Eric Lamballais

Experimental and numerical studies of the flow over a circular cylinder at Reynolds number 3900

This work contributes to the study of flow over a circular cylinder at Reynolds number Re=3900. Although this classical flow is widely documented in the literature, especially for this precise Reynolds number that leads to a subcritical flow regime, there is no consensus about the turbulence statistics immediately just behind the obstacle. Here, the flow is investigated both numerically with large eddy simulation and experimentally with hot-wire anemometry and particle image velocimetry. The numerical simulation is performed using high-order schemes and a specific immersed boundary method. The present study focuses on turbulence statistics and power spectra in the near wake up to ten diameters. Statistical estimation is shown to need large integration times increasing the computational cost and leading to an uncertainty of about 10% for most flow characteristics considered in this study. The present numerical and experimental results are found to be in good agreement with previous large eddy simulation da...

High-order compact schemes for incompressible flows: A simple and efficient method with quasi-spectral accuracy

In this paper, a finite difference code for Direct and Large Eddy Simulation (DNS/LES) of incompressible flows is presented. This code is an intermediate tool between fully spectral Navier-Stokes solvers (limited to academic geometry through Fourier or Chebyshev representation) and more versatile codes based on standard numerical schemes (typically only second-order accurate). The interest of high-order schemes is discussed in terms of implementation easiness, computational efficiency and accuracy improvement considered through simplified benchmark problems and practical calculations. The equivalence rules between operations in physical and spectral spaces are efficiently used to solve the Poisson equation introduced by the projection method. It is shown that for the pressure treatment, an accurate Fourier representation can be used for more flexible boundary conditions than periodicity or free-slip. Using the concept of the modified wave number, the incompressibility can be enforced up to the machine accuracy. The benefit offered by this alternative method is found to be very satisfactory, even when a formal second-order error is introduced locally by boundary conditions that are neither periodic nor symmetric. The usefulness of high-order schemes combined with an immersed boundary method (IBM) is also demonstrated despite the second-order accuracy introduced by this wall modelling strategy. In particular, the interest of a partially staggered mesh is exhibited in this specific context. Three-dimensional calculations of transitional and turbulent channel flows emphasize the ability of present high-order schemes to reduce the computational cost for a given accuracy. The main conclusion of this paper is that finite difference schemes with quasi-spectral accuracy can be very efficient for DNS/LES of incompressible flows, while allowing flexibility for the boundary conditions and easiness in the code development. Therefore, this compromise fits particularly well for very high-resolution simulations of turbulent flows with relatively complex geometries without requiring heavy numerical developments.

Large- and small-scale stirring of vorticity and a passive scalar in a 3-D temporal mixing layer

We investigate, with the aid of a three‐dimensional direct‐numerical simulation at high resolution, the origin and topology of the longitudinal vortex filaments which appear in the temporally growing mixing layer. The basic velocity field is a hyperbolic‐tangent profile U tanh(2y/δi), with a Reynolds number of Uδi/ν =100. The calculation uses pseudospectral methods, and is carried out at a resolution of 1283 grid points in a cubic box of size L containing four fundamental most‐amplified wavelengths (L=4λa). The initial velocity field is the basic velocity, upon which is superposed a three‐dimensional Gaussian perturbation of wide spectrum, peaking at ka=1/2πλa, with kinetic energy equal to 10−4U2, modulated by a Gaussian exp[−(y/δi)2] in the transverse direction. A passive‐scalar transport equation is solved as well, with the same initial profile as the basic velocity profile. Isosurfaces of the passive scalar and three vorticity components are visualized, permitting the 3‐D vortex structure of the flows ...

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 ...

Subsonic jet noise reduction by fluidic control : The interaction region and the global effect

A microjet arrangement comprising both penetration (or immersion) and convergence (jets oriented such that two jets of a pair interact with one another) is used to control a subsonic turbulent jet with a view to noise reduction. The acoustic effect of the so-called fluidevron system is comparable to chevrons and nonconverging microjets as far as the noise reduction is concerned. Detailed experimental measurements are performed for a main jet with Mach and Reynolds numbers of 0.3 and 310 000, respectively. A direct numerical simulation study is performed for a model, plane mixing-layer problem using the immersed-boundary method, in order to help understand the topological features of the fluidevron–mixing-layer interaction. In terms of modifications produced in the flow, two relatively distinct regions are identified: the near-nozzle region, 0 1, where the jet recovers many of the uncontrolled-jet flow ch...

Short note: Straightforward high-order numerical dissipation via the viscous term for direct and large eddy simulation

In this short note, we show how to use a highly accurate finite-difference scheme to compute second derivatives in the Navier-Stokes equations while ensuring targeted numerical dissipation. This approach, essentially non conservative, is shown to be close to an upwind method and is straightforward to implement with a negligible computational extra cost. The benefit offered by the resulting discrete operator is illustrated for the direct computation of sound in aeroacoustics and in the more general context of large-eddy simulation through connections with hyperviscosity and spectral vanishing viscosity.

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...

Direct numerical simulations of vortex shedding behind cylinders with spanwise linear nonuniformity

Three-dimensional direct numerical simulations of vortex shedding behind cylinders have been performed when the body diameter and the incoming flow involved spanwise linear nonuniformity. Four configurations were considered: the shear flow, the tapered cylinder and their combination which gave rise to namely the adverse and aiding cases. In contrast with the observations of other investigators, these computations highlighted distinct vortical features between the shear case and the tapered case. In addition, it was observed that the shear case and the adverse case (respectively tapered case and aiding case), yielded similarities in flow topology. This phenomenon was explained by the spanwise variations of U/D which seemed to govern these flows. Indeed, it was observed that large spanwise variations of U/D seemed to enhance three dimensionality, through the appearance of vortex adhesions and dislocations. Spanwise cellular pattern of vortex shedding was identified. Their modifications in cell size, junctio...

Combination of the immersed boundary method with compact schemes for DNS of flows in complex geometry

We present a direct forcing method better suited for the use of compact finite difference schemes in Direct Numerical Simulation. The new forcing creates inside the body an artificial flow preserving the no-slip condition at the surface but reducing the step-like change of the velocity derivatives across the immersed boundary. This modification is shown to improve results both qualitatively and quantitatively for conventional and complex flow geometries.

Direct numerical simulation of interactions between a mixing layer and a wake around a cylinder

Direct numerical simulations of turbulent flows around a cylinder are performed using the virtual boundary technique to model the presence of the obstacle. This method consists of the imposition of a no-slip boundary condition within the flow field, using a specific forcing term added to the momentum equation. In this paper, two different inflow conditions are considered upstream from the cylinder. In the first case, where the inflow conditions correspond to a constant velocity flow, common features of the cylinder wake dynamics are well recovered (three-dimensional vortex shedding) while turbulent statistics (mean velocity and Reynolds stresses) are in good agreement with previous experimental and numerical results. This clearly shows that a code based on high-order finite difference schemes combined with the virtual boundary method can lead to reliable results even if the grid is not well designed for the shape of the obstacle. In the second case, the inflow conditions correspond to a spatially developi...