Aircraft Engine Nacelles Research Articles

VALIDATION OF SPECIES TRANSPORT MODEL TO SIMULATE AGENT TRANSPORT IN A NACELLE

This is in continuation of work done to study the parameters that affect fire suppressant delivery and distribution in an idealized aircraft engine nacelle, based on the quarter scale model used at WPAFB. The primary objective of this paper is to outline the study of the computational fluid dynamics (CFD) approach to study the injection and spread of an inert fire suppressant in an idealized aircraft engine nacelle and to validate the model used. The inert gas (Nitrogen) is used as the fire suppressant. Initially, the work done by Hammins is used as the basis for validating the computational model. The model settings are chosen based on work done by Crawford and then modified to better match the experimental data. The computational model and assumptions used are documented and suggestions for improved accuracy of CFD model are suggested before applying the CFD model to the WPAFB geometry to conduct preliminary studies of agent spread in the nacelle.

Feasibility of an acoustic liner applied to a Fenestron: experimentation

A helicopter's anti-torque system is a significant contributor to radiated noise. The Fenestron, used on Airbus Helicopters, allows to mitigate the anti-torque noise by masking effect and modulation of the blade distribution. However, unlike aircraft engine nacelles, it does not contain acoustic liners inside the duct to absorb the noise radiated by the rotor blades. This paper describes: first, the (aero-)acoustic tests applied to different acoustic liners optimised to be integrated into a 1/3-scale fenestron mock-up, in order to provide a specific impedance (measurement in impedance tube and aeroacoustic bench); second, the configuration of a mock-up installed in an anechoic room, with an acoustic antenna; third, the acoustic pressure mitigation obtained for several diffusers equiped with liners (variation of RPM and pitch for the purpose of the experiment). It appears that the tested liners are likely to provide a high absorption coefficient in a large frequency range and a significant attenuation when installed in the Fenestron mock-up, in accordance with preliminary simulations. Another paper completes the study with a preliminary numerical assessment of the effect of an absorbing treatment introduced inside the collector or the diffuser, and a design optimization applied to different acoustic liner types, adapted to the mock-up.

Feasibility of an acoustic liner applied to a Fenestron: simulation

A helicopter's anti-torque system is a significant contributor to radiated noise. The Fenestron, used on Airbus Helicopters, allows to mitigate the anti-torque noise by masking effect and modulation of the blade distribution. However, unlike aircraft engine nacelles, it does not contain acoustic liners inside the duct to absorb the noise radiated by the rotor blades. This paper describes: first, the FEM modelization of the noise radiation for a 1/3-scale fenestron mock-up at the 1st BPFs, in static conditions; second, the assessment of the effect of an absorbing treatment introduced inside the collector or the diffuser and defined by its acoustic specific impedance; third, the design optimization applied to different acoustic liner types, adapted to the diffuser of the mock-up. Because of the limited duct length, forward and afterward radiations interact, generating interference pressure patterns. Several types of liners (conventional and unconventional), designed to deliver a high absorption, appear adapted to dimensional constraints: SDOF or DDOF type, with a perforated plate above cavities or based on the concept LEONAR ("Long Elastic Open Neck Acoustic Resonator"). Another paper completes the study with (aero-)acoustic tests, applied to elementary samples to validate the impedance target, and performed in an anechoic room with the mock-up.

Evaluating an Additive Manufactured Acoustic Metamaterial Using the Advanced Noise Control Fan

This paper examines the performance of a 3D printed acoustic metamaterial as an acoustic treatment for aircraft engine nacelles in the Advanced Noise Control Fan. As the level of air travel continues to increase, so too does the demand for better noise-reduction technologies for aircraft. Engines are one of the two main sources of noise generated by aircraft, with fan noise, in particular, being of concern due to its broadband and tonal contributions. Small and lightweight methods of addressing both broadband and tonal noise are necessary due to the limitations presented by the current engine design. Presented in this paper is a novel acoustic metamaterial that has undergone design optimization for broadband noise reduction. The final design was produced using 3D printing and tested using the Advanced Noise Control Fan at the University of Notre Dame. It was found that the material is capable of reducing the first harmonic of the blade passing frequency by up to 18.5 dB, with an overall noise reduction of 3.7 dB.

Shear flow effects in a 2D duct: Influence on wave propagation and direct impedance eduction

Impedance eduction of acoustic liners with flow is commonly performed under the assumption of a uniform velocity profile. In practice, the flow profile is not uniform and a significant mean flow shear is present close to the duct wall. The present paper aims to assess the validity of the uniform flow assumption, and more generally to study the effect of the mean flow shear on the impedance eduction process. This is particularly relevant when considering large ducts, high frequencies, high-order acoustic modes and flow velocities representative of aircraft engine nacelles. A numerical mode-matching model is used to compute the sound field in a 2D lined duct with different flow profiles. A detailed parametric study evaluates the influence of the mean flow shear on the axial wavenumbers of the acoustic modes and on the educed impedance. Various parameters are considered: the flow velocity profile (boundary layer thickness and mean Mach number), the direction of acoustic propagation relative to the flow, two different liner configurations and the presence of noise in the pressure data. A number of recommendations are given to achieve robust impedance eduction in large-duct facilities and at high-frequencies.

Super-cooled droplet impact and ice accretion on an aircraft engine nacelle lip

This manuscript concentrated on the icing phenomenon on the lips of an engine nacelle. Numerical simulation was employed to establish the airflow, supercooled droplet impact, and engine nacelle ice accretion models. The study investigated the airflow field near the lip, the characteristics of supercooled droplet impact, and the ice accretion near the lip under different flight conditions. The computational results indicated that due to the effects of nozzle contraction and flow extraction, a low-pressure, high-velocity, and low-temperature region appeared on the inner side of the lip, leading to ice accretion on the nearby wall. The maximum collection efficiency on the lip surface at 20, 000 ft was slightly higher than that at 10, 000 ft, with a maximum value of approximately 0.6. In the computed conditions of this study, as the flight altitude and Mach number increased, the ice thickness on the lip surface increased and the ice spread inward, posing a significant risk of ingestion after detachment.

Analysis of a fire extinguishing model with a p-Laplacian operator and with non-linear advection and reaction

We provide a new model to describe the fire extinguisher dynamics in an aircraft engine nacelle. We connect the mathematical properties introduced by a non-linear operator with the physical mechanisms of a fire extinguishing process. Some discussions about diffusion and non-linear parabolic operators are carried out to justify the governing equation. In addition, the model introduces phenomena like forced convection and fast reaction to account for the discharge of pressurized fire suppressant agent that is released from the fire bottles. The driving equation is firstly discussed analytically to obtain existence and uniqueness results concerning weak solutions. Afterward, we explore profiles of solutions and a characterization of the propagating extinguisher front is introduced. In order to provide a validity note on the analytical conceptions used, a flight test has been executed. Based on this, we obtain particular values for the parameters involved in the model. The proposed assessments permit to obtain different time levels, for which the suppressant agent extinguishes a fire. Such times depend strongly on the involved physical processes connected with diffusion, forced convection and reaction.

Prediction of ice accretion and aerodynamic performance analysis of NACA 2412 aerofoil

AbstractAdverse meteorological conditions often contribute to the formation of ice on aircraft wing section, engine nacelle and other parts leading to the loss of lift coefficient and increase in drag coefficient affecting aircraft control and stability. This paper addresses the problem of in-flight icing on an asymmetric aerofoil under three different ambient and cloud conditions. The study involves prediction of the leading-edge ice thickness using a numerical model developed from the mass and energy conservation law and Messinger freezing fraction model at the same Reynolds number. Later on, degradation in the aerodynamic performance of the iced aerofoil was also investigated using the computational fluid dynamics (CFD) technique, taking the flow field around a 2D aerofoil geometry into account. The aerodynamic study indicates that cumulus clouds embedded with stratified clouds contribute to the formation of mixed ice on aerofoil leading edge and causes the worst icing scenario reducing the lift coefficient to 90% and increasing the drag coefficient to 800% for the same ambient conditions.

Influence of Micro-Blowing Technique Hole Parameters on Drag Reduction of Civil Aircraft Engine Nacelle: A Computational Study

The numerical parametric analysis conducted to analyze the impact of micro-blowing technique (MBT) hole-parameters are quite few at the present stage. The main aim of this research paper is to analyze the effect of micro blowing flow rate and its different hole-parameters on the skin friction drag reduction of an aircraft engine nacelle operating at cruise conditions. The primary tasks are focused to understand the behavior of the flow characteristics at the vicinity of the micro-porous holes by means of different types of MBT configurations. The interaction between main-stream flow and the micro-channel flow is numerically solved by using the Reynolds average Navier-Stokes equation and the k-omega SST is used to model the turbulent flow at the vicinity of the wall region. The hole-pattern is kept aligned in a single-row channel and the shape of the hole cross-section is kept straight to obtain an overall simplicity of the simulation model. The influences of the micro blowing technique are quite clearly seen from the simulation results, as there is a significant reduction in the velocity gradient between the solid engine nacelle surface and all the MBT configurations. The porous engine nacelle surface with zero blowing velocity is capable to reduce the skin friction drag by 7.045 % than of its solid surface, implying that the presence of the micro-porous holes possesses low effective surface roughness, and it is an effective method to manipulate the turbulent boundary layer. The optimum skin friction drag reduction is observed when the geometrical characteristics of the holes possess small diameter and high aspect ratio.

Simulation of Fire Extinguishing Agent Transport and Dispersion in Aircraft Engine Nacelle

The flow and dispersion characteristics of the fire extinguishing agent in the pipings and the concentration distribution in the nacelle are essential for optimizing the aircraft fire extinguishing system. In the present work, we developed a three-dimensional CFD model to simulate the transport and dispersion of the agent in piping and nacelle. The results show that the length and structure of the pipings near the nozzles affect the concentration, pressure, flow rate, and flow distribution of the extinguishing agent. The smaller the bend of the pipings near the nozzles and the angle of connection with the main piping, the less time it takes for the agent to reach the nozzles and the more mass flow rate of the agent is injected, which is more conducive to extinguishing fire rapidly. External ventilation and the blockage of the nacelle’s ribs and other components impact the concentration distribution of the fire extinguishing agent in the nacelle. The agent is mainly concentrated in the middle and rear areas of the engine nacelle. Agent concentration tests were carried out in the simulated engine nacelle. The experimental result is similar to the simulation result, which verifies the feasibility of the simulation method. The simulation method can be used to increase the concentration of fire extinguishing agent to meet the safety requirements by changing the outside ventilation and increasing the filling amount of fire extinguishing agent, so as to achieve the optimization of the fire extinguishing system.

Tiny hole inspection of aircraft engine nacelle in 3D point cloud via robust statistical fitting

In the aircraft manufacturing industry, drilling hole inspection is a vital task for the noise reduction ability and the aircraft structure stability. Due to the complexity of the inner surface of the engine nacelle, inspecting drilling holes of a small size and a large number is fairly challenging. In this paper, we propose a framework for automatic hole inspection on composite flat parts. The raw data of traditional 3D laser scanning usually contain considerable noise and outliers, which has a great influence on tiny hole inspection. First, to perform measurement efficiently and to get raw data of good quality, we design a measurement platform to perform automatic measurement on a preset path. Instead of applying common boundary detection methods, we design a method to detect boundary points by evaluating the boundary possibility of each point, which is more likely to detect the points on a real hole boundary than noise. Based on the characteristics of laser scanning data, we then combine an effective algebraic circle fitting method with an iterative fitting strategy and develop a robust circle fitting method. The improved circle fitting method is capable of estimating a circle closest to the ground truth. Experiments demonstrate that our fitting method performs better than other common fitting methods for comparison under both synthetic and real scanning data.

An acoustic liner design methodology based on a statistical source model

The attenuation of fan tones remains an important aspect of fan noise reduction for high bypass ratio turbofan engines. However, as fan design considerations have evolved, the simultaneous reduction of broadband fan noise levels has gained interest. Advanced manufacturing techniques have also opened new possibilities for the practical implementation of broadband liner concepts. To effectively address these elements, practical acoustic liner design methodologies must provide the capability to efficiently predict the acoustic benefits of novel liner configurations. This paper describes such a methodology to design and evaluate multiple candidate liner configurations using realistic, three dimensional geometries for which minimal source information is available. The development of the design methodology has been guided by a series of studies culminating in the design and flight test of a low drag, broadband inlet liner. The excellent component and system noise benefits obtained in this test demonstrate the effectiveness of the broadband liner design process. They also illustrate the value of the approach in concurrently evaluating multiple liner designs and their application to various locations within the aircraft engine nacelle. Thus, the design methodology may be utilized with increased confidence to investigate novel liner configurations in future design studies.

Adaptive-surrogate-based robust optimization of transonic natural laminar flow nacelle

Natural Laminar Flow (NLF) technology is very effective for reducing the skin friction drag of aircraft engine nacelle, but the aerodynamic performance of NLF nacelle is highly sensitive to uncertain working conditions. Therefore, it’s imperative to incorporate uncertainties into the design of NLF nacelle. In this study, for a robust optimization of NLF nacelle and for improving its efficiency, an adaptive-surrogate-based robust optimization strategy is established, which is an iterative optimization process where the surrogate model is updated to obtain the real Pareto front of multi-objective optimization problem. A case study is carried out to validate its feasibility and effectiveness. The results show that the optimization increases the favorable pressure gradient region and the volume ratio of the nacelle by increasing its lip radius and reducing its maximum diameter. And the aerodynamic robustness of the NLF nacelle is mainly determined by the lip radius, maximum diameter of nacelle and location of the maximum diameter. Compared to the initial nacelle, the optimized nacelle maintains a wide range of low drag and high laminar flow ratio in the disturbance space, which extends the average laminar flow region to 21.6% and facilitates a decrease of 1.98 counts in the average drag coefficient.

Modeling of an aircraft fire extinguishing process with a porous medium equation

The aim of this work is to provide a formulation of a non-linear diffusion model in the form of a Porous Medium Equation (PME) with application to a fire extinguishing process in an aircraft engine nacelle. The work starts by describing some relevant publications currently related to fire suppression modelling methods with emphasis in the aerospace sector. The PME is then introduced highlighting some key relevant features (particularly the finite speed of propagation) compared to the classical Heat Equation (HE). We will refer as u to the extinguisher or suppressor concentration in the media, which is postulated to be governed by a PME equation of the form: 1 $$\begin{aligned} u_{t}=\Delta u^m + \vert x \vert ^\sigma u^p, \end{aligned}$$ 2 $$\begin{aligned} u\left( x,0\right) = u_0(x) \in \mathbb {L}_{loc}^\infty (\mathbb {R^N}), \end{aligned}$$ where 3 $$\begin{aligned} m>1, \sigma >0, p<1, \end{aligned}$$ 4 $$\begin{aligned} (x,t) \in Q_T = \mathbb {R^N} \times (0,T) \end{aligned}$$ Without losing generality and in virtue of the mass transfer application, we will consider that any solution is $$u \ge 0$$ . The set of equation and conditions expressed from (1) to (4) will be referred as problem P. From a mathematical perspective, the main areas of analysis are related to the existence of solutions, the obtaining of particular solutions as asymptotic approach and the application or particularization to a representative aircraft engine nacelle domain where a fire may happen.

Burn-Through Responses of Aero-Engine Nacelle using Fiber Metal Composites by ISO2685 Propane-Air Burner

The paper presents experimental investigation on the behavior and burn through responses of a flat plate composite of fiber-metal laminates composites of aluminum alloy 2024-T3 with carbon and natural fibers subjected to environmental conditions in fire designated zone of an aircraft engine exposed to fire. The main purpose of this study is to know the different burn through time responses of different forms of aluminum alloy with synthetic and natural fibers. The composites were designed and developed to improve the flame fire resistance in a fire designated zone of an aircraft engine in near future for prolonged burning through time that will at least withstand the fire flame for 15 minutes according to the ISO 2685 standard. The composites are fabricated by hand lay-up method in a mold of 300 mm x 300 mm and compressed by compression machine. Based on the obtained results, some of the composites are indicated to be fireproof while some others are fire resistant. It is shown that the carbon fiber reinforced aluminum alloy laminate with only aluminum at top and bottom (CF+AA) of the composites can resist more flame temperature than the carbon fiber reinforced aluminum alloy laminate with alternate layers of aluminum and carbon fiber (CARALL) with 7.86%, the sandwich of carbon fiber with flax with 18.2% and the sandwich of carbon fiber with kenaf with 23.43%. In conclusion, the study reveals that aluminum alloy 2024-T3 with carbon fiber and some types of natural fibers composites possess good properties of resisting high temperature of fire designated zone of an aircraft engine nacelle.

Analysis of a Polynomial Chaos-Kriging Metamodel for Uncertainty Quantification in Aerodynamics

New aerospace aerodynamic bodies increasingly require robust design methods that demand data of the key problem variables in the presence of uncertainty. When these bodies are subject to complex physics, such as turbulence, separation, or secondary flows, the uncertainty data become more difficult to produce economically. Metamodels (surrogate models) can be used to produce data in the presence of uncertainty more efficiently by propagating the uncertainty from the model parameters to the outputs. However, the chief difficulty of metamodels is in consistently producing statistical data of the full system from a sparse number of evaluations. Recently, the polynomial chaos and kriging metamodeling approaches have been combined to take advantage of both their benefits. This research explores the combined method’s effectiveness on airfoil and aircraft engine nacelle examples. It demonstrates that, although the combined method can produce more accurate results than either method alone, there is always a compromise between accuracy and efficiency.

Surrogate model of complex non-linear data for preliminary nacelle design

Most response surface methods typically work on isotropically sampled data to predict a single variable and fitted with the aim of minimizing overall error. This study develops a metamodel for application in preliminary design of aircraft engine nacelles which is fitted to full-factorial data on two of the eight independent variables, and a Latin hypercube sampling on the other six. The specific set of accuracy requirements for the key nacelle aerodynamic performance metrics demand faithful reproduction of parts of the data to allow accurate prediction of gradients of the dependent variable, but permit less accuracy on other parts. The model is used to predict not just the independent variable but also its derivatives, and the Mach number, an independent variable, at which a certain condition is met. A simple Gaussian process model is shown to be unsuitable for this task. The new response surface method meets the requirements by normalizing the input data to exploit self-similarities in the data. It then decomposes the input data to interpolate orthogonal aerodynamic properties of nacelles independently of each other, and uses a set of filters and transformations to focus accuracy on predictions at relevant operating conditions. The new method meets all the requirements and presents a marked improvement over published preliminary nacelle design methods.

Determining the Depth of Occurrence of Defects in Multilayer PCM Structures by Acoustic Methods Based on the Mechanical Impedance Value

Dedicated low-frequency acoustic-testing methods are widely used to detect flaws in three-, five-, and seven-layer structures with a honeycomb core made of polymer composite materials (PCM), with the impedance technique being the main one. However, this technique allows one to establish only the mere fact of the presence of a flaw but not the depth of its occurrence. To obtain information on the depth of occurrence, it is necessary to numerically measure the mechanical impedance on the surface. The complete impedance-transducer frame system has been simulated by the electromechanical analogy method in order to establish the relationship (and derive respective dependences) between the external load applied to the sensor, expressed as the total mechanical impedance on the article surface, and the change in the values of measured electrical parameters on sensor’s piezoelectric elements. Relevant dependences have been derived for the transmission factor, which is the modulus of the ratio of voltages across receiving and emitting piezoelectric elements, and for the phase shift between these voltages. By combining these dependences, hodographs have been produced that represent the graphs of the dependences in the amplitude-phase plane, similar to how information is displayed in state-of-the-art impedance flaw detectors. The dynamic contact compliance (significantly affecting the efficiency of impedance testing technique) of a dry point contact between materials of the outer PCM layers and widely distributed and commercially available impedance sensors (for example, PADI-8) has been determined. Model hodographs constructed with allowance for contact compliance were used to run an experiment on revealing delamination and starved-joint flaws at various depths in a seven-layer honeycomb structure of an aircraft engine nacelle. It has been confirmed that the depths of defects are effectively discriminated both by the magnitude of mechanical impedances and by the characteristic indication from cell walls on the C-scan over the entire article surface.

Duct modes damping through an adjustable electroacoustic liner under grazing incidence

This paper deals with active sound attenuation in lined ducts with flow and its application to duct modes damping in aircraft engine nacelles. It presents an active lining concept based on an arrangement of electroacoustic absorbers flush mounted in the duct wall. Such feedback-controlled loudspeaker membranes are used to achieve locally reacting impedances with adjustable resistance and reactance. A broadband impedance model is formulated from the loudspeaker parameters and a design procedure is proposed to achieve specified acoustic resistances and reactances. The performance is studied for multimodal excitation by simulation using the finite element method and the results are compared to measurements made in a flow duct facility. This electroacoustic liner has an attenuation potential comparable to that of a conventional passive liner, but also offers greater flexibility to achieve the target acoustic impedance in the low frequencies. In addition, it is adaptive in real time to track variable engine speeds. It is shown with the liner prototype that the duct modes can be attenuated over a bandwidth of two octaves around the resonance frequency of the loudspeakers.

Numerical Investigation of the Influence of the Configuration Parameters of a Supersonic Passenger Aircraft on the Intensity of Sonic Boom

Results of calculations of the sonic boom produced by a supersonic passenger aircraft in a cruising regime of flight at the Mach number M = 2.03 are presented. Consideration is given to the influence of the lateral dihedral of the wings and the angle of their setting, and also of different locations of the aircraft engine nacelles on the wing. An analysis of parametric calculations has shown that the intensities of sonic boom generated by a configuration with a dihedral rear wing and by a configuration with set wings remain constant, in practice, and correspond to the intensity level created by the optimum configuration. Comparative assessments of sonic boom for tandem configurations with different locations of the engine nacelles on the wing surface have shown that the intensity of sonic boom generated by the configuration with an engine nacelle on the windward side can be reduced by ~14% compared to the configuration without engine nacelles. In the case of the configuration with engine nacelles on the leeward size of the wing, the profile of the sonic-boom wave degenerates into an N-wave, in which the intensity of the bow shock is significantly reduced.