Unsteady Flow Processes Research Articles

Structural evolution and rheology of continuous shear-induced dense granular flow in unsteady state

The structural properties of particulate matter can significantly affect the rheology of the system. We report the structural evolution and flow properties of dense granular flows induced by Couette shear and try to reveal the relationship between them. In unidirectional shear, monodisperse particles undergo a transformation from disorder to order. Throughout this continuous process, both the velocity and shear strain rate of the particles experience alterations. By filling with particles of varying polydispersity, the structural potential of the system can be controlled, thereby influencing the extent of structural transformations. The results indicate that the transition in flow characteristics is suppressed as the initial filling in the system approaches from high to low structural potential. The results based on the local volume fraction and relative positions of particles suggest that it is due to the weakening of the structural thinning effect caused by order. We found that both fixed shear paths and more rotatable local structures caused a significant reduction in the contact force to transfer energy. Inertia number and apparent viscosity vary with volume fraction, indicating a transition in dense granular flow after volume fraction φ≳ 0.62, with the onset of significant structural thinning effects. We have revealed the physical mechanisms influencing fluidity from a local structural perspective and established the relationship between fluidity g and φ in the continuous process of unsteady flow.

Flow physics during durge of an axial-centrifugal compressor

Aerodynamic instability design is one of the crucial elements when designing a compressor. Unstable phenomenon such as surge harms the compressor in both performance and structure integrity. The complex unsteady flow process during instability strongly relates to the compressor design. This paper investigates the transient instability process using URANS on a three-stage-axial and one-stage-centrifugal combined compressor under its off-design conditions. Results show that the compressor suffers two surge patterns. The mild surge happens first, with a higher frequency than the traditional mild surge. Then, with the decrease of valve opening, the deep surge is initiated, and the axial stages work at the peak pressure ratio because the mass flow rate is limited by the choked radial diffuser. Moreover, analysis of aerodynamic loads reveals that IGV and the first rotor have the largest unsteady force among all blade rows. According to the flow field, the rotor root and stator tip suffer the most serious impulse caused by reversed flow during surge conditions.

Cross-Scale Seepage Characteristics of Fractured Horizontal Wells in Tight Gas Reservoirs.

Tight gas reservoirs are rich in microscale pores and fractures. The effect of microscale gas seepage in tight sandstone matrix on gas well productivity cannot be ignored. At present, the effect of microscale gas flow on the well testing model of fractured horizontal wells has not been systematically studied. In this paper, first, the coupling influence mechanism of "multicomponent gas effect-fracture geometry characteristics" is expounded, second, a seepage model coupling the flow in the fracture and the flow in the gas reservoir is established on the fracture wall surface, and third, an unsteady pressure analysis model for fractured horizontal wells in tight gas reservoirs is established. Results show that (a) when the microscale fracture is at a quite small level, the Knudsen diffusion plays a dominant role at a wide range of pressures; (b) the mass transfer rate of multicomponent shale gas through macrofractures increases with increasing CO2 fraction; (c) the unsteady flow process is divided into six stages in turn: microscale diffusion stage, linear flow stage in fractures, linear flow stage between fractures, early radial flow stage in gas reservoirs, linear flow stage in gas reservoirs, and late radial flow stage in gas reservoirs; and (d) with an increase of dimensionless bottom-hole storage coefficient, the second and third stages of flow are gradually covered up.

Chemical reactive process of unsteady bioconvective magneto Williamson nanofluid flow across wedge with nonlinearly thermal radiation: Darcy–Forchheimer model

This article examines the chemically reactive process of time-dependent bioconvective magnetohydrodynamic Williamson nanofluid flowing toward a wedge with radiative heat in the presence of gyrotactic motile microorganisms. The main goal is to boost heat transport. Nanoparticles are the most significant and widely utilized source of heat in both science and industrial mechanisms. Nanotechnology with remarkable thermal properties has a variety of roles in laser diode, energy transport, boilers, power generation, MEMS, medication, cool automobile engines, and electronic cooling mechanisms, among many others. The current prominent problem considering this stream is involved partial differential equations (PDEs). A suitable modification is utilized to change PDEs to ordinary differential equations (ODEs), which are computationally solved by mean of R-K fourth-order approach based on shooting technique. The prominent characteristics of nanoparticles such as liquid velocity, concentricity, thermal, drag friction factor, motile organisms, heat and mass transport, and density of motile microorganisms are examined concerning numerous variables. It is noticeable that the velocity curve declined for the escalated valuation of the Hartmann number and bioconvective Rayleigh number. The motile density decayed by climbing valuation of Lewis number and microorganisms difference factor. A review of the available research in the field verifies the findings stated above.

The role of phase transition by inception and surface reactions for the synthesis of silicon nanoparticles in a hot-wall reactor – Simulation and experiment

A novel Eulerian particle model tailored for the application in computational fluid dynamics (CFD) is presented. The model is based on a bivariate moment method and features a full coupling with the gas-phase by kinetic formulations for inception, and activated and non-activated surface reactions. A new formulation for the particle composition accounts for the volatile content and its emission. The model is applied in the context of silicon nanopowder synthesis from monosilane, where a validation is performed with a well-established hot-wall reactor experiment from the literature. It is characterized by high precursor concentrations at moderate heating temperature of 853 K. Further, a new experiment was conducted for the comparison with the new model approach in a three-dimensional, transient simulation. The semi-industrial hot wall reactor features an unsteady complex flow field with buoyancy driven recirculation. The simulations enable new insights into the unsteady flow, mixing and nanoparticle formation process and contribute to their understanding. For the validation, residence-time dependent data for the precursor concentration, the primary particle size, and the particle number concentration are compared with results of the simulation at different stages. A comparison with the results of stochastic methods serves as further benchmark for the presented model. An excellent agreement with the experimental data is obtained, if essential growth mechanisms are considered. For the semi-industrial hot-wall reactor experiment, the BET surface and particle diameter, as well and the precursor consumption, are compared to the results of the new model and to a description with inception, coagulation, and sintering only. Due to the absence of crucial activated and non-activated surface reactions, the latter model predicts values that deviate more than one order of magnitude from the experimental data, while the new model shows an overall good agreement.

On the Unsteady Rotationally Symmetric Flow Between a Stationary and a Finite Rotating Disk With a Given Change in the Axial Velocity

Abstract We consider the scenario of an unsteady viscous flow between two coaxial finite disks, one stationary and the other rotating with an axial velocity changing impulsively from zero to a constant value. The three-dimensional (3D) incompressible Navier–Stokes equations are analytically solved by postulating the polynomial profiles for the axial and circumferential velocity components and by employing the open-end condition of zero pressure difference and an integral approach. It is shown that the time-dependent squeezing of the fluid between the disks and the edge effects of the finite open-ended disks in the flow domain are determined by the compressing Reynolds number and the rotating Reynolds number for the case of laminar flow at low Reynolds numbers and small aspect ratios. The general explicit formulae are derived for the velocity and pressure distributions as a function of the compressing and rotating Reynolds numbers in this unsteady flow process (and the steady-state solutions are then obtainable as the compressing Reynolds number vanishes). A simple theoretical relationship between the radial and axial pressure gradients is deduced to hinge the radial and circumferential velocity components together. The values of the compression and rotation Reynolds numbers suitable to this theory are also suggested for the problem of rotating disk flows at low Reynolds numbers. The validity of the theoretical predictions for the circumferential, radial, and axial velocity components is partially verified through comparison with previous steady experimental and numerical results. These analytical results have the immediate engineering applications of fluid flows with varying gap widths, including wet brakes, wet clutches, hydrostatic bearings, face seals, and rotating heat exchangers.

Model predictive control of long Transfer-line cooling process based on Back-Propagation neural network

• A model predictive control (MPC) method based on back-propagation (BP) neural network as a surrogate inversion model was designed to improve the automation of the long transfer-line pre-cooling process. • A one-dimensional simulation model based on unsteady flow and heat transfer process was used to generate training datasets for BP neural networks, and reasonable cleaning rules are the key to obtain better training results. • Simulation and test results show that the MPC method can be applied to the automatic control of nonlinear large hysteresis dynamical systems such as the long transfer-line pre-cooling process. As the scale of large cryogenic systems continues to expand, the thermal inertia and nonlinear characteristics of the pre-cooling process of long-distance cryogenic transfer-line become obvious, and the traditional control methods are less effective in controlling such nonlinear large hysteresis time-varying systems. To improve the automation of the pre-cooling process, a Model Predictive Control (MPC) method based on Back-Propagation (BP) neural network as a surrogate inversion model was designed and deployed on a large helium cryogenic system of the Platform of Advanced Photon Source (PAPS). Simulation and test results show that the MPC method can be applied to the automatic control of nonlinear large hysteresis dynamical systems; the BP neural network as a surrogate model can invert the one-dimensional flow heat transfer model better. The actual test results on the PAPS cryogenic system show that the method can realize the automatic pre-cooling of long transfer-line, and the overall cooling effect is stable and efficient, with the maximum absolute temperature difference of no more than 3.2 K and the maximum relative temperature difference of no more than 2.1% from the ideal cooling line.

Analysis of the effect of unsteady reservoir flow and sand excavation on the channel conditions of the Yangtze River (Yibin-Chongqing)

This paper systematically describes the current research methods for analyzing unsteady flows in an open channel. In particular, it focuses on elaborating a discrete method to solve the one-dimensional Saint–Venant equations for unsteady flow in an open channel, compares the advantages and disadvantages of different discrete methods, and proposes a new implicit iterative mathematical model applicable to the calculation of the multiwavelength unsteady flow in the Yangtze River from Yibin to Chongqing. This paper also simulates three typical unsteady flow processes and analyzes the effect of sand excavation on the water level in the upper reaches of the Yangtze River from Yibin to Chongqing considering flood, normal-water, and low-water conditions. The spreading characteristics of the daily adjusted unsteady flow were obtained in the Shuifu-Chongqing river reach. Under the conditions of the daily minimum unsteady discharge flow rate, the designed water level of the reach from Yibin to Chongqing increases by 0.19 m compared to the natural water level. With the increase in concentrated sand excavation in the upper reaches of the Yangtze River, a large number of shingle beaches have disappeared, and the riverbed has been drastically lowered. Affected by the excessive and unordered sand excavation in the river channel, the water level of the upper reaches of the Yangtze River from Chongqing to Yibin has decreased by 0.5–2.0 m.

Моделирование течения слабо сжимаемой жидкости при условии проскальзывания на стенке трубопровода

The process of unsteady flow of a weakly compressible fluid through a pipeline described by a one-dimensional model in the form of a nonlinear system of partial differential equations is considered. It is assumed that the pipeline wall meets the slip condition according to Navier’s law. Within the framework of this model, the inverse problemof determining the flow velocity on the pipeline wall is posed. After discretization of the model in time, the initial problem for each discrete value of the time variable is split into two sequentially solved problems. In the first problem, a spatial variable is discretized and a special representation using the local regularization method is proposedto solve the resulting system of difference equations. As a result, an explicit formula for determining the flow velocity on the pipeline wall is obtained. After determining the distribution of the fluid flow velocity along the lengthof the pipeline, the pressure distribution is determined from the solution of the second problem.

One-dimensional morphodynamic model for a tail reach of the Yellow River estuary

A one-dimensional morphodynamic model was improved to study the unsteady flow and sediment transport processes in a tail reach of the Yellow River estuary (YRE). In this model, the modified equations for flow hydrodynamics take the effects of sediment concentration and bed evolution into consideration. The improved model was validated using measurements in the tail reach of the YRE for the years 2003 and 2015, and the model showing an excellent data fit for discharge, water level, suspended sediment concentration and the cumulative volume of channel evolution. The effect of different cross-sectional spacings was also studied using the model results in the tail reach, with various numerical tests of the 2015 flood event conducted. It was found that an appropriate cross-sectional distance of less than 3.14 km should be adopted when simulating the process of bed deformation. Moreover, the interaction between runoff and tide in 2015 was studied on the basis of the simulated water levels and an empirical relation for the location of tidal limit was established. The results show that the tidal limit in 2015 was 15.84 km downstream of Xihekou.

A Study on the Unsteady Flow Characteristics and Energy Conversion in the Volute of a Pump-as-Turbine Device

To study the unsteady flow and related energy conversion process in the volute of a pump-as-turbine (PAT) device, six different working conditions have been considered. Through numerical calculation, the spatio-temporal varia... | Find, read and cite all the research you need on Tech Science Press

Research on viscoelastic fluid unsteady flow model based on torque loss correction

To address the complexity of unsteady shear flow in the initial start-up stage of rheometers, a model considering torque losses in motors and fixtures was developed based on the true torque balance relationship at the rotor boundary of the rheometer and the constitutive equation of the viscoelastic rheological model. The viscoelastic fluid transient flow model was established, and the real wall stress expression of unsteady shear flow of the viscoelastic fluid in the initial stage was obtained. The reliability of the model was verified by comparing the result from the analytical model with numerical calculation. The impact of the geometric dimensions of the test system and the material viscoelastic characteristics on the transient test was analyzed. The results show that the non-uniform velocity distribution has little effect on the initial strain response in the gap of the actual test system of the rheometer. The oscillation curves of strain and stress obtained by numerical and analytical methods coincide completely, proving the reliability of the model in characterizing the transient flow. For a Jeffery fluid, the elastic modulus G mainly affects the oscillation period and amplitude, the parallel viscosity pots η1 mainly impact the amplitude and oscillation attenuation speed, and the series clay pots η2 mainly influence the slope of the strain curve. The greater the system inertia, the more significant the creep oscillation in the unsteady flow process. The coupling between the test system inertia and the viscoelasticity fluid is the essential cause of the creep oscillation. Combined with the mechanical response characteristics of rheometer, a Jeffery model based on short term correction and torque loss was established. The model can accurately describe the strain time curve of gelled crude oil in the initial unsteady flow stage. The results of this study can help understand the rheological properties of materials in the transient start-up process.

Optimal reservoir operation based on hydrological and hydraulic methods incorporating the multiple water replenishment process

A reservoir usually functions as multi-objectives. Different objective has various requirement in water volume and temporals process due to the fact that some objectives require different constant base flow in different months and some objectives demand ephemeral flood pulse. How to balance reservoir functioning and water allocation of multiple targets has become increasingly significant at present. In the past, the multi-objectives operation was usually designed based on water quantity balance, neglecting the coupling with the hydrodynamic process. Sometimes the coupling is so important for some ecological factors and the neglecting may result in failure to meet the hydraulic requirements of the downstream ecological environment. By coupling the hydrologic and hydrodynamic methods, an integrated reservoir model for multiple-objectives optimal operation was presented including water balance model, one-dimensional unsteady flow model, two-dimensional depth-averaged model and ecological baseflow model. The different process demands include water allocation at the dam site, unsteady flow process at different sections downstream the dam site. Taking a reservoir in northern China as an example, the minimum change of the hydrological process in the river mouth section was set as the objective function for the optimal operation. Regarding the demand for aquatic organisms, the low and high baseflow schemes were suggested by the comprehensive use of different ecological flow methods (the Tennant method, the flow duration curve method and the wetted perimeter method). According to the ecological flooding during the wetland water supplement, the Wetland Water Replenishment Model was applied to calculate the replenishment process under different flood cases. The Lake Water Replenishment Model was used for flow scheduling of the downstream lake. Regarding the wet years, the objective functions at the river mouth section were 16.0% with the low baseflow scheme and 15.5% with the high baseflow scheme, whereas for the dry years, it is 18.2% with the low baseflow scheme and 15.5% with the high baseflow scheme. Comparison showed that the high baseflow scheme was optimal for the reservoir operation. The result could provide a reference for decision-makers in reservoir scheduling design. It shows that the coupling of hydraulic and hydrological methods could provide a more reliable basis for reservoir optimal operation.

Study on Unsteady Flow Based on Optimized Water Distribution Model in Irrigation District

The canal delivery system is the main infrastructure of agricultural irrigation. The efficiency of water use in agriculture can be achieved by mastering the dynamic process of unsteady flow in the channel. In this study, an unsteady flow model for the calculation of the water flow transition information of the river channel during the water distribution process was established, based on the water distribution scheme given by the backtracking-search algorithm (BSA). This model was more suitable for areas with inefficient channel systems. The research areas included the main irrigation channels in Xiying City, which is one of the typical agricultural areas in northwestern China. The scheme obtained by optimal solution proposed for Xiying Irrigation District was feasible. According to the results of the flow simulations, the sluice gate calculation correlation could determine the change process of the gate opening of each channel, which provided a basis to realize the modernization of the irrigation area.

INVESTIGATING THE STABILITY OF LONG ONE- DIMENSIONAL WAVES IN A SLOPING RUNOFF


 
 
 The paper is dedicated to the problem of the influence of waves in shallow-water slope flows on the intensity of soil erosion that has not been considered earlier. The stability of one-dimensional continuous waves on the free surface of the sloping runoff is analyzed, both at constant, and with the variable flow along the way. Attention is drawn towards the unsteady flow process and the shape of the free surface in various planes.
 
 

Numerical method of pipeline hydraulics identification at turbulent flow of viscous liquids

A mathematical model for the process of incompressible viscous liquid unsteady turbulent flow in a pipeline is proposed, based on the semi-empirical theory of Prandtl turbulence. As part of this model, the problem of identifying the pipeline hydraulics is formulated. It is assumed that slippage of the flow on the pipeline wall meets the condition of Navier law. This problem belongs to the class of inverse problems related to the restoration of the parabolic equations right parts’ dependence on time. Next, a difference analogue of the formulated problem is constructed, and a special representation associated with the solution of two linear difference problems of the second order is proposed to solve the obtained difference problem. The result is an explicit formula to determine the approximate value of the differential pressure along the length of the pipeline for each discrete value of the time variable. On the basis of the proposed computational algorithm, numerical experiments for model problems were carried out.

Real-time simulation of hydrodynamic and scalar transport in large river-lake systems

Real-time coupling of a hydro-environmental model and an optimization model is difficult because hydrodynamic model (HDM) and scalar transport model (STM) are both time-consuming, especially when the computational domain is large (e.g., river-lake system). This study presents a new 1D hydro-environmental model for free-surface flows and scalar transport in large river-lake systems. Using a prediction–correction method, hydrodynamic is simulated by solving local linear systems based on domain decomposition. A flux-form Eulerian-Lagrangian method (ELM) is constructed to solve advective transport of scalars in 1D grid systems, and a nested technique is proposed to reduce its startup cost. The resulting STM and HDM both allow large time steps for which the Courant–Friedrichs–Lewy number (CFL) is greater than 1, and they are parallelized using the open multiprocessing technique (OpenMP). Moreover, the STM is good at solving multiscalar transport and has low startup cost. The new model is tested using the Jing-Dongting (JDT) system which is covered by a grid of 2382 cells (with 113,600 sub-grids). Stable and accurate simulations are achieved at large time steps for which the CFL can be larger than 5. A sequential run of the new model runs tens of times faster than that of a conventional 1D model such as the Mike11. Moreover, the efficiency of the new model can be further improved by the OpenMP parallel technique. In the test of scale property of the HDM + STM model (using 32 kinds of scalars and 16 cores), a parallel run is 11.8 times faster than a sequential run, and it only takes the new model 33.3 s to complete a simulation of a 1-year process of unsteady flow and scalar transport in the JDT system.

Unsteady Flow Process in Mixed Waterjet Propulsion Pumps with Nozzle Based on Computational Fluid Dynamics

The unsteady flow process of waterjet pumps is related to the comprehensive performance and phenomenon of rotating stall and cavitation. To analyze the unsteady flow process on the unsteady condition, a computational domain containing nozzle, impeller, outlet guide vane (OGV), and shaft is established. The surface vortex of the blade is unstable at the valley point of the hydraulic unstable zone. The vortex core and morphological characteristics of the vortex will change in a small range with time. The flow of the best efficiency point and the start point of the hydraulic unstable zone on each turbo surface is relatively stable. At the valley point of the hydraulic unstable zone, the flow and pressure fields are unstable, which causes the flow on each turbo surface to change with time. The hydraulic performance parameters are measured by establishing the double cycle test loop of a waterjet propulsion device compared with numerical simulated data. The verification results show that the numerical simulation method is credible. In this paper, the outcome is helpful to comprehend the unsteady flow mechanism in the pump of waterjet propulsion devices, and improve and benefit their design and comprehensive performance.

A Prediction–Correction Solver for Real-Time Simulation of Free-Surface Flows in River Networks

A prediction–correction solver is presented here for rapid simulation of free-surface flows in dendritic and looped river networks. Rather than solving a large global algebraic system over the entire domain of river networks, the model solves subsystems for subdomains of branches in two steps: prediction and correction. With the help of the prediction–correction method (PCM), the partial linearization technique, the semi-implicit method, and the Eulerian–Lagrangian method (ELM), the model only needs to solve tridiagonal linear systems for branches and is free of any iteration. The new model was tested using a hypothetical looped river network with regular cross sections, the Three Gorges Reservoir (TGR) dendritic river network, and the Jing-south looped river system (with seasonally flooding branches). In the first test, a time-step sensitivity study was conducted and the model was revealed to produce accurate simulations at large time steps when the condition for application of the PCM to river networks was satisfied. In the TGR test, the PCM model provided almost the same histories of water levels and discharges as those simulated by the HEC-RAS model. In the Jing-south test, the mean absolute error in simulated water levels was 0.07–0.24 m, and the relative error in simulated cross-section water flux was 0.5–4.9% compared with field data; the conservation error was generally 2 × 10−4 to 3 × 10−4. The PCM model was revealed to be 2–4 times as fast as a reported model, which solves local nonlinear subsystems using two-layer iterations, and 1.2–1.4 times as fast as the HEC-RAS. Using a time step of 1200 s, it took the sequential code 26.8 and 23.1 s to complete a simulation of a one-year unsteady flow process, respectively, in the TGR river networks (with 588 cells) and the Jing-south river system (with 662 cells).

Tracer-free laser-induced grating spectroscopy using a pulse burst laser at 100 kHz.

This work shows the first application of a burst laser for laser-induced grating spectroscopy (LIGS) diagnostics. High repetition rate (100 kHz) LIGS is performed in non reacting and reacting flows using the fundamental harmonic of a Nd:YAG pulse-burst laser as pump. In the first part of the paper, we demonstrate the first time-resolved, high repetition rate electrostrictive LIGS measurements in a sinusoidally-modulated helium jet, allowed by the highly energetic pulses delivered by the burst laser (around 130 mJ per pulse). In the second part of the paper, we perform thermal LIGS measurements in a premixed laminar methane/air flame. Thermal gratings are generated in the flame products from the water vapour, which weakly absorbs 1064 nm light. Thus, this work demonstrates the potential of seeding-free high repetition rate LIGS as a technique to detect and time-resolve the instantaneous speed of sound, temperature, and composition in unsteady flow processes.