Wind turbine stability: Comparison of state-of-the-art aeroelastic simulation tools

As rotor diameters and blade flexibility are increasing, current and future generation wind turbines are more susceptible to aeroelastic instabilities. It is thus important to know the prediction capabilities of state-of-the-art simulation tools in regards of the onset of aeroelastic instability. This article presents results of a code-to-code comparison of five different simulation codes using a representative wind turbine model. It is shown that the models are in good agreement in terms of isolated structural dynamics and steady state aeroelastics. The more complex the test cases become, the more significant are the differences in the results. In the final step of comparison, the aeroelastic stability limit is determined through a run-away analysis. The instability onset is predicted at different wind speeds and the underlying mechanisms differ between the tools. A Campbell diagram is used to correlate the findings of time domain simulation tools with those of a linear analysis in the frequency domain

An open-source framework for the uncertainty quantification of aeroelastic wind turbine simulation tools

The uncertainty quantification of aeroelastic wind turbine simulations is an active research topic. This paper presents a dedicated, open-source framework for this purpose. The framework is built around the uncertainpy package, likewise available as open source. Uncertainty quantification is done with a non-intrusive, global and variance-based surrogate model, using PCE (i.e., polynomial chaos expansion). Two methods to handle the uncertain parameter distribution along the blades are presented. The framework is demonstrated on the basis of an aeroelastic stability analysis. A sensitivity analysis is performed on the influence of the flapwise, edgewise and torsional stiffness of the blades on the damping of the most critical mode for both a Bladed linearization and a Bladed time domain simulation. The sensitivities of both models are in excellent agreement and the PCE surrogate models are shown to be accurate approximations of the true models

Relationship between videofluoroscopic and subjective (physician- and patient- rated) assessment of late swallowing dysfunction after (chemo) radiation:Results of a prospective observational study

BACKGROUND AND PURPOSE: Primary (chemo)radiation (CHRT) for HNC may lead to late dysphagia. The purpose of this study was to assess the pattern of swallowing disorders based on prospectively collected objective videofluoroscopic (VF) assessment and to assess the correlations between VF findings and subjective (physician- and patient-rated) swallowing measures. MATERIAL AND METHODS: 189 consecutive HNC patients receiving (CH)RT were included. Swallowing evaluation at baseline and 6 months after treatment (T6) encompassed: CTCAE v.4.0 scores (aspiration/dysphagia), PROMs: SWAL QOL/ EORTC QLQ-H&N35 (swallowing domain) questionnaires and VF evaluation: Penetration Aspiration Scale, semi-quantitative swallowing pathophysiology evaluation, temporal measures and oral/pharyngeal residue quantification. Aspiration specific PROMs (aPROMs) were selected. Correlations between late penetration/aspiration (PA_T6) and: clinical factors, CTCAE and aPROMs were assessed using uni- and multivariable analysis. RESULTS: Prevalence of PA increased from 20% at baseline to 43% after treatment (p<0.001).The most relevant baseline predictors for PA_T6 were: PA_T0, age, disease stage III-IV, bilateral RT and baseline aPROM 'Choking when drinking' (AUC: 0.84). In general aPROMs correlated better with VF-based PA than CTCAE scores. The most of physiological swallowing components significantly correlated and predictive for PA (i.e. Laryngeal Vestibular Closure, Laryngeal Elevation and Pharyngeal Contraction) were prone to radiation damage. CONCLUSION: The risk of RT-induced PA is substantial. Presented prediction models for late penetration/aspiration may support patient selection for baseline and follow-up VF examination. Furthermore, all aspiration related OARs involved in aforementioned swallowing components should be addressed in swallowing sparing strategies. The dose to these structures as well as baseline PROMs should be included in future NTCP models for aspiration

Patterns of long-term swallowing dysfunction after definitive radiotherapy or chemoradiation

Objectives: To identify patterns of long-term, radiation-induced swallowing dysfunction after definitive radiotherapy with or without chemotherapy (RT or CHRT) and to determine which factors may explain these patterns over time.Material and methods: The study population consisted of 238 consecutive head and neck cancer patients treated with RT or CHRT. The primary endpoint was &gt;= grade 2 swallowing dysfunction at 6, 12, 18 and 24 months after treatment. Cluster analysis was used to identify different patterns over time. The differences between the mean dose to the swallowing organs at risk for each pattern were determined by using dose maps.Results: The cluster analysis revealed five patterns of swallowing dysfunction: low persistent, intermediate persistent, severe persistent, transient and progressive. Patients with high dose to the upper pharyngeal, laryngeal and lower pharyngeal region had the highest risk of severe persistent swallowing dysfunction. Transient problems mainly occurred after high dose to the laryngeal and lower pharyngeal regions, combined with moderate dose to the upper pharyngeal region. The progressive pattern was mainly seen after moderate dose to the upper pharyngeal region.Conclusions: Various patterns of swallowing dysfunction after definitive RT or CHRT can be identified over time. This could reflect different underlying biological processes. (C) 2015 The Authors. Published by Elsevier Ireland Ltd.</p

Development and validation of a prediction model for tube feeding dependence after curative (chemo-) radiation in head and neck cancer

BACKGROUND: Curative radiotherapy or chemoradiation for head and neck cancer (HNC) may result in severe acute and late side effects, including tube feeding dependence. The purpose of this prospective cohort study was to develop a prediction model for tube feeding dependence 6 months (TUBEM6) after curative (chemo-) radiotherapy in HNC patients. PATIENTS AND METHODS: Tube feeding dependence was scored prospectively. To develop the multivariable model, a group LASSO analysis was carried out, with TUBEM6 as the primary endpoint (n = 427). The model was then validated in a test cohort (n = 183). The training cohort was divided into three groups based on the risk of TUBEM6 to test whether the model could be extrapolated to later time points (12, 18 and 24 months). RESULTS: Most important predictors for TUBEM6 were weight loss prior to treatment, advanced T-stage, positive N-stage, bilateral neck irradiation, accelerated radiotherapy and chemoradiation. Model performance was good, with an Area under the Curve of 0.86 in the training cohort and 0.82 in the test cohort. The TUBEM6-based risk groups were significantly associated with tube feeding dependence at later time points (p<0.001). CONCLUSION: We established an externally validated predictive model for tube feeding dependence after curative radiotherapy or chemoradiation, which can be used to predict TUBEM6

Improvements on the application of direct-CFD in unsteady aeroelastic simulations

Aircraft manufacturers have to prove a flutter free design for all operational cases within the complete flight envelope plus a safety margin. This certification process relies on validated flutter computations which have to be made for all flight conditions including variations in the aircraft's loading and failure cases. The aerodynamic component of these unsteady aeroelastic simulations is restricted to fast computational methods due to the large parameter space. Linear, inviscid models were the industry standard for this application. They had to be corrected by wind-tunnel or CFD data to cover transonic flow phenomena. Recently, high-fidelity aerodynamic models have been introduced which, in contrast to the previous methods, have an inherent quality to represent the important transonic flow phenomena for all flight conditions. However, the significant computational cost of these models restricts the computation of unsteady aerodynamics to a limited set of reference elastic modes. These unsteady aerodynamic reference results are subsequently mapped to all 'production' flutter computations with a least-squares method. This thesis report presents an investigation on the possibility of accuracy or robustness improvements in the implementation of this new, high-fidelity direct-CFD method in unsteady aeroelastic simulations. An error estimation study quantified the impact of approximation errors of the least-squares method on the frequency and damping curves of the 'production' computational case. Modal basis quality criteria are established and their performance is compared for a test case. In contrast to the proposed hypothesis, global mode assurance criteria are sufficient to predict the errors. Local or aerodynamically weighted quality criteria show similar performance and can therefore be considered redundant for the presented test case. In case of a non-satisfactory reference set, the modal basis can be enriched automatically and effectively in order to eliminate the approximation error. Additionally, a performance study of two reference selection methods on four computational test cases has been conducted. The application of the elastic modes of a nominal structural lay-out as reference is satisfactory for nominal structure flutter computations at different load distributions and for failure case simulations without strongly deviating mode shapes. However, this reference selection method can be insufficient regarding the approximation error for critical failure cases with strongly deviating mode shapes with respect to the nominal structure modes. Yet, for all test cases the error estimators are able to predict this approximation error, such that they can be eliminated by modal basis enrichment. On the other hand, a new method is proposed which uses the POD (Proper Orthogonal Decomposition) theory to decompose a wide range of modes for different failure and load cases into a reference set. This method performed satisfactory for all test cases, without any enrichment necessity. The prerequisite for a good performance of this reference selection method is a well-considered selection of the POD setup and the presence of the considered failure cases in the POD input.Aerospace Engineerin

Comparison of Unsteady Low- and Mid-Fidelity Propeller Aerodynamic Methods for Whirl Flutter Applications

Aircraft configurations with propellers have been drawing more attention in recent times, partly due to new propulsion concepts based on hydrogen fuel cells and electric motors. These configurations are prone to whirl flutter, which is an aeroelastic instability affecting airframes with elastically supported propellers. It commonly needs to be mitigated already during the design phase of such configurations, requiring, among other things, unsteady aerodynamic transfer functions for the propeller. However, no comprehensive assessment of unsteady propeller aerodynamics for aeroelastic analysis is available in the literature. This paper provides a detailed comparison of nine different low- to mid-fidelity aerodynamic methods, demonstrating their impact on linear, unsteady aerodynamics, as well as whirl flutter stability prediction. Quasi-steady and unsteady methods for blade lift with or without coupling to blade element momentum theory are evaluated and compared to mid-fidelity potential flow solvers (UPM and DUST) and classical, derivative-based methods. Time-domain identification of frequency-domain transfer functions for the unsteady propeller hub loads is used to compare the different methods. Predictions of the minimum required pylon stiffness for stability show good agreement among the mid-fidelity methods. The differences in the stability predictions for the low-fidelity methods are higher. Most methods studied yield a more unstable system than classical, derivative-based whirl flutter analysis, indicating that the use of more sophisticated aerodynamic modeling techniques might be required for accurate whirl flutter prediction

Framework for uncertainty quantification of aeroelastic wind turbine simulations

Framework for uncertainty quantification of aeroelastic wind turbine simulations. The code for a case study on the influence of blade beam properties on the wind turbine aeroelastic stability is available with interfaces to multiple aeroelastic tools. The framework is a wrapper around the uncertainpy package which provides methods for surrogate model based uncertainty quantification

Code-to-code comparison of realistic wind turbine instability phenomena

Preceding studies (Hach et al, 2020) show significant differences in the prediction of dynamic aeroelastic instabilities with state-of-the-art simulation tools. Instabilities were provoked by an overspeed scenario where the wind speed increases, but no counteracting generator moment was imposed. Significant differences between the tools were found in the critical speed and instability behavior. A negative side effect of the runaway procedure is that the operating conditions at which the instability occurred could be vastly different. Differences in the instability mechanisms could therefore be in part allocated to these differing operating conditions. The aim of this study is to find a critical configuration model which becomes unstable under nominal, controlled operating conditions. The expectation was that the establishing instability mechanism would be more representative for potential aeroelastic phenomena which could be experienced on current or future turbines (Volk et al, 2020). State-of-the-art simulation models are used to give insight in the instability phenomena and to compare their respective modeling capabilities. These included two general purpose multi-body simulation tools (alaska/Wind and Simpack) and three industry relevant turbine simulation tools (Bladed, HAWC2, OpenFAST). Simulations were executed both in the time domain (all tools) and in the frequency domain (Bladed and HAWCStab2). The publicly available reference wind turbine model IWT-7.5-164 served as reference for the comparison. The global flapwise, edgewise and torsional stiffnesses were reduced over the full blade to enforce instabilities under nominal operating conditions. A recomputation of the stiffness matrices was performed with the adjusted input in order to assure equivalent stiffness reductions across all tools. Consistency across the models was verified by a blade and full model modal analysis, static structural deformation tests and a steady aeroelastic deformation test. The final stability analysis was performed for multiple points along the nominal control curve. Unstable time domain simulations were analyzed by a frequency analysis to determine the instability mechanism. This also allowed a comparison between the time domain and the frequency domain simulation tools. A comparison is shown for the aeroelastic modes which play a role in the instability mechanisms. The agreement between the linearization results of Bladed and HAWCStab2 is satisfying. All simulation tools exhibit comparable unstable behavior with a significant participation of the 1st and/or 2nd edge bending modes of the blades. Large differences are observed between critical wind speeds in the time domain simulation results. Preliminary analyses reveal discrepancies in the damping associated with the vibrations. The presented comparison demonstrates that the observed instability mechanism matches across the aeroelastic simulation tools if the operating conditions are close enough - which is not always the case in a runaway analysis. To provoke an instability in the normal operating range the stiffness of the rotor blades was decreased significantly. It was found that the stiffness matrices needed to be re-computed since independently reducing the principle stiffnesses in flap/edge/torsion leads to inconsistent models

The course of swallowing problems in the first 2 years after diagnosis of head and neck cancer

INTRODUCTION: Head and neck cancer (HNC) and its treatment often negatively impact swallowing function. The aim was to investigate the course of patient-reported swallowing problems from diagnosis to 3, 6, 12, and 24 months after treatment, in relation to demographic, clinical, and lifestyle factors. METHODS: Data were used of the Netherlands Quality of Life and Biomedical Cohort Study in head and neck cancer research (NET-QUBIC). The primary outcome measures were the subscales of the Swallowing Quality of Life Questionnaire (SWAL-QOL). Linear mixed-effects models (LMM) were conducted to investigate changes over time and associations with patient, clinical, and lifestyle parameters as assessed at baseline. RESULTS: Data were available of 603 patients. There was a significant change over time on all subscales. Before treatment, 53% of patients reported swallowing problems. This number increased to 70% at M3 and decreased to 59% at M6, 50% at M12, and 48% at M24. Swallowing problems (i.e., longer eating duration) were more pronounced in the case of female, current smoking, weight loss prior to treatment, and stage III or IV tumor, and were more prevalent at 3 to 6 months after treatment. Especially patients with an oropharynx and oral cavity tumor, and patients receiving (C)RT following surgery or CRT only showed a longer eating duration after treatment, which did not return to baseline levels. CONCLUSION: Half of the patients with HNC report swallowing problems before treatment. Eating duration was associated with sex, smoking, weight loss, tumor site and stage, and treatment modality, and was more pronounced 3 to 6 months after treatment