Optimization of Fan Flow Deflection for Supersonic Turbofan Engines

Noise reduction is a primary area of study in the automotive sector and reduction of aerodynamically generated noise due to flow past bluff bodies is highly desired. Bluff bodies are found in various daily applications like poles, wires, components of vehicles, etc. where they are faced with oncoming flows. The aeroacoustic noise generated due to flow past such bodies is a impact of the vortex shedding phenomenon occurring in the wake of the flows and hence control of the vortex shedding phenomenon is of high importance. In the present study, flow past a square cylinder has been modelled at different Reynolds numbers and the noise generated has been recorded. Modification of the geometry by addition of a splitter plate has been performed and its effect on the flow properties and noise generation has been observed. Flow simulations have been performed using commercial CFD software, ANSYS Fluent. Unsteady two- dimensional analyses have been performed by solving governing equations for incompressible isothermal viscous flow. Large Eddy Simulation (LES) with Smagorinsky-Lilly subgrid model has been employed for turbulence modelling and acoustic modelling has been performed using Ffowcs-Williams and Hawkings model. Flow properties such as coefficients of lift and drag and aeroacoustic parameters such as Overall Sound Pressure Level (OASPL) and Sound Pressure Level (SPL) have been studied. The results are found to be in good agreement with available experimental data.

Numerical simulations were performed to investigate the transient compressible flow through lobed mixers inside the internal mixing nozzles of turbofan engines. The Lattice Boltzmann Method (LBM) was used. A detailed model of the mixer and the nozzle was created to capture both the internal flow through the mixer and the external jet plume. The mean flow characteristics were obtained both inside the nozzle and within the jet plume. The transient behavior of the streamwise vortices at the nozzle exit was visualized and quantified. A confluent mixer was selected as a baseline to investigate the performance of both the low and high penetration lobed mixers. The goal was to better understand the detailed noise reduction mechanisms of lobed mixers, as well as thrust enhancements. The Reynolds number based on nozzle exit diameter was 1.36×10 6 and the peak Mach number reached 0.5. The low-Mach setting is to abide by the constraints of the 19-stage LBM algorithm used in this study. The sub grid scales were modeled using the renormalization group (RNG) forms of the standard k-e equations. Far-field sound was computed using the porous Ffwocs William-Hawkings (FWH) surface integral method. The results suggested an increase in thrust coefficient as expected for the lobed mixers. The far-field sound analysis showed considerable low-frequency noise reduction (i.e. ~4-5 dB) for the lobed mixers, as well as about 3dB reduction in the overall sound pressure level (OASPL) in comparison with the confluent nozzle. Both near field and far field results and trends were as expected based on available subsonic experimental data.

Computational simulation and analysis of double-swept blade in BVI noise reduction

There is a wide body of evidence that suggests that the turbulent mixing noise from high-speed jets consists of two components. These two components are generated by sources/mechanisms associated with the fine-scale turbulence and the large-scale structures of the jet plume. The noise generated by the large-scale structures radiates predominantly to the aft quadrant and typically peaks at angles close to the jet axis. The noise from the fine-scale turbulence is dominant in the forward quadrant and at near-normal angles to the jet axis. The initial objective of this study is to find a means to alter this basic radiation process in a high-speed jet. The ultimate goal of this study, however, is the achievement of noise reduction by accomplishing the preceding objective. A simple concept, ab eveled nozzle, is proposed, and detailed aeroacoustic measurements are performed on two nozzles of different bevel angles. Noise measurements, over a wide range of polar angles, are made at several azimuthal angles to map the azimuthal variations. The performances of the beveled nozzles are assessed against a reference round nozzle. The beveled nozzles introduce significant azimuthal variations in the spectra, resulting in major differences in the polar directivities of the overall sound pressure levels at different azimuthal angles; these differences become pronounced when the jet velocity is increased. It is demonstrated that significant noise reduction is achieved in the azimuthal directions below the longer lip of the beveled nozzle, principally in the polar angular range of ∼110 to ∼140 deg. Furthermore, this reduction is observed at all frequencies, with a low performance penalty of ∼1%. The magnitude of the noise reduction is a strong function of the jet velocity, with progressively higher reductions as the jet velocity is increased. With proper orientation of the nozzle, the noise footprint on the ground can be reduced. It is shown that the noise reduction is caused by the modification of the noise generated by the large-scale structures in the jet plume. Thus, noise reduction is achieved through the manipulation of the generation mechanisms. HERE is a wide body of evidence that suggests that the turbulent mixing noise from high-speed jets consists of two components. These two components are generated by sources/mechanisms associated with the fine-scale turbulence and the large-scale structures of the jet plume. The noise generated by the large-scale structures radiates predominantly to the aft quadrant and typically peaks at angles close to the jet axis. The noise from the fine-scale turbulence is dominant in the forward quadrant and at near-normal angles to the jet axis. As the jet velocity is increased, there is a broadening of the angular sector in which peak radiation occurs, with more noise being radiated to lower polar angles. (All polar angles are measured from the jet inlet axis, with the jet exhaust axis corresponding to

This paper describes an experimental work to investigate the effect of a reflector on supersonic jet noise radiated from a convergent-divergent nozzle with a design Mach number 2.0. In the present study, a metal reflector and reflectors made of three different sound-absorbing materials (grass wool and polyurethane foam) were employed, and the reflector size was varied. Acoustic measurement is carried out to obtain the acoustic characteristics such as frequency, amplitude of screech tone and overall sound pressure level (OASPL). A high-quality schlieren optical system is used to visualize the detailed structure of supersonic jet. The results obtained show that the acoustic characteristics of supersonic jet noise are strongly dependent upon the jet pressure ratio and the reflector size. It is also found that the reflector with sound-absorbing material reduces the screech tone amplitude by about 5–13dB and the overall sound pressure levels by about 2–5dB, compared with those of the metal reflector.

Short conical solid/perforated plug-nozzle as supersonic jet noise supressor

Design approach for tilt propellers of UAM/eVTOLs for cruise and hover considering aerodynamic and aeroacoustic characteristics via a multi-fidelity model

Large eddy simulation (LES) is performed to study high subsonic round and chevron jets with a diameterbased Reynolds number Re=10⁵.2 different chevron jet flows are considered, which are with 6 lobes and 4 lobes respectively.The simulation results are checked by comparing mean axial velocity profiles and overall sound pressure levels (OASPLs) of the round jet with the existent experimental and LES results, and they are in good agreement with each other. Then the properties of chevron jets are compared with those of round jets. The chevron jets show higher radial expansion rate in the nearnozzle region and shorter potential core lengths. The OASPLs of the chevron jets decrease as high as 4 dB at the shallow angles compared with the round jets and without apparent noise increment at the sideline. 3 azimuthal modes,m=0,1,2of the farfield pressure fluctuations from both the round and chevron jets are investigated. All cases show similar OASPL distribution profiles vs. the polar angles for each azimuthal mode. But, apparent noise increment is found at high polar angles in the zeroth and first azimuthal modes for the chevron jet with 4 lobes when compared with the round jet. By performing proper orthogonal decomposition (POD), the most energetic coherent structures at specified coupled azimuthal wavenumbers are extracted. The wavepacket features associated with coherent structures of round and chevron jets are analyzed in detail to explain changes of noise properties in the far fields.

The experimental article focuses on the flow characteristics and acoustic features of a jet issuing from a Convergent Divergent (C-D) nozzle with a cruciform (cross-shaped) cross-section. The design Mach number is 1.5. The cruciform nozzle has an equivalent exit diameter of 5.42 mm, and its features are compared with the equivalent circular and square nozzles. The operating Nozzle Pressure Ratio (NPR) is varied from 2 to 6. The pitot probe and microphone, in conjunction with data acquisition instruments, provide the total pressure and acoustic measurements of the jet flow. The cruciform jet exhibits a shorter supersonic core length compared to both the circular and square jets for all operating NPRs. Compared to circular and square jets, the cruciform jet has a maximum Overall Sound Pressure Level (OASPL) reduction of 7.06 dB and 7.73 dB at NPR 5. The screech is suppressed and is only discernible at NPR 5 in the cruciform jets, whereas it exists from NPR 3 to 6 in the other two jets. The higher screech amplitudes are localized to lower emission angles from 45° to 75°. Together, the OASPL, and screech are suppressed in cruciform jets.

There are many approaches to determine the sound propagated from turbulent flows. In hybrid methods, the turbulent noise source field is computed or modeled separately from the far-field calculations. To have an initial and quick estimation of the sound propagation, less computationally intensive methods can be developed using stochastic models of the turbulent fluctuations. In this paper, turbulent mean flow of a two dimensional, compressible, cold-jet at Mach 0.56 is computed using RANS with 2 equation k-e RNG model. The above mean-flow quantities are then used in a stochastic model to generate the details of the turbulent velocity fluctuations. This method is based on the use of classical Langevin equation to model the details of fluctuating flow field superimposed on the averaged computed quantities. The resulting sound field due to the generated unsteady flow is then evaluated using Lighthill's acoustic analogy. Our results are validated by comparing the directivity and the overall sound pressure level (OASPL) magnitudes with the available experimental data. Numerical results show reasonable agreement with the experiments, both in maximum directivity and the magnitude of the OASPL.

An active noise control work in jets from contoured and conical supersonic axisymmetric nozzles (henceforth contoured and conical jets, respectively) with plasma actuators is conducted. The Mach 1.65 contoured nozzle is designed by using the method of characteristics for shock-free jet. The Mach 1.65 conical nozzle has converging and diverging conical sections with a sharp throat. Eight actuators, distributed uniformly around the nozzle exit, are used and the jet is forced with azimuthal modes of 0-3, and ±4 and at forcing Strouhal numbers ranging from 0.18 to 4.0. Far-field acoustic is measured by a linear microphone array covering polar angles from 25 to 80q relative to the jet axis. At a far-field polar angle of 30q, more noise reduction is observed at a higher azimuthal mode of 3 or ±4 in both jets. The maximum reduction in overall sound pressure level is 1.5 and 2.6 dB for the contoured and conical jets, respectively. At a polar angle of 80q (near sideline), the reduction in noise level is negligible. In the conical jet, there is a competition for energy between the naturally-occurring structures and those generated by forcing, similar to the process taking place in forcing an imperfectly-expanded supersonic jet. At moderate forcing Strouhal numbers, the structures generated by forcing prevail and thus the naturally-occurring structures are suppressed or weakened. This seems to be responsible for the reduced or suppressed screech tones in the conical jet.

The effect of rotational speed and pressure rise on the leakage flow noise radiated by a low-speed axial fan, provided with rotating shroud, has been systematically investigated. The flow in the gap region has been studied by means of particle image velocimetry (PIV) measurements taken in the meridional plane. At low blade loading, the leakage flow is restrained close to the rotor ring and, at higher loading, it forms a wide recirculation zone. In the latter conditions, an unsteady flow separation likely takes place in the blade tip region which may be observed in the instantaneous flow field only. The leakage flow noise generally increases with the blade loading, but is non-monotonic, as the overall sound pressure level (OASPL) growth is interrupted by local minima; such a trend is qualitatively independent of the rotational speed. As the loading increases, the sound pressure level (SPL) spectrum shows important modifications, since the characteristic frequency of the subharmonic narrowband humps related to the leakage noise decreases; furthermore, height and width of the humps vary non-monotonically. Such a complicated behavior is likely related to the modifications in the leakage flow pattern and also to the appearance of the flow separation at the blade tip.

Noise barrier with acoustic metamaterial has application prospect in power transformer noise reduction due to its low frequency noise reduction advantages. The acoustic diffraction along the edges of the noise barrier decreases the overall noise reduction obviously. An active noise control (ANC) method combined with noise barrier is proposed to counteract the diffracting sound waves near the edges of the noise barrier. The noise barrier is composed of the Hilbert fractal acoustic metamaterial (HFAMM) units with good sound insulation characteristics at low frequencies. The transmission loss (TL) of the HFAMM unit is higher than 20dB at the main frequencies of the transformer noise theoretically. The experiment system including the acoustic metamaterial noise barrier and the ANC system is established. The sound pressure level with ANC on the specified points in the target area is more than 10dB lower than that without ANC in the experiment. The results prove that the new method is effective for improving noise reduction performance of the noise barriers.

Pantograph system of high-speed trains become significant source of aerodynamic noise when travelling speed exceeds 300 km/h. In this paper, a hybrid method of non-linear acoustic solver (NLAS) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy is used to predict the aerodynamic noise of pantograph system in this speed range. When the simulation method is validated by a benchmark problem of flows around a cylinder of finite span, we calculate the near flow field and far acoustic field surrounding the pantograph system. And then, the frequency spectra and acoustic attenuation with distance are analyzed, showing that the pantograph system noise is a typical broadband one with most acoustic power restricted in the medium-high frequency range from 200 Hz to 5 kHz. The aerodynamic noise of pantograph systems radiates outwards in the form of spherical waves in the far field. Analysis of the overall sound pressure level (OASPL) at different speeds exhibits that the acoustic power grows approximately as the 4th power of train speed. The comparison of noise reduction effects for four types of pantograph covers demonstrates that only case 1 can lessen the total noise by about 3 dB as baffles on both sides can shield sound wave in the spanwise direction. The covers produce additional aerodynamic noise themselves in the other three cases and lead to the rise of OASPLs.

Data-driven prediction of aerodynamic noise of transonic buffeting over an airfoil