François Garnier

Analysis of Wake Vortex Decay Mechanisms in the Atmosphere

Abstract Results of high-resolution numerical simulations of aircraft wake vortex evolution and decay in different regimes and atmospheric conditions are presented. The different cases comprise (i) the near field interaction of a trailing vortex with an exhaust jet, (ii) the evolution of single vortices and counter-rotating vortex pairs in homogeneous isotropic turbulence, as well as (iii) the decay of wake vortices in a turbulent stably stratified atmosphere, and (iv) in a weakly turbulent sheared environment. The different cases are used to analyse common aspects of vortex dynamics and decay mechanisms. In all scenarios the formation of coherent secondary vorticity structures that enclose the primary vortices is observed. These secondary vorticity structures deform and weaken the primary vortices and in some cases lead to rapid vortex decay. It is shown that the mean swirling flow effectively rearranges and intensifies any secondary vorticity by tilting and stretching. The secondary vorticity may either originate from the turbine jet, ambient turbulence or may be produced baroclinically. Based on the observed phenomena, eleven postulates are established that pinpoint fundamental aspects of the observed decay mechanisms.

Numerical Investigation of Turbulent Mixing in a Jet/Wake Vortex Interaction

A three-dimensional temporal numerical simulation of the interaction between a jet and a vortex wake is described to contribute to the understanding and modeling of engine jet dispersion of an aircraft in cruise condition. First, a comparison of the numerical results with a wind-tunnel experiment is presented. Experimental data comprise turbulent rates measured by a laser Doppler velocimeter and mean temperature e elds delivered by thermocouples. Thecalculation isin quite good agreement with the experiment. Second, the passivescalar distribution (initially included in the jet )in the vortex wake is focused on. The turbulence induced by the jet e owe eld interacts with the vortex, resulting in large-scale structures generated outside the vortex core, whereas the region very close to the core keeps its laminar state. The passive scalar concentrates in the large-scale structures where azimuthal and axial (positive and negative ) vortices are combined. The resulting scalar e eld distribution shows no scalar inside the vortex core.

Large-eddy simulation of a turbulent jet and a vortex sheet interaction : particle formation and evolution in the near field of an aircraft wake

Aircraft are prolific sources of particles (soot, liquid aerosols and contrails) that can impact cloudiness and affect the Earths radiative budget balance. In order to study the formation and evolution of these particles, a numerical approach has been developed combining large-eddy simulation (LES) and a detailed microphysical model. Generally very detailed microphysical models are run along a single average trajectory, without any temperature fluctuation. However, this approach may lead to significant differences in particle properties and particle size distribution as it oversimplifies dynamical and mixing processes compared to multidimensional descriptions of aircraft wakes. This may affect the initialisation of meso-scale models, such as, for example, the formation of cloud condensation nuclei from persistent contrails, and heterogeneous chemical reactions. In this paper, we present the results of detailed microphysical processes calculations applied to a large number of fluid parcels trajectories, generated by a LES two-phase flow solver.

Spatial Simulation of Contrail Formation in Near-Field of Commercial Aircraft

During this study, three-dimensional numerical simulations of contrails generated by commercial aircraft in cruise conditions were performed. The objective was to study the early development of contrails in the near field of an aircraft, including engine core and bypass flows. Computational fluid dynamics simulations, based on the three-dimensional Reynolds-averaged Navier–Stokes approach, were carried out on a realistic aircraft geometry. A microphysical model was implemented in the computational fluid dynamics code to simulate particle growth using an Eulerian approach. Results showed that the mixing processes in the bulk plume were in good agreement with the literature. Then, the early growth of a contrail in the near wake of the aircraft was investigated for two ambient relative-humidity conditions. This parameter played a significant role in contrail local properties. Increased relative humidity led to an increase in the fraction of particles under the supersaturated condition and condensation rate. ...

Calibration strategy of diesel-fuel spray atomization models using a design of experiment method

The Reitz and Diwakar and the KHRT atomization models are widely used for high-pressure diesel-fuel spray. The constants in both models must be calibrated to correctly predict the injection process based on the nozzle geometry, injection conditions, and fuel. Calibration can be significantly time-consuming given the four constants in both models. This paper suggests a strategy to assess the impact of models’ constants on spray tip penetration and mean droplet-diameter predictions on a reference case, with an injection pressure of 700 bar, to characterize the influence of the atomization model’s calibration. The assessment used a design of experiment method (DOE), which demonstrated the important interaction between constants on the results. Obtained calibrations were used for comparing the models’ performances qualitatively and quantitatively by accounting for spray and air-entrainment characteristics. Both models gave similar results, but the KHRT model yielded a better spray shape. Finally, based on DOE results, a method is proposed to modify the model’s constants for higher pressures (900 and 1300 bar).

Effect of Compressibility on Contrail Ice Particle Growth in an Engine Jet

Abstract In order to understand the formation process of condensation trails (contrails), the flow in the near field of an aircraft engine jet is studied by using the three-dimensional Large Eddy Simulation technique. The configuration consists of a hot round jet laden with soot particles. The particles are tracked using the Lagrangian approach, and their growth is calculated by a microphysics water vapour deposition model. A series of simulations are performed at a realistic Reynolds number (Re = 3.2 · 106) for two different jet Mach numbers: quasi-incompressible jet flow (M = 0.2) and compressible jet flow (M = 1). Whatever the Mach number used the ice crystals first appear at the edges of the jet where the hot and moist flow mixes with the cold and dry ambient air. Both the thermal transfers and the mass coupling, which are more significant for the quasi-incompressible jet flow, control the growth process.

Numerical simulation of aerosols in an aircraft wake using a 3D LES solver and a detailed microphysical model

The global impact of aviation on the atmosphere is generally determined using the total amount of gas and particulate matter emitted at the aircraft engine exit but generally ignore some of the physical transformations occurring at much smaller scales in the aircraft wake. In this work, we present an offline alternative method based on the use of a detailed plume aerosol model combined to fluid trajectories calculated from 3D large-eddy simulations (LESs). The study has been limited to the first 10 s behind a type Airbus 340 aircraft. The results have been compared to those obtained from a one-way coupling approach including a simple microphysics water vapour deposition model on soot cores. The respective evolutions of average ice particles radius are in good agreement. Furthermore, different types of aerosol properties are examined including the charged volatile particles, the dry and activated soot and the ice crystals from homogeneous and heterogeneous freezing. The variability of the aerosol size distribution clearly illustrates the influence of the mixing, as a function of the position in the aircraft plume. Finally, the volatile particles distribution exhibits a bimodal shape resulting from the presence of charges, in agreement with that observed experimentally.

Automated thermal and stress preliminary analyses applied to a turbine rotor

Abstract The use of Multidisciplinary Design Optimization (MDO) techniques at the preliminary design phase (PMDO) of a gas turbine engine allows investing more effort at the pre-detailed phase in order to prevent the selection of an unsatisfactory concept early in the design process. Considering the impact of the turbine tip clearance on an engines efficiency, an accurate tool to predict the tip gap is a mandatory step towards the implementation of a full PMDO system for the turbine design. Tip clearance calculation is a good candidate for PMDO technique implementation considering that it implies various analyses conducted on both the rotor and stator. As a first step to the development of such tip clearance calculator satisfying PMDO principles, the present work explores the automation feasibility of the whole analysis phase of a turbine rotor preliminary design process and the potential increase in the accuracy of results and time gains. The proposed conceptual system integrates a thermal boundary conditions automated calculator and interacts with a simplified air system generator and with several design tools based on parameterized CAD models. Great improvements were found when comparing this works analysis results with regular pre-detailed level tools, as they revealed to be close to the one generated by the detailed design tools used as target. Moreover, this design process revealed to be faster than a common preliminary design phase while leading to a reduction of time spent at the detailed design phase. By requiring fewer user inputs, this system decreases the risk of human errors while entirely leaving the important decisions to the designer.

Numerical Prediction of Gaseous Aerosol Precursors and Particles in an Aircraft Engine

Aviation-produced particulate matter has a direct impact on climate, atmospheric composition at flight altitudes, and local air quality in the vicinity of airports. The formation of soot and gaseous aerosol precursors inside the combustor and during gas expansion in turbine stages and nozzles must be addressed before the real impact of aircraft engines with respect to particulate matter emissions can be assessed. To design strategies to reduce particulate matter emissions, the development of a zero-/one-dimensional gas-turbine model is proposed, taking into account combustor and postcombustor flow operating over the landing/takeoff cycles with a detailed kerosene jet-A1 kinetics scheme, including a soot-dynamics model. This approach is very efficient computationally and may be clearly satisfying for parametric studies or in a predesign step. First, the model’s predictive capacity for capturing the main features of gas-turbine combustion as well as the expansion of combustion products in the turbine and no...

A Preliminary Design System for Turbine Discs

In order to improve product development cycle, design engineers use multi-disciplinary analysis tools which allow better productivity. This paper covers the development of new tools to improve the preliminary design phase of turbine disc, being a critical part of aircraft engines. First, a new single platform D&A (Design & Analysis) tool integrating commercial CAD (Computer Aided Design) and FEA (Finite Element Analysis) software processing in batch mode is presented. This integrated architecture leads to a real improvement enabling a cohesive single integrated simulation environment that offers significant time reduction on user manipulation and execution. An optimization of disc geometry is then performed by using different optimization algorithms and configurations for a given disc parameterized model. The results show potential improvement over the current preliminary rotor discs for life and burst limited design. Finally, optimal curves obtained by developing HPT (High Pressure Turbine) disc reference charts, indicate how to get the minimum weight for given mechanical performance without running any structural analysis. These new tools supporting disc design have allowed improvement of disc life and durability leading to reduction of preliminary design phase duration.© 2014 ASME