Stability and control of unsteady phenomena in rotor/stator cavities using Large Eddy Simulation

Unsteady phenomena in rotor/stator cavity are well known to be the source of dangerous vibrations in space turbopump. Even though many palliative measures have been taken during their design, experimental campaigns often reveal high flow oscillations that can jeopardize turbomachinery components and even the rocket engine. Today, the origin of such flow instabilities usually called ’pressure band phenomenon’(PBP) is not well understood and difficult to predict numerically. The main goal of this thesis is to investigate such phenomenon mechanism to find technical solutions so as to control it. This problematic is addressed here trough two types of configuration: an academic rotor/stator cavity and a space turbopump cavity. When it comes to cavity flows, their rotating boundary layers are known to be three dimensional and receptive to several instabilities taking the form of spirals or annuli. Reynolds Averaged Navier-Stokes Simulations (RANS)failed to predict such unsteady systems. However, Large Eddy Simulation (LES) proved to be a relevant alternative in many similar applications and is therefore chosen for the present work. Using Power Spectral Analysis (PSD) and Dynamic Mode Decomposition (DMD) on LES predictions, one shows that the PBP is retrieved in an annular smooth rotor/stator cavity and it is composed of three modes driving all the system dynamics. To investigate these mode organization and their possible interactions, a new tool called Dynamic Mode Tracking /Control (DMT/DMTC) is introduced. DMT is constructed so as to extract "on-thefly" flow coherent structures with a given frequency on the basis of LES. Furthermore, augmenting the Navier-Stokes equations with a relaxation term coupled to DMT, DMTC allows to control and follow the evolution of a controlled mode as well as non controlled ones and thereby observe interactions. This strategy after validation is applied to the annular rotating cavity and shows that the low frequency mode is generated by the dominant mode of the system. To go further, Global Linear Stability Analysis (GLSA) augmented with adjoint methods is used to shed some light on all mode origins and points out that the low frequency and dominant modes are coming from the stationary boundary layer. In order to set up control strategies, the GLSA framework is further developed introducing the concept of the sensitivity to base flow modifications which gives the location where the flow should be modified if one wants to stabilize or at least shift a frequency mode. Applied to the academic cavity, one shows that contrary to most studies in the literature, controlling the stator boundary layer is the more efficient way to damp the PBP through suction/injection devices. Finally, gathering all the previous understanding of this flow, the LES framework enables to validate the control strategies proposed and to stabilize the PBP for very low suction amplitudes. To finish, the PBP is analyzed in real space turbompump cavities. In particular, the sensitivity of this specific phenomenon to geometry changes is investigated through two configurations: one without and one with the blades of the stator of the turbopump. Even though the introduction of the blades in the LES creates a more complex flow with the presence of shocks, similar pressure fluctuation spectra are retrieved in both configurations but with azimuthal wavenumber modes that are shifted. Following the studies on the academic cavity, an adapted GLSA to the non-linear dynamics of the turbopump enables to point out that even though the PBP modes are particularly marked in the mainstream of the system, the source of these modes is located in the subcavity in the rotor-stator wheel space. In particular, GLSA results indicate that two possible ways to control the phenomenon are possible: modifying the flow around the seal rim and or modifying the leak around the hub

Computational fluid dynamics of vortex flow controls at low flow rates

PublishedJournal ArticleA vortex flow control with differing outlet shapes is investigated computationally at low flow rates. The volume of fluid method was utilised to track the moving free surface. In order to achieve a smooth free surface, interface compression coupled with the inter-gamma compressive scheme was used. The turbulent evolution of the two-phase flow was modelled by solving the Reynolds-averaged Navier-Stokes equations with the k-ε model for turbulent quantities. Validation of the results was carried out by analysing the total head and discharge coefficient for the three outlet shapes at various flow rates and comparing these results with experimental data. Very good agreement with the experimental data was obtained

An economic model for offshore cultivation of macroalgae

Algae biomass is considered as a potential non-fossil source of raw materials to produce fuel, feed, chemicals and materials. For this purpose microalgae as well as macroalgae can be used, and in this report we focus on the latter. More than 99% of the world production of aquatic plants is produced in Asia (FAO 2012, Table 1). From the remaining 1% about 4% is cultivated in Europe. Important European countries with commercial seaweed cultivation are Denmark, Ireland and France. Depending on their pigmentation seaweed species are commonly grouped in brown, red and green seaweeds

Extraction and Fractionation of Pigments from Saccharina latissima (Linnaeus, 2006) Using an Ionic Liquid plus Oil plus Water System

There is a strong industrial interest in the development of greener and more sustainable processes based on the use of renewable resources, and a biorefinery based on marine resources, such as macroalgae, stands as a major opportunity toward that end. In this work, Saccharina latissima (Linnaeus), a brown macroalga, was used as a source of pigments to develop an integrated platform that is able to promote the extraction and separation of chlorophyll and fucoxanthin in one single step. The process was studied, and its operational conditions were optimized with yields of extraction of chlorophyll and fucoxanthin of 4.93 ± 0.22 mgchl·gdry biomass–1 and 1956 ± 84 μgfuco·gdry biomass–1, respectively. These results were achieved with extraction systems composed of 84% of an aqueous solution of a tensioactive phosphonium-based ionic liquid (IL) at 350 mM + 16% of sunflower oil, during 40 min, using a solid–liquid ratio of 0.017 gdry biomass·mLsolvent–1. After the separation of both aqueous IL-rich and oil-rich phases, the IL content in both phases was investigated, the oil phase being free of IL. Envisioning the industrial potential of the process developed in this work, the recovery of the IL from the aqueous IL-rich phase of the initial system was attempted by a back-extraction using organic solvents immiscible in water, being shown that 82% of the IL can be recovered and reused in new cycles of extraction. The environmental and economic impacts of the final process proposed for the extraction and fractionation of chlorophyll and fucoxanthin were evaluated. Different scenarios were considered, but summing up the main results, the solvents’ recycling allowed better results, proving the economic and environmental viability of the overall process

Influence of process parameters on the particle–matrix interaction of WC-Co metal matrix composites produced by laser-directed energy deposition

The prediction of the in-service behaviour of metal-matrix composites produced by laser-directed energy deposition is a fundamental challenge in additive manufacturing. The interaction between the reinforcement phase and the matrix has a major impact on the micro and macroscopic properties of these materials. This interaction is fostered by the exposition of the materials to high temperatures. Hence, it is highly influenced by the thermal cycle of the manufacturing process. In this work, an experimental approach is adopted to determine the influence of the main process parameters on the properties of metal-matrix composites. Statistical regression models are employed to consider the role of the most relevant parameters, from exploration to exploitation. The obtained trends are further corroborated by the corresponding microstructural, SEM, and EDS analyses. In terms of surface hardness, the DOE reveals different trends of the response depending on the composition of the feedstock employed. It is concluded that the strengthening behaviour of the material varies throughout the experimental domain studied. When high WC% feedstocks are employed, the main strengthening mechanism responsible for the increase of hardness is the solid-solution of tungsten and carbide precipitation. On the contrary, when low WC%s are employed, grain refinement becomes the main strengthening mechanism.publishedVersionPeer reviewe

Potential process 'hurdles' in the use of macroalgae as feedstock for biofuel production in the British Isles

This review examines the potential technical and energy balance hurdles in the production of seaweed biofuel, and particular for the MacroBioCrude processing pipeline for the sustainable manufacture of liquid hydrocarbon fuels from seaweed in the UK. The production of biofuel from seaweed is economically, energetically and technically challenging at scale. Any successful process appears to require both a method of preserving the seaweed for continuous feedstock availability and a method exploiting the entire biomass. Ensiling and gasification offer a potential solution to these two requirements. However there is need for more data particularly at a commercial scal

Modelling of vortex flow controls at high drainage flow rates

A number of vortex flow control (VFC) devices for urban drainage systems are investigated computationally at high flow rates, for which a confined vortex dominates the flow regime. A range of turbulence models, including both eddy viscosity and Reynolds stress closures, are compared with in-house experimental measurements of head loss and internal pressure measurements. Single-phase and multi-phase (free surface) calculations are also compared. Very good agreement with the experimental data was obtained when the swirl parameter of the device was below 3.14 for predictions made using the Reynolds stress closure formulations. For devices with swirl parameters above this value, the computational methodology was found to under-predict the head loss of the device. This was attributed to poor calibration of the turbulence model for swirling flow scenarios in which the pressure gradient and diffusive (turbulent) forces in the flow are comparable

Stabilité et contrôle des phénomènes instationnaires dans les cavités de type rotor/stator par simulation aux grandes échelles

Les phénomènes instationnaires dans les cavités rotor/stator sont connus pour être la source de dangereuses vibrations dans les turbopompes spatiales. Bien que plusieurs mesures palliatives aient été prises en comptes durant les phases de conception, des campagnes d’essais ont mis en évidence de fortes oscillations des écoulements internes pouvant menacer le moteur cryogénique des lanceurs. Aujourd’hui, l’origine de ce phénomène, appelé «bandes de pression », est peu compris et difficile à prédire numériquement. L’objectif de cette thèse est d’analyser les mécanismes physiques de ce phénomène afin d’apporter des solutions pour le contrôler. Pour répondre à cette problématique, deux types de configuration sont étudiés: une cavité rotor/stator annulaire et une cavité de turbopompe spatiale. Les couches limites tournantes dans ces cavités sont connues pour être 3D et réceptives à plusieurs types d’instabilités prenant entre autre la forme de spirales ou d’anneaux. Les simulations basées sur la moyenne de Reynolds (RANS) ont par le passé échoué à prédire ce type d’écoulement. Cependant, les Simulations aux Grandes Echelles (SGE) se sont avérées être une alternative à ce problème et sont donc été utilisées tout au long de cette thèse. Des Densités Spectrales de Puissance (DSP) ainsi que des Décompositions modales dynamiques (DMD) appliqués aux résultats SGE, ont permis de montrer que le phénomène de bandes de pression est visible également dans une cavité annulaire de type académique et composé de trois modes dictant toute la dynamique du système. Afin d’étudier les interactions de ces modes, une nouvelle méthode appelée Dynamic Mode Tracking/Control (DMT/DMTC) a été proposée durant cette these. La DMT est construite pour extraire des structures cohérentes dans une simulation SGE. De plus, en ajoutant un terme de relaxation dans les équations de Navier-Stokes couplées avec la DMT, sa variante appelée DMTC permet de contrôler et de suivre en temps réelle un ou plusieurs modes et donc de pouvoir analyser de possibles interactions. Appliqué à la cavité académique annulaire, cette méthode a permis de montrer que le mode basse fréquence est généré dans l’écoulement par le mode dominant du système. Pour aller plus loin, des analyses de stabilité linéaire de type global (GLSA), sont effectuées sur la cavité académique. Grâce à des méthodes adjointes, la GLSA a permis de mettre en avant l’origine spatiale de chacun des trois modes. Deplus, afin de mettre en place des stratégies de contrôle, la sensibilité à la modification de l’écoulement de base, obtenue par GLSA, a permis d’identifier la région à modifier pour stabiliser un mode donné ou décaler sa fréquence. Appliqué au cas académique, il est montré et que contrôler la couche limite du stator est le moyen le plus efficace de supprimer le phénomène de bandes de pression à travers des injections/aspirations. Pour finir, le phénomène de bandes de pression est analysé dans une cavité de turbopompe spatiale. En particulier, la sensibilité de ce phénomène aux changements géométriques est abordée à travers deux configurations: une première sans les aubes du stator de la turbopompe et une deuxième avec. Bien que les aubes génèrent un écoulement complexe, des fréquences similaires de fluctuation de pression sont retrouvées dans les deux configurations avec cependant des nombres azimutaux caractéristiques différents. En se basant sur les études faites sur la cavité académique, une version adaptée de GLSA pour la dynamique de la turbopompe permet de mettre en avant que malgré que le phénomène de bandes pression soit particulièrement présent dans la veine de la turbopompe, la source de ces modes se situe dans les cavités inférieures entre le rotor et le stator. De plus les résultats de GLSA mettent en avant que deux moyens de contrôle pourraient être appliqués pour supprimer le phénomène de bandes de pression dans ce cas industriel: modifier le joint d’étanchéité ou modifier la fuite du moyeu.Unsteady phenomena in rotor/stator cavity are well known to be the source of dangerous vibrations in space turbopump. Even though many palliative measures have been taken during their design, experimental campaigns often reveal high flow oscillations that can jeopardize turbomachinery components and even the rocket engine. Today, the origin of such flow instabilities usually called ’pressure band phenomenon’(PBP) is not well understood and difficult to predict numerically. The main goal of this thesis is to investigate such phenomenon mechanism to find technical solutions so as to control it. This problematic is addressed here trough two types of configuration: an academic rotor/stator cavity and a space turbopump cavity. When it comes to cavity flows, their rotating boundary layers are known to be three dimensional and receptive to several instabilities taking the form of spirals or annuli. Reynolds Averaged Navier-Stokes Simulations (RANS)failed to predict such unsteady systems. However, Large Eddy Simulation (LES) proved to be a relevant alternative in many similar applications and is therefore chosen for the present work. Using Power Spectral Analysis (PSD) and Dynamic Mode Decomposition (DMD) on LES predictions, one shows that the PBP is retrieved in an annular smooth rotor/stator cavity and it is composed of three modes driving all the system dynamics. To investigate these mode organization and their possible interactions, a new tool called Dynamic Mode Tracking /Control (DMT/DMTC) is introduced. DMT is constructed so as to extract "on-thefly" flow coherent structures with a given frequency on the basis of LES. Furthermore, augmenting the Navier-Stokes equations with a relaxation term coupled to DMT, DMTC allows to control and follow the evolution of a controlled mode as well as non controlled ones and thereby observe interactions. This strategy after validation is applied to the annular rotating cavity and shows that the low frequency mode is generated by the dominant mode of the system. To go further, Global Linear Stability Analysis (GLSA) augmented with adjoint methods is used to shed some light on all mode origins and points out that the low frequency and dominant modes are coming from the stationary boundary layer. In order to set up control strategies, the GLSA framework is further developed introducing the concept of the sensitivity to base flow modifications which gives the location where the flow should be modified if one wants to stabilize or at least shift a frequency mode. Applied to the academic cavity, one shows that contrary to most studies in the literature, controlling the stator boundary layer is the more efficient way to damp the PBP through suction/injection devices. Finally, gathering all the previous understanding of this flow, the LES framework enables to validate the control strategies proposed and to stabilize the PBP for very low suction amplitudes. To finish, the PBP is analyzed in real space turbompump cavities. In particular, the sensitivity of this specific phenomenon to geometry changes is investigated through two configurations: one without and one with the blades of the stator of the turbopump. Even though the introduction of the blades in the LES creates a more complex flow with the presence of shocks, similar pressure fluctuation spectra are retrieved in both configurations but with azimuthal wavenumber modes that are shifted. Following the studies on the academic cavity, an adapted GLSA to the non-linear dynamics of the turbopump enables to point out that even though the PBP modes are particularly marked in the mainstream of the system, the source of these modes is located in the subcavity in the rotor-stator wheel space. In particular, GLSA results indicate that two possible ways to control the phenomenon are possible: modifying the flow around the seal rim and or modifying the leak around the hub

Qualité de vie du parkinsonien

CLERMONT FD-BCIU-Santé (631132104) / SudocLYON1-BU Santé (693882101) / SudocSudocFranceF