Crossflow instability in rotor-stator flows with throughflow

We study the formation of new spiral patterns, denoted SRJ2 at the periphery of an enclosed rotor-stator cavity when an axial inward or outward throughflow is superimposed

Shear-layer instability in a rotating system

International audienceThe shear-layer instability in the flow over a rotating disk with a free surface is investigated experimentally by flow visualizations for a large range of the flow control parameters: the aspect ratio G of the cavity, the rotationnal Reynolds number Re and the radius ratio s between the inner and outer radii of the rotating disk. This instability develops along the cylindrical shroud as sharp-cornered polygonal patterns characterized by the number of vortices m. This number m can be scaled by considering an Ekman number based on the water depth, which confirms that the shroud boundary layer is of Stewartson type. The appearance threshold of the first polygonal mode is constant by considering the mixed Reynolds number introduced by Niino and Misawa (1984) based on both the water depth at rest and the rotating disk radius. For large values of s, the instability patterns appear along the hub as small stationary cells

Instabilité d'une couche de cisaillement au-dessus d'un disque en rotation

International audienceCette étude expérimentale porte sur l'instabilité d'une couche de cisaillement au-dessus d'un disque en rotation avec surface libre. Cette instabilité est caractérisée par visualisations de l'écoulement pour une large gamme des paramètres de contrôle: le rapport d'aspect G de la cavité, le nombre de Reynolds global Re et le rapport s entre les rayons intérieur et extérieur du disque tournant. Cette instabilité spectaculaire se développe le long du cylindre extérieur sous la forme de polygones à m côtés. Ce nombre m dépend d'un nombre d'Ekman basé sur la hauteur d'eau au repos, ce qui confirme la nature « stewartsonienne » de la couche limite. Le seuil d'apparition du premier mode est constant si l'on considère le nombre de Reynolds mixte proposé par Niino and Misawa (1984). Pour des larges valeurs de s, une instabilité se développe le long du cylindre intérieur sous la forme de petites cellules

Large eddy simulation and measurements of turbulent enclosed rotor-stator flows

International audienceTurbulent flows are studied in an actual enclosed rotor-stator configuration with a rotating hub and a stationary shroud. Besides its fundamental importance - the disk boundary layer is one of the simplest platforms for investigating the underlying structure of three-dimensional boundary layers - this cavity models more complex configurations relevant to rotating machinery. Large Eddy Simulation (LES) is performed using a Spectral Vanishing Viscosity (SVV) technique which is shown leading to stable discretizations without sacrificing the formal accuracy of the spectral approximation. Numerical results and velocity measurements have been favorably compared for a large range of rotational Reynolds numbers up to one million in an annular cavity of curvature parameter Rm=(b+a)/(b-a)=1.8 and of aspect ratio G=(b-a)/h=5, where a and b are respectively the inner and outer radii of the rotating disk and h is the interdisk spacing. In the detailed picture of the flow structure that emerges, the turbulence is mainly confined in the boundary layers including in the Stewartson layer along the external cylinder. For Reynolds numbers larger than 0.1 million, the stator boundary layer is turbulent over most of the cavity. On the other hand, the rotor layer becomes progressively turbulent from the outer radial locations although the rotating hub is shown to destabilize the inner part of the boundary layers. The isosurface maps of the Q-criterion reveal that the three-dimensional spiral arms observed in the unstable laminar regime evolve to more axisymmetric structures when turbulence occurs. At Re equal to one million, the flow is fully turbulent and the anisotropy invariant map highlights turbulence structuring, which can be either a ``cigar-shaped'' structuring aligned on the tangential direction or a ``pancake-shaped'' structuring depending on the axial location. The reduction of the structural parameter a1 (the ratio of the magnitude of the shear stress vector to twice the turbulence kinetic energy) under the typical limit 0.15, as well as the misalignment between the shear stress vector and the mean velocity gradient vector, highlight the three-dimensional nature of both rotor and stator boundary layers with a degree of three-dimensionality much higher than in the idealized system studied by Lygren and Andersson (2001-2006)

Large eddy simulation and measurements in a turbulent rotor-stator flow

International audienceThere have been numerous numerical simulations and experimental studies of flow between rotating and stationary discs with a stationary shroud and no throughflow (a “rotor-stator cavity”) (see references in Serre et al. 2001; Poncet et al. 2005; Randriamampianina & Poncet 2006). The flow has significant industrial applications, such as internal gas-turbine flows and computer hard disks, and the geometry is relatively simple. A characteristic feature of such flows is the coexistence of adjacent coupled flow regions that are radically different in terms of the flow properties (Serre et al. 2004). Moreover, the confinement, the flow curvature and the rotation effects create a strongly inhomogeneous and anisotropic turbulence. Consequently, these flows are very challenging for numerical modelling particularly in turbulent regimes (see a review in Crespo del Arco et al. 2005). Turbulent regimes are investigated here in an annular rotor-stator cavity, using experimental measurements as well as Large-Eddy Simulation (LES). At our knowledge, there has been no efficient investigation of turbulent rotor-stator flows within a closed interdisk cavity using LES. The mean flow is mainly governed by three control parameters: the aspect ratio of the cavity G(=(b-a)/h)=5, the rotational Reynolds number Re based on the outer radius b of the rotating disk and the radius ratio s(=a/b)=0.286. In this work, LES and experimental measurements have been used to characterize statistical properties of turbulent rotor-stator flows for Reynolds numbers up to one million. Till now, LES predictions have compared very favourably with experimental measurements for Reynolds number up to 0.7 million. In the oral presentation of this work it will be possible to show computations still in progress at the moment at Re equal to one million

A coupled numerical/experimental investigation of a turbulent rotor-stator flow

International audienceTurbulent incompressible flows are studied both numerically and experimentally within an annular rotor-stator cavity of aspect ratio G=(b-a)/h=5 and radius ratio a/b=0.286 (where a and b are the inner and outer radii of the rotating disk and h the interdisk spacing). Besides its fundamental importance as a three-dimensional prototype flow, such flows are crude models of flows arising in many industrial devices, especially in turbomachinary ap-plications. Our aim is to investigate turbulent regimes at three Reynolds numbers up to one million (Ω the rotation speed of the rotor and ν the kinematic viscosity of the fluid) corresponding to different flow prop-erties as the rotation of the rotor is increased. Experimental measurements have been obtained using a laser Dop-pler anemometer (LDA) technique. Numerical modelling is based on a Large Eddy Simulation (LES) using a spectral vanishing viscosity (SVV) technique implemented in a Chebyshev-collocation Fourier-Galerkin pseudo-spectral code. As far as the authors are aware, LES of fully turbulent flow in an actual shrouded ro-tor-stator cavity have not been performed before. Turbulent quantities are shown to compare very favourably with experimental measurements and are shown of interest in understanding the physics of turbulent rotor-stator flows from transitional to turbulent regimes. Moreover, averaged results may provide target data for workers employing RANS schemes

Comparative kinetic analysis of two fungal β-glucosidases

<p>Abstract</p> <p>Background</p> <p>The enzymatic hydrolysis of cellulose is still considered as one of the main limiting steps of the biological production of biofuels from lignocellulosic biomass. It is a complex multistep process, and various kinetic models have been proposed. The cellulase enzymatic cocktail secreted by <it>Trichoderma reesei </it>has been intensively investigated. β-glucosidases are one of a number of cellulolytic enzymes, and catalyze the last step releasing glucose from the inhibitory cellobiose. β-glucosidase (BGL1) is very poorly secreted by <it>Trichoderma reesei </it>strains, and complete hydrolysis of cellulose often requires supplementation with a commercial β-glucosidase preparation such as that from <it>Aspergillus niger </it>(Novozymes SP188). Surprisingly, kinetic modeling of β-glucosidases lacks reliable data, and the possible differences between native <it>T. reesei </it>and supplemented β-glucosidases are not taken into consideration, possibly because of the difficulty of purifying BGL1.</p> <p>Results</p> <p>A comparative kinetic analysis of β-glucosidase from <it>Aspergillus niger </it>and BGL1 from <it>Trichoderma reesei</it>, purified using a new and efficient fast protein liquid chromatography protocol, was performed. This purification is characterized by two major steps, including the adsorption of the major cellulases onto crystalline cellulose, and a final purification factor of 53. Quantitative analysis of the resulting β-glucosidase fraction from <it>T. reesei </it>showed it to be 95% pure. Kinetic parameters were determined using cellobiose and a chromogenic artificial substrate. A new method allowing easy and rapid determination of the kinetic parameters was also developed. β-Glucosidase SP188 (K_m= 0.57 mM; K_p= 2.70 mM) has a lower specific activity than BGL1 (K_m= 0.38 mM; K_p= 3.25 mM) and is also more sensitive to glucose inhibition. A Michaelis-Menten model integrating competitive inhibition by the product (glucose) has been validated and is able to predict the β-glucosidase activity of both enzymes.</p> <p>Conclusions</p> <p>This article provides a useful comparison between the activity of β-glucosidases from two different fungi, and shows the importance of fully characterizing both enzymes. A Michaelis-Menten model was developed, including glucose inhibition and kinetic parameters, which were accurately determined and compared. This model can be further integrated into a cellulose hydrolysis model dissociating β-glucosidase activity from that of other cellulases. It can also help to define the optimal enzymatic cocktails for new β-glucosidase activities.</p

Sur les écoulements de disque tournant

Les écoulements de disque(s) tournant(s) trouvent des applications dans des domaines très variés comme l'astrophysique, la géophysique et surtout les turbomachines. Une majeure partie de cette étude concerne d'ailleurs les écoulements turbulents de type rotor-stator soumis à un flux axial, écoulements que l'on retrouve dans la turbopompe à hydrogène liquide du moteur Vulcain d'Ariane V. D'un point de vue fondamental, ils offrent une géométrie simple ou la rotation influence la structure de la turbulence. Cet article présente une revue sur les écoulements confinés entre un disque tournant et un disque fixe avec ou sans flux imposé pour une gamme de rapport d'aspect couvrant les écoulements de type Batchelor à couches limites séparées et les écoulements de type Couette de torsion à couches limites jointes. Ces écoulements ont été abordés sous différents aspects: stabilité, transition vers la turbulence et écoulements pleinement turbulents, que ce soit expérimentalement ou par simulation numérique. Des résultats concernant des écoulements turbulents avec transferts de chaleur, plus proches des écoulements industriels, sont également présentés