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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

SU-E-T-454: Impact of Calculation Grid Size On Dosimetry and Radiobiological Parameters for Head and Neck IMRT

Purpose: IMRT has become standard of care for complex treatments to optimize dose to target and spare normal tissues. However, the impact of calculation grid size is not widely known especially dose distribution, tumor control probability (TCP) and normal tissue complication probability (NTCP) which is investigated in this study. Methods: Ten head and neck IMRT patients treated with 6 MV photons were chosen for this study. Using Eclipse TPS, treatment plans were generated for different grid sizes in the range 1–5 mm for the same optimization criterion with specific dose-volume constraints. The dose volume histogram (DVH) was calculated for all IMRT plans and dosimetric data were compared. ICRU-83 dose points such as D2%, D50%, D98%, as well as the homogeneity and conformity indices (HI, CI) were calculated. In addition, TCP and NTCP were calculated from DVH data. Results: The PTV mean dose and TCP decreases with increasing grid size with an average decrease in mean dose by 2% and TCP by 3% respectively. Increasing grid size from 1–5 mm grid size, the average mean dose and NTCP for left parotid was increased by 6.0% and 8.0% respectively. Similar patterns were observed for other OARs such as cochlea, parotids and spinal cord. The HI increases up to 60% and CI decreases on average by 3.5% between 1 and 5 mm grid that resulted in decreased TCP and increased NTCP values. The number of points meeting the gamma criteria of ±3% dose difference and ±3mm DTA was higher with a 1 mm on average (97.2%) than with a 5 mm grid (91.3%). Conclusion: A smaller calculation grid provides superior dosimetry with improved TCP and reduced NTCP values. The effect is more pronounced for smaller OARs. Thus, the smallest possible grid size should be used for accurate dose calculation especially in H and N planning.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Comparison of Equivalence Ratio Transients on Combustion Instability in Single-Nozzle and Multi-Nozzle Combustors

Low-emissions gas turbine combustion, achieved through the use of lean, premixed fueling strategies, is susceptible to combustion instability. The driving mechanism for this instability arises from fluctuations of pressure, fuel/air flow rate, and heat release rate. If these fluctuations are relatively in-phase, the combustion system will evolve to a self-excited state. The self-excited instability frequency and amplitude depend mainly on the operating condition and the geometry of the combustor. In this study, we consider the onset and decay of self-excited instabilities, resulting from transients in fuel/air ratio, in both single-nozzle and multi-nozzle combustors. In particular, we examine the differences in the instability onset and decay processes between these two flame configurations, as most gas turbine combustors have multiple nozzles, but most gas turbine combustor experiments utilize a single-nozzle. A nonlinear logistic regression analysis is applied to study the timescales of the decay and onset transients. Variations in the equivalence ratio change the heat release rate distribution inside the combustor, which is captured using chemiluminescence imaging. The normalized Rayleigh index, which shows the spatial distribution of the instability driving, is calculated to analyze the driving strength in different regions of the flame. Comparisons between the single- and multi-nozzle flame transients, including both center and outer flames for the multi-nozzle combustor, suggest that both confinement from the wall and flame-flame interaction are crucial to determining flame dynamics as the equivalence ratio transient changes the heat release rate distribution near corner recirculation zone and flame shear layers.

Chen, Xiaoling↗
DOE OSTI2018

Comparison of equivalence ratio transients on combustion instability in single-nozzle and multi-nozzle combustors

Low-emissions gas turbine combustion, achieved through the use of lean, premixed fueling strategies, is susceptible to combustion instability. The driving mechanism for this instability arises from fluctuations of pressure, fuel/air flow rate, and heat release rate. If these fluctuations are relatively in-phase, the combustion system will evolve to a self-excited state. The self-excited instability frequency and amplitude depend mainly on the operating condition and the geometry of the combustor. In this study, we consider the onset and decay of self-excited instabilities, resulting from transients in fuel/air ratio, in both single-nozzle and multi-nozzle combustors. In particular, we examine the differences in the instability onset and decay processes between these two flame configurations, as most gas turbine combustors have multiple nozzles, but most gas turbine combustor experiments utilize a single-nozzle. A nonlinear logistic regression analysis is applied to study the timescales of the decay and onset transients. Variations in the equivalence ratio change the heat release rate distribution inside the combustor, which is captured using chemiluminescence imaging. The normalized Rayleigh index, which shows the spatial distribution of the instability driving, is calculated to analyze the driving strength in different regions of the flame. Comparisons between the single- and multi-nozzle flame transients, including both center and outer flames for the multi-nozzle combustor, suggest that both confinements from the wall and flame-flame interaction are crucial to determining flame dynamics as the equivalence ratio transient changes the heat release rate distribution near corner recirculation zone and flame shear layers.

20 FOSSIL-FUELED POWER PLANTS↗
DOE OSTI2017

Parametric Evaluation of SiC/SiC Composite Cladding with UO 2 Fuel for LWR Applications: Fuel Rod Interactions and Impact of Nonuniform Power Profile in Fuel Rod

SiC/SiC composites are candidates for accident tolerant fuel cladding in light water reactors. In the extreme nuclear reactor environment, SiC-based fuel cladding will be exposed to neutron damage, significant heat flux, and a corrosive environment. Here, to ensure reliable and safe operation of accident tolerant fuel cladding concepts such as SiC-based materials, it is important to assess thermo-mechanical performance under in-reactor conditions including irradiation and realistic temperature distributions. The effect of non-uniform dimensional changes caused by neutron irradiation with spatially varying temperatures, along with the closing of the fuel–cladding gap, on the stress development in the cladding over the course of irradiation were evaluated. The effect of non-uniform circumferential power profile in the fuel rod on the mechanical performance of the cladding is also evaluated. These analyses have been performed using the BISON fuel performance modeling code and the commercial finite element analysis code Abaqus. A constitutive model is constructed and solved numerically to predict the stress distribution in the cladding under normal operating conditions. The dependence of dimensions and thermophysical properties on irradiation dose and temperature has been incorporated into the models. Initial scoping results from parametric analyses provide time varying stress distributions in the cladding as well as the interaction of fuel rod with the cladding under different conditions of initial fuel rod-cladding gap and linear heat rate. It is found that a non-uniform circumferential power profile in the fuel rod may cause significant lateral bowing in the cladding, and motivates further analysis and evaluation.

Accident tolerant fuel↗
DOE OSTI2018

Mobility of the Two-Dimensional Electron Gas in DA-pHEMT Heterostructures with Various δ–n-Layer Profile Widths

The effect of the silicon-atom distribution profile in donor δ-layers of AlGaAs/InGaAs/AlGaAs heterostructures with donor–acceptor doping on the mobility of the two-dimensional electron gas is studied. The parameters of the δ-layer profiles are determined using the normal approximation of the spatial distributions of silicon atoms, measured by secondary-ion mass spectroscopy. It is shown that the standard deviation σ of the δ-layer profile can be reduced from 3.4 to 2.5 nm by the proper selection of growth conditions. Measurements of the magnetic-field dependences of the Hall effect and conductivity show that such a decrease in σ allowed an increase in the mobility of the two-dimensional electron gas in heterostructures by 4000 cm{sup 2}/(V s) at 77 K and 600 cm{sup 2}/(V s) at 300 K. The mobility calculation taking into account filling of the first two size-quantization subbands shows that an increase in the mobility is well explained by a reduction in the Coulomb scattering at ionized donors due to an increase in the effective thickness of the spacer layer with decreasing σ of the δ-layer profile.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Kickstart Your Federal Microgrid Resiliency Project: Financing Opportunities and Best Practices

Recent high-impact events such as hurricanes and wildfires have resulted in the loss of utility grid power to federal sites, highlighting the need for energy system resilience. To sustain critical functions, site loads may need to operate during an electric grid outage or at least have power restored quickly after the event. A microgrid can provide reliable backup power to critical loads when electric utility power is interrupted, and may also provide value during normal operations.

DER↗
DOE OSTI2016

Theoretical Benefits of Dynamic Collimation in Pencil Beam Scanning Proton Therapy for Brain Tumors: Dosimetric and Radiobiological Metrics

Purpose: To quantify the dosimetric benefit of using a dynamic collimation system (DCS) for penumbra reduction during the treatment of brain tumors by pencil beam scanning proton therapy (PBS PT). Methods and Materials: Collimated and uncollimated brain treatment plans were created for 5 patients previously treated with PBS PT and retrospectively enrolled in an institutional review board–approved study. The in-house treatment planning system, RDX, was used to generate the plans because it is capable of modeling both collimated and uncollimated beamlets. The clinically delivered plans were reproduced with uncollimated plans in terms of target coverage and organ at risk (OAR) sparing to ensure a clinically relevant starting point, and collimated plans were generated to improve the OAR sparing while maintaining target coverage. Physical and biological comparison metrics, such as dose distribution conformity, mean and maximum doses, normal tissue complication probability, and risk of secondary brain cancer, were used to evaluate the plans. Results: The DCS systematically improved the dose distribution conformity while preserving the target coverage. The average reduction of the mean dose to the 10-mm ring surrounding the target and the healthy brain were 13.7% (95% confidence interval [CI] 11.6%-15.7%; P<.0001) and 25.1% (95% CI 16.8%-33.4%; P<.001), respectively. This yielded an average reduction of 24.8% (95% CI 0.8%-48.8%; P<.05) for the brain necrosis normal tissue complication probability using the Flickinger model, and 25.1% (95% CI 16.8%-33.4%; P<.001) for the risk of secondary brain cancer. A general improvement of the OAR sparing was also observed. Conclusion: The lateral penumbra reduction afforded by the DCS increases the normal tissue sparing capabilities of PBS PT for brain cancer treatment while preserving target coverage.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

{sup 103}Pd strings: Monte Carlo assessment of a new approach to brachytherapy source design

Purpose: A new type of{sup 103}Pd source (CivaString and CivaThin by CivaTech Oncology, Inc.) is examined. The source contains {sup 103}Pd and Au radio-opaque marker(s), all contained within low-Z{sub eff} organic polymers that permit source flexibility. The CivaString source is available in lengths L of 10, 20, 30, 40, 50, and 60 mm, and referred to in the current study as CS10–CS60, respectively. A thinner design, CivaThin, has sources designated as CT10–CT60, respectively. The CivaString and CivaThin sources are 0.85 and 0.60 mm in diameter, respectively. The source design is novel and offers an opportunity to examine its interesting dosimetric properties in comparison to conventional {sup 103}Pd seeds. Methods: The MCNP5 radiation transport code was used to estimate air-kerma rate and dose rate distributions with polar and cylindrical coordinate systems. Doses in water and prostate tissue phantoms were compared to determine differences between the TG-43 formalism and realistic clinical circumstances. The influence of Ti encapsulation and 2.7 keV photons was examined. The accuracy of superposition of dose distributions from shorter sources to create longer source dose distributions was also assessed. Results: The normalized air-kerma rate was not highly dependent onL or the polar angle θ, with results being nearly identical between the CivaString and CivaThin sources for common L. The air-kerma strength was also weakly dependent on L. The uncertainty analysis established a standard uncertainty of 1.3% for the dose-rate constant Λ, where the largest contributors were μ{sub en}/ρ and μ/ρ. The Λ values decreased with increasing L, which was largely explained by differences in solid angle. The radial dose function did not substantially vary among the CivaString and CivaThin sources for r ≥ 1 cm. However, behavior for r < 1 cm indicated that the Au marker(s) shielded radiation for the sources having L = 10, 30, and 50 mm. The 2D anisotropy function exhibited peaks and valleys that corresponded to positions adjacent to {sup 103}Pd wells and Au markers, respectively. Dose distributions of both source types had minimal anisotropy in comparison to conventional {sup 103}Pd seeds. Contributions by 2.7 keV photons comprised ≤0.1% of the dose from all photons at positions farther than 0.13 mm from the polymer source surface. Differences between absorbed dose to water and prostate became more substantial as distance from the sources increased, with prostate dose being about 13% lower for r = 5 cm. Using a cylindrical coordinate system, dose superposition of small length sources to replicate the dose distribution for a long length source proved to be a robust technique; a 2.0% tolerance compared with the reference dose distribution did not exceed 0.1 cm{sup 3} for any of the examined source combinations. Conclusions: By design, the CivaString and CivaThin sources have novel dosimetric characteristics in comparison to Ti-encapsulated{sup 103}Pd seeds. The dosimetric characterization has determined the reasons for these differences through analysis using Monte Carlo-based radiation transport simulations.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2024

Particle Acceleration in Relativistic Alfvénic Turbulence

Abstract Strong magnetically dominated Alfvénic turbulence is an efficient engine of nonthermal particle acceleration in a relativistic collisionless plasma. We argue that in the limit of strong magnetization, the type of energy distribution attained by accelerated particles depends on the relative strengths of turbulent fluctuations δ B 0 and the guide field B 0 . If δ B 0 ≪ B 0 , the particle magnetic moments are conserved, and the acceleration is provided by magnetic curvature drifts. Curvature acceleration energizes particles in the direction parallel to the magnetic field lines, resulting in log-normal tails of particle energy distribution functions. Conversely, if δ B 0 ≳ B 0 , interactions of energetic particles with intense turbulent structures can scatter particles, creating a population with large pitch angles. In this case, magnetic mirror effects become important, and turbulent acceleration leads to power-law tails of the energy distribution functions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SU-E-I-05: A Correction Algorithm for Kilovoltage Cone-Beam Computed Tomography Dose Calculations in Cervical Cancer Patients

Purpose: To investigate the accuracy and feasibility of dose calculations using kilovoltage cone beam computed tomography in cervical cancer radiotherapy using a correction algorithm. Methods: The Hounsfield units (HU) and electron density (HU-density) curve was obtained for both planning CT (pCT) and kilovoltage cone beam CT (CBCT) using a CIRS-062 calibration phantom. The pCT and kV-CBCT images have different HU values, and if the HU-density curve of CBCT was directly used to calculate dose in CBCT images may have a deviation on dose distribution. It is necessary to normalize the different HU values between pCT and CBCT. A HU correction algorithm was used for CBCT images (cCBCT). Fifteen intensity-modulated radiation therapy (IMRT) plans of cervical cancer were chosen, and the plans were transferred to the pCT and cCBCT data sets without any changes for dose calculations. Phantom and patient studies were carried out. The dose differences and dose distributions were compared between cCBCT plan and pCT plan. Results: The HU number of CBCT was measured by several times, and the maximum change was less than 2%. To compare with pCT, the CBCT and cCBCT has a discrepancy, the dose differences in CBCT and cCBCT images were 2.48%±0.65% (range: 1.3%∼3.8%) and 0.48%±0.21% (range: 0.1%∼0.82%) for phantom study, respectively. For dose calculation in patient images, the dose differences were 2.25%±0.43% (range: 1.4%∼3.4%) and 0.63%±0.35% (range: 0.13%∼0.97%), respectively. And for the dose distributions, the passing rate of cCBCT was higher than the CBCTs. Conclusion: The CBCT image for dose calculation is feasible in cervical cancer radiotherapy, and the correction algorithm offers acceptable accuracy. It will become a useful tool for adaptive radiation therapy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

TU-EF-304-11: Therapeutic Benefits of Collimation in Spot Scanning Proton Therapy in the Treatment of Brain Cancer

Purpose: A dynamic collimation system (DCS) based on two orthogonal pairs of mobile trimmer blades has recently been proposed to reduce the lateral penumbra in spot scanning proton therapy (SSPT). The purpose of this work is to quantify the therapeutic benefit of using the DCS for SSPT of brain cancer by comparing un-collimated and collimated treatment plans. Methods: Un-collimated and collimated brain treatment plans were created for five patients, previously treated with SSPT, using an in-house treatment planning system capable of modeling collimated and un-collimated beamlets. Un-collimated plans reproduced the clinically delivered plans in terms of target coverage and organ-at-risk (OAR) sparing, whereas collimated plans were re-optimized to improve the organ-at-risk sparing while maintaining target coverage. Physical and biological comparison metrics such as dose distribution conformity, mean and maximum doses, normal tissue complication probability (NTCP) and risk of secondary brain cancer were used to evaluate the plans. Results: The DCS systematically improved the dose distribution conformity while preserving the target coverage. The average reduction of the mean dose to the 10-mm ring surrounding the target and the healthy brain were 7.1% (95% CI: 4.2%–9.9%; p<0.01) and 14.3% (95% CI: 7.8%–20.8%; p<0.01), respectively. This yielded an average reduction of 12.0% (95% CI: 8.2%–15.7%; p<0.01) for the brain necrosis NTCP using the Flickinger model, and 14.2% (95% CI: 7.7%–20.8%; p<0.01) for the risk of secondary brain cancer. The average maximum dose reductions for the brainstem, chiasm, optic nerves, cochleae and pituitary gland when comparing un-collimated and collimated plans were 14.3%, 10.4%, 11.2%, 13.0%, 12.9% and 3.4%, respectively. Evaluating individual plans using the Lyman-Kutcher-Burman NTCP model also yielded improvements. Conclusion: The lateral penumbra reduction performed by the DCS increases the normal tissue sparing capabilities of SSPT for brain tumor treatment while preserving the target coverage. This research was financially supported by Ion Beam Applications S.A. (IBA, Louvain-La-Neuve, Belgium)

60 APPLIED LIFE SCIENCES↗
DOE OSTI2024

Hydrogen Transportation and Distributed Energy Systems Risk and Feasibility Assessment for Airport Facilities - CRADA 555 (Abstract)

This CRADA presents the strategy that Pacific Northwest National Laboratory (PNNL) and Sandia National Laboratories (SNL) will take to support PDX in performing a risk assessment for several hydrogen deployment scenarios. It is intended that the deployment of hydrogen at PDX could serve several roles, including decarbonizing its bus fleet and building out a distributed energy system which could also be used to facilitate emergency operations. PDX’s initial plans concern replacing their current fleet of 28 busses with FCEBs. To do so, PDX will need to deploy the hydrogen systems needed to support the buses. This includes hydrogen storage, production, delivery, filling, compression, and maintenance. PDX also has long range plans to build a distributed energy system to support normal and resilient operations. This study will investigate the risks of each plan individually, and a combined plan which serves both. Additionally, PDX is in an area with the potential for high seismic activity, as such this work will also include a seismic risk assessment. A detailed risk assessment using PDX as a test case is necessary to ensure the deployment of large-scale hydrogen is successful and to understand the benefits or drawbacks in contrast to electric only technologies. Many technologies have been proposed that claim to utilize hydrogen for both transportation and distributed energy systems; however, these technologies need to be analyzed as they apply to an actual site. The physical infrastructure and hydrogen use cases for PDX will be analyzed, a risk assessment will be performed on the hydrogen infrastructure and distributed energy system. These assessments will be useful for understanding how each of these technologies would perform as applied to a large public facility and public safety. The results of this CRADA, and an impact of the DOE funding, will evaluate hydrogen as a key fuel for PDX and will accelerate by several years availability of hydrogen, fuel cells, and other low-carbon technologies.

08 HYDROGEN↗
DOE OSTI2025

Lattice QCD Benchmark of Proton Helicity and Flavor-Dependent Unpolarized Transverse Momentum-Dependent Parton Distribution Functions at Physical Quark Masses

We present the first lattice QCD calculations of the isovector helicity transverse momentum-dependent parton distribution function (TMDPDF) and the flavor-dependent unpolarized TMDPDFs for up and down quarks in the proton. Our computations utilize domain-wall fermion discretization with physical quark masses. Employing Coulomb-gauge-fixed quark correlation functions within the large-momentum effective theory framework, we access nonperturbative transverse quark separations 𝑏 𝑇 up to approximately 1 fm, corresponding to transverse momenta as low as 200 MeV. Based on the quasi-TMD factorization theorem, we construct renormalization-group–invariant ratios that are equal to the corresponding light-cone TMDPDF ratios. At moderate 𝑥, our results reveal that the isovector helicity and unpolarized TMDPDFs exhibit nearly identical transverse structure up to a normalization factor, and the unpolarized distributions display only mild flavor dependence. These findings not only support key trends observed in recent global analyses but also provide robust, nonperturbative constraints that can distinguish between different parameterizations. This Letter establishes a first-principles benchmark for TMDPDFs, offering valuable input for ongoing and future experimental efforts to map the proton’s three-dimensional structure.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

cdf_normalize

The code performs image intensity normalization. A new intensity normalization method is presented, which works well with images whose pixel intensity distribution follow a Gamma Probability Distribution. Some examples are provided to show the normalization effect. This new intensity normalization method is also compared to some other well-established intensity normalization methods.

Mundhenk, TerrellN.↗
DOE OSTI2024

Parametric description of intermittent probability distribution functions in solar wind and magnetohydrodynamic turbulence

ABSTRACT In this work, we find empirical evidence that the scale-dependent statistical properties of solar wind and magnetohydrodynamic (MHD) turbulence can be described in terms of a family of parametric probability distribution functions (PDFs) known as Normal Inverse Gaussian (NIG). Understanding these PDFs is one of the most important goals in turbulence theory, as they are inherently connected to the intermittent properties of solar wind turbulence. We investigate the properties of PDFs of Elsasser increments based on a large statistical sample from solar wind observations and high-resolution numerical simulations of MHD turbulence. In order to measure the PDFs and their corresponding properties, three experiments are presented: fast and slow solar wind for experimental data and a simulation of reduced MHD (RMHD) turbulence. Conditional statistics on a 23-yr-long sample of WIND data near 1 au and high-resolution pseudo-spectral simulation of steadily driven RMHD turbulence on a $2048^3$ mesh are used to construct scale-dependent PDFs. The empirical PDFs are fitted to NIG distributions, which depend on four free parameters. Our analysis shows that NIG distributions accurately capture the evolution of the PDFs, with scale-dependent parameters, from large scales characterized by a Gaussian distribution, turning to exponential tails within the inertial range and stretched exponentials at dissipative scales. We also show that empirically-measured NIG parameters exhibit well-defined scaling properties that are similar across the three empirical data sets, which may be indicative of universal behaviour.

Astronomy & Astrophysics↗
DOE OSTI2018

Convergence and Efficiency of Adaptive Importance Sampling Techniques with Partial Biasing

We propose a new Monte Carlo method to efficiently sample a multimodal distribution (known up to a normalization constant). We consider a generalization of the discrete-time Self Healing Umbrella Sampling method, which can also be seen as a generalization of well-tempered metadynamics. The dynamics is based on an adaptive importance technique. The importance function relies on the weights (namely the relative probabilities) of disjoint sets which form a partition of the space. These weights are unknown but are learnt on the fly yielding an adaptive algorithm. In the context of computational statistical physics, the logarithm of these weights is, up to an additive constant, the free-energy, and the discrete valued function defining the partition is called the collective variable. The algorithm falls into the general class of Wang–Landau type methods, and is a generalization of the original Self Healing Umbrella Sampling method in two ways: (i) the updating strategy leads to a larger penalization strength of already visited sets in order to escape more quickly from metastable states, and (ii) the target distribution is biased using only a fraction of the free-energy, in order to increase the effective sample size and reduce the variance of importance sampling estimators. We prove the convergence of the algorithm and analyze numerically its efficiency on a toy example.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Secondary particle emission from a ferromagnetic binary compound

Secondary ion emission from an ion-bombarded binary compound in the ferro- and paramagnetic states has been studied using experimental methods and molecular dynamics simulations. The experiments were performed with a widely used NiPd binary compound, which was bombarded by obliquely incident 10-keV Ar ions. It is established that the intensity of Ni{sup +} and Pd{sup +} ion emission from a polycrystalline NiPd sample decreases significantly when it passes from the ferromagnetic to paramagnetic state. This effect is explained by a change in the surface binding energy and density of surface states at the Fermi level and by a cumulative process related to sputtering. The energies and directions of emission of secondary particles from poly- and single-crystalline NiPd samples in the ferromagnetic state have been jointly studied. It is established that the maximum of the polar angular distribution of secondary particles deviates toward the normal to the irradiated surface with a decrease in their energy. A difference in the azimuthal distribution of emitted Ni and Pd particles has been observed during the bombardment of a (001)NiPd crystal face and explained by specific features of correlated collisions. The best agreement of experimental data with the results of molecular dynamics simulations is obtained if the calculations are performed with allowance for an experimentally established modified composition of the three uppermost surface layers [39].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Sensitivity of fatigue reliability in wind turbines: effects of design turbulence and the Wöhler exponent

Fatigue assessment of wind turbines involves three main sources of uncertainty: material resistance, load, and the damage accumulation model. Many studies focus on increasing the accuracy of fatigue load assessment to improve the fatigue reliability. Probabilistic modeling of the wind's turbulence standard deviation is an example of an approach used for this purpose. Editions 3 and 4 of the IEC standard for the design of wind energy generation systems (IEC 61400-1) suggest different probability distributions as alternatives for the representative turbulence in the normal turbulence model (NTM) of edition 1. There are debates on whether the suggested distributions provide conservative reliability levels, as the established design safety factors are calibrated based on the representative turbulence approach. The current study addresses the debate by comparing annual reliability based on different scenarios of NTM using a probabilistic approach. More importantly, it elaborates on the relative importance of load assessment accuracy in defining the fatigue reliability. Using the DTU 10 MW reference wind turbine and the first-order reliability method (FORM), we study the changes in the annual reliability level and its sensitivity to the three main random inputs. We perform the study considering the blade root flapwise and the tower base fore–aft moments, assuming different fatigue exponents in each load channel. The results show that integration over distributions of turbulence in each mean wind speed results in less conservative annual reliability levels than representative turbulence. The difference in the reliability levels varies according to turbulence distribution and the fatigue exponent. In the case of the tower base, the difference in the annual reliability index after 20 years can be up to 50 %. However, the model and material uncertainty have much higher effects on the reliability levels compared to load uncertainty. Knowledge about such differences in the reliability levels due to the choice of turbulence distribution is especially important, as it impacts the extent of lifetime extension through reliability reassessments.

17 WIND ENERGY↗