Entire Flow Domain Research Articles

Head-on collision of gravity currents of unequal strengths: Large-eddy simulations and laboratory experiments

The collision of two counterflowing gravity currents with unequal strengths was investigated through large-eddy simulations and laboratory experiments. The collisions were initiated by releasing currents from two partial-depth locks at identical heights but with different densities, characterized by the reduced gravity ratio, γg. By varying γg, we elucidate the transport processes of colliding gravity currents, spanning from comparable driving strengths (γg=1.0) to markedly disparate driving strengths (γg≪1). Three distinct regimes of colliding gravity currents were identified based on kinematic features derived from integrated measures. For γg≥0.92, the collisions are driven by counterflowing gravity currents with comparable driving strengths, leading to nearly symmetrical collisions with negligible impact on evolved flow patterns. In the intermediate regime when 0.4<γg<0.92, the collisions are weakly asymmetric, characterized by differing contact surface steepness and insensitive maximum vertical displacement of ascending motions to γg. For γg≤0.4, strongly asymmetric collisions dominate, featuring minimal vertical convective fluxes in the collision region rather than typical colliding currents. In this flow regime, the denser fluid mass intrudes beneath the less dense one, akin to the propagation of intrusive lock-exchange gravity currents. Additionally, mixing rates over the entire flow domain were quantified using background potential energy calculations. The results reveal that intense diapycnal mixing is predominantly driven by stirring processes before collision, with the mixing rate increasing as γg decreases. From the collision stage onward, currents with the gain of inertia converge within the collided region and move upward with the distinct opposite effect of negative buoyancy. Notably, the mixing rate stabilizes regardless of convective process variations and decreases consistently as the currents slump away from the collision region.

Structure of the streaming flow generated by a sphere in a fluid undergoing rectilinear oscillation

A solid body in a viscous fluid undergoing oscillatory motion naturally produces a steady secondary flow due to convective inertia. This phenomenon is embodied in the streaming flow generated by a sphere in an unbounded fluid executing rectilinear oscillations. We review the considerable literature on this canonical problem and summarise exact and asymptotic formulas in the small-amplitude limit. These analytical formulas are used to explore the characteristic flow structure of this problem and clarify previously unreported features. A single, toroidal-shaped vortex exists in each hemisphere regardless of the oscillation frequency, which can drive a counter-flow away from the sphere. The vortex centre moves monotonically away from the sphere with decreasing oscillation frequency, and engulfs the entire flow domain for $\beta \equiv \omega R^2/\nu < 16.317$ , where $\omega$ is the angular oscillation frequency, $R$ the sphere radius, and $\nu$ the fluid kinematic viscosity. This seemingly abrupt change in flow structure at the critical frequency $\beta _{critical} =16.317$ , and its quantification, appear to have not been reported previously. We perform a direct numerical simulation of the Navier–Stokes equations, to (1) confirm existence of this critical frequency at finite amplitude, and (2) examine its variation with amplitude. This reveals a universal relationship between the critical frequency and oscillation amplitude, clarifying previous reports on the structure of this streaming flow. The critical frequency is shown to be identical for the streaming flow and the cycle-averaged particle paths, establishing that the critical frequency is accessible directly using standard measurements.

Comparison of Immersed Boundary Simulations of Heart Valve Hemodynamics Against In Vitro 4D Flow MRI Data.

The immersed boundary (IB) method is a mathematical framework for fluid-structure interaction problems (FSI) that was originally developed to simulate flows around heart valves. Direct comparison of FSI simulations around heart valves against experimental data is challenging, however, due to the difficulty of performing robust and effective simulations, the complications of modeling a specific physical experiment, and the need to acquire experimental data that is directly comparable to simulation data. Such comparators are a necessary precursor for further formal validation studies of FSI simulations involving heart valves. In this work, we performed physical experiments of flow through a pulmonary valve in an in vitro pulse duplicator, and measured the corresponding velocity field using 4D flow MRI (4-dimensional flow magnetic resonance imaging). We constructed a computer model of this pulmonary artery setup, including modeling valve geometry and material properties via a technique called design-based elasticity, and simulated flow through it with the IB method. The simulated flow fields showed excellent qualitative agreement with experiments, excellent agreement on integral metrics, and reasonable relative error in the entire flow domain and on slices of interest. These results illustrate how to construct a computational model of a physical experiment for use as a comparator.

Unsteady Simulation and Analysis of Energy Loss Mechanism of a Vertical Volute Pump Under Stall Conditions

AbstractThis study comprehensively investigates the flow features and energy loss mechanisms under stall conditions based on computational fluid dynamics (CFD) and hydraulic loss visualization techniques. The three-dimensional unsteady Reynolds-averaged Navier– Stokes (URANS) equations were solved using the shear stress transport (SST) k–ω turbulence model. Furthermore, the entire flow domain of the vertical volute pump was considered to capture the boundary flow behavior accurately. The results illustrate that the critical stall condition occurs at 0.7Qd, where the H–Q curve exhibits a positive slope, and the deep stall condition occurs at 0.65Qd. The growth rate of energy loss from critical stall to deep stall is 182.2%. In the stall condition, a secondary vortex appears at the impeller inlet. A high energy loss occurs at the suction side and trailing edge in the impeller caused by the reduction in the effective inflow area. The energy loss in the blade suction side guide vane is primarily due to the friction loss under the critical stall condition. By contrast, under the deep stall condition, the energy loss in the outlet of the guide vane is mainly the impact loss from the volute of the rear gunner. The impact effect can result in high energy losses near the volute tongue. The entropy production analysis demonstrates that the hydraulic losses in the diffuser are literately greater than that in the impeller and inlet pipe. Hence, optimization of such components can be taken into consideration in future works for performance improvement.

Modeling of reverse water–gas shift reaction in a membrane integrated microreactor

CO2 hydrogenation to syngas via reverse water–gas shift (RWGS) is modeled in a membrane decorated microchannel reactor. Coated CuO/ZnO/Al2O3 catalyst exists in the reaction channel which is physically separated from the neighboring permeate channel by a layer of supported sodalite (SOD) membrane selective to H2O and H2 transport. Pure H2 is used as the sweep gas. Two–dimensional, steady state, isothermal operation of the reactor is quantified by the momentum and mass conservations in the entire flow domain, membrane material transfer and catalytic reaction. Upon its benchmarking with the literature–based experimental data, the reactor model is used to study the effects of inlet velocities and partitioning of the reactive mixture and sweep streams, reactor pressure, and molar inlet H2:CO2 ratio. At 523 K, 15 bar and H2:CO2 = 3, the per cent CO2 conversion exceeds the thermodynamic limit of 16.8% and can be maximized to 52% when the inlet velocity of the H2 + CO2 mixture is minimized. Decreased dosing of the reactive mixture, however, limits the molar amount of CO2 converted and reduces reactor capacity. Both metrics are positively correlated with reactor pressure and the inlet velocity of the sweep gas but remain insensitive to flow direction. Syngas composition (molar exit (H2 – CO2)/(CO + CO2) ratio) can be varied between 1.7 and 9.8. The proposed reactor can deliver ∼2.5 times higher volume and catalyst mass–based productivity and ∼33% higher CO2 conversion than an equivalently operated packed–bed membrane reactor.

Real-time flow measurement system: physics-informed reconstruction and sampling strategy

In this work, we focus on a multi-hole pressure-probe-based flow measurement system for wind tunnel measurements that provides real-time feedback to a robot probe-manipulator, rendering the system autonomous. The system relies on a novel, computationally efficient flow analysis technique that translates the probe's point measurements of velocity and pressure into an updatable mean flow map that is accompanied by an uncertainty metric. The latter provides guidance to the manipulator when planning the optimal probe path. The probe is then guided by the robot in the flow domain until an available time budget has been exhausted, or until the uncertainty metric falls below a prescribed target threshold in the entire flow domain. We assess the capabilities of our new measurement system using computational fluid dynamics data, for which the ground truth is available in the form of a mean flow field. An application in a real wind tunnel setting is provided as well.

Field synergy principle for compressible laminar flow and the application for drag reduction in microchannel

Considering the density variation of compressible fluid, the field synergy principle for compressible laminar flow is presented based on the incompressible flow field synergy principle. The flow resistance is related to the synergy of velocity field and density logarithmic gradient field. Based on the principle of minimum mechanical energy dissipation, the compressible laminar flow field synergy equation is derived. The field synergy principle is verified by an example of the microchannel with expansion cross section, and the structure is optimized. The results indicate that the viscous dissipation value in the entire flow domain of the optimized structure can be reduced by 13.5%, and the angle between the velocity vector and the gradient field of logarithm of density increases. In fluid flows, it will reduce the fluid flow drag to decrease the synergy angle between the velocity vector and the gradient field of logarithm of density.

Forward and inverse modeling of water flow in unsaturated soils with discontinuous hydraulic conductivities using physics-informed neural networks with domain decomposition

Abstract. Modeling water flow in unsaturated soils is vital for describing various hydrological and ecological phenomena. Soil water dynamics is described by well-established physical laws (Richardson–Richards equation – RRE). Solving the RRE is difficult due to the inherent nonlinearity of the processes, and various numerical methods have been proposed to solve the issue. However, applying the methods to practical situations is very challenging because they require well-defined initial and boundary conditions. Recent advances in machine learning and the growing availability of soil moisture data provide new opportunities for addressing the lingering challenges. Specifically, physics-informed machine learning allows both the known physics and data-driven modeling to be taken advantage of. Here, we present a physics-informed neural network (PINN) method that approximates the solution to the RRE using neural networks while concurrently matching available soil moisture data. Although the ability of PINNs to solve partial differential equations, including the RRE, has been demonstrated previously, its potential applications and limitations are not fully known. This study conducted a comprehensive analysis of PINNs and carefully tested the accuracy of the solutions by comparing them with analytical solutions and accepted traditional numerical solutions. We demonstrated that the solutions by PINNs with adaptive activation functions are comparable with those by traditional methods. Furthermore, while a single neural network (NN) is adequate to represent a homogeneous soil, we showed that soil moisture dynamics in layered soils with discontinuous hydraulic conductivities are correctly simulated by PINNs with domain decomposition (using separate NNs for each unique layer). A key advantage of PINNs is the absence of the strict requirement for precisely prescribed initial and boundary conditions. In addition, unlike traditional numerical methods, PINNs provide an inverse solution without repeatedly solving the forward problem. We demonstrated the application of these advantages by successfully simulating infiltration and redistribution constrained by sparse soil moisture measurements. As a free by-product, we gain knowledge of the water flux over the entire flow domain, including the unspecified upper and bottom boundary conditions. Nevertheless, there remain challenges that require further development. Chiefly, PINNs are sensitive to the initialization of NNs and are significantly slower than traditional numerical methods.

Coherent Structures of a Turbulent Flow Bounded by a Compact Permeable Wall

In order to optimize the use of compact porous media as flow and heat transfer devices, it is imperative to understand those coherent structures of the associated flow that generate and sustain turbulence. Given the deficiency of data regarding this area in the literature, this study has been carried out to fill this need. To this end, a series of particle image velocimetry measurements were conducted to capture a turbulent flow field bounded by a model permeable medium of 85% porosity. The bulk Reynolds numbers based on the bulk velocity through the entire flow domain and the depth of flow over the permeable boundary are approximately 5.0 × 103 and 2.0 × 104. By applying velocity gradient eigenanalysis, quadrant decomposition, multi-point correlations, and proper orthogonal decomposition, requisite information about the coherent structures of the flow field is extracted. The results indicate the existence of spatial structures whose order, size, and orientation are dependent on the Reynolds number and location along the permeable boundary. While the largest scales are marked by sweeps, ejections, and high vortex activity, there is evidence of inward and outward interactive events at the upstream portions of the permeable boundary layer flow. This work helps to clarify some observations made on turbulent flow over the compact permeable boundary.

Pressure from data-driven estimation of velocity fields using snapshot PIV and fast probes

The most explored path to obtain pressure fields from Particle Image Velocimetry (PIV) data roots its basis on accurate measurement of instantaneous velocity fields and their corresponding time derivatives. This requires time-resolved measurements, which are often difficult to achieve due to hardware limitations and expensive to implement. In alternative, snapshot PIV experiments are more affordable but require enforcing physical constraints (e.g. Taylor’s hypothesis) to extract the time derivative of the velocity field. In this work, we propose the use of data-driven techniques to retrieve time resolution from the combination of snapshot PIV and high-repetition-rate sensors measuring flow quantities in a limited set of spatial points. The instantaneous pressure fields can thus be computed by leveraging the Navier–Stokes equations as if the measurement were time-resolved. Extended Proper Orthogonal Decomposition, which can be regarded as one of the simplest algorithm for estimating velocity fields from a finite number of sensors, is used in this paper to prove the feasibility of this concept. The method is fully data-driven and, after training, it requires only probe data to obtain field information of velocity and pressure in the entire flow domain. This is certainly an advantage since model-based methods can retrieve pressure in an observed snapshot, but show increasing error as the field information is propagated over time. The performances of the proposed method are tested on datasets of increasing complexity, including synthetic test cases of the wake of a fluidic pinball and a channel flow, and experimental measurements in the wake of a wing. The results show that the data-driven pressure estimation is effective in flows with compact POD spectrum. In the cases where Taylor’s hypothesis holds well, the in-sample pressure field estimation can be more accurate for model-based methods; nonetheless, the proposed data-driven approach reaches a better accuracy for out-of-sample estimation after less than 0.20 convective times in all tested cases.

Effect of inflow conditions on transonic turbine airfoil limit loading

To better understand airfoil limit loading and the effect inflow conditions have on local efficiency, a computational fluid dynamic investigation was performed for four different transonic turbine airfoils under sub-critical, critical, and supercritical conditions. A computational baseline was established using data previously collected at the Pratt and Whitney Canada High-Speed Wind Tunnel at Carleton University near design conditions using the Reynolds-Averaged Naiver-Stokes shear stress transport k − ω turbulence model with γ transition. The effects of inflow conditions on aerodynamic performance were examined by varying incidence by ± 20°, mainstream turbulence intensity from 5 to 20% and mainstream turbulent length scale from 1 to 100% of the airfoil pitch. Quantitative data of mass-flow averaged Mach numbers, mass-flow averaged flow angles, surface isentropic Mach number distributions and mass-flow averaged total pressure loss coefficients were collected and are presented alongside flow visualizations of numerical Schlieren images to allow for a detailed description of the entire flow domain. Similar to previous experimental work the limit loading pressure ratio and the mass-flow averaged outlet flow angle were strongly correlated with the airfoil outlet metal angle. The influence of inflow conditions was minimal on the exit flow profile with the exception of the mass-flow averaged total pressure loss coefficients. Results show incidence variation to change the total pressure loss coefficient depending on the airfoil, whereas, turbulence intensity and turbulent length scale predicted a drastic increase in loss with increased turbulence level for all airfoils considered.

Numerical simulation of the complex flow during material extrusion in fused filament fabrication

This paper presents a CFD simulation of the non-Newtonian and non-isothermal polymer flow through the nozzle of a fused filament fabrication printer. The modeling approach is based on a melt fraction model which allows to cover the entire flow domain from the solid filament entering the nozzle to the melt state at the nozzle exit. The numerical results were compared with a specially developed setup that allows the measurement of the feeding force required to push the filament through the nozzle. The comparison is carried out over a wide range of filament velocities using different nozzle temperatures and nozzle geometries. It is shown that the assumptions used for the simulation are suitable to predict the melting and flow behavior in the relevant processing range. The model represents both the process region where the filament has sufficient time to melt before reaching the nozzle tip and the region of high filament velocities where there is insufficient time for complete melting. In the latter region, the model predicts that the molten region is reduced to a thin melt film at the tip of the filament as it is pushed against the conical transition zone of the nozzle. Furthermore, the simulation results show that the temperature at the nozzle exit decreases with increasing filament velocity. Based on this modeling approach, melting within the additive manufacturing process of material extrusion can be improved. Furthermore, the findings can be used to optimize the strand deposition.

Secondary instability of channel-confined transition around dual-circular cylinders in tandem

The incompressible viscous flows around the dual-circular cylinders in tandem confined in a plane channel were simulated by direct numerical simulations (DNS) with the Reynolds numbers (Re) at 200, 400 and 800. The blocking ratio and cylinder center-to-center distance are set at 1/4 and 2.5. The secondary instabilities and the corresponding vortex evolution in transition were investigated by the dynamics of third-generation vortex identification quantities of liutex scalar and vector, which are governed by the vorticity transport equations, or Helmholtz equations, and the liutex-shear decomposition. The studies, for the first time, discover the influence of liutex stretching effect on the rigid rotating strength, which gives rise to the two-dimensional non-uniform deformations of the liutex isosurface in the secondary transition. The simulations also demonstrate that the liutex vector is mainly produced by the newly-defined shear allocating effect at the early stage of vortex generation. The quasi-streamwise vortices in the entire flow domain are found being generated in the region where the shear presents a strong strength. With the distinctive elliptic instability in nature, the vortices in the gap are confined by the cylinder shear layers for the Re=400 flow and the vortex bulges structures are firstly observed as the start to transit from the 2-D transient vortex to the more complex 3-D vortical structures in gap, namely the secondary instability. In the wake region, the study reported the two prominent developing stages of Mode B vortex pairs corresponding to the shear instability and vortex stretching, which are governed by their own corresponding balancing mechanisms in the liutex vector dynamic equations. The similar two-stages developing processes are also confirmed in the bluff-body wake transitions with the Mode B instability in nature. With the increase of Re, the shear layers start to interact with the gap vortices and the new secondary-instability is found causing by the hairpin vortices, which are originated from the gap and subsequently lead to the rapid three-dimensional transitions in both gap and wake regions. These studies of vortex structures and dynamics push the front of the current understandings regarding the secondary instability and the subsequent transition to fully-developed turbulence.

Fluid Flow Characteristics of Healthy and Calcified Aortic Valves Using Three-Dimensional Lagrangian Coherent Structures Analysis

Aortic valve calcification is an important cardiovascular disorder that deteriorates the accurate functioning of the valve leaflets. The increasing stiffness due to the calcification prevents the complete closure of the valve and therefore leads to significant hemodynamic alterations. Computational fluid dynamics (CFD) modeling enables the investigation of the entire flow domain by processing medical images from aortic valve patients. In this study, we computationally modeled and simulated a 3D aortic valve using patient-specific dimensions of the aortic root and aortic sinus. Leaflet stiffness is deteriorated in aortic valve disease due to calcification. In order to investigate the influence of leaflet calcification on flow dynamics, three different leaflet-stiffness values were considered for healthy, mildly calcified, and severely calcified leaflets. Time-dependent CFD results were used for applying the Lagrangian coherent structures (LCS) technique by performing finite-time Lyapunov exponent (FTLE) computations along with Lagrangian particle residence time (PRT) analysis to identify unique vortex structures at the front and backside of the leaflets. Obtained results indicated that the peak flow velocity at the valve orifice increased with the calcification rate. For the healthy aortic valve, a low-pressure field was observed at the leaflet tips. This low-pressure field gradually expanded through the entire aortic sinus as the calcification level increased. FTLE field plots of the healthy and calcified valves showed a variety of differences in terms of flow structures. When the number of fluid particles in the healthy valve model was taken as reference, 1.59 and 1.74 times more particles accumulated in the mildly and severely calcified valves, respectively, indicating that the calcified valves were not sufficiently opened to allow normal mass flow rates.

An insulating plate drifting over a thermally convecting fluid: the effect of plate size on plate motion, coupling modes and flow structure

Abstract

Characteristics of complex interaction between two oppositely-moving laminar buoyant-plumes within slender vertical-channels

The article presents laminar plumes, cellular structures and spatial-waviness in free convective flow within a vertical channel under anti-symmetric heating, and the subtle flow-features during the transition to anti-asymmetry. We perform numerical simulations for a wide range of cross-sectional Grashof numbers (262.4≤ Grb ≤33025), channel aspect ratio (0.0375≤ψ ≤0.1255) and isothermal boundary conditions (anti-asymmetric heating to anti-symmetric heating). The chosen working fluid is air; hence the Prandtl number (Pr) is fixed at 0.715. Certain modified parameters are defined for the present study, such as the channel Grashof number (Gr) and symmetry ratio (εC), to govern the working condition of the channel. The physics of the interaction between oppositely moving wall plumes is studied thoroughly, and the highly sensitive, spatially-oscillatory nature of the flow near the plume interface is shown with the help of velocity and temperature contours. Analytical expressions for the primary velocity and temperature have been derived for the wall plumes for the developed region. Also, the crucial role of the secondary velocity field on the heat and mass transfer across the plume interface and convection cell formation is highlighted, which fails to find much attention in the available literature. Motivated by the flow-features, a novel vorticity-mapping is performed, and the entire flow domain is divided into two regimes, viz. a highly rotational core region and a unidirectional near-wall region, which are separated and illustrated using limiting streamlines. The rotational, stagnant core region seems to expand as Gr increases, resulting in a drop in pumping performance. The Gr is varied by changing both the ψ as well as the hot wall temperature ΘH. The relative effects of these methods on the channel pumping, and thermal performance are studied and an optimum method of Gr variation is proposed for the channel. Finally, the effect of the degree of opposing buoyancy forces is studied by varying the cold wall temperature to attain thermal boundary conditions ranging from anti-asymmetry to anti-symmetry. As the opposing buoyancy increases, the heat transfer becomes more convection dominant near the colder wall and less convection dominant near the hotter wall. Also, the pumping performance seems to be affected adversely, as εC is increased. A peculiar change in the channel performance is also highlighted during the transition to anti-symmetric heating conditions near εC=0.9.

Approximate analytical approach to unsteady conducting micropolar fluid flow in the presence of buoyant forces and reactive species

AbstractThe present analysis studied the significance of free convective flow of an unsteady non‐Newtonian fluid through a semi‐infinite vertical porous plate embedded in a porous medium. The thermal transport equation is enhanced by incorporating the heat source/sink and mass transfer in the chemical reaction parameters. The plate moves at a constant velocity and the velocity near the free stream varies exponentially, which allows perturbation laws. Such a model has relevance to a few industrial and engineering applications. In particular, a micropolar fluid is used as a lubricant in various gadgets, also, in polymeric suspensions as well as in animal blood, the phenomena are used to define the local structure and micromotion of the particles. Transformed nondimensional governing equations are solved analytically employing the perturbation method. Influences of the characterizing parameters are presented via graphs and computations for the coefficients of quantities of interest obtained and shown in a table. Also, validation of the present results in a particular case such as the case of Newtonian fluid is obtained with an earlier study and found to be in good agreement. The major findings are laid down here; an increase in plate velocity enhances the velocity profile near the plate and a destructive chemical reaction restrains the fluid concentration in the entire flow domain.

Hall and ion slip effects on MHD laminar flow of an elastico‐viscous (Walter's‐B) fluid

AbstractWe investigated the magnetohydrodynamic (MHD) laminar flow of an elastico‐viscous electrically conducting (Walter's‐B) fluid through a circular cylinder or pipe, loosely packed with a porous material subjected to Hall and ion‐slip effects. The innovation of the study is to consider the entire flow domain without boundary layer approximation in the governing equations. Fully developed solutions of the velocity and pressure drop are obtained making use of perturbation approximation and computationally discussed with reference to flow governing parameters. It is quite exciting that the elastic parameter almost reduces the speed of the liquid in the center of the channel and then continuously expands into the cylinder. For engineering interest, we found the analytical solution and then computationally discussed for skin friction. The occurrence of a magnetic field and a porous matrix gives a fairly uneven flow between the pipes. Elasticity and suction are resistant to experience greater skin friction and are therefore useful for controlling flow separation. A porch has been made to include studies of non‐Newtonian fluids with Hall and ion‐slip effects due to the vast number of possible engineering applications, like power generators, MHD accelerators, refrigeration coils, electric transformers, and heating elements.

A general definition of formation time for starting jets and forced plumes at low Richardson number

As an important dimensionless parameter for the vortex formation process, the general form of the formation time defined by Dabiri (Annu. Rev. Fluid Mech., vol. 41, 2009, pp. 17–33) is refined so as to provide better normalization for various vortex generator configurations. Our proposed definition utilizes the total circulation over the entire flow domain rather than that of the forming vortex ring alone. It adopts an integral form by considering the instantaneous infinitesimal increment in the formation time so that the effect of temporally varying properties of the flow configuration can be accounted for properly. By including the effect of buoyancy, the specific form of the general formation time for the starting forced plumes with negative and positive buoyancy is derived. A theoretical prediction based on the Kelvin–Benjamin variational principle shows that the general formation time manifests the invariance of the critical time scale, i.e. the formation number, under the influence of a source–ambient density difference. It demonstrates that the general formation time, based on the circulation production over the entire flow field, could take into account the effect of various vorticity production mechanisms, such as from a flux term or in a baroclinic fluid, on the critical formation number. The proposed definition may, therefore, serve as a guideline for deriving the specific form of the formation time in other types of starting/pulsatile flows.

Drill cuttings transport and deposition in complex annular geometries of deviated oil and gas wells: A multiphase flow analysis of positional variability

In this study, the two-fluid Eulerian–Eulerian multiphase flow model in ANSYS Fluent (v17.1) is adopted for the simulation of cuttings transport in a deviated annular geometry using two different non-Newtonian drilling fluids (described using the power law and Herschel–Bulkley models). The Syamlal-O'Brien interphase momentum exchange coefficient is implemented to capture non-spherical effects of the drill cuttings. The analysis conducted is based on the hypothesis that a position-dependent profile evaluation is expected to yield more insight into the transport process compared to a volume-averaged analysis over the entire flow domain. This is because the adopted simulation approach takes into account the microscopic particle properties which significantly affect the overall particle transport mechanism. However, this requires the application of robust postprocessing functionalities for data extraction from desired annular regions. Particle sizes considered are in the range of 0.002m–0.008m with a sphericity range of 0.5 to 1.0. A rotational effect is incorporated in our model to describe the drillpipe motion in an annular wellbore with a vertical eccentricity of 0.6. The considered geometry contains a vertical, inclined and horizontal section with interconnecting upper and lower bends. The analysis of the particle velocity and concentration profiles revealed that the mud rheology, particle sphericity and particle sizes play vital roles in determining the cuttings removal process. It is also particularly observed that the lower annular region of the upper bend is the most susceptible to particle deposition with the lowest transport velocities observed. Our positional variability analysis has shown that the variable dominance of non-spherical and spherical particles’ velocities significantly depends on the nature of the flow (i.e. dense granular flow or dilute annular flow in the upper and lower sections, respectively).