Sylvain Lardeau

Direct numerical simulation of a jet controlled by fluid injection

This study is concerned with direct numerical simulations of jet control carried out by means of secondary jets. The use of realistic inflow conditions enables us to examine the influence of the side jets on the vortical structure dynamics in the main jet. Four control configurations are presented for several positions and flow rates of the control jets. It can be seen that the jet expansion is strongly modified by fluid injection as previously observed experimentally and numerically, with a significant reduction in the potential core length. Secondary jets induce strong anisotropy on the mean flow, whereas the global effect on turbulent fluctuations is more limited. Contrary to an a priori point of view, structural modifications of the main flow due to a fluid injection do not lead to an important increase in the enstrophy production. The strong distortion of the main flow seems to be related to the appearance of large-scale vortices. These structures, organized in two counter-rotating vortex pairs, are ...

Unsteady Reynolds-Averaged Navier-Stokes Computations of Transitional Wake/Blade Interaction

The interaction between wakes generated by a moving array of cylindrical bars and the transitional boundary layer over a low-pressure turbine blade located downstream of the bars is studied by means of an unsteady Reynold-averaged Navier-Stokes (RANS) method that incorporates a recentlow-Reynolds-number, nonlinear eddy-viscosity turbulence model. The context of the study is the modelling of unsteady rotor/stator-interaction phenomena. A number of issues pertaining to the ability of the unsteady RANS approach to simulate this interaction within a phase-averaged framework are investigated, including numerical accuracy and resolution, the consequence of vortex shedding from the bars, the characteristics of the wake within the blade passage, and the ability of the model to return the transitional response of the boundary layer to the dynamics of and the turbulence transported by the wakes

Direct numerical simulation of a mixing layer downstream a thick splitter plate

In this numerical study, the flow obtained behind a trailing edge separating two streams of different velocities is studied by means of direct numerical simulation. The main originality of this work is that the splitter plate itself is included in the computational domain using an immersed boundary method. The influence of the trailing-edge shape is considered through the analysis of the destabilizing mechanisms and their resulting effect on the spatial development of the flow. The streamwise evolution of the different flows is found to be very different for each of the configurations considered, both in terms of mean quantities and flow dynamics. Present results suggest that the wake component, which dominates the flow close to the trailing edge, is still influential further downstream, as already observed in pure wake flows but only conjectured in mixing layer. A detailed analysis of the vortex dynamics is proposed using instantaneous visualizations, statistical/stability analysis considerations, and pr...

Large-eddy simulation of turbulent boundary layer separation from a rounded step

Highly resolved large-eddy simulation (LES) is used to investigate the characteristics of a canonical boundary layer separating from a curved step in a channel of height 8.5 times that of the step. The flow is treated as statistically spanwise homogeneous, in line with the conditions of a related experimental study in a large aspect ratio channel, undertaken within a companion research programme. Primary attention focuses on the details of the separation process and the properties of the separated region, including reattachment. Results are reported and analysed, from a flow physical perspective, for a wide variety of properties, including wall pressure and skin friction, mean velocity, Reynolds stresses and related anisotropy maps, two-point-correlation functions, unsteadiness indicators, budgets of the Reynolds stresses and length scales characterising the turbulence, and mean strain fields. The study highlights a range of distinctive features of separation from gently curved surfaces: the separation pr...

Large Eddy Simulation of Transitional Boundary Layers at High Free-Stream Turbulence Intensity and Implications for RANS Modeling

Large-eddy simulations of transitional flows over a flat plate have been performed for different sets of free-stream-turbulence conditions. Interest focuses, in particular, on the unsteady processes in the boundary layer before transition occurs and as it evolves, the practical context being the flow over low-pressure turbine blades. These considerations are motivated by the wish to study the realism of a RANS-type model designed to return the laminar fluctuation energy observed well upstream of the location at which transition sets in. The assumptions underlying the model are discussed in the light of turbulence-energy budgets deduced from the simulations. It is shown that the pretransitional field is characterized by elongated streaky structures which, notwithstanding their very different structural properties relative to fully established turbulence, lead to the amplification of fluctuations by conventional shear-stress/shear-strain interaction, rather than by pressure diffusion, the latter being the process underpinning the RANS-type transitional model being investigated.

Simulation of slot and round synthetic jets in the context of boundary-layer separation control.

Synthetic jets—also referred to as mass-less jets—offer the potential of effective, on-demand, fluid-based control of separating boundary layers on highly loaded aerodynamic surfaces, without the need for a mass source. However, the control authority that may optimally be derived from such jets, and any generality of the underlying flow physics are obscured by the wide range of geometric and flow parameters that contribute to their performance characteristics. The present article reviews the state-of-the art in the area of computational modelling and simulation of synthetic jets, with emphasis placed on key fluid-mechanics phenomena. The review is divided into two principal parts, one focusing on slot jets and the other on round jets. Within the latter part, ongoing research by the authors on the simulation of synthetic jets discharged into a separated boundary layer is highlighted as an example of the current status in this area.

The streamwise drag-reduction response of a boundary layer subjected to a sudden imposition of transverse oscillatory wall motion

A direct numerical simulation study is presented, which examines the response of a spatially developing boundary layer to oscillatory spanwise wall motion imposed over a limited streamwise stretch. At the heart of the study is the dependence of the streamwise variations in skin friction and turbulence properties on the period of the oscillatory motion, with particular emphasis placed on the behaviour downstream of the start of the actuation. The friction Reynolds number just upstream of the actuation is Reτ = 520, and the wall-scaled actuation period, T+ = Tuτ2/ν, covers the range 80–200. In contrast to channel flow, the present configuration allows the processes during the transition stretch from the unactuated state to the low-drag state and the recovery from the low-drag state to be studied. Attention focuses primarily on the former. Results are included for the time-averaged turbulent stresses, their budgets and probability-density functions, as well as a range of phase-averaged properties. The study ...

Modeling of Wake-Induced Transition in Linear Low-Pressure Turbine Cascades

An unsteady Reynolds-averaged Navier-Stokes (URANS) strategy is applied to the problem of wake-induced transition at high freestream turbulence on the suction side of two blades representative of those used in low-pressure turbines. Experimentally, the blades are arranged in high-aspect-ratio linear cascades, with upstream circular bars generating passing wakes, and two-dimensional flow conditions are, thus, assumed. The strategy combines an explicit algebraic Reynolds-stress turbulence model with transition-specific modifications targeted at capturing the effects of high freestream turbulence and of pretransitional laminar fluctuations. Close attention is paid to numerical accuracy, and grids of up to 140,000 cells are used in combination with 800 time steps per pitchwise traverse to resolve small-scale features in the blade boundary layers that are associated with the unsteady interaction. The computational results demonstrate that the combined model returns a good representation of the response of the suction-side boundary layer to the passing wakes in both blades. Specifically, in the boundary layer of one of the two blades, the wakes are observed to cause a periodic upstream shift in the transition onset and, thus, correspondingly periodic attachment and calming. In the other, no separation occurs, and the wakes are shown to produce a significant periodic reduction in shape factor and increase in skin friction in the blade boundary layer, again as a consequence of the upstream shift in the transition location.

Analysis of a jet–mixing layer interaction

Abstract The improvement of mixing in free-shear flows via external jets has been proven efficient in subsonic and supersonic flows as well. However, the hyper-mixing process is not well known. The present study deals with an experimental and a numerical approach of the interaction of an external control jet with a turbulent mixing layer. The main conclusion is that an intermittent penetration of the control jet occurs both in supersonic and subsonic configurations. Moreover, all results tend to show that the control jet flapping frequency and the spacing between the structures involved downstream of the interaction are respectively very close to the frequency and wavelength of the Kelvin–Helmholtz structures at the impact location. Two hypotheses are provided in order to explain the mechanism of the interaction. The first one is based upon the interaction with the passage of Kelvin–Helmholtz structures in the mixing layer, the other deals with an intrinsic instability of such a flow configuration.

Three-dimensional direct numerical simulation of electromagnetically driven multiscale shallow layer flows: Numerical modeling and physical properties

Three-dimensional (3D) direct numerical simulations of a flow driven by multiscale electromagnetic forcing are performed in order to reproduce with maximum accuracy the quasi-two-dimensional (2D) flow generated by the same multiscale forcing in the laboratory. The method presented is based on a 3D description of the flow and the electromagnetic forcing. Very good agreements between our simulations and the experiments are found both on velocity and acceleration field, this last comparison being, to our knowledge, done for the first time. Such agreement requires that both experiments and simulations are carefully performed and, more importantly, that the underlying simplification to model the experiments and the multiscale electromagnetic forcing do not introduce significant errors. The results presented in this paper differ significantly from previous 2D direct numerical simulation in which a classical linear Rayleigh friction modeling term was used to mimic the effect of the wall-normal friction. Indeed, ...