Publisher version : http://pof.aip.org/resource/1/phfle6/v22/i1/p014102_s1?isAuthorized=no; The effects of non-normality and nonlinearity of the two-dimensional Navier–Stokes differential operator on the dynamics of a large laminar separation bubble over a flat plate have been studied in both subcritical and slightly supercritical conditions. The global eigenvalue analysis and direct numerical simulations have been employed in order to investigate the linear and nonlinear stability of the flow. The steady-state solutions of the Navier–Stokes equations at supercritical and slightly subcritical Reynolds numbers have been computed by means of a continuation procedure. Topological flow changes on the base flow have been found to occur close to transition, supporting the hypothesis of some authors that unsteadiness of separated flows could be due to structural changes within the bubble. The global eigenvalue analysis and numerical simulations initialized with small amplitude perturbations have shown that the non-normality of convective modes allows the bubble to act as a strong amplifier of small disturbances. For subcritical conditions, nonlinear effects have been found to induce saturation of such an amplification, originating a wave-packet cycle similar to the one established in supercritical conditions...
Publisher version : http://pof.aip.org/resource/1/phfle6/v22/i11/p114102_s1?isAuthorized=no; The three-dimensional stability dynamics of a separation bubble over a flat plate has been studied in both linear and nonlinear conditions. Using a global eigenvalue analysis, two centrifugal global modes are identified: an asymptotically unstable three-dimensional weakly growing mode which appears to be originated by a Rayleigh instability; a marginally stable three-dimensional steady mode which is originated by a convective Gortler instability. Direct numerical simulations show that both modes play a role in the route to transition toward the turbulent flow. A structural sensitivity analysis is used to investigate the mechanism of selection of the path toward transition when small perturbations are considered. Finally, a scenario of transition via Gortler modes breakdown is studied in detail, revealing the formation of trains of hairpin vortices in streamwise succession.
Recent studies have suggested that in some cases transition can be triggered by some purely nonlinear
mechanisms. Here we aim at verifying such an hypothesis, looking for a localized perturbation able to lead a
boundary-layer flow to a chaotic state, following a nonlinear route. Nonlinear optimal localized perturbations
have been computed by means of an energy optimization which includes the nonlinear terms of the Navier-
Stokes equations. Such perturbations lie on the turbulent side of the laminar-turbulent boundary, whereas, for
the same value of the initial energy, their linear counterparts do not. The evolution of these perturbations
toward a turbulent flow involves the presence of streamwise-inclined vortices at short times and of hairpin
structures prior to breakdown.
This paper provides the analysis of bursting and transition to turbulence in a Couette flow, based on the growth of nonlinear optimal disturbances. We use a global variational procedure to identify such optimal disturbances, defined as those initial perturbations yielding the largest energy growth at a given target time, for given Reynolds number and initial energy. The nonlinear optimal disturbances are found to be characterized by a basic structure, composed of inclined streamwise vortices along localized regions of low and high momentum. This basic structure closely recalls that found in boundary-layer flow (Cherubini et al., J. Fluid Mech., vol. 689, 2011, pp. 221–253), indicating that this structure may be considered the most ‘energetic’ one at short target times. However, small differences in the shape of these optimal perturbations, due to different levels of the initial energy or target time assigned in the optimization process, may produce remarkable differences in their evolution
towards turbulence. In particular, direct numerical simulations have shown that optimal disturbances obtained for large initial energies and target times induce bursting events, whereas for lower values of these parameters the flow is directly attracted towards the turbulent state. For this reason...
Publisher version : http://iopscience.iop.org/1873-7005/44/3/031404; The understanding of transition in shear flows has recently progressed
along new paradigms based on the central role of coherent flow structures
and their nonlinear interactions. We follow such paradigms to identify, by
means of a nonlinear optimization of the energy growth at short time, the
initial perturbation which most easily induces transition in a boundary layer.
Moreover, a bisection procedure has been used to identify localized flow
structures living on the edge of chaos, found to be populated by hairpin vortices and streaks. Such an edge structure appears to act as a relative attractor for the trajectory of the laminar base state perturbed by the initial finite-amplitude disturbances, mediating the route to turbulence of the flow, via the triggering of a regeneration cycle of Lambda and hairpin structures at different space and time scales. These findings introduce a new, purely nonlinear scenario of transition in a boundary-layer flow.
Publisher version : http://pof.aip.org/resource/1/phfle6/v23/i5/p051705_s1?isAuthorized=no; The understanding of laminar-turbulent transition in shear flows has recently progressed along new paradigms based on the central role of nonlinear exact coherent states. We follow such paradigms to identify, for the first time in a spatially developing flow, localized flow structures living on the edge of chaos, which are the precursors of turbulence. These coherent structures are constituted by hairpin vortices and streamwise streaks. The results reported here extend the dynamical systems description of transition to spatially developing flows.
version post-print de l'article :
JLA Vol : 24 Iss:3. 2D longitudinal modeling of heat transfer and fluid flow during multilayered direct laser metal deposition process; Derived from laser cladding, the Direct Laser Metal Deposition (DLMD) process is
based upon a laser beam – powder – melt pool interaction, and enables the manufacturing
of complex 3D shapes much faster than conventional processes. However, the surface
finish remains critical, and DLMD parts usually necessitate post-machining steps. Within
this context, the focus of our work is to improve the understanding of the phenomena
responsible for deleterious surface finish by using numerical simulation. Mass,
momentum, and energy conservation equations are solved using COMSOL
Multiphysics® in a 2D transient model including filler material with surface tension and
thermocapillary effects at the free surface. The dynamic shape of the molten zone is
explicitly described by a moving mesh based on an Arbitrary Lagrangian Eulerian
method (ALE). This model is used to analyze the influence of the process parameters,
such as laser power, scanning speed, and powder feed rate, on the melt pool behavior.
The simulations of a single layer and multilayer claddings are presented. The numerical
results are compared with experimental data...
The authors wish to thank Region Nord-Pas de Calais and CNRS for their financial support in the frame of the CISIT program; The paper presents analysis of the performance and the internal flow behaviour in the vaned diffusor of a radial flow pump using PIV (Particle Image Velocimetry) and pressure probe traverses. PIV measurements have already been performed at mid height inside one diffusor channel passage for a given speed of rotation and various mass flow rates. These results have been already presented in several previous communications. New experiments have been performed using a 3 holes pressure probe traverses from hub to shroud diffusor width at different radial locations between the two diffusor geometrical throats. Numerical simulations are also realized with the commercial codes Star CCM+ 7.02.011 and CFX. Frozen rotor and fully unsteady calculations of the whole pump have been performed. Comparisons between numerical results, previous experimental PIV results and new probe traverses one’s are presented and discussed for one mass flow rate. In this respect, a first attempt to take into account fluid leakages between the rotating and fixed part of the pump has been checked since it may affect the real flow structure inside the diffuser
In this study unsteady numerical simulations with software Star-CCM+ were conducted to understand clearly the Savonius wind turbine operation, especially by comparing static results (motionless wind turbine) and dynamic results (wind turbine in rotation). This study provides a new vision of its behavior. Indeed, the Savonius is so-called “drag” as its torque is maximum when the blades are perpendicular to the flow direction (maximum projected area). This fact is actually verfyed with a static study. However, the dynamic study shows that the maximum torque is obtained when the axis of the blade is parallel to the wind, so the Savonius wind turbine would be “lift”. This study opens new perspectives for the optimization of the Savonius rotors.; Cluster éolien W4F
This work provides a global optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of smooth three-dimensional roughness elements. Amplification mechanisms are described which can bypass the asymptotical growth of Tollmien–Schlichting waves. Smooth axisymmetric roughness elements of different height have been studied, at different Reynolds numbers. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can localize the optimal disturbance characterizing the Blasius boundary-layer flow. Moreover, for large enough
bump heights and Reynolds numbers, a strong amplification mechanism has been recovered, inducing an increase of several orders of magnitude of the energy gain with respect to the Blasius case. In particular, the highest value of the energy gain is obtained for an initial varicose perturbation, differently to what found for a streaky parallel flow. Optimal varicose perturbations grow very rapidly by transporting the strong wall-normal shear of the base flow, which is localized in the wake of the bump. Such optimal disturbances are found to lead to transition for initial energies and amplitudes considerably smaller than sinuous optimal ones...
The present work provides an optimal control strategy, based on the nonlinear
Navier–Stokes equations, aimed at hampering the rapid growth of unsteady finite-amplitude perturbations in a Blasius boundary-layer flow. A variational procedure is used to find the blowing and suction control law at the wall providing the maximum damping of the energy of a given perturbation at a given target time, with the final aim of leading the flow back to the laminar state. Two optimally growing finite-amplitude initial perturbations capable of leading very rapidly to transition have been used to initialize the flow. The nonlinear control procedure has been found able to drive such perturbations back to the laminar state, provided that the target time of the minimization and the region in which the blowing and suction is applied have been suitably chosen. On the other hand, an equivalent control procedure based on the linearized Navier–Stokes equations has been found much less effective, being not able to lead the flow to the laminar state when finite-amplitude disturbances are considered. Regions of strong sensitivity to blowing and suction have been also identified for the given initial perturbations: when the control is actuated in such regions...
This work provides a three-dimensional energy optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of roughness elements. The immersed boundary technique has been coupled with a Lagrangian optimization in a three-dimensional framework. Four roughness elements with different heights have been studied, inducing amplification mechanisms that bypass the asymptotical growth of Tollmien-Schlichting waves. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can strongly localize the optimal disturbance. Moreover, the highest value of the energy gain is obtained for a varicose perturbation. This result demonstrates the relevance of varicose instabilities for such a flow and shows a different behavior with respect to the secondary instability theory of boundary layer streaks.
This paper provides an investigation of the structure of the stable manifold of the lower branch steady state for the plane Couette flow. Minimal energy perturbations to the laminar state are computed, which approach within a prescribed tolerance the lower branch steady state in a finite time. For small times, such minimal-energy perturbations maintain at least one of the symmetries characterizing the lower branch state. For a sufficiently large time horizon, such symmetries are broken and the minimal-energy perturbations on the stable manifold are formed by localized asymmetrical vortical structures. These minimal-energy perturbations could be employed to develop a control procedure aiming at stabilizing the low-dissipation lower branch state.
We use direct numerical simulations in the presence of free-stream turbulence having different values of intensity, T u, and integral length scale, L, in order to determine which kind of structures are involved in the path to transition of a boundary-layer flow. The main aim is to determine under which conditions the path to transition involves structures similar to the linear or non-linear optimal perturbations. For high values of T u and L, we observe a large-amplitude path to transition characterized by localized vortical structures and patches of high- and low-momentum fluctuations. Such a scenario is found to correlate well with the L and hairpin structures resulting from the time evolution of non-linear optimal perturbations, whereas, for lower T u and L, a larger correlation is found with respect to linear optimal disturbances. This indicates that a large-amplitude path to transition exists, different from the one characterized by elongated streaks undergoing secondary instability. To distinguish between the two transition scenarios, a simple parameter linked to the streamwise localisation of high- and low-momentum zones is introduced. Finally, an accurate law to predict the transition location is provided, taking into account both T u and L...
This paper is devoted to the 3D extension of a sharp interface multimaterial method.
We present preliminary results for 3D stiff cases of shock air/helium/water interaction and impacts of projectiles on elastic bodies immersed in air.
Recently, the need for smaller axial‐flow fans with high specific speeds leads to the design of counter‐rotating axial fans. The design of this type of machines, which have promising aerodynamic performances, suffers from a lack of knowledge about their aerodynamics. Counter‐rotating rotors, widely studied in aeronautics, are an effective alternative to conventional machines and offer many advantages: rotation ratio and diameter reduction, and high flexibility in use. However, a better understanding of their working and of the rotors interaction is required to enhance their design and to make them widely integrated in current applications. This experimental research work investigates
on a ducted counter‐rotating stage designed with a home code, MFT based on an inverse design method for rotors and rotor‐stator stages, and to which a rapid and simple method is implemented to design the rear rotor. The study focuses on the effects of the rotation ratio and on the axial spacing between rotors. It highlights several aspects of the rotors interaction through global performance and local unsteady measurements.
L’étude a pour objet l’aide au développement d’une méthode de pré-dimensionnement des ventilateurs axiaux opérant en champs contraints. L’influence d’un obstacle situé à l’aval d’un ventilateur est étudiée expérimentalement sur quatre rotors de vitesses angulaires spécifiques différentes. Les rotors sont testés sur un banc d’essais normalisé de type caisson aspirant. Les rotors fonctionnent en champ libre et en champs
contraints par 15 configurations de blocage. Les blocages sont réalisés par une plaque plane, perpendiculaire à l’axe de rotation du rotor, modélisant par exemple l’obstruction d’un bloc moteur automobile à combustion interne. Les quatre jeux de performances globales (élévation de pression, débit et
rendement statique) évoluant avec la distance à l’obstacle sont comparés pour mettre en évidence leurs similarités. On observe des variations d’élévation de pression en fonction du débit quand l’obstacle se rapproche du ventilateur. Les courbes caractéristiques semblent évoluer autour d’un pivot dont la position
par rapport au débit nominal du rotor en champ libre pourrait être en relation avec la vitesse angulaire spécifique.
This study deals with the development of an efficient method of design for subsonic conter-rotating axial flow turbomachineries. From specifications of a reference axial pump with only one rotor, two configurations of counter-rotating systems are studied : a system with a lower specific speed, and a system with
a smaller specific radius. The numerical study is conducted in stationary regime, with a RANS-type turbulence model. Numerical simulations of the flow in stationary cavitating conditions use an homogeneous model at the thermodynamic equilibrium state. Results obtained for the two counter-rotating
systems increase the yield of 12% for the same operating point. The first configuration decreases the size of 25%, which is a favorable for the manufacturing cost and the use in the system. The second configuration allows to obtain a lower speed of rotation (35%) which is beneficial for the increase of
the suction capacity.
The paper presents the results of numerical analysis on the local and global internal flow behaviour at the inlet of the vaned diffuser of a radial flow pump model, taking into account the effect of fluid leakages for various flow rates and a given rotation of speed.
For each flow rate, numerical calculations were performed both with two different boundary conditions:
- Without any leakage effects.
- With calculated leakage effects
The numerical simulations were realized with the two commercial codes: i-Star CCM+ 8.02.011 (at LML), ii-CFX 10.0 (at University of Padova). RANS unsteady calculations, with a k- RNG model were performed with Star CCM+. Fully unsteady calculations of the whole pump were done with CFX with DES turbulentmethod (Cavazzini ).
For each flow rate, different angular positions of the impeller are considered.
First part of the paper shows global comparisons between numerical and experimental results already presented in ref [2-3], for which the effects of fluid leakage due to the gap between the rotating and fixed part of the pump model were found to be interesting to be analysed.
Second part is devoted to the local comparisons of flow structures at the inlet section of the diffuser, without and with leakages only with Star CCM+.
In water supply installations, noise pollution often occurs. As a basic component of a system,
a flush valve may frequently be a source of noise and vibration generated by cavitation or high turbulence
levels. During valve closing or valve opening, cavitation can be a problem. In order to decrease the noise
and to improve the design inside a flush valve, some experimental and numerical analyses were carried
out in our laboratories. These analyses led to some improvements in the design of the valves. Cavitation
occurrence was more specifically addressed, using numerical simulation, and this is the main aim of the
present paper. Particularly, the use of a simplified numerical test without cavitation model is compared
with one using a cavitation model. In order to define potential cavitation risks in some parts of the valve, it
has been found that a simplified approach provides an accurate overview. Computational Fluid Dynamics
(CFD) simulations of cavitating flow of water through an industrial flush valve were performed using
the Reynolds averaged Navier-Stokes (RANS) equations with a near-wall turbulence model. The flow was
assumed turbulent, incompressible and steady. Two commercial CFD codes (Fluent 6.3 and Star CCM+
3.04.009) were used to analyse the effects of inlet pressure as well as mesh size and mesh type on cavitation
intensity in the flush valve.; contrat Arts