Florent Ravelet

Multistability and memory effect in a highly turbulent flow: experimental evidence for a global bifurcation.

We report experimental evidence of a global bifurcation on a highly turbulent von Kármán flow. The mean flow presents multiple solutions: the canonical symmetric solution becomes marginally unstable towards a flow which breaks the basic symmetry of the driving apparatus even at very large Reynolds numbers. The global bifurcation between these states is highly subcritical and the system thus keeps a memory of its history. The transition recalls low-dimension dynamical system transitions and exhibits very peculiar statistics. We discuss the role of turbulence in two ways: the multiplicity of hydrodynamical solutions and the effect of fluctuations on the nature of transitions.

Toward an experimental von Kármán dynamo : Numerical studies for an optimized design

Numerical studies of a kinematic dynamo based on von Karman type flows between two counterrotating disks in a finite cylinder are reported. The flow has been optimized using a water model experiment, varying the driving impellers’ configuration. A solution leading to dynamo action for the mean flow has been found. This solution may be achieved in VKS2, the new sodium experiment to be performed in Cadarache, France. The optimization process is described and discussed; then the effects of adding a stationary conducting layer around the flow on the threshold, on the shape of the neutral mode and on the magnetic energy balance are studied. Finally, the possible processes involved in kinematic dynamo action in a von Karman flow are reviewed and discussed. Among the possible processes, we highlight the joint effect of the boundary-layer radial velocity shear and of the Ohmic dissipation localized at the flow/outer-shell boundary.

On the dynamics and breakup of a bubble rising in a turbulent flow

Experimental investigations of the dynamics of a deformable bubble rising in a uniform turbulent flow are reported. The turbulence is characterized by fast particle image velocimetry. Time-resolved evolutions of bubble translation, rotation, and deformation are determined by three-dimensional shape recognition from three perpendicular camera views. The bubble dynamics involves three mechanisms fairly decoupled: (1) average shape is imposed by the mean motion of the bubble relative to liquid; (2) wake instability generates almost periodic oscillations of velocity and orientation; (3) turbulence causes random deformations that sometimes lead to breakup. The deformation dynamics is radically different from that observed in the absence of a significant sliding motion due to buoyancy. Large deformations that lead to breakup are not axisymmetric and correspond to elongations in the horizontal direction. The timescale of decay of shape oscillations is of the same order as their natural frequency f2, so that brea...

Experimental Study of the Instationary Flow Between Two Ducted Counter-Rotating Rotors

The purpose of this work is to study experimentally the aerodynamic characteristics of a subsonic counter-rotating axial-flow fans system operating in a ducted configuration. The fans of diameter D = 375 mm were designed to match the specification point using an original iterative method: the front rotor blade cascade is designed with a conventional inverse method, setting the radial distribution of the Euler work. The through-flow is then computed using an axisymmetric and radial equilibrium asumption, with empirical models of losses. The rear rotor is not conventional but is designed to straighten the radial profile of the tangential velocity. The design of the front rotor is then modified until the stage meets the requirements. The experimental setup is arranged such that the rotation rate of each fan is independently controlled and that the axial distance between the rotors can be varied from 17% to 310% of the mid-span chord length. Systematic measurements of the global performances and local measurements of the velocity field and of the wall pressure fluctuations are performed, in order to first validate the design method, and to explore the effects of the two specific free parameters of the system: the axial spacing and the ratio of rotation rates. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The developed design method slightly over-predicts the pressure rise and slightly under-predicts the best ratio of rotation rates. Flow angle measurements downstream of the stage show that the outflow is not completely straightened at the design point. Finally, the system is highly efficient on a wide range of flow-rates and pressure rises: this system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space.

Design and Experimental Validation of a Ducted Counter-rotating Axial-flow Fans System

An experimental study on the design of counter-rotating axial-flowfans was carried out. The fans were designed using an inversemethod. In particular, the system is designed to have a pure axialdischarge flow. The counter-rotating fans operate in a ducted-flowconfiguration and the overall performances are measured in a nor-malized test bench. The rotation rate of each fan is independentlycontrolled. The relative axial spacing between fans can vary from17% to 310%. The results show that the efficiency is stronglyincreased compared to a conventional rotor or to a rotor-statorstage. The effects of varying the rotation rates ratio on the overallperformances are studied and show that the system has a very flexi-ble use, with a large patch of high efficient operating points in theparameter space. The increase of axial spacing causes only a smalldecrease of the efficiency. [DOI: 10.1115/1.4007591]

Evidence for Forcing-Dependent Steady States in a Turbulent Swirling Flow

We study the influence of the forcing on the steady turbulent states of a von Kármán swirling flow, at constant impeller speed, or at constant torque. We find that the different forcing conditions change the nature of the stability of the steady states and reveal dynamical regimes that bear similarities with low-dimensional systems. We suggest that this forcing dependence may be an outof-equilibrium analogue of the ensemble inequivalence observed in long-range interacting statistical systems, and that it may be applicable to other turbulent systems.

Study of the cavitating instability on a grooved Venturi profile

Cavitation is a limiting phenomenon in many domains of fluid mechanics. Instabilities of a partial cavity developed on an hydrofoil, a converging-diverging step or in an inter-blade channel in turbomachinery, have already been investigated and described in many previous works. The aim of this study is to evaluate a passive control method of the sheet cavity. According to operating conditions, cavitation can be described by two different regimes: an unstable regime with a cloud cavitation shedding and a stable regime with only a pulsating sheet cavity. Avoiding cloud cavitation can limit structure damages since a pulsating sheet cavity is less agressive. The surface condition of a converging-diverging step, like a Venturi-type obstacle, is here studied as a solution for a passive control of the cavitation. This study discusses the effect of an organized roughness, in the shape of longitudinal grooves, on the developed sheet cavity. Analyzes conducted with Laser Doppler Velocimetry, visualisations and pressure measurements show that the grooves geometry, and especially the groove depth, acts on the sheet cavity dynamics. Results show that modifying the surface condition, by varying the grooves geometry, can reduce cavity sheet length and even suppress the cloud cavitation shedding.

POD study of aerated cavitation in a venturi nozzle

The fact of injecting bubbles into a cavitating flow influences typical cavitating behavior. Cavitation and aerated cavitation experiments has been carried out on a symmetrical venturi nozzle with convergent/divergent angles of 18° and 8°, respectively. A snapshot Proper Orthogonal Decomposition (POD) technique is used to identify different modes in terms of discharge flow velocity, pressure and injected quantity of air. The energy spectrum per given mode is also presented. The first four modes are outlined in the present paper for an aerated and non-aerated cavitating flows.

Influence of Reynolds number and forcing type in a turbulent von Kármán flow

We present a detailed study of of a global bifurcation occuring in a turbulent von Karman swirling flow. In this system, the statistically steady states progressively display hysteretic behaviour when the Reynolds number is increased above the transition to turbulence. We examine in detail this hysteresis using asymmetric forcing conditions -- rotating the impellers at different speeds. For very high Reynolds numbers, we study the sensitivity of the hysteresis cycle -- using complementary Particle Image Velocimetry (PIV) and global mechanical measurements -- to the forcing nature, imposing either the torque or the speed of the impellers. New mean states, displaying multiple quasi- steady states and negative differential responses, are experimentally observed in torque control. A simple analogy with electrical circuits is performed to understand the link between multi-stability and negative responses. The system is compared to other, similar bulk systems, to understand some relevant ingredients of negative differential responses, and studied in the framework of thermodynamics of long-range interacting systems. The experimental results are eventually compared to the related problem of Rayleigh-Benard turbulence.

Study of the Aerodynamics/Aeroacoustics of an Axial-Flow Fan: Experimental Validation of a LES/LPCE/Brinkman Penalization Method

Department of Mechanical Engineering, Korea University, Seoul, 136-713, KoreaThe seek for an efficient aerodynamic and aeroacoustic design of axial-flow fans is animportant field of investigation for both academic and applied research. Improvementscan only be made with a better understanding of the physical mechanisms arising in thesemachines that combine strong interactions between rotating and non-rotating parts ofhighly complex geometries. One way is to couple well-suited experimental investigationsand innovative computational methods, that overtake the weaknesses of methods based forinstance on aeroacoustic analogy. In this paper we study an axial fan using a new numericalmethod based on LES/LPCE Brinkman Penalization Method. This method is developedin the Department of Mechanical Engineering at Korea University. The experimental testsand validations are performed in the Laboratory of Fluid Dynamics at Arts & M´etiersParisTech in Paris. Detailed analysis of numerical and experimental results are in progresswithin the two partner teams. In this paper we present preliminary encouraging results.