Lagrangian and spectral analysis of the forced flow past a circular cylinder using pulsed tangential jets

We numerically investigate the influence of pulsed tangential jets on the flow past a circular cylinder. To this end a spectral-Lagrangian dual approach is completed on the basis of time-series data. The analysis reveals that the flow response to unsteady for- cing is driven by strong interactions between shear layers and pulsed jets. The latter preferentially lead to either the lock-on regime or the quasi-steady vortex feeding regime whether the excitation frequency is of the order of, or significantly greater than the fre- quency of the natural instability. The intensity of the wake vortices is mainly influenced by the momentum coefficient through the introduction of opposite-sign vorticity in the shear layers. This feature is emphasized using a modal-based time reconstruction, i.e. by reconstructing the flow field upon a specific harmonic spectrum associated with a charac- teristic time-scale. The quasi-steady regime exhibits small-scale counter-rotating vortices that circumscribe the separated region. In the lock-on regime, atypical wake patterns as 2P or P+S can be observed, depending on the forcing frequency and the momentum coefficient, highlighting remarkable analogies with oscillating cylinders

Experimental determination of the growth rate of Richtmyer-Meshkov induced turbulent mixing after reshock

The time evolution of the width of the turbulent mixing zone arising from the late development of Richtmyer-Meshkov instability is investigated in this work. This is achieved by means of the analysis of time-resolved Schlieren images obtained with a given set of shock-tube experiments. The post-reshock growth rate of the mixing zone width is found to be nearly insensitive to the development state of the mixing at the time of reshock

Study of the turbulent mixing zone induced by the Richtmyer-Meshkov instability using Laser Doppler Velocimetry and Schlieren visualizations

An experimental study of the compressible mixing generated by the Richtmyer-Meshkov instability (RMI) is carried out in a vertical shock tube by means of two-components Laser Doppler Velocimetry (2C-LDV) measurements and Time-resolved Schlieren visualizations. An attempt is made to quantify the RMI-induced air/sulphurhexafluoride (SF6) mixing by measuring turbulence levels inside the mixing zone at a given stage of its development and by extracting the growth rate of the mixing zone from the Schlieren images

Investigation of the interface stretching within a reshocked mixing zone produced by the Richtmyer Meshkov Instability

The spatio-temporal evolution of a bi-dimensional (2D) and three-dimensional (3D) air/SF6 mixing layer issued from the development of a Richtmyer-Meshkov instability (RMI) under reshock is investigated using direct numerical simulations (DNS) at moderate Mach number (M=1.2) and high Atwood number (A=0.67). This study discusses the relevance of an original criterion based on the measurement of the gaseous interface stretching in the analysis of the mixing process. The first part of the work provides an estimation of the validity of a 2D approach in time for the retained simulation cases. To this avail, a 2D simulation for one typical parameter set is compared to its 3D counterpart. As a means of comparing the development of the mixing layer in both simulations, the classical criterion relying on the evaluation of the mixing layer thickness has been chosen. This criterion is commonly used to characterize baroclinic instability as it is intuitive and easy to compute and to analyze. However, this criterion only provides the mixing zone frontiers but does not provide information about the length scale content and its evolution on the interface. In order to tackle this issue, it is proposed to adapt a still documented criterion for the determination of the interface stretching, based on the computation of the temporal evolution of the mixing interface length for the study of various cases involving different initial interface perturbations, with reshock consideration

WO 2017/153393 A1 Dispositif d'atténuation de sillage tourbillonnaire / Vortex wake attenuation device

The present invention relates to a device for attenuating the vortex wake generated in the tail section of an aircraft (100), the aircraft having at least one wing (108) and an afterbody (106) with an asymmetric cross-section that sharply decreases towards the top of the rear fuselage (112). The device is positioned downstream of the wing (108) of the aircraft (100), symmetrically relative to the longitudinal plane of the latter. The device comprises aerodynamic vortex-generating appendages (200) suitable for being deployed between a folded position where the aerodynamic appendages are folded substantially in the direction of the fuselage (110) and a deployed position calculated to generate vortex structures having an intensity and following a path that modify the local pressure field in order to interact with the vortex wake and attenuate it, and to move the upsweep vortices away from the longitudinal plane of the aircraft (100)

Transitions to chaos in the wake of an axisymmetric bluff body

This letter aims at understanding the dynamical process that leads to the onset of chaos in the flow past a blunt-based axisymmetric bluff body. On the basis of direct numerical simulations, conducted for Reynolds numbers ranging from 100 to 900, we show that the flow undergoes multiple transitions, successively giving rise to the SS, RSPa, RSPb, RSPc and RSB wake states. In particular, the RSPc state, revealed in this work via long-term computations, is characterized by intermittent vortex stretching denoting the onset of chaos before the symmetry breaking and the occurence of the RSB state

Flow Structure Past a Canonical Shape of Engine/Pylon/Wing Installation for Takeoff/Landing Conditions

Here, a canonical model is proposed, which is able to represent the flow past a wing equipped with a pylon-mounted engine at low speed/moderate angle of attack. The vortices that develop past this model are described numerically and experimentally. For such configurations, the presence of a power-plant installation under the wing initiates a complex and unsteady vortical flowfield at the nacelle/pylon/wing junctions, responsible for a drop in aircraft performances. To gain insight into the underlying physics, the geometry is simplified into a symmetric two-dimensional extruded wing equipped with a symmetric, hemispheric-ended cylinder. The study was conducted at a Reynolds number of 200,000, based on the wing chord and on the freestream velocity. Two angle of attack α=sideslip angle β configurations are investigated on the basis of unsteady Reynolds-averaged Navier–Stokes computations, oil-flow visualizations, and stereoscopic particle image velocimetry. The vortex dynamics thus produced is described in terms of vortex core position, intensity, and size. The analysis of the velocity flowfields obtained from the wind-tunnel measurements and the numerical computations highlights the influence of the longitudinal vortex initiated at the pylon/wing junction on the separation process of the boundary layer near the upper-wing leading edge

Steady and Unsteady compressible Reduced-Order Models of a Zero-Net Mass-Flux Synthetic Jet Actuator

International audienceIn the framework of an optimization study of a Zero-Net-Mass-Flux fluidic, synthetic jet actuator, based on a multi-objective optimization formulation, the consideration of optimization parameters such as the actuator location and the outlet design implies a re-meshing procedure that adds complexity. It is still the case even if the actuator is modeled with simple boundary conditions at the jet orifice exit since, locally, the re-mesh is still required. This strongly impacts the global computational cost, in particular if the considered geometry is complex. In a previous study, we proposed an alternative method to model Zero-Net Mass Flux synthetic jet actuators through the implementation of volumetric reduced-order models (ROM) as additional source terms. The previous reduced-order model consisted in a simplified ROM model where a constant-in-space momentum quantity was imposed in the ROM formulation and the compressible effects were neglected. In this paper, we propose to extend the previous work in an attempt to apply this ROM strategy to higher Mach number flows, where compressibility effects at the outlet of the pulsed jets can no more be neglected, while improving the early interaction of the pulsed jet with the surrounding flow by considering the starting jet influence when the actuators are operated in a pulsed manner

Can a starting vortex boost the turbulent transition of a Richtmyer-Meshkov instability-induced air/helium mixing zone?

International audienceThis work is part of the study of the turbulent mixture produced by the Richtmyer- Meshkov instability. It focuses on the characterization of the initial conditions and on their persistence when the mixing zone develops into a turbulent flow, in other words the so-called initial condition memory effect. A Direct Numerical Simulation of the flow arising from the rotation of series of blades initially separating two gases of different density is conducted. The analysis relies on the impact of the resulting vortical flow on the generation of a more or less diffused and disturbed initial gaseous interface as the rotational speed of the blades is varied. Results highlight different flow regimes, associated with the redistribution of the total vorticity between the different vortex cores promoted by the opening of the blades, as both the rotational speed and the gaseous medium in which the blade tips are moving into vary

Direct numerical simulation of particles dispersion in supersonic shear layers

Particles dispersion plays an important role in many industrial applications such as combustion, pollution control and also in experimental measurements like Laser Doppler Velocimetry. In this last case, particles are supposed to have the same behavior as fluid particles in order to give relevance to the experimental measure. However it has been shown (Jacquin et al. 1991) that noticeable errors can appear in the rms velocity measurement of supersonic jet or shear layer, even if care has been taken concerning particle seeding of the flow. The aim of this paper is to use direct numerical simulation of particle-gas flow to investigate this phenomenon