Nacelle Geometry Research Articles

Nacelle optimisation through multi-fidelity neural networks

PurposeAerodynamic shape optimisation is a complex problem usually governed by transonic non-linear aerodynamics, a high dimensional design space and high computational cost. Consequently, the use of a numerical simulation approach can become prohibitive for some applications. This paper aims to propose a computationally efficient multi-fidelity method for the optimisation of two-dimensional axisymmetric aero-engine nacelles.Design/methodology/approachThe nacelle optimisation approach combines a gradient-free algorithm with a multi-fidelity surrogate model. Machine learning based on artificial neural networks (ANN) is used as the modelling technique because of its ability to handle non-linear behaviour. The multi-fidelity method combines Reynolds-averaged Navier Stokes and Euler CFD calculations as high- and low-fidelity, respectively.FindingsRatios of low- and high-fidelity training samples to degrees of freedom of nLF/nDOFs = 50 and nHF/nDOFs = 12.5 provided a surrogate model with a root mean squared error less than 5% and a similar convergence to the optimal design space when compared with the equivalent CFD-in-the-loop optimisation. Similar nacelle geometries and aerodynamic flow topologies were obtained for down-selected designs with a reduction of 92% in the computational cost. This highlights the potential benefits of this multi-fidelity approach for aerodynamic optimisation within a preliminary design stage.Originality/valueThe application of a multi-fidelity technique based on ANN to the aerodynamic shape optimisation problem of isolated nacelles is the key novelty of this work. The multi-fidelity aspect of the method advances current practices based on single-fidelity surrogate models and offers further reductions in computational cost to meet industrial design timescales. Additionally, guidelines in terms of low- and high-fidelity sample sizes relative to the number of design variables have been established.

Numerical investigation of the vortical structures in the near wake of a model wind turbine

It is well known that the vortex system in a horizontal axis wind turbine wake is highly relevant in terms of fatigue loads and performance of wind turbines located in the wake of other wind turbines. The breakdown process of tip vortices particularly influences the mixing process of the low-speed wake region with the undisturbed flow outside the wake. As a collaboration with the Technische Universität Berlin (TUB), a major goal in a joint research is to study the effects involved in tip vortex breakdown. TUB designed a model turbine that will be towed through a large water tank to analyse and to control the tip vortex decay. To accommodate the measurement equipment, the ratio of blade length to nacelle length is unconventionally small, leading to uncertainty regarding the effect on the breakdown of the tip vortex. Due to the dimensions of the model wind turbine the root vortex system and the nacelle wake are not comparable to former studies and should therefore be investigated in detail. To do so computational fluid dynamics, in particular delayed detached eddy simulations, are conducted for the model turbine with the compressible flow solver FLOWer and the two-equation Menter-SST turbulence model. Two simulations are conducted with and without the nacelle. The results indicate that the root vortices propagate downstream and interact with each other. Additionally, these vortical structures are also influenced by the geometry of the turbine nacelle which causes faster decay of the root vortices compared to a configuration without the nacelle. However, the root vortex breakdown does not influence the tip vortices as the turbulent intensity matches for the area where tip vortices are present. In short, this investigation shows that the influence of the nacelle geometry and root vortex system on the tip vortices is negligible. Thus, the model wind turbine designed by TUB is suitable for the investigation of tip vortices.

Impact of 2D engine nacelle flow on buffet

Numerical studies on the interaction of nacelle and airfoil shock dynamics are performed. To keep the computational cost at an acceptable level, first 2D investigations on the interaction of nacelle and airfoil are performed that cover basic dynamic phenomena of this configuration. The transonic flow around the OAT15A airfoil is computed at buffet conditions, i.e., freestream Mach number Ma∞=0.73\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Ma{_\\infty } = 0.73$$\\end{document}, chord-based freestream Reynolds number Rec=2·106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Re_c = 2\\cdot 10^6$$\\end{document}, and angle of attack α=3.5∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha = 3.5^\\circ $$\\end{document} using wall-modeled LES. Two configurations are considered, one which includes a generic 2D ultra-high bypass ratio (UHBR) engine nacelle geometry and one without an engine, which is denoted the baseline case. In addition to the airfoil shock, the flow field of the nacelle configuration is characterized by a shock wave on the upper part of the nacelle. Furthermore, the introduction of the UHBR-engine nacelle leads to a reduced effective angle of attack and Mach number in the flow to the airfoil. The changes in the topology of the flow to the airfoil caused by the nacelle lead to a reduced strength of the airfoil shock and a less developed buffet, which resembles the behavior close to the stability limit. The reduced shock dynamics yields lower pressure fluctuations at the airfoil trailing edge. A frequency analysis of time series data from the airfoil shock location shows a reduction of the buffet frequency for the nacelle configuration. Further investigations of the flow field dynamics using sparsity-promoting dynamic mode decomposition reveal a mutual mode between the airfoil shock and the nacelle shock. The existence of this mode has consequences for future investigations of nacelle airfoil interaction in transonic flow.

Numerical Investigations on the Fluid Behavior in the Near Wake of an Experimental Wind Turbine Model in the Presence of the Nacelle

Accurate predictions of the near wake of horizontal-axis wind turbines are critical in estimating and optimizing the energy production of wind farms. Consequently, accurate aerodynamic models of an isolated wind turbine are required. In this paper, the steady-state flow around an experimental horizontal-axis wind turbine (known as the MEXICO model) is investigated using full-geometry computational fluid dynamics (CFD) simulations. The simulations are performed using Reynolds-Averaged Navier-Stokes (RANS) equations in combination with the transitional k-kl-w turbulence model. The multiple reference frame (MRF) approach is used to allow the rotation of the blades. The impacts of the nacelle and blade rotation on the induction region and near wake are highlighted. Simulation cases under attached and detached flow conditions with and without the nacelle were compared to the detailed particle image velocimetry (PIV) measurements. The axial and radial flow behaviors at the induction region have been analyzed in detail. This study attempts to highlight the nacelle effects on the near wake flow and on numerical prediction accuracy under various conditions, as well as the possible reasons for these effects. According to simulation results, the rotation of blades dominates the near wake region, and including the nacelle geometry can improve both axial and radial flow prediction accuracy by up to 15% at high wind speeds. At low wind speeds, the nacelle effects can be ignored. The presence of the nacelle has also been shown to increase flow separation at the trailing edges of the blade airfoils, increasing both root and tip vorticities. Finally, small nacelle diameters are recommended to reduce flow separation on the blades and increase the average velocity downstream of the rotor, thereby optimizing wind farm output power.

Framework of nacelle inverse design method based on improved generative adversarial networks

Nowadays, inverse design has been widely used in aerodynamic design. However, the traditional inverse design method is limited to ensure the optimality, accuracy, and reality of the target distribution, which restricts the further development of the inverse design. To address these issues, the framework of nacelle inverse design based on improved Generative Adversarial Networks (GAN) is firstly proposed in this paper. The pressure distribution database obtained by CFD is used for training the GAN model, and the well-trained generator producing a large number of samples is employed for searching the optimal pressure distribution. Especially, in order to enhance the optimization ability of the traditional GAN model, an improved GAN model is proposed by modifying the loss function of the generator and discriminator. Pressure distribution-aerodynamic parameters ANN model and pressure distribution-geometry ANN model are established to evaluate the aerodynamic performance of the target distribution and obtain the nacelle geometry corresponding to the target distribution. The inverse design of the nacelle profile is carried out using the traditional GAN model and the improved GAN model and applied for the spin-forming and non-spin-forming nacelles, respectively. The results show that the inverse design framework with the improved GAN model achieves better optimal results.

Loss assessment of the NASA SDT configuration using LES with phase-lagged assumption

This paper presents the numerical study of the Source Diagnostic Test fan rig of the NASA Glenn (NASA SDT). Large-Eddy Simulations (LES) based on a finite volume approach are performed for the three different Outlet Guide Vane (OGV) geometries (baseline, low-count and low-noise) and three rotational speeds corresponding to approach, cutback and sideline operating conditions respectively. The full stage and nacelle geometries are considered in the numerical simulations, and results are compared to available measurements. The NASA SDT configuration is equipped respectively with 22 fan blades and either 26 of 54 vanes depending on the OGV geometry. The simulation domain could only be reduced to half of the full annulus and would still be a significant cost for the LES. In order to reduce computational cost, an LES with phase-lagged assumption approach is used. This method allows to perform unsteady simulations of multistage turbomachinery configurations including multiple frequency flows with a reduced computational domain composed of one single blade passage for each row. The large data storage required by the phase-lagged approach is handled by a compression method based on a Proper Orthogonal Decomposition replacing the traditional Fourier series decomposition. This compression method improves the signal spectral content especially at high frequency. Based on the numerical simulations, the flow field is described and used to assess the losses generated in the turbofan configuration based on an entropy approach. The results show different flow topologies for the fan depending on the rotational speed with a leading edge shock at high rotational speed. The fan boundary layer contributes strongly to losses with the majority of the losses being generated close to the leading edge for the dissipation due to mean strains and close to the recirculation zone occurring on the suction side for the turbulent kinetic energy production.

Investigation of the nacelle blockage effect for a downwind turbine

As downwind turbines garner increasing research interest, nacelle blockage becomes an important consideration. This paper examines nacelle blockage effects under a variety of Reynolds numbers, turbulence intensities, and nacelle geometries and proposes a computationally inexpensive nacelle blockage engineering model. Results show minimal nacelle blockage impacts on rotor loading and performance, except for tall (low-aspect-ratio) nacelles, which can cause an increase in rotor Cp of up to 0.6%. In all cases, nacelle blockage increases rotor loading and performance metrics, however little. Finally, (more expensive) geometry-resolved and (less expensive) body-force computational fluid dynamics modeling techniques are compared, and body-force models are found to need improvement to adequately model nacelle blockage.

Framework for estimation of nacelle drag on isolated aero-engines with separate jets

Typically, the evaluation of nacelle drag in preliminary design is required to find an overall optimum engine cycle and flight trajectory. This work focuses on the drag characteristics of aero-engine nacelles with separate jet exhausts. The main body of analysis comes from 3D numerical simulations. A new near-field method to compute the post-exit force of a nacelle is presented and evaluated. The effects of the engine size, Mach number, mass flow capture ratio and angle of attack are assessed. The results obtained from the numerical assessments were used to evaluate conventional reduced-order models for the estimation of nacelle drag. Within this context, the effect of the engine size is typically estimated by the scaling ratio between the maximum areas and Reynolds numbers. The effect of the angle of attack on nacelle drag is mostly a function of the nacelle geometry and angle of attack. In general, typical low-order models based on skin friction and form factor can underestimate the friction drag by up to 15% at cruise operating point. Similarly, reduced-order models based solely on Reynolds number, and Mach number can underestimate the overall nacelle drag by up to 74% for free stream Mach number larger than the drag rise Mach number.

Numerical analyses and optimizations on the flow in the nacelle region of a wind turbine

Abstract. The present study investigates flow dynamics in the hub region of a wind turbine focusing on the influence of nacelle geometry on the root aerodynamics by means of Reynolds averaged Navier–Stokes simulations with the code FLOWer. The turbine considered is a generic version of the Enercon E44 converter incorporating blades with flat-back-profiled root sections. First, a comparison is drawn between an isolated rotor assumption and a setup including the baseline nacelle geometry in order to elaborate the basic flow features of the blade root. It was found that the nacelle reduces the trailed circulation of the root vortices and improves aerodynamic efficiency for the inner portion of the rotor; on the other hand, it induces a complex vortex system at the juncture to the blade that causes flow separation. The origin of these effects is analyzed in detail. In a second step, the effects of basic geometric parameters describing the nacelle have been analyzed with the purpose of increasing the aerodynamic efficiency in the root region. Therefore, three modification categories have been addressed: the first alters the nacelle diameter, the second varies the blade position relative to the nacelle and the third comprises modifications in the vicinity of the blade–nacelle junction. The impact of the geometrical modifications on the local flow physics are discussed and assessed with respect to aerodynamic performance in the blade root region. It was found that increasing the nacelle diameter deteriorates the root aerodynamics, since the flow separation becomes more pronounced. Possible solutions identified to reduce the flow separation are a shift of the blade in the direction of the rotation or the installation of a fairing fillet in the junction between the blade and the nacelle.

Retrospective on the Common Research Model for Computational Fluid Dynamics Validation Studies

The development of a wing–body–nacelle–pylon–horizontal-tail configuration for a common research model is presented, with the focus on the aerodynamic design of the wing. A contemporary transonic supercritical wing design is developed with aerodynamic characteristics that are well behaved and of high performance for configurations with and without the nacelle–pylon group. The horizontal tail is robustly designed for dive Mach number conditions and is suitably sized for typical stability and control requirements. The fuselage is representative of a wide-body commercial transport aircraft; it includes a wing–body fairing, as well as a scrubbing seal for the horizontal tail. The nacelle is a single-cowl high-bypass-ratio flowthrough design with an exit area sized to achieve a natural unforced mass flow ratio typical of commercial aircraft engines at cruise. The simplicity of this un-bifurcated unpowered nacelle geometry facilitates experimental data collection and grid generation efforts for computational fluid dynamics (CFD) validations. Detailed aerodynamic performance data are provided; however, this information was originally presented in such a manner as to not bias CFD predictions planned for the fourth AIAA CFD Drag Prediction Workshop in 2009 (DPW-IV); the DPW-IV incorporated this common research model into its blind test cases. The fifth AIAA CFD Drag Prediction Workshop in 2012 and the sixth AIAA CFD Drag Prediction Workshop in 2016 also used this configuration as the basis of their test cases. The CFD results presented include wing pressure distributions with and without the nacelle–pylon Mach lift-to-drag ratio (ML/D) trend lines, as well as the drag-divergence curves. The design point for the wing–body configuration is within 1% of its maximum ML/D. The original plans to test the common research model in the National Transonic Facility at NASA Langley Research Center and the 11-by-11 ft wind tunnel at NASA Ames Research Center are discussed. Furthermore, wind-tunnel models have been replicated by the Japan Aerospace Exploration Agency, ONERA–The French Aerospace Lab, the University of Washington, as well as by numerous other laboratories. A Web site has been established to collect, archive, and freely provide wind-tunnel data from these various experimental campaigns. The geometry definition has been extended to include a high-lift system. It has been used to form the basis for experimental swept-wing icing studies. Finite element models have been developed for the wind-tunnel environment, as well as to be more representative of a full-scale aircraft in flight. A simple search for technical papers related to this configuration yields thousands of results. Needless to say, with a decade in retrospect, the NASA Common Research Model has significantly exceeded the initial most-optimistic goals, and it has become the de facto standard test case for present-day transonic CFD validations.

Uncertainty quantification and robust design optimization applied to aircraft propulsion systems

Abstract The standard way of formulating optimization problems applied to aircraft design is based on the assumption that the underlying system is deterministic, i.e., that the knowledge associated with the design variables and with the system dynamic is not characterized by uncertainty. However, in real conditions randomness impacts the formulation of the design process in multiple ways and the system outputs (i.e., the key performance indicators and the design constraints) are also affected by uncertainty. A system designed under deterministic assumptions may therefore have an unreliable behavior due to the fluctuations associated with the input random variables. This problem can be tackled by adopting a probabilistic approach and re-formulating the design optimization problem with an additional set of constraints associated with the robustness / reliability of the target system. This work addresses the problem of optimizing the geometry of a turbofan engine nacelle subject on reliability constraints. An advanced, machine-learning based framework is adopted in order to (a) investigate the system behavior through an adaptive design of experiments technique and (b) build accurate surrogate models of the system dynamics. These surrogate models are then employed to run a set of probabilistic studies at an affordable computational cost. The results of these investigations include (a) an extensive suite of analyses aimed at characterizing the uncertainty associated with the output quantities of interest; (b) a robust optimization of the engine nacelle geometry and (c) an assessment of the reliability of the optimized design. The improved performance and reliability of the design, together with the limited number of overall system evaluations required to run the analyses, demonstrate the effectiveness and the engineering applicability of the proposed approach.

Turbofan Broadband Noise Prediction Using the Lattice Boltzmann Method

The present work describes a numerical reproduction of the 22-in source diagnostic test fan rig of the NASA Glenn Research Center. Numerical flow simulations are performed for three different rotor/stator configurations and one rotational speed, representative of an approach operating condition, by using the lattice-Boltzmann solver PowerFLOW. The full stage and nacelle geometries are considered, and results are compared to available measurements. Tripping the rotor blades results in a slightly more accurate noise prediction as a consequence of a more accurate prediction of the velocity fluctuations in the rotor wake over the whole blade span. Fourier circumferential analyses are performed for an intake and a bypass duct section with the intent of explaining the origin of some tonal noise components and comparing the present results to available literature results. Finally, the effects of adding an acoustic treatment in the intake is shown by directly resolving the unsteady flowfield in a single-degree-of-freedom honeycomb layer.

Algorithms and analyses for stochastic optimization for turbofan noise reduction using parallel reduced-order modeling

Simulation-based optimization of acoustic liner design in a turbofan engine nacelle for noise reduction purposes can dramatically reduce the cost and time needed for experimental designs. Because uncertainties are inevitable in the design process, a stochastic optimization algorithm is posed based on the conditional value-at-risk measure so that an ideal acoustic liner impedance is determined that is robust in the presence of uncertainties. A parallel reduced-order modeling framework is developed that dramatically improves the computational efficiency of the stochastic optimization solver for a realistic nacelle geometry. The reduced stochastic optimization solver takes less than 500 s to execute. In addition, well-posedness and finite element error analyses of the state system and optimization problem are provided.

Study on Aircraft Lift Estimation

This chapter describes how lift can be derived from a known aircraft geometry and conversely, how the selection of aircraft layout influences the generation of lift. As the fuselage shape is predetermined by the payload requirements, the nacelle geometry by the engine size, and the tail surfaces by stability considerations, the wing parameters are the principal variables to be considered in respect to lift aspects. To this end, guidance is given on the selection of wing section profile and platform geometry to provide specified performance requirements. A lift estimation method covering all the aircraft components is also provided. Such methods are suitable for use in the conceptual design phase and provide an introduction to more detailed estimation procedures.

Computational analysis of the effects of a boundary layer ingesting propulsion system in transonic flow

Continuous requirements for more efficient aircrafts lead to the design and analysis of novel propulsion configurations, with an example being the boundary layer ingestion. The complexity and integration challenges in such aircraft synergistic propulsion system characterize the research in this field, driven by the potential benefits. The aim of this article is to investigate the effects of boundary layer ingestion on the aerodynamics of a transonic wing, together with the quality of the flow ingested by the propulsion system. A two-dimensional computational model of a transonic airfoil with boundary layer ingesting propulsion system is developed in order to assess boundary layer ingestion for a commercial air transport at cruise conditions and highlight the complex integration issues arising from such configuration. A parametric analysis of the effects of flight conditions, nacelle geometry and engine operating point, on lift, pressure recovery, distortion, total pressure and velocity distribution at the intake, comes to enhance understanding of the performance of this configuration. The pressure distribution around the airfoil and the boundary layer growth are both substantially affected by the engine operating condition, which is represented by the mass flow ratio, with a direct impact on pressure recovery and lift. Mach number and angle of attack influences on lift and drag ingested are also investigated. Intake size and position on the airfoil appear to have significant effects on lift and losses ingested. In general, the results of this study include several aspects related to wing aerodynamics and ingested flow quality, which may facilitate design and integration of the boundary layer ingestion propulsion system for future commercial aircraft.

2D and 3D numerical simulation of the wind-rotor/nacelle interaction in an atmospheric boundary layer

Two-dimensional axisymmetric and three-dimensional steady turbulent flow computations around two horizontal-axis wind turbines (Nordex N80 and Jeumont J48) are carried out to investigate the wind-rotor/nacelle interaction and quantify its effects on the wind speed at the nacelle anemometry. The actuator disk concept has been used to model the action of the blades. For both turbines, the geometry of the nacelle was reproduced as faithfully as possible. The terrain was represented by an appropriate law of the wall to account for roughness with particular attention paid to the boundary conditions in order to reproduce the neutral atmospheric boundary layer. The calculated velocity field in the vicinity of the nacelle exhibits good agreement with available experimental data. The results also show that for a complex nacelle geometry, like that of the N80, a three-dimensional calculation is necessary to obtain a good prediction of the velocity field in the near wake. The hub height effect is evaluated for the J48 by raising the nacelle from a height of 36 to 60 m. No significant impact is noted on the ratio nacelle wind speed/freestream wind speed.

Computational aero-acoustics for fan duct propagation and radiation. Current status and application to turbofan liner optimisation

Novel techniques are presented to reduce noise from turbofan aircraft engines by optimising the acoustic treatment in engine ducts. The application of Computational Aero-Acoustics (CAA) to predict acoustic propagation and absorption in turbofan ducts is reviewed and a critical assessment of performance indicates that validated and accurate techniques are now available for realistic engine predictions. A procedure for integrating CAA methods with state of the art optimisation techniques is proposed in the remainder of the article. This is achieved by embedding advanced computational methods for noise prediction within automated and semi-automated optimisation schemes. Two different strategies are described and applied to realistic nacelle geometries and fan sources to demonstrate the feasibility of this approach for industry scale problems.

Characterization of the unsteady flow in the nacelle region of a modern wind turbine

AbstractA three‐dimensional Navier–Stokes solver has been used to investigate the flow in the nacelle region of a wind turbine where anemometers are typically placed to measure the flow speed and the turbine yaw angle. A 500 kW turbine was modelled with rotor and nacelle geometry in order to capture the complex separated flow in the blade root region of the rotor. A number of steady state and unsteady simulations were carried out for wind speeds ranging from 6 m s−1 to 16 m s−1 as well as two yaw and tilt angles. The flow in the nacelle region was found to be highly unsteady, dominated by unsteady vortex shedding from the cylindrical part of the blades, which interacted with the root vortices from each blade, generating high‐velocity gradients. As a consequence, the nacelle wind speed and the nacelle flow angle were found to vary significantly with the height above the nacelle surface. The nacelle anemometry showed significant dependence on both yaw and tilt angles with yaw errors of up to 10 degrees when operating in a tilted inflow. Copyright © 2010 John Wiley & Sons, Ltd.

Evaluation of nacelle drag using computational fluid dynamics

Thrust and drag components must be defined and properly accounted in order to estimate aircraft performance, and this hard task is particularty essential for propulsion system where drag components are functions of engine operating conditions. The present work describes a numerical method used to calculate the drag in different nacelles, long and short ducted. Two- and three-dimensional calculations were performed, solving the Reynolds Averaged Navier-Stokes (RANS) equations with a commercial Computational Fluid Dynamics (CFD) code. It is then possible to obtain four drag components: wave, induced, viscous and spurious drag using a far-field formulation. An expression in terms of entropy variations was shown and drag for different nacelle geometries was estimated.

A review of CAA for fan duct propagation and radiation, with application to liner optimisation

This article reviews the application of Computational Aero-Acoustics (CAA) to the prediction of acoustic propagation in turbofan ducts. The challenge of resolving the radiated sound field for realistic nacelle geometries is discussed and evidence is presented to indicate that current CAA is able to represent the effect of in-duct liners on far-field measured rig and engine data. Two approaches are presented for incorporating CAA predictions within liner optimisation schemes for a turbofan intake.