Investigation of the noise emitting zones of a cold jet via causality correlations

Within the framework of lighthill's acoustic analogy the causality method proposed by Ribner and Siddon is used to identify equivalent noise sources inside a cold jet. An exploration of a few cross-sections shows that a two-dimensional investigation suffices in a first approach for integrating the source function provided the upper frequency limit does not exceed a Strouhal number approximately equal to 0·5. Furthermore the transverse distribution of the source term shows the jet region located on the microphone side to be dominant; the effective diameter of the source region is comparable with that of the nozzle. It is shown that in a direction with an angle of 30° to the jet axis, the “shear noise” is dominant (about 70% of the noise measured in the same frequency range). The noise emanates essentially from the transition region and from layers located between $\text{r}\text{D}\text{= ±0·25}$ and $\text{r}\text{D}\text{= ±0·375}$. This analysis is suitable for frequency range bounded above by St = 0·54. For the direction with an angle of 45° to the jet axis comparable results are obtained in a frequency range also limited at St = 0·54. However, this range contains only 40% of the total acoustic energy. The source region of the “shear noise” (near 70% of the total energy) and that of the “self-noise” remain always in the transition region located at 4–11 D. Radially the main part of the noise originates from the layers located on the microphone side between $\text{r}\text{D}\text{= 0·25}$ and 0·375. For the direction with an angle of 60° to the jet axis the “shear noise” is no longer measurable and the calculated “self-noise” represents only a few percent of the noise measured. For an acoustically excited jet (white noise filtered between St = 0·39 and St = 0·52, 320 Pa at the nozzle) another type of correlation appears which is believed to be related to a coherent structure travelling inside the jet at 0·75 v_j. The study of the source term shows that this structure must be related to noise originating from the nozzle outlet.     

A numerical study of transient flow around a cylinder and aerodynamic sound radiation

The fully 3D turbulent incompressible flow around a cylinder and in its wake at a Reynolds number Re = = 9×10⁴ based on the cylinder diameter and Mach number M = 0.1 is calculated using Large Eddy Simulations (LES). Encouraging results are found in comparison to experimental data for the fluctuating lift and drag forces. The acoustic pressure in far-field is commutated through the surface integral formulation of the Ffowcs Williams and Hawkings (FWH) equation in acoustic analogy. Five different sound sources, the cylinder wall and four permeable surfaces in the flow fields, are employed. The spectra of the sound pressure are generally in quantitative agreement with the measured one though the acoustic sources are pseudo-sound regarding the incompressible flow simulation. The acoustic component at the Strouhal number related to vortex shedding has been predicted accurately. For the broad band sound, the permeable surfaces in the near wake region give qualitative enough accuracy level of predictions, while the cylinder wall surface shows a noticeable under-prediction. The sound radiation of the volumetric sources based on Lighthill tensors at vortex shedding is also studied. Its far-field directivity is of lateral quadrupoles with the weak radiations in the flow and cross-flow directions.     

On whistling of pipes with a corrugated segment: Experiment and theory

Corrugated pipes are commonly used because of their local rigidity combined with global flexibility. The flow through such a pipe can induce strong whistling tones, which is an environmental nuisance and can be a threat to the mechanical integrity of the system. This paper considers the use of a composite pipe: a shorter corrugated pipe segment embedded between smooth pipe segments. Such a pipe retains some flexibility, while the acoustical damping in the smooth pipe reduces whistling tones. Whistling is the result of coherent vortex shedding at the cavities in the wall. This vortex shedding is synchronized by longitudinal acoustic waves traveling along the pipe. The acoustic waves trigger the vortex shedding, which reinforces the acoustic field for a critical range of the Strouhal number values. A linear theory for plane wave propagation and the sound production is proposed, which allows a prediction of the Mach number at the threshold of whistling in such pipes. A semi-empirical approach is chosen to determine the sound source in this model. This source corresponds to a fluctuating force acting on the fluid as a consequence of the vortex shedding. The functional form of the Strouhal number dependency of the dimensionless sound source amplitude is based on numerical simulations. The magnitude of the source and the Strouhal number range in which it can drive whistling are determined by matching the model to results for a specific corrugated pipe segment length. This semi-empirical source model is then applied to composite pipes with different corrugated segment lengths. In addition, the effect of inlet acoustical convective losses due to flow separation is considered. The Mach number at the threshold of whistling is predicted within a factor 2.     

Review of sound induced by vortex shedding from cylinders

Sound induced by periodic vortex shedding from cylinders has been studied more-or-less continuously since the first quantitative study by Strouhal in 1878. Measurements have shown that vortex shedding is a dipole source of sound. Theoretical models for aeroacoustic sound in a free space, mostly inspired by Lighthill's work, have been developed which can replicate the measurements once the vortex shedding force, coherence, and periodicity are experimentally measured. Vortex shedding from tubes in heat exchanger tube bundles can reach damaging intensities because the acoustic mode is bound by the lower speed of sound within the tube bank itself. However, the amplitude and occurrence of the resonance can only be approximately predicted at present.     

The spectrum of combustion-generated noise

A calculation of the energy release rate resulting from the combustion of propane-air mixtures is presented and the result is used to calculate the far field noise spectrum for an open flame by using appropriate Fourier transform techniques. The results illustrate the broad band nature of combustion noise and show that, for the range of parameters indicated, the peak frequency in the $\text{1}\text{3}$ octave band is in the range 400–1000 Hz. The results also indicate that the shape of spectrum is influenced by the time history of the heat release rate and the turbulence intensity and length scales; on the other hand, the peak frequency is a function of the heat release per unit mass of fuel which is essentially the same for hydrocarbon fuels.     

Interaction of induced sound with flow past a square leading edged plate in a duct

The interaction of resonant sounds with the flow past a thick, blunt, flat plate in a rigid walled square duct has been examined. Sound pressure levels of up to 146 dB (re 20 μPa) have been recorded. It has been established that the resonant sound can initially be excited at a harmonic of the normal vortex shedding frequency. In some cases, the sound “feeds back” on the vortex shedding process causing a step change in the shedding frequency, increasing the Strouhal number for the plate by up to twice the normal value. This excited vortex shedding and associated resonances can be suppressed by locating the plate at incidence to the air flow direction. Complex duct modes can be generated by the vortex shedding resulting in different regions of the plate shedding at different Strouhal numbers.     

Experimental characterization of onset of acoustic instability in a nonpremixed half-dump combustor

This paper reports work on a nonpremixed half-dump combustor, in which methane is injected at the backward-facing step, and mixes and burns with the air flowing past the step in the unsteady recirculation zone. The flow and geometric parameters are widely varied, to gradually change from conditions of low-amplitude noise to excitation of high-amplitude discrete tones. The purpose of the work is to focus on the transition from the former condition to the latter, and to mark the onset of instability. Dimensionless groups such as the Helmholtz and Strouhal numbers are formed based on the observed dominant frequencies, whose variation with the air flow Reynolds number is used to identify the oscillations as those due to the natural acoustic modes or the vortex shedding process. High-speed chemiluminescence imaging reveals shedding of vortical structures in the flame zone. With variation in the conditions, flow-acoustic lock-on and transition from one vortex shedding mode to another is marked by nonlinearity in the corresponding amplitude variations. Such conditions are identified as the onset of instability in terms of the ratio of the flow time scale to the acoustic time scale and mapped against the operating fuel-air equivalence ratio of the combustor.     

Modal content of noise generated by a coaxial jet in a pipe

The problem investigated was that of noise generated by air flow through a coaxial obstruction in a long, straight pipe of inside diameter, D = 97 mm. Downstream modal pressure spectra in the 200–6000 Hz frequency range were measured by a new technique [1] for orifices and nozzles of diameter d where $\text{0·03 ? (}\text{d}\text{D}\text{) ? 0·52}$. The Mach numbers of the flow through the restrictions ranged from 0·15 to choked conditions. The shape of the modal frequency spectrum was found to be determined by the frequency ratio $\text{f}{}_{\mathrm{r}}\text{=}\text{He}\text{St}\text{=}\text{U}{}_{\mathrm{i}}\text{D}\text{a}{}_0\text{d}$, where U_i is the jet velocity and a₀ is the speed of sound in the gas downstream of the restriction. This parameter is the ratio of two non-dimensional frequencies: namely, He, which controls acoustic propagation inside circular ducts, and St, which scales the jet noise spectrum shape. At low f_r(<3) the higher modes dominate the noise spectrum above their cut-off frequencies, while for higher f_r all modes are approximately of equal amplitude. The nature of large scale turbulence structures in the region of the jet near the nozzle exit may be used to explain these phenomena. The measured modal pressure spectra were converted to modal power spectra and integrated over the frequency range 200–6000 Hz. The acoustic efficiency levels (acoustic power normalized by jet kinetic energy flow), when plotted vs. jet Mach number, depend strongly on the ratio of restriction diameter to pipe diameter ($\text{d}\text{D}$). Dividing the efficiency levels by the area ratio, $\text{(}\text{d}\text{D}\text{)}{}^2$, correlated the results over a moderate range of ($\text{d}\text{D}$).     

Environmental acoustic quality in Jeddah urban sites

Surveys of physical exposure to noise at urban sites in Jeddah city indicate that noise from road traffic is very intensive. Relatively high instant sound levels (90 dB(A) and higher) were recorded on a numbr of congested, as well as freely-flowing traffic, roads. Values of the statistical indicators $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>50</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>90</mtext>}}$ and $\text{L}{}_{\mathrm{<mtext>dn</mtext>}}$ were determined for indoor domestic noise and comparisons with current western standards show that the levels determined exceeded the limits of dissatisfaction given by those standards. Overall sound pressure levels measured inside typical university offices indicate that the presence of individual room-units of air-conditioning impairs the acoustic quality in those environments. The results of a social survey on noise perception in residential areas are in good agreement with the above findings, indicating that 89 per cent of people interviewed were substantially disturbed by traffic noise. The results of this research, however, demonstrate the necessity for the application of a traffic noise control programme on Jeddah main roads and also the need for attention to be paid to the indoor acoustic quality of homes and offices.     

Prediction of aerodynamic sound from circular rods via spanwise statistical modelling

The Ffowcs Williams and Hawkings’ acoustic analogy is combined in the time domain with a statistical model in order to take into account the three-dimensional character of the vortex shedding process from a rod in a uniform stream. By applying the model to a two-dimensional unsteady Reynolds averaged Navier-Stokes flow computation, it is shown that the three-dimensional effects, like spectral broadening around the shedding frequency, are partially recovered. The ad hoc statistical model relates a spanwise random distribution of the vortex shedding phase and wall pressure modulations to an arbitrary spanwise correlations. The phase distribution is applied to the tonal pressure signals of the computation and the resulting ad hoc signals are fed into the acoustic analogy. The study is carried out for a rod based Reynolds number of 2.2×10⁴ for which the rod wake is turbulent. Numerical results compare favourably to those of an accompanying experiment.     

Effects of mean flow convection,quadrupole sources and vortex shedding on airfoil overall sound pressure level

This paper presents a further analysis of results of airfoil self-noise prediction obtained in the previous work using large eddy simulation and acoustic analogy. The physical mechanisms responsible for airfoil noise generation in the aerodynamic flows analyzed are a combination of turbulent and laminar boundary layers, as well as vortex shedding (VS) originated due to trailing edge bluntness. The primary interest here consists of evaluating the effects of mean flow convection, quadrupole sources and vortex shedding tonal noise on the overall sound pressure level (OASPL) of a NACA0012 airfoil at low and moderate freestream Mach numbers. The overall sound pressure level is the measured quantity which eventually would be the main concern in terms of noise generation for aircraft and wind energy companies, and regulating agencies. The Reynolds number based on the airfoil chord is fixed at _Rec=408,000${\mathit{Re}}_c=408,000$ for all flow configurations studied. The results demonstrate that, for moderate Mach numbers, mean flow effects and quadrupole sources considerably increase OASPL and, therefore, should be taken into account in the acoustic prediction. For a low Mach number flow with vortex shedding, it is observed that OASPL is higher when laminar boundary layer separation is the VS driving mechanism compared to trailing edge bluntness.     

Vortex shedding noise of low tip speed,axial flow fans

Noise and performance tests were conducted on three low tip speed, half-stage, axial flow fans to determine the nature of the vortex shedding noise mechanism. Each fan was 356 mm in diameter and had eight equally spaced, variable pitch blades. The noise measurements were made in a free field environment and the fan back pressure and speed were varied during the tests. An acenaphthene coating on the blades was used to determine the regions of laminar and turbulent flow.Vortex shedding can be a significant source of noise when the fan is operated in a lightly loaded condition. Essentially it is due to instabilities in the laminar boundary layer on the suction side of the blade where these instabilities are in the form of Tollmien-Schlichting (T-S) waves. These instabilities interact with the trailing edge of the blade and generate acoustic waves which radiate from the trailing edge and form a feedback loop with the source of the instabilities. Vortex shedding noise can contribute as much as 5 dB in overall noise level and up to 22 dB at higher frequencies (8–14 kHz).Serrations located at the leading edge, at the mid-chord, or near the trailing edge on the suction side were found to reduce the vortex shedding noise significantly. The mid-chord location was found to be the most satisfactory because, as well as eliminating the noise, the serrations provided a 3% improvement in peak efficiency. This improvement occurred because separation of the laminar boundary layer was prevented on the suction side. On the other hand, serrations placed at the other two locations tended to degrade fan performance.     

Application of the short-term Leqto the identification and quantification of several sources constituting an environment

A technique applicable to situations with different noise sources is developed on the basis of a series of limited data (short-term $\text{L}{}_{\mathrm{<mtext>eq</mtext>}}$).This technique makes acoustic monitoring of a site possible, identifying and evaluating the respective contributions of the different components of the environment studied.     

High energy xps using a monochromated Ag Lα source: resolution,sensitivity and photoelectric cross sections

Resolution, sensitivity and calibration data are presented for a novel high energy XPS source, monochromated Ag Lα radiation (hv = 2984.3 eV). Adequate resolution is attainable for good signal/noise spectra, whilst values for experimental sensitivity factors agree well with theoretical cross section values calculated by Nefedov. This allows an evaluation of ESCA 3 Mk. II transmission function up to 3000 eV, which appears to obey an approximate E^{$\text{?1}\text{2}$} dependence. Monochromated Ag Lα (linewidth 1.3 eV) overcomes the problem of broad natural linewidths for high energy sources, such that chemical state information can be gained. Various new core level and Auger peaks are developed, a notable feature being the 1s core level and KLL Auger transition capability from Al through to Cl. Improved sensitivity is experienced for elements whose major peaks occur in the 1500–3000 eV BE range, whilst there is no serious reduction of sensitivity in the conventional XPS energy range.     

The acoustic spectrum of axial flow machines

Rotor spectra from a variety of axial flow machines have been compared and acoustic trends examined. Types of rotor considered in the investigation included aircraft propellers, helicopter rotors, jet engine compressor and by-pass fans and a range of domestic, automotive and industrial cooling fans. Rotor sizes varied from 15 in to 60 ft in diameter and tip speeds from 200 ft/s to transonic speeds.The study showed that the acoustic output from these various rotor devices can be described by a common characteristic spectrum. In particular, the minimum broad band noise generated by ducted and free field rotors corresponds to laminar and turbulent boundary layer shedding, and discrete excess noise is generated by various degrees of impulsive blade loading. Finally the study showed that aerodynamic details can be interpreted from the acoustic spectra, and as a result, new insight into the mechanisms of rotor noise has been formed.     

Experimental study on the symmetric modes of vibration of a four bolt clamped square plate

The response of structural components excited by turbulent flow noise depends on the natural resonance frequencies and displacement modes of those components. An experimental program is described which determined those resonance frequencies and modes for a flat plate clamped in the middle of its four sides and also for the case when one of the four bolts had failed. Thirty-three resonances in the range 30·5 to 736 Hz have been identified for the four-bolt normal operation and forty-two in the range 12·5 to 496·1 Hz for the three-bolt accidental situation. Of particular interest to the turbulent flow noise response, the modes symmetrical to the $\text{x-y}$ and diagonal axes have been identified and classified into a two-dimensional matrix form for the $\text{mn + nm}$ and $\text{mn - nm}$ modes combination ($\text{m}$ and $\text{n}$ being the pure modes along the $\text{x-y}$ axis).     

The reactions 18O(p,p)18O and 18O(p,p')18O1(1.98 MeV) for Ep = 3.8–6.1 MeV

Angular distributions of cross sections and analyzing powers have been measured for ¹⁸O(p, p)¹⁸O and ¹⁸O(p, p₁)¹⁸O¹ (1.98 MeV) in 25 keV intervals for proton energies between 3.8 and 6.1 MeV. A phase-shift analysis of the elastic scattering data was carried out, yielding resonance parameters for 16 levels in ¹⁹F in the excitation energy region 11.6–13.8 MeV. The results generally are in good agreement with previous work. On the basis of spin, parity, excitation energy and a comparison of reduced proton widths with reduced neutron widths of levels in ¹⁹O, an assignment of $\text{T =}\text{3}\text{2}$ could be made to at least five of the levels, including the analog of the broad $\text{3}\text{2}{}^{\mathrm{+}}$ level in ¹⁹O at 5.45 MeV. A Legendre-polynomial analysis of the inelastic scattering data suggests that the cross section for proton energies between 5.0 and 5.5 MeV is dominated by the broad $\text{3}\text{2}$⁺ resonance at E_p = 5.15 MeV.     

Theory of charge exchange scattering from surfaces

We present a simple model calculation of the charge exchange probability for ions scattering from solid surfaces, including interactions with both broad band delocalized electrons and with localized core electrons within an Anderson type Hamiltonian with time dependent parameters. The results exhibit some of the major observed experimental features, including characteristic oscillations as a function of incident velocity (from quasi-resonant electron interchange with the core levels) and overall exponential decay $\text{～exp}\text{(?A}\text{v)}$, with v the incident ion velocity, from interactions with the broad band electrons.     

Vortex dynamics of a trapezoidal bluff body placed inside a circular pipe

The influence of Reynolds number and blockage ratio on the vortex dynamics of a trapezoidal bluff body placed inside a circular pipe is studied experimentally and numerically. Low aspect ratio, high blockage ratio, curved end conditions (junction of pipe and bluff body), axisymmetric upstream flow with shear and turbulence are some of the intrinsic features of this class of bluff body flows which have been scarcely addressed in the literature. A large range (200:200,000) of Reynolds number (Re_D) is covered in this study, encompassing all the three pipe flow regimes (laminar, transition and turbulent). Four different flow regimes are defined based on the distinct features of Strouhal number (St)–Re_D relation: steady, laminar irregular, transition and turbulent. The wake in the steady regime is stationary with no oscillations in the shear layer. The laminar regime is termed as irregular owing to irregular vortex shedding. The vortex shedding in this regime is observed to be symmetric. The emergence of separation bubble downstream of the bluff body on either side is another interesting feature of this regime, which is further observed to be symmetric. Two pairs of mean streamwise vortices are noticed in the near-wake regime, which are termed as reverse dipole-type wake topology. Beyond the irregular laminar regime, the Strouhal number falls gradually and vortex shedding becomes more periodic. This regime is named transition and occurs close to the Reynolds number at which transition to turbulence takes place in a fully developed pipe. The turbulent regime is characterised by a nearly constant Strouhal number. Typical Karman-type vortex shedding is noticed in this regime. The convection velocity, wake width formation length and irrecoverable pressure loss are quantified to highlight the influence of blockage ratio. These results will be useful to develop basic understanding of vortex dynamics of confined bluff body flow for several practical applications.     

Phonon observations from 1ƒ noise measurements

From mobility fluctuation $\text{1}\text{?}$ noise parameter measurements five phononic energies corresponding to LO, TO vibrational modes as well as TA + S₂ or O, TO + TA, TO + O combined modes were identified. They stand for phonon replicas of non-radiative recombination processes. It is thus demonstrated that the carrier-phonon interaction is the source of $\text{1}\text{?}$ noise in semiconductors.