Simulation of turbulent aircraft wake vortex flows and their impact on the signals returned by a coherent Doppler LIDAR system

This thesis concerns the numerical simulation and the remote sensing of aircraft wake vortex flows. Due to its lift force, an aircraft releases large scale swirling flows (vortices) in its wake. As these vortices can impact significantly the trajectory of a following aircraft, their study is of great importance for practical applications concerning safety of air traffic management. The investigation carried here is twofold: it concerns, on one hand, the physics and the numerical simulation of aircraft wake vortices and, on the other hand, the technique to detect those vortices and measure their properties.The numerical simulation of aircraft wake vortices requires high order and energy conserving codes. Moreover, as aircraft wake vortex flows are turbulent,subgrid scale (SGS) models are required to perform Large Eddy Simulation (LES) of these flows. In the first part of this work, the numerical codes used (here spectral and high order finite differences)are presented and validated. Several SGS models are presented and their ability to perform LES of vortical flows, also in presence of a ground is assessed. In particular a new “multiscale” model with a natural wall damping behaviour has been developed and investigated: its performance is very good. In the second part, two flows relevant to the wake vortex problem are analyzed. The LES of a wake vortex system in a weakly turbulent atmosphere allowed highlighting the turbulent equilibrium state of such a flow. LES of wake vortices in ground effect, with and without wind, were also carried out. These simulations help to better understand the physics of wake vortices. In the last part, we simulate the remote sensing of aircraft wake vortices using a ground based LIDAR (Light Detection And Ranging) system. The aim of this LIDAR is to sense aircraft wake vortices and turbulent winds. As the LIDAR signals are simulated using realistic parameters and realistic turbulent flows, this work serves as support in the design of an airport based LIDAR system.(FSA 3) -- UCL, 200

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

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

A multiscale subgrid model for both free vortex flows and wall-bounded flows

A new subgrid-scale (SGS) model which has an adequate behavior in both vortical flows and wall-bounded flows is proposed. In wall-bounded flows with "wall-resolved" large eddy simulation (LES), the theory predicts that the SGS dissipation should vanish as y(+3) near the wall. In free vortex flows, one needs to have models which do not dissipate energy in the strongly vortical and essentially laminar part of the flow, e. g., in the vortex core regions. The wall adapting local eddy (WALE) viscosity model of Nicoud and Ducros (Flow, Turbul. Combust. 62, 183 (1999)] has the correct near-wall behavior. It is, however, demonstrated here that it produces values of effective eddy viscosity which are too high in vortical flows: this constitutes a major drawback for LES of vortex flows. On the other hand, the regularized variational multiscale models are suitable to simulate vortical flows as demonstrated by Cocle et al. [Complex Effects in LES (Springer, New York, 2007), p. 56], but they do not have a correct behavior in wall-bounded flows as shown by Jeanmart and Winckelmans [Phys. Fluids 19, 055110 (2007)]. The model presented here aims at combining the strengths of the two models: it is a multiscale model, thus acting on the high pass filtered LES field, and for which the SGS viscosity is evaluated using the WALE model, itself also computed using the high pass filtered field. Hence, this model is only active when there is locally a significant high wavenumber content in the flow and it has a natural near-wall damping behavior. The ability of this model to simulate vortex and wall-bounded flows is demonstrated on three test cases. The first case is the turbulent channel flow at Re-tau=395 and Re-tau=590. The second test concerns a counter-rotating four-vortex system at Re-Gamma=20 000. The third case concerns a two-vortex system in ground effect at Re-Gamma=20 000. It is shown that the model allows to perform successfully the LES of these flows with the proper dissipative behavior in both the near-wall and the vortical regions. (C) 2009 American Institute of Physics. [doi:10.1063/1.3241991

Scale dependence and asymptotic very high Reynolds number spectral behavior of multiscale subgrid models

This paper investigates the spectral response of recent multiscale subgrid models, all of eddy viscosity type, in large-eddy simulation (LES) of fully developed turbulence, from moderate to very high Reynolds (Re) number. The objective of this work is to provide useful results about the behavior of the subgrid scale (SGS) models and, in particular, their asymptotic behavior. Such information is indeed important for practitioners using LES to simulate truly high Reynolds number flows. Specifically, we consider LES of homogeneous isotropic turbulence at very high Re and where the LES cutoff (here the grid Delta) is taken well into the inertial range (i.e., Delta/eta >= 100 with eta the Kolmogorov scale). Large LES grids (128(3) and 256(3)) are also used in order to compare and attain the true asymptotic behavior of each SGS model, something not fully observable in LES on smaller grids. An analysis is also carried out to obtain the scale dependence of each model coefficient in the viscous range of turbulence using LES run on several grids and compared to direct numerical simulation. The results provide C for each model and for various Delta/eta. A convenient fit then also provides C/C-infinity. as a function of Delta/eta, where C-infinity is the asymptotic coefficient. The comparisons are supported using the evolution of resolved energy (global and spectrum), resolved enstrophy, and effective dissipation. It is shown that the multiscale models acting on the high wavenumber part of the LES field are indeed able to provide a significant kappa(-5/3) inertial subrange, yet it is always followed by an energy pileup effect also called "bottleneck." This effect is also characterized for the various models. (C) 2009 American Institute of Physics. [DOI:10.1063/1.3194302

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

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.J05174

A reinforcement-learning approach for individual pitch control

Individual pitch control has shown great capability of alleviating the oscillating loads experienced by wind turbine blades due to wind shear, atmospheric turbulence, yaw misalignment or wake impingement. This work presents a novel controller structure that relies on the separation of low-level control tasks and high-level ones. It is based on a neural network that modulates basic periodic pitch angle signals. This neural network is trained with reinforcement learning, a trial and error way of acquiring skills, in a low-fidelity environment exempt from turbulence. The trained controller is further deployed in large eddy simulations to assess its performances in turbulent and waked flows. Results show that the method enables the neural network to learn how to reduce fatigue loads and to exploit that knowledge to complex turbulent flows. When compared to a state-of-the-art individual pitch controller, the one introduced here presents similar load alleviation capacities at reasonable turbulence intensity levels, while displaying very smooth pitching commands by nature

Experimental study on the wake meandering within a scale model wind farm subject to a wind-tunnel flow simulating an atmospheric boundary layer

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 3 × 3 scaled wind farm composed of three-bladed rotating wind-turbine models is subject to a neutral boundary layer over a slightly-rough surface, i.e. corresponding to offshore conditions. Particle-image-velocimetry measurements are performed in a horizontal plane at hub height in the wakes of the three wind turbines occupying the wind-farm centreline. These measurements allow determination of the wake centrelines, with spectral analysis indicating the characteristic wavelength of the wake-meandering phenomenon. In addition, measurements with hot-wire anemometry are performed along a vertical line in the wakes of the same wind turbines, with both techniques revealing the presence of wake meandering behind all three turbines. The spectral analysis performed with the spatial and temporal signals obtained from these two measurement techniques indicates a Strouhal number of ≈ 0.20 - 0.22 based on the characteristic wake-meandering frequency, the rotor diameter and the flow speed at hub height

Can Water Dilution Avoid Flashback on a Hydrogen-Enriched Micro-Gas Turbine Combustion? - A Large Eddy Simulations Study

11 pages.International audienceConsidering the growing interest in Power-to-Fuel, i.e., production of H 2 using electrolysis to store excess renewable electricity, combustion-based technologies still have a role to play in the future of power generation. Especially in a decentralized production with small-scale cogeneration, micro-Gas Turbines (mGTs) offer great advantages related to their high adaptability and flexibility, in terms of operation and fuel. Hydrogen (or hydrogen enriched methane) combustion is well-known to lead to flame and combustion instabilities. The high temperatures and reaction rates reached in the combustor can potentially lead to flashback. In the past, combustion air humidification (i.e., water addition) has proven effective to reduce temperatures and reaction rates, leading to significant NO x emission reductions. Therefore, combustion air humidification can open a path to stabilize hydrogen combustion in a classical mGT combustor. However accurate data assessing the impact of humidification on the combustion is still missing for real mGT combustor geometries and operating conditions. In this framework, this paper presents a comparison between pure methane and hydrogen enriched methane/air combustions, with and without combustion air humidification, in a typical mGT combustion chamber (Turbec T100) using Large Eddy Simulations (LES) analysis. In a first step, the necessary minimal water dilution, to reach stable and low emissions combustion with hydrogen, was assessed using a one-dimensional (1D) approach. The one-dimensional unstretched laminar flame is computed for both pure methane (reference case) and hydrogen enriched methane/air combustion cases. The results of this comparison show that, for the hydrogen enriched combustion, the same level of flame speed as in the reference case can be reached by adding 10% (in mass fraction) of water. In a second step, the feasibility and flexibility of humidified hydrogen enriched methane/air combustion in an industrial mGT combustor have been demonstrated by performing high fidelity LES on a 3D geometry. Results show that steam dilution helped to lower the reactivity of hydrogen, and thus prevents flashback, enabling the use of hydrogen blends in the mGT at similar CO levels, compared to the reference case. These results will help to design future combustor toward more stability

Characterisation in water experiments of a “detached flow” free surface spallation target

In the development of accelerator driven systems, ADS, free surface lead–bismuth spallation targets are considered as promising solutions due to their possibility for compactness, their lifetime, and their ability to transport the heat deposited by the proton beam away from the spallation zone. Experiments to characterise the hydraulics of the targets are needed to allow the validation of numerical models and to improve the design. Such experiments have been performed in water on a new concept labelled “detached flow” geometry. This name was chosen because the liquid undergoes a free fall between the nozzle exit where the main free surface (that separating the void of the beam line from the liquid) is created and a second free surface downstream. The void surrounding the liquid jet plays the role of a buffer. The experiments show that a very stable main free surface with a small recirculation is obtained using this geometry thanks to the presence of the second free surface and the nozzle geometry. The experiments confirm that the level of the second free surface has no influence on the characteristics of the main free surface, improving the main free surface control. The influences of the mass flow rate and of the inlet velocity are evaluated. The free surface level rises linearly with an increase in mass flow rate. The recirculation zone is also stronger in this case. The opposite is found when the mass flow rate is decreased. For all mass flow rates studied, a stable free surface is obtained. Moreover, the outer shape of the liquid jet is similar at all mass flow rates. It is only dictated by the nozzle exit angle. Increasing slightly the inlet velocity for a given mass flow rate has a positive effect on the recirculation stability. The “detached flow” target is a promising design for ADS