Direct numerical simulations of a vortex pair embedded in a stable atmospheric boundary layer are presented. The eeects of various crosswind conditions are studied. These computations demonstrate the creation of secondary vortices for the range of Reynolds numbers (3770 Re ? 7540). The physics of wake vortex interactions with the ground for diierent values of crosswind are discussed. The redistribution of vorticity between the atmospheric boundary layer and the vorticity induced by the primary vortices may explain the vortex tilting phenomenon. A parametrization of the eeect of crosswind on the minimum altitude reached by the two vortices is given.

Body fitted structured grid generation is a crucial step in the computation of flow around complex aircraft configurations. The structured body fitted approach inevitably leads to dense meshes in zones where gradients are expected to be low. The challenge is to develop an efficient way to simplify the grid generation for complex configurations in transonic flow without wasting points. In this paper, a 3D patched grid algorithm is described using Jameson's central scheme for the inviscid part of the flux and a 2nd order centered discretization for the viscous part of the flux. The patched grid algorithm is associated with the LU-SGS scheme for implicit time integration. The turbulence terms are estimated by the Badwin Lomax algebraic model or by the one equation Spalart Allmaras model. We show the efficiency of the patched grid algorithm with several referenced test cases. Finally, we present computations over the AS28G aircraft (fuselage, wing, pylon, nacelle) using the patched grid algorithm.

The objective of this study is to perform direct numerical simulations (DNS) of the three-dimensional short-wavelength elliptic instability developing in a counter-rotating vortex pair, and to reproduce numerically a water-tank experiment. The main features of the elliptic instability are recovered by the simulations. In particular, the spatial structure and the temporal evolution of the most amplified perturbation mode during the linear regime correspond to both experimental measurements and theoretical predictions. The long-term evolution is also simulated, and the stages leading to transition to turbulence are described. Some elements resulting from simulations related to the interaction between the short-wavelength elliptic instability and the long-wavelength Crow instability are provided.

The effects of ambient turbulence on vortex dynamics are studied using a validated,threedimensional, large-eddy simulation model. The objective of this study is to understand the mechanisms of interaction between vortices and turbulence and to obtain quantitative data about the aging process. Numerical simulations are performed in order to isolate the effects of ambient turbulence on a configuration consisting of a single vortex or vortex pair. A set of simulations has been conducted by placing vortices in a field of homogeneous isotropic turbulence for a range of turbulence intensities. The results reveal typical phenomena of vortex/turbulence interaction such as gradual, quasilinear circulation decay due to turbulent diffusion, vortex deformation, appearance of asimuthal vertical structures and the occurrence of longand short-wavelength instabilities. Several parameter studies have been performed in order to quantify a critical parameter which affects the strength of vortices and/or predicts the occurrence of instabilities.

This paper presents the development of an efficient implicit upwind multi-domain block-structured solver for aeroelastic computations of unsteady turbulent transonic flows around airfoils in arbitrary motion. The resolution of the structural system resolution is performed with an unconditionnally stable algorithm. Computations of two test cases with forced oscillation and one with flutter are analyzed.

The aim of this paper is to demonstrate that trailing wake vortices can be reliably detected from an axial point of view using Doppler lidar. Three-dimensional large eddy simulations of wake vortices are performed in order to investigate the performances of an airborne Doppler Lidar based wake vortex detection system, known as the MFLAME system. Three test cases were investigated: i) Crow instabilities, ii) wake vortex decay in isotropic homogeneous turbulence and iii) wake vortex collapse in convective atmospheric boundary layer. In all cases, the axial velocity is not initialized. Once the appropriate flowfield is computed, it is inputted into the MFLAME system simulator. The results indicate that a forward looking Doppler Lidar system is capable of detecting wake vortex signatures from several ages of the applied vortices.

This paper presents three-dimensional large eddy simulations (LES) and two-dimensional direct numerical simulation (DNS) of wake vortices of an aircraft in a realistic atmospheric boundary layer (ABL). These calculations are performed in order to parametrize an operational model aiming at managing the separations between aircraft at take-off and landing. The computation considered here is taken from twelve selected cases corresponding to trials made at Idaho Falls sponsored by the Federal Aviation Administration (FAA) in 1990. After computing the evolution of a convective atmospheric boundary layer with a non-hydrostatic meteorological model, the evolution of the wake vortices (with their characteristics measured during the experiments) is computed. The threedimensional behavior of the wake vortices as well as the effects of the atmospheric parameters are shown. The results of the simulations are compared to experimental data.

The work done at Service Technique de la Navigation Arienne (STNA) on wake vortices has led to the establishment of relationships between wind characteristics and potential hazards for a following aircraft. The methods that define these relationships and their validation are presented. Separation between aircraft is defined from our model as varying between 80 and 30 s for either a total wind of 12 to 20 kt or a crosswind of 5 to 8 kt. Based on these results, a first version of SYAGE (SYsteme Anticipatif de Gestion des Espacements) has been defined which considers only a temporary change of the current separation matrix. This version of SYAGE will be applied only to take-off from a single runway because of the difficulty of forecasting wind characteristics, as would be required for a system applied also to landing. To optimize the system’s efficiency, it is planned to link SYAGE 1 to a meteorological interface and to the Surface Movement and Guidance Control System (SMGCS) now under development. The objective is to realize a wake vortex advisory system (WVAS) for departing aircraft. Prior to implementing the WVAS at Orly Airport, a demonstration is planned to provide experience for controllers and pilots.

An approximate model of wake vortices behavior is presented. We have introduced modifications In the Greene's model to take into account the effects of ground (divergence, rebound) and crosswind (advection, shear). Direct numerical simulations of laminar flows with and without lateral wind are performed to validate these extensions, The first results are encouraging and efforts are carried on to derive a more reliable model. The capability of this model to mimic the reality is shown by comparison with an experimental test case.

Among the various meteorological parameters acting on the lifespan of the wake vortices, the atmospheric turbulence is one of major importance. Direct numerical simulations are one of the best tools to study turbulence even if the Reynolds number used in the computations is far bellow the Reynolds number of the phenomenon. Respecting as much as possible the scales of the real flow field, namely the integral length scale of atmospheric turbulence and the r.m.s. velocity, phenomena explaining the decay and the lifetime of trailing vortices are studied. It is shown in this paper that the interaction between the vortices and the ambient turbulence creates ring of vorticity through a tridimensional mechanism. The first effect of these rings is to generate axial velocities as great than velocities induced by the vortices. Furthermore, the vortices are curved and the vortex pair could become unstable. Nomenclat ure d0 = initial spacing between vortices. Kd = wavenumber of the maximum in dissipation spectrum Ke = wavennmber of the maximum in energy spectrum Le = length scale associated to Kf rc = core radius Re = Reynolds number t = time, f * = trc/V0 Tc = turbulence turn-over time TV = vortex time scale a' = r.m.s. turbulent velocity V0 = maximal vortex velocity x,y,z = coordinates x = horizontal cross-plane direction y = vertical cross-plane direction z — axial direction e = turbulence dissipation rate A = integral length of turbulence v — kinematic viscosity fl! = vorticity vector Q* = |Q| rc/V0 = l/2rot({7) •Researcher, email: rissoQiraft.enseeiht.fr ^Research Engineer, Member AIAA, email: corjonQcerfacs.fr. *Research Engineer, Member AIAA, email: stoesselOifp.fr. Copyright ©1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. INTRODUCTION Acting for the DGAC (Direction Generale de PAviation Civile), the STNA (Service Technique de la Navigation Aerienne) has defined objectives to study wake vortices. The first goal is to decrease the delays at landing and take off with an advisory system for ATC (Air Traffic Control) based on meteorological parameters forecasting. This system would allow to reduce momentarily the separations to specified Radar minima. In the long term, an automatic system optimizing separations between aircraft would be integrated in an advanced ATC system and more realistic categorization criteria would be defined than the MTOW (Maximum Take-Off Weight). Both these systems require to take into account the effects of atmospheric turbulence on the lifespan of the wake vortices. For the more simple system only turbulence classes are necessary while accurate forecasting are needed for the optimal system. It was decided to develop first semi-empirical models which allow to estimate the formation of the trailing vortices, to simply take into account the effect of meteorological conditions on the vortices behavior and to study the interaction between a vortex and an aircraft. This engineering model was presented in a former paper and is called VORTEX. From an operational point of view, it is necessary to develop simple models which could be used in real time. But these models require a large amount of validation because many of their parameters are difficult to fix. Therefore it was decided to develop not only operational models but also more realistic tools based on a Navier-Stokes solver. For certain phenomena important in the wake vortex proccess, these solvers allow cheaper validation than experiments and could provide crucial informations to improve for example the modeling of the vortex decay process. The ambient air turbulence is one of the major effects influencing the decay of the trailing vortices. The operational model uses the results of Donaldson and Bilanin which relate exponentionally the decay of the vortices and the r.m.s. velocity of the turbulence (see equation 10). The first aim of this work is to study this relationship by comparing the results of direct numerical simulations made with NTMIX3D and the simple model. Secondly, it is important to understand what is exactly the process of trailing vortices decay. Several authors have shown that the most important, vortex-decay mechanism works from outside of the vortex to the center. NUMERICAL TOOL The present NTMIX3D code is solving 3D unsteady compressible Navier-Stokes equations with multi-species, transport and chemistry. Obviously, for this study, multispecies and chemistry capabilities are not used. A highly accurate finite difference method over a cartesian grid is used to fully handle most of the different scales in the 3D flow: spatial derivatives are computed with a 6 order compact scheme (Fade scheme) which has the property of being low-dispersive and non-dissipative. These schemes even if they are quite expensive are allowing some quasispectral accuracy. Time advancement is achieved with a 3 order Runge-Kutta method. The code is supporting accurately many different type of boundary conditions thanks to the use of the NSCBC method . In particular, nonreflecting open boundary conditions as well as adiabatic or isothermal solid walls are implemented. The cost in CPU and memory of a 3D simulation is so high that it requires the use of MPP computers to be affordable. Thus, the code has been written directly in parallel with a Message Passing Paradigm. Specific domain decomposition methods have been developped for the parallel treatment of compact schemes and are allowing the availability of a highly efficient version of the code on a wide range of distributed memory parallel computers such as IBM SP2, Fujitsu VPP500 and Cray T3D. The simulation presented in the following has required 10 hours 20 min on a IBM SP2 (32 ns/GridPoint/Timestep).