bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure-aware diffusion for low-dose CT imaging
Authors: Du Wenchao , Cui HuanHuan , He LinChao , Chen Hu , Zhang Yi , Yang Hongyu
Published: June 2024

Reducing the radiation dose leads to the x-ray computed tomography (CT) images suffering from heavy noise and artifacts, which inevitably interferes with the subsequent clinic diagnostic and analysis. Leading works have explored diffusion models for low-dose CT imaging to avoid the structure degeneration and blurring effects of previous deep denoising models. However, most of them always begin their generative processes with Gaussian noise, which has little or no structure priors of the clean data distribution, thereby leading to long-time inference and unpleasant reconstruction quality. To alleviate these problems, this paper presents a Structure-Aware Diffusion model (SAD), an end-to-end self-guided learning framework for high-fidelity CT image reconstruction. First, SAD builds a nonlinear diffusion bridge between clean and degraded data distributions, which could directly learn the implicit physical degradation prior from observed measurements. Second, SAD integrates the prompt learning mechanism and implicit neural representation into the diffusion process, where rich and diverse structure representations extracted by degraded inputs are exploited as prompts, which provides global and local structure priors, to guide CT image reconstruction. Finally, we devise an efficient self-guided diffusion architecture using an iterative updated strategy, which further refines structural prompts during each generative step to drive finer image reconstruction. Extensive experiments on AAPM-Mayo and LoDoPaB-CT datasets demonstrate that our SAD could achieve superior performance in terms of noise removal, structure preservation, and blind-dose generalization, with few generative steps, even one step only.
Convergence of resonant state expansions for transmission in double delta resonators
Authors: Villavicencio Jorge , Hernández-Maldonado Alberto
Published: June 2024

We investigate the transmission features of symmetric double Dirac delta resonators using a non-Hermitian quantum approach, based on resonant state expansions involving complex energy eigenvalues. We focus on the convergence properties of these expansions, which involve a sum of resonance and anti-resonance terms We demonstrate that systems with transmission profiles featuring sharp and isolated resonances converge more rapidly, whereas those with overlapping resonances exhibit slower convergence. We also show that for the former systems, by taking into account only the resonance terms, we can accurately describe the transmission coefficient as a sum of Breit-Wigner resonances, each distinctly characterized by its energy and width. We demonstrate that there is a one-to-one correlation between transmission peaks and the resonance energies of the system. We also compare the convergence properties of transmission using Mittag-Leffler expansions, noting their slower rates compared to resonant state expansions. These findings emphasize the advantage of using non-Hermitian resonant expansions for analyzing quantum mechanical systems, providing a clearer understanding of systems characterized by resonant features.
Molecular dynamics study on the effect of cementite structure on the mechanical properties of pearlite
Authors: Zhang Haichao , Chen Yinli , Sun Yanhui
Published: June 2024

The effect of cementite structure on the mechanical properties of pearlite were investigated using molecular dynamics simulations. Three types of cementite structures were considered: single crystal, nanocrystalline, and a mixture of nanocrystalline and amorphous structures. The study found that regardless of the cementite structure, the ferrite phase in the pearlite exhibited plastic deformation first due to the activation of its internal {112} 〈111〉 slip system during tensile loading, resulting in macroscopic yielding of the pearlite. However, the difficulty of plastic deformation in the ferrite phase varied with different cementite structures. Compared to the other two types of cementite structures, the ferrite phase in the pearlite containing single crystal cementite exhibited the most difficult plastic deformation, leading to the highest peak stress. As the strain increased, the plastic deformation in the ferrite transferred through the ferrite-cementite interface to the cementite. After the plastic deformation transferred to the cementite, the different cementite structures exhibited distinct mechanical responses: the single crystal cementite structure experienced cleavage fracture, the nanocrystalline cementite structure underwent plastic deformation induced by grain boundary slip, while the nanocrystalline-amorphous mixture cementite structure showed shear deformation in the amorphous cementite and plastic deformation induced by grain boundary slip in the nanocrystalline cementite. Due to the influence of cementite mechanical response mechanism, the flow stress of the pearlite was minimal when the cementite in the pearlite were in single crystal structure. The study also revealed that when the cementite was in nanocrystalline structure, the flow stress of pearlite decreases with the grain size decreasing from 4.25 nm to 2.68 nm. When the cementite is a mixture of nanocrystalline and amorphous structure, the flow stress of pearlite decreases as the thickness of amorphous layer increases from 1 nm to 2 nm.
Design and analysis of Minkowski fractal shaped metasurface absorber with broadband and polarization-insensitive characteristics using a tunable graphene layer for terahertz applications
Authors: Rekha Vutukuri Sarvani Duti , Prasad Nagandla , Madhav B T P , Alathbah Moath
Published: June 2024

The proposed research article’s main goal is to demonstrate a terahertz (THz) broadband absorber for high-speed wireless communication applications. The proposed structure is a compact one and possesses three layers. The ground layer acts as a metal reflector, lossy silicon acts as a dielectric material and finally a graphene layer in the shape of a minkowski fractal acts as a radiating patch for the proposed design. We have chosen a thickness of 5 μm for the lossy silicon which has a dielectric constant of 11.90. The bottom layer of the proposed design contains a good conductive material like gold with a conductivity of 4.561 e+007 s m−1 with a thickness of 0.2 μm. The thickness of a monolayer graphene is one nanometer, and the overall unit cell size of the proposed structure is 12 × 12 μm2. Because of its symmetrical nature, the proposed absorber offers a broad response to both TM and TE modes irrespective of any polarization angle. The proposed absorber can operate in the terahertz frequency range and has achieved two broad frequency bands from 3.34–3.98 THz and 4.6–5.30 THz, with an absorption percentage greater than 90. We can observe peak absorption frequencies at 3.60 THz and 5.04 THz, which exhibit an absorption percentage close to unity. Additionally, we validated the proposed broadband absorber using an equivalent circuit approach and verified it using the ADS tool.
Hydrodynamics of (1+1) dimensional fluid in the presence of gravitational anomaly from first order thermodynamics
Authors: Seenivasan Abhinove Nagarajan , Chakrabarti Sayan , Majhi Bibhas Ranjan
Published: June 2024

Considering (1+1)-dimensional fluid in presence of gravitational trace anomaly, as an effective description of higher-dimensional fluid, the hydrodynamics is discussed through a first order thermodynamic description. Contrary to the existing approaches which are second order in nature, the fluid velocity is identified through the auxiliary field required to describe the Polyakov action for the effective description of relevant energy-momentum tensor. The thermodynamic and fluid quantities, on a static black hole spacetime, are calculated both near the horizon as well as at the asymptotic infinity. The Unruh vacuum appears to be suitable one for the present analysis, in contrast to Israel-Hartle-Hawking vacuum which is consistent with second order description. As in anomaly cancellation approach to find the Hawking flux the Unruh vacuum is consistent with the required conditions, in reverse way we interpret this fluid description as an alternative approach to find these required conditions to calculate the same in anomaly cancellation approach.
Constraints on Bianchi-I type universe with SH0ES anchored Pantheon+ SNIa data
Authors: Verma Anshul , Patel Sanjeet K. , Aluri Pavan K. , Panda Sukanta , Mota David F.
Published: June 2024

We study the Bianchi-I cosmological model motivated by signals of statistical isotropy violation seen in cosmic microwave background (CMB) observations and others. To that end, we consider various kinds of anisotropic matter that source anisotropy in our model, specifically Cosmic strings, Magnetic fields, Domain walls and Lorentz violation generated magnetic fields. These anisotropic matter sources, taking one at a time, are studied for their co-evolution with standard model (isotropic) sources viz., dust-like (dark/normal) matter, and dark energy modelled as cosmological constant. We constrain the Hubble parameter, density fractions of anisotropic matter, cold dark matter (CDM), and dark energy (Λ) in a Bianchi-I universe with planar symmetry i.e., which has a global ellipsoidal geometry, and try to find signatures of a cosmic preferred axis if any.
The latest compilation of Type Ia Supernova (SNIa) data from Pantheon+SH0ES collaboration is used in our analysis to obtain constraints on cosmological parameters and any preferred axis for our universe.
In our analysis, we found mild evidence for a cosmic preferred axis. It is interesting to note that this preferred axis lies broadly in the vicinity of other prominent cosmic anisotropy axes reported in the literature from diverse data sets.
Also we find some evidence for non-zero (negative) cosmic shear and eccentricity that characterize different expansion rates in different directions and deviation from an isotropic scale factor respectively.
The energy density fractions of two of the sources considered are found to be non-zero at a
2σ confidence level.
To be more conclusive, we require more SNIa host galaxy data for tighter constraints on distance and absolute magnitude calibration which are expected to be available from the future JWST observations and others.
Fate of oscillating homogeneous ℤ2-symmetric scalar condensates in the early Universe
Authors: Ai Wen-Yuan , Wang Zi-Liang
Published: June 2024

Dark matter, if represented by a ℤ2-symmetric scalar field, can manifest as
both particles and condensates. In this paper, we study the evolution of an oscillating
homogeneous condensate of a ℤ2-symmetric scalar field in a thermal plasma in an FLRW
universe. We focus on the perturbative regime where the oscillation amplitude is sufficiently
small so that parametric resonance is inefficient. This perturbative regime necessarily comprises
the late stage of the condensate decay and determines its fate. The coupled coarse-grained
equations of motion for the condensate, radiation, and spacetime are derived from first principles
using nonequilibrium quantum field theory. We obtain analytical expressions for the relevant
microscopic quantities that enter the equations of motion and solve the latter numerically. We
find that there is always a nonvanishing relic abundance for a condensate with a ℤ2 
symmetry that is not spontaneously broken. This is because its decay rate decreases faster than
the Hubble parameter at late times due to either the amplitude dependence or the temperature
dependence in the condensate decay rate. Consequently, accounting for the condensate contribution
to the overall dark matter relic density is essential for ℤ2 scalar singlet
dark matter.
Data-driven modelling of visual receptive fields: comparison between the generalized quadratic model and the nonlinear input model
Authors: Almasi Ali , Sun Shi H , Jung Young Jun , Ibbotson Michael , Meffin Hamish
Published: June 2024

Objective: Neurons in primary visual cortex (V1) display a range of sensitivity in their response to translations of their preferred visual features within their receptive field: from high specificity to a precise position through to complete invariance. This visual feature selectivity and invariance is frequently modeled by applying a selection of linear spatial filters to the input image, that define the feature selectivity, followed by a nonlinear function that combines the filter outputs, that defines the invariance, to predict the neural response. We compare two such classes of model, that are both popular and parsimonious, the generalized quadratic model (GQM) and the nonlinear input model (NIM). These two classes of model differ primarily in that the NIM can accommodate a greater diversity in the form of nonlinearity that is applied to the outputs of the filters. Approach: We compare the two model types by applying them to data from multielectrode recordings from cat primary visual cortex in response to spatially white Gaussian noise After fitting both classes of model to a database of 342 single units (SUs), we analyze the qualitative and quantitative differences in the visual feature processing performed by the two models and their ability to predict neural response. Main results: We find that the NIM predicts response rates on a held-out data at least as well as the GQM for 95% of SUs. Superior performance occurs predominantly for those units with above average spike rates and is largely due to the NIMs ability to capture aspects of the model’s nonlinear function cannot be captured with the GQM rather than differences in the visual features being processed by the two different models. Significance: These results can help guide model choice for data-driven receptive field modelling.
Analysis of taVNS effects on autonomic and central nervous systems in healthy young adults based on HRV, EEG parameters
Authors: Kang Donghun , Choi Youngseok , Lee Jongshill , Park Eunkyoung , Kim In Young
Published: June 2024

Objective. Transcutaneous auricular vagus nerve stimulation (taVNS), a non-invasive method of stimulating the vagus nerve, simultaneously affects the autonomic nervous system (ANS) and central nervous system (CNS) through efferent and afferent pathways. The purpose of this study is to analyze the effect of taVNS on the ANS and CNS through heart rate variability (HRV) and electroencephalography (EEG) parameters of identified responders. Approach. Two sets of data were collected from each of 10 healthy adult male subjects in their 20 s, and five HRV parameters from the time domain (RMSSD, pNN50, pNN30, pNN20, ppNNx) and two EEG parameters (power of alpha band, power of delta band) were extracted. Main results. Based on pNN50, responders to taVNS were identified; among them, pNN50 (p = 0.0041) and ppNNx (p = 0.0037) showed significant differences before and after taVNS. At the same time, for alpha power and delta power of EEG, significant difference (p < 0.05) was observed in most channels after taVNS compared to before stimulation. Significance. This study demonstrated the validity of identifying responders using pNN50 and the influence of taVNS on both the ANS and CNS. We conclude that taVNS can be used to treat a variety of diseases and as a tool to help control the ANS and CNS.
Numerical simulation of train-induced aerodynamic pressure on railway noise barriers
Authors: Liu Dongyun , Wang Chao , Gonzalez-Libreros Jaime , Enoksson Ola , Hojsten Tommy , Tu Yongming , Elfgren Lennart , Sas Gabriel
Published: June 2024

Noise barriers built parallel to the railway to reduce noise pollution, will be subjected to strong aerodynamic pressure from high-speed trains and have significant dynamic responses under such pressure. Based on computational fluid dynamics (CFD), the numerical simulation of train-induce aerodynamic pressure on noise barriers was performed. Time-varying pressure and its distribution along height direction of noise barriers were analysed, and the effect of different factors on results, i.e., the distance from noise barriers to track centre and height of noise barrier, were discussed. Results show that the geometric changes in train head and tail cause the obvious transient pressure pulse, and the pressure magnitude from head is higher than that from tail. With the measuring height increases, the pressure gradually decreases, which can be well characterized by height coefficient equation from Germany DB code. The pressure magnitude increases non-linearly with the distance to track centre decreases. Importantly, height of noise barrier is also an important factor affecting pressure magnitude on noise barriers and with the height of noise barrier increases, the pressure magnitude gradually increases but tends to be stable at higher height. Exponential equation can well characterize such effect of height of noise barrier on train-induced aerodynamic pressure.
Methods for reducing the output scatter of results for determining realistic damping factors of railway bridges
Authors: Stollwitzer A , Bettinelli L , Fink J
Published: June 2024

A realistic and economical computational assessment of the dynamic behaviour of railway bridges requires input parameters that correspond to reality. In this context, the computationally applied damping factor has a decisive influence on the results. However, the standard prescribes damping factors regarded as highly conservative, which makes in-situ measurements on structures often necessary to classify a bridge categorized as critical in prior as non-critical. Regarding in-situ tests, a measurement-based determination of the damping factor is inevitably accompanied by a scattering of the results due to the measurement method used and results from the individual scope of action of the person evaluating the test. This contribution presents methods and analysis tools for determining the damping factor in the frequency and time domain, intending to reduce the scatter of results and limit the scope of action of the evaluating person. Based on a single-degree-of-freedom system, more advanced procedures and methods are presented and explained using an exemplary steel railway bridge. The basic idea is to use the least square method to adjust the recorded amplitude-frequency response (frequency domain) or the decay process (time domain) based on in-situ measurements by a mathematically defined curve.
Detection and assessment of rail discontinuities using a multibody vehicle-track model
Authors: Vishwakarma Abhilash , Dasgupta Anirvan , Racherla Vikranth
Published: June 2024

In this work, a method is proposed to detect and assess discontinuous/fractured rail by analyzing the axle-box acceleration. The method uses a combination of unsupervised-machine learning algorithm and time-frequency analysis to detect the defect. In the previous work, fishplate rail joints modeling and impact loading induced by the track discontinuities was analyzed [1, 2]. Most of the past work has been reported on fishplate joints, and not much work is found related to the broken welded rail joint. Hence, a study is performed to detect the rail discontinuities using axle-box acceleration. A multibody vehicle-track model is used to generate the acceleration data. The multibody vehicle-track model is developed in SIMPACK. The vehicle model consists of a coach, two bogies, and four axles. Linear spring and damper system is used to model the primary and secondary suspension of the vehicle. The equivalent stiffness of the track along the length of the track is calculated and imported into the SIMPACK model. A finite element-based Euler-Bernoulli beam model is used to calculate the equivalent vertical stiffness of the rail and its support. Sleepers/rail fasteners stiffness is modeled using equispaced springs that support the overhanging portion of the rail. These equispaced springs have stiffness equal to the combined stiffness of the railpad, sleeper, and ballast. Track vertical irregularity of levels five is modeled. These track irregularities are generated from the power spectral density function obtained by the Federal Railway Administration (FRA) of America. Results are obtained for different vehicle speeds, axle loads, and overhanging lengths. To detect the defect, the axle-box acceleration is processed in two stages. In the first stage, a clustering algorithm is applied to locate the rail joint. Statistical features are calculated for the axle-box acceleration. Feature selection is done by the principal component analysis (PCA). The clustering algorithm works very well in locating the rail joints for fractured rail from the rest of the track irrespective of vehicle speed, axle-load, and different overhanging portions of rail. After locating the rail joint, in the second stage, continuous wavelet transform method is applied to the data to measure the level of the defect. The continuous wavelet transform efficiently classifies the severity of the defect in terms of the frequency content in the response.
A framework recommendation for updating running safety design criteria of non-ballasted railway bridges using statistical investigations
Authors: Allahvirdizadeh R , Andersson A , Karoumi R
Published: June 2024

As far as the authors are aware, the threshold for vertical acceleration of the bridge deck was chosen based on the assumption that the induced dynamic loads would overcome gravity at higher accelerations, resulting in loss of contact between wheels and rail; however, the previous studies do not support this hypothesis. Considering these inconsistencies, a better understanding of the simplified design criteria is essential before conducting further studies such as the calibration of partial safety factors. Therefore, this study considers a set of representative design scenarios to statistically compare wheel-rail contact loss with other criteria that can be derived from moving load models, such as vertical accelerations and bridge deck deflections. Based on the analyzes performed, deflection seems to be a better criterion than acceleration to control the running safety limit-state; although the results presented do not necessarily show a very strong correlation between these two criteria. Therefore, the k-means clustering approach is used together with 5% lower quantiles of the collected data to propose potential new thresholds. It should be noted that due to the limited number of analyzes, the approach presented in this study can be considered as a possible framework for further updates of the current design method rather than drawing general conclusions.
Finite element model updating for the dynamic response of continuous deck railway viaducts leading to efficient digital twins
Authors: Tarque Nicola , Goicolea José María , Cuevas Rocío G. , Martínez José Luis
Published: June 2024

Bridges and viaducts are key infrastructure elements within high speed railway lines, for which dynamic actions due to traffic are one of the main actions for design of new structures or evaluation of existing ones. This paper describes techniques based on experimental measurements and numerical finite element models leading to efficient digital twins, to be used for predictive maintenance, within the framework of an innovative prototype for a continuous predictive maintenance system installed in the La Marota 381 m long viaduct, in the high-speed line between Córdoba and Málaga in Spain. This system includes both model-based methods as well as data-based methods for maintenance decisions. Here we focus on the work for model-based acceleration performance, comparing in real time measurements from accelerometer sensors and predictions from a digital twin. The digital twin is an autonomous structural modal analysis software derived from a finite element model previously calibrated with Operational Modal Analysis experimental data. Results obtained from 3 representative high-speed trains are shown and compared in terms of root-mean-square (RMS) and moving window RMS of accelerations between the measured and numerical predictions. These allow to establish criteria and thresholds for predictive maintenance alerts.
Field measurement data set of wind turbine tower for enhanced calculation of vortex-induced vibration
Authors: Kurniawati Ika , Lupi Francesca , Seidel Marc , Höffer Rüdiger , Niemann Hans-Jürgen
Published: June 2024

From July 2021, a full-scale measurement campaign is carried out on two wind turbine towers with smooth surfaces (k/D ≈ 1.0·10-6). Tower response (through accelerations) and wind pressure on the surface are measured. Throughout the one-year monitoring period, there were challenges in transferring full-scale data into a ready-to-use VIV data set. For example, the spatially varying wind conditions despite the small distance between the met masts and the towers. This work aims to provide a ready-to-use full-scale data set for the development of VIV prediction models. A data selection approach based on three simultaneous measurements of wind conditions and maximum moving standard deviation of oscillations s proposed. In addition, the load parameters obtained from the wind pressure measurements in the field and from wind tunnel tests are considered. An enhanced calculation is carried out by defining local aerodynamic damping for sectional heights of the tower. A comparison of the data set with the calculated response is presented to assess the calculation method and to give ideas on sensitive input parameters considering the full-scale condition.
Torsional aero-elastic oscillations of an elastically mounted circular cylinder
Authors: Demchenko Yaroslav , Vedeneev Vasily , Zubkov Alexander
Published: June 2024

The object of this study is a system consisting of a rigid thin-walled cylinder of finite span, mounted on an elastic cantilever beam transversely to the direction of the subsonic air flow in a wind tunnel. The purpose is to identify and analyze the mechanisms of various types of aero-elastic resonant excitation of the system for the use in energy harvesting designs based on vortex-induced vibrations. The results of an experimental study are presented. In contrast to similar works with a similar configuration of the model that performs transverse translational oscillations, we discovered previously unexplored VIVs type in which the cylinder rotates around the cantilever support.
Wind-tunnel experimental study on identification of modal aerodynamic damping matrix for operating wind turbines
Authors: Wang Zhaowei , Chen Chao , Hua Xugang
Published: June 2024

This study aimed to design a scaled wind turbine model, with a geometric scaling factor of 1/75 and based on the NREL-5MW wind turbine prototype, in order to conduct an experimental study on the aerodynamic damping identification under the operating condition. First, the dynamic characteristics of the scaled model such as natural frequency and damping ratio were tested under the parked condition. Then, wind tunnel force measurements were conducted to verify the similarity between the rotor thrust coefficients of the scaled model and prototype, ensuring a reasonable rotor design. Subsequently, identification of the modal aerodynamic damping matrix of the operating scaled wind turbine model was performed using the identification method based on artificial harmonic excitations and frequency response functions. Experimental results show that both the identified diagonal and off-diagonal terms in the modal damping matrix agree well with the theoretical values when the rotor rotation speed is 150 rpm. However, apparent differences between the identified values and theoretical values can be observed when the rotor rotation speed and wind speed change. Generally, the wind tunnel test confirms the availability of the theoretical modal damping matrix expression in capturing the damping characteristics and the feasibility of the identification method for operating wind turbines.
Multivariate prediction on wake-affected wind turbines using graph neural networks
Authors: Santos Francisco de N , Duthé Gregory , Abdallah Imad , Réthoré Pierre-Élouan , Weijtjens Wout , Chatzi Eleni , Devriendt Christof
Published: June 2024

Modern wind turbines are large and slender dynamical structures with a fatigue loading profile of complex nature. The guarantee of their structural integrity is paramount for materializing cost efficient and more reliable wind energy. The measurement of the global dynamic response and loads of wind turbines is fundamental for achieving this goal. However, an industry-wide, cost-effective direct sensing framework is yet to arise. Moreover, deploying physical sensors and measurement systems on every structural component of interest of a wind turbine induces prohibitive costs in deployment, maintenance and data management. Considering that direct fluid-structure interaction simulations on a farm level are not computationally feasible, the preferred path for structural response estimation on wind farms has been surrogate modelling. Within this landscape, new model architectures have risen in recent years which are able to take into account graph structured data (i.e. non-euclidean data). Wind turbines positioned in a farm, where there is a layout- and topology-dependant interplay of aerodynamic wake affecting the loading profile and power production, lend themselves perfectly to this paradigm. Thus, in this contribution, we introduce the use of graph neural networks (GNN) for layout agnostic saptio-temporal joint modelling of fatigue loads effects, rotor-averaged wind speed and power production on individual turbines of wind farms. To this end, we generate stochastic dependent samples of inflow conditions for wind speed, wind direction, wind shear and nacelle yaw angles. Additionally, wind farm layouts are randomly generated based on different geometric shapes (rectangle, triangle, ellipse and sparse circles) with random parametrization (varying orientations, length/width ratio) for different numbers of turbines and minimal distance (based on the rotor diameter). Both the arbitrary layouts and the random inflow conditions are used as inputs for PyWake, a wind farm simulation tool capable of calculating wind farm flow fields, power and fatigue loads. In our analysis, we develop and compare the performance of the GENeralized Aggregation Networks (GEN), the Graph Attention Networks (GAT) and the Graph Isomorphism Network with Edges (GINE) in their accuracy and ability to generalize their joint predictions for unseen layouts, uncertain inflow conditions and fatigue load estimation on the blade root, tower top and tower base of any wind turbine in the farm. Our results indicate that the GEN layer yields the best performance, followed by GINE, while the GAT layer under-performs and is unable to differentiate between different wake conditions. We further observe that the GAT layer causes a latent space collapse, due to the coupled effect of the manner in which we initialise node features and the way in which its messages are computed.
An Autonomous Data Acquisition and System Identification Framework for In-Service Wind Turbine Using p-LSCF Method
Authors: Ozturkoglu Onur , Ozcelik Ozgur , Gunel Serkan , Yurtseven Veysel , Taner Yasar
Published: June 2024

Wind energy has increased its importance among renewable energy sources in recent years due to its high energy production capacity and the return of investment cost in a short time and has become a better alternative to conventional energy sources. The share of relatively large wind turbines in the renewable energy market, which can produce more energy in a short time, is increasing rapidly. Being able to accurately and consistently define the dynamic characteristics of turbines is crucial to ensure that these structures remain operational throughout their economic life and even serve more than their planned lifespan. In this study a novel autonomous data acquisition and system identification framework is developed. The subject of the work is an in-service 2.5 MW horizontal axis wind turbine with three rotor blades in Izmir/Turkey. Vibration, temperature, and relative humidity data is recorded through data acquisition system designed distributed along the turbine tower height (80 m.). Data acquisition system contain three types of sensors at different levels: One tri-axial, three uniaxial accelerometers, temperature, and relative humidity sensors at foundation level; two uniaxial accelerometers at 20-meter level; two uniaxial accelerometers and temperature sensor at 40meter level; two uniaxial accelerometers at 60-meter level; two uniaxial accelerometers, temperature, and relative humidity sensors at 80-meter level. Preprocessed vibration data is transmitted from wind farm to the university campus with operational and environmental data (e.g., wind speed, wind direction, temperature, humidity, rotor speed, nacelle direction, and pitch angle) collecting from turbine SCADA system synchronously. Modal parameters are estimated using p-LSCF method providing more clear stabilization chart than other operational modal analysis methods. A clear stabilization chart makes more easier to choose stable system poles that is significant to automate system identification. Whole data acquisition and system identification process is done without any user interference. A preliminary correlation work is done between the dynamic characteristics of the turbine and operational/environmental factors.
Substructure optimization of a 10MW floating wind turbine for installations in the Mediterranean Sea
Authors: Ferri Giulio , Borri Claudio , Marino Enzo
Published: June 2024

In this contribution, a multi-objective, site-specific optimization procedure aimed at finding the optimal substructure configuration for a 10MW FOWT is presented. An in-house developed Frequency Domain (FD) model is adopted for the simulation of the coupled system. An installation site near to the Italian coastline is selected. The probability distributions of wind speed, significant wave height and peak spectral period are calculated based on a metocean 20-year database of wind and wave records. The multi-objective optimization concerns the joint minimization of the tower structural response under an extreme event and the manufacturing cost of the substructure. Constraints on the admissible platform displacements, cables geometry, and anchor loads are considered. Results show that the optimized solutions significantly reduce the cost of the system with a moderate increase of stresses, opening interesting perspectives for the reduction of the Levelised Cost of Energy (LCOE) in sites characterized by mild sea states and low wind resource.
Validation of an engineering model for vortex generators in a viscous-inviscid interaction method for airfoil analysis
Authors: Sahoo Abhratej , Ferreira Carlos Simao , Ravishankara Akshay Koodly , Schepers Gerard , Yu Wei
Published: June 2024

As the demand for renewable energy increases, wind turbine rotors will become larger with slender blades. Vortex Generators (VGs) are used for passive flow control to avoid flow separation and reduce unsteady loading on the thick root section of slender blades due to their simplicity, inexpensiveness, and the ability to retrofit them to blades. Aerodynamic load calculations for VGs involve long experimental campaigns or resource intensive CFD calculations. Due to their inherent time-intensive nature aeroelastic optimisation and design tools prefer to use simplified but accurate analysis tools for aerodynamic load calculations of clean airfoils. One such class of tools are viscous-inviscid interaction solvers that use Integral Boundary Layer (IBL) methods for viscous calculations in the boundary layer coupled with an inviscid solver for the rest of the domain. The most popular example of this tool is XFOIL. An engineering model for VGs in IBL methods has previously been developed and implemented in XFOIL. In this research, the model parameters are tuned for implementation in another tool RFOIL based on XFOIL. RFOIL has been developed for accurate and robust analysis of wind turbine airfoils with improvements for thick airfoils and rotational corrections. The aerodynamic performance predicted the VG model in both XFOIL and RFOIL is then validated with an extensive database of airfoil data consisting of airfoils between 21% to 60% thickness, as well as Reynolds numbers between 1 million to 14 million, equipped with and without VGs. Finally, the computation time for the VG model is compared with that for a clean airfoil analysis in the same viscous-inviscid interaction solvers RFOIL and XFOIL. The investigation provides an overview of the usability of the engineering model in airfoil design methodologies in the wind turbine industry.
Unsteady aerodynamic modeling of whirl flutter on a bending wing
Authors: Gaudemaris Vincent de , Schotté Jean-Sébastien , Placzek Antoine , Blanc Laurent , Thouverez Fabrice
Published: June 2024

With the new generation of propeller engines, aircrafts become more prone to whirl flutter, an aeroelastic instability possibly causing irreversible structural failures. The prediction of the motion induced aerodynamic loads on the propeller is essential to catch such kind of instability. Classical theory uses a quasi-steady aerodynamic model to obtain analytically the dependence of the forces and moments on the movement. This paper proposes to use an unsteady aerodynamic theory and to take into account the inflow disturbance, to obtain a more realistic model. This leads to a complex dependency of the aerodynamic loads on the vibratory frequency, which is bypassed by the use of a rational matrix approximation (RMA) of the aerodynamic transfer function to obtain a linearized stability problem. Aeroelastic stability studies are performed on a classical two degrees of freedom (dof) structural model, and on another one including one more degree of freedom to represent the wing bending. Results demonstrate a strong dependence of the stability boundaries on the aerodynamic model used, and highlight the precision and convenience of the method involving a RMA of the aerodynamic transfer function. Wing flexibility is of importance, as some unconventional stability boundaries are obtained in comparison to the classical 2-dof model.
A general approach to model human-structure interaction in the case of slender structures
Authors: Vanali M. , Manzoni S. , Berardengo M. , Lucà F.
Published: June 2024

The paper deals with a general approach for modeling human-structure interaction in slender structures. Vertical vibrations are addressed in the paper. The proposed approach relies on the model of the complete system and on a solution procedure capable to account for the most complex dynamic conditions. The effects of changes in pedestrians positions along the structure and of changes in pedestrians postures during motion are combined, therefore taking into account different and time varying mechanical impedances.Moreover, the developed model describes the coupled system (i.e., structure plus people on the structure) through the definition of two different actions of pedestrians: passive and active forces. This approach was used to evaluate the dynamic interactions between a light slender structure and pedestrians. The proposed approach was validated by carrying out experimental tests on a staircase and results show a good match between the predicted vibrations and the experimentally found ones.
Field-testing and serviceability assessment of a lively footbridge
Authors: Mulas Maria Gabriella , Fortis Cristina , Lastrico Giulia
Published: June 2024

Following a previous work, the paper describes the serviceability assessment of a footbridge over the Lambro River near Milano (Italy), based on the comparative analysis of on-field tests and of the outcome of the Hyvoss guidelines. The 3-span footbridge, for a bicycle-pedestrian mixed use, is 107 m long, 4.4 m wide and roughly symmetric about both mid-span and the longitudinal axis. A reinforced concrete (RC) deck is supported by a steel structure: two longitudinal welded I-profiles, braced in their lower part, support the transverse beams that, in turn, support the RC deck. Ambient vibration tests identified the footbridge modal properties, detecting the fundamental bending mode, with the maximum amplitude recorded at mid-span, at 1.75 Hz, well within the critical range of excitation from walking pedestrians. Hence, a series of forced vibration tests was subsequently performed, involving single pedestrians or groups of up to 12 people. Sensors, located as in AVTs, recorded footbridge acceleration, both horizontal and vertical. In all the tests, pedestrians walked in resonance conditions with the first mode frequency along straight trajectories. To excite further the first mode, their spatial configurations were symmetric about the longitudinal axis of the bridge. Difference among tests concerns not only the number of test subjects (TSs), but also their spatial configuration and/or the TSs involved in each test. The comparison among tests sharing similar spatial configurations, with TSs either in a row or in a column, allows a serviceability assessment directly from the experimental footbridge response. The numerical analyses are based on a FE model previously developed, showing an excellent reproduction of the first bending mode, but a poor simulation of the second torsional mode. The outcome of the analyses performed according to HiVoSS guideline is in good agreement with the experimental results, in spite of the very low crowd density of the on-field tests and the unsatisfactory reproduction of the torsional mode within the FE model.
A first step towards the development of load models for the vibration serviceability assessment of footbridges under running actions
Authors: Strobbe Senne , Lottefier Jasper , Broeck Peter Van den , Nimmen Katrien Van
Published: June 2024

The state of the art involving dynamic running actions is limited to single-person load models valid for running on a rigid laboratory floor. Little to no expertise is available on running excitation as a load scenario for civil engineering structures. The development of realistic load scenarios requires input on (1) intra-person variabilities, (2) inter-person variabilities and (3) human-structure interaction phenomena, and their impact on the resulting structural response. This contribution uses vibration measurements on the lower back of running persons to identify and characterize intra- and inter-person variabilities of the dynamic running load. Based on these results, the impact of these variabilities on the resulting structural response is investigated numerically. The results show that as the impact of intra-person variability is negligible, it can be disregarded in design calculations. However, the impact of inter-person variability is shown to be considerable, reducing the resulting structural response with 64% and more. Realistic ranges of inter-person variability should therefore be considered in design calculations.
Extended vibration serviceability assessment of a footbridge that considers the modal coupling
Authors: Bastin T , de Ville de Goyet V , Duchêne Y
Published: June 2024

The evolution of pedestrian bridge design is tending towards increasingly slender and flexible structures which are more sensitive to vibrations caused by pedestrians. The accelerations induced by the dynamic response are checked to fulfil the comfort criteria. The loads proposed in the guides are sinusoidal and harmonic. Since only stationary vibrations are of interest, the system is solved in the frequency domain. The calculation is done in the modal base and the phasing of pedestrian load is associated with each mode to maximise the modal load. Traditionally, engineers calculate for each mode the dynamic peak response with a static analysis using an equivalent resonant loading. Hence, implementing equivalent resonant load could be time-consuming for complex models. Furthermore, this methodology does not consider the cross correlation between the modes that can be enhanced by the presence of Tuned Mass Dampers (TMDs). An automatic load procedure is implemented in our FEM software FinelG which adapts the phasing and surface size of pedestrian loading to obtain the load cases that maximise each modal loading. The dynamic response of the structure for a range of pedestrian walking frequency and phasing corresponding to each eigenmode is solved through an appropriate transfer function matrix. Both methods are compared to the “L’enjambée” footbridge located in Namur, Belgium.
Codifiable probability-based model for footbridges response to random walkers
Authors: Picozzi V , Avossa A M , Ricciardelli F
Published: June 2024

The response to the transient action of a single pedestrian or a small group of pedestrians is required for the serviceability assessment of human-induced footbridge vibrations. The approach adopted by standards and guidelines is proved to lead to a non-uniform level of reliability. This is in contrast with both the stochastic nature of pedestrian gait parameters and the probabilistic approach adopted by modern structural codes. In this paper, firstly the analysis of peak accelerations is developed according to the Eurocode for a number of simply-supported footbridges, both in vertical and lateral directions. These are shown to correspond to different level of probability of exceedance, that is larger for short-span footbridges and smaller for long-span footbridges. Then, a model for the assessment of the characteristic footbridge acceleration is provided. This is based on the prediction of the basic acceleration, i.e. the response of simply-supported footbridges to single pedestrian crossings, and the application of an acceleration ratio factor for footbridge configurations other than simply-supported.
Coupled Flutter Behavior and Mechanism of Flexible Suspended Footbridge
Authors: Lee S H , Hwang Y , Kim H K
Published: June 2024

Flexible suspended footbridges are more prone to flutter than road bridges due to their lightweight, flexible, and slender structural features. Understanding the flutter generation mechanism and developing effective suppression methods are essential for the safe and reliable design of such footbridges. This study conducted complex eigenvalue analysis and wind tunnel tests to investigate the flutter characteristics of a flexible suspended footbridge with a solid deck. The results revealed that the aeroelastic coupling effect between torsional vibration and torsional-driven vertical vibration lead to torsional flutter. The prototype flexible suspended bridge in this paper was constructed by applying open grating to the entire deck to ensure wind stability.