Cascade Simulations Research Articles

Experiment and numerical simulation of the performance on scaled compressor cascade and development of a prediction model

In this study, performance variations and predictions of scaled compressor cascades were investigated. A high-subsonic profile from a rotor section of a certain axial compressor was studied. Tests as well as two-dimensional (2D) and three-dimensional (3D) CFD simulations of cascades of different scale ratios were performed to obtain the flow data of 2D and 3D flow conditions. According to the results, a scaling criterion based on aspect ratio of cascade test for the full range of incidence angles was proposed. Test results shows that aspect ratio of 2.16 is sufficient only in the minimum loss condition for test to reflected the performance of the 2D profile. When the aspect ratio was larger than 4.32, the flow at the mid-span of the tests and 3D-CFD results had good two-dimensionality at the range of twice-minimum-loss incidence angles. The relative deviation of 2D and 3D performance was less than 5%. The total pressure loss and deviation angle increased after scaling down. These results were attributed to the change in the Reynolds number corresponding to the scale. Prediction models were proposed for the calculation of the total pressure loss and deviation angle of the original profiles using the results of the scaled profiles based on the boundary layer theory. The models mainly take into account the Reynolds number, and they are applicable for conditions in which the Reynolds number exceeds the critical Reynolds number.

Efficient multi-fidelity computation of blood coagulation under flow.

Clot formation is a crucial process that prevents bleeding, but can lead to severe disorders when imbalanced. This process is regulated by the coagulation cascade, a biochemical network that controls the enzyme thrombin, which converts soluble fibrinogen into the fibrin fibers that constitute clots. Coagulation cascade models are typically complex and involve dozens of partial differential equations (PDEs) representing various chemical species' transport, reaction kinetics, and diffusion. Solving these PDE systems computationally is challenging, due to their large size and multi-scale nature. We propose a multi-fidelity strategy to increase the efficiency of coagulation cascade simulations. Leveraging the slower dynamics of molecular diffusion, we transform the governing PDEs into ordinary differential equations (ODEs) representing the evolution of species concentrations versus blood residence time. We then Taylor-expand the ODE solution around the zero-diffusivity limit to obtain spatiotemporal maps of species concentrations in terms of the statistical moments of residence time, [Formula: see text], and provide the governing PDEs for [Formula: see text]. This strategy replaces a high-fidelity system of N PDEs representing the coagulation cascade of N chemical species by N ODEs and p PDEs governing the residence time statistical moments. The multi-fidelity order (p) allows balancing accuracy and computational cost providing a speedup of over N/p compared to high-fidelity models. Moreover, this cost becomes independent of the number of chemical species in the large computational meshes typical of the arterial and cardiac chamber simulations. Using a coagulation network with N = 9 and an idealized aneurysm geometry with a pulsatile flow as a benchmark, we demonstrate favorable accuracy for low-order models of p = 1 and p = 2. The thrombin concentration in these models departs from the high-fidelity solution by under 20% (p = 1) and 2% (p = 2) after 20 cardiac cycles. These multi-fidelity models could enable new coagulation analyses in complex flow scenarios and extensive reaction networks. Furthermore, it could be generalized to advance our understanding of other reacting systems affected by flow.

Molecular dynamics study of primary radiation damage in TiVTa concentrated solid-solution alloy

The primary radiation damage in pure V and TiVTa concentrated solid-solution alloy (CSA) was studied using a molecular dynamics method. We have performed displacement cascade simulations to explore the generation and evolution behavior of irradiation defects. The results demonstrate that the defect accumulation and agglomeration in TiVTa CSA are significantly suppressed compared to pure V. The peak value of Frenkel pairs during cascade collisions in TiVTa CSA is much higher than that in pure V due to the lower formation energy of point defects. Meanwhile, the longer lifetime of the thermal spike relaxation and slow energy dissipation capability of TiVTa CSA can facilitate the recombination of point defects. The defect agglomeration rate in TiVTa CSA is much lower due to the lower binding energy of interstitial clusters and reduced interstitial diffusivity. Furthermore, the occurrence probability of dislocation loops in TiVTa CSA is lower than that in pure V. The reduction in primary radiation damage may enhance the radiation resistance of TiVTa CSA, and the improved radiation tolerance is primarily attributed to the relaxation stage and long-term defect evolution rather than the ballistic stage. These results can provide fundamental insights into irradiation-induced defects evolution in refractory CSAs.

Atomic Insights into the Structural Properties and Displacement Cascades in Ytterbium Titanate Pyrochlore (Yb2Ti2O7) and High-Entropy Pyrochlores

Pyrochlore oxides (A2B2O7) are potential nuclear waste substrate materials due to their superior radiation resistance properties. We performed molecular dynamics simulations to study the structural properties and displacement cascades in ytterbium titanate pyrochlore (Yb2Ti2O7) and high-entropy alloys (HEPy), e.g., YbYTiZrO7, YbGdTiZrO7, and Yb0.5Y0.5Eu0.5Gd0.5TiZrO7. We computed lattice constants (LC) (ao) and threshold displacement energy (Ed). Furthermore, the calculation for ao and ionic radius (rionic) were performed by substituting a combination of cations at the A and B sites of the original pyrochlore structure. Our simulation results have demonstrated that the lattice constant is proportional to the ionic radius, i.e., ao α rionic. Moreover, the effect of displacement cascades of recoils of energies 1 keV, 2 keV, 5 keV, and 10 keV in different crystallographic directions ([100], [110], [111]) was studied. The number of defects is found to be proportional to the energy of incident primary knock-on atoms (PKA). Additionally, the Ed of pyrochlore exhibits anisotropy. We also observed that HEPy has a larger Ed as compared with Yb2Ti2O7. This establishes that Yb2Ti2O7 has characteristics of lower radiation damage resistance than HEPy. Our displacement cascade simulation result proposes that HEPy alloys have more tendency for trapping defects. This work will provide atomic insights into developing substrate materials for nuclear waste applications.

Model and Data Driven Machine Learning Approach for Analyzing the Vulnerability to Cascading Outages With Random Initial States in Power Systems

In this paper, a hybrid machine learning model is applied to evaluate the relationship between random initial states and the power system’s vulnerability to cascading outages. A cascading outage simulator (CS), which uses off-line AC power flows, is proposed for generating training data. The initial states are randomly selected and the CS model is deployed for each initial state, where power system generation and loads are adjusted dynamically and power flows are redistributed to quantify the vulnerability metric. Furthermore, the proposed hybrid machine learning model deploys a combined Support Vector Machine (SVM) classification and Gradient Boosting Regression (GBR) to improve the learning precision. The classification model is trained by SVM, which divides the data into two categories with and without load shedding. Then, GBR is adopted only for the data with load shedding to determine the relationship between input power outage states and the vulnerability metric. The proposed vulnerability analysis approach is applied to several test systems and the results are analyzed. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —The power system vulnerability can be quantified by cascading outage simulations. However, there are two challenges: i) there are a huge number of possible initial states and we cannot enumerate all these initial states for the cascading outage simulation. Neither can we precisely quantify the bus vulnerability. ii) The cascading outage simulation may be time-consuming for large-scale power systems, which is challenging for the online application. To address the above challenges, we expect to design a machine learning technique to predict the power system vulnerability, which can train the model in an offline way and then use it for the online application. Firstly, since there is not enough operation data from practical power systems, we develop a cascading outage simulator, using off-line AC power flows, for generating synthetic training data. Secondly, we observe that the training precision by directly applying the regression model may be very poor because the output of the machine learning model may take on an uneven distribution concerning input parameters. Thus, we propose a hybrid machine learning model with a combined classification and regression method, where the classification model is employed to remove the data without the load shedding, and the regression model then determines the relationship between input power outage states and the vulnerability metric. The proposed model and method have been tested on several systems including a practical large-scale Polish power system to show the effectiveness.

Capturing cascading effects under urban flooding: A new framework in the lens of heterogeneity

Increasing urban flooding posed serious threat to urban safety and sustainable development. The heterogeneity of hazard-bearing bodies is one of the important reasons for the variability of cascading failure scenarios in floods. Most previous studies have focused on failure propagation risks among specific types of hazard-bearing bodies, making it impossible to conduct a systematic analysis of the damage caused by persistent flooding in cities. In contrast to conventional graph representations, the heterogeneous hazard-bearing bodies network (HHBBN) based on heterogeneous functional graph theory was constructed in this paper and used to capture the relevant associations of urban resources in a flood. The HHBBN takes into account heterogeneity such as sensitivity and service diffusivity, and functionalities related to hazard-bearing bodies are represented as nodes, with service relationships and geographical associations depicted as edges. To further study the performance change of HHBBN in urban areas during rainstorm flooding, the cascading failure of HHBBN under continuous disturbance was also simulated. The proposed network modeling and cascading failure simulation methods were tested in Futian District of Shenzhen City, China. The case results demonstrate that the heterogeneous hazard-bearing bodies are interdependent and networked, which causes flood damage to spread more quickly and widely. Furthermore, the larger degree and more propagating hazard-bearing bodies are the key factors affecting the cascading failure of heterogeneous hazard-bearing bodies.

Trigger based on fuzzy logic for the detection of neutrino-induced extensive air showers in water-Cherenkov detectors

A fuzzy logic trigger is an alternative approach to triggering a discrete cosine transform or a trigger based on artificial neural networks developed for the surface detector of the Pierre Auger Observatory. The aim of the article is to report on the development of estimators that would allow one distinguishing the signal profiles from the Auger photomultipliers of the water-Cherenkov detectors originating from the neutrino-induced atmospheric showers with a high background of the proton showers. Fuzzy Logic Toolbox provides MATLAB functions, apps, and a Simulink block for analyzing, designing, and simulating systems based on fuzzy logic. The next step is to implement trace recognition algorithms in the FPGA structure to generate a real-time trigger with reasonable use of FPGA resources and with sufficient time margin for 120 MHz sampling. The fuzzy logic trigger will support existing triggers in the Auger surface detector but is optimized to identify neutrino-generated showers. The next step in our research is the simulation of electromagnetic and hadronic cascades in the atmosphere, the propagation of Cherenkov light in the water detector, and the digitalization of the signals in the ADC if the preliminary results obtained from the artificially generated profiles are satisfactory.

Flood risk cascade analysis and vulnerability assessment of watershed based on Bayesian network

Under the dual impact of climate change and accelerated urbanization, urban flood disasters are frequent, resulting in severe losses, and there has been widespread concern. Through constructing the topological structure of flood control network and collecting the hydrological information from hydrological stations, this study proposes a Bayesian flood network model of Haihe river basin (Beisanhe and Yongdinghe river basin) to assess the flood risk. Flood Probability (FP) and Flood Network Cascade (flood network cascade) are proposed to characterize the vulnerability and flood cascade of flood control networks respectively. The accuracy of the flood cascade simulation of the Beisanhe river basin exceeds 80%, and the accuracy of the Yongdinghe river basin exceeds 85%. It is proved that the Bayesian network model proposed in this study has good simulation performance. The derived vulnerability assessment indicates that the central and southern regions of the Beisanhe River and Yongdinghe River basin are easily affected by floods at other nodes. The proposed Bayesian network modeling framework can also simulate the flood cascade in flood control infrastructure, so it can be used for scenario planning and real-time flood forecasting to provide information for flood management and operation.

Simulation and stability analysis of a thermocompressor cascade with overdriven free displacers

Simulation and stability analysis of a thermocompressor cascade with overdriven free displacers

Insights into radiation resistance of titanium alloys from displacement cascade simulations

Radiation damage in beam window materials limits the use of high power proton beams in high energy physics research. The alloy Ti-6Al-4V is presently used as a beam window material but a prospective alternative, Ti-15V-3Cr-3Sn-3Al has been proposed. Since both these alloys contain dual phases at room temperature, we compare the radiation damage in the α and β-phases of these two materials via primary knock-on atom (PKA) cascade simulations in the 10–40 keV energy range. At PKA energies 30 and 40 keV, the number of Frenkel pairs in the ballistic stage is higher in the β-phase of Ti-6Al-4V than that in the β-phase of Ti-15V-3Cr-3Sn-3Al almost by a factor of 2. The α-phase, of both these alloys, by far outperforms the β-phases of the two alloys, both in terms of damage during the ballistic stage and in terms of the surviving defects. The average displacement threshold energy (Ed) in the α-phase of both alloys was found to be 66 eV while that in the β-phase of Ti-15-3-3 was 55 eV and in the β-phase of Ti-6-4 was 46. While the number of surviving defects is almost equal in both alloys, the vacancy and interstitial clustering mechanisms differ notably, which can impact the degree of radiation hardening and loss of ductility. Our simulations show larger vacancy and interstitial clusters form in Ti-6Al-4V as compared to that in Ti-15-3-3-3 alloy. These results indicate that Ti-15-3-3-3 alloys may be a promising candidate for next generation beam window material with a higher radiation tolerance than the existing Ti-6-4 alloy.

Final ion yields upon the cascade decay of single K, L, M, and N vacancies in atomic silver

Final ion yields and mean final ion charges upon the cascade decay of single vacancies in the K, L, M, and N shells of atomic silver are calculated using the method of construction and analysis of the decay trees with accounting for the shake-offs caused by the change of atomic core potential due to cascade transitions. The shake-off effect on the final ion yields is found to be comparatively small, the largest increase of the mean final ion charge being 0.20e in the case of initial 2s vacancy. Total semi-relativistic Pauli–Fock energies of cascade-produced ionic configurations were used to calculate cascade transition energies, which made it possible to accurately account for the closing of some non-radiative transitions in multivacancy cascade configurations. For the transitions with overlapping multiplets of initial and final ionic configurations, only energetically allowed transitions between the multiplet components were counted when calculating mean transition energies and transitions branching ratios. Comparison of calculated charge spectra with those reported earlier by other authors demonstrated crucial importance of accurate calculation of transition energies and accounting for multiplet splitting in simulation of cascades in heavy atoms.

MD Simulations of Collision Cascades in α-Ti. Cluster Statistics and Governing Mechanisms of Point Defect Cluster Formation

MD Simulations of Collision Cascades in α-Ti. Cluster Statistics and Governing Mechanisms of Point Defect Cluster Formation

MD Simulations of Collision Cascades in α-Ti. Statistics and Governing Mechanisms of Point Defect Cluster Formation

The outcomes of molecular dynamics simulations of primary damage formation in collision cascades initiated by primary knock-on atoms (PKA) with PKA energy 5 keV ≤Epka≤ 25 keV in α-titanium at 100 K ≤T≤ 900 K temperatures have been analysed. The fraction of vacancies, εv, and self-interstitial atoms (SIA), εi, in point defect clusters created in isolated collision cascades was evaluated. The corresponding averages ⟨εv⟩ and ⟨εi⟩ over cascade series with the same (Epka, T) parameters, the average size ⟨Nvac⟩ and ⟨Nsia⟩ of vacancy and SIA clusters, respectively, and the average vacancy ⟨Yvac ⟩ and SIA ⟨Ysia ⟩ cluster-per-cascade yield were found as well. Possible governing mechanisms have been suggested to explain ⟨εv⟩, ⟨εi⟩ , ⟨Nvac⟩, ⟨Nsia⟩, ⟨Yvac⟩ and ⟨Ysia⟩ dependence on (Epka,T).

MD Simulations of Collision Cascades in α-Ti. The Residual Number of Radiation Defects, Cascade Relaxation Time, and Displacement Cascade Region Morphology

MD Simulations of Collision Cascades in α-Ti. The Residual Number of Radiation Defects, Cascade Relaxation Time, and Displacement Cascade Region Morphology

MD Simulations of Collision Cascades in α-Ti. The Residual Number of Radiation Defects, Cascade Relaxation Time and Displacement Cascade Region Morphology

Molecular dynamics (MD) simulations were applied to study radiation damage formation in collision cascades created by the recoil of primary knock-on atoms (PKA) with PKA energies Epka = 5, 10, 15, 20 and 25 keV in α-Ti at temperatures T = 100, 300, 600 and 900 K. A series of 24 collision cascades with the same set of (Epka, T) parameters has been simulated to generate representative sampling. Sapling size was justified a posteriori. The number of Frenkel pairs Nfp and cascade relaxation time were obtained as functions of (Epka, T). It was shown that the average ⟨Nfp(Epka,T)⟩ fits within 0.3NRT, provided the threshold displacement energy is chosen in 28-40 eV range depending on the irradiation temperature. Under high PKA energy/low irradiation temperature conditions, displacement cascade region splits into subcascades expended along high-energy recoil trajectories. Cascade relaxation time does not depend on Epka in this case. Contrary, under low PKA energy/ high temperature simulation conditions, most of collision cascades possess equiaxial shape, and their relaxation time grows with PKA energy increase.

Analysis of Interaction between Leakage Flow and Upstream Wake by Proper Orthogonal Decomposition Applied

The turbine blade generally works in severe aerodynamic environments such as high temperature and high pressure. Various forms of secondary flow, unsteady potential flow, periodic wake in the blade passage, and strong interactions among these flow structures affect the efficiency of the turbine seriously. This paper performs a numerical simulation of the T106 cascade to investigate the flow characteristics and performance under the interaction between upstream wake and leakage flow. The cascade flow with and without upstream wake are decomposed by proper orthogonal decomposition, and the mode characteristics of leakage flow and wake are obtained. The results indicate that periodic wake can significantly reduce the energy loss in the channel and tip leakage flow has a greater impact in the region near the shroud plane. According to the proper orthogonal decomposition, the second to seventh modes account for 96.14% of the total energy except for the time-averaged mode. The second mode is identified as the wake mode and leakage flow can significantly increase vorticity in the tip region. The harmonics of the wake mode are greatly affected by leakage flow, especially at the trailing edge region.

Simulation of defect build-up in tungsten during low energy irradiation

Tungsten is the material chosen to be used in some of the most demanding parts of fusion test reactors and planned fusion power plants. The irradiation present will degrade the material over time, which lead to the need for component replacement. In order to understand the material degradation and to be able to predict the need for replacement, there is a need to understand how defects are produced in the material. Even though there will be high energy recoils in the material, there will be a substantial amount of recoils in the low energy regime. We study the effect of irradiation on the hundred eV scale and compare it to the commonly used Frenkel-pair insertion method. By conducting this investigation we can investigate if this insertion method is a possible speed up to full cascade simulations. These low energy simulations are also comparable with the energy given to recoils during electron irradiation. We found that the low energy cascades will produce damage as expected and that Frenkel-pair insertion will qualitatively give a similar result. However, we found, quantitatively in the form of clustering, that the results differ between methods, especially for the higher energies investigated.

Study of Circumferential Grooved Casing Treatment on Cascade Aerodynamic Performance

To explore the influence of circumferential grooved casing treatment on subsonic cascade performance, a numerical simulation of subsonic cascade was conducted. In contrast to traditional research on variable single parameters for casing treatment, this paper used the Latin hypercube sampling method to randomly sample multiple geometric parameters of casing treatment and compared many sample data with the total pressure loss of the cascade as a measurement standard. After selecting several typical cases of high and low total pressure loss cases for in-depth flow field analysis, it was found that casing treatment affects the strength and structure of the leakage vortex, thereby reducing the blockage of fluid in the passage of the cascade. Changes in the total pressure loss and in the margin of the cascade meant that casing treatment affected cascade performance. This prompted analysis of the correlation between the casing treatment parameters and total pressure loss of the cascade. The clearance height and groove depth had the greatest influence on the total pressure loss of the cascade.

Primary radiation damage in tungsten-based high-entropy alloy: Interatomic potential and collision cascade simulations

Tungsten (W)-based high-entropy alloys (HEAs) have shown promising properties as nuclear fusion materials. Characterizing the primary damage is a critical step in describing and revealing the irradiation-induced damage and radiation resistance mechanism. However, there are a limited number of studies on collision cascades and primary radiation damage in W-based HEAs, owing to the large amount of calculations involved and a lack of appropriate interatomic interaction potentials. In this work, we developed a semi-empirical interatomic potential for W–Ta–Cr–V. By using the developed potential, molecular dynamics simulations of collision cascades in W-based HEAs were performed to assess the primary damage due to irradiation. Based on experimental samples, we reported defect production in W38Ta36Cr15V11 and compared it to pure W for primary knock-on atom with energies ranging from 1 keV to 100 keV. Our findings showed that the number of FPs at the thermal spike and the number of surviving FPs at the end of the cascade in W38Ta36Cr15V11 are more than those in pure W, mainly due to the lower threshold displacement energy, melting temperature, and formation energy of point defects. Collision cascades in the W38Ta36Cr15V11 are less likely to result in the formation of dislocation loops compared to pure W. After collision cascade, in W38Ta36Cr15V11, the concentrations of Cr and V atoms in defects is significantly higher than their corresponding concentrations in the system, showing an aggregation tendency. The current collision cascade results provide insights into the primary damage of W-based HEA system under irradiation and should provide reliable guidance for describing the primary damage source terms needed in the kinetic models used to simulate radiation-induced microstructural evolution.

Development of the microstructure of 2D graphene sheets in the molecular dynamics simulation of irradiation damage cascades

Two-dimensional graphene is widely used in the nuclear industry, computer simulation of the physical process of graphene irradiation damage can assist the study of microscopic dynamic processes that cannot be directly observed in irradiation experiments. The irradiation-induced microstructural change of 2D graphene sheets was studied using molecular dynamics simulations with a maximum primary knock-on atoms (PKA) energy of 180[Formula: see text]keV. The results show that using direct observation, the defects generated during the cascade collision process can be divided into three types according to the generation mechanism: defects by PKA damage (DPD), instantaneous defects by shock wave rippling (DSR), and permanent defects by shock wave overlapping (DSO). The atomic configuration of the final stage consists of various topological polygons; in general, octagons and polygons with fewer than eight sides are bulged, while polygons with more than eight sides are depressed. The largest topological polygon found by the simulation is a 13-sided polygon. In addition to atoms with a coordination number of 3 (originating from the perfect lattice structure of graphene), atoms with coordination numbers of 2 and 4 are produced. Atoms with a coordination number of 4 are generally enclosed by four polygons that have no more than six sides, while atoms with a coordination number of 2 are generally located between two polygons that have at least six sides.