Yannick Bury

Experimental study of the behavior of confined variable density jets in a time varying crossflow

An experimental work and a physical analysis dedicated to the study of a low density jet subjected to a time varying crossflow with high acceleration/deceleration levels are presented in this paper. Relevant nondimensional numbers are derived and show that unsteady effects associated with the presence of the jet in the acceleration field have noticeable consequences on the flapping of the jet. The Schlieren technique is applied in the test section of a square duct to obtain time resolved images of the jet. Analysis of the results is focused on the influence of the unsteady effects on the global dynamic behaviour of the jet in the near field. The interaction between the jet and the crossflow is analysed in three contrasted situations corresponding to different values of the jet outlet velocity U0. We predict and observe an increase of the jet deflection during the acceleration phase and a competition between drag and acceleration during the deceleration. This competition is particularly clear for the two lowest ejection velocities of the jet and we have shown that the jet is initially deflected upstream the nozzle. The influence of exit jet injection angle is finally considered. We show that upstream or downstream injections induce a very strong modification of the mixing process of the jet fluid with the pulsed crossflow.

Wake Instabilities behind an Axisymmetric Bluff Body at Low Reynolds Numbers

This paper aims at understanding the mechanisms that lead to the onset of chaos in the wake of blunt based axisymmetric bluff body. On the basis of direct numerical simulations, conducted for Reynolds numbers ranging from 100 to 800, we show that the flow undergoes multiple transitions, successively giving rise to the Steady State SS and to the Reflectional Symmetry Preserving RSP a , RSP b and RSP c wake states. In particular, the RSP c state is characterized by intermittent vortex stretching denoting the onset of chaos and the potential occurence of a third instability that superimposes to the first and second instability associated with state RSP a and RSP b respectively. Interestingly, the reflectional symmetry plane that characterizes the RSP states is still retained. Hence, chaos is triggered before the symmetry breaking and the occurence of the Reflectional Symmetry Breaking RSB state observed at higher Reynolds numbers.

Laser Doppler Velocimetry Measurements in Turbulent Gaseous Mixing Induced by the Richtmyer–Meshkov Instability: Statistical Convergence Issues and Turbulence Quantification

A statistical characterization of the turbulent flow produced in a vertical shock tube dedicated to the study of the Richtmyer-Meshkov instability (RMI) is carried out using Laser Doppler Velocimetry (LDV), time-resolved Schlieren images and pressure histories. The time evolution of the phase-averaged velocity field and the fluctuating velocity levels produced behind the shock wave are first investigated for different configurations of a pure air, homogeneous medium. This allows us to determine the background turbulence of the experimental apparatus. Second, the RMI-induced turbulent Air/SF6 mixing zone (TMZ) is studied both in its early stage of development and after its interaction with a reflected shock wave (reshock phenomenon). Here the gaseous interface is initially produced by a thin nitrocellulosic membrane trapped between two grids. One of the most consistent issue regarding such a process is the generation of a large number of fragments when the incident shock wave crosses the interface. These fragments are likely to corrupt the optical measurements and to interact with the flow. This work seeks to clarify the influence of these fragments on the statistical determination of the velocity field. In particular it is shown that statistical convergence cannot be achieved when the fragments are crossing the LDV measurement volume, even if a significant number of identical experiments are superimposed. Some specific locations for the LDV measurements are however identified to be more favourable than others in the Air/SF6 mixing configuration. This finally allows us to quantify the surplus of turbulence induced by the reshock phenomenon.

Round Turbulent Air Jet Submitted to a Pulsed Coflow

Mean and turbulent properties of an unsteady round air jet submitted to a pulsed coflowing airstream were studied from laser Doppler Velocimetry measurements. The ejection velocity of the jet is kept at a constant value, whereas the coflowing stream is pulsed. These measurements revealed that the unsteadiness leads to a longitudinal partition of the jet. Near the jet exit, the flow is a quasi-steady jet flow. Farther downstream, the flow is unsteady with the creation of a large and propagative structure in the jet flow. The objective of the study is comprehen- sive understanding of the main physical mechanisms induced by the coflow unsteadiness. Consequences of the entrainment process are also discussed.

Experimental and numerical investigation of the growth of an air/SF6 turbulent mixing zone in a shock tube

Shock-induced mixing experiments have been conducted in a vertical shock tube of 130mm square cross section located at ISAE. A shock wave traveling at Mach 1.2 in air hits a geometrically disturbed interface separating air and SF6, a gas five times heavier than air, filling a chamber of length L up to the end of the shock tube. Both gases are initially separated by a 0.5 lm thick nitrocellulose membrane maintained parallel to the shock front by two wire grids: an upper one with mesh spacing equal to either ms=1.8mm or 12.1 mm, and a lower one with a mesh spacing equal to ml=1 mm. Weak dependence of the mixing zone growth after reshock (interaction of the mixing zone with the shock wave reflected from the top end of the test chamber) with respect to L and ms is observed despite a clear imprint of the mesh spacing ms in the schlieren images. Numerical simulations representative of these configurations are conducted: the simulations successfully replicate the experimentally observed weak dependence on L, but are unable to show the experimentally observed independence with respect to ms while matching the morphological features of the schlieren pictures.

Experimental Characterization of Turbulence Produced in a Shock Tube: A Preliminary Work for the Study of the Turbulent Gaseous Mixing Induced by the Richtmyer-Meshkov Instability

The Richtmyer-Meshkov Instability (RMI) occurs in several physical and technological processes such as supernova explosion, supersonic combustion, detonics or inertial confinement fusion. This instability develops when interfacial perturbations, between two fluids of different densities, grow because of a shock wave induced impulsive acceleration. The basic mechanism for the initial growth of perturbations on the interface is the baroclinic generation of vorticity which results from the misalignment of the pressure and density gradientswhen the shock crosses the interface. Early time linear and following nonlinear growth of the RMI have been, and are still widely investigated, either theoretically, numerically and experimentally [1]. Nevertheless, experimental investigation of the Turbulent Mixing Zone (TMZ) induced by a rapidly growing RMI is still nowadays poorly documented, even if we can mention for instance the work of Leinov et al. [2] who characterized the growth of the MZ with time following the passage of the re-shock (with an emphasis on the influence of the initial amplitude of the MZ and the reshock strength), and the study of Poggi et al. [3] in which the production of turbulence by the TMZ has been investigated in a vertical shock tube using two-components Laser Doppler Velocimetry (LDV).

Experimental and Computational Investigation of a Simplified Geometry of an Engine/Pylon/Wing Installation at High-Lift Flight Conditions

This paper focuses on the numerical and experimental characterization of the vortex structures that develop along a simplified geometry of a wing equipped with pylon-mounted engine at low speed/high angle of attack flight conditions. In these conditions, the presence of the engine installation under the wing induces a complex and unsteady vortical flow field at the nacelle/pylon/wing junction which interacts with the upper wing boundary layer and leads to a drop of aircraft performances. In order to gain insight into the physics driving this interaction, it is proposed to isolate its fundamental mechanisms by simplifying the problem. The parameters of interest that led to the simplification of the model are first described. As a first step into a more comprehensive knowledge of this complex physics, this study is initially conducted at a Reynolds number of 200000, based on the chord wing and on the free stream velocity. Two configurations of angle of attack α and sideslip angles β (α=8°/β=0° and α=8°/β=30°) have been investigated. This work relies on unsteady RANS computations, oil flow visualizations and 3C-PIV measurements. The vortex dynamics thus produced is described in terms of vortex core position, intensity, size and turbulent intensity thanks to a vortex tracking post-processing algorithm. In addition, the analysis of the velocity flow field obtained from the PIV measurements will highlight the influence of the longitudinal vortex issued from the pylon/wing junction on the separation process of the boundary layer near the upper wing leading-edge.

Experimental Determination of the Growth Rate of Richtmyer-Meshkov Induced Turbulent Mixing after Reshock

The Richtmyer-Meshkov Instability (RMI) develops when a shock wave impulsively accelerates an initially perturbed interface between two gases of different density, promoting their mixing inside a delimited zone, hereafter denoted the mixing zone (MZ). This mixing is a key issue for inertial confinement fusion process. It also finds applications in many different scientific and engineering issues, e.g. in supernova explosion or supersonic combustion [1]. In the context of inertial confinement fusion, the characterization of the RMI-induced mixing zone initiated by a shock and further amplified by a reshock is largely based on the temporal evolution of integral parameters such as the width of the MZ. This is classically achieved through shock tube experiments involving the time-resolved acquisition of Schlieren images.