Direct shear tests on a series of fractured Beishan granite samples were performed under different normal stresses, to investigate the characteristics of asperity degradation during shear and to ...quantify their effects on solute retardation in fractures. A three-dimensional laser scanner and a laser diffraction particle size analyzer were used to examine the shear-induced asperity volume loss and the size distribution of sheared-off fragments. The results identified two main modes of asperity degradation during shear, i.e., instantaneous failure of asperities occurring at the peak shear stress, and crushing of the generated fragments with further shear displacement. Based on the volume of asperity degradation, a new joint damage coefficient was proposed. The size of the sheared-off fragments under different normal stresses was found to follow a Weibull distribution. Combining the proposed joint damage coefficient and the Weibull size distribution of the sheared-off fragments can approximately predict the potential effects of sheared-off fragments on solute retardation coefficient in rock fractures. The results showed that the shear-induced asperity degradation significantly increases the values of the solute retardation coefficient, by offering more sorption surfaces in fracture voids.
The capacitor voltage imbalance is a critical issue of five-level diode-clamped converters (5L-DCC). To address this issue, an inner-hexagon-vector-decomposition-based space-vector modulation ...(VDSVM-H1) approach is provided in the literature, which obtains the capacitor voltage balancing with high modulation index and high power factor, but renders some drawbacks. To overcome these shortcomings, a novel capacitor voltage balancing method is proposed here. First, the previous VDSVM-H1 approach is modified by introducing six new vector sequences to each triangle and applying a new vector selection rule such that the converter will not violate the 5L-DCC switching mechanism in all operating conditions. Second, the variation of the line-to-line voltage output in one sampling period is restricted to one- or two-level in the optimized region, instead of the three-level in the previous VDSVM-H1 approach, which means less harmonics generating in the ac-side outputs. Finally, the simulation and experimental results show that the proposed method can improve the VDSVM-H1 with convincing results.
•A mechanical-hydraulic-solute transport model during shear is developed.•The roles played by dilation and retardation on solute transport are studied.•Transport behaviors through shallow and ...deep-seated fractures are different.
The present study developed a two dimensional mechanical-hydraulic-solute transport model for rough-walled rock fractures under constant normal stiffness boundary conditions. First, the mechanical responses of a fracture during shear including the shear stress, the normal stress, the dilation and the sheared-off area of asperities at each shear step were calculated in a mechanical module. The surface roughness was characterized by two-order asperities, i.e., waviness and unevenness, and the degradation of asperities was estimated based on the principle of wear. The mechanical module was validated by comparison with laboratory experiments. The surface geometry and the dilation behavior were subsequently incorporated into a hydraulic module to estimate the hydraulic aperture and flow rate based on the cubic law. Finally, these data were input into a solute transport module to investigate the influences of the mechanical boundary conditions on the solute transport in the fracture and the matrix at different shear displacements and time. The model linked the complex mechanisms involved in the mechanical, hydraulic and solute transport processes for fractures subjected to shear, and revealed a controlling effect of normal stiffness on the transport behavior. The results show that the normal displacement, normal stress, cumulative sheared-off area and porosity of matrix change quickly in the initial stage of shear, and gradually reach some constant values when the fracture surface is sufficiently smoothed in the residual stage. In the entire shear process, the major asperities are substantially damaged, producing abundant gouge particles that contribute to the retardation of solute transport. The shear-induced dilation and the retardation induced by gouge particles from damaged asperities played competitive roles in solute transport that resulted in the nonlinear variations in the coefficient of retardation. The model established a solid platform for estimating the mechanical-hydraulic-solute transport processes in fractures subjected to shear, upon which more sophisticated modules such as pressure solution and clogging by particles may be developed to improve the understanding on the outstanding issues involved in the coupled processes.
The effective stress coefficient is critical to accurately predict the hydromechanical coupling behavior of single water-bearing rock fractures. The physical meaning of the effective stress ...coefficient in a rough rock fracture is understood as the ratio of the nominal fracture surface occupied by water to the total fracture surface area. To overcome the difficulty of measuring contact ratio changes in rough rock fractures under normal loading and water pressure, a new effective stress coefficient model for single rough water-bearing fractures is proposed in terms of two mechanical parameters, initial normal stiffness and maximum normal closure. By incorporating the new effective stress coefficient model into the Barton-Bandis constitutive model, a hydromechanical coupling model was built and verified by laboratory and in-situ experimental data. The results indicate that the effective stress coefficient is less than 1 for single rough rock fractures and decreases with increasing difference between normal stress and water pressure. When the normal stress is close to or considerably higher than the fracture water pressure, both the newly built model and Terzaghi's model obtain similar normal displacement. However, for moderate normal stress, the normal displacements predicted by the newly built model and Terzaghi's model differ significantly. The implications of the newly built model in subsurface engineering are discussed.
•The effective stress coefficient for single rough water-bearing fractures is presented.•The proposed expression is validated by laboratory and in-situ data.•The effective stress coefficient decreases nonlinearly with increasing difference between normal stress and water pressure.
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•Proposed van der Pol–Duffing oscillator based weak signal detection method.•The signal-to-noise ratio of the weak signal can be detected −45dB.•Compared to Duffing oscillator more ...robust against the different frequency signal.
This study presents a new weak signal detection method based on the van der Pol–Duffing oscillator. The principle of the proposed method is described. A weak signal is detected through the transition from the chaotic to the periodic state. Numerical simulation shows that the van der Pol–Duffing oscillator is sensitive to a weak signal under strong noise conditions. Several aspects of the proposed method, including the noise influence, influence of different frequency signals, and influence of the phase shift, are studied in detail. Results indicate that the application of the van der Pol–Duffing oscillator to weak signal detection is feasible.
Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems. The complicated convective heat transfer behavior in intersected fractures is critical to the heat ...recovery in fractured geothermal reservoirs. A series of three-dimensional intersected fracture models is constructed to perform the flow-through heat transfer simulations. The geometry effects of dead-end fractures (DEFs) on the heat transfer are evaluated in terms of intersected angles, apertures, lengths, and the connectivity. The results indicate that annular streamlines appear in the rough DEF and cause an ellipse distribution of the cold front. Compared to plate DEFs, the fluid flow in the rough DEF enhances the heat transfer. Both the increment of outlet water temperature ΔTout and the ratio of heat production Qr present the largest at the intersected angle of 90° while decline with the decrease of the intersected angle between the main flow fracture (MFF) and the DEFs. The extension of the length of intersected DEFs is beneficial to heat production while enhancing its aperture is not needed. Solely increasing the number of intersected DEFs induces a little increase of heat extraction, and more significant heat production can be obtained through connecting these DEFs with the MFF forming the flow network.
•Coupling hydrothermal behaviors in three-dimensional intersected plate and rough fractures are compared.•Fluid flow in intersected rough dead-end fractures can enhance the heat recovery in the main flow fracture.•Influences of several geometries parameters of intersected dead-end fractures on the heat transfer are evaluated.
The behavior of fluid flow through rock fractures at varying temperatures is a critical issue in many subsurface geo-energy projects. Previous work has considered the thermal effects on fracture ...permeability, but not in isolation of the chemical effects as well. Therefore, to quantitatively assess the mechanical influences on fracture permeability, we present results from permeability tests of five cylindrical Beishan granite samples, each with a single artificial fracture, at different temperatures. Three samples were tested at a constant confining pressure of 5 MPa and temperatures of 22 °C, 90 °C and 150 °C for 22 days, to examine creep-induced changes in fracture permeability. Fracture permeability decreases with time until about 10 days, and the eventual magnitudes of fracture permeability reduction are much greater at 90 °C and 150 °C than at 22 °C. Two samples were subjected to three heating cycles (30 °C to 150 °C) at a constant confining pressure of 5 MPa. Fracture permeability decreases as temperatures increase from 30 °C to 150 °C, and then slight changes in permeability occur as the sample is cooled to 30 °C. A similar tendency appears in subsequent temperature cycles, while the magnitude of fracture permeability reduction decreases with increasing heating-cooling cycles. A coupled thermal-mechanical model considering asperity damage is developed to describe the thermally-induced changes in fracture permeability, which properly predicts the experimental results. The residual deformation of asperities and temperature dependent Young's modulus play an important role in thermally-induced changes in fracture permeability.
This paper proposes a new method that can identify and predict financial fraud among listed companies based on machine learning. We collected 18,060 transactions and 363 indicators of finance, ...including 362 financial variables and a class variable. Then, we eliminated 9 indicators which were not related to financial fraud and processed the missing values. After that, we extracted 13 indicators from 353 indicators which have a big impact on financial fraud based on multiple feature selection models and the frequency of occurrence of features in all algorithms. Then, we established five single classification models and three ensemble models for the prediction of financial fraud records of listed companies, including LR, RF, XGBOOST, SVM, and DT and ensemble models with a voting classifier. Finally, we chose the optimal single model from five machine learning algorithms and the best ensemble model among all hybrid models. In choosing the model parameter, optimal parameters were selected by using the grid search method and comparing several evaluation metrics of models. The results determined the accuracy of the optimal single model to be in a range from 97% to 99%, and that of the ensemble models as higher than 99%. This shows that the optimal ensemble model performs well and can efficiently predict and detect fraudulent activity of companies. Thus, a hybrid model which combines a logistic regression model with an XGBOOST model is the best among all models. In the future, it will not only be able to predict fraudulent behavior in company management but also reduce the burden of doing so.
Increasing the deeper understanding of the thermal damages and failure mechanisms of sandstone undergoing thermal treatments at different temperatures is a key concern for deep-mining and underground ...coal gasification processes. In this research study, a scanning electron microscope (SEM) apparatus, JSM-5410LV, which was equipped with a built-in digital electro-hydraulic servo loading system, was applied to carry out a series of three-point bending tests on Pingdingshan sandstone following heat treatments at elevated and high temperatures ranging from 25 to 600 °C. The subcritical crack initiation load, peak load, and elastic modulus were found to increase with the increases in the thermal treatment temperatures until a maximum was achieved at 125 °C, after which decreases were observed. However, it should be noted that there were sudden drops observed for the specimens after the thermal treatment temperature reached 150 °C due to the thermal diffusivity of the cement. The subcritical crack growth length was theoretically calculated, and a digital speckle correlation method (DSCM) was applied to verify the initial load and subcritical crack growth length. It was found that the fracture toughness fluctuated significantly when the thermal treatment temperature ranged from 25 to 125 °C, and reached a peak of 47.45 MPa mm
0.5
. It was also observed that, as the temperature was raised from 175 to 600 °C, the fracture toughness gradually decreased. The subcritical crack growth mode was determined to be intra-granular cracking following the thermal treatments below 125 °C, while a mixture of intra-granular and trans-granular cracking occurred in the specimens which experienced thermal treatments of 175 °C. The relationship between heat treatment temperature and subcritical crack growth was derived, which could be used to develop geothermal energy extraction from the critical temperature resources.
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