Non-uniform Conditions Research Articles

Determination of Skin Friction Factor in Gravel Bed Rivers: Considering the Effect of Large-Scale Topographic Forms in Non-Uniform Flows

Determination of skin friction factor has been a controversial topic, particularly in gravel-bed rivers where total flow resistance is influenced by the existence of small-scale skin roughness and large-scale topographic forms. The accuracy of existing models predicting skin friction factors in conditions where small-scale skin roughness and large-scale topographic forms exist is very low. The objective of this study is to develop a modified model that improves the accuracy of the determination of skin friction factors in gravel-bed rivers. To this end, 100 velocity profile data obtained from eight gravel-bed rivers were utilized to develop an analytical method that considers the momentum thickness of the boundary layer and its deviation in large-scale topographic bedforms in a 1D force-balance model. The results show that the accuracy of the skin friction factors is enhanced when (1) the model is in the form of an exponential function of energy slope, and (2) the deviation of momentum thickness is considered in the model. The proposed model results in high accuracy of the predicted skin friction factors for energy slopes between 0.001 and 0.1, which exist in most gravel-bed rivers with different morphologies. Additionally, this study model was used to modify the classic Einstein–Strickler equation. The modified equation resulted in improved accuracy of the predicted skin friction factors in non-uniform flow conditions even when velocity profiles and energy slope were not available.

Towards green energy for sustainable development: Machine learning based MPPT approach for thermoelectric generator

The rapidly increasing demand for electrical energy and the depletion of fossil fuels ignited research in renewable energy generation and management. Thermoelectric generation (TEG) systems as a green energy resource are designed to recover waste heat recovery. Concentrated solar tackles the disadvantages of low power generation efficiency. This paper investigates the feasibility of using machine learning (ML) based MPPT technique, to harvest maximum power of a centralized TEG system under various operating conditions. The environment-dependent issues such as the non-constant distribution of heat, loss to the heat transfer coating between sinks and sources, and mechanical faults create non-uniform current generation and impedance mismatch causing power loss. In this article, we have proposed a feed-forward neural network (FNN) trained by a novel flow direction algorithm (FDA) with a tuned PID controller to harvest the energy under non-uniform temperature gradient conditions. The non-uniformity of physical conditions generates multiple local maxima's (LMs) in electrical characteristics of TEG systems. Common gradient-based MPPT techniques are unlikely to track true GMPP. As a solution to this non-linear problem, novel FNN-FDA is tested under non uniformly distributed temperature (NTUD) over heat source surfaces under varying load and temperature conditions. In this study, certain contributions to the field of TEG systems were made by tackling the issues such as GMPP tracking, low efficiency, and oscillations around GMPP. The results are compared to the particle swarm optimization (PSO), Barnacle mating optimization (BMO), and grey wolf optimization (GWO) algorithm. Five experiments under different weather conditions are performed. Results are validated and proved with experiments and MATLAB/SIMULINK. The experimental results demonstrated that FNN-FDA performs significantly better when compared with other control techniques.

Estimating of aqueduct water withdrawal via a wavelet-hybrid soft-computing approach under uniform and non-uniform climatic conditions

Estimating of aqueduct water withdrawal via a wavelet-hybrid soft-computing approach under uniform and non-uniform climatic conditions

Influence of Xe-gas concentration on the amplification of macroscopic instabilities in Ne/Xe/HCl pulsed discharge

The effect of varying xenon content in Ne/Xe/HCl mixtures on the discharge characteristics, with a non-uniform preionization conditions, is clarified in the frame of this work. With these mixtures, the excimer lasers and lamps are generated. The present study is performed using a transverse one-dimensional description of the plasma discharge. In this description, a set of resistances in a parallel connection represents the plasma, and in which the value of each one depending on the electron density. The computational results show clearly, that the xenon content can also contribute to the worsening of the macroscopic non-uniformity occurring in such mixtures. At the plasma center, the densities of current and deposited power pass from values 0.4 kA/cm2, 9.7 MW/cm3 for [Xe]=0.5% to values 0.75 kA/cm2, 24.6 MW/cm3 for [Xe]=5%, respectively.

A novel two-stage MPPT method based on inflection voltages

ABSTRACT The electrical characteristic of a photovoltaic system is directly dependent on atmospheric conditions such as temperature, solar insolation, and even shading conditions. Hence, utilizing a maximum power point tracking hardware with a fast and accurate algorithm is crucial to obtaining the highest possible photovoltaic power. Several algorithms have been proposed in the literature to obtain maximum photovoltaic power, such as particle swarm optimization and perturb & observe variants. However, these algorithms provide less performance than optimal under partial shading. This study proposes a novel two-stage maximum power point tracking algorithm that equipped inflection voltages method and conventional perturb & observe algorithms. The proposed algorithm displays superior properties of both the inflection voltages method and the perturb & observe algorithms, such as predicting the location of maximum power points and high-speed climbing to these power points. A set of computer simulations are conducted in the PSIM environment to compare the performance of the proposed algorithm with particle swarm optimization, perturb & observe, and inflection voltages algorithms. Furthermore, a DC/DC converter-based experimental setup was produced and connected to a BK precision solar array simulator for experimental verification. The analysis results have shown that the proposed algorithm outperforms perturb & observe, inflection voltages, and particle swarm optimization methods in convergence speed and rate. Experimental and simulation results also show that the proposed algorithm successfully tracks the maximum peak in uniform and non-uniform irradiance conditions when others fail occasionally. On top of that, over 99% tracking efficiency is achieved, and the convergence time is also under 1 second.

A new scheme of polar Fast Fourier Transform Code for iris recognition through symbolic modelling approach

The rich random structure of the iris, non-invasiveness and temporal stability properties makes it one of the most powerful biometric trait for person identification. Iris recognition techniques have demonstrated very low false acceptance rates and very high matching efficiency in large iris datasets. However, iris recognition systems face several challenges including non-uniform lighting conditions, occlusion of the eye by the eyelid, variation of head pose (causes dimensional inconsistencies of iris part), the reflection of surrounding objects and many others. The paper has addressed some of the reported issues of iris recognition technique by exploring new feature extraction technique and devising a new symbolic modelling approach for feature representation and classification, along with other classification techniques namely k-NN, SVM and PNN. The symbolic approach employed in the proposed research to represent iris feature reduces the dimensionality of feature space and also reduces the actual time required for iris recognition. The symbolic approach with new feature extraction technique has attained recognition accuracy of 100%, 98.26%, 99.25% and 96.67% on CASIA 1.1, CASIA 4.0 Interval, SGGSIE&T and the newly formed VISA Iris datasets respectively. Additionally, authors deployed previous algorithms and their findings of the results are discussed in comparison with proposed techniques.

Mathematical modeling of unsteady flow with uniform/non-uniform temperature and magnetic intensity in a half-moon shaped domain

The mathematical modeling of two-dimensional unsteady free convective flow and thermal transport inside a half-moon shaped domain charged in the presence of uniform/non-uniform temperature and magnetic effects with Brownian motion of the nanoparticles has been conducted. Thirty-two types of nanofluids in a combination of various nanoparticles and base fluids having different sizes, shapes, and solid concentrations of nanoparticles are chosen to examine the better performance of heat transfer. The circular boundary is cooled while the diameter boundary is heated with uniform/non-uniform temperature. An external uniform/non-uniform/periodic magnetic field is imposed along diameter. The powerful partial differential equations solver, finite element method of Galerkin type, has been engaged in numerical simulation. The numerical solution's heat transfer mechanism reaches a steady state from the unsteady situation within a very short dimensionless time of about 0.65. The thermal transport rate enhances for increasing buoyancy force whereas decreases with higher magnetic intensity. The uniform thermal condition along the diameter of half-moon gives a higher thermal transport rate compared to non-uniform heating conditions. The non-uniform magnetic field provides greater values of the mean Nusselt number than the uniform field. In addition, the outcomes also predict that a better rate of temperature transport for kerosene-based nanofluid than water-based, ethylene glycol-based, and engine oil-based nanofluid. The heat transfer rate is observed at about 67.86 and 23.78% using Co-Kerosene and Co-water nanofluids, respectively, with an additional 1% nanoparticles volume fraction. The blade shape nanoparticles provide a better heat transfer rate than spherical, cylindrical, brick, and platelet shapes. Small size nanoparticles confirm a higher value of average Nusselt number than big size. Mean Nusselt number increases 22.1 and 5.4% using 1% concentrated Cu-water and Cu-engine oil nanofluid, respectively than base fluid.

Mass transfer from a sheared spherical rigid capsule

Solute mass transfer from a spherical fluid-filled rigid capsule subjected to shear flow is studied numerically, while considering unsteady, continuous, and nonuniform boundary conditions on its surface. Here, the capsule acts as a reservoir with its initially encapsulated solute concentration decaying over time. This scenario differs from the classical case study of either constant concentration or constant mass flux at the surface of the particle. The flow and the concentration field are computed using fully three-dimensional lattice Boltzmann simulations, where the fluid-structure two-way coupling is achieved by the immersed boundary method. The effects of the flow and the boundary conditions on mass transfer efficacy are quantified by the Sherwood number (the dimensionless mass transfer coefficient), which is found to increase due to the combined effects of forced convection and the rotation of the capsule. Having continuity of both the concentration and the mass flux on the capsule significantly decreases the Sherwood number as compared to the case with constant and uniform boundary condition. All the obtained results can be applied to heat transfer in the case of cooling an initially hot spherical particle, for which the concentration must be replaced by the temperature and the Sherwood number by the Nusselt number.

Drag Coefficient of Submerged Flexible Vegetation Patches in Gravel Bed Rivers

Vegetation patches and strips either along riverbanks or in channel beds are essential for the protection of erosion and sedimentation processes. In the present study, the drag coefficient Cdv of submerged flexible vegetation patches in gravel bed rivers was investigated. A total of 13 vegetation patches with different densities were studied in disparate reaches of the Padena Marbor and Beheshtabad gravel bed rivers in Iran. Water depths, flow velocities, and particle grain sizes around these vegetation patches were collected. The Saint-Venant equation and various empirical equations for estimating the drag coefficient were applied to study hydrodynamics in the presence of vegetation patches under nonuniform flow conditions. Furthermore, the drag coefficient factor of flexible vegetation was used to represent the flexibility of vegetation patches and drag characteristics, which were explored from the perspective of material mechanics. The results showed that the calculated values of Cdv exhibited nonuniform variations with the increase in the Reynolds number along the streamwise direction due to flow nonuniformity. Two effects caused by flexible vegetation patches were observed, namely, the sheltering effect (for Red>580) and blockage effect (for Red<450). In most of the vegetated patches, the sheltering effect was dominant, which reduced the drag coefficient. Finally, a fitting formula was proposed based on the drag coefficient factor and Cauchy number.

An Improved Partial Shading Detection Strategy Based on Chimp Optimization Algorithm to Find Global Maximum Power Point of Solar Array System

A PV system’s operation highly depends on weather conditions. In case of varying irradiances or load changes, there is a power mismatch between various modules of the PV array. This power mismatch causes instability in the output of the PV system and deteriorates the overall system efficiency. To overcome instability and lower efficiency problems, and to extract maximum power from the PV system, various maximum power point tracking (MPPT) techniques are employed. The success of these techniques depends on the identification of the actual operating conditions of the system. This article proposes a hybrid maximum power point tracking (MPPT) technique that is capable of efficiently differentiating between uniform irradiance, non-uniform irradiance, and load variations on the PV system. Based on the identified operating conditions, the proposed method uses modified perturb and observe (Modified P&O) to cope with uniform irradiance variations and chimp optimization algorithms (ChOA) for non-uniform conditions to track the oscillation free maximum power-point. The proposed method is implemented and verified using a 4 × 3 PV array model in MATLAB Simulink software. Different cases of uniformly changing irradiance and non-uniformly changing irradiance are applied to test the performance of the proposed hybrid technique. The load varying conditions are performed by applying a variable load resistor. The authenticity of the proposed hybrid technique is critically evaluated against the well-known and most widely used optimization techniques of modified perturb and observe (Modified P&O), particle swarm optimization (PSO), flower pollination algorithm (FPA), and grey wolf optimization (GWO). The results demonstrate the superiority of the proposed technique in oscillation-free tracking of global maximum power point (GMPP) in a minimum tracking time of 0.4 s and 0.15 s, and steady-state MPPT efficiency of 96.92% and 99.54% under uniform and non-uniform irradiance conditions, respectively.

Effect of trapezoidal shapes and non-uniform edge loads on buckling behaviour of plates with cutouts

Crucial buckling loads of composite laminates are typically determined by employing analytical techniques based on the presumptions that the plates are susceptible to uniform edge loads, ignoring the fact that the practical assemblies are regularly exposed to different non-uniform edge loads. The current research paper is centered on the buckling behaviour of different patterns of trapezoidal composite laminates with cutouts under the application of non-uniform in-plane edge loads by employing finite element technique. To achieve this, the panel is discretized by using 9-noded heterosis plate element by employing the impact of shear deformation and rotary inertia. For a given load and the shape of panel, the stress concentration within the panel is highly non-uniform in nature. Hence, a dynamic approach has been employed to calculate the buckling loads. The accuracy of the method is verified by relating the obtained results with existent research. The impact of various variables including the size of the cutout, non-uniform edge loads, boundary conditions, ply alignments and panel trapezoidal shape ratio has been considered to examine the buckling behavior of trapezoidal panels. Buckling load factor optimization using CCD and RSM has also been carried out in detail.

Incipient motion of sediment particles in the presence of bed forms under decelerating and accelerating flows

Abstract This paper investigates the incipient motion of sediment particles under non-uniform flow in river and laboratory. In rivers, the non-uniform flow is often observed due to the presence of various bed forms. Threshold condition has been examined by using the Shields diagram based on the uniform flow assumption, however, this approach can be led to fallacious results for non-uniform flows where the effect of pressure gradient is significant due to bed forms. This study investigates the chronological order of incipient motion of the particles, the average threshold velocity (Ucr ), and Shields parameter for non-uniform flows. River data collection with gravel is used for investigating the incipient motion of surface layer of river bed and the laboratory data collection is considered studying the incipient motion of sub-surface layer of river. Both river and laboratory data collections are conducted in the presence of bed forms. Results reveal that the Shields diagram underestimates the particle incipient motion under accelerating and decelerating flows for the both case of laboratory and river. In both weak and general motion in the laboratory, the values of the critical Shields parameter are located below the Shields diagram, showing no particle motion. Our analysis shows that the incipient motion in river is affected by the presence of bed forms, river width changes, and flow non-uniformity conditions. The results show that in the accelerating flow (the bed form exit with a negative slope), the incipient motion is greater than the decelerating flow (the bed form entrance with a positive slope).

Influence of specimen nonuniformity and end restraint conditions on drained triaxial compression test results in sand

Drained triaxial compression tests on sand are common in geotechnical engineering due to their application in determining strength and critical state properties, and calibration of constitutive models. However, questions around reliability of test results due to strain localization and specimen nonuniformity have sparked debate around the adequacy of various specimen preparation techniques and configuration of top cap and bottom pedestal, among other variables. A rigorously calibrated and validated three-dimensional finite element model using an advanced constitutive model, NorSand, was used to address some of these concerns. The results generally confirmed that common specimen preparation techniques, moist tamping, and (air or water) pluviation, if done properly, can reasonably estimate the properties of a perfectly uniform specimen with the same density. Unsurprisingly, it was also confirmed that the post-peak response of dense specimens is by far the most sensitive part of sand behaviour characterized by these tests. It was suggested that for two-to-one height-to-diameter ratio specimens, end platen lubrication does not necessarily improve test results and can cause concentration of localized deformations near the platens. A fixed cap appeared to enforce a better boundary condition than a free-to-rotate one by preventing cap rotation driven by the formation of a single failure plane.

Investigation on the Degradations of Parallel-Connected 4H-SiC MOSFETs Under Repetitive UIS Stresses

Parallel devices in nonuniform conditions can easily lead to reduced overall reliability or even failure due to uneven energy distribution. Especially under long-term avalanche stresses, the degradation of the device may have a great impact on the electrical parameters and performance. To study the degradation tendency and aging mechanism of parallel-connected silicon carbide (SiC) MOSFET, the repetitive unclamped inductive switching (UIS) experiments are carried out in this article. Technology computer-aided design (TCAD) simulations are utilized to simulate the temperature change and current distribution of the chip during an avalanche. The results showed that for planar and trench devices, the electrical parameters drifts are different during repetitive avalanches. The degradation rates are unequal for parallel devices with different case temperatures. Two degradation mechanisms, including hot holes injection and trapping in the gate oxide and metal aging, are found to dominate in the repetitive avalanche process. The findings are helpful for better understanding of degradations for parallel repetitive avalanches in device applications.

A New Loss Generation Body Force Model for Fan/Compressor Blade Rows: Application to Uniform and Non-Uniform Inflow in Rotor 67

Abstract Despite advances in computational power, the cost of time-accurate flows in axial compressor and fan stages with spatially non-uniform inflow is still too high for design-stage use in industry. Body force modeling reduces the computation time to practical levels, mainly by reducing the problem to a steady one. These computations are important to determine efficiency penalties associated with non-uniform inflows. Previous studies of body force methods have, in most cases, relied on computations with the presence of the blades to calibrate loss models. In some recent studies, uncalibrated models have been used, but such models can drop off in accuracy at conditions where separation would occur on the blade surfaces. In this paper, a neural-network-based loss model introduced in a recent paper by the authors is implemented for NASA rotor 67 for both uniform and non-uniform inflow conditions. For uniform inflow, the spanwise trend of entropy variation is generally captured with the new body force model. Although there are discrepancies at some span fractions, the present model generally predicts the compressor’s isentropic efficiency to within 3% compared to bladed Reynolds-averaged Navier–Stokes simulations. For non-uniform inflow, we consider a stagnation pressure profile representative of boundary layer ingestion. The results show that the region of maximum entropy generation is captured by the present model and the prediction of isentropic efficiency penalty due to the non-uniform inflow is only 0.2 points less than that determined from bladed computations.

Adjoint-Based Boundary Condition Sensitivity Analysis

Turbine performance in modern aeroengines is heavily affected by the nonuniform temperature distribution generated in the combustor. The conventional method to study the effect of nonuniform boundary conditions (BCs) is to compare flowfields and performance metrics with the nonuniformity at several typical positions. A much more efficient and intuitive method to analyze the BC sensitivity is proposed in this paper. An inlet guide vane is analyzed as a demonstration. The sensitivity analysis is achieved by pointwise derivatives of objectives with respect to inlet BCs, which are obtained by discrete adjoint method. Furthermore, to study the variation of gradients, a random sampling method that is based on the Gaussian process is proposed to explore the high-dimensional parameter space. When the gradient is invariant within the parameter space, the relationship between objectives and BCs is linear, and the effect of any perturbation of BCs can be directly predicted. In this case, the computational cost of the sensitivity analysis is drastically reduced to one computational fluid dynamics and one adjoint simulation. Interestingly, some gradients are nearly invariant across the parameter space. When the gradient varies, the active subspace method is introduced to reduce the number of dimensions. Then, several models, including the kriging, gradient-enhanced kriging (GEK), and Taylor expansions, are compared to predict objectives. The ordinary kriging model is recommended for its accuracy and stability. The GEK model can also predict the gradient and is a good choice for small BC variations. The method presented here offers a paradigm that extends the gradient-based sensitivity analysis to the global sense, which is suitable for high-dimensional problems such as BCs and spatially varying model parameters.

Non-uniform illumination image enhancement for surface damage detection of wind turbine blades

Vision-based monitoring technology has been exploited for the surface damage detection of wind turbine blades (WTBs). However, the image quality is often significantly influenced by environmental illumination conditions, imposing difficulties for obtaining high detection accuracy for large-scale WTB surfaces. To improve the image quality and guarantee reliable damage detection on WTB surfaces, this study presents an image processing method for enhancing the images captured under non-uniform illumination conditions. First, cartoon and texture maps of the WTB images are constructed by cartoon texture decomposition. Second, an illumination model is established on the cartoon map from the Gaussian scale-space, to remove the non-uniform illumination. Third, the WTB images are enhanced by utilizing a multi-directional Gabor transformation to increase the contrast between the surface damage and image background. Finally, the WTB surface damages are detected using a gradient threshold segmentation method. The experimental results indicate that the damage detection accuracy of the WTB surfaces is significantly improved by using image enhancement. The F-measure and the intersection-over-union values of the damage detection are increased by 28.05% and 41.61%, respectively, relative to those detected from the input images. Therefore, this vision-based detection method for WTB surface damage inspection under non-uniform illumination has potential application values in practice.

Experimental study on fixed-bed combustion and agglomeration of sawdust–polyethylene mixtures

ABSTRACT Fixed-bed conversion of polymer-containing waste is one of the most common ways of their disposal. One of the problems with such conversion is the cross-sectional non-uniformity of the bed, the formation of agglomerates, and burnouts. The agglomeration process essentially depends on the weight fraction of melting components (plastic materials). Usually, the non-uniformity of the bed is associated with the random nature of the packing (fluctuations of local concentrations and percolation properties). In the present work, we investigate the agglomeration process caused by non-uniform conditions of heat transfer. To this end, combustion of model waste (mixtures of sawdust and polyethylene with a content of the latter of 20–80) is experimentally studied in a laboratory setup with a bed height of about 10 cm. Due to the heat supply through the wall, airflow, non-uniformities developed, associated with different gas permeability in different areas of the bed. The transition from intense thermal decomposition to smoldering is achieved in 5–15 min (the higher the polyethylene content, the shorter the transition time). When the polyethylene content is high, the permeable area decreases so that the air velocity becomes high enough to entrain sawdust particles. As the polymer decomposes, tarry substances are released and form dense deposits in the gas cooling and cleaning system.

Balancing Supercapacitor Voltages in Modular Bidirectional DC–DC Converter Circuits

At present, passive balancing methods dominate energy storage applications, however, they suffer from a long balancing duration. In this article, we took advantage of a modular architecture, where several modular power converters replace a central dc–dc converter for fast charging and balancing of a supercapacitor stack. A strategy has been proposed to control how power is shared among the converters during the charging period in order to balance the supercapacitors. However, some converters enter control saturation due to voltage differences between supercapacitors caused by their nonuniform conditions and characteristics. The originality of this article lies in taking into account the saturation by modifying an energy-based strategy to correct the power shares and make balancing the supercapacitors possible. Simulation and experimental case studies were used to demonstrate the strategy's performance and limitations.

From discrete modeling to explicit FE models for port-Hamiltonian systems of conservation laws

Mixed finite element (FE) approaches have proven very useful for the structure-preserving discretization of port-Hamiltonian (PH) distributed parameter systems, but non-uniform boundary conditions (BCs) were treated in an implicit manner up to now. We apply our recent approach from structure mechanics, which relies on the weak imposition of both Neumann and Dirichlet BCs based on a suitable variational principle, to the class of PH systems of two conservation laws. We illustrate (a) starting with the integral conservation laws the transition to an exterior calculus representation suitable for FE approximation according to Farle et al. (2013). Based thereon, we show (b) the variational formulation with weakly imposed BCs of both types. We discuss (c) on a simple example on a quadrilateral mesh the structure and the variables of the resulting FE models compared to the equations derived from a direct discrete approach on dual cell complexes. We (d) provide the corresponding FEniCS code for download.