Prediction of the unsteady turbulent flow in an axial compressor stage. Part 2: Analysis of unsteady RANS and LES data

This paper presents the analysis of URANS and LES database in a stage of an axial subsonic compressor. Details about numerical methods and comparison with experiments can be found in a companion paper. The analysis here focuses on the transition processes that take place in the rotor and stator rows. In the rotor, LES and URANS show that transition develops at mid-chord and is induced by the adverse pressure gradient. In the stator, the flow behavior is more complex since the transition is influenced by the rotor passing wakes, a laminar separation bubble on the suction side and the accumulation of rotor wakes on the pressure side. The analysis also investigates the unsteady flow patterns at the rotor/stator interface, from mid-span to the casing. In the tip region, LES shows the development of frequencies that are not correlated to the blade passing frequency, while URANS only predicts multiples of the blade passing frequency

Regularized characteristic boundary condition for the Lattice Boltzmann methods at high Reynolds number flows

This paper reports the investigations done to adapt the Characteristic Boundary Conditions (CBC) to the Lattice-Boltzmann formalism for high Reynolds number applications. Three CBC formalisms are implemented and tested in an open source LBM code: the baseline one-dimension inviscid (BL-LODI) approach, its extension including the effects of the transverse terms (CBC-2D) and a local streamline approach in which the problem is reformulated in the incident wave framework (LS-LODI). Then all implementations of the CBC methods are tested for a variety of test cases, ranging from canonical problems (such as 2D plane and spherical waves and 2D vortices) to a 2D NACA profile at high Reynolds number (Re = 100,000), representative of aeronautic applications. The LS-LODI approach provides the best results for pure acoustics waves (plane and spherical waves). However, it is not well suited to the outflow of a convected vortex for which the CBC-2D associated with a relaxation on density and transverse waves provides the best results. As regards numerical stability, a regularized adaptation is necessary to increase the Reynolds number. The so-called regularized FD adaptation, a modified regularized approach where the off-equilibrium part of the stress tensor is computed thanks to a finite difference scheme, is the only tested adaptation that can handle the high Reynolds computation

Reynolds, Mach, and Freestream Turbulence Effects on the Flow in a Low-Pressure Turbine

This article presents the large-eddy simulation (LES) of a low-pressure turbine (LPT) nozzle guide vane (NGV) for different Reynolds (Re) and Mach (Ma) numbers with or without inlet turbulence prescribed. The analysis is based on a slice of an LPT blading representative of a midspan flow, where secondary flows, hub, and shroud effects are lower. The characteristic Re of the LPT can vary by a factor of four between take-off and cruise conditions. In addition, the LPT operates at different Ma values, and the incident flow can have significant levels of turbulence due to upstream blade wakes. This article investigates numerically using LES the flow around an LPT blading with three different Reynolds number Re= 175,000 (cruise), 280,000 (mid-level altitude), and 500,000 (take-off) keeping the same characteristic Mach number Ma = 0.2 and three different Mach number Ma= 0.2, 0.5, and 0.8 keeping the same Reynolds number Re= 280,000. These different simulations are performed with 0% freestream turbulence (FST) followed by inlet turbulence (6% FST). The study focuses on the influence of these three parameters (Re, Ma, and upstream turbulence) on different flow characteristics: pressure distribution around the blade, near-wall flow behavior, loss generation, and turbulent kinetic energy (TKE) budget. The results show an earlier boundary layer separation on the aft region of the blade suction side when the Re is increased, while the increase of the Ma delays separation, similar to freestream turbulence. The TKE budget led on the different cases shows the predominant effect of the turbulent production and diffusion in the wake, the axial evolution of these different terms being relatively insensitive to Re and Ma

Discharge coefficient of an orifice jet in cross flow: influence of inlet conditions and optimum velocity ratio

International audienceThe present work aims to characterize the discharge performance of aircraft door vent flaps. For this purpose, three different configurations with increasing complexity are studied with a RANS and a LES solver. The first configuration consists of an orifice plate in a duct for which experimental pressure loss data are available in the literature. This configuration is used as a reference for the validation of the RANS and LES setups. The duct placed downstream of the orifice is then removed to produce an unconfined geometry in which the orifice jet discharges either into an open atmosphere or a transverse flow. Finally, a classic jet in cross flow is also studied. The main objective is to analyze the discharge coefficient variations depending on three key parameters: (i) the jet Reynolds number, (ii) the inlet velocity profile, and (iii) the velocity ratio between the jet and the cross flow. Results show that for cases without cross flow, the jet Reynolds number has no influence on the discharge performance whereas a steady decrease of the orifice pressure loss is observed as the duct inlet velocity profile is deformed from that of a flat profile. The Poiseuille profile is found to minimize the pressure loss. In addition, numerical data of the reference configuration compare well with experimental values when such a profile is prescribed. Finally, simulations with a cross flow evidence an optimal velocity ratio for which the discharge coefficient is maximum and exceeds the freejet value

Conception efficace de soufflante sous distorsion d'entrée d'air

International audienceDesigning a turbofan to operate in distorted inlet conditions is an issue of growing interest. In such conditions however, fan design can be computationally challenging. Indeed, subject to neither axi-symmetrical nor periodic inlet conditions, computations must be carried out throughout the whole circumferential domain i.e. 360 •. Besides, the classical CFD approach implies the use of URANS computations so as to capture the distortion transfer across the fan stage. Eventually, computations are too onerous to be used in design loops. In this context, this paper presents a methodology to effectively assess a fan blade design domain in distorted conditions. This methodology is based on a body-force source term approach formulated in order to accurately recreate deviations, loads and losses across the fan stage. It notably enables to gain two orders in terms of restitution time and thus the possibility to use optimization tools. The design domain of this study is based on variations of profile chord, blade leading and trailing edges angles applied at two different relative heights. A Latin Hypercube Sampling (LHS) associated with a meta-model based on Radial Basis Functions (RBF) enables to assess the impact of geometric variations on performance and operability. Although this study emphasizes that some modeling improvements are still necessary, it also demonstrates the potential of the body-force methodology to conduct fan design when subject to inlet distortion.La conception de soufflante sous distorsion d'entrée d'air est un problème de plus en plus abordé. Cependant, sous ces conditions, les simulations de soufflante peuvent être complexes. En effet, sous conditions ni périodiques ni axisymétriques, les simulations doivent a priori être exécutée sur l'ensemble de la circonférence. Cela impose aussi l'utilisation de méthodes instationnaires type U-RANS afin de reproduire le transfère de la distorsion à travers l'étage. En partant de ce contexte, cette publication présente une méthodologie efficace pour mener une conception de soufflante sous distorsion d'entrée d'air. Cette méthodologie se base sur l'utilisation de termes sources alias "body-force" afin de correctement reproduire les effets de déviation, de charge et de pertes à travers l'étage de soufflante. Cela permet en outre une diminution du temps de simulation de deux ordres de grandeur et ainsi l'utilisation de méthodologie d'optimisation. Le domaine de conception de cette étude inclut des variations d'angle squelette de bord d'attaque et de bord de fuite ainsi que des variations de corde sur deux hauteurs de contrôle. Une surface de réponse définie par une méthode LHS associée à un méta-modèle basé sur des fonctions RBF ont permis d'évaluer l'impact de variations géométriques sur les performances et l'opérabilité d'une soufflante UHBR sous distorsion de pression d'arrêt. Bien que cette étude conclut sur le fait que de nombreux points sont à perfectionner, elle démontre aussi le fort potentiel de ces méthodes "body-force" au sein des bureau d'étude des concepteurs de soufflante, notamment lorsque les conditions à l'amont ne sont plus homogènes

Aerodynamic analysis of 3D multi-elements wings : an application to wingsails of flying boats

International audienceA study of multi-elements wings applied to a wingsail of a america's cup yacht

Large Eddy Simulation of flows in industrial compressors: a path from 2015 to 2035

A better understanding of turbulent unsteady flows is a necessary step towards a breakthrough in the design of modern compressors. Due to high Reynolds numbers and very complex geometry, the flow that develops in such industrial machines is extremely hard to predict. At this time, the most popular method to simulate these flows is still based on a Reynolds Averaged Navier-Stokes (RANS) approach. However there is some evidence that this formalism is not accurate for these components, especially when a description of time-dependent turbulent flows is desired. With the increase in computing power, Large Eddy Simulation (LES) emerges as a promising technique to improve both knowledge of complex physics and reliability of flow solver predictions. The objective of the paper is thus to give an overview of the current status of LES for industrial compressor flows as well as to propose future research axes regarding the use of LES for compressor design. While the use of wall-resolved LES for industrial multistage compressors at realistic Reynolds number should not be ready before 2035, some possibilities exist to reduce the cost of LES, such as wall-modelling and the adaptation of the phase lag condition. This paper also points out the necessity to combine LES to techniques able to tackle complex geometries. Indeed LES alone, i.e. without prior knowledge of such flows for grid construction or the prohibitive yet ideal use of fully homogeneous meshes to predict compressor flows, is quite limited today

Delineating loss sources within a linear cascade with upstream cavity and purge flow

Purge air is injected in cavities at hub of axial turbines to prevent hot mainstream gas ingestion into inter-stage gaps. This process induces additional losses for the turbine due to an interaction between purge and mainstream flow. This paper investigates the flow in a low-speed linear cascade rig with upstream hub cavity at a Reynolds number commonly observed in modern low pressure turbine stages by the use of numerical simulation. Numerical predictions are validated by comparing against experimental data available. Three different purge mass flow rates are tested using three different rim seal geometries. Numerical simulations are performed using a Large Eddy Simulation (LES) solver on structured grids. An investigation of the different mechanisms associated to turbine flow including cavity and purge air is intended through this simplified configuration. The underlying mechanisms of loss are tracked using an entropy formulation. Once described for a baseline case, the influence of purge flow and rim seal geometry on flow mechanisms and loss generation are described with the emphasis to obtain design parameters for losses reduction. The study quantifies loss generation due to boundary layer on wetted surfaces and secondary vortices developing in the passage. The analysis shows different paths by which purge flow and rim seal geometry can change loss generation including a modification of the shear layer between purge and mainstream, interaction with secondary vortices and a modification of the flow behavior close to hub compared to a smooth configuration. The study shows the influence of purge flow rate and swirl on the strengthening of secondary vortices in the passage and the ability of axial overlapping rim seal to delay the development of secondary vortices compared to simple axial gaps

Simulation numérique de l'écoulement en régime de pompage dans un compresseur axial multi-étage

Dans le contexte économique et environnemental actuel, la prochaine génération de moteurs d avion devra offrir opérabilité, compacité et hauts rendements. Les compresseurs demeurent une des pièces critiques de ces moteurs, et leur conception un challenge. À débit réduit, leur plage de fonctionnement est contrainte par la limite de pompage, phénomène hautement instable et dangereux. À ce jour, peu d études expérimentales sur un compresseur en situation de pompage ont été réalisées, étant donné le danger inhérent pour les installations. Dans ce cadre, la simulation numérique peut apporter des informations sur le développement des instabilités aérodynamiques et aider à la prévision de la limite de pompage. L objectif du travail présenté dans cette thèse est de mettre en place une méthode afin de simuler numériquement l entrée en pompage et un cycle complet de l instabilité avec le code elsA. Le cas test retenu est le compresseur de recherche axial multi-étage CREATE dessiné par Snecma, et étudié expérimentalement par le LMFA. Des études antérieures ont montré le rôle joué par les volumes entourant le compresseur ; l originalité de cette étude réside donc dans l inclusion des volumes du banc d essai dans la simulation du compresseur. Une des difficultés inhérentes à la simulation de ces instabilités est leur temps caractéristique, qui représente plus d une centaine de rotations de la machine. Le calcul a donc nécessité le recours à une approche massivement parallèle ; environ un million d heures CPU ont été utilisées pour décrire le cycle. Enfin, compte tenu du retournement de l écoulement dans le compresseur, les conditions aux limites ont été modifiées pour pouvoir s adapter aux changements de sens de l écoulement. La simulation a permis de décrire l entrée en pompage et un cycle complet de l instabilité. La comparaison avec les données expérimentales montre que les caractéristiques du cycle sont correctement prédites (phénomènes physiques précurseurs de l instabilité, durée du cycle..). En parallèle, une étude acoustique a été menée afin de mettre en évidence les modes propres du banc d essai. L analyse de ces résultats a notamment montré le rôle de l acoustique dans le déclenchement du pompage. Les différentes phases du cycle de pompage sont ensuite étudiées, et caractérisées (déclenchement, débit inversé, récupération et recompression). Ce travail a généré une base de données qui permet de mieux comprendre les instabilités qui se développent dans ce type de machine. À terme, ces résultats pourront être utilisés pour élaborer et valider des modélisations du phénomène de pompage moins coûteuses, pouvant intervenir dans un cycle de conception.In order to deal with the current economical and environmental context, the next engine generation will need to offer great operability, compactness and high efficiency. In aircraft engines, the compressor remains one of the critical components, and its design is still a challenging task. At low massflow rate, their operability is bounded by the surge limit, surge being a highly unstable and dangerous phenomenon. Today, few experimental studies on compressor surge are available because of the inherent threat to the facility. In that context, numerical simulation can bring about information on the onset of aerodynamic instabilities and help to predict the surge limit. The work presented in this PhD thesis aims at setting up a method to perform the numerical simulation of surge inception and of an entire cycle of the instability with the CFD code elsA. The chosen test case is the axial multistage research compressor CREATE designed and built by Snecma, and experimentally studied at LMFA. Previous studies have pointed out the role of the volumes adjacent to the compressor ; the originality of this work is thus the inclusion of the volumes of the test-rig in the simulation of the compressor. One of the difficulties inherent to the simulation of those instabilities is their characteristic time of at least one hundred revolutions of the machine. Hence the computation has required a massively parallel approach and about one million CPU hours. Finally, given that the flow reverses during a surge cycle, the boundary conditions have been modified to be able to cope with the flow inversions. The simulation was able to capture surge inception and the entire cycle of the instability. The comparison with the experimental data showed that the main patterns of the cycle are correctly predicted (precursor phenomena of surge, duration of the cycle...). In the meantime, an acoustic study has been performed in order to isolate the eigenmodes of the test-rig. The analysis of the results pointed out the role of acoustic phenomena in surge inception. The different phases of the cycle are then studied and characterized (surge inception, reversed-flow phase, recovery and repressurization). This work has incremented a database that allows a better understanding of the instabilities that develop in this kind of machine. From now on, those results may help to elaborate and validate cheaper models of the surge phenomenon to be used in the design process.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Fluid–Structure Interactions and Unsteady Kinematics of a Low-Reynolds-Number Rotor

Micro air vehicles are used for both civil and military applications, like rescue or surveillance. The aerodynamic performance of the rotor is known to be lower than for classical large rotors, due to increased drag at low Reynolds numbers. However, the rotor performance can be improved by taking advantage of the flow unsteadiness and considering unsteady rotor kinematics, like a periodic variation of the rotor pitch. To study such behaviors, it is necessary to develop numerical methods adapted to these fluid–structure interaction phenomena, which are the main objectives of this paper. The method relies on the implementation of fluid–structure interaction capabilities in a lattice–Boltzmann flow solver, which is implemented in a monolithic fashion using generalized coordinates. The validation is first conducted on a vortex-induced vibration test case. Then, numerical simulations are performed for a rotor test case 1) with a forced motion and 2) by coupling the flow with the equation of the dynamics. Some semianalytical models are derived and validated against the numerical simulations to predict the effects of pitching, flapping, and surging on the thrust. The results show that flapping and surging significantly increase the rotor thrust, but at the price of a penalty on the power consumption