Flow Field Variables Research Articles

Wind-driven hydrodynamic characteristics of Lake Taihu, a large shallow lake in China.

As an essential drinking water source and one of the largest eutrophic shallow lakes in China, the management of Lake Taihu requires an adequate understanding of its hydrodynamic characteristics. Studying the hydrodynamic characteristics of Lake Taihu based on field observations is limited owing to its large area and the lack of flow field stability. Previous studies using hydrodynamic models experienced challenges, such as dimensionality and lack of dynamic response analysis between flow field and realistic wind; therefore, the results were still inconclusive. In this study, a 3D model of Lake Taihu, calibrated and validated based on field observations, was used to simulate and compare three scenarios: windless, steady wind, and realistic wind. The hydrodynamic characteristics of Lake Taihu were analyzed as close to the actual conditions as possible. The results showed that wind-driven currents dominated the flow field in Lake Taihu, and the horizontal velocity driven by wind was more than 6 times that without wind. Observing a stable flow field in Lake Taihu was difficult because of the variability of realistic wind. The hydrodynamic characteristics of Lake Taihu were defined as "strongly affected by wind," "higher on the surface and smaller at the bottom," and "difference between the surface and the bottom." Vertical turbulent kinetic energy can be used to characterize the variable flow field of a wind-driven lake and has a positive correlation with wind speed. Therefore, it could be used as a key component to predict water blooms with practical implications.

The impact of pulsating parameters on particle dynamics in vertical pipe during hydraulic conveying with pulsating inlet flow

This paper presents a numerical investigation of the hydraulic conveying performance of coarse particles with the inlet pulsating fluid velocity in a vertical pipe. The influence of pulsating parameters, in terms of pulsating amplitudes and frequencies, on hydraulic conveying is mainly considered. The pulsations of particle and slip velocity are as periodic and regular as the inlet pulsating velocity. The variation of the normalized force in one pulsating time cycle is also analyzed. Then the laws of particle group distribution with transient time and vertical distance are obtained: the plug phenomenon is more obvious at the larger pulsating amplitude, further leading to pipe blockage, while a larger pulsating frequency is beneficial to the weakening of the plugs. Particle relaxation time is introduced to explain the changes in particle group distribution. Additionally, the temporal variations of flow fields are also used to illustrate the propagation of kinematic waves.

Bathymetry Inversion Using a Deep‐Learning‐Based Surrogate for Shallow Water Equations Solvers

AbstractRiver bathymetry is critical for many aspects of water resources management. We propose and demonstrate a bathymetry inversion method using a deep‐learning‐based surrogate for shallow water equations solvers. The surrogate uses the convolutional autoencoder with a shared‐encoder, separate‐decoder architecture. It encodes the input bathymetry and decodes to separate outputs for flow field variables. Utilizing the differentiability of the surrogate, a gradient‐based optimizer is used to perform bathymetry inversion. Two physically based constraints on the ranges of both bed elevation and slope have to be added as inversion loss regularizations to contract the solution space. Using the “L‐curve” criterion, a heuristic approach is proposed to determine the regularization parameters. Both the surrogate model and the inversion algorithm show good performance. The bathymetry inversion progresses in two distinctive stages, which resembles the sculptural process of initial broad‐brush calving and final fine detailing. The inversion loss due to flow prediction error and the two regularizations play dominant roles in the initial and final stages, respectively. The surrogate architecture (whether with both velocity and water surface elevation or velocity only as outputs) does not have significant impact on inversion result. The methodology proposed in this work, an example of differentiable parameter learning, can be similarly used in the inversion of other important distributed parameters such as roughness coefficient.

Reynolds-average Navier-Stokes turbulence models assessment: A case study of CH4/H2/N2-air reacting jet

Computational Fluid Dynamics has become a very powerful tool for developing engineering combustion devices, such as burners and furnaces. However, there are a wide variety of turbulence models, and some of them have proven to be more effective for some turbulent flow configurations than others. A reacting turbulent jet is a common flow configuration found in combustion engineering devices like burners. The present work assesses Reynolds-Average Navier-Stokes turbulence models, being tested on a CH4/H2/N2-Air reacting jet. Eight two-equation eddy-viscosity and three five-equation turbulence models were tested in the studied turbulent flow. Computational results were compared against experimental measurements in terms of flow field variables, mean mixture fraction, temperature, and species mass fraction. The findings suggest a strong influence of the turbulence model perforce on the mean mixture fraction as well as on the turbulence-chemistry interaction model. The modified version of the standard k-ε model proves to be the more reliable choice for this reactive flow configurations. Specially, where the flow patterns of the jet dictate the general flow physics. Near the fuel nozzle, both the Reynolds stress model with stress baseline k-ω (RSM-SBSL) and the standard k-ω model exhibit better agreement with experimental data than the conventional modified k-ε model. Moreover, findings from the standard modified k-ε model indicate a significant underestimation of spreading rates for radial samples in regions where jet spreading intensifies.

Numerical Study on the Impact of Central Venous Catheter Placement on Blood Flow in the Cavo-Atrial Junction.

An in silico study is performed to investigate fluid dynamic effects of central venous catheter (CVC) placement within patient-specific cavo-atrial junctions. Prior studies show the CVC infusing a liquid, but this study focuses on the placement without any liquid emerging from the CVC. A 7 or 15-French double-lumen CVC is placed virtually in two patient-specific models; the CVC tip location is altered to understand its effect on the venous flow field. Results show that the CVC impact is trivial on flow in the superior vena cava when the catheter-to-vein ratio ranges from 0.15 to 0.33. Results further demonstrate that when the CVC tip is directly in the right atrium, flow vortices in the right atrium result in elevated wall shear stress near the tip hole. A recirculation region characterizes a spatially variable flow field inside the CVC side hole. Furthermore, flow stagnation is present near the internal side hole corners but an elevated wall shear stress near the curvature of the side hole's exit. These results suggest that optimal CVC tip location is within the superior vena cava, so as to lower the potential for platelet activation due to elevated shear stresses and that CVC geometry and location depth in the central vein significantly influences the local CVC fluid dynamics. A thrombosis model also shows thrombus formation at the side hole and tip hole. After modifying the catheter design, the hemodynamics change, which alter thrombus formation. Future studies are warranted to study CVC design and placement location in an effort to minimize CVC-induced thrombosis incidence.

Statistical analysis of flow field variations via independent component analysis

The link between in-cylinder flow and subsequent combustion in a single-cylinder gasoline spark-ignition engine is analyzed via independent component analysis (ICA). Experimentally, the two in-plane components of the velocity are measured in the central cylinder plane by high-speed particle image velocimetry (PIV) with the engine running slightly lean at 1500 rpm in skip-fired mode. In 213 cycles, measurements are made during the late compression stroke before ignition with approximately 1° crank-angle temporal resolution. ICA then decomposes the set of 213 flow fields at each time step, yielding a set of “source” flow patterns—the independent components (IC). The temporal coherence between the ICs is then examined in a persistence analysis, comparing each IC with the one from the previous time step starting at ignition timing and going backwards in time. The results show which ICs persist how long throughout the compression stroke. To investigate the link between the ICs and combustion, the crank angle at which 10% of the fuel are burned (CA10) in each cycle is correlated with the extent to which a given IC can be found in each flow field. The most persistent IC can be traced over more than half of the 70 degrees crank angle over which images were acquired. The IC that correlates best with CA10 visually more resembles some of the flow features found in conditional averaging of fast-burning versus slow-burning cycles.

GPU parallel computing based on PF-LBM method for simulating dendrites growth under natural convection conditions

This study introduces a GPU-based parallel computing approach that combines the phase-field model (PF) and the lattice Boltzmann model (LBM). By establishing a coupled multiphase field model incorporating physical external fields such as flow field, temperature field, and solute field, the research simulates the growth of single grains and multiple grains under the influence of natural convection. The variations in dendritic morphology, flow field, and solute field during dendritic solidification processes are observed. Initially, the study analyzes the morphology of equiaxed dendrites and the growth patterns of primary dendrites arms under natural convection conditions. The evolution of equiaxed dendrites in single grains and multiple grains under various conditions is investigated. Furthermore, the study explores the impact of different anisotropy strengths on the growth of single grains and multiple grains under natural convection. Notably, a distinct “necking” phenomenon is observed when the anisotropy strength of a single grain is 0.05. In the case of multiple grains, where competition between dendrites is present in addition to the influence of natural convection, a pronounced “necking” phenomenon is evident at an anisotropy strength of 0.03. Moreover, OpenCL parallel technology is designed on the GPU platform to accelerate the solution of the model. The parallelization of the phase-field model coupled with the LBM model on the GPU demonstrates a clear advantage. The parallel computation based on GPU not only exhibits absolute superiority but also shows more significant acceleration effects as the computational domain increases.

Impacts of the Bottom Vortex on the Surrounding Flow Characteristics of a Semi-Submerged Rectangular Cylinder Under Four Aspect Ratios

Abstract Vortex has attracted attention because it is the main factor affecting the flow characteristics around offshore structures. This paper researches the flow field variations around a semi-submerged rectangular cylinder (SRC) under four aspect ratios, thereby revealing the bottom vortex effects on the flow field at Reynolds number = 36,300. According to the particle image velocimetry (PIV) data, the renormalization group (RNG) k–ε model is selected as an applicable turbulence model for studying SRC flow characteristics. From the results, the bottom vortex generation and streamwise velocity are inseparable. As the draft increases, the increased range of the bottom vortex would directly interfere with the flow characteristic around the SRC. Since the wake vortex forms a reattachment state when the SRC pierces the free surface, its flow field could be disturbed by the bottom vortex. Furthermore, the bottom vortex also affects the extreme point position of the boundary layer characteristic at the SRC bottom.

Modeling of heat transfer in target material during the electron beam evaporation process

Numerical simulation provides a new method to study the heat and mass transfer behavior during the electron beam evaporation (EBE) process, which can obtain some physical quantities that cannot be accurately measured in experiments. In this study, a mathematical model is established to investigate the evaporation behavior, the variation of temperature field and flow field of target material during the EBE process, and verified through experiments. The results show that the evaporation rate of target material is extremely non-uniform on the molten pool surface, which the calculation error of the evaporation rate was 2.78 %. The solid-liquid interface obtained by numerical simulation is basically consistent with the experiment, which the calculation error of the molten pool depth was 3.57 % and the calculation error of the molten pool width was 1.18 %. In addition, the surface temperature and flow rate of the molten pool increased with the increase of the electron beam power, the depth and the width of the molten pool increased linearly with the increase of the electron beam power and then level off. The model can accurately describe the variations of temperature field and flow field during the EBE process, and then make reliable predictions based on the simulation results.

Bubble-particle detachment behavior during bubble coalescence: Role of bubble size

Studies have reported that bubble coalescence causes particle–bubble detachment in the flotation pulp and foam phases. Most coalescence experiments have been conducted using bubble pairs with the same diameter; however, this is not the case in real flotation. In this study, particle–bubble detachment behavior during the coalescence of two bubbles with different sizes was investigated using a high–speed camera, and the variation of the coalescing bubble area was fitted to a damped oscillatory equation and analyzed. To explore the intrinsic detachment mechanism of the bubble size effect, the flow field variation in the bubble periphery was simulated using COMSOL 6.1. Results show that the particle–bubble detachment probability decreased with increasing bubble size difference. The released energy is greatest when the two equal bubbles coalesced, thus leading to the strongest oscillation and highest detachment probability. The simulation results show that turbulent kinetic energy affects particle-bubble stability and is an important reason for inducing particle-bubble detachment. Notably, particle–bubble detachment did not occur at the maximum turbulent energy, because there was a delay due to the time required for the sliding contraction of the three–phase contact line. The research finding provides new insights into the mechanism of particle–bubble detachment.

Fast flow field prediction of three-dimensional hypersonic vehicles using an improved Gaussian process regression algorithm

Conducting large-scale numerical computations to obtain flow field during the hypersonic vehicle engineering design phase can be excessively costly. Although deep learning algorithms enable rapid flow field prediction with high-precision, they require a significant investment in training samples, contradicting the motivation of reducing the cost of acquiring flow field. The combination of feature extraction algorithms and regression algorithms can also achieve high-precision prediction of flow fields, which is more suitable to tackle three-dimensional flow prediction with a small dataset. In this study, we propose a reduced-order model (ROM) for the three-dimensional hypersonic vehicle flow prediction utilizing proper orthogonal decomposition to extract representative features and Gaussian process regression with improved automatic kernel construction (AKC-GPR) to perform a nonlinear mapping of physical features for prediction. The selection of variables is based on sensitivity analysis and modal assurance criterion. The underlying relationship is unveiled between flow field variables and inflow conditions. The ROM exhibits high predictive accuracy, with mean absolute percentage error (MAPE) of total field less than 3.5%, when varying altitudes and Mach numbers. During angle of attack variations, the ROM only effectively reconstructs flow distribution by interpolation with a MAPE of 7.02%. The excellent small-sample fitting capability of our improved AKC-GPR algorithm is demonstrated by comparing with original AKC-GPRs with a maximum reduction in a MAPE of 35.28%. These promising findings suggest that the proposed ROM can serve as an effective approach for rapid and accurate vehicle flow predicting, enabling its application in engineering design analysis.

Research on simulation method of multiple time scales dissolution process in tube electrode pulse electrochemical machining

Tube electrode pulsed electrochemical machining (TE-PECM) has received widespread attention as an electrochemical perforation technique that is widely used for machining deep small holes/oblique small holes such as air film holes in blades. However, the TE-PECM process is characterized by complex multi-physics field coupling relationships and multiple time scales for multi-physics interactions, making the simulation of the pulsed electrolytic processing process computationally huge and difficult to perform. To accurately describe the multi-physical field coupling relationship for pulsed electrochemical processing. In this paper, a k-ε gas-liquid two-phase flow model has been introduced to model the interpolar gas-liquid two-phase flow field variation law, and on this basis, a coupled multi-physics field model for pulsed electrochemical machining processing is established to provide an accurate description of the interpolar multi-physics interaction mechanism. A quasi-steady-state iterative simplification algorithm based on heat source averaging is proposed to solve the problem that the time scale span of the dissolution process leads to a huge amount of simulation calculations. The simulation analyzes the dissolution characteristics at the micro time scale and macro time scale and compares them with the full precision method. The results show that the simulation accuracy of the two methods is the same, but the simulation calculation time of the simplified algorithm is reduced by two-thirds. The experimental platform was designed and built to carry out a series of experiments under different processing times, and the results effectively verified the correctness of the simulation method.

NN-augmented k-ω shear stress transport turbulence model for high-speed flows with shock-wave/boundary layer interaction

Menter k-ω shear stress transport (SST) turbulence model demonstrates excellent performance for incompressible, subsonic and transonic flows with mild separation but shows overprediction of the separation bubble of supersonic shock-wave/boundary layer interaction (SWBLI). Some efforts focus on the effect of the structure parameter in stress limiter in an ad-hoc way. Few studies attempt to construct the relation between the structure parameter and flow field variables. The motivation of this work is to construct such a relation to augment the prediction performance of the SST model by introducing a correction factor. Machine learning methods are used since the physical mechanism of SWBLI is complex and unclear. The simulation results show that the constructed relation enhances the structure parameter near the shock wave in the boundary layer when applied to the SST model. Compared with direct numerical simulation and experimental data, the pressure and skin friction coefficients along the wall and the velocity field are significantly improved. In addition, the introduced correction factor can automatically degrade for the subsonic benchmark case of NACA4412 airfoil and maintain the prediction accuracy of the original SST model, but delay the shock location of the transonic case.

Numerical analysis on the deformation of scroll teeth of variable base circle radius scroll expander

For research on the effect of different working loads on the deformation of variable base circle radius scroll expander scroll teeth, a multi-field-coupled calculation method is proposed to reveal the deformation law of scroll teeth under different working loads. In this paper, the equations of the variable base radius profile are derived using the normal isometric method to establish a three-dimensional model of the variable base radius scroll expander. The CFD dynamic mesh technique is used to simulate the working process of a variable base radius scroll expander to obtain the flow field variation in the inner part of the working chamber. The results of the simulation are applied to the scroll teeth by interpolation to analyze the deformation of the scroll under single-field loading and multi-field coupling, to obtain the deformation law of the scroll teeth. The results of the study show that the deformation of the scroll teeth does not decrease linearly with the involute spreading angle from the head to the tail of the teeth due to the uneven temperature distribution of the scroll expander. The thermal stress of the temperature load impacts the deformation trend and amount of the scroll tooth, whereas the gas force load just modifies the amount of scroll tooth deformation and has a lower effect on the scroll tooth's deformation trend. Under different loads, the same deformation and different deformation on the scroll teeth can interact with each other to reduce the scroll tooth deformation to a certain extent.

Tailoring Parallel Channel Flow Fields for Efficient Mass Transport and Compression in Proton Exchange Membrane Fuel Cells

The adverse effects of global warming have made it critical to transition our reliance on fossil fuels to more sustainable energy sources. Renewable energy can be stored in the form of hydrogen to continuously produce clean energy using a proton exchange membrane (PEM) fuel cell. However, to accelerate commercialization, especially in heavy-duty applications, the performance and durability of the fuel cell needs to be improved. Particularly, PEM fuel cell performance is strongly dependent on the flow fields which should be designed to optimize the transport of reactants and byproducts while maintaining uniform compression of the fuel cell materials. An important flow field design parameter is the channel aspect ratio (channel width by height) which directly influences compression and the transport of reactants and products. Although novel flow field configurations have been studied previously, an investigation on the effects of channel aspect ratio on cell performance using electrochemical testing and operando X-ray imaging has yet to be performed. In this study, we compared the water saturation and electrochemical performance across varying flow field channel aspect ratios from 0.5-2.0 with a fixed active area. The ohmic and mass transport resistances were quantified using electrochemical impedance spectroscopy. Operando X-ray imaging was performed to spatially resolve water saturation under the land and channel regions of the flow fields. We observed that lower channel aspect ratios (or higher number of channels for the given active area) led to lower ohmic resistances but resulted in significant water saturation at higher current densities due to ineffective water removal from the smaller channels. Notably, from these results we elucidated that there exists an ideal channel to rib width ratio to facilitate an effective balance of enhanced water removal using the channels and contact between the porous microstructures under the lands of the flow fields.

Study on the influence of non-newtonian fluid characteristics on the hydraulic performance and internal flow field of multiphase pump

As a pivotal prospective clean energy source, the secure and efficient extraction of natural gas hydrate has emerged as a forefront concern within the global oil drilling industry in recent years. In this study, a self-developed multiphase pump adapted to "gas-liquid-solid" three-phase mixing is used to extract natural gas hydrate slurries which have non-Newtonian characteristics. Prior research on pumps has primarily focused on water, introducing a significant disparity in handling slurries containing hydrates. This paper investigates the hydraulic performance and flow field variations of a multiphase pump using three different concentrations of Herschel-Bulkley non-Newtonian fluids that exhibit rheological properties similar to natural gas hydrate slurries. Additionally, a viscous Newtonian fluid with seawater as the liquid phase medium is studied for comparison. The results show that the shear-thinning properties of the non-Newtonian fluid improve the efficiency of the pump. However, excessive concentration negatively affects the hydraulic properties. The flow field characteristics such as apparent viscosity and turbulent kinetic energy are closely related to the pump structure and fluid properties. Compared with viscous Newtonian fluid, the non-Newtonian fluid's also have a greater effect on the distribution of gas-solid phases.

Performance evaluation on full-scale proton exchange membrane fuel cell: Mutual validation of one-dimensional, three-dimensional and experimental investigations

The coupling of multiple physical processes such as gas multi-component transport, gas–liquid multiphase flow, and electrochemical reactions within the proton exchange membrane fuel cell (PEMFC) makes its hydrothermal management exceptionally complicated. Therefore, the complex heat and mass transfer processes in the full-scale PEMFC are investigated in single cell with single channel serpentine flow field (FF). Firstly, the reliability of the proposed three-dimensional (3D) computational fluid dynamics (CFD) numerical model is verified by using the experimental test results of a single cell. Among them, the trend of simulation and experiment performance curves is consistent, and the minimum error of current density is 0.665%. Secondly, the ionomer model of cathode catalyst layer (CCL) is considered in CFD simulation, which is closer to the actual working condition of fuel cell (FC). The resistance of ionomer film and liquid water film mainly affects the diffusion rate of oxygen in CCL, resulting in a decrease in both current density values and power density values. Subsequently, the electrochemical reaction platform of one-dimensional (1D) FC is constructed, and the performance curves of multi-scale FCs are analyzed. The results demonstrate that the error of stack level is the largest, followed by single cell and single channel is the smallest. Finally, the 1D numerical model and the 3D PEMFC multiphase flow CFD model are utilized to investigate the cell voltage losses in the bionic spider flow field, the variable path flow field, and the serpentine flow field, respectively, under different operating conditions.

Cycle-to-cycle variations in cross-flow turbine performance and flow fields

Cycle-to-cycle variations in cross-flow turbine performance and flow fields

Dynamic numerical simulation method and flow characteristic analysis on the mode transition in variable cycle compression system

Studying the dynamic aerodynamic characteristics and flow field characteristics for the dynamic mode transition in the variable cycle compression system (VCCS) can guarantee that the variable cycle engine (VCE) achieves high performance and stable mode transition. The common quasi-steady state analysis method cannot reflect the transient performance of the dynamic mode transition. Moreover, the dynamic simulation based on the dynamic grid technology will encounter the problem of negative volume grids because the adjustment range of variable components for VCCS is usually very large. To solve the problem, a dynamic simulation method based on real-time variation for geometry and aerodynamic boundaries of the dynamic mode transition in VCCS is first established in this paper. Using this method, the dynamic simulation was completed for the couple adjustment process of the outer bypass valve and variable stators of the dynamic mode transition in VCCS. The results of the dynamic simulation are compared with the quasi-steady state results. The results show that the rotor work ability and stator separation degree are affected by the stator variable angle degree and the mass flow rate (MFR) during the dynamic opening process of the outer bypass valve. For the MFR variation of 14.8%, when the stator variable angle is small (less than 3.3° in this study example), the flow separation on the variable stator blade pressure surface is increased during the dynamic opening process of the outer bypass valve. And the work ability of the downstream rotor is weakened. Therefore, in order to reduce the flow separation of stator blades with a small variable angle during the dynamic opening process of the outer bypass valve, a larger positive incidence angle can be adopted for stators at a small bypass ratio. For the whole dynamic process, the variation trend of the rotor work ability and stator separation state is opposite in the opening and closing process of the outer bypass valve. However, there is a dynamic effect of the flow field variation lagging behind the geometry variation in the opening and closing process. Therefore, the transient flow field in the opening and closing process is different from the quasi-steady state results, and the difference is more significant in the case of a small valve opening. Under a small valve opening, the work ability of the stage before the outer bypass entrance in the open process is significantly smaller than that in the closing process. And the difference in the flow field is mainly that the stator separation state of the stage before the outer bypass entrance in the open process is significantly weaker than that in the closing process. Therefore, the dynamic effect of the opening and closing process is more significant under a small valve opening, and the flow stability problem is more likely to occur during the dynamic mode transition in VCCS.

Investigating the impact of variable aspect ratio cathode flow field on temperature distribution and performance in a PEM fuel cell

This study numerically investigates the effect of variable aspect ratio serpentine cathode flow field on proton exchange membrane fuel cell (PEMFC) performance and temperature distribution. By varying the proportion of two aspect ratios, 0.6 and 1, three modified cathode (MC) designs MC-1, MC-2, and MC-3, alongside a conventional cathode (CC) is considered. With 0.45 W cm−2, MC-2 exhibited 15.39% higher performance and 10.16 K lower temperature than CC. The transition region position, equal number of narrow and wider channels enhanced the current and temperature uniformity in MC-2. Furthermore, experimental results are consistent with the numerical results obtained from MC-2.