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Abstract: For the evaluation of future aircraft architectures or novel technologies, a well-understood and sound reference to compare to is essential to prove the impact of intended changes. A reference aircraft serves as a basis for calibration of the aircraft and is used as a starting point for subsequent investigations, sensitivity studies or optimizations. The prime need for such a reference arose from the European Clean-Sky-2 project HLFC-Win (Hybrid Laminar Flow Control—Wing), where no industrial reference data for a conventional aircraft were available. The project investigates a HLFC system integrated into the wing aiming to reduce the wing drag and consequently reducing the fuel consumption. Therefore, this paper presents the D300-XRF1 designed by DLR, which is based on the AIRBUS research aircraft XRF1 (eXternal Research Forum). It is intended to be used as a reference aircraft database representing aircraft for medium- and long-range missions. A variable fidelity multidisciplinary design analysis and optimization (MDAO) aircraft design environment is set up to provide a consistent estimate of the geometry, mass breakdown, propulsion system, aerodynamics and aircraft performance. A conceptual aircraft design tool is utilized in the aircraft design environment to initialize the process, and higher-fidelity modules are used to enhance the results. The presented results describe the overall aircraft characteristics of the D300-XRF1 based on 2010 entry into service technology level and is offered to be used as a reference within the aviation research community, hopefully reducing similar design efforts in other research projects.The aircraft operates at a design cruise Mach number of 0.83, has a design range of 5500 NM and transports a payload of 31.5 t (300 PAX at 105 kg/PAX). The D300-XRF1 serves as a consistent reference aircraft database and provides a holistic overview of the aircraft’s performance that has been presented to and approved by AIRBUS. Therefore, this reference aircraft design can be used for future studies and to assess new technologies on a sophisticated level. PubDate: 2023-05-18
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Abstract: Recent developments in the field of data analysis offer enormous potential to derive predictions of future aircraft maintenance necessities from observable aircraft attributes. This procedure is called Predictive Maintenance. The Predictive Maintenance generates considerable business benefits compared to alternatives like reactive or preventive maintenance for various reasons. Nevertheless, recognizing a maintenance necessity is only the first step, since the execution of the maintenance action has to fit into the airlines operational schedule. An essential step to realize financial benefits is to integrate Predictive Maintenance into the airlines optimization processes. To contribute to this task, we expand a well-known Tail Assignment model for the assignment of a potentially heterogeneous fleet of aircraft to a schedule of flights by a number of part failure scenarios. This results in a stochastic mixed integer linear optimization program, for which an optimization algorithm with solution guarantees is developed. This optimization algorithm is based on Benders Decomposition, which is concretized for the Tail Assignment problem and optimized for this task. Using this algorithm, a large proportion of the recovery costs for almost all instances tested is saved. The algorithm solves all instances in a reasonable amount of time. The algorithm uses a state-of-the-art mixed-integer linear solver, implementing a decomposition-based solution procedure for stochastic programs, called the L-shaped method. To demonstrate the potential of the approach, we benchmark our results using four strategies, motivated by commonly used preventive, reactive and passive approaches. Our approach leads to considerable cost savings when compared to each of the four benchmark approaches. The algorithms are tested on a set of instances with up to 80 flights based on a pre-pandemic schedule of a larger German airline. To the best of our knowledge, this article presents the first attempt to implement an exact optimization approach integrating malfunction predictions of the form presented into a Tail Assignment model. PubDate: 2023-05-13
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Abstract: This report describes the approach to create permanently updated 3D models of research aircraft and laboratory facilities. Therefore, optical metrology scans the research environment in its raw or as-delivered condition. The result is a virtual model of the actual geometry and, in comparison to reference data (e.g. CAD-data), the smallest inaccuracies can be identified and analyzed. The exact position of non-rigid components, like riser ducts, electronics or isolation, can be determined in the models. Further changes to the layout of these facilities are permanently digitized and added to the virtual model of the environment. This can be a new recording of the entire facility or of individual areas that are affected by the changes. The individual, newly recorded models are then integrated into the existing model. This creates an always up-to-date 3D model of the research environment, which is added to its digital twin and can be observed there. In combination with CAD data, future conversion and installation measures are planned in advance and analyzed virtually in relation to the up-to-date geometry and installation space data. In addition, the virtual models of the aircraft cabins can be used to support the lengthy approval and certification process at an early stage. PubDate: 2023-05-09
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Abstract: Urban Air Mobility has the potential to substantially reduce travel times in some cases of urban-related transportation. Travel time savings strongly depend on fast processing at vertiports, which presents a key challenge considering demand levels’ vertiports would experience when becoming an established mode of transport. This article sheds light on the passenger throughput vertiport airfields can manage and how the operations are sensitive to changes. One main contribution of this article is the introduction of hourly passenger throughput per area as a performance indicator that allows to compare vertiports of different sizes. VoloCity is studied as a reference vehicle and the resulting space requirement of the carefully specified baseline scenario is 188 square-meters per passenger per hour. A total of 13 prominent eVTOL designs are included in the study from which the current design space between maximum vehicle dimension and number of seats is deducted. The study shows that vehicles with a small maximum dimension yield the highest passenger throughput capacity. CityAirbus performs best (46.3 m2/PAX/h) with a diameter of 7.92 m and Archer Maker performs worst (221 m2/PAX/h) with a diameter of 12.2 m. How the performance indicators can be used as rules-of-thumb in the first-order estimations of vertiport throughput capacity or space requirement is described by means of illustrative examples. The insights presented in this paper might be useful for researches, vehicle developers, and municipalities alike. PubDate: 2023-05-06
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Abstract: The North Atlantic is one of the world’s airspaces accommodating a very high aircraft density while at the same time no radio coverage or radar surveillance is available. Beside satellite communication, one approach to enable data communication between aircraft and ground in the North Atlantic region is to establish ad-hoc networks build up by direct data links between the aircraft that are acting as communication nodes. In this paper we, therefore, present a modeling approach to model air traffic and ad-hoc networks in the North Atlantic region using up-to-date flight plans and trajectory modeling techniques and to assess the connectivity provided by such networks. Assuming an applicable set of ground stations that provide data transfer to and from this airborne network, we assess the connectivity by time-series analysis and in total for a set of different fractions of all aircraft assumed to be equipped with the necessary systems as well as for a variation of the air-to-air communication range. In addition, we present average link durations, average amounts of hops to reach ground and numbers of connected aircraft for the different scenarios and identify general relations between the different factors and metrics. We will show, that communication range and equipage fraction significantly influence the connectivity of such networks. PubDate: 2023-05-02
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Abstract: A gradient-based aeroelastic shape optimization framework making use of a reduced order model to substitute a parameterization based on computer-aided design software is presented. This parameterization concept is not novel in principle, but it is embedded here in a complex high-fidelity optimization process and proven for a high-dimensional design space. The design software is used initially to generate a parametric model of a three-dimensional transport aircraft configuration. To streamline the actual optimization process, the computer-aided design model is replaced with a parametric reduced order model based on proper orthogonal decomposition that is capable of predicting discrete surface displacement fields as a function of the design parameters. During the optimization, surface displacements are computed according to the current design parameters and applied on the baseline shape. In every optimization step, the aircraft's steady-state equilibrium of forces and moments are satisfied by a trimming algorithm and the Reynolds-averaged Navier–Stokes solver TAU is coupled with a linear structural finite-element method model. Gradients are computed analytically using geometric sensitivities provided by the reduced order model and by applying the adjoint method to the flow solver and the mesh deformation tool. The workflow is embedded within FlowSimulator, a multiphysics environment for high performance computing. The optimization process is demonstrated for a high-dimensional wing parameterization with 126 degrees of freedom. The aircraft cruise drag could be significantly reduced by 6% on a series of three continuously refined meshes for the aerodynamic analysis. For an accurate representation of the optimal shape by the computer-aided design software after the optimization, the approximation error introduced by the reduced order modelling approach must be sufficiently small. Therefore, the accuracy of the predictions was analyzed. The results identify the main source of the geometric error and quantify their effect on the drag reduction gained by the optimization. We dedicate this article to the memory of our colleague and friend Arno Ronzheimer, whose devotion to CAD modeling was unsurpassed. PubDate: 2023-04-24
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Abstract: A novel process to determine an aircraft performance model from operational flight data with limited a priori knowledge is developed. The given big data problem is solved by application of fundamental engineering knowledge and a specific data evaluation strategy. The resulting smart data approach is fundamentally different from existing deep learning methods to solve such big data problems. A given aerodynamic model is updated to represent the characteristics of an Airbus A320neo aircraft based on a given large database of operational flights. The updated aerodynamic model implementation for one specific flap/slat configuration is exemplarily compared to the information available from flight data and the results are discussed in terms of model quality. PubDate: 2023-04-20
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Abstract: A low-noise low-pressure ultra-high-bypass-ratio fan stage to be implemented in the next generation of aircraft engines is described and evaluated acoustically with semi-empirical and analytical methods suited for preliminary design. As expected, good reduction potentials are observed for the jet noise and fan tonal noise components when the UHBR design is compared to current fans in service. However, concerns are identified for fan broadband noise, which are attributed to the off-design operation of the UHBR fan too close from its stability limit. By unloading the fan and thus reducing the size of the rotor wakes, the variable-area nozzle provides a substantial fan broadband noise reduction with a nozzle opened by around 15% from its design value. Alternatively, with the variable-pitch fan, closing the rotor blades by roughly 5° turns out to be an even more effective method to reduce fan noise, as the unloading mechanism is combined with a stronger tilting of the rotor wakes and a lower intra-stage flow Mach number. Opening the nozzle or closing the blades beyond the setting that provides the best fan efficiency is not recommended as the acoustic benefit progressively vanishes, whereas technical feasibility becomes more challenging. Finally, the presence of one of these systems may allow for the design of a low-solidity rotor, with a smaller contribution from the rotor wakes and thus a weaker fan noise emission. PubDate: 2023-04-13
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Abstract: Electric airplanes powered by fuel cells are a promising alternative for green aviation. However, many technological obstacles still have to be overcome for this type of aircraft to be commercially viable. For example, the relatively low output power capacity of fuel cells still constrains the application of these devices to power heavier aircraft. These limitations make performance optimization an even more important subject. This article presents a novel formulation for the analysis of electric current, fuel and oxidizer consumption of fuel-cell-powered airplanes driven by fixed-pitch propellers. Due to its simplicity, lighter weight and lower cost, the fixed-pitch propeller is widely used on aircraft with small flight speed variation. Also, since the efficiency of this type of propeller is known to vary with airspeed, a dedicated method is necessary. A parametric description of the relevant aircraft systems is developed, and expressions for instantaneous and total consumption are formulated. Optimal conditions for these performance metrics are investigated, and analytical solutions are derived and presented in closed form. The formulation employs a parametric model for the fixed-pitch propeller and eliminates the need to determine the motor and propeller rotation speed. A numerical approach for method validation is used, and simulations are provided to give the reader a quantitative insight into the problem. The results show that the optimal speeds for minimum fuel consumption of fuel-cell-powered aircraft driven by fixed-pitch propellers can be higher than what is predicted by traditional methods. Mathematical proofs are provided to show that this can happen. PubDate: 2023-04-12
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Abstract: The eVTOL industry is a rapidly growing mass market expected to start in 2024. eVTOL compete, caused by their predicted missions, with ground-based transportation modes, including mainly passenger cars. Therefore, the automotive and classical aircraft design process is reviewed and compared to highlight advantages for eVTOL development. A special focus is on ergonomic comfort and safety. The need for further investigation of eVTOL’s crashworthiness is outlined by, first, specifying the relevance of passive safety via accident statistics and customer perception analysis; second, comparing the current state of regulation and certification; and third, discussing the advantages of integral safety and applying the automotive safety approach for eVTOL development. Integral safety links active and passive safety, while the automotive safety approach means implementing standardized mandatory full-vehicle crash tests for future eVTOL. Subsequently, possible crash impact conditions are analyzed, and three full-vehicle crash load cases are presented. PubDate: 2023-04-05
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Abstract: The geometric assessment of physical demonstrators are an integral part of several research projects conducted at the Chair of Aircraft Design at the Technical University of Munich. The projects range from several research UAVs, a sailplane morphing wing to propellers. There are different project objectives like the assessment of manufacturing deviations, design and function validation as well as reverse engineering of aerodynamic surfaces for model adaptation and simulation in the projects. Nevertheless, mutual approaches and solutions have been identified. Therefore joint development efforts are undertaken using 3D-scanning technology for data collection and evaluation. This technology captures the surface of a given object typically as a point cloud with comparably high accuracy. Since a manual evaluation process bears disadvantages in terms of reproducibility, custom post-processing software tools are developed. Global geometry data, like wing platform data, as well as airfoils can be extracted from a surface point cloud to analyze UAV wings or propellers. Airfoils can be derotated, normed and smoothed for aerodynamic analysis with low-fidelity aerodynamic tools, such as XFLR5 or XFOIL. For the analysis of morphing airfoil structures, the scanned geometry is aligned with the desired design airfoil shape so they can be compared. In this paper, analysis methods and several example results are presented. PubDate: 2023-04-03
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Abstract: Information exchange via aeronautical data communication is of increasing importance for the communication between pilots and air traffic control, providing the basis for surveillance of aircraft in oceanic or remote airspaces, as well as enabling the communication between an airlines’ fleet and its operational control. The aeronautical data communication that is being transmitted via data link encompasses, among others, surveillance-related aircraft position updates, clearances for flight path change requests, maintenance-related status reports, estimated arrival times and weather information in accordance with the required performance, that is set by technical standards. Aeronautical data communication events are driven by an aircraft’s flight phase, the current airspace or may occur in a randomized manner throughout the flight. As the usage of aeronautical data communication is expected to grow in future, ample data link technologies are being evaluated and developed. The usability and operational value of new data link technologies for aeronautical applications can be evaluated by applying suitable models of the respective data link communication pattern combined with operational simulations. Current models for aeronautical data communication demand support the design and evaluation of direct aircraft-to-ground communication networks. The geographical location of the data communication demand is secondary for these models, since coverage areas can be defined, where direct communication links are available. New data link technologies offer the opportunity of direct data transfer between aircraft and forwarding of messages from sending aircraft to a ground entity via ad-hoc communication networks between aircraft. This is of special interest for the North Atlantic oceanic airspace, an airspace with high traffic density and little ground infrastructure, where communication currently relies mostly on satellite-based systems. For these airborne ad-hoc networks the definition of coverage areas around ground or space-based entities is not possible. To assess the new data link technology a model for aeronautical data link communication demand is needed, that accounts for operationally derived communication events such as handover procedures at boundaries of oceanic control areas or status reports at route-specific waypoints, which cause an accumulation in certain geographical regions that pose a challenge to the dynamic connectivity coverage of aeronautical ad-hoc networks. Addressing this issue, we present a new modelling approach for air traffic service communication that considers the operational context of the simulated airspace and provides a geospatial data communication demand distribution, which is derived from air traffic management procedures, airspace geometries and events inherent to each flight path. The air traffic service communication is then validated based on 2019 air traffic and performance monitoring data provided by ICAO supplemented by an existing model for communication related to airline operational control. In a next step, the communication demand per area in the North Atlantic Oceanic airspace is being assessed. The aeronautical data traffic model shows deviations of less than one message per aircraft and airspace when compared to recorded data traffic from 2019 for two key services in the most frequented North Atlantic oceanic control areas. Therefore, it is assumed to be suitable for the evaluation of network-based data link technology and operational impact assessments. PubDate: 2023-03-25
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Abstract: The aerodynamic performance of propellers strongly depends on their geometry and, consequently, on aeroelastic deformations. Knowledge of the extent of the impact is crucial for overall aircraft performance. An integrated simulation environment for steady aeroelastic propeller simulations is presented. The simulation environment is applied to determine the impact of elastic deformations on the aerodynamic propeller performance. The aerodynamic module includes a blade element momentum approach to calculate aerodynamic loads. The structural module is based on finite beam elements, according to Timoshenko theory, including moderate deflections. Several fixed-pitch propellers with thin-walled cross sections made of both isotropic and non-isotropic materials are investigated. The essential parameters are varied: diameter, disc loading, sweep, material, rotational, and flight velocity. The relative change of thrust between rigid and elastic blades quantifies the impact of propeller elasticity. Swept propellers of large diameters or low disc loadings can decrease the thrust significantly. High flight velocities and low material stiffness amplify this tendency. Performance calculations without consideration of propeller elasticity can lead to decreased efficiency. To avoid cost- and time-intense redesigns, propeller elasticity should be considered for swept planforms and low disc loadings. PubDate: 2023-03-21 DOI: 10.1007/s13272-023-00649-y
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Abstract: In the present study, a hybrid deep learning reduced-order model (ROM) is applied for the prediction of wing buffet pressure distributions on a civil aircraft configuration. The hybrid model is compound of a convolutional variational neural network autoencoder (CNN-VAR-AE) and a long short-term memory (LSTM) neural network. The CNN-VAR-AE is used for the reduction of the high-dimensional flow field data, whereas the LSTM is applied to predict the temporal evolution of the pressure distributions. For training the neural network, experimental buffet data obtained by unsteady pressure sensitive paint measurement (iPSP), is applied. As a test case, the Airbus XRF-1 configuration is selected, considering two different experimental setups. The first setup is defined by a wind tunnel model with a clean wing, whereas the second setup includes an ultra high bypass ratio engine nacelle on each wing. Both configurations have been tested in the European Transonic Windtunnel, considering several transonic buffet conditions. Finalizing the training of the hybrid neural networks, the trained models are applied for the prediction of buffet flow conditions which are not included in the training data set. A comparison of the experimental results and the pressure distributions predicted by the hybrid ROMs indicate a precise prediction performance. Considering both aircraft configurations, the main buffet flow features are captured by the hybrid ROMs. PubDate: 2023-03-14 DOI: 10.1007/s13272-023-00641-6
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Abstract: Generating turbulent inflow data is a challenging task in zonal large eddy simulation (zLES) and often relies on predefined DNS data to generate synthetic turbulence with the correct statistics. The more accurate, but more involved alternative is to use instantaneous data from a precursor simulation. Using instantaneous data as an inflow condition allows to conduct high fidelity simulations of subdomains of, e.g. an aircraft including all non-stationary or rare events. In this paper, we introduce a toolchain that is capable of interchanging highly resolved spatial and temporal data between flow solvers with different discretization schemes. To accomplish this, we use interpolation algorithms suitable for scattered data in order to interpolate spatially. In time, we use one-dimensional interpolation schemes for each degree of freedom. The results show that we can get stable simulations that map all flow features from the source data into a new target domain. Thus, the coupling is capable of mapping arbitrary data distributions and formats into a new domain while also recovering and conserving turbulent structures and scales. The necessary time and space resolution requirements can be defined knowing the resolution requirements of the used numerical scheme in the target domain. PubDate: 2023-03-09 DOI: 10.1007/s13272-023-00638-1
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Abstract: A new converter technology allows suitably equipped aircraft to use data provided by the satellite-based augmentation system in receivers originally designed for the ground-based augmentation landing system. For these aircraft, that system enables a lower decision altitude and, hence, improved access to airports. To make this technology usable, air crews require an operational concept and the flight crew has to be presented with the appropriate information in the form of approach charts. Two different possibilities for an operational concept were developed and the corresponding approach charts created. One option is a modified area navigation approach chart, to which the specific information is added. The other chart is an entirely separate procedure for the approach. These two options were tested with airline pilots in an Airbus A320 full-flight training simulator. During the simulator flights, aircraft performance data was recorded and the participants filled in questionnaires regarding workload and quality of the operational concept. The results show different behavior during the intercept of the final course, but all approaches remained within the required limits. The questionnaires revealed that the workload is higher during the area navigation variant and that all participants prefer the separate ground-based augmentation landing system variant. PubDate: 2023-03-01
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Abstract: The paper investigates the effect of inter-blade dampers with generic in-plane and out-of-plane attachment offsets on ground resonance stability proneness. An analytical formulation, considering dampers with radial offsets only is initially proposed. Sensitivity analyses show that the increase of radial offset reduces the cyclic lead-lag damping and stiffness, providing a non-zero contribution to collective terms. The analytical formulation is suitable, in a preliminary design phase, to define the optimal location of the inter-blade attachment points to avoid ground resonance phenomena and to stabilize the engine drive-train dynamics. A more detailed numerical approach is then presented to consider generic in-plane and out-of-plane attachment offsets. Ground resonance stability analyses are performed also for cases with dissimilar dampers. It is found that out-of-plane offset leads to a modification on the blade pitch-lag coupling, acting on the helicopter stability margins. However, to capture these effects it is necessary to include the overall blade motions, considering flap, lag, and pitch dynamics, together with the corresponding generalized aerodynamics forces, usually neglected in classical ground resonance analysis. Finally, the periodic stability with one damper inoperative shows how, with the radial offsets, the hybridized lead-lag collective and cyclic modes may fall into resonance conditions due to super-harmonics. PubDate: 2023-03-01
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Abstract: Small unmanned aerial vehicles are a threat for manned aviation. Their increased use by hobby pilots and within the commercial sector has been accompanied by an increase in incidents involving manned aircraft. The problem is that current aircraft structures are designed to resist collisions with birds. They are not designed to withstand drone impacts. The composition of drones differs significantly from previously known load cases. Drones consist of several components with various materials. This means, that there is no analytic model to determine the impact force of such drone strikes with aircraft structures. Within this work, a novel reduced order model for drone impacts is developed. It is validated with high velocity impact test data and explicit finite element simulations. The impact of fragmenting components of the drone are modelled with the aircraft impact model. The impact of non-fragmenting components like motors are described with a spring-mass model. The results show that the approach of superimposing a spring-mass model with the aircraft impact model leads to good results. In case of a rigid target only minor deviations occur within the validity range of the model. Damage and degradation of the target is not included in the model what leads to larger deviations in case of an impact with deformable structures. Nevertheless, the model is very well suited for rapid load estimation and can qualitatively reproduce contact force curves. It can be used for preliminary design of aircraft structures without conducting time and cost intensive tests and simulations. PubDate: 2023-03-01
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Abstract: The present paper proposes a method for analyzing reinforced thin-walled structures based on high-order one-, two- and three-dimensional finite elements (FE). Refined finite elements are developed in the domain of the Carrera unified formulation (CUF). The node-dependent kinematic approach (NDK), which allows to connect in an easy manner elements with incompatible kinematics, has been used to connect elements with different dimensions without the need of ad hoc connection techniques. The formulation ensures the continuity of the displacement at the interface preventing the onset of singularities that lead to inaccurate results when beam, plate and solid elements have to be coupled to solve complex structures. The effectiveness of the present method has been confirmed by comparing the results with those from literature and with those obtained using commercial finite element codes. Static and free-vibration analyses of reinforced panels have been carried out to demonstrate the capabilities of the present models. The results show that the limits of classical structural models can be easily overcome using the present approach, and at the same time, a quasi three-dimensional solution can be obtained with a large computational cost saving. PubDate: 2023-02-27 DOI: 10.1007/s13272-023-00648-z
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Abstract: The dispersion of aerosols originating from one source, the ‘index’ passenger, within the cabin of the aircraft Do728 is studied experimentally using an aerosol-exhaling thermal manikin and in Reynolds-averaged Navier–Stokes simulations (RANS). The overall aim of the present study is the experimental determination of the aerosol spreading for the state-of-the-art mixing ventilation (MV) and to evaluate the potential of alternative ventilation concepts for controlling the aerosol spreading in RANS. For MV, the experiments showed that the ratio of inhaled to exhaled aerosol particles drops below 0.06% (volume ratio) for distances larger than two seat rows from the source. However, within a single row, the observed ratio is higher. Further, the dispersion is much weaker for a standing than for a seated index passenger. High air exchange rates and a well-guided flow prevent a dispersion of the aerosols in high concentrations over larger distances. Additionally, the positive effect of a mask and an increased air flow rate, and especially their combination are shown. In the complementary conducted RANS, the advantages of floor-based cabin displacement ventilation (CDV) which is alternative ventilation concept to MV, regarding spreading lengths and the dwell time of the aerosols in the cabin were determined. The obtained results also underline the importance of the flow field for the aerosol dispersion. Further, additional unsteady RANS (URANS) simulations of the short-term process of the initial aerosol cloud formation highlighted that the momentum decay of the breathing and the evaporation processes take place within a few seconds only. PubDate: 2023-02-15 DOI: 10.1007/s13272-023-00644-3
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