Laurent Bricteux

Pulsed 1.5-

In this paper, we present the development of an axial aircraft wake vortex light detection and ranging (LIDAR) sensor, working in Mie scattering regime, based on pulsed 1.5-mu m high-brightness large-core fiber amplifier. An end-to-end Doppler heterodyne LIDAR simulator is used for the LIDAR design. The simulation includes the observation geometry, the wake vortex velocity image, the scanning pattern, the LIDAR instrument, the wind turbulence outside the vortex, and the signal processing. An innovative high-brightness pulsed 1.5-mum laser source is described, based on a master oscillator power fiber amplifier (MOPFA) architecture with a large-core fiber. The obtained beam quality is excellent (M 2 = 1.3), and achieved pulsed energy is 120 muJ with a pulse repetition frequency of 12 kHz and a pulse duration of 800 ns. A Doppler heterodyne LIDAR is developed based on this laser source with a high-isolation free-space circulator. The LIDAR includes a real-time display of the wind field. Wind dispersion is postprocessed. Field tests carried out at Orly airport in April 2008 are reported. Axial aircraft wake vortex signatures have been successfully observed and acquired at a range of 1.2 km with axial resolution of 75 m for the first time with fiber laser source.

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This study investigates the influence, on aircraft wake vortex behavior, of atmospheres that are stably stratified (neutral to very strong) and weakly turbulent, by means of large-eddy simulations at very high Reynolds number and on relatively fine grids. The atmospheric fields are first generated using large-eddy simulations of forced and stratified turbulence reaching a statistically stationary state. The obtained fields are shown to be realistic and consistent with the literature. A pair of counter-rotating vortices, with relatively tight cores, is then put in the obtained fields and evolved. The evolution of the wake vortex topology, transport, and decay is analyzed in depth by measuring the wake vortex characteristics in all cross planes. The vortex transport and deformation are related to the stratification and turbulence levels. Stratification combined with weak turbulence is seen to strongly affect the Crow instability development. Different decay mechanisms are identified, related to the interactions with the turbulence, the turbulent baroclinic vorticity, and/or between the vortices themselves. Finally, improved simplified models of vortex altitude evolution and of vortex decay (with two phases) are proposed and calibrated on the present results. They yield good agreement with the large-eddy simulation results and are usefully integrated in our operational models. Read More: http://arc.aiaa.org/doi/abs/10.2514/1.J051742

m LIDAR for Axial Aircraft Wake Vortex Detection Based on High-Brightness Large-Core Fiber Amplifier

This study is concerned with the investigation of two-vortex systems (2VS) of various strengths that are released near the ground and evolve in the presence of a turbulent crosswind. We analyze the physics of the vortices interactions with the turbulent wind and with the ground during the rebound phase, and that lead to the fully developed turbulent flow and interactions. The transport and decay of the vortices are also analyzed. The turbulent wind itself is obtained by direct numerical simulation using a half channel flow. The flow is then supplemented with the 2VS, using vortices with a circulation distribution that is representative of vortices after roll-up of a near wake. The vortex strengths, Γ0, are such that ReΓ = Γ0/ν = 2.0 × 104 for the baseline; there is then a case with twice weaker vortices, and a case with twice stronger vortices. The simulations are run in wall-resolved Large Eddy Simulation (LES) mode. The baseline is in line with the wall-resolved LES study of a similar case [A. Stephan e...

Aircraft wake vortices in stably stratified and weakly turbulent atmospheres: simulation and modeling

The dynamic procedure for LES performed solely in physical space (i.e.,no Fourier transform) is considered. It amounts to a procedure working at the force (vector) level that is natural and quite general: it only requires a numerical tool for restriction of the discrete LES field and forces to a coarser level. It is here investigated using finite differences and with restriction done by sampling. It gives good results on flows with homogenous directions: Burger’s turbulence and homogeneous isotropic turbulence. Preliminary results on the turbulent channel flow are also presented: they are encouraging but not yet satisfactory (velocity profile underpredicted). The obtained profile of CΔ2 is found to have the proper near and far wall behaviors,but with too low amplitude. Further improvements are required: they might include some filtering (using tensor-product stencil-3 discrete filters, also iterated) prior to the sampling, to mitigate the aliasing effects due to the sampling; they might also require to modify the procedure itself, following what was done by others when using the classical procedure expressed at the force level.

LES investigation of the transport and decay of various-strengths wake vortices in ground effect and subjected to a turbulent crosswind

This paper is concerned with comparing different approaches for the numericalprediction of heat transfer in a turbulent channel flow at very low Prandtl number and highReynolds number. Results obtained with a Reynolds Averaged Navier Stokes (RANS) approachat relevant Reynolds numbers for the liquid metal reactor problematic (Reτ = 590, Reτ = 1020),are presented and discussed. Original results obtained with a wall resolved Large EddySimulations (LES) at Reτ = 2000 are provided. The velocity profile agrees very well withthat of a reference Direct Numerical simulation (DNS). The obtained temperature profile canserve as a reference as the energy equation is computed without any Subgrid Scale (SGS) modelat this low Pr.

The sampling-based dynamic procedure for LES: investigations using finite differences

This study investigates the capabilities of various recent subgrid-scale (SGS) models (the so-called “multiscale” models) for large-eddy simulation (LES), used either in a vortex-in-cell (VIC) method or in a pseudo-spectral (PS) method, and their applicability to the simulation of decaying homogeneous isotropic turbulence (HIT) in the limit of very high Reynolds number (i.e. LES on a large grid and where the molecular viscosity dissipation is negligible compared to the SGS dissipation). The proper coefficient value for each model investigated was obtained by a calibration performed in an earlier study. Various large grid resolutions (1283, 2563 and 5123) are used to compare and to indeed obtain the asymptotic spectral behavior of each model. We are then able to emphasize the behavior of the models, that is not necessarily observable in “small” LES (i.e. in LES at moderate Reynolds number and/or using a smaller mesh). In particular, we show that the multiscale models perform significantly better than the Smagorinsky model: a much wider inertial range is obtained.

Numerical prediction of turbulent heat transfer at low prandlt number

A new spectral model of the return signal from a LIDAR Doppler wake vortex detector is proposed. It has been experimentally discovered during ground-based and flight test campaigns but suffered a lack of theoretical evidence. Using high resolution fluid simulations of wake vortices, we highlight the physical meaning of this model. Comparisons with the traditional single Gaussian model show the superiority of this new approach is consistent with previous experimental results.

Spectral behavior of various subgrid-scale models in LES at very high Reynolds number

High pressure turbine (HPT) blades are cooled partly by guiding air from the compressor through internal ducts.

Simulation of LIDAR-based aircraft wake vortex detection using a bi-gaussian spectral model

We investigate several algorithms to detect the centerline of a wind turbine wake. First, we apply these methods during Large Eddy Simulations of an isolated wind turbine subject to a uniform (TI = 0%) and a synthetic turbulent inflow (TI = 10%). The simulations are performed using a vortex-particle mesh method with the blades modeled using immersed lifting lines. The most robust algorithm is then applied to investigate the wakes positions inside a wind farm. At wind farm scale, the simulations are performed with a fourth-order finite difference code inside which the wind turbines are accounted for through advanced actuator disks.

DNS and ILES of Wall Bounded Flows Using a Discontinuous Galerkin Method and Inlet Synthetic Turbulence

The phenomenon of meandering of the wind-turbine wake comprises the motion of the wake as a whole in both horizontal and vertical directions as it is advected downstream. The oscillatory motion of the wake is a crucial factor in wind farms, because it increases the fatigue loads, and, in particular, the yaw loads on downstream turbines. To address this phenomenon, experimental investigations are carried out in a wind-tunnel flow simulating an atmospheric boundary layer with the Coriolis effect neglected. A