Homogeneous sound-absorbing structures for aircraft engine ducts

As applied to the ducts of aircraft engines, a new method is studied for extending the frequency range of sound absorption by using special homogeneous materials of a rigid structure. A through- or blind-hole perforation of such a homogeneous material is for the first time suggested with a view to substantially extend its capabilities. A theory is developed for sound-absorbing structures of perforated homogeneous material that allows for computing their wave parameters and impedance on the basis of those of the starting material. Based on this theory, one can calculate the impedance of any, no matter how complex a structure built up of several layers differing in thickness and perforation percentage and diameter. The results of calculations made for the impedance and sound absorption coefficient of single and multiple layer samples show good agreement with experimental data.     

A possible approach to optimization of parameters of sound-absorbing structures for multimode waveguides

A method of allowing for the spatial sound field structure in designing the sound-absorbing structures for turbojet aircraft engine ducts is proposed. The acoustic impedance of a duct should be chosen so as to prevent the reflection of the primary sound field, which is generated by the sound source in the absence of the duct, from the duct walls.     

Effectiveness of combined aircraft engine noise suppressors

We consider the design features of fan noise suppressors in application to air intakes and the bypass duct of a turbofan engine. A combined liner is developed that has increased acoustic efficiency in comparison to conventional honeycomb liner. We demonstrate the important role of the area of the sound-absorbing liner between fan Rotor and Stator ensuring significant noise reduction.     

IR signature study of aircraft engine for variation in nozzle exit area

In general, jet engines operate with choked nozzle during take-off, climb and cruise, whereas unchoking occurs while landing and taxiing (when engine is not running at full power). Appropriate thrust in an aircraft in all stages of the flight, i.e., take-off, climb, cruise, descent and landing is achieved through variation in the nozzle exit area. This paper describes the effect on thrust and IR radiance of a turbojet engine due to variation in the exit area of a just choked converging nozzle (M_e = 1). The variations in the nozzle exit area result in either choking or unchoking of a just choked converging nozzle. Results for the change in nozzle exit area are analyzed in terms of thrust, mass flow rate and specific fuel consumption. The solid angle subtended (Ω) by the exhaust system is estimated analytically, for the variation in nozzle exit area (A_ne), as it affects the visibility of the hot engine parts from the rear aspect. For constant design point thrust, IR radiance is studied from the boresight (ϕ = 0°, directly from the rear side) for various percentage changes in nozzle exit area (%ΔA_ne), in the 1.9–2.9 μm and 3–5 μm bands.     

Raman and fluorescence lidar measurements of aircraft engine emissions

Laser induced Raman and fluorescent measurements were made in the exhaust of a gas turbine engine with a new field portable instrument devised specifically for gas turbine exhaust measurements. The gas turbine exhaust was analyzed by conventional instruments for CO, CO₂, NO, NO_x, total hydrocarbons, smoke and temperature, and these data were used as a ‘calibration’ standard for the evaluation of the laser Raman instrument. Results thus far indicate good correlations for CO₂, O₂, smoke, hydrocarbons and temperature. The instrument was not sensitive enough for NO detection but the data analysis indicates that 100 ppm may be detectable with instrument improvements. CO analysis was not attempted, but it is expected that CO could be detected with further research. NO₂ (or NO_x) was not attempted because theoretical and experimental laboratory analysis indicated severe interference with CO₂. The conclusion was that laser Raman shows a good potential for aircraft gas turbine emission analysis.     

Prediction of the angular vibration of aircraft structures

With the Bernoulli-Euler beam used as a theoretical model, qualitative relationships have been developed to predict the angular vibration from a prescribed linear vibration response. Based on laboratory and flight test data, the overall accuracy of predictions is within ±20% of the angular vibration measurement. Such a prediction capability may be considered very significant in that the prediction schemes are intended only for qualitative purposes. In addition, due to the difficulties in obtaining accurate angular measurements, the present vibration data are limited to a gross confirmation of predictions.     

Two-dimensional modeling of an aircraft engine structural bladed disk-casing modal interaction

In modern turbo machines such as aircraft jet engines, structural contacts between the casing and bladed disk may occur through a variety of mechanisms: coincidence of vibration modes, thermal deformation of the casing, rotor imbalance due to design uncertainties to name a few. These nonlinear interactions may result in severe damage to both structures and it is important to understand the physical circumstances under which they occur. In this study, we focus on a modal coincidence during which the vibrations of each structure take the form of a k-nodal diameter traveling wave characteristic of axi-symmetric geometries. A realistic two-dimensional model of the casing and bladed disk is introduced in order to predict the occurrence of this very specific interaction phenomenon versus the rotation speed of the engine. The equations of motion are solved using an explicit time integration scheme in conjunction with the Lagrange multiplier method where friction is accounted for. This model is validated from the comparison with an analytical solution. The numerical results show that the structures may experience different kinds of behaviors (namely damped, sustained and divergent motions) mainly depending on the rotational velocity of the bladed disk.     

Noise simulation of aircraft engine fans by the boundary element method

Numerical simulation results of the civil aircraft engine fan stage noise in the far field are presented. Non-steady-state rotor–stator interaction is calculated the commercial software that solves the Navier–Stokes equations using differentturbulence models. Noise propagation to the far acoustic field is calculated by the boundary element method using acoustic Lighthill analogies without taking into account the mean current in the air inlet duct. The calculated sound pressure levels at points 50 m from the engine are presented, and the directional patterns of the acoustic radiation are shown. The use of the eddy resolving turbulence model to calculate rotor–stator interaction increases the accuracy in predicting fan stage noise.     

High-frequency guided ultrasonic waves for hidden defect detection in multi-layered aircraft structures

Aerospace structures often contain multi-layered metallic components where hidden defects such as fatigue cracks and localized disbonds can develop, necessitating non-destructive testing. Employing standard wedge transducers, high frequency guided ultrasonic waves that penetrate through the complete thickness were generated in a model structure consisting of two adhesively bonded aluminium plates. Interference occurs between the wave modes during propagation along the structure, resulting in a frequency dependent variation of the energy through the thickness with distance. The wave propagation along the specimen was measured experimentally using a laser interferometer. Good agreement with theoretical predictions and two-dimensional finite element simulations was found. Significant propagation distance with a strong, non-dispersive main wave pulse was achieved. The interaction of the high frequency guided ultrasonic waves with small notches in the aluminium layer facing the sealant and on the bottom surface of the multilayer structure was investigated. Standard pulse-echo measurements were conducted to verify the detection sensitivity and the influence of the stand-off distance predicted from the finite element simulations. The results demonstrated the potential of high frequency guided waves for hidden defect detection at critical and difficult to access locations in aerospace structures from a stand-off distance.     

Fan noise reduction of an aircraft engine by inclining the outlet guide vanes

Acoustical Physics - The paper presents the results of an experimental study of the acoustic characteristics of an aircraft engine fan in the design of which the outlet guide vanes are leaned in...     

Three-dimensional detonation cellular structures in rectangular ducts using an improved CESE scheme

The three-dimensional premixed H_2-O_2 detonation propagation in rectangular ducts is simulated using an in-house parallel detonation code based on the second-order space-time conservation element and solution element(CE/SE) scheme.The simulation reproduces three typical cellular structures by setting appropriate cross-sectional size and initial perturbation in square tubes.As the cross-sectional size decreases,critical cellular structures transforming the rectangular or diagonal mode into the spinning mode are obtained and discussed in the perspective of phase variation as well as decreasing of triple point lines.Furthermore,multiple cellular structures are observed through examples with typical aspect ratios.Utilizing the visualization of detailed three-dimensional structures,their formation mechanism is further analyzed.     

Use of the bertin aérotrain for the investigation of flight effects on aircraft engine exhaust noise

A prototype of the Aérotrain has been modified by SNECMA and Société Bertin to investigate flight effects on jet noise and jet suppressor performance. To this end, special attention was given to the reduction of parasitic noise from the vehicle and internal noise from the GE-J85 turbojet engine which powers the Aérotrain. The vehicle, its performance, the operational techniques, the measurement and analysis procedures used are fully described, together with the results of the calibration tests of this unique and very flexible facility which presents many advantages compared to aircraft in flight. Typical results, consistent with the trends shown by clean aircraft noise data, are presented.     

Adhesive bonding of composite aircraft structures: Challenges and recent developments

In this review paper,the challenges and some recent developments of adhesive bonding technology in composite aircraft structures are discussed.The durability of bonded joints is defined and presented for parameters that may influence bonding quality.Presented is also,a numerical design approach for composite joining profiles used to realize adhesive bonding.It is shown that environmental ageing and pre-bond contamination of bonding surfaces may degrade significantly fracture toughness of bonded joints.Moreover,it is obvious that additional research is needed in order to design joining profiles that will enable load transfer through shearing of the bondline.These findings,together with the limited capabilities of existing non-destructive testing techniques,can partially explain the confined use of adhesive bonding in primary composite aircraft structural parts.     

Fan beam and double crosshole Lamb wave tomography for mapping flaws in aging aircraft structures

As the worldwide aviation fleet continues to age, methods for accurately predicting the presence of structural flaws-such as hidden corrosion and disbonds-that compromise airworthiness become increasingly necessary. Ultrasonic guided waves, Lamb waves, allow large sections of aircraft structures to be rapidly inspected. However, extracting quantitative information from Lamb wave data has always involved highly trained personnel with a detailed knowledge of mechanical waveguide physics. The work summarized here focuses on a variety of different tomographic reconstruction techniques to graphically represent the Lamb wave data in quantitative maps that can be easily interpreted by technicians. Because the velocity of Lamb waves depends on thickness, for example, the traveltimes of the fundamental Lamb modes can be converted into a thickness map of the inspection region. This article describes two potentially practical implementations of Lamb wave tomographic imaging techniques that can be optimized for in-the-field testing of large-area aircraft structures. Laboratory measurements discussed here demonstrate that Lamb wave tomography using either a ring of transducers with fan beam reconstructions, or a square array of transducers with algebraic reconstruction tomography, is appropriate for detecting flaws in multilayer aircraft materials. The speed and fidelity of the reconstruction algorithms as well as practical considerations for person-portable array-based systems are discussed in this article.     

Numerical predictions and experiments for optimizing hidden corrosion detection in aircraft structures using Lamb modes

To increase the sensitivity of Lamb waves to hidden corrosion in aircraft structures, a preliminary step is to understand the phenomena governing this interaction. A hybrid model combining a finite element approach and a modal decomposition method is used to investigate the interaction of Lamb modes with corrosion pits. The finite element mesh is used to describe the region surrounding the corrosion pits while the modal decomposition method permits to determine the waves reflected and transmitted by the damaged area. Simulations make easier the interpretation of some parts of the measured waveform corresponding to superposition of waves diffracted by the corroded area. Numerical results permit to extract significant information from the transmitted waveform and thus to optimize the signal processing for the detection of corrosion at an early stage. Now, we are able to detect corrosion pits down to 80-μm depth distributed randomly on a square centimeter of an aluminum plate. Moreover, thickness variations present on aircraft structures can be discriminated from a slightly corroded area. Finally, using this experimental setup, aircraft structures have been tested.     

Wide-band intracavity microwave cells for laser-microwave double resonance spectroscopy

The design and use of two wide-band microwave cells suitable for double resonance experiments within the resonator of a cw CO₂-laser are described. The high sensitivity of the intracavity arrangement allowed very strong double-resonance and two-photon transition signals to be observed with high microwave-frequency scan-rates (1 GHz/min), and low microwave pumping powers (≦10mW/cm²). Both cells were used over a range 15 to 63 GHz, with transmission and VSWR measurements made over the range 18 to 26.5 GHz.     

Application of supersonic particle deposition to enhance the structural integrity of aircraft structures

Aircraft metal components and structures are susceptible to environmental degradation throughout their original design life and in many cases their extended lives.This paper summarizes the results of an experimental program to evaluate the ability of Supersonic Particle Deposition(SPD),also known as cold spray,to extend the limit of validity(LOV)of aircraft structural components and to restore the structural integrity of corroded panels.In this study the potential for the SPD to seal the mechanically fastened joints and for this seal to remain intact even in the presence of multi-site damage(MSD)has been evaluated.By sealing the joint the onset of corrosion damage in the joint can be significantly retarded,possibly even eliminated,thereby dramatically extending the LOV of mechanically fastened joints.The study also shows that SPD can dramatically increase the damage tolerance of badly corroded wing skins.