Lionel Agostini

Lionel Agostini
French National Centre for Scientific Research | CNRS · Institut des sciences de l'ingénierie et des systèmes (INSIS)

Phd

About

52
Publications
9,148
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633
Citations
Additional affiliations
August 2014 - January 2016
The Ohio State University
Position
  • Research Associate
April 2012 - July 2014
Imperial College London
Position
  • Research Associate

Publications

Publications (52)
Article
Full-text available
Direct numerical simulation data for channel flow at a friction Reynolds number of 4200, generated by Lozano-Durán and Jiménez [J. Fluid Mech. 759, 432 (2014)], are used to examine the properties of near-wall turbulence within subranges of eddy-length scale. Attention is primarily focused on the intermediate layer (mesolayer) covering the logarithm...
Article
Full-text available
Several recent studies discuss of role of skewness of the turbulent velocity fluctuations in near-wall shear layers, in the context of quantifying the correlation between large-scale motions and amplitude variations of small-scale fluctuations -- referred to as ``modulation''. The present study is based on the premise that the skewness of the small...
Article
Full-text available
A large-eddy simulation-based study is presented and focuses on different unsteadiness-source features in a Mach 2.3 shock reflection with separation. The sources of unsteadiness are localized and the path taken by disturbance as it spreads out to the whole field is defined. It is shown that the phenomena arising inside the recirculation bubble gov...
Article
Full-text available
A direct-numerical-simulation-based study is presented, which focuses on the response of near-wall turbulence and skin friction to the imposition of an oscillatory spanwise wall motion in channel flow. One point of contrast to earlier studies is the relatively high Reynolds number of the flow, namely \$Re_\tau =1000\$ in the unforced baseline flow....
Article
Full-text available
Full flow-field data derived from a direct numerical simulation for channel flow subjected to drag-reducing oscillatory spanwise motion are analysed by means of a recently developed methodology, which consolidates the entire simulation data set within multiple-variable joint-probability-density functions (PDFs). A wide variety of statistical data o...
Article
Full-text available
Machine-learning (ML) algorithms offer a new path for investigating high-dimensional, nonlinear problems, such as flow-dynamical systems. The development of ML methods, associated with the abundance of data and combined with fluid-dynamics knowledge, offers a unique opportunity for achieving significant breakthroughs in terms of advances in flow pr...
Preprint
Full-text available
Machine-learning (ML) algorithms offer a new path for investigating high-dimensional, nonlinear problems, such as flow-dynamical systems. The development of ML methods, associated with the abundance of data and combined with fluid-dynamics knowledge, offers an unique opportunity for achieving major breakthroughs in terms of advances in flows predic...
Article
Full-text available
Although several mechanisms have been suggested as explanations for the low-frequency unsteadiness in shock wave/turbulent boundary layer interactions, questions remain on causes and effects. In this effort, we examine the observed asymmetry in large-scale shock motions to highlight which of the suggested mechanisms is most consistent with shock-sp...
Article
Full-text available
Data from a direct numerical simulation for channel flow at a friction Reynolds number of 1000 are analysed to derive statistical properties that offer insight into the mechanisms by which large-scale structures in the log-law region affect the small-scale turbulence field close to the wall and the statistical skin-friction properties. The data com...
Article
Full-text available
An examination is undertaken of the validity and limitations of the quasi-steady hypothesis of near-wall turbulence. This hypothesis is based on the supposition that the statistics of the turbulent fluctuations are universal if scaled by the local, instantaneous, wall shear when its variations are determined from footprints of large-scale, energeti...
Preprint
Full-text available
An examination is undertaken of the validity and limitations of the Quasi-Steady Hypothesis of near-wall turbulence. This Hypothesis is based on the supposition that the statistics of the turbulent fluctuations are universal if scaled by the local, instantaneous, wall-shear when its variations are determined from footprints of large-scale, energeti...
Preprint
Full-text available
Data from a direct numerical simulation for channel flow at a friction Reynolds number of 1000 are analysed to derive statistical properties that offer insight into the mechanisms by which large-scale structures in the log-law region affect the small-scale turbulence field close to the wall and the statistical skin-friction properties. The data com...
Article
Full-text available
This study is motivated by the observation that the drag-reduction effectiveness achieved by the imposition of oscillatory spanwise wall motion declines with Reynolds number. The question thus posed is whether the decline is linked to the increasingly strong influence of large-scale outer structures in the log layer on the near-wall turbulence, in...
Conference Paper
Full-text available
The paper examines various features of the premultiplied spectra , the premultiplied derivatives of the second-order structure function (PMDS2) and three scalar parameters that characterize the anisotropic or isotropic state of the various length-scale sub-ranges within the spectra for channel flow at Re τ = 4200, generated by Lozano-Duran and Jime...
Article
Full-text available
The properties of spectral subranges of scales in a boundary layer at Mach=2.3 and friction Reynolds number Reτ = 570 are investigated by analysing DNS data. One major aim is to examine whether footprinting and modulation of small-scale near-wall motions by outer large structures, observed at high Reynolds numbers, also pertain to this low-Reynolds...
Article
Local fluctuations in a Mach 1.3 cold jet are tracked to understand the genesis of nearfield directivity and intermittency. A newly developed approach leveraging two synchronized large-eddy simulations is employed to solve the forced Navier–Stokes equations, linearized about the evolving unsteady base flow. The results are summarized by exposing th...
Article
Full-text available
The “quasi-steady hypothesis,” as understood in the context of large-scale/small-scale interactions in near-wall turbulence, rests on the assumption that the small scales near the wall react within very short time scales to changes imposed on them by energetic large scales whose length scales differ by at least one order of magnitude and whose ener...
Article
Full-text available
A phenomenological model is provided, based on post-processing Direct Numerical Simulation (DNS) data at Reτ=1020, which permits the near-wall-turbulence statistics to be predicted from a “universal signal,” free from the effects of large-scale motions, in combination with information on the large-scale motions in the outer log-law region. The sepa...
Conference Paper
Full-text available
The features of the interaction between an oblique shock wave and a Mach 2.3 turbulent boundary layer are examined in an effort to differentiate the dynamics of bubble dilation (upstream shock motions) from bubble collapse (downstream shock motions). A dataset obtained from Large Eddy Simulations, which has been validated against experiments is use...
Conference Paper
Nearfield intermittency and spatio-temporal correlations of a Mach 1.3 cold jet are analyzed to highlight the filtering and directivity of turbulence scales. The intermittent wavepackets at the end of the potential core are found to occur primarily between St = 0.5 and St = 1 from the Hilbert Spectrum and the spec-trogram. The component scales of c...
Research
Full-text available
Several recent studies discuss of role of skewness of the turbulent velocity fluctuations in near-wall shear layers, in the context of quantifying the correlation between large-scale motions and amplitude variations of small-scale fluctuations – referred to as “modulation”. The present study is based on the remise that skewness should be accounted...
Conference Paper
Full-text available
A phenomenological model is provided, based on post-processing Direct Numerical Simulation (DNS) data at Reτ = 1020, which permits the near-wall-turbulence statistics to be predicted from a "universal signal," free from the effects of large-scale motions, in combination with information on the large-scale motions in the outer log-law region. The se...
Research
Full-text available
Separated flow are often strongly unsteady ; the aim of this thesis is to localize and identify the sources of the unsteadiness and to understand the physical phenomena governing the information transfer from these source zones to the rest of the flow. To do this, data used for this analysis have been obtained from numerical simulations (LES). Both...
Conference Paper
The effects of turbulent fluctuations on the axis and lip-line of a Mach 1.3 cold jet are analyzed with a synchronized pair of Large Eddy Simulations. The method uses two LESs to effectively boost naturally existing perturbations in one or more selected regions (" sources ") of the jet and track their evolution through the jet turbulence. The metho...
Article
Full-text available
Direct Numerical Simulation (DNS) data for channel flow at 1025 are used to analyse the interaction between large outer scales in the log-law region - referred to as super-streaks - and the small-scale, streaky, streamwise-velocity fluctuations in the viscosity-affected near-wall layer. The study is inspired by extensive experimental investigations...
Conference Paper
Full-text available
A framework for active wall-transpiration control of spatially developing, pre-transitional, boundary-layer flows over flat and concave walls (in the streamwise direction) is developed. The controller is designed to suppress the energy growth of streaks and Görtler vortices (with emphasis on the latter) induced by free-stream vortical disturbances...
Article
Full-text available
Shock-wave / boundary layer interactions usually exhibit unsteadiness with strong low frequency content. The present work aims at analyzing the low frequency shock motion using high resolution data from long-time large-eddy simulations. Three different flow configurations are considered yielding incipient to full separation of the boundary layer in...
Article
Full-text available
Large-eddy simulations (LES) of a shock impinging on a turbulent boundary layer are carried out and they demonstrate good agreement with the experiments. A special emphasis is put on the analysis of the three-dimensional (3D) modulations of the flow in order to clarify the origin of the mean vortices located in the separation region highlighted by...
Thesis
Full-text available
Separated flow are often strongly unsteady ; the aim of this thesis is to localize and identify the sources of the unsteadiness and to understand the physical phenomena governing the information transfer from these source zones to the rest of the flow. To do this, data used for this analysis have been obtained from numerical simulations (LES). Both...
Article
Full-text available
This paper aims at describing the main features of a separated boundary layer in shock reflection at a Mach number of 2.3. The data used for this analysis were taken from large eddy simulations of the interactions for three-shock intensities, resulting in flow cases from incipient to fully separated. Computational results were validated versus expe...
Article
Large-eddy simulations are carried out in order to clarify the origin of the large-scale spanwise modulations found experimentally within the separation bubble induced by a shock impinging on a turbulent boundary layer. Structures of various spanwise lengthscales are highlighted by means of short time averaged flowfield and spatial Fourier transfor...
Article
Full-text available
A key feature of rotating turbulence is the anisotropic transfer of energy towards horizontal modes, normal to the rotation axis Omega = Omega hat z eventually driving turbulence to a quasi-two-dimensional state [1, 2, 3]. While nonrotating homogeneous and isotropic turbulence shows direct energy cascade towards small scales, the energy may be tran...
Article
Full-text available
A Large-eddy simulation of a shock impinging on a turbulent boundary layer is carried out and demonstrates good agreement with the experiments. A special emphasis is put on the analysis of the three-dimensional modulation of the flow in order to clarify the origin of the mean vortices located in the separation region highlighted by the experiment

Projects