Thibault BRIDEL-BERTOMEU, Ph.D.

Direct numerical simulations of two-dimensional inviscid mixing layers have been conducted to help understanding the different struc- tures composing 2D turbulence and the interactions between the vortices over time. Because of its intrinsic multiscale nature, nu- merically simulating turbulence stemming from the Euler equations can be a very challenging task. Common workaround for this chal- lenge are the increase of degrees of freedom, the use of a high-order reconstruction scheme (see e.g… Mehr anzeigen

Direct numerical simulations of two-dimensional inviscid mixing layers have been conducted to help understanding the different struc- tures composing 2D turbulence and the interactions between the vortices over time. Because of its intrinsic multiscale nature, nu- merically simulating turbulence stemming from the Euler equations can be a very challenging task. Common workaround for this chal- lenge are the increase of degrees of freedom, the use of a high-order reconstruction scheme (see e.g. [6, 12, 15]) and/or the use of a low- dissipation Riemann solver (see e.g. [18]) for finite-volume fluxes. However, there are many choices available to researchers and any of these ingredients increases the walltime of the simulation. There- fore, a thorough study of what each brings to the resolution of 2D turbulence needs to be done in order to find the best combination yielding the best resolution with the least computational time. Weniger anzeigen

Hypersonic boundary layer receptivity and transition to turbulence have been demonstrated to be highly sensi tive to the presence of roughness elements on the wall surface, particularly for blunt-body geometries, used for atmospheric-reentry. To get a better understanding of the transition process occurring on such geometries, a necessary step is the analysis of roughness effects on the re- ceptivity of the hypersonic boundary-layer. The present paper aims to describe recent advances of an… Mehr anzeigen

Hypersonic boundary layer receptivity and transition to turbulence have been demonstrated to be highly sensi tive to the presence of roughness elements on the wall surface, particularly for blunt-body geometries, used for atmospheric-reentry. To get a better understanding of the transition process occurring on such geometries, a necessary step is the analysis of roughness effects on the re- ceptivity of the hypersonic boundary-layer. The present paper aims to describe recent advances of an ongoing work on roughness effects on the hypersonic boundary layer transition. Direct numerical simulations of isolated roughness elements of two different heights immersed in a boundary layer are performed for a Mach number of M∞ = 6, and a Reynolds number based on the displacement thickness of Reδ ∗ = 28000. Broadband forcing of varying amplitudes is added and a preliminary analysis of the instabilities arising from the disturbed boundary layer is made. This analysis highlights the different disturbances exhibited by either roughness heights and the associated main frequencies. Weniger anzeigen

In this study, we present an immersed boundary reconstruction method for the simulation of viscous compressible flows presenting strong discontinuities in the vicinity of the immersed body. The technique we propose is based on a weighted least-square reconstruction. Its major feature is that the weights are automatically adapted to the presence of a discontinuity within the least-square stencil by using a shock-sensor approach. In so doing, the reconstruction becomes ENO-like and mathematically… Mehr anzeigen

In this study, we present an immersed boundary reconstruction method for the simulation of viscous compressible flows presenting strong discontinuities in the vicinity of the immersed body. The technique we propose is based on a weighted least-square reconstruction. Its major feature is that the weights are automatically adapted to the presence of a discontinuity within the least-square stencil by using a shock-sensor approach. In so doing, the reconstruction becomes ENO-like and mathematically consistent even in the presence of strong discontinuities. The objective of this paper is to demonstrate that it then becomes possible to use an immersed boundary method for the simulation of hypersonic flows and that it is robust, even for flows around complex geometries. To do so, two-dimensional compressible Navier-Stokes equations are solved to study canonical hypersonic flows and it is showed that the results are in good agreement with either experimental data and/or body-fitted approaches and that the impact of the immersed boundary technique on the overall spatial order of convergence is reasonable. Weniger anzeigen

In this paper, we present the hydrodynamic mechanisms which occur between a low-hypersonic shock wave and a millimetric water droplet. To do so, two numerical models, based respectively on the Volume of Fluid (VOF) and Diffuse Interfaces (DI) approaches, are developed. The goal is to compare the results obtained with the models in order to evaluate which is the most accurate to describe the evolution of the physical phenomena. The studied Mach number and initial droplet diameter are 4.25 and… Mehr anzeigen

In this paper, we present the hydrodynamic mechanisms which occur between a low-hypersonic shock wave and a millimetric water droplet. To do so, two numerical models, based respectively on the Volume of Fluid (VOF) and Diffuse Interfaces (DI) approaches, are developed. The goal is to compare the results obtained with the models in order to evaluate which is the most accurate to describe the evolution of the physical phenomena. The studied Mach number and initial droplet diameter are 4.25 and 1.135 mm, respectively. Each model allows the compressible Euler equations to be solved in a 2D-axi-symmetric configuration. The evolution of both air and liquid phases is modelled by a stiffened gas equation of state. For qualitative validation, the numerical results are compared to experimental data recently presented in the literature. In this work, the authors used a shock tube test facility and a shad-owgraph visualization technique to observe the phenomenology over a long time. Their investigation shows that the droplet deformation, detached bow shock and recompression waves are well captured by the two models until a Rayleigh dimensionless time of 1.5. Beyond this critical time, and up to 3, some differences appear between the two numerical approaches, especially on the droplet deformation. Globally, the droplet deformation is better described with the VOF model, while the DI model appears to be more accurate when it comes to the evaluation of the position of the bow shock. In the discussion section, some ideas are proposed to improve the models. Weniger anzeigen

This paper focuses on the explorationof a compressible turbulent 2D flow, to better understand the flightbehavior of an object. Topological data analysis (TDA) is used toimprove understanding and avoid costly traditional methods such as 3D modal decomposition algorithms or highly technical hydrodynamic stability codes. The attention is put on the large eddies shed behind a cylinder hit by a crossflow. Thanks to TDA the tracking of the eddies, the identification of their origin and the evolution… Mehr anzeigen

This paper focuses on the explorationof a compressible turbulent 2D flow, to better understand the flightbehavior of an object. Topological data analysis (TDA) is used toimprove understanding and avoid costly traditional methods such as 3D modal decomposition algorithms or highly technical hydrodynamic stability codes. The attention is put on the large eddies shed behind a cylinder hit by a crossflow. Thanks to TDA the tracking of the eddies, the identification of their origin and the evolution of their amplitude with the downstream distance are facilitated. Weniger anzeigen

Although rotating cavity flows are essential components of industrial applications, their dynamics is still largely misunderstood. From computer hard-drives to turbo-pumps of space launchers, designed devices often produce flow oscillations that can either destroy the component prematurely or produce disturbing noise or undesired operating modes of the system. Fundamentals of encountered static and rotating flow boundary layers have evidenced, a long time ago now, the presence of specific… Mehr anzeigen

Although rotating cavity flows are essential components of industrial applications, their dynamics is still largely misunderstood. From computer hard-drives to turbo-pumps of space launchers, designed devices often produce flow oscillations that can either destroy the component prematurely or produce disturbing noise or undesired operating modes of the system. Fundamentals of encountered static and rotating flow boundary layers have evidenced, a long time ago now, the presence of specific boundary layer instabilities and structures for low Reynolds number. For higher Reynolds numbers and fully enclosed systems, features are however more complex. For these flows, Large Eddy Simulation (LES) has illustrated the capacity of reproducing features and limit cycles. However, identifying the origin and region within these flows that are responsible for mode selections remains difficult if not impossible using such Computational Fluid Dynamics (CFD) tools. The present contribution evaluates a LES and a global stability analysis framework to identify the triggering instability mechanisms responsible for the observed limit-cycles of two types of rotor-stator cavities. In particular, the presence of a central body or shaft and its impact on the instability selection is of interest here: i.e. the identification of the regions of mode activation for a cylindrical as well as an annular cavity are detailed. Results issued by the conjunct use of Dynamical Mode Decomposition (DMD) and Global Linear Stability Analysis (GLSA) confirm the observed LES dynamics. Most importantly GLSA gives access to the triggering mechanisms at the root of the limit-cycles expression as well as hints on the mode selection. Weniger anzeigen

The present investigation targets the generation of airfoil trailing-edge broadband noise that arises from the interaction of turbulent boundary layer with the airfoil trailing edge. Large-eddy simulations, carried out using a massively parallel compressible solver CharLESX, are conducted for a Controlled-Diffusion (CD) airfoil with rounded trailing edge for sevenconfigurations, characterized with a Reynolds number, angle of attack and Mach number. An analysis of the unsteady pressure signals… Mehr anzeigen

The present investigation targets the generation of airfoil trailing-edge broadband noise that arises from the interaction of turbulent boundary layer with the airfoil trailing edge. Large-eddy simulations, carried out using a massively parallel compressible solver CharLESX, are conducted for a Controlled-Diffusion (CD) airfoil with rounded trailing edge for sevenconfigurations, characterized with a Reynolds number, angle of attack and Mach number. An analysis of the unsteady pressure signals in the boundary layer is proposed in regard to classical trailing edge noise modelling ingredients. Weniger anzeigen

Self-sustained oscillations of rotor/stator cavity flows are well known to industry. This unsteady phenomenon can be very dangerous and jeopardize the structural integrity of aeronautical engines by damaging turbomachinery components or turbopumps in the context of space applications. Today, the origin of such flow instability and resulting limit-cycle is not well understood and still difficult to predict numerically. In order to have more insight of this phenomenon dynamic, an academic… Mehr anzeigen

Self-sustained oscillations of rotor/stator cavity flows are well known to industry. This unsteady phenomenon can be very dangerous and jeopardize the structural integrity of aeronautical engines by damaging turbomachinery components or turbopumps in the context of space applications. Today, the origin of such flow instability and resulting limit-cycle is not well understood and still difficult to predict numerically. In order to have more insight of this phenomenon dynamic, an academic rotor/stator cavity is investigated in the present paper. The main motivation of this study is to highlight the benefit of conjunct numerical strategies relying on Large Eddy Simulations (LES) and flow stability analyses to understand driving instability mechanisms. More specifically, results of a local and global methods are devised and compared to a Dynamic Mode Decomposition (DMD) of LES predictions. Good agreements between the stability methods studied and the present features in the LES limitcycle are found. On this basis, a sensitivity and receptivity analysis of the flow is realized to point the origin of the two most unstable modes: i.e the position within the flow where the problem issues. Weniger anzeigen

Rotating cavity flows are essential components of industrial applications but their dynamics are still not fully understood when it comes to the relation between the fluid organization and monitored pressure fluctuations. From computer hard-drives to turbo-pumps of space launchers, designed devices often produce flow oscillations that can either destroy the component prematurely or produce too much noise. In such a context, large scale dynamics of high Reynolds number rotor/stator cavities need… Mehr anzeigen

Rotating cavity flows are essential components of industrial applications but their dynamics are still not fully understood when it comes to the relation between the fluid organization and monitored pressure fluctuations. From computer hard-drives to turbo-pumps of space launchers, designed devices often produce flow oscillations that can either destroy the component prematurely or produce too much noise. In such a context, large scale dynamics of high Reynolds number rotor/stator cavities need better understanding especially at the flow limit-cycle or associated statistically stationary state. In particular, the influence of curvature as well as cavity aspect ratio on the large scale organization and flow stability at a fixed rotating disc Reynolds number is fundamental. To probe such flows, wall-resolved large eddy simulation is applied to two different rotor/stator cylindrical cavities and one annular cavity. Validation of the predictions proves the method to be suited and to capture the disc boundary layer patterns reported in the literature. It is then shown that in complement to these disc boundary layer analyses, at the limit-cycle the rotating flows exhibit characteristic patterns at mid-height in the homogeneous core pointing the importance of large scale features. Indeed, dynamic modal decomposition reveals that the entire flow dynamics are driven by only a handful of atomic modes whose combination links the oscillatory patterns observed in the boundary layers as well as in the core of the cavity. These fluctuations form macro-structures, born in the unstable stator boundary layer and extending through the homogeneous inviscid core to the rotating disc boundary layer, causing its instability under some conditions. More importantly, the macro-structures significantly differ depending on the configuration pointing the need for deeper understanding of the influence of geometrical parameters as well as operating conditions. Weniger anzeigen

Large scale dynamics of high Reynolds industrial rotor/stator cavities is today poorly understood although it remains critical in determining operating margins for many devices. For space applications, the unsteady pressure phenomena of turbopumps called "pressure bands" (PB) depend on geometrical and thermal parameters. Such dangerous operating modes are today not captured by CFD: the present primary goal is to investigate the ability of LES to accurately reproduce such flows and their… Mehr anzeigen

Large scale dynamics of high Reynolds industrial rotor/stator cavities is today poorly understood although it remains critical in determining operating margins for many devices. For space applications, the unsteady pressure phenomena of turbopumps called "pressure bands" (PB) depend on geometrical and thermal parameters. Such dangerous operating modes are today not captured by CFD: the present primary goal is to investigate the ability of LES to accurately reproduce such flows and their sensitivity in terms of large scale motions and spectral content. After validation of LES based on two geometries and thermalizations, we show that LES reproduces the spectral content overhaul triggered by the change of thermalization. The second objective is a study of the activity recorded in the different cases to improve our understanding of the PB phenomena. To do so, Dynamic Modal Decomposition (DMD) reveals that the dynamics of the flow is driven by some atomic modes whose combination explains the oscillatory signals registered by pointwise-probes. These modes form macro-structures occupying the interdisk space and are at the origin of the large-scale flow dynamics. Finally, they are observed to significantly differ depending on the configuration, confirming the dependency of PB phenomena on the operating condition and geometrical parameters. Weniger anzeigen

In pumps, cavities are naturally present at the junction of fixed and rotating parts. Mastering the flow stability in rotor/stator cavities is essential to avoid imposing too large flow variations which could lead to miss-tuned operating conditions in the engine and a drastic loss life-span. To cope with such complex geometry, fully unsteady flows, the so-called Large Eddy Simulation (LES) approach appears as a very promising method. However and for such high Reynolds number industrial… Mehr anzeigen

In pumps, cavities are naturally present at the junction of fixed and rotating parts. Mastering the flow stability in rotor/stator cavities is essential to avoid imposing too large flow variations which could lead to miss-tuned operating conditions in the engine and a drastic loss life-span. To cope with such complex geometry, fully unsteady flows, the so-called Large Eddy Simulation (LES) approach appears as a very promising method. However and for such high Reynolds number industrial applications, wall modeling still remains a necessity to alleviate the computational cost of LES. Understanding the impact of such a strong modeling hypothesis on the mean flow features and unsteady energetic content is hence mandatory and the objective of the present discussion. To do so, the present paper studies LES flow solutions in an enclosed rotor-stator configuration with a stationary shroud. These simulations are performed using a two-steps Taylor-Galerkin finite-element scheme coupled to a WALE subgrid scale model giving a better prediction of the eddy viscosity in zones of strong shear. This allows capturing the unsteady structures known to be present in the boundary layers. Two sets of statoric boundary conditions are investigated: the dynamics of the boundary layer is either resolved using a fine mesh grid or modeled using a classic boundary layer law of the wall. It is shown that the modeling of the stationary disc layer induces an underestimation of the flow velocity at low radii and loss of accuracy in the radial description of the boundary layer structures. Despite these differences, the most energetic structures are found to have the same azimuthal organization as in the wall resolved configuration as well as the same pulsation, which in turn produces a boundary layer with the same spectral content as in the wall resolved test case. Weniger anzeigen

Although impressive advances have been achieved in the area of tabulated chemistry for turbulent combustion modeling, table generation remains a tedious task, sometimes as costly as the computation itself. In this work, we intend to offer an alternative to tabulation, applied to the MIL turbulent combustion model. An explicit expression for chemical times, function of pressure, temperature, and mixture fraction (Boivin et al., 2011), is successfully implemented and used instead of tabulated… Mehr anzeigen

Although impressive advances have been achieved in the area of tabulated chemistry for turbulent combustion modeling, table generation remains a tedious task, sometimes as costly as the computation itself. In this work, we intend to offer an alternative to tabulation, applied to the MIL turbulent combustion model. An explicit expression for chemical times, function of pressure, temperature, and mixture fraction (Boivin et al., 2011), is successfully implemented and used instead of tabulated chemistry in the simulation of a supersonic H2–O2 lifted flame. Weniger anzeigen