Yannick Hoarau

Organized modes and the three-dimensional transition to turbulence in the incompressible flow around a NACA0012 wing

The transition to turbulence in the incompressible flow around a NACA0012 wingat high incidence is studied by DNS in the Reynolds number range 800–10000. Twomain routes are identified for the two-dimensional transition mechanisms: that toaperiodicity beyond the von Karm´ an mode via a period-doubling scenario and the´development of a shear-layer instability, forced by the fundamental oscillation ofthe separation point downstream of the leading edge. The evolution of the globalparameters as well as the variation law of the shear-layer instability wavelengthare quantified. The history of the three-dimensional transition mechanisms from anominally two-dimensional flow structure is identified beyond the first bifurcation, aswell as the preferred spanwise wavelengths.

The three-dimensional transition in the flow around a rotating cylinder

The flow around a circular cylinder rotating with a constant angular velocity, placed in a uniform stream, is investigated by means of two- and three-dimensional direct numerical simulations. The successive changes in the flow pattern are studied as a function of the rotation rate. Suppression of vortex shedding occurs as the rotation rate increases (>2). A second kind of instabilty appears for higher rotation speed where a series of counter-clockwise vortices is shed in the upper shear layer. Threedimensional computations are carried out to analyse the three-dimensional transition under the effect of rotation for low rotation rates. The rotation attenuates the secondary instability and increases the critical Reynolds number for the appearance of this instability. The linear and nonlinear parts of the three-dimensional transition have been quantified by means of the amplitude evolution versus time, using the Landau global oscillator model. Proper orthogonal decomposition of the three-dimensional fields allowed identification of the most energetic modes and three-dimensional flow reconstruction involving a reduced number of modes.

Prediction of Transonic Buffet by Delayed Detached-Eddy Simulation

A delayed detached-eddy simulation of the transonic buffet over a supercritical airfoil is performed. The turbulence modeling approach is based on a one-equation closure, and the results are compared to an unsteady Reynolds-averaged Navier–Stokes simulation using the same baseline model as well as experimental data. The delayed detached-eddy simulation successfully predicts the self-sustained unsteady shock-wave/boundary-layer interaction associated with buffet. When separation occurs, the flow exhibits alternate vortex shedding and a spanwise undulation. The method also captures secondary fluctuations in the boundary layer that are not predicted by unsteady Reynolds-averaged Navier–Stokes simulation. A map of flow separation emphasizes the differences between the delayed detached-eddy simulation and unsteady Reynolds-averaged Navier–Stokes flow topologies. Statistical pressure distributions and velocity profiles help assess the performance of each model. They indicate that the delayed detached-eddy simul...

Intensifying the ATRP synthesis of statistical copolymers by continuous micromixing flow techniques1

Abstract The impact of micromixers on copolymers’ characteristics in a continuous-flow microprocess was studied. A stainless steel coiled tube was used as the microreactor. Several micromixers with different working principles, like bilamination, multilamination and impact jet, were used to mix the reactants’ streams before entering the reactor. (Co)polymers of 2-dimethyl amino ethyl methacrylate (DMAEMA) and benzyl methacrylate (BzMA) were synthesized by the atom transfer radical polymerization (ATRP) technique, with two different compositions of BzMA (20% and 40%). A faster polymerization rate was observed in case of microprocess, as compared to batch process, highlighting the inherent intensification nature of microfluidic-assisted processes. Despite equal conversion for the three micromixers, a remarkable difference in molecular weight was observed. The highest molecular weights with lowest polydispersity indices (PDIs) were obtained when the multilamination micromixer was used, while the bilamination gave polymers with high PDIs and low molecular weights. Diffusion constraints arising from the increase in viscosity was clearly visible for highest residence times in the microreactor, resulting in a deviation of molecular weight from the theoretical value.

Recent Developments of the Navier Stokes Multi Block (NSMB) CFD solver.

The Navier Stokes Multi Block solver NSMB was initially developed in 1992 at the Swiss Federal Institute of Technology (EPFL) in Lausanne, and from 1993 onwards in the NSMB consortium composed of different universities, research establishments and industries. Today NSMB is developed by IMF-Toulouse (IMFT), ICUBE (Strasbourg), University of Munchen (TUM, Germany), University of the Army in Munchen, Ecole Polytechnique Montreal, Airbus Defence and Space, RUAG Aviation and CFS Engineering. At the Aerospace Sciences Meeting in 1998 an overview of the developments of NSMB was given . Since then various papers have been published on NSMB, examples are . This paper will present several recent developments of NSMB and the use of NSMB for industrial test cases.

Chimera method applied to the simulation of a freely falling cylinder in a channel

In this paper, we study the motion of a circular cylinder freely falling in a channel under the action of gravity parallel to the wall. The fixed parameters of the study are the cylinder diameter to channel width ratio, D/d = 3.3, and the fluid to particle density ratio, ß = 2. The varying parameters are the initial position (in or out of the middle axis) and the Galileo number (151. Ga. 300). An automatic chimera method is implemented in a Navier-Stokes solver to simulate this moving confined configuration. The presence of the wall accelerates the oscillations of the motion. The initial position has an influence on the amplification of transverse oscillations. If the cylinder is out of the middle axis, transverse oscillations appear earlier and reach rapidly the amplitude of the terminal periodic oscillations. A relation between the Strouhal and Reynolds numbers is proposed.

Unsteady Flow Around a NACA0021 Airfoil Beyond Stall at 60° Angle of Attack

The flow around thick and symmetric NACA0021 airfoil at angle of attack 60°, is studied at the Reynolds number based on the chord length \(Re = \frac{{U_\infty C}}{\nu } = 2.7 \times 10^5 \). Hybrid Detached Eddy Simulation (DES) approaches are used, with improvement by means of Organised Eddy Simulation (OES) in the context of the URANS part.

Development of a three-dimensional icing simulation code in the NSMB flow solver

An icing model is developed in the Navier-Stokes multi-block (NSMB) compressible solver. The code implemented in this study is based on an Eulerian formulation for droplets tracking solved implicitly by means of CGSTAB or SIP methods, a modified iterative Messinger model using an improved water runback scheme for ice thickness calculation and three-dimensional mesh deformation to track the ice/air interface through time. The whole process is parallelised with MPI for efficient calculations. Validation is performed on several test cases including NACA23012 and NACA00012 airfoils and ONERA-M6 swept wing. Pressure coefficients around iced 23012 airfoil are compared with experimental data. Rime icing is computed on a DLR-F6 wing/body configuration. In all studied cases, the results are in good agreement with the literature.

Improved Implicit Immersed Boundary Method via Operator Splitting

We present an implicit immersed boundary method via operator splitting technique for simulating fluid flow over moving solid with complex shape. An additional moving force equation is derived in order to impose the interface velocity condition exactly on the immersed surface. The moving force matrix is formulated to be symmetric and positive definite, thus its calculation is computational inexpensive by using the conjugate gradient method. Moreover, the proposed immersed boundary method is incorporated into the rotational incremental projection method as a plug-in. No numerical boundary layers will be generated towards the velocity and pressure during the calculation. The method is validated through various benchmark tests.

Hybrid RANS-LES Modeling of a Strongly Detached Turbulent Flow around a Tandem Cylinders Configuration

The turbulent flow around a generic configuration of a landing gear (’the tandem cylinder’) is simulated and analysed physically at Re = 1.66x105, by means of hybrid RANS-LES turbulence modelling approaches. In the present study, the Delayed Detached Eddy Simulation (DDES) approach has been employed. The DDES-OES modelling has been considered, especially involving turbulence length scale reconsiderations in the statistical part, by means of the Organised Eddy Simulation, (OES), to take into account non-equilibrium turbulence effects. The DDES-k-ω SST model is also considered. The results, obtained by means of two different time steps are compared with experiments carried out at the NASA-Langley Research Centre in the context of ATAAC EU-program in which the tandem cylinders is one of the ‘stepping stones’. In the present study, the benefits of these hybrid approaches have been discussed for capturing the vortex dynamics and frequency modes responsible for aerodynamic noise production in the context of landing gear configurations.