Time-dependent Flow Rate Research Articles

Evolution of large-scale flow structures in an axial-flow pump during performance breakdown

This paper presents a very large eddy simulation analysis of the unsteady flow in the pre-stall to stall transition process of an axial-flow pump, with the aim to elucidate the spatiotemporal evolution of large-scale flow structures during the performance breakdown of the pump. The transient flow is investigated utilizing a time-dependent flow rate computation scheme. The results demonstrate that, as the flow rate is dynamically reduced, the reduction in pump head is found lags behind the reduction in flow rate by approximately 15 impeller revolutions. The leading edge separation on the blade suction side (SS) evolves into a leading edge separation vortex (LSV) in conjunction with the dynamic reduction in flow rate. The attached flow on the SS in the vicinity of the hub and blade trailing edge squeezes the mainstream outwards, resulting in the formation of a cross passage vortex (CPV) on the tip side of the passage. The combined effect of the LSV, CPV, and tip-clearance flow induces a penetrating upstream flow in the tip region of the impeller, which gives rise to a swirling backflow within the inlet pipe. At stall, the CPV is stably attached to the SS and extends upstream of the leading edge of the neighboring blade. Furthermore, a trailing edge backflow is observed near the junction of the blade trailing edge and the hub, and it collides with the inflow near the hub, resulting in the formation of a hub-attached vortex.

Effect of stress conditions on concentrated leak erosion resistant of fine-grained soils with different characteristics

AbstractInternal erosion is one of the most important factors that cause earth structures that retain water, such as embankment dams, to collapse. Concentrated leak erosion, one of the forms of internal erosion, occurs in cracked fine-grained soils and pressurized flow conditions. To evaluate the concentrated leak erosion risk of cracks/voids, it is necessary to ascertain the erosion resistance of these materials. The erosion rate and critical shear stresses determine internal erosion resistance in concentrated leak erosion. This study determined soil’s concentrated leak erosion resistance using test equipment that allowed the flow to pass through a hole with stress-free (no loading), anisotropic-compression stress, anisotropic-expansion stress, and isotropic stress conditions. The stresses that developed in the samples’ hole wall where erosion occurred were determined with numerical modeling as pre-experimental stress conditions. The experiments were performed under a single hydraulic head on four selected cohesive soils with different erosion sensitivity. Time-dependent flow rates obtained from the test system can be used to determine hydraulic parameters, such as energy grade lines, with the help of basic theorems of pipe hydraulics in theoretical hydraulic models. Moreover, the erosion rates were quantitatively determined using the continuity equation, while critical shear stresses were qualitatively compared for concentrated leak erosion developed by the dispersion mechanism. As a result of the experiments, stress conditions influence the concentrated leak erosion resistance in the soil samples with dispersive erosion. Moreover, the shear strength in the Mohr–Coulomb hypothesis can explain the erosion resistance in these soils under stress conditions depending on the sand/clay ratio.

Control of interfacial instabilities through variable injection rate in a radial Hele-Shaw cell: A nonlinear approach for late-time analysis.

In this paper, the nonlinear behavior of immiscible viscous fingering in a circular Hele-Shaw cell under the action of different time-dependent injection flow rate schemes is assessed numerically. Unlike previous studies which addressed the infinite viscosity ratio (inviscid-viscous flow), the problem is tackled by paying special attention to flows with finite viscosity ratio (viscous flow) in which the viscosity of the displacing and the displaced fluids can have any arbitrary value. Systematic numerical simulations based on a complex-variable formulation of Cauchy-Green barycentric coordinates are performed at different mobility ratios and capillary numbers with a focus on the late-time fully nonlinear regime. Additionally, numerical optimization is used to obtain the optimal flow rate schedule through a second-order weakly nonlinear stability analysis in contrast to previous studies in which the optimal flow rate was obtained entirely based on linear stability analysis. It is demonstrated that, irrespective of the values of the mobility ratio and/or the capillary number, for patterns whose constant injection counterpart exhibits linear flow regime, the curvature-driven relaxation time is comparable with the operational time of the time-dependent injection flow rate controlling schemes, and most of the controlling schemes work very well and suppress the fingering phenomenon remarkably with the maximum recovery improvement of 15%. As the nonlinearity of the system increases, the schemes may still perform well, but their effectiveness is more pronounced in patterns with less nonlinearity in their constant injection counterpart than those with higher nonlinearity. As the nonlinearity increases, the curvature-driven relaxation time becomes longer than the operational time of the schemes, leading to a reduction in their effectiveness. Additionally, it is shown that employment of the second-order weakly nonlinear stability analysis to formulate the objective function does not result in any remarkable variation in the obtained optimal flow rate schedule.

P84: The impact of temporal synchronization on platelet activation in LVAD patients: CFD and PIV study for the HeartMate3 pulsatility mode.

Study: LVAD patients continue to experience high rates of stroke. Stasis and high shear stresses contribute to platelet activation in the left ventricle, that can lead to thrombus formation. A pulsatility mode was incorporated into the HeartMate3 to improve in-pump washout. The pump speed modulation occurs every two seconds, independently of the cardiac cycle even in patients with preserved native contractility. This study examines intraventricular washout under different LVAD pulsatility/native cardiac cycle synchronization. Methods: A patient-specific 3D time-resolved computational fluid dynamics (CFD) simulation captures the hemodynamics in the left ventricle implanted with an LVAD. Lagrangian particle tracking models platelet motion from the mitral valve, across the left ventricle and out through the outflow cannula. The residence time and shear stress history for each platelet is tracked to determine its risk of activation. Four different temporal synchronizations between the LVAD pulsatility and the native contractility are studied: contraction, peak contraction, relaxation, and peak relaxation. The flow rate through the cannula is prescribed from flow meter data in the experimental campaign. Particle image velocimetry (PIV) experiments measured the in-plane and through plane velocity fields at two orthogonal planes in the ventricle. This PIV data is compared to the CFD to benchmark the simulations. Results: The different pulsatility mode synchronizations result in significantly different time-dependent flow rates through the cannula. The difference between these flow rates results in different hemodynamics patterns in the ventricle. The velocity fields from the CFD models agree closely with the results from the PIV experiments, and take these in a new direction with the use of Lagrangian platelet tracking. Recirculation zones develop underneath the aortic valve and around the cannula/apex pocket, resulting in areas of stasis for the platelets. By identifying platelets that experience both high shear stress history and long residence time, the thrombogenicity of the different synchronizations is evaluated.

GANSim-surrogate: An integrated framework for stochastic conditional geomodelling

Stochastic conditional geomodelling requires effective integration of geological patterns and various types of data, which is crucial but challenging. To address this, we propose a deep-learning framework (GANSim-surrogate) for conditioning geomodels to static well facies data, facies probability maps, and non-spatial global features, as well as dynamic time-dependent pressure or flow rate data observed at wells. The framework consists of a Convolutional Neural Network (CNN) generator trained from GANSim (a Generative Adversarial Network-based geomodelling simulation approach), a CNN-based surrogate, and options for searching appropriate input latent vectors for the generator. The four search methods investigated are Markov Chain Monte Carlo, Iterative Ensemble Smoother, gradient descent, and gradual deformation. The framework is validated with channelized reservoirs. First, a generator is trained using GANSim to generate geological facies models; in addition, a flow simulation surrogate is trained using a physics-informed approach. Then, given well facies data, facies probability maps, global facies proportions, and dynamic bottomhole pressure data (BHP), the trained generator takes the first three static conditioning data and a latent vector as inputs and produces a random realistic facies model conditioned to the three static data. To condition to the dynamic data, the produced facies model is converted to permeability property and mapped to BHP data by the trained surrogate. Finally, the mismatch between the surrogate-produced and the observed BHP data is minimized to obtain appropriate input latent vectors which are further mapped into appropriate facies models through the generator. These facies models prove to be realistic and consistent with all of the conditioning data, and the framework is computationally efficient.

Application of machine learning in predicting blood flow and red cell distribution in capillary vessel networks.

Capillary blood vessels in the body partake in the exchange of gas and nutrients with tissues. They are interconnected via multiple vascular junctions forming the microvascular network. Distributions of blood flow and red cells (RBCs) in such networks are spatially uneven and vary in time. Since they dictate the pathophysiology of tissues, their knowledge is important. Theoretical models used to obtain flow and RBC distribution in large networks have limitations as they treat each vessel as a one-dimensional segment and do not explicitly consider cell-cell and cell-vessel interactions. High-fidelity computational models that accurately model each individual RBC are computationally too expensive to predict haemodynamics in large vascular networks and over a long time. Here we investigate the applicability of machine learning (ML) techniques to predict blood flow and RBC distributions in physiologically realistic vascular networks. We acquire data from high-fidelity simulations of deformable RBC suspension flowing in the networks. With the flow and haematocrit specified at an inlet of vasculature, the ML models predict the time-averaged flow rate and RBC distributions in the entire network, time-dependent flow rate and haematocrit in each vessel and vascular bifurcation in isolation over a long time, and finally, simultaneous spatially and temporally evolving quantities through the vessel hierarchy in the networks.

Determination of Dispersive Erosion Resistance in Fine-Grained Soils with Newly Developed Test Equipment

Abstract Dispersive soils cause great damage to hydrophilic earth structures and unprotected slope surfaces because of their high erosion sensitivity. In fine-grained soils, erosion resistance decreases with increased dispersibility. The dispersion mechanism of clays is controlled by the clay mineralogy and by the physicochemical repulsive forces in the clay–water system. The erosion resistance of dispersive soils can be determined by surface and internal erosion tests. In this study, the internal erosion resistance of soils was determined by using new test equipment that allowed the flow to pass through a hole when the dispersion mechanism in fine-grained soils was activated. The experiments for this study were performed under a single hydraulic head on two different natural dispersive soils with similar clay mineralogy. In this experimental system, both uniform and fully developed flow conditions were achieved. Time-dependent flow rates obtained from the experimental system can be used to determine hydraulic parameters, such as energy grade line, at very low error rates with the help of basic theorems of pipe hydraulics in theoretical hydraulic models, which were formed using a physical hydraulic model. Moreover, the erosion rates were quantitatively determined by using the continuity equation, and critical shear stresses were qualitatively compared for internal erosion developed by the dispersion mechanism. The sand/clay ratio determined the erosion resistance and behavior of the dispersive soils.

Non-stationary rarefied gas flow in a plane channel with a series of oscillating barriers

Present paper is dedicated to the numerical study of transient free-molecular gas flow through the microchannel with a series of barriers, oscillating perpendicular to the axis of the channel. Phases of barriers motion are arranged in a specific way to obtain a “wave-like” motion of barriers system. Time-dependent flow rate pattern is studied for different values of problem parameters — oscillation frequencies, number of barriers, distance between them, pressure drop in the channel. It is obtained that pattern of gas flow rate significantly depends on the ratio of characteristic “wavespeed” of barriers system motion and the thermal speed of molecules. It is shown that frequency of output pulsatile flow may be significantly higher than the frequency of barriers mechanical oscillations, depending on the number of barriers, which can be used in practice for creation of high-frequency pulsatile flows. Similarly to authors previous paper Kosyanchuk, et al. (2017) it was obtained that motion of barriers system can lead to separation of gas mixtures with different molecular masses. In contrast to “quasi-stationary” (time-periodic) flow regime (at times around 100-1000 oscillation periods), it is shown that in transient regime (at times less than 10-100 oscillation periods) separation factor can be significantly higher. Moreover, separation effect in transient regime is observed even for the values of defining parameters, which do not grant separation in quasi-stationary flow regime.

Modeling Left Ventricle Perfusion in Healthy and Stenotic Conditions.

A theoretical fluid mechanical model is proposed for the investigation of myocardial perfusion in healthy and stenotic conditions. The model hinges on Terzaghi’s consolidation theory and reformulates the related unsteady flow equation for the simulation of the swelling–drainage alternation characterizing the diastolic–systolic phases. When compared with the outcome of experimental in vivo observations in terms of left ventricle transmural perfusion ratio (T.P.R.), the analytical solution provided by the present study for the time-dependent blood pressure and flow rate across the ventricle wall proves to consistently reproduce the basic mechanisms of both healthy and ischemic perfusion. Therefore, it could constitute a useful interpretative support to improve the comprehension of the basic hemodynamic mechanisms leading to the most common cardiac diseases. Additionally, it could represent the mathematical basis for the application of inverse methods aimed at estimating the characteristic parameters of ischemic perfusion (i.e., location and severity of coronary stenoses) via downstream ventricular measurements, possibly inspiring their assessment via non-invasive myocardial imaging techniques.

Computational fluid dynamics simulation of time-resolved blood flow in Budd-Chiari syndrome with inferior vena cava stenosis and its implication for postoperative efficacy assessment

BackgroundThis study aimed to adopt computational fluid dynamics to simulate the blood flow dynamics in inferior vena cava stenosis based on time-dependent patient-specific models of Budd-Chiari syndrome as well as a normal model. It could offer valuable references for a retrospective insight into the underlying mechanisms of Budd-Chiari syndrome pathogenesis as well as more accurate evaluation of postoperative efficacy. MethodsThree-dimensional inferior vena cava models of Budd-Chiari syndrome patient-specific (preoperative and postoperative) and normal morphology model were reconstructed as per magnetic resonance images using Simpleware. Moreover, computational fluid dynamics of time-resolved inferior vena cava blood flow were simulated using actual patient-specific measurements to reflect time-dependent flow rates. FindingsThe assessment of the preoperative model revealed the dramatic variations of hemodynamic parameters of the stenotic inferior vena cava. Moreover, the comparison of the preoperative and postoperative models with the normal model as benchmark showed that postoperative hemodynamic parameters were markedly ameliorated via stenting, with the attenuation of overall velocity and wall shear stress, and the increase of pressure. However, the comparative analysis of the patient-specific simulations revealed that some postoperative hemodynamic profiles still bore some resemblance to the preoperative ones, indicating potential risks of restenosis. InterpretationComputational fluid dynamics simulation of time-resolved blood flow could reveal the tight correlation between the hemodynamic characteristics and the pathological mechanisms of inferior vena cava stenosis. Furthermore, such time-resolved hemodynamic profiles could provide a quantitative approach to diagnosis, operative regimen and postoperative evaluation of Budd-Chiari syndrome with inferior vena cava stenosis.

Groundwater flow characterization of an ophiolitic hard-rock aquifer from cross-borehole multi-level hydraulic experiments

Ophiolitic formations play a critical role in the groundwater resource of numerous countries and areas. Previous studies show that the structural heterogeneities of these rocks, coming from the presence of both different lithological units and multi-scale discontinuities, result in complex hydrogeological features that are not well characterized yet. In particular, there is a need for understanding how these heterogeneities impact the hydrodynamic properties of ophiolitic aquifers and the highly variable chemical composition of the water. To this end, we conduct various kinds of pumping experiments between two boreholes 15 m apart in the ophiolitic formation of the Batin (BA1) site in the wadi Tayin massif of the Sultanate of Oman. Cross-borehole open pumping experiments, as well as multi-level pumping and monitoring hydraulic tests, are performed in conductive zones that were identified from temperature and flowmeter data, but also in low-permeability zones requiring to manage very low pumping flow rates. The collected data are interpreted with a model implementing non-integral flow dimension, leakage and time-dependent pumping flow rates. The considered modeling concepts and the estimated hydrogeological properties show that the multi-directional structural heterogeneities of ophiolitic aquifers are key features that must be considered in future hydrogeological models because they drive the hydraulic responses of these systems.

A time-dependent vapor flow model of asphalt concrete

ABSTRACT It is straightforward to determine vapour flow for a non-absorbing medium using Darcy’s law. However, asphalt concrete (AC) sample absorbs moisture over time if it is exposed to a humid environment which causes swelling of the binder materials. This action reduces pore connectivity. As such, the flow properties of AC exposed to moisture can change with time due to clogging of capillary pores and reduced pore connectivity by swelling. To this end, this study develops a model by modifying Darcy’s law to quantify vapour permeability of an AC sample. The resulting model considers asphalt’s swelling using air voids and sample thickness (represents flow length) in addition to regular parameters of Darcy’s law. As a first step, the effects of air voids and thickness on vapour flow are evaluated. It is shown that vapour flow increases with an increase in air voids and with a decrease in thickness. Vapor flow shows a logarithmic correlation with time. As a second step, a permeability model is developed considering the time-dependent vapour flow rate in Darcy’s law. Next, the model is validated using laboratory tested data. The model is found to have good agreements with the tested data. The developed model is advantageous over simpler Darcy’s model for determining asphalt permeability considering binder swelling, pore connectivity and clogging.

Estimating timescale of unstirred layer formation in osmotically driven flow

The osmotically driven flow across a semipermeable membrane under a constant static pressure difference is revisited with reference to previous reports on reverse osmosis. The osmosis due to the local solute concentration difference across the membrane induces solvent seeping flow, which advects solutes in a layer in contact with the membrane and reduces the osmosis itself as a result of nonlinear feedback. A few mathematical techniques for obtaining approximate solutions of the seeping flow and the layer thickness, such as that for inverse problems used in the field of heat transfer, are presented with an emphasis on nonlinear boundary conditions and the time-dependent solvent flow rate. We determined that the layer on the membrane forms rapidly and spontaneously through osmosis at a timescale of and that the layer thickness increases with no upper limit in an infinite time interval. Based on the obtained solution, we also discuss the thermodynamic output work in an irreversible process, which is extracted from the seeping flow as an osmotic engine.

Buoyancy-driven natural ventilation in one storey connected with an atrium

A macroscopic zone model is used to analyze buoyancy-driven natural ventilation in one storey connected with an atrium. The height ratio between the storey and atrium can be variable as well as the heat ratio between the two heat sources, one in storey and the other in atrium. Both the steady-state and time-dependent flow rate through the storey and atrium are investigated. The transient process from different initial conditions to different steady-state flow patterns are numerically performed by means of Euler method, especially the detailed investigation of time evolution of flow rate between different flow patterns from ‘zero’ initial conditions. Seven classical cases of time development flow rate are given to explain flow pattern can evolve from one to another and the implications of natural ventilation driven by the two opposing buoyancy forces in atrium building are discussed. Furthermore, small-scale experiments were conducted to evaluate switch of flow pattern in transient flow. Stability analysis and bifurcation point of multiple flow patterns are also considered on the basis of fundamental nonlinear dynamics.

On singular solutions of the initial boundary value problem for the Stokes system in a power cusp domain

The initial boundary value problem for the non-steady Stokes system is considered in bounded domains with the boundary having a peak-type singularity (power cusp singularity). The case of the boundary value with a nonzero time-dependent flow rate is studied. The formal asymptotic expansion of the solution near the singular point is constructed. This expansion contains both the outer asymptotic expansion and the boundary-layer-in-time corrector with the ‘fast time’ variable depending on the distance to the cusp point. The solution of the problem is constructed as the sum of the asymptotic expansion and the term with finite energy.

Joule heating and zeta potential effects on peristaltic blood flow through porous micro vessels altered by electrohydrodynamic

In most of the medical therapies, electromagnetic field plays important role to modulate the blood flow and to reduce the pain of human body. With this fact, this paper presents a mathematical model to study the peristaltic blood flow through porous microvessels in the presence of electrohydrodynamics. The effects of Joule heating and different zeta potential are also considered. Darcy law is employed for porous medium. The mathematical analysis is carried out in the form of electroosmosis, flow analysis and heat transfer analysis. Velocity slip conditions are imposed to solve momentum equation and thermal energy equation. Time dependent volumetric flow rate is considered which varies exponentially. Closed form solutions for potential function is obtained under Debye-Hückel approximation and velocity and temperature fields are obtained under low Reynolds number and large wavelength approximations. The influence of Hartmann number, electroosmotic parameter, slip parameters, Zeta potential, and couple stress parameter on flow characteristics, pumping characteristics and trapping phenomenon is computed. The effects of thermal slip parameters, Joule heating parameter, and Brinkman number on heat transfer characteristics are also presented graphically. Finally, the effect of Brinkman number on a graph between Nusselt number and Joule heating parameter is examined.

Investigation of a fast transition from pump mode to generating mode in a model scale reversible pump turbine

Pumped storage power plants are an efficient way to store energy at a large scale. In the last years, the changes between pump and turbine mode have become more and more frequent and the necessity of fast changes has increased. This paper analyses the flow in a model scale pump turbine during a fast transition from pump mode to generating mode by means of CFD. Results will be compared between two different mesh sizes and between simulation and measurement. A linear variation of rotational speed over time is chosen. A time-dependent flow rate through the machine is prescribed at the inlet. Due to the varying conditions, a fully transient analysis is carried out using the open-source code OpenFOAM®. The state of the machine at certain points of time during the transient is compared to the results for steady state simulations with identical boundary conditions. To characterize the phenomena in the guide vane channels, torque on selected guide vanes is evaluated as well as pressure at predefined locations. In the runner, pressure sensors are evaluated near the leading edge on pressure and suction side. In the draft tube, four dynamic pressure sensors in a plane below the runner are analysed. Frequencies and amplitudes are compared to simulation.

Numerical research on unsteady flow rate characteristics of pump as turbine

Pump as turbine (PAT), an effective and simple way to recover energy from high pressure fluid, has been one of hot issues in scientific research and engineering applications. Unlike previous computational fluid dynamics (CFD) work on PAT's velocity and pressure fields, this paper first focused on the internal flow rate characteristics, attempted to reveal the periodic rules of flow rate distribution. The transient CFD calculation was performed on a conventional centrifugal pump as turbine, time dependent flow rate was captured and analyzed in computational fluid domains by Lagrangian and Eulerian viewpoints. The results show that time-averaged velocity varies in an overall decline trend from cross section 1–8 of the volute. Net fluid passing into in certain some sections and out in other sections in the gap region between the rotor and volute. The frequent bi-directional flow in the regions of the gap and the lateral chambers is found. The flow rate is significant out of the rotor at section 8 on prediction of turbine performance when ignoring the bilateral chambers in CFD. Experiments were carried out to verify the accuracy of numerical results for three specific speeds of PAT.

First 3D numerical simulations validated with experimental measurements during a LOVA reproduction inside the new facility STARDUST-Upgrade

The aim of this work is to simulate a Loss of Vacuum Accident (LOVA) in the STARDUST (Small Tank for Aerosol Removal and DUST)-UPGRADE facility. These events are one of the major safety concerns in Tokamaks, since they can cause the mobilization and the dispersion of radioactive dust contained in a fusion reactor. The first step in the study of a LOVA event is the estimation, by means of numerical simulations, of the pressurization transient in the vacuum chamber. The STARDUST-UPGRADE facility, which has a cylindrical shape, is considered as a case study. An air inlet is located in a radial position with respect to the facility, so the numerical domain is symmetric and, therefore, only a half of it has been considered in the simulation. A time-dependent mass flow rate is imposed at the inlet, in a range consistent with experimental estimates. The simulation takes 20s and the attention is focused on the mean pressure value over time and on the Mach number distribution. The results are presented and discussed in the perspective of simulating LOVAs in ITER (International Thermonuclear Experimental Reactor).

Enhancing nano-scale computational fluid dynamics with molecular pre-simulations: Unsteady problems and design optimisation

We demonstrate that a computational fluid dynamics (CFD) model enhanced with molecular-level information can accurately predict unsteady nano-scale flows in non-trivial geometries, while being efficient enough to be used for design optimisation. We first consider a converging–diverging nano-scale channel driven by a time-varying body force. The time-dependent mass flow rate predicted by our enhanced CFD agrees well with a full molecular dynamics (MD) simulation of the same configuration, and is achieved at a fraction of the computational cost. Conventional CFD predictions of the same case are wholly inadequate. We then demonstrate the application of enhanced CFD as a design optimisation tool on a bifurcating two-dimensional channel, with the target of maximising mass flow rate for a fixed total volume and applied pressure. At macro scales the optimised geometry agrees well with Murray’s Law for optimal branching of vascular networks; however, at nanoscales, the optimum result deviates from Murray’s Law, and a corrected equation is presented.