Fluid Coupling Research Articles

Transcranial photoacoustic computed tomography based on a layered back-projection method

A major challenge of transcranial human brain photoacoustic computed tomography (PACT) is correcting for the acoustic aberration induced by the skull. Here, we present a modified universal back-projection (UBP) method, termed layered UBP (L-UBP), that can de-aberrate the transcranial PA signals by accommodating the skull heterogeneity into conventional UBP. In L-UBP, the acoustic medium is divided into multiple layers: the acoustic coupling fluid layer between the skull and detectors, the skull layer, and the brain tissue layer, which are assigned different acoustic properties. The transmission coefficients and wave conversion are considered at the fluid–skull and skull–tissue interfaces. Simulations of transcranial PACT using L-UBP were conducted to validate the method. Ex vivo experiments with a newly developed three-dimensional PACT system with 1-MHz center frequency demonstrated that L-UBP can substantially improve the image quality compared to conventional UBP.

Modeling tracer flowback behaviour for a fractured vertical well in a tight formation by coupling fluid flow and geomechanical dynamics

In this work, a pragmatic technique has been developed to describe tracer flowback behaviour for a vertically fractured well in a tight formation by coupling fluid flow and geomechanical dynamics. More specifically, the Barton-Bandis model is employed to describe the relationship between effective stress and fracture permeability, while tracer flowback profiles, which can reveal the fracture properties, are quantified by taking tracer dispersion and adsorption into account. Subsequently, this method considering two main mechanisms (i.e., tracer flow behaviour and fracture propagation dynamics) is separately validated with previous analytical solutions and then extends its application to a field case. In addition to tracer flowback concentration, the tracer recovery factor (TRF) is generated to analyze the tracer flowback behaviour. With considering geomechanics, the TRF is nearly 20% higher than that without considering geomechanics. During the flowback period, tracer concentration profiles appear to be unimodal for reservoirs with a single fracture, while tracer concentration increases quickly at the initial stage and then decreases slowly as time proceeds. An increase in matrix permeability increases tracer flowback concentration and the TRF. A larger tracer dispersion coefficient leads to earlier arrival time for the tracer flowback concentration peak together with a lower TRF. Also, the stronger the tracer adsorption is, the lower the tracer flowback concentration and the TRF will be. A higher Young's modulus results in a lower tracer flowback concentration and TRF. An increase in minimum horizontal stress increases tracer flowback concentration. Other parameters, including maximum horizontal stress, fracture closure permeability, and normal fracture stiffness, are also examined and analyzed though less sensitive.

Simulation system for collisions and two-way coupling of non-Newtonian fluids and solids

To solve the problem of unreasonable boundary conditions of the existing smoothed particle hydrodynamics (SPH) non-Newtonian fluid simulation, a simulation system for collisions and two-way coupling of non-Newtonian fluids and solids is proposed to alleviate the loss of physical properties of simulated non-Newtonian fluid. In this paper, solids are sampled as single-layer boundary particles, and the corresponding density calculation formula is given. Fluid particles are interacting directly with the boundary particles under our interaction force calculation formula. The velocity of fluid particles near the boundary is iteratively updated using Coulomb friction boundary conditions. This method can be integrated into the existing SPH non-Newtonian fluid simulation system. Experiments show that the friction coefficient of the fluid–solid boundary can be adjusted according to the user’s experimental needs. Compared to simulation under free-slip boundary conditions, in our method non-Newtonian fluids can exhibit more diverse physical characteristics and have a good sense of reality effect.

Development of a hybrid model based on mesh and meshfree methods and its application to fluid–elastic structure interaction for free surface waves

In this paper, a hybrid scheme, Fluid–Fluid–Elastic Structure (FFES) model was developed in the time domain to address the wave breaking impact on the structure. The model is developed based on the partitioned approach with different governing equations that describe various regions of the model domain. The fluid–fluid model denotes that two different fluid models were used to describe fluid in the actual physical domain. The method is a physics-based approximation to reduce the computational time, i.e. in the far-field inviscid fluid (fully nonlinear potential flow theory model), and near to the structure, viscous fluid (Navier Stokes model) is used. The coupled model then interacts with the elastic structure (based on Euler–Bernoulli beam theory). The system of equations is strongly coupled both in space and time. The Fluid–Fluid coupling uses an implicit predictor–corrector scheme, and the fluid–structure coupling works based on an iterative scheme. This approach makes the method more robust and for future extension. Three different possibilities for introducing the coupling was identified and implemented. The model was validated against results from the analytical solution and literature. The method proposed is a reliable, robust, and efficient alternative for simulating fluid–structure interaction problems.

Density ratio effects on the topology of coherent turbulent structures in two-way coupled particle-laden channel flows

This investigation considers the effect of density ratio on the modification of coherent turbulent structures in particle-laden channel flows at a shear Reynolds number of Reτ = 180. Direct numerical simulation and Lagrangian particle tracking are used to accurately predict the motion of particles dispersed within the flow at three Stokes numbers, St+ = 0.1, 50, and 92. Particle–fluid coupling is achieved through a local element-based force feedback field in the Navier–Stokes equations, which are solved using a seventh-order accurate spectral element method. After an initial transitory period wherein the effects of particle–fluid interaction are emphasized, the low density ratio particles are found to enhance the turbulence field, increasing the frequency of Q-criterion satisfying regions, while the inertial particles suppress the turbulence, reducing the number of quasistreamwise vortices. Results indicate that the topology of the quasistreamwise vortices is altered by the presence of the particles in the viscous sublayer, the buffer layer, and the log-law region such that the distribution of the third invariant of the deviatoric tensor, R, is widened by the presence of tracer-like particles and made thinner by the inertial particles. This effect reduces the amount of unstable focus/compressing regions and stable focus/stretching regions, which account for the streamwise vortical structures observed in these types of flow. Investigating the instantaneous coupling force field surrounding turbulent structures and low speed streaks shows that particles exert their greatest influence on the fluid in the regions noted, and mechanisms by which the particulate phase interacts with turbulent vortices are analyzed.

Three‐dimensional complex fracture propagation simulation: Implications for rapid decline of production capacity

AbstractIn this study, a simulation model of fracture network geometry in fractured and porous elastic reservoirs is established based on the globally embedded three‐dimensional cohesive zone model (3D CZM). The effects of stress interference between natural fractures (NFs), horizontal stress difference (HSD), injection rate, and fluid viscosity on fracture geometry were studied and the phenomenon of rapid failure of unconventional reservoir productivity was explained from a coupling fluid flow/geomechanics perspective. The intersection behavior of hydraulic fracture (HF) and NF is simulated; the simulation results are in good agreement with Blanton's criterion, proving the reliability of the 3D CZM. The research results show that in the process of hydraulic fracturing, in addition to the strong stress interference between HFs, there is also strong stress interference between NFs. Stress interference has an important impact on the traction separation of cohesive elements (CEs) and a continuous dynamic impact on the NF opening. For reservoirs with a small HSD, higher injection rate and fluid viscosity produce smaller stimulated reservoir volume (SRV) length, but larger SRV width. Complex fracture networks tend to develop in the vicinity of horizontal wellbores, which leads to rapid failure of productivity. For this kind of reservoir, minimizing the impact of stress interference with a small injection rate is recommended instead of maximizing the injection rate and fluid viscosity. For reservoirs with a large HSD, lower injection rate and fluid viscosity produce larger SRV length and smaller SRV width. The fracture geometries tend to develop simple straight fractures, which leads to low initial productivity. Thus, adopting a relatively large injection rate and fluid viscosity is recommended to make full use of stress interference effects to increase the development of complex fractures near horizontal wellbores. The results of this study can serve as a guide in evaluating fracture complexity, SRV, proppant migration, drainage reservoir volume (DRV), and well completion design; they can also promote the in‐depth understanding of the coupling effect between fluid flow and geomechanics.

Design optimization of hydrodynamic coupling applying response surface method combined with CFD technique

Hydrodynamic coupling is a type of fluid machinery that uses fluid kinetic energy to transmit power. In this paper, meridional profile optimization of fully filled fluid coupling was performed to improve transmitted torque. The response surface method, a global optimization method, combined with CFD was introduced to find optimum meridional profile of blades. The torque coefficient was applied as an objective function. The response surface method was adopted with three design variables: hydraulic diameter ratio of blades, central core position, and meridional profile of the hub. Results showed that the best diameter ratio occurred at d/D =0.67. To specify an optimal position for the central core, a dimensionless parameter (b/d) was introduced. With regression analysis, it was found that the optimal point occurs at about b/d =0.37. It was confirmed with the results of contour and vector plots. Three types of curves (circular, elliptical, and cubic Bezier curve) were employed to optimize the meridional shape of blades. The circular curve was applied in the design of the hub in the reference geometry. Relative to the circular curve hub, the use of elliptic curve had no significant effect on the objective function. Two dimensionless parameters, l1/d and l2/d, were defined to investigate the effect of Bezier curves on the performance of hydrodynamic coupling. The optimum values for design variables of l1/d and l2/d were obtained 0.80 and 0.88, respectively.

Influence of holdup on gas and particle flow patterns in a spiral jet mill

Particle size reduction in spiral jet mills is induced by high velocity gas jets, causing interparticle and particle wall collisions leading to breakage. Despite extensive research on the design and operational parameters, the underlying mechanics of size reduction is still poorly understood. Discrete Element Method and Computational Fluid Dynamics are used here to analyse particle and fluid motions. A fast shearing dense particle bed is formed on the wall, with a transition to lean phase towards the centre of the mill. This necessitates four-way coupling of fluid and particle interactions for analysis. It is shown that increasing the depth of the particle bed reduces the fluid phase tangential velocity in the proximity of the classifier, as momentum is exchanged with circulating particles. The energy dissipation through particle collisions occurs mainly along the bed surface and in front of the grinding jet nozzles.

Revisiting steady viscous flow of a generalized Newtonian fluid through a slender elastic tube using shell theory

AbstractA flow vessel with an elastic wall can deform significantly due to viscous fluid flow within it, even at vanishing Reynolds number (no fluid inertia). Deformation leads to an enhancement of throughput due to the change in cross‐sectional area. The latter gives rise to a non‐constant pressure gradient in the flow‐wise direction and, hence, to a nonlinear flow rate–pressure drop relation (unlike the Hagen–Poiseuille law for a rigid tube). Many biofluids are non‐Newtonian, and are well approximated by generalized Newtonian (say, power‐law) rheological models. Consequently, we analyze the problem of steady low Reynolds number flow of a generalized Newtonian fluid through a slender elastic tube by coupling fluid lubrication theory to a structural problem posed in terms of Donnell shell theory. A perturbative approach (in the slenderness parameter) yields analytical solutions for both the flow and the deformation. Using matched asymptotics, we obtain a uniformly valid solution for the tube's radial displacement, which features both a boundary layer and a corner layer caused by localized bending near the clamped ends. In doing so, we obtain a “generalized Hagen–Poiseuille law” for soft microtubes. We benchmark the mathematical predictions against three‐dimensional two‐way coupled direct numerical simulations (DNS) of flow and deformation performed using the commercial computational engineering platform by ANSYS. The simulations show good agreement and establish the range of validity of the theory. Finally, we discuss the implications of the theory on the problem of the flow‐induced deformation of a blood vessel, which is featured in some textbooks.

Vehicle contact network based on fluid coupling analysis of load of operational status detection device

Based on the analysis of aerodynamic load and structural strength of the vehicle contact network monitoring device, the distribution of aerodynamic moments and surface pressure of the monitoring device is calculated by means of fluid-solid coupling. The calculated load is applied to the surface of the device for structural analysis, mainly to investigate the maximum deformation and maximum equivalent stress of the device. The study found that air flow separation occurred above the ear region when driving forward, the pressure was higher, and the air flow below the ear region flowed around the ear region. The pressure difference between the upper and lower ears produced a downward force of about 150N in a single ear region, and produced a large rolling moment, after checking its connection strength with the central body to meet the requirements for use.

Thermal physical process in a liquid oxygen tank under different sloshing excitations

Fluid sloshing usually results in some unpredictable effects, including additional sloshing force and moment, and thermodynamic imbalances. It is necessary to conduct in-depth investigations on fluid sloshing to ensure the safe storage of cryogenic propellants. From our own perspectives, the thermal physical performance during sloshing in cryogenic storage tanks is often ignored and few reported. In this study, a numerical model is built to study the coupling thermal physical and fluid sloshing process with the volume of fluid method adopted. Both the external heat invasion and interfacial phase change are considered to predict the thermal performance during fluid sloshing. With the sloshing excitation realized by the user-defined function, the effect of the excitation type on fluid sloshing, has been studied. The results show the sloshing excitations have great influences on the thermal physical process in cryogenic storage tanks. With some valuable conclusions achieved, the present study is significant to depth understanding fluid sloshing thermal behavior.

Numerical Computing Paradigm for Investigation of Micropolar Nanofluid Flow Between Parallel Plates System with Impact of Electrical MHD and Hall Current

The current study develops a novel application of numerical computing paradigm to inspect the cumulative effects of both electric and magnetic field on a micropolar nanofluid bounded by two parallel plates in a rotating system by using Adams and Explicit Runge–Kutta (RK) solvers. The steady state micropolar nanofluidics flow has been considered between parallel plates under the Hall current effect. The conventional expressions for the governing flow in terms of system of ODEs are engaged. Adams and Explicit RK-based numerical solvers have been exploited for approximate solution of the system dynamics. Accuracy, convergence and stability analyses of both numerical schemes established their correctness. The impact of the Sherwood number on the concentration profile, Nusselt number and coefficient of skin friction on temperature and velocity profiles, respectively, have been analyzed numerically along with thermophoresis investigation, rotation, Hall current and Brownian motion of micropolar nanofluid. The effect of physical quantities including viscosity, magnetic, rotating, coupling, Brownian motion, thermophoretic and micropolar fluid parameters as well as Prandtl and Schmidt numbers has been presented.

Parametric Analysis and Optimization of Leaning Angle in Torque Converters

Abstract Torque converters are durable fluid couplings that can provide output torque multiplication. Blade leaning angle represents the angular position of a blade chord with respect to its radial reference line, and it is an important blade variable regarding both hydrodynamic performance and manufacturability of a torque converter. In traditional design processes, blade leaning angles are often determined based on experiences of engineers; hence, this study proposed a design approach using the combination of computational fluid dynamics (CFD) and optimization. Two CFD models were developed to design blade leaning angles. A steady-state periodic CFD model was employed for the parameter study and the optimization, and a transient full three-dimensional (3D) model was performed to study the flow mechanism and evaluate the performance with higher accuracy. Design of experiment (DOE) technique was employed to investigate the relationship between blade leaning angles and hydrodynamic performance, and a reduced cubic model was derived from the results. It was found that blade leaning angles had profound effects on torque converter performance; a large blade leaning angle intensified the flow blockage effect, thus resulting in a lower mass flowrate and torque capacity. Seven torque converters with different blade leaning angles were tested to validate the obtained numerical results, and the test data were found to be in good agreement with the CFD predictions. Finally, the hydrodynamic performance of the base model torque converter was optimized by a multi-objective genetic algorithm.

Viscoplastic constitutive equations for modeling fluid loading and damage evolution during warm medium forming

A unified viscoplastic constitutive model is proposed coupling fluid pressure rate control and damage evolution for warm medium forming (WMF), which also includes flow stress function, hardening law and dislocation density evolution. The pressure rate and damage behavior are investigated based on the experimental data of Ref. (Lang et al., 2013) [10] and the fracture morphology observations of specimens. The constitutive equations are discretized and derived through the fourth-order Runge-Kutta method. Meanwhile, the material constants of this model are determined using the Genetic Algorithm. The proposed constitutive model is applied to the WMF process of AA7075-O sheet and compared with the results of Ref. [10]. The error between calculated and experimental data is quantified using the average absolute relative error (AARE). Results indicate that the prediction accuracy of the proposed constitutive model is significantly improved compared with Ref. [10], also, the predictions compare well with the experimental results. The new constitutive model can effectively predict the flow behavior of AA7075-O in WMF.

Numerical Simulation for the Influence of EMS Position on Fluid Flow and Inclusion Removal in a Slab Continuous Casting Mold

Electromagnetic stirring (EMS) has been used to improve the steel quality in continuous casting process and EMS position is a key factor in optimizing the stirring performance. In order to clarify the issue that where to install the electromagnetic stirrer, a mathematical model, coupling electromagnetic field and fluid flow field, was developed in this paper. Three cases differing in EMS position have been investigated: the stirrer was installed above the submerged entry nozzle (SEN) port; the stirrer was installed near it; and the stirrer was installed below it. The model was validated by the comparison of electromagnetic flux density. The influence of EMS position on electromagnetic force, flow field and inclusion removal was analyzed. Index Rc was introduced to quantify the possibility of slag entrapment. The results shows that as the stirrer moves downwards, the maximum electromagnetic force increases, index Rc increases first and then turns to decrease, and the electromagnetic force causes the jet to bend more horizontally, resulting in an inactive zone when the stirrer is installed near the SEN port. Furthermore, under our computed condition, to improve the inclusion removal, the stirrer is suggested to be installed below the SEN port.

Coupling multi-body dynamics and fluid dynamics to model lubricated spherical joints

A new approach of coupling multibody dynamics and fluid dynamics is developed to model hydrodynamic lubrication of spherical clearance joints with thin fluid film and relative multidirectional motion. The model accounts for dynamics motion of articulating components as well as both squeeze- and wedge-film actions of the synovial fluid. Multibody dynamics methodology is employed to derive the motion equations and Reynolds equation governs the fluid dynamics. The finite difference method is utilized to discretize the governing equation of lubricant and the multi-grid method augments computational efficiency to acquire outcomes employing a Gauss–Seidel relaxation scheme. Fluid–structure interaction is incorporated into the methodology using a partitioned formulation embedded in a high-order Runge–Kutta time integrators for integrating the nonlinear equations of the coupled system over time of interest. A demonstrative example of total hip arthroplasty is considered and the developed model is assessed against outcomes available in the literature. The effect of initial conditions on the pressure, film thickness and dynamics of the lubricated spherical joint is analyzed and discussed. It is illustrated that maximum fluid pressure is undergone by the hip implant at the first walking cycle of movement due to an unstable state, which is strongly dependent upon the initial condition. Finally, the approach presented in this research work is a robust dynamic model to study hydrodynamic lubrication of spherical joints.

Evaluation of dynamic compaction to improve saturated foundation based on the fluid-solid coupled method with soil cap model

Abstract This paper presented a dynamic fluid-solid coupled finite element (FE) method incorporating the soil cap yield hardening model to analyze the improvement on saturated foundation under dynamic compaction (DC). Biot’s dynamic u–U–p formulation was employed to describe the coupling of pore fluid and solid phases, which was discretized into finite elements by application of the Galerkin method and viscous Cartesian connectors. The proposed numerical model showed reasonably good agreement with the existing analytical solutions of one-dimensional transient loading problems and field measurement of dynamic compaction. The effects of groundwater table and soil permeability were examined via the development of excess pore water pressure, effective soil stress and void ratio. Results showed that groundwater table had a significant effect on the foundation improvement by DC. Higher groundwater table resulted in larger excess pore water pressure, and effective soil stress was not found to develop under the groundwater table during the compaction process. The decrement in the void ratio was only limited to soils located above the groundwater table, and a critical depth of groundwater table existed for the DC reinforcement in fine soil foundation, which was suggested to be 6 m for the DC reinforcement with the tamping energy of 2500 kN·m per blow. Four soil permeability coefficients of k = 10−2 m/s, 10−3 m/s, 10−5 m/s and 10−7 m/s were chosen in the parametric analysis, which represented gravel, coarse sand, silt and silty clay, respectively. Higher permeability resulted in lower excess pore water pressure and larger decrease in void ratio. However, the saturated foundation was not suitable to reinforce by dynamic compaction, no matter how much the soil permeability was. Compared to the increase in soil permeability, it was more crucial to lower the groundwater table before implementing dynamic compaction. Moreover, the gravel columns were preferred to be used to accelerate the drainage of groundwater than the sand columns.

Linkage Analysis between Solid-Fluid Coupling and the Strength Reduction Method for Karst Cave Water Inrush in Mines

The present paper aims to study the mechanical mechanism and characteristics of water irruption from Karst cave. Combining the nonlinear seepage-pipe coupling model with the strength reduction method, the linkage analysis of fluid solid coupling and strength reduction method are constructed to study the whole process of confined Karst cave water inrush. Taking the water inrush accident of Shibaijing of the Qiyi mine in south China as an example, the instability mechanism of the water-proof rock pillar and evolution of water inrush are discussed. It is suggested that water discharge on the working face augments with the increase in the reduction factor of the water-proof rock pillar before the rock pillar loses its stability. Once the rock pillar becomes unstable, Karst water bursts from confined Karst cave in a pipe flow shape, and the water irruption quantity reaches the peak value in a short time by adopting the pipe flow to simulate and then decreases slowly. The hydraulic rough flow at the initial stage changes into pipe laminar flow finally in the process of Karst water inrush, due to the constraint of Karst cave water reserve. The conception for the safety factor of the water-proof rock pillar introduced, the relation of the safety factor, Karst cave water pressure, and thickness of the water-proof rock pillar are studied. It is proposed that thickness of the water-proof rock pillar whose safety factor equals 1.5 is regarded as the calculating safety thickness of the water-proof rock pillar, and the safety thickness of the water-proof rock pillar setting in mining engineering should be equal to the sum of the blasthole depth, blasting disturbance depth, and the calculating safety thickness. The reason leading to Karst water inrush of Qiyi Mine is that without advanced boreholes, the water-proof rock pillar is set so small that it could not possess safety margin, so the confined Karst cave water breaks the water-proof rock pillar and bursts out. Combining the solid fluid coupling theory, pipe flow theory, and strength reduction method, the nonlinear mechanical response of confined Karst cave water inrush is studied, which provides a new study method for the whole process of confined Karst cave water inrush.

Study on induced stress of hydraulic fracturing in fractured‐porous elastic media

AbstractThe study on induced stress of hydraulic fracturing is an important part of hydraulic fracturing design, temporary plugging and steering fracturing design, and completion design in oil and gas development. Based on fracture continuum method (FCM), cohesive unit method, and finite volume method (FVM), the propagation behavior of multiple hydraulic fractures (HFs) in fractured‐porous media reservoir is simulated from the perspective of fluid and geological stress coupling. The influence of natural fractures (NFs) on stress distribution and stress inversion sensitive factors are studied. NFs in the model are equivalent to physical models controlled by tension and shear bonds. The proposed model can deal with any number of NFs, which greatly improves the computational efficiency of the model. Then PKN analytic solution and Abaqus software results were used to verify the correctness of our model. The results show that the NF will make the reservoir stress show strong heterogeneity after fracturing, and the prominent feature is the regional stress mutation. The sudden change of stress shows the sudden increase or decrease in stress; with the increase in the initial stress difference, the stress inversion becomes more difficult. When the stress difference exceeds 3 MPa, the stress inversion area is almost 0. The stress interference increases with the decrease in cluster spacing. The stress inversion region between HFs decreases, while the external stress inversion region increases; Biot coefficient has great influence on stress and stress inversion. With the decrease in Biot coefficient, stress inversion region will decrease obviously. When the Biot coefficient is 0.4, there is no stress inversion area in the whole reservoir. Poisson's ratio has little influence on stress inversion. When Poisson's ratio changes between 0.1 and 0.3, the change range of stress is within 1 MPa, and the stress inversion area is almost unchanged. Our results can be helpful for understanding the stress distribution after hydraulic fracturing in fractured‐porous media.

Improving energy performance of hydraulic torque converters

Torque converters are widely used in the transmissions of self-propelled vehicles for various purposes on wheeled and tracked chassis. A significant drawback of torque converters compared to mechanical transmissions is their relatively low efficiency. Therefore, the task of ensuring high efficiency values of torque converters in the entire range of gear ratios has been of importance since the invention of these hydraulic machines. Torque converters have the lowest efficiency values in the torque transformation mode among low and medium gear ratios from 0 to 0.7. In addition, the nature of the change in efficiency for an integrated torque converter in automatic transition to the fluid coupling mode also usually decreases sharply. This article discusses a number of motor and tractor torque converter designs aimed at obtaining high values of efficiency in the range of small or medium gear ratios. The considered technical solutions are associated with a reduction in energy losses when the working fluid enters the pump wheel of the torque converter.