1,720,970 research outputs found
Representative-in-class vehicles for fleet-level aviation noise analysis
No full textGlobal air traffic demand is projected to nearly double by 2035 (7.2 billion passengers) compared to the 3.8 billion passengers in 2016. At such a growth rate, the aviation sector might cause an important detriment of the welfare of those living around airports via a substantial increase in noise. For addressing such a concern, the aviation industry is required to assess a significant number of aviation scenarios, involving different technology platforms and operational procedures, in order to define the strategies that ensure the higher reduction in aircraft noise impact. A common approach to reduce the combinatorial nature of fleet-level studies and enable more flexibility for exploring multiple aviation scenarios, is to simplify the fleet into a number of representative-in-class vehicles that capture the noise performance of the various classes within the fleet. In this paper, a statistical classification process is implemented for reducing the UK commercial fleet into a number of representative-in-class vehicles based on aircraft noise characteristics. The optimal number of representative-in-class aircraft is analysed for three airports in the UK (London Gatwick, Heathrow and Stansted), with significant differences in aircraft movements and fleet composition, on the basis of the accuracy vs. computational time when calculating noise contour areas. Finally, it is discussed the use of these representative-in-class vehicles as baseline models for projecting the reduction in aviation noise impact with future technology implementation.</p
Propeller noise
No full textPropellers consist of a number of rotating blades and produce both tonal (or harmonic) and broadband noise. However, the former is by far the dominant source as it affects community noise, certification noise, and cabin noise. Hence, considerable effort has been expended advancing efficient methods of calculating it. Isolated propellers produce tones at multiples of the blade passing frequency, whereas counter-rotating propellers (CRPs) produce tones at frequencies corresponding to “sum and difference” frequencies of the blade passing frequencies of the front and rear rotors. In most instances, quadrupole noise can be neglected, and analytic expressions can be derived for predicting the noise from the remaining sources, which is due to the thickness of, and the loading on, the propeller blades. While the principles are well understood, good predictions depend on an accurate modeling of the flow fields.<br/
Counter-rotation propeller tip vortex interaction noise
No full textCounter-rotation propeller tip vortex interaction noise occurs when tip vortices, shedfrom each of the upstream propeller blades, interact with the blades on the downstreampropeller. This paper describes two separate models for calculating the noise produced bythis interaction. The first model approximates each tip vortex as a helical vortex tube ofinfinite extent while the second is based on a previously published study and uses a twodimensionalapproximation to model the tip vortex interaction and represents the velocityfield induced by the tip vortices as a piecewise function for which the flow incident on thedownstream blade row must be calculated numerically. The unsteady loading on thedownstream propeller blades is determined from the incident flow predicted using either ofthe models and is used to calculate the radiated sound field using an analytic propeller noiseformul
Experimental investigation into the turbulence flowfield of in-flight round jets
No full textIn this paper, insight is provided into the modeling of single-point and two-point statistics of a subsonic round jet discharged into a moving ambient medium. An experimental campaign has been performed comprising two round, unheated air flows: a central jet at Mach number equal to 0.6 surrounded by a slower flight flow. Constant-temperature hot-wire anemometry was used to measure both the axial and radial velocity fluctuations within the turbulent jet flowfield. A Mach 0.6 jet was discharged into the flight flow, which ranged from zero up to Mach 0.3. The data show that the degree to which the jet stretches with increasing flight velocity can be discerned with the knowledge of the decay of the mean velocity field downstream of the end of the jet’s potential core. This stretching factor can then be used to predict the changes in the static jet turbulence statistics for the in-flight case. Additionally, in the region of high-turbulence kinetic energy, the two-point statistics can be estimated using information about the single-point statistics and the local mean velocity. Empirical models for the in-flight jet’s shear stresses, cross-correlations, and power spectral density functions are presented and compared with those derived for the static jet case
Experimental investigation into the turbulence flow field of in-flight jets
No full textIn this paper, the velocity field of jets under flight condition is investigated. The aim is to provide insight into the modelling of single-point statistics (e.g. mean velocity, turbulence intensity and other central moments) and two-point statistics (e.g. cross-correlation and coherence coefficients) of subsonic jets discharged into a moving medium. An experimental campaign was performed in a recently built facility comprising an open jet wind tunnel and a jet rig. Hot-wire anemometry was used to measure the velocity fluctuations in the jet turbulence field. A moderate subsonic jet (nozzle exit Mach number equals to 0.6) was discharged into an ambient medium of speeds ranging from 0 to 100 m/s. Experimental data shows that the stretching of the jet with increasing flight-stream velocity can be calculated from the mean velocity decay downstream of the end of the jet potential core. The stretching factor is used to predict the changes in the jet turbulence statistics with the flight-stream speed. Additionally, in the region of high turbulence kinetic energy, the two-point statistics can be recovered accurately from information of the single-point statistics and the local mean velocity. Empirical models for the shear-stress, cross-correlation, and coherence functions are presented and compared to static jet data. Finally, the changes due to forward flight seen in the jet turbulence agrees well with experimental data in the near field and far field of model-scale in-flight jets. This result suggests open jet wind tunnels are capable of reproducing the full-scale problem, that is, the noise from the exhausted jet of a moving engine/nozzle.</p
Interaction between a flat plate and a circular subsonic jet
Full text linkThis paper reports an extensive near- and far-field analysis of the noise generated by an isothermal, subsonic, circular jet in the presence of a solid, flat plate shield. Far-field polar and azimuthal acoustic images are presented initially to characterize the interaction noise source. Near-field streamwise microphone phase analysis along the plate trailing edge reveals a deeper understanding of the link between the jet hydrodynamic field (both linear and non-linear regions) and the mechanisms behind interaction noise generation. Near-field point spectrum data have also been used successfully to validate Amiet's far-field trailing edge dipole prediction code for low-speed jet acoustic Mach numbers.</p
A ray tracing method applied to the propagation of jet noise
No full textAircraft noise remains a major obstacle for air traffic growth as new aircraft must meet ever more stringent certification requirements for noise emissions around airports. A large part of aircraft noise arises from the engine and manufacturers have taken steps to make them quieter. In the case of turbofan aircraft this has been achieved largely by increasing the bypass ratio. Today, there is little room left for further reduction of noise in this way and more novel solutions must be found. One such method is to redesign exhaust nozzles in order manipulate the flow in a way that gives an acoustic benefit, for example in non-circular nozzles a larger flow field may shield the noise sources more efficiently. In turn, this brings a requirement for more advanced noise prediction tools. In this paper, a jet noise prediction method based on Lighthill’s Acoustic Analogy coupled with a Ray-Tracing theory is presented. A full 3D Ray-Tracing method is developed which provides information about the refraction effects due to wave propagation in a non-axisymmetric jet flow. Using the turbulence information obtained from a RANS CFD simulation and the refractions effects obtained from the Ray-Tracing, the method calculates the far-field noise using a modified Lighthill’s equation (LRT). The classic jet noise prediction method, known as MGBK, is also used here for comparison. Results are presented for subsonic single-stream jets operating at Mach 0.75, 0.9 and different temperatures. The refraction results obtained using the ray tracing method are compared with those found using Lilley’s wave propagation equation. Comparisons have shown that the ray tracing method works well for all polar angles outside the zone of silence. The far-field noise comparisons have shown that the LRT method is capable of capturing the peak frequency much better than the MGBK method. The general trend of the spectrum at high and low frequencies as obtained using the new method is also better than those found using MGBK. <br/
Subjective dominance as a basis for selecting frequency weightings
No full textThe objective of this paper is to propose and illustrate a simple approach for the selection of frequencyweightings for the assessment of environmental and transportation noise. In recent years,the A-frequency weighting has become almost universal except where existing standards and regulationsmandate the use of alternative weightings and/or frequency summation procedures, but evenwhere this has been based on extensive research, no real consensus has been achieved. The proposedapproach is based on the concept of subjective dominance, which does not always conformto the physically dominant frequencies identified by the A- or other frequency weightings and summationprocedures used in measurements and/or predictions. The proposed approach is illustratedby the results of a limited series of five listening tests that clearly demonstrate that no single objectivefrequency weighting or summation procedure is capable of providing the best-fit to subjectiveresponses across a range of different contexts. Subjective dominance varies across different listeningcontexts and situations, and should, therefore, be considered whenever noise management andcontrol decisions are being made. The proposed approach will naturally require further researchbecause of the wide range of different contexts and situations in which it might need to be applied
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