Jet-flap Interaction Noise Research Articles

Wavepacket Modelling of Jet-Flap Interaction Noise: from Laboratory to Full-Scale Aircraft

Wavepacket Modelling of Jet-Flap Interaction Noise: from Laboratory to Full-Scale Aircraft

Quantitative criteria for designing thrust gates to mitigate jet-flap interaction noise

Thrust gates have been reported as a promising concept to reduce jet-flap interaction noise. This manuscript proposes two quantitative criteria for the design of thrust gates based on time-averaged flow parameters that can be extracted from steady-state simulations. Computations were carried out by solving the explicit, transient, compressible lattice Boltzmann equation, while the acoustic far-field was obtained via the Ffowcs-Williams and Hawkings analogy. After being validated based on the truncation error and comparisons with measurements for the far-field acoustics, the simulation model was used in a parametric analysis of thrust gates, varying the width (W/DJ = 1,2,3,4) for a fixed depth B/CF = 1, where DJ is the nozzle exit diameter and CF is the flap chord. Also, two acoustic Mach numbers (Ma = 0.5 and Ma = 0.7) and two flap deflection angles (α = 7° and α = 30°) were considered. The results show that thrust gates with W/DJ > 1 and W/DJ > 2 do not reduce significantly the far-field noise in weak (α = 7°) and strong (α = 30°) jet-flap interaction configurations for both Mach numbers. In this context, the jet-spreading angle and pressure-load are proposed as criteria for designing thrust gates that are effective for noise reduction but with minimum cutout on the flap. Thrust gates designed according to the pressure-load criterion were quieter by around 0.89 dB (α = 7°) and 4.25 dB (α = 30°) for Ma = 0.5, and 0.94 dB (α = 7°) and 3.53 dB (α = 30°) for Ma = 0.7 in comparison with those designed following the jet-spreading criterion. Additionally, the pressure-load criterion was also shown to be effective in reducing the cutout on the flap.

Jet–flap interaction tones

Motivated by the problem of jet–flap interaction noise, we study the tonal dynamics that occurs when an isothermal turbulent jet grazes a sharp edge. We perform hydrodynamic and acoustic pressure measurements to characterise the tones as a function of Mach number and streamwise edge position. The observed distribution of spectral peaks cannot be explained using the usual edge-tone model, in which resonance is underpinned by coupling between downstream-travelling Kelvin–Helmholtz wavepackets and upstream-travelling sound waves. We show, rather, that the strongest tones are due to coupling between Kelvin–Helmholtz wavepackets and a family of trapped, upstream-travelling acoustic modes in the potential core, recently studied by Towneet al. (J. Fluid Mech.vol. 825, 2017) and Schmidtet al. (J. Fluid Mech.vol. 825, 2017). We also study the band-limited nature of the resonance, showing the high-frequency cutoff to be due to the frequency dependence of the upstream-travelling waves. Specifically, at high Mach number, these modes become evanescent above a certain frequency, whereas at low Mach number they become progressively trapped with increasing frequency, which inhibits their reflection in the nozzle plane.

Predictive Large Eddy Simulation for Jet Aeroacoustics–Current Approach and Industrial Application

The major techniques for measuring jet noise have significant drawbacks, especially when including engine installation effects such as jet–flap interaction noise. Numerical methods including low order correlations and Reynolds-averaged Navier–Stokes (RANS) are known to be deficient for complex configurations and even simple jet flows. Using high fidelity numerical methods such as large eddy simulation (LES) allows conditions to be carefully controlled and quantified. LES methods are more practical and affordable than experimental campaigns. The potential to use LES methods to predict noise, identify noise risks, and thus modify designs before an engine or aircraft is built is a possibility in the near future. This is particularly true for applications at lower Reynolds numbers such as jet noise of business jets and jet-flap interaction noise for under-wing engine installations. Hence, we introduce our current approaches to predicting jet noise reliably and contrast the cost of RANS–numerical-LES (RANS–NLES) with traditional methods. Our own predictions and existing literature are used to provide a current guide, encompassing numerical aspects, meshing, and acoustics processing. Other approaches are also briefly considered. We also tackle the crucial issues of how codes can be validated and verified for acoustics and how LES-based methods can be introduced into industry. We consider that hybrid RANS–(N)LES is now of use to industry and contrast costs, indicating the clear advantages of eddy resolving methods.

Studying the effect of flap angle on the noise of interaction of a high-bypass jet with a swept wing in a co-flow

Jet–flap interaction noise for a small-scale swept wing model of a trendsetter plane has been studied experimentally, with a double-stream nozzle installed nearby. The effect of the flap angle on the noise of jet–wing interaction has been analyzed. It has been discovered that the flap angle considerably affects the interaction noise in a wide frequency range in such a way that the noise intensity in each frequency band grows exponentially as the flap edge approaches the shear-layer boundary (on a logarithmic scale, this corresponds to linear scaling of the spectrum depending on the flap angle). The exponential noise reduction as the flap angle decreases agrees with the known theoretical simplified-configuration models that are based on the effect of interaction of the near field of instability wave with the edge. Not only does this agreement show that noise increase/reduction mechanism may be associated with similar processes, it also provides an effective tool for controlling the interaction noise.

Flight test of the 747-JT9D for airframe noise

This paper describes the method used to isolate the individual airframe noise components and determine their relative contribution to the total noise radiated by a 747 aircraft with JT9D-7A engines at approach. The individual components are isolated using ensemble averaged flight test data. The spectral data are normalized on the basis of altitude, airspeed, temperature, and relative humidity. The noise radiated at approach is reconstructed from addition of individual components and compares very accurately in level, spectral shape, and directivity pattern to the actual flyover. A comparison is made between each individual airframe noise component, their logarithmic sum representing the synthesized total airframe component, and Fink's prediction method. The results of this study show that the landing gear, when isolated from configurations having the flaps retracted, is the major contributor to airframe noise. If the effect of flap deployment was included in the isolation of the landing gear noise component, this result could be altered. No evidence of jet-flap interaction noise is found for the 747-JT9D aircraft.

Installation and airspeed effects on jet shock-associated noise

Experimental acoustic data are presented to illustrate, at model scale, the effect of varying the nozzle-wing installation on jet shock associated noise, statically and with airspeed. The variation in installations included nozzle only, nozzle under the wing (with and without flaps deflected), and nozzle over the wing (unattached flow). The nozzles used were a conical and a six-tube mixer nozzle with a cold-flow nozzle pressure ratio of 2.1. A 33-cm diameter free jet was used to simulate airspeed. With the nozzle only, shock-wave noise dominated the spectra in the forward quadrant, while jet-mixing noise dominated in the rearward quadrant. Similar trends were observed when a wing (flaps retracted) was included. Shock noise was attenuated with an over-the-wing configuration and increased with an under-the-wing configuration (due to reflection from the wing surface). With increasing flap deflection (under-the-wing) configuration, the jet-flap interaction noise exceeded the shock noise and became dominant in both quadrants. The free-jet results showed that airspeed had no effect on shock noise. The free-jet noise data were corrected for convective amplification to approximate flight and comparisons between the various configurations are made.