications and fatigue, as well as passenger comfort. This presentation will overview research in active noise and vibration control relating to interior noise being investigated by NASA. The research to be presented includes: active control of aircraft fuselage sidewall transmission due to turbulent boundary layer or jet noise excitation, active control of interior tones due to propeller excitation of aircraft structures, and adaptive stiffening of structures for noise, vibration, and fatigue control. Work on actuator technology ranging from piezoelectrics, shape memory actuators, and fluidic actuators will be described including applications. Control system technology will be included that is experimentally based, real-time, and adaptive.

Chaotic Response of Panel Vibrations Forced by Turbulent Boundary Layer and Sound

Experimental data are presented to show evidence of chaotic response of two adjacent aircraft panels forced by a turbulent boundary layer and pure tone sound. The experiments are a simulation of boundary-layer and fan noise loads on a fuselage sidewall with Reynolds number per meter of 2 :85 £ 10 5 . The response of the panels is purely random and assumed linear when forced by the turbulent boundary-layer e ow and clearly becomes nonlinear with the appearance of the interspersed periodic to chaotic motion when forced by the boundary layer with superimposed pure tone sound. The initial periodic response of two tori of two commensurate frequencies changes with an increase in pure tone sound level. The response of period-doubling bifurcations then makes a transition to chaos, which alternates with quasiperiodic response as the wave loses the spatial homogeneity. The objective is to demonstrate the existence of strong nonlinear effects on the structure response, which is not yet well understood. I. Background M OST studies of nonlinear deterministic and stochastic dynamic problems examine externally excited systems. A typical example of an externally excited system is an aircraft fuselage structure interacting with a turbulent boundary layer and jet engine noise. Periodic, aperiodic, and chaotic responses can occur along the sidewall of the fuselage structure during the acceleration from takeoff, as well as at cruise altitude. One type of load is the socalled buzz-saw noise in high-bypass-ratio turbofan engines. The present experiment is designed to simulate such loads, as well as structural nonlinear responses that result from turbulent boundarylayer e ow and high-intensity sound interaction. Such experiments must be conducted in a wind tunnel with an anechoic test section to prevent standing wave formation between the test panel surface and the opposite sidewall of the tunnel. As a rule, the panel tension and curvaturedependon theloading. Thistension, therefore, constitutes a coupling between the loading and the response. One manifestation of this coupling is the spontaneous surface deformation of the panel, giving rise e rst to the regular and then to the irregular spatial patterns as the load increases. In the previous experiments, panels with periodic nonlinear responses to sound and e ows of constant or accelerated speeds and their active control were considered. 1;2 The present study simulates the abnormal processes of e ow and sound loads, the unsteady loads of the boundary-layer pressure e uctuations coupled with the panel responses,andthesoundradiationbythepanel.Nonlinearbehaviors result fromincreasing levels of pure tone sound as evidenced by the response changes in the panel from periodic motions to broadband chaos. In the past, chaotic signals were not recognized as a physical behavior but were hidden in the broad view given by stochastic processes. At present, the interest is to distinguish between periodic, quasiperiodic,and nonperiodic responses. 3‐6 In the experiments reported herein, the input of the acoustic load superimposed on the turbulent boundary-layer e ow was gradually increased. First, the boundary-layer instability and then responses changing from periodic to quasiperiodic and e nally to chaotic were observed, analo

Boundary-layer noise from high-speed vehicles: Theory and measurement

Wayside noise emitted by the turbulent boundary layer (TBL) on a moving vehicle represents its minimal acoustic radiation. When wayside noise levels for aerodynamically clean vehicles (particularly maglev ones) begin to approach within 3 dB of the TBL noise level, further abatement measures become ineffectual. Knowledge of this lower bound to wayside noise is important to avoid expenditures for useless attempts at further lowering sound levels. The acoustic energy of TBL noise per unit area is low, but its cumulative contribution on a long vehicle could be significant. The present paper comprises two parts. One is a semi-empirical theory of TBL noise. Although Lighthill’s acoustic analogy correctly describes TBL noise, this equation cannot be solved without knowledge of the (as yet) unknown source terms. Furthermore, the fluctuating density, the desired solution, appears on both sides of the equation. Hence, a model approach becomes necessary. The present offering is based upon extrapolated parameters describing the burst-sweep cycle within a flat-plate TBL. The second part of the paper summarizes results of array measurements made above the smooth roof of the maglev vehicle Transrapid 07 at speeds up to 400 km/h. Predictions show reasonable agreement with measurements.

Turbulent boundary layer noise transmission into an aircraft fuselage—Modeling and analysis

The turbulent boundary layer (TBL) pressure fluctuations on the exterior of an aircraft excite the fuselage structure which in turn radiates noise inside the passenger cabin. In most of the previous analytical and experimental studies of sound transmission loss of structures, only acoustic excitation is considered. It is then assumed that the transmission loss of a structure due to the TBL pressure field will be the same as that for the acoustic field. The main objective of this paper is to examine the effects of the TBL pressure field on the transmission loss of a fuselage structure and compare it with the acoustic transmission loss. The vibration and acoustic response of a circular cylindrical shell due to the TBL excitation is first modeled with a finite-element-based computational approach. The finite-element model of the cylindrical shell panel is validated with the results from classical analytical model. The vibration and acoustic response of an aircraft fuselage due to the TBL pressure field is then calculated using the finite-element model. The incident TBL pressure field is simulated by a cross-spectral formulation of the excitation field. The noise reduction characteristics of the fuselage due to acoustic and TBL pressure fields are compared.

Active control of transmission of turbulent boundary layer noise using smart foam elements

Results concerning the active control of transmission of turbulent boundary layer noise using smart foam elements will be presented. Smart foam consists of a cylindrical shaped PVDF film embedded within partially reticulated polyurethane acoustic foam. A smart foam element is an active–passive control device utilizing the passive absorption characteristics of the foam (effective at high frequencies) and the electrically driven PVDF film as the active element (effective at lower frequencies). This lightweight, compact arrangement lends itself well to the control of aircraft cabin noise resulting from an exterior turbulent boundary layer. Six smart foam elements (each 2 in. thick×4 in.×5 in.) are used to control the sound transmission through a 0.05 in. thick×10 in.×20 in. aluminum plate mounted in a wind tunnel. The sound transmission is monitored by measuring the pressure inside an externally mounted anechoic box that encloses the plate. Various control strategies are tested and results compared.

Aeroelastic structural acoustic coupling: Implications on the control of turbulent boundary-layer noise transmission

A method of formulating a model to evaluate the aeroelastic structural acoustic response of a panel subjected to turbulent boundary layer (TBL) noise sources and coupled with full potential flow aerodynamics is presented. Reduced-order models of both the aerodynamics and the structural acoustic coupling are presented such that a state-variable realization of the entire system dynamics can be developed for future active control system design and synthesis with modern and robust control theory. Results from this study demonstrate the importance of including aeroelastic coupling in modeling the structural acoustic response of panels for interior noise control on modern aircraft. At subsonic flow conditions, the aeroelastic coupling serves to increase the transmission loss across the panel with increasing Mach number; however, the power spectrum of the TBL noise source increases with increasing Mach number as well and thus offsets this benefit to some degree. Results from this study also serve to demonstrate that for future analysis of robust stability and performance, variations in the plant dynamics due to variations in flow conditions must be considered in the design of broadband, feedback, active structural acoustic control systems.

Active control of turbulent boundary layer noise transmission through an aeroelastic plate

The reduction of sound transmission to the interior fuselage of modern jet aircraft is gaining increasing concern in an ever-competitive international market. In an attempt to identify effective, weight-saving noise control strategies, active control of sound transmission through elastic plates has received much attention in the literature. While many of these investigations have demonstrated promising results, none of them have considered the effects on plate dynamics by convected fluid loading. Past investigations into the effects of convected fluid loading on plates (i.e., aeroelasticity) have demonstrated significant effects on plate response in the high subsonic and supersonic Mach number range. This is the typical operating range for current and future commercial aircraft. The objective of this investigation is to demonstrate the effectiveness of active control on the transmission of turbulent boundary layer noise through an aeroelastic plate and to highlight the importance of including convected fluid loading in the model. Development of a fully coupled model will be presented for a plate subjected to convected fluid loading on one side and acoustic loading on the other will be presented. Finally, the effectiveness of active control employing co-located piezoelectric transducers on the plate will be presented.

Multivariable structural acoustic control with static compensation

The active control of turbulent boundary layer noise in commercial aircraft has gained significant interest in recent years, and, due to the stochastic nature of the disturbance, feedback control strategies are required. Linear quadratic Gaussian control has been proposed in the past [Baumann et al., J. Acoust. Soc. Am. (1991)], but compensators resulting from such control approaches are typically quite involved. The thrust of the current effort is to demonstrate that acceptable performance in structural acoustic control can be achieved with static compensation implemented through a multiple input, multiple output (MIMO) array of colocated transducers. Since static compensation requires a matrix of constant gains, it is easily realized in analog or digital hardware. By augmenting the system dynamics to include radiation filters, the dual Levine–Athans algorithm was implemented to design the appropriate static compensator. Results for various MIMO control cases are presented.

Spacing effects in large-area hydrophone arrays

It has long been known that large hydrophones filter out the high-wave number, convective components of turbulent boundary layer (TBL) noise. A two-dimensional array constructed of N×M hydrophones might be expected to increase the filtering by a factor of N×M, but, in practice, inter-hydrophone spaces (‘‘gaps’’) diminish this benefit. An analysis, based on a wave-vector approach, has been done to study the effect of gaps in two-dimensional arrays. It is shown that the effect of gaps is to allow leakage into the array response function at wave numbers corresponding to aliasing lobes. This leakage can extend to moderately high wave numbers, although the details of the leakage are sensitive to the exact gap size. Numerical estimates of array self-noise due to flow noise are made using models of the TBL wall pressure wave-vector spectrum. It is shown that, at low values of k0L (k0 is the acoustic wave number and L is characteristic array length), the leakage from the convective and subsonic components of the TBL can be substantial, even for relatively small gaps.

Nonlinear response of composite panels of high speed aircraft

The surface thermal protection systems of advanced high speed aircraft such as the National Aerospace Plane (N.A.S.P.) will be constructed from high temperature resistant composite and intermetallic materials. The various dynamic response studies indicate that under severe aerodynamic, acoustic and thermal loads the response of these surface panels is nonlinear. The equations of motion for nonlinear response of composite surface panels are developed in the space-time domain. A time domain Monte Carlo type approach is used to determine the nonlinear deflection and stress response solutions. The formulation includes surface loads due to nonsteady aerodynamic flow, turbulent boundary layer and engine exhaust noise, oscillating surface shocks and aerodynamic heating. Numerical results are presented for a simply supported laminated composite panel.

Viscous Effects on Turbulent Boundary-Layer Noise of Ship's Sonar Dome in a Water Tunnel

Turbulent boundary-layer (TBL) wall pressure fluctuations of a body measured in a water tunnel need correction to obtain unbounded free-field values. Besides blockage effects in a tunnel which are easily accounted for, viscous effects on TBL noise are to be evaluated to quantify this correction. An analytical method using suitable wave vector spectrum modeling to estimate the correction needed due to viscous effects is presented. A sonar dome body is considered as a typical example.

Nonlinear analysis of transitional and turbulent boundary layer noise

The boundary layer over an axisymmetric surface is characterized by three types of flow; laminar, transitional, and turbulent. The latter stages of the laminar boundary layer are characterized by infinitesimal linear wavelike disturbances, the so-called Tollmien-Schlichting (TS) waves that amplify spatially in the downstream direction. The nonlinear nature of transition produces spectra containing not only the TS frequencies, but sum and difference frequencies as well. Moreover, by applying nonlinear techniques (chaos theory) to evaluate the fluid dynamics within the boundary layer, there is some experimental evidence of the persistence of an underlying structure from transitional to full boundary layer turbulence, that may be associated with the TS mode. The analysis of both theoretical and experimental work in a boundary layer flow leading to these results is the subject of this paper.

Planar turbulent boundary layer (TBL) pressure field emulation with a reduced degree of freedom array

Planar arrays being developed for producing one‐dimensional convective pressure fields with prescribed spatial wavenumber content, slow phase velocity, and temporal frequency are described in the preceding paper. Such devices are capable of “frozen” turbulence emulation but allow no specified correlation properties such as those assumed to exist beneath Turbulent Boundary Layers (TBL). Extending the one‐dimensional technique (involving N independently driven stripes) to two dimensions (with N2 independently driven rectangular areas) becomes unfeasible as the array size becomes large due to the extremely high bandwidth required. In this paper, an alternate approach is proposed and the trade‐off between performance and implementation is discussed. The conclusions reached in this study include the types of TBL second order statistics which the device can produce within implementation constraints and methodologies for calibration of extended area sensors to TBL noise in a repeatable fashion.

Theoretical predictions of turbulent boundary layer pressure fluctuations transmitted into a viscoelastic‐fluid composite layer

The objective of this paper is to develop a model for evaluating the turbulent boundary layer pressure fluctuations transmitted into a viscoelastic‐fluid composite layer. The theoretical model considered here is a plane layer of viscoelastic‐fluid composite structure backed by a rigid plane surface. The other side of the composite layer is exposed to turbulent flow. The transmitted flow noise received by a rectangular hydrophone embedded in the composite layer was calculated for the turbulent boundary layer wall pressure spectrum proposed by Corcos. The transmitted flow noise was characterized by the frequency spectral density expressed in decibels. The main results presented in this paper are turbulent boundary layer noise reductions, which are given relative to the noise levels evaluated in the absence of the composite layer. Effects of the viscoelastic layer thickness, the fluid layer thickness, properties of the viscoelastic layer, and flow conditions on the noise reduction are discussed.

Optimal delay estimation in a multiple sensor array having spatially correlated noise

The maximal likelihood (ML) estimation of time-of-arrival differences for signals from a single source or target arriving at M \geq 2 sensors has been the subject of a large number of papers in recent years. These time differences or delays enable target location. Nearly all previous work has assumed noises which are independent among all sensors. Herein, noises are taken to have complex correlation between sensors. A set of nonlinear equations in the unknown delays is derived and the Fisher information matrix (FIM) for the estimates is also derived. The Cramer-Rao matrix bound (CRMB), which is the inverse of FIM, shows optimal estimator covariances. Computer evaluations are plotted for CRMB elements with varied SNR and noise covariance values typical of turbulent boundary layer noise in towed arrays and signal sources at infinite range (plane-wave fronts). Maximum changes in the bound are within ±3 dB for complex noise correlations with magnitudes up to 0.4, which we tested.

Noise transmission into semicylindrical enclosures through discretely stiffened curved panels

An analytical study of sound transmission into semicylindrical enclosures through discretely stiffened curved elastic panels is presented. The transmitted sound is estimated by solving the acoustic wave equation for the interior acoustic field, a Galerkin-like method being used. This solution is then coupled to the vibration of the stiffened panels. The response characteristics of these panels are determined by using a modal analysis where the modes are obtained by the finite element-strip method. Numerical results include spectra of the interior sound pressure due to white noise, turbulent boundary layer and propeller noise inputs.

Effect of tactile vibration on annoyance to synthesized propfan noise

A research program at NASA-Langley Research Center is being conducted to provide design information that maximizes passenger comfort for proposed propfan aircraft. Particular emphasis in this study is being placed on predicting noise and vibration environments and the resultant passenger acceptability. Previous ride quality research of this program has indicated that vibrations of sufficient intensity to produce whole body movements (at frequencies less than 30 Hz) cause passenger discomfort and annoyance. Within this complex interior environment, this type of vibration interacts additively with noise to produce discomfort. However, recent questions have arisen concerning the effect of high frequency tactile vibration (i.e., greater than 30 Hz) on passenger reactions. The current study addressed this question through obtaining passenger reactions to a wide range of noise and tactile vibration environments. The investigation was conducted in the passenger ride quality simulator located at the NASA-LaRC using 96 subjects who evaluated either synthesized propeller noises only, or these noises combined with seat/arm vibration. The noises ranging from 80–100 dB consisted of a turbulent boundary layer noise with a factorial combination of five blade passage frequencies (50–200 Hz), two harmonic rolloffs, and three tone/noise ratios. The results indicate that passenger reaction (annoyance) to noise is not significantly changed in the presence of tactile vibration.

Acceleration canceling pressure gradient hydrophone

An acceleration canceling pressure gradient hydrophone is described. The unique difference between the conventional pressure gradient hydrophone and the current phone is a plumbing and counterweight arrangement whereby acceleration induced forces from the enclosed fluid are balanced by counterweight forces. Cancelation ratios of 35 dB flat from 10 Hz up are reported from tank measurements. One axis decoupling techniques are described to enhance this ratio at 12 dB/octave from decoupler resonance. The effects of some hydrophone outer housings on the decoupler are shown. Three field tests are described: two using a new radial arm device in a quiet lake, and one using a bottom mounted cable in a tidal estuary. The first radial arm test indicated a cross‐over between structure borne and turbulent boundary layer noise as flow rates rise above about 12knot. The tidal estuary test showed large enhancements of gradient acoustic output near irregular boundaries. This is attributed to the excess pressure gradients in curved wavefronts. Tests of a cardioid version of the hydrophone are included. [Work supported by NAVELEXSYSCOM.]

Optimum signal processing with suboptimum realizations to reject turbulent boundary layer noise

Optimum beamforming or filtering of a line array of either two- or three-element closely spaced hydrophone clusters is considered. The particular topic of the paper is the cancellation of Turbulent Boundary Layer (TBL) noise but the signal processing approach applies to the general subsonic noise problem. Array gain for clusters of unweighted hydrophones and clusters of optimally weighted hydrophones are compared. Array gain improvement for two-hydrophone optimally processed clusters with high interelement TBL noise coherence relative to conventional (time delay and sum) single element beam-forming is shown to increase linearly with the TBL-to-uncorrelated noise ratio and quadratically with the magnitude coherence of the TBL process. Suboptimum realizations of the optimum cluster-element combining schemes such as noise cancellation are proposed. Those realizations are shown to result in very simple hardware implementations. The general noise cancelling approach for high TBL-to-uncorrelated noise ratios uses outputs of a subset of hydrophones in a cluster to estimate TBL noise in a single hydrophone. The estimate is subtracted from the single hydrophone resulting in a subtractor output that is ideally insensitive to the TBL process. It is shown that using three or more hydrophones in an optimally processed cluster is relatively less effective than the two-element cluster as the TBL noise level increases. This suggests that a simple two-element optimally processed cluster would be a cost effective technique for suppressing high level TBL noise because processing complexity increases as the square of the number of hydrophones in a cluster.

On the role of aerodynamically generated sound in determining wayside noise levels from high speed trains

The relative contributions of aerodynamic and wheel/rail noise to railway wayside noise levels are not well understood. Methods for predicting these contributions discussed in this paper include (i) an equation for turbulent boundary layer noise (the minimum wayside noise), (ii) an empirical formula for total aerodynamic noise based on airframe noise studies, and (iii) the Peters equation for wheel/rail interaction noise. Comparisons are made between predicted and measured noise levels for (i) a buoyant vehicle, (ii) the Linear Induction Motor Research Vehicle (LIMRV), and (iii) a magnetically levitated vehicle. Analysis of these results indicates that aerodynamic fluctuations could become the dominant source 3f wayside noise at train speeds of 240–280 km/h. This prognosis is for new high speed railway vehicles equipped with disc brakes and other innovations that reduce the wheel/rail noise contribution.