Broadband Noise Generation Research Articles

Direct numerical simulation of turbulent flow past a trailing edge and the associated noise generation

Direct numerical simulations (DNS) are conducted of turbulent flow passing an infinitely thin trailing edge. The objective is to investigate the turbulent flow field in the vicinity of the trailing edge and the associated broadband noise generation. To generate a turbulent boundary layer a short distance from the inflow boundary, high-amplitude lifted streaks and disturbances that can be associated with coherent outer-layer vortices are introduced at the inflow boundary. A rapid increase in skin friction and a decrease in boundary layer thickness and pressure fluctuations is observed at the trailing edge. It is demonstrated that the behaviour of the hydrodynamic field in the vicinity of the trailing edge can be predicted with reasonable accuracy using triple-deck theory if the eddy viscosity is accounted for. Point spectra of surface pressure difference are shown to vary considerably towards the trailing edge, with a significant reduction of amplitude occurring in the low-frequency range. The acoustic pressure obtained from the DNS is compared with predictions from two- and three-dimensional acoustic analogies and the classical trailing-edge theory of Amiet. For low frequencies, two-dimensional theory succeeds in predicting the acoustic pressure in the far field with reasonable accuracy due to a significant spanwise coherence of the surface pressure difference and predominantly two-dimensional sound radiation. For higher frequencies, however, the full three-dimensional theory is required for an accurate prediction of the acoustic far field. DNS data are used to test some of the key assumptions invoked by Amiet for the derivation of the classical trailing-edge theory. Even though most of the approximations are shown to be reasonable, they collectively lead to a deviation from the DNS results, in particular for higher frequencies. Moreover, because the three-dimensional acoustic analogy does not provide significantly improved results, it is suggested that some of the discrepancies can be attributed to the approach of evaluating the far-field sound using a Kirchhoff-type integration of the surface pressure difference.

Experimental investigation of an inversion technique for the determination of broadband duct mode amplitudes by the use of near-field sensor arrays

This paper is an experimental investigation of an inverse technique for deducing the amplitudes of the modes radiated from a turbofan engine, including schemes for stablizing the solution. The detection of broadband modes generated by a laboratory-scaled fan inlet is performed using a near-field array of microphones arranged in a geodesic geometry. This array geometry is shown to allow a robust and accurate modal inversion. The sound power radiated from the fan inlet and the coherence function between different modal amplitudes are also presented. The knowledge of such modal content is useful in helping to characterize the source mechanisms of fan broadband noise generation, for determining the most appropriate mode distribution model for duct liner predictions, and for making sound power measurements of the radiated sound field.

Flux flow noise and dynamical transitions in a flux line lattice.

Generation of low frequency broadband noise is studied in the low- ${T}_{c}$ superconductor 2 $H$- ${\mathrm{NbSe}}_{2}$ near the ``peak effect.'' A striking dependence on both current and magnetic field is observed in the power, spectral shape, and non-Gaussian nature of the noise. The noise is very large in the plastic flow regime of the dynamics, and small in both the elastic flow and fluid flow regimes. The results show a rapid decrease in the velocity correlation length, a measure of the spatial homogeneity of the moving flux line assembly, as the upper critical field is approached.

Periodic velocity fluctuations in the near wake of a rotating flat-plate blade and their role in the generation of broadband noise

The effect of the shape near the trailing edge of a rotating flat-plate blade on the periodic velocity fluctuation of the wake of the blade has been experimentally investigated. The results show that the characteristics of the near wake in the rotating flow field are similar to those in the uniform flow field; i.e., periodic velocity fluctuation does not occur when the suction surface of the blade is beveled near the trailing edge, but occurs if the pressure surface is beveled. In the latter case, two types of trailing edge separation generally occur: that is, the flow always adheres up to the trailing edge of the suction-side-beveled blade. On the other hand, the flow separation alternately and randomly occurs at two different locations, either the starting point of the beveling of the blade or a certain fixed point on the beveled surface in the case of a pressure side beveled blade. The important role of the periodic velocity fluctuation in the generation of broadband noise from a rotating flat-plate blade is also verified.

An experimental investigation of the sources of propeller noise due to the ingestion of turbulence at low speeds

Noise radiation from a four bladed, 10 in. diameter propeller operating in air at a rotational speed of 3000 RPM and a freestream velocity of 33 ft/s was experimentally analyzed using hot-wire and microphone measurements in an anechoic wind tunnel. Turbulence levels from 0.2 to 5.5% at the propeller location were generated by square-mesh grids upstream of the propeller. Autobicoherence measurements behind the blade trailing edges near the hub and tip showed regions of high phase-coherence between the blade-passage harmonics and the broadband frequencies. Inflow turbulence reduced this coherence. By relating the fluctuation velocities in the propeller wake to the unsteady blade forces, the primary regions of tonal noise generation have been identified as the hub and tip regions, while the midspan has been identified as a region responsible for broadband noise generation. These measurements were complimented by cross-spectra between the propeller wake-flow and the measured sound. The effect of turbulence on the radiated noise level showed an overall increase of 2 dB in the broadband levels for every 1% increase in turbulence. This effect varied for different frequency bands in the acoustic spectrum.

Generation of Broadband Noise due to Periodic Velocity Fluctuation in the Near Wake of a Rotating Flat-Plate Blade.

Generation of Broadband Noise due to Periodic Velocity Fluctuation in the Near Wake of a Rotating Flat-Plate Blade.

Evidence for two-dimentional quantum Wigner crystal.

We report observations of an electric-field threshold conduction and of related ac voltage (broad-band noise) generation in low-disorder two-dimensioanl electron systems in the extreme magnetic quantum limit. We interpret these phenomena as definitive evidence for formation of a pinned quantum Wigner crystal and determine its melting phase diagram from the disappearance of threshold and noise behavior at higher temperatures.

A parametric study of broadband noise generation by small centrifugal blowers

An experimental program has been conducted to identify the influence of several design elements on the broadband noise generated by small centrifugal blowers having impeller diameters of about 0.15 m. The design parameters considered in this study include: blade type, axial and radial inlet clearance, scroll development angle, scroll development length, and cutoff clearance. The noise generated by each blower was measured using the INCE plenum in a reverberation room (as standardized in ANSI S12.11 1987), and the aerodynamic performance of each was determined using a test tank built according to AMCA Standard 210-85. Pure tones generated by blade-cutoff interaction and by the driving motor were removed during data acquisition so that the A-weighted and 1/3 oct, broadband sound-power levels associated with each blower configuration could be established. In addition, a scaling procedure was developed to adjust blower sound-power levels to compensate for level differences resulting simply from aerodynamic performance differences. In this way, it has been possible to identify “design-related” noise differences unequivocally. Design guidelines for low-noise centrifugal blowers have been developed and will be summarized.

Comment on “Generation of broadband noise in the magnetotail by the beam acoustic instability” by P. B. Dusenbery

It is shown here that of the three plasma models for the generation of broadband noise in the magnetotail proposed by Dusenbery and Lyons (1985), two result in the same types of instabilities that are also excited in the Grabbe and Eastman (GE, 1984) model, while the third model introduces other modes not present in the GE model. For the plasma parameters given in the GE model, the ion/ion acoustic instability is a nonresonant one, even though the phase velocity of the excited waves falls within the distribution function of the core ions. The nonlinear saturation mechanism of the ion/ion acoustic instability is the trapping of both the core and the beam ions. In a reply, Dusenbery addresses several ongoing controversies which have resulted from studies of wave particle interactions in the plasma sheet boundary layer.

Reply [to “Comment on “Generation of broadband noise in the magnetotail by the beam acoustic instability” by P. B. Dusenbery”

Reply [to “Comment on “Generation of broadband noise in the magnetotail by the beam acoustic instability” by P. B. Dusenbery”

Noise generation in bench-top ultrasonic cleaners

A theory of audible noise generation in bench-top ultrasonic cleaners is presented. Strong audible noise is generated when the ultrasound intensity exceeds a cavitation threshold with harmonic, subharmonic and broad-band noise generation. In conventional bench-top cleaners the highest intensities occur in the immediate vicinity of the ultrasonic transducers. According to the proposed theory the audible noise is produced there. A method of avoiding this ultrasound concentration and audible noise generation using point source transducers is described, and experimental results supporting a theory of their operation are given.

Generation of broadband noise in the magnetotail by the beam acoustic instability

Analytic properties of the ion beam acoustic instability are evaluated for reasonable magnetotail conditions and are found to be consistent with observations of broadband turbulence not only in the plasma sheet boundary layer but also in the lobes and central plasma sheet regions. Numerical calculations confirm many of the analytic results. Specifically, the dependence of the ion beam acoustic mode on particle distribution models is investigated. Without cold ions present the acoustic mode is stable. With cold streaming ions present the acoustic mode is unstable, but the growth rate is significantly enhanced when warm streaming boundary layer ions are simultaneously present. These results suggest that the interaction between the warm and cold ion streams is responsible for the peak in the electrostatic wave intensities observed within the plasma sheet boundary layer. In addition, the beam acoustic instability, for magnetotail models, is found to be due to resonant, not nonresonant, interactions. Relative streaming effects and dependence on ion species are considered and found to play significant roles in the behavior of beam acoustic growth as a function of wave normal angle and wave number.

Nonlinear Conductivity and Broad Band Noise of Monoclinic TaS3

We have studied the nonlinear conductivity and the accompanied broad band noise of monoclinic TaS 3 . The noise voltage becomes maximum just above the threshold field E T of the nonlinear conduction. For comparison, the same measurements were made for NbSe 3 and the similar results were obtained. The generation of broad band noise is explained in terms of two valued process, and the number of degrees of freedom of the CDW motion, N , is estimated to be 10 9-10 /cm 3 in both of monoclinic TaS 3 which is semiconducting at low temperatures, and NbSe 3 which is metallic. This result indicates that the motion of the CDW's is not described as that of a rigid body. N is found to decrease with decreasing temperature in monoclinic TaS 3 .

Broadband Noise Generation by a Vortex Model of Cavity Flow

This paper presents a potential flow model of two-dimensional cavity flow in which the shear layer is represented by discrete rectilinear vortices. The model compares favorably with physical features of real cavity flows. A theory is developed through which the noise generation by the cavity may be evaluated. Calculations of the spectra and directivity of the quadrupole noise source determined by this theory explain frequency and directivity phenomena observed in airframe noise testing of real aircraft.

Basic Flow Analyses Used in the Study of Broad-Band Noise in Compressors

The propagation of sound in a moving compressible fluid displays interesting features which are important in the problem of noise generation in compressors. Without considering the discrete frequency noise generated by interacting rotating blades and stationary parts, a perturbation method applied to the equations of flow motion in an idealized continuous medium leads to an equation of density waves propagation in a nonhomogeneous moving fluid. The right-hand side is considered as describing distributed noise sources which involve velocity fluctuations. On the left-hand side there appears a negative diffusive effect due to a negative divergence of the velocity field, which downstream entails a streamwise amplification of the intensity of the sound generated upstream. Further, there is a dispersive effect entailing, for a given wave number, a group velocity which in a simple example is shown to be larger than the phase velocity and to become imaginary for a velocity divergence sufficiently high in absolute value. This, together with the amplification effect, may explain the relative importance of the high frequency band in the actual noise spectra of compressors. An attempt at determining the coefficients of the acoustical equation for compressors is made in the schematic Beltrami-Gromeka case of a helicoidal axisymmetrical flow. With simplified assumptions on the behavior of density, depending either solely on the axial coordinate or only on the radial one, both types of axial and radial compressors are considered simultaneously and the method of analytical solution applied. It is emphasized that this treatment is restricted to the mechanical aspect of the broad-band noise generation.