Quasi-static Flow Research Articles

Enhancing PV Hosting Capacity of Electricity Distribution Networks Using Deep Reinforcement Learning-Based Coordinated Voltage Control

Coordinated voltage control enables the active management of voltage levels throughout electricity distribution networks by leveraging the voltage support capabilities of existing grid-connected PV inverters. The efficient management of power flows and precise voltage regulation through coordinated voltage control schemes facilitate the increased adoption of rooftop PV systems and enhance the hosting capacity of electricity distribution networks. The research work presented in this paper proposes a coordinated voltage control scheme and evaluates the enhanced hosting capacity utilizing a deep reinforcement learning-based approach. A comparative analysis of the proposed algorithm is presented, and the performance is benchmarked against existing local voltage control schemes. The proposed coordinated voltage control scheme in this paper is evaluated using simulations on a real-world low-voltage electricity distribution network. The evaluation involves quasi-static time series power flow simulations for assessing performance. Furthermore, a discussion is presented that reflects on the strengths and limitations of the proposed scheme based on the results observed from the case study.

Probabilistic maximization of time-dependent capacities in a gas network

AbstractThe determination of free technical capacities belongs to the core tasks of a gas network owner. Since gas loads are uncertain by nature, it makes sense to understand this as a probabilistic problem provided that stochastic modeling of available historical data is possible. Future clients, however, do not have a history or they do not behave in a random way, as is the case, for instance, in gas reservoir management. Therefore, capacity maximization becomes an optimization problem with uncertainty-related constraints which are partially of probabilistic and partially of robust (worst case) type. While previous attempts to solve this problem were devoted to models with static (time-independent) gas flow, we aim at considering here transient gas flow subordinate to the isothermal Euler equations. The basic challenge addressed in the manuscript is two-fold: first, a proper way of formulating probabilistic constraints in terms of the differential equations has to be provided. This will be realized on the basis of the so-called spherical-radial decomposition of Gaussian random vectors. Second, a suitable characterization of the worst-case load behaviour of future customers has to be found. It will be shown, that this is possible for quasi-static flow and can be transferred to the transient case. The complexity of the problem forces us to constrain ourselves in this first analysis to simple pipes or to a V-like structure of the network. Numerical solutions are presented and show that the differences between quasi-static and transient solutions are small, at least in these elementary examples.

Shear rate dependency on flowing granular biomass material

The commercialization of bioenergy has been significantly limited by various material handling issues due to the poor flowability of granular biomass materials. Addressing these issues demands a fundamental understanding of flow physics and robust models to predict the multi-regime flow behavior of biomass particles. In this study, we investigated the multi-regime flow behavior of loblolly pine chips, a widely used bioenergy feedstock, through combined experiments and simulations. A quasi-static hypoplastic model and a cross-regime Drucker–Prager (DP)-μ(I) model were utilized and validated against inclined plane flow experiments to understand the quasi-static and dense flow behavior of milled pine chips. The results show that along the inclined plane, loblolly pine chips mainly present in heap flow (varying thickness along the plane), and in plane flow (iso-thickness along the plane) only when the inclination closes to the material’s critical state internal friction angle. The results also show that the multi-regime DP-μ(I) model predicts a completely different velocity profile from the hypoplastic model. DP-μ(I) model can capture the flow behavior of pine chips in both flow regimes well but at the cost of extra parameter determination, and the shear-rate dependent rheology model can successfully capture the flow of elongated high-frictional particles. These findings advance the scientific understanding of the multi-regime flow behavior of granular biomass materials and shed light on formulating novel constitutive models to assist granular biomass handling in the bioenergy industry.

Clinical Implementation of Combined Quasi-Static Ultrasound Elastographic and Regional Blood Flow Assessment Techniques, in Areas of Forearm Myofascial Dysfunction, Before and After Osteopathic Physical Manipulation

A fast-growing area of high-resolution ultrasound imaging is localizing painful regions termed myofascial tender points (MTP). To mitigate pain and reduce musculoskeletal nodularity and stiffness, osteopathic physicians perform loco-regional osteopathic manipulative procedures (OMT). Current MTP-diagnostic and clinical efficacy criteria using sono-elastographic, or regional flow biomarkers are poorly characterized. This research aims to develop a well-grounded approach to identify forearm MTPs and track changes post-physical interventions, using elastography combined with regional power Doppler. A 44-subject clinical study cohort, was randomly divided in OMT, mild exercise and “rest” groups: imaging was performed using an open-platform SonixTouch Q + . During controlled tissue deformation (4.5-5 Hz), ultrasound elastography was performed by computing root-main-square (RMS) strain of beamformed ultrasound echoes (13 MHz, 15fps) for 3 s. Similarly, power Doppler maps were averaged over 5 seconds (5–12 fps), to evaluate regional blood flow. Data acquisition and processing sequence was repeated post-intervention. Changes in tissue stiffness as well as regional blood flow were compared pre- and post-intervention for all three human subject groups. With the experimental technique used in this study, a significant reduction in tissue stiffness, and increase in regional blood flow is demonstrated in the OMT group compared to the exercise and rest group.

Effects of flow regimes on the interaction between granular flow and flexible barrier.

Flexible barriers are widely used to mitigate granular flows. In practice, flow regimes may keep changing along a flow path after the initiation of granular flows. The effects of flow regimes should be considered in the design of flexible barriers to intercept granular flow. In this study, flow regimes are divided into three types: dilute flow; dense flow; and quasistatic flow. The impact mechanisms of dense granular flows and dilute granular flows against flexible barriers are investigated using flume tests and the discrete element method. Influences of the ratio of the average particle size to the mesh size of a flexible barrier and particle segregation on the interaction between the flexible barrier and the granular flow are revealed. Differences of the impact mechanisms between rockfall and granular flow are compared. Results show that the impact force of dense granular flow against a flexible barrier will not increase linearly with the average particle size. The tensile force of the bottom cable is usually the maximum tensile force among all cables of the flexible barrier. Particle segregation will lead to increase in impact force of dense flows and tensile force of the upper cables. Impact force of the dilute granular flow increases with the average particle size. Different from the failure of a flexible barrier under the impact of the dense flow, the middle and upper cables are easier to break. Based on these findings, a useful reference for the future design of flexible barriers was proposed.

Characterization of Flow Parameters in Shale Nano-Porous Media Using Pore Network Model: A Field Example from Shale Oil Reservoir in Songliao Basin, China

The pore-throat radius of the shale oil reservoir is extremely small, and it is difficult to accurately obtain the absolute permeability and oil–water two-phase relative permeability of the actual oil reservoir through conventional core experiments. However, these parameters are very important for reservoir numerical simulation. In this paper, a method for characterizing flow parameters based on a pore network model that considers differential pressure flow and diffusion flow is proposed. Firstly, a digital core was reconstructed using focused ion beam scanning electron microscopy (FIB-SEM) from the Gulong shale reservoir in the Songliao Basin, China, and a pore network model was extracted. Secondly, quasi-static single-phase flow and two-phase flow equations considering diffusion were established in the pore network model. Finally, pore-throat parameters, absolute permeability, and oil–water two-phase permeability curves were calculated, respectively. The results show that the pore-throat distribution of the Gulong shale reservoir is mainly concentrated in the nanometer scale; the mean pore radius is 87 nm, the mean throat radius is 41 nm, and the mean coordination number is 3.97. The calculated permeability considering diffusion is 0.000124 mD, which is approximately twice the permeability calculated without considering diffusion. The irreducible water saturation of the Gulong shale reservoir is approximately 0.4, and the residual oil saturation is approximately 0.35. The method proposed in this paper can provide an important approach for characterizing the flow parameters of similar shale oil reservoirs.

The Transient Evolution of Flow Due to the Excitation Pulse in Oscillating Drop Experiments in Microgravity Electromagnetic Levitation

In oscillating drop experiments, surface oscillations in a molten sample are captured and analyzed to determine the surface tension and viscosity of a melt without the need to contact the liquid sample. In electromagnetic levitation, surface oscillations are initiated using an excitation pulse in the electromagnetic field. The variation in the electromagnetic force field drives rapid acceleration in the melt while also changing the flow pattern. During the quasi-static flow conditions prior to the excitation pulse, the flow displays a “positioner-dominated” flow pattern with 4 recirculation loops in the sample hemisphere. However, the accelerating flow of the excitation pulse transitions into a “heater-dominated flow” pattern in which there are only 2 recirculation loops in the sample hemisphere. Following the excitation pulse, the flow rapidly slows and quickly returns to the conditions present before the excitation pulse. For many combinations of parameters, the transition in the flow pattern results in a very complicated variation in velocity with time; that variation is the topic of this paper.

Experimentally validated axisymmetric simulation for thermo-fluid performance of an active flat-plate solar water heater at low flow rates

ABSTRACT In this work, the thermo-fluid performance of a liquid flat-plate collector (FPC) has been analyzed using experimental and computational methods. A closed-loop active flat-plate solar water heater (AFPSWH) was tested under quasi-static and laminar flow operating conditions, where a maximum instantaneous collector efficiency of 62.82% and an outlet temperature of 45.2°C were attained in the winter with flow rates of 0.012 and 0.004 kg/s, respectively. Raising the flow rates from 0.004 kg/s to 0.008 and 0.012 kg/s reduces the energy loss parameter [FRUL] by 23.77% and 34.61%, whereas the absorbed energy parameter [FR(τα)n] improves by 10.48% and 21.55%, respectively. The suggested axisymmetric numerical model was validated with outcomes of an in-house developed lab scale experimental setup, with less than 1.5% deviations for all studied cases. In addition, the overall thermal efficiency of the solar thermal system (STS), Nusselt number (Nu), and friction factor (f) have been analyzed for a comprehensive understanding of the system’s thermo-fluid performance.

Development of the Johnson-Cook flow stress and damage parameters for the impact response of polycarbonate: Experimental and numerical approach

The Johnson and Cook (JC) flow stress and damage model parameters of a polycarbonate (PC) plate were determined by the mechanical tests and numerical simulations of the tests. The experimental tests included quasi-static and high strain rate tension and compression, quasi-static notched-specimen tension, quasi-static indentation (QSI), low velocity impact (LVI) and projectile impact (PI). Initially, five different quasi-static flow stress-strain equations were extracted from the experimental and numerical tests. The flow stress equation determined from the experimental average true stress-true strain curve well agreed with the effective stress-strain obtained from the quasi-static numerical tension test. The numerical QSI force-displacement curve based on the experimental average true stress-true strain equation was further shown to be very similar to that of the experiment. The LVI and PI test simulations were then continued with the experimental average true stress-true strain equation using five different flow stress-strain rate relations: JC, Huh and Kang (HK), Allen-Rule and Jones (ARJ), Cowper-Symonds (CS) and the nonlinear rate approach (NLA). The rate sensitivity parameters of these relations were extracted from the quasi-static and high strain rate tests. The LVI test simulations using the stress-strain rate relations exhibited force-displacement curves higher than those of the experiments. The detected almost no strain rate sensitivity in the LVI tests was ascribed to low strain rate dependency of the flow stress at these intermediate strain rates and large strains involved. On the other side, all the stress-strain rate relations investigated nearly predicted the experimental damage types: dishing at 100 and 140 m s−1 and petalling at 160 m s−1, except the CS relation which predicted the fracture of the plate at 140 m s−1. The experimental average projectile exit velocity at 160 m s−1 was further well predicted by the used stress-strain rate relations while the experimental average petal thicknesses were under estimated by the models. The absorbed energy at 160 m s−1 PI test was determined 1.6 times that of the QSI test, which proved an increased energy absorption capability of the tested PC at the investigated impact velocities.

Flowstress and Overstress Approaches to Dynamic Viscoplasticity

Viscoplasticity is mostly modelled by the flowstress approach, where the flowstress (Y) is a function of pressure, temperature, plastic strain and strain rate Y (P, T, ). For dynamic viscoelasticity the flowstress approach is used in hydrocodes together with the radial return algorithm, to determine deviatoric stress components in each computational cell and for each time step. The flowstress approach assumes that during plastic loading, the flowstress in stress space follows the current stress point (current Y). Unloading of a computational cell is therefore always elastic. The overstress approach to dynamic viscoplasticity was used in various versions in the 1950s and early 1960s, before the advent of hydrocodes. By the overstress approach a state point may move out of the quasistatic flow surface upon loading, and hence the term overstress. When this happens, the state point tends to fall back (or relax) onto the quasistatic flow surface through plastic flow, and the rate of this relaxation is an increasing function of the amount of overstress. In this paper we first outline in detail how these two approaches to dynamic viscoplasticity work, and then show an example for which the overstress approach has an advantage over the flowstress approach. The example has to do with elastic precursor decay in planar impact, and with the phenomenon of anomalous thermal strengthening, revealed recently in planar impact tests. The overstress approach has an advantage whenever plastic flow during unloading is of importance.

Quasi-static Power Flow and Fault Analysis of Photovoltaic Farm

This report first analysis quasi static power flow for photovoltaic power plant. Then, fault analysis for different symmetrical and asymmetrical fault is done. The system used is IEEE 39 bus New England system that a Photovoltaic farm with 500 MW power is connected to bus 24. Newton-Raphson Power flow method is used with Gauss-Jordan elimination method for matrix inversion in this power flow analysis. Different operation condition including faults are also considered for fault at bus 16. The results are validated with simulation in MATLAB software.

How dirt cones form on glaciers: Field observation, laboratory experiments, and modeling.

Dirt cones are meter-scale structures encountered at the surface of glaciers, which consist of ice cones covered by a thin layer of ashes, sand, or gravel, and which form naturally from an initial patch of debris. In this article, we report field observations of cone formation in the French Alps, laboratory-scale experiments reproducing these structures in a controlled environment, and two-dimensional discrete-element-method-finite-element-method numerical simulations coupling the grain mechanics and thermal effects. We show that cone formation originates from the insulating properties of the granular layer, which reduces ice melting underneath as compared to bare ice melting. This differential ablation deforms the ice surface and induces a quasistatic flow of grains that leads to a conic shape, as the thermal length become small compared to the structure size. The cone grows until it reaches a steady state in which the insulation provided by the dirt layer exactly compensates for the heat flux coming from the increased external surface of the structure. These results allowed us to identify the key physical mechanisms at play and to develop a model able to quantitatively reproduce the various field observations and experimental findings.

Quantitative Understanding of the Onset of Dense Granular Flows.

The question of when and how dense granular materials start to flow under stress, despite many industrial and geophysical applications, remains largely unresolved. We develop and test a simple equation for the onset of quasistatic flows of granular materials which is based on the frictional aging of the granular packing. The result is a nonmonotonic stress-strain relation which-akin to classical friction-is independent of the shear rate. This relation suffices to understand the quasistatic deformations of aging granular media, and its solid-to-liquid transition. Our results also elucidate the (flow) history dependence of the mechanical properties, and the sensitivity to the initial preparation of granular media.

Influence of the thermal effect on quasi-static shear characteristics of calcareous soil: an experimental study

In principle, the mechanical properties of soil particles are irreversibly changed after particles are subjected to heating. Accordingly, this study performed ring-shear tests on calcareous sand samples subjected to high temperatures to qualitatively investigate the influence exerted by the degradation of the calcareous sand, caused by the thermal effect, on the large displacement shear characteristics of the samples. The effects of the shear velocity and normal stress on the quasi-static shear behavior of the calcareous sand samples were analyzed. The influence of the thermal effect on the quasi-static shear flow behavior of the samples is primarily reflected in the change in the particle mineral composition, particle hardness, and sample density. These variations result in changes in the shear strength, residual shear stress, macroscopic friction coefficient, and other shear characteristics of the calcareous sand samples. Both the shear velocity and the high temperature affect the fluctuation amplitude of the residual shear stress. The results have great theoretical and practical significance in terms of explaining the instability mechanism of a slope. Moreover, a feasible and effective technique is proposed to investigate the large-displacement shear behavior of soil subjected to the thermal effect exerted by a long-runout landslide.

Anisotropization of quasistatic magnetohydrodynamic turbulence with an increasing magnetic field: Transition from three to two dimensions

A systematic description of turbulence behavior in response to the impact of external forces is one of the important outstanding problems of fluid dynamics. We study the process of turbulence anisotropization in quasi-static magnetohydrodynamic flows with an increasing magnetic field using quasi-normal scale elimination analytical theory and direct numerical simulations. The transition from isotropic turbulence at zero field to two-dimensional turbulence at strong field is investigated. Intermediate states and critical parameters of this transition are identified.

Formation mechanisms of fracture plugging zone and optimization of plugging particles

As formation mechanisms of plugging zone and criteria for fracture plugging remain unclear, plugging experiments and methods testing granular material mechanical properties are used to study the formation process of the plugging zone in fractured formations, analyze composition and ratios of different sizes of particles in the plugging zone, and reveal the essence and driving energy of the formation and damage of the plugging zone. New criteria for selecting lost circulation materials are proposed. The research results show that the formation of the plugging zone has undergone a process from inertial flow, elastic flow, to quasi-static flow. The plugging zone is composed of fracture mouth plugging particles, bridging particles and filling particles, and the proportion of the three types of particles is an important basis for designing drilling fluid loss control formula. The essence of the construction of the plugging zone is non-equilibrium Jamming phase transition. The response of the plugging zone particle system to pressure is driven by entropy force; the greater the entropy, the more stable the plugging zone. Lost circulation control formula optimized according to the new criteria has better plugging effect than the formula made according to conventional plugging rules and effectively improves the pressure-bearing capacity of the plugging zone. The research results provide a theoretical and technical basis for the lost circulation control of fractured formations.

Using Direct Numerical Simulation of Pore-Level Events to Improve Pore-Network Models for Prediction of Residual Trapping of CO2

Direct numerical simulation and pore-network modeling are common approaches to study the physics of two-phase flow through natural rocks. For assessment of the long-term performance of geological sequestration of CO2, it is important to model the full drainage-imbibition cycle to provide an accurate estimate of the trapped CO2. While direct numerical simulation using pore geometry from micro-CT rock images accurately models two-phase flow physics, it is computationally prohibitive for large rock volumes. On the other hand, pore-network modeling on networks extracted from micro-CT rock images is computationally efficient but utilizes simplified physics in idealized geometric pore elements. This study uses the lattice-Boltzmann method for direct numerical simulation of CO2-brine flow in idealized pore elements to develop a new set of pore-level flow models for the pore-body filling and snap-off events in pore-network modeling of imbibition. Lattice-Boltzmann simulations are conducted on typical idealized pore-network configurations, and the interface evolution and local capillary pressure are evaluated to develop modified equations of local threshold capillary pressure of pore elements as a function of shape factor and other geometrical parameters. The modified equations are then incorporated into a quasi-static pore-network flow solver. The modified model is applied on extracted pore-network of sandstone samples, and saturation of residual trapped CO2 is computed for a drainage-imbibition cycle. The modified model yields different statistics of pore-level events compared with the original model; in particular, the occurrence of snap-off in pore-throats is reduced resulting in a more frontal displacement pattern along the main injection direction. Compared to the original model, the modified model is in closer agreement with the residual trapped CO2 obtained from core flow experiments and direct numerical simulation.

Liquid film distribution around long gas bubbles propagating in rectangular capillaries

We present a systematic analysis of the bubble and liquid film dynamics corresponding to the propagation of long, isolated gas bubbles, within rectangular capillary channels of cross-sectional aspect-ratio ranging from 1 to 8. Direct numerical simulations of the flow are performed using ESI-OpenFOAM v.1812 and its geometric Volume-Of-Fluid solver isoAdvector. The interface curvature, which enters the calculation of the surface tension force in the momentum equation, is calculated with a parabolic reconstruction method. This study covers a range of capillary and Reynolds numbers of, respectively, 0.005≤Ca≤1 and 1≤Re≲1000. The lubrication film surrounding the bubble is always resolved by the computational mesh, and thus the present results are representative of a perfectly wetting fluid. This study shows that rectangular cross-sections promote the formation of an extended liquid film covering the longer wall of the channel. This liquid film exhibits a saddle-like shape and its streamwise evolution varies depending on the channel shape and flow conditions. Although cross-sectional liquid film profiles and corresponding thicknesses are not constant along the bubble, in general the film deposited upon the shorter wall becomes thicker for increasing values of the aspect-ratio, while the thickness of the film deposited upon the longer wall obeys a Ca2/3/(1+Ca2/3) law which, provided that the channel hydraulic radius is the same, is independent of the aspect-ratio at sufficiently small Ca. An empirical correlation is proposed to predict the cross-sectional gas fraction and bubble speed as a function of a modified capillary number, embedding dependencies on both Ca and aspect-ratio, and converging to the asymptotic limit for a quasi-static flow when Ca→0.

Gravity Governs Shear Localization in Confined Dense Granular Flows.

The prediction of flow profiles of slowly sheared granular materials is a major geophysical and industrial challenge. Understanding the role of gravity is particularly important for future planetary exploration in varying gravitational environments. Using the principle of minimization of energy dissipation, and combining experiments and variational analysis, we disentangle the contributions of the gravitational acceleration, confining pressure, and layer thickness on shear strain localization induced by moving fault boundaries at the bottom of a granular layer. The flow profile is independent of the gravity for geometries with a free top surface. However, under a confining pressure or if the sheared layer withstands the weight of the upper layers, increasing gravity promotes the transition from closed shear zones buried in the bulk to open ones that intersect the top surface. We show that the center position and width of the shear zone and the axial angular velocity at the top surface follow universal scaling laws when properly scaled by the gravity, applied pressure, and layer thickness. Our finding that the flow profiles lie on a universal master curve opens the possibility to predict the quasistatic shear flow of granular materials in varying gravitational environments.

Collapse dynamics of dry granular columns: From free-fall to quasistatic flow.

Gravity-driven collapses involving large amounts of dense granular material, such as landslides, avalanches, or rock falls, in a geophysical context, represent significant natural hazards. Understanding their complex dynamics is hence a key concern for risk assessment. In the present work, we report experiments on the collapse of quasi-two-dimensional dry granular columns under the effect of gravity, where both the velocity at which the grains are released and the aspect ratio of the column are varied to investigate the dynamics of the falling grains. At high release velocity, classical power laws for the final deposit are recovered, meaning those are representative of a free-fall-like regime. For sufficiently high aspect ratios, the top of the column undergoes an overall free-fall-like motion. In addition, for all experiments, the falling grains also spread horizontally in a free-fall-like motion, and the characteristic time of spreading is related to the horizontal extension reached by the deposit at all altitudes. At low release velocity, a quasistatic state is observed, with scaling laws for the final geometry identical to those of the viscous regime of granular-fluid flow. The velocity at which the grains are released governs the collapse dynamics. Between these two asymptotic regimes, higher release velocity correlates with smaller impact on the collapse dynamics. The criterion V[over ¯]≥0.4sqrt[gH_{0}], where H_{0} is the initial height of the column, is found for the mean release velocity V[over ¯] not to influence the granular collapse.