Fluid/Structure Interactions

This Special Issue contains 12 papers devoted to fluid/structure interaction (FSI) problems. The main feature of the problems is an interface on which consistent boundary conditions for both the liquid and the solid regions are formulated. The presented studies cover a wide range of problems and methods for their solution, including problems of weak, or one-way interaction, in which the effect of interface deformation on the fluid flow can be neglected, as well as problems of the strong interaction, for which the interface change affects both the flow and the structure behaviour. The interest in FSI problems is very great due to their practical importance. Recent developments in engineering have led to advanced formulations of FSI problems. Some of them could not be formulated several years ago. The presented papers demonstrate progress in both numerical algorithms, mathematical apparatus and advanced computational techniques. In this issue, we have tried to collect different FSI problems, new mathematical and numerical approaches, new numerical techniques and, of course, new results, which can provide an insight into FSI processes

Investigation of the self-excited vibrations in a Francis runner in transient conditions of load rejection

Tableau d'honneur de la Faculté des études supérieures et postdorales, 2015-2016Les travaux réalisés dans le cadre de ce mémoire de maîtrise visent à décrire la source hydraulique de vibrations mesurées lors de la mise en service d’une roue Francis. Lors de l’essai de rejet de charge à partir de la puissance maximale, il a été relevé que la roue subissait des vibrations autoexcitées à l’une de ses fréquences naturelles. L’objectif de ces travaux est d’identifier le phénomène hydraulique causant l’entrée en vibrations auto-excitées de la roue submergée. Puisque la cavitation semble jouer un rôle dans cette problématique, les capacités du solveur fluide ANSYS CFX à simuler de la cavitation instationnaire ont été évaluées. De ce fait, deux géométries ayant été étudiées expérimentalement ont été simulées. Cette étude a permis de conclure que l’approche homogène, plus stable et couramment utilisée dans la littérature, ne permet pas de reproduire de façon fiable les fluctuations de pression causées par l’instationarité des cavités de vapeur. Il a cependant été montré que les simulations permettent de prédire la forme, la localisation ainsi que les mécanismes entraînant la présence de vapeur dans l’écoulement. De plus, il a été demontré que la cavitation est un phénomène particulièrement sensible et sujet à répondre en phase à une excitation oscillatoire, par exemple la vibration de la roue. Les simulations numériques réalisées durant différentes phases du rejet de charge transitoire ont entre autres permis d’identifier que lorsque les vibrations apparaissent, une forte région de vapeur se crée au bord de fuite de l’aubage, près du plafond. En augmentant le temps de fermeture du distributeur, le partenaire industriel a réussis à éliminer les vibrations problématiques. En réalisant des simulations avec différents temps de fermeture, il a été démontré que la solution proposée permet d’augmenter le niveau de pression dans le canal inter-aubes, réduisant ainsi la quantité de vapeur s’y trouvant. Cela laisse suggérer que la cause d’entrée en vibrations de la roue est la cavitation se formant durant le rejet de charge. Cependant, il est à noter que les différentes méthodologies proposées n’ont pas permis d’obtenir les fréquences d’excitation mesurées expérimentalement, essentiellement à cause de limitations liées à la modélisation de la cavitation.The work presented in this master degree thesis aims to identify the hydraulic cause of mechanical vibrations measured during the commissioning of a Francis runner. During the test of load rejection from maximal power output regime, it was noticed that the runner entered a state of self-excited vibrations at one of its natural frequencies. The purposes of this work is to investigate the hydraulic phenomenon which causes the submerged runner to enter self-excited vibrations. Since cavitation is expected to play a role in this problematics, there was a need to evaluate the capabilities of the fluid solver ANSYS CFX to solve unsteady cavitating flows. Two geometries which had been investigated experimentally were thus simulated. It was concluded that the homogeneous approach, more robust and widely used in the literature, does not lead to a reliable prediction of the pressure fluctuations caused by cavitation. It was however shown that the simulations allowed to predict the shape, the location as well as the physical mechanisms responsible for the presence of vapor in the flow. It was also demonstrated that cavitation is a phenomenon particularly sensitive and subject to respond in phase to oscillatory perturbations, for instance the vibrating runner. The numerical simulations carried out at different phases of the load rejection transient have established that when the vibrations appear, a wide region of vapor forms at the trailing edge of the blade, near the crown. By increasing the distributor closing time, the industrial partner in this work had success in eliminating such problematic vibrations. In our case, when performing simulations with increased closing times, it was demonstrated that the solution proposed allows to increase the pressure level in the inter-blade channel, lowering the quantity of vapor it contains. This strongly suggests that the hydraulic cause of the vibrations is the cavitation forming during the load rejection. However, one can note that the proposed methodologies have not permitted to predict the excitation frequencies as measured experimentally, essentially due to limitations in the modeling of cavitation

MARE-WINT: New Materials and Reliability in Offshore Wind Turbine Technology

renewable; green; energy; environment; law; polic

Aeroelastic Analysis of a Wind Turbine Blade Using the Harmonic Balance Method

Most current wind turbine aeroelastic codes rely on the blade element momentum method with empirical corrections to compute aerodynamic forces on the wind turbine blades. While efficient, this method relies on experimental data and does not allow designers much flexibility for alternative blade designs. Unsteady solutions to the Navier-Stokes equations offer a significant improvement in aerodynamic modeling, but these are currently too computationally expensive to be useful in a design situation. However, steady-state solutions to the Navier-Stokes equations are possible with reasonable computation times. The harmonic balance method provides a way to represent unsteady, periodic flows through coupled a set of steady-state solutions. This method offers the possibility of unsteady flow solutions at a computational cost on the order of a few steady-state solutions. By coupling a harmonic balance driven aerodynamic model with a mode shape-based structural dynamics model, an efficient aeroelastic model for a wind turbine blade driven by the Navier-Stokes equations is developed in this dissertation. For wind turbine flows, turbulence modeling is essential, especially in the transition of the boundary layer from laminar to turbulent. As part of this dissertation, the Spalart-Allmaras turbulence model and the gamma-Re\_theta-t transition model are included in the aerodynamic model. This marks the first time that this transition model, turbulence model, and the harmonic balance method have been coupled to study unsteady wind turbine aerodynamics. Results show that the transition model matches experimental data more closely than a fully turbulent model for the onset of both static and dynamic stall. Flutter is of particular interest as turbines continue to increase in size, and longer and softer blades continue to enter the field. In this dissertation, flutter is investigated for the 1.5 MW WindPACT rotor blade. The aeroelastic model created, which incorporates the harmonic balance method and a fully turbulent aerodynamic model, is the first of its kind for wind turbine flutter analysis. Predictions match those of other aeroelastic models for the 1.5 MW WindPACT blade, and the first flapwise and edgewise modes are shown to dominate flutter for the rotor speeds considered

Uncertainty quantification of film cooling effectiveness in gas turbines

In this study the effect of uncertainty of velocity ratio on jet in crossflow and particual- rly film cooling performance is studied. Direct numerical simulations have been combined with a stochastic collocation approach where the parametric space is discretized using Multi-Element general Polynomial Chaos (ME-gPC) method. Velocity ratio serves as a bifurcation parameter in a jet in a crossflow and the dynamical system is shown to have several bifurcations. As a result of the bifurcations, the target functional is observed to have low-regularity with respect to the paramteric space. In that sense, ME-gPC is particularly effective in discretizing the parametric space. One particular case of a jet in a crossflow is numerically solved with the velocity ratio variations assumed to have a truncated Gaus- sian distribution with mean of 1.5 and the standard variation of approximately 0.5. Five elements are used to discretize the parametric space using ME-gPC method. Within each element general polynomial chaos of order 3 is used. A fast convergence of the polynomial expansion in the parametric space was observed. Time-dependent Navier-Stokes equations are sampled at Gauss-quadrature points using spectral/hp element method implemented in NEKTAR. Overall due to the low-regularity of the response surface, ME-gPC is observed to be a computationally effective strategy to study the effect of uncertainty in a jet in a crossflow when velocity ratio is the random parameter

Design Optimization of Wind Energy Conversion Systems with Applications

Modern and larger horizontal-axis wind turbines with power capacity reaching 15 MW and rotors of more than 235-meter diameter are under continuous development for the merit of minimizing the unit cost of energy production (total annual cost/annual energy produced). Such valuable advances in this competitive source of clean energy have made numerous research contributions in developing wind industry technologies worldwide. This book provides important information on the optimum design of wind energy conversion systems (WECS) with a comprehensive and self-contained handling of design fundamentals of wind turbines. Section I deals with optimal production of energy, multi-disciplinary optimization of wind turbines, aerodynamic and structural dynamic optimization and aeroelasticity of the rotating blades. Section II considers operational monitoring, reliability and optimal control of wind turbine components

Aeronautical engineering: A continuing bibliography with indexes (supplement 295)

This bibliography lists 581 reports, articles, and other documents introduced into the NASA Scientific and Technical Information System in Sep. 1993. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

SIRM 2017

This volume contains selected papers presented at the 12th International Conference on vibrations in rotating machines, SIRM, which took place February 15-17, 2017 at the campus of the Graz University of Technology. By all meaningful measures, SIRM was a great success, attracting about 120 participants (ranging from senior colleagues to graduate students) from 14 countries. Latest trends in theoretical research, development, design and machine maintenance have been discussed between machine manufacturers, machine operators and scientific representatives in the field of rotor dynamics. SIRM 2017 included thematic sessions on the following topics: Rotordynamics, Stability, Friction, Monitoring, Electrical Machines, Torsional Vibrations, Blade Vibrations, Balancing, Parametric Excitation, and Bearings. The papers struck an admirable balance between theory, analysis, computation and experiment, thus contributing a richly diverse set of perspectives and methods to the audience of the conference

Aeronautical Engineering: A continuing bibliography, supplement 120

This bibliography contains abstracts for 297 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1980

Development of numerical and data models for the support of digital twins in offshore wind engineering

Error on title page. Date of award is 2022.As offshore wind farms grow there is a continued demand for reduced costs. Maintenance costs and downtime can be reduced through greater information on the asset in relation to its operational loads and structural resistance to damage and so there is an increasing interest in digital twin technologies. Through digital twins, an operational asset can be replicated computationally, thus providing more information. Modelling these aspects requires a wide variety of models in different fields. To advance the feasibility of digital twin technology this thesis aims to develop the multi-disciplinary set of modelling domains which help form the basis of future digital twins. Throughout this work, results have been validated against operational data recorded from sensors on offshore structures. This has provided value and confidence to the results as it shows how well the mix of state-of-the art models compare to real world engineering systems. This research presents a portfolio of five research areas which have been published in a mix of peer-reviewed journal articles and conference papers. These areas are: 1) A computational fluid dynamics (CFD) model of an offshore wind farm conducted using a modified solver in the opensource software. This work implements actuator disk turbine models and uses Reynolds averaged Naiver Stokes approaches to represent the turbulence. This investigates the impact of modelling choices and demonstrates the impact of varied model parameters. The results are compared to operational site data and the modelling errors are quantified. There is good agreement between the models and site data. 2) An expansion on traditional CFD approaches through incorporating machine learning (ML). These ML models are used to approximate the results of the CFD and thereby allow for further analysis which retains the fidelity of CFD at comparatively negligible computational cost. The results are compared to operational site data and the errors at each step are quantified for validation. 3) A time-series forecasting of weather variables based on past measured data. A novel approach for forecasting time-series is developed and compared to two existing methods: Markov-Chains and Gradient Boosting. While this new method is more complex and requires more time to train, it has the desirable feature of incorporating seasonality at multiple timescales and thus providing a more representative time-series. 4) An investigation of the change in modal parameters in an offshore wind jacket structure from damages or from changing operational conditions. In this work the detailed design model of the structure from Ramboll is used. This section relates the measurable modal parameters to the operational condition through a modelling approach. 5) A study conducted using accelerometer data from an Offshore Substation located in a wind farm site. Operational data from 12 accelerometers is used to investigate the efficacy of several potential sensor layouts and therefore to quantify the consequence of placement decisions. The results of these developments are an overall improvement in the modelling approaches necessary towards the realisation of digital twins as well as useful development in each of the component areas. Both areas related to wind loading as well as structural dynamics have been related to operational data. The validation of this link between the measured and the modelled domains facilitates operators and those in maintenance in gaining more information and greater insights into the conditions of their assets.As offshore wind farms grow there is a continued demand for reduced costs. Maintenance costs and downtime can be reduced through greater information on the asset in relation to its operational loads and structural resistance to damage and so there is an increasing interest in digital twin technologies. Through digital twins, an operational asset can be replicated computationally, thus providing more information. Modelling these aspects requires a wide variety of models in different fields. To advance the feasibility of digital twin technology this thesis aims to develop the multi-disciplinary set of modelling domains which help form the basis of future digital twins. Throughout this work, results have been validated against operational data recorded from sensors on offshore structures. This has provided value and confidence to the results as it shows how well the mix of state-of-the art models compare to real world engineering systems. This research presents a portfolio of five research areas which have been published in a mix of peer-reviewed journal articles and conference papers. These areas are: 1) A computational fluid dynamics (CFD) model of an offshore wind farm conducted using a modified solver in the opensource software. This work implements actuator disk turbine models and uses Reynolds averaged Naiver Stokes approaches to represent the turbulence. This investigates the impact of modelling choices and demonstrates the impact of varied model parameters. The results are compared to operational site data and the modelling errors are quantified. There is good agreement between the models and site data. 2) An expansion on traditional CFD approaches through incorporating machine learning (ML). These ML models are used to approximate the results of the CFD and thereby allow for further analysis which retains the fidelity of CFD at comparatively negligible computational cost. The results are compared to operational site data and the errors at each step are quantified for validation. 3) A time-series forecasting of weather variables based on past measured data. A novel approach for forecasting time-series is developed and compared to two existing methods: Markov-Chains and Gradient Boosting. While this new method is more complex and requires more time to train, it has the desirable feature of incorporating seasonality at multiple timescales and thus providing a more representative time-series. 4) An investigation of the change in modal parameters in an offshore wind jacket structure from damages or from changing operational conditions. In this work the detailed design model of the structure from Ramboll is used. This section relates the measurable modal parameters to the operational condition through a modelling approach. 5) A study conducted using accelerometer data from an Offshore Substation located in a wind farm site. Operational data from 12 accelerometers is used to investigate the efficacy of several potential sensor layouts and therefore to quantify the consequence of placement decisions. The results of these developments are an overall improvement in the modelling approaches necessary towards the realisation of digital twins as well as useful development in each of the component areas. Both areas related to wind loading as well as structural dynamics have been related to operational data. The validation of this link between the measured and the modelled domains facilitates operators and those in maintenance in gaining more information and greater insights into the conditions of their assets