Flow Computational Fluid Research Articles

Numerical flow simulation of pump stability for pumped storage units with analysis and visualization of the dynamic flow patterns

With the increasing participation of alternative energy conversion sources in the energy market, the demand for energy storage and grid regulation services keep steadily growing. Pumped storage stations stand out currently as the most suitable and effective solution for these purposes. This leads to tougher requirements regarding the operation flexibility, regulation band and operating range of pumped storage units with reversible pump-turbines or ternary sets (with single or multi-stage pumps), operating at fixed or variable speed. Through the extension of the pump stable region, the pump regulation band can be extended (assuming the proper cavitation safety is provided) and in the case of reversible units the design of the hydraulic machine for turbine mode operation can be accordingly adjusted. The objective here is to model and examine details of the flow phenomena occurring as the pump goes through the unstable region at flows below minimum operating flow. A detailed understanding of the flow phenomena in this region can provide insights to improve the hydraulic design and extend the stable region of operation. Traditional model tests only provide data about the pump characteristic curve. The focus here will be transient computational fluid dynamic (CFD) simulations of a pump-turbine as it goes through the unstable low flow pumping region. The finite volume model includes the complete hydraulic machine from draft tube to spiral case and hybrid turbulence models were used, in this case scale adaptive simulation (SAS). For the analysis and assessment of the transient fluid flow at the different pump operating conditions, integral quantities such as the lift head were evaluated, the velocity field was visualized with flow streamlines, the pressure field with contour plots at the hydraulic surfaces and the vortical structures were identified, quantified and plotted using the criterion. The transient fluid flow calculation was started at an operating point in the pump stable operating range. The simulated pump flow was progressively reduced during the given numerical simulation until the set-in of the pump instability with the break-down of the lift head and pump flow continued to be reduced until the lift head started rising again. The numerical simulations were carried out for a low specific-speed reversible pump-turbine. Detailed model test results were available for the selected design and have been used to validate the numerical simulations.

Upstream events dictate interfacial slip in geometrically converging nanopores.

Continuum computations of fluid flow in conduits approaching molecular scales are often executed with a certain level of abstractions via the imposition of a pre-defined slip condition at the wall. However, in reality, the interfacial slip may not be affixed a priori as a direct one-to-one mapping with the surface wettability and charge but is implicitly interconnected with the concomitant dynamical events that may be effectively captured only under flow conditions. The flow in nanofluidic channels with axially varying cross sections hallmarks such situations in which the effective slip at the wall gets dynamically modulated by upstream flow conditions and cannot be trivially stamped as guided by localized intermolecular interactions over interfacial scales alone. In an effort to capture such flows without resorting to full-domain molecular dynamics simulations, here we bring out advancements on hybrid molecular-continuum simulations and report predictions that closely capture molecular dynamics based predictions of water transport through converging nanopores. Our results turn out to be of significant implications toward designing of emerging nanoscale devices of multifarious applications ranging from miniaturized reactors to highly targeted drug delivery systems.

Flow in a Simulated Turbine Blade Cooling Channel With Spatially Varying Roughness Caused by Additive Manufacturing Orientation

Abstract Because of the effects of gravity acting on the melt region created during the laser sintering process, additively manufactured surfaces that are pointed upward have been shown to exhibit roughness characteristics different from those seen on surfaces that point downward. For this investigation, the roughness internal flow tunnel (RIFT) and computational fluid dynamics models were used to investigate flow in channels with different roughness on opposing walls of the channel. Three rough surfaces were employed for the investigation. Two of the surfaces were created using scaled, structured-light scans of the upskin and downskin surfaces of an Inconel 718 component which was created at a 45 deg angle to the printing surface and documented by Snyder et al. (2015). A third rough surface was created for the RIFT investigation using a structured-light scan of a surface similar to the Inconel 718 downskin surface, but a different scaling was used to provide larger roughness elements in the RIFT. The resulting roughness dimensions (Rq/Dh) of the three surfaces used were 0.0064, 0.0156, and 0.0405. The friction coefficients were measured over the range of 10,000 < ReDh < 70,000 for each surface opposed by a smooth wall and opposed by each of the other rough walls. At multiple ReDh values, x-array hot-film anemometry was used to characterize the velocity and turbulence profiles for each roughness combination. The friction factor variations for each rough wall opposed by a smooth wall approached complete turbulence. However, when rough surfaces were opposed, the surfaces did not reach complete turbulence over the Reynolds number range investigated. The results of inner variable analysis demonstrate that the roughness function (ΔU+) becomes independent of the roughness condition of the opposing wall providing evidence that Townsend’s hypothesis holds for the relative roughness values expected for additively manufactured turbine blade cooling passages.

Boundary Vorticity Analysis and Shedding Dynamics of Transient Cavitation Flow Around a Twisted Hydrofoil

AbstractThe objective of this paper is to investigate the dynamic characteristics of transient cavitating flow over a twisted NACA0009 hydrofoil. The large eddy simulation (LES) approach is selected for the computation of fluid flow and the Zwart model is used for the mass transfer due to cavitation. Moreover, the skin-friction coefficient and boundary-vorticity flux (BVF) are used to study the flow separation. Numerical results show that the attached shear layer separates from the boundary layer and then squeezes to form the separation line under the obstruction of the reentrant jet. The analysis based on the terms of vorticity transport equation demonstrates that vortex stretching and vortex dilatation terms dominate the evolution of the multiscale vortex. Moreover, the secondary shedding induced by the side-entrant jet enhances the instability of partial cavities and the underlying mechanism is comprehensively revealed. Furthermore, the feature of the pressure fluctuation indicates that high pressure generated by the cavity collapse at the tail simultaneously propagates to the leading edge and downstream of the hydrofoil. This enhances the intensity of the reentrant jet and side-entrant jet, promoting occurrences of flow separation near the suction surface and cavity shedding to a certain extent.

On the lattice Boltzmann method and its application to turbulent, multiphase flows of various fluids including cryogens: A review

Cryogenic fluids are used in a myriad of different applications not limited to green fuels, medical devices, spacecraft, and cryoelectronics. In this review, we elaborate on these applications and synthesize recent lattice Boltzmann methods (LBMs) including collision operators, boundary conditions, grid-refinement techniques, and multiphase models that have enabled the simulation of turbulence, thermodynamic phase change, and non-isothermal effects in a wide array of fluids, including cryogens. The LBM has reached a mature state over the last three decades and become a strong alternative to the conventional Navier–Stokes equations for simulating complex, rarefied, thermal, multiphase fluid systems. Moreover, the method's scalability boosts the efficiency of large-scale fluid flow computations on parallel clusters, including heterogeneous clusters with graphics card-based accelerators. Despite this maturity, the LBM has only recently experienced limited use in the study of cryogenic fluid systems. Therefore, it is fitting to emphasize the usefulness of the LBM for simulating computationally prohibitive, complex cryogenic flows. We expect that the method will be employed more extensively in the future owing to its simple representation of molecular interaction and consequently thermodynamic changes of state, surface tension effects, non-ideal effects, and boundary treatments, among others.

Validation of a three-phase Eulerian CFD model to account for cavitation and spray atomization phenomena

Cavitation phase change phenomenon appears in many engineering applications, often eroding and damaging surfaces, so deteriorating the performance of devices. Therefore, it is a phenomenon of great interest for the research and industry communities. In this work, three different cavitation models, the Homogeneous Relaxation Model (HRM), the Schnerr and Sauer, and the Kunz, are implemented in a Eulerian multiphase homogeneous flow Computational Fluid Dynamics (CFD) solver previously developed for simulating fully atomized sprays. The improved solver can be used then to study not only cases with cavitation, such a hydrofoil, but also situations where cavitation occurs together with liquid atomization, such as high pressure injection systems. Validation of this solver is carried out for three different cases under diverse operating conditions: a two-dimmensional throttle, a hydrofoil and a single-hole fuel injector. The Reynolds-Averaged Navier-Stokes (RANS) approach is employed for taking into account the turbulence effects. Simulation results are compared to experimental data available in the literature. Among the tested cavitation models, the HRM is the one that provides the best accuracy in the three validation cases. Nevertheless, the onset of cavitation and the area occupied by the vapor cavities are always underpredicted, by all cavitation models in all validation cases. This can be associated to the unsteady and turbulent nature of the cavitation phenomenon. Even so, the computational prediction of several parameters, such as mass flow rate through the nozzles or spray spreading angle, has an error below 5-10%, which proves the capabilities of the solver.

Near-surface real-time seismic imaging using parsimonious interferometry

Results are presented for real-time seismic imaging of subsurface fluid flow by parsimonious refraction and surface-wave interferometry. Each subsurface velocity image inverted from time-lapse seismic data only requires several minutes of recording time, which is less than the time-scale of the fluid-induced changes in the rock properties. In this sense this is real-time imaging. The images are P-velocity tomograms inverted from the first-arrival times and the S-velocity tomograms inverted from dispersion curves. Compared to conventional seismic imaging, parsimonious interferometry reduces the recording time and increases the temporal resolution of time-lapse seismic images by more than an order-of-magnitude. In our seismic experiment, we recorded 90 sparse data sets over 4.5 h while injecting 12-tons of water into a sand dune. Results show that the percolation of water is mostly along layered boundaries down to a depth of a few meters, which is consistent with our 3D computational fluid flow simulations and laboratory experiments. The significance of parsimonious interferometry is that it provides more than an order-of-magnitude increase of temporal resolution in time-lapse seismic imaging. We believe that real-time seismic imaging will have important applications for non-destructive characterization in environmental, biomedical, and subsurface imaging.

Computation of Steady Free Convective Boundary Layer Viscous Fluid Flow and Heat Transfer towards the Moving Flat subjected to Suction/Injection Effects

The steady free convective boundary layer viscous fluid flow and heat transfer towards the moving flat plate is investigated. It is important to study the fluid flow and heat transfer problems in the presence of suction and injection effects due to an extensive variety of applications in engineering and industry. Thus, the main objective of the present study is to analyse the impacts of the suction and injection parameter on the velocity and temperature of the fluid as well as the skin friction and the Nusselt number coefficients. The problem has coupled partial differential equations which are converted into ordinary differential equations by employing similarity transformation and adopting of the strong wall suction. The ordinary differential equations thus obtain are handled numerically by utilizing Maple software simulation. The main findings concerning the behaviours of velocity and temperature against various values of the emerging physical parameter such as suction/injection are presented clearly in this numerical examination via graphical illustrations whose physical explanation are discussed thoroughly based on a strong theoretical basis. Furthermore, skin friction and Nusselt number results are studied at different values of pressure gradient. The detail geometry reveals that the velocity and temperature of the fluid decreases with increase of suction/injection parameter. Both skin friction coefficient and Nusselt number decreases with an increase of suction/injection parameter.

Ocean Energy Systems Wave Energy Modeling Task 10.4: Numerical Modeling of a Fixed Oscillating Water Column

This paper reports on an ongoing international effort to establish guidelines for numerical modeling of wave energy converters, initiated by the International Energy Agency Technology Collaboration Program for Ocean Energy Systems. Initial results for point absorbers were presented in previous work, and here we present results for a breakwater-mounted Oscillating Water Column (OWC) device. The experimental model is at scale 1:4 relative to a full-scale installation in a water depth of 12.8 m. The power-extracting air turbine is modeled by an orifice plate of 1–2% of the internal chamber surface area. Measurements of chamber surface elevation, air flow through the orifice, and pressure difference across the orifice are compared with numerical calculations using both weakly-nonlinear potential flow theory and computational fluid dynamics. Both compressible- and incompressible-flow models are considered, and the effects of air compressibility are found to have a significant influence on the motion of the internal chamber surface. Recommendations are made for reducing uncertainties in future experimental campaigns, which are critical to enable firm conclusions to be drawn about the relative accuracy of the numerical models. It is well-known that boundary element method solutions of the linear potential flow problem (e.g., WAMIT) are singular at infinite frequency when panels are placed directly on the free surface. This is problematic for time-domain solutions where the value of the added mass matrix at infinite frequency is critical, especially for OWC chambers, which are modeled by zero-mass elements on the free surface. A straightforward rational procedure is described to replace ad-hoc solutions to this problem that have been proposed in the literature.

Meta-Heuristic Algorithms-Tuned Elman vs. Jordan Recurrent Neural Networks for Modeling of Electron Beam Welding Process

A boost in the preference of high energy beam, such as electron beam, laser beam etc. has led to the requirement of its automation through accurate input–output modelling. Modeling of electron beam welding is conducted in the present study through Elman and Jordan recurrent neural networks (RNNs), both having a single feed-back loop, to meet the said requirement. The RNNs are trained using some nature-inspired optimization tools, namely cuckoo search, firefly, flower pollination, and crow search utilizing input–output welding data, obtained from a computational fluid dynamics-based heat transfer and fluid flow welding model. RNN predictions are validated through real experiments. Thus, the effect of change in the position of the feed-back loop on the accuracy of prediction of RNNs is investigated. In addition, a few popular statistical tests have been used to evaluate the performances of the RNNs tuned by various optimization algorithms, where flower pollination-tuned Jordan RNN is observed to yield the best results.

Parallel finite volume method-based fluid flow computations using OpenMP and CUDA applying different schemes

Parallelization of inhouse develpoed code for numerical computations on heterogeneous computing machine is becoming common. As the numerical solvers and problem complexity are evolving, the parallel computing facilities are also growing. This research study deals with the use of four different schemes to parallelly compute the numerical flow equations based on the finite volume method. The different schemes proposed are applied for parallelization using central processing units and graphical processing units. Open multiprocessing (OpenMP) and compute unified device architecture (CUDA) are the parallel computing tools used for parallelization of the code applying four schemes, viz. red and black successive over-relaxation (RBSOR), wavefront, combined RBSOR and wavefront, and alternate RBSOR and wavefront scheme. The flow analysis is carried out for internal and external flow at different Reynolds numbers on dissimilar machines having their individual computational capability. Speedup obtained and parallel efficiency achieved using the proposed unusual parallelization method are investigated separately. The grid size for both the flow conditions is fixed during the parallel computation performance analysis. The RBSOR scheme provided the maximum speedup in all cases of flow, scheme, and tool used. The wavefront scheme provides the lowest speedup and parallel efficiency. The alternate scheme is better than the wavefront scheme and combined scheme using OpenMP. The speedup achieved and parallel efficiency obtained for the CUDA parallelized code are in the range of 200 × and 70%, respectively, applying the RBSOR scheme.

A Partitioned Solution Algorithm for Concurrent Computation of Stress–Strain and Fluid Flow in Continuous Casting Process

Control of macrosegregation phenomena and deformation-related defects is a main issue in steel continuous casting. Numerical simulation could help industrial engineers to master these defects. However, as a first step, it is essential to achieve a concurrent computation of fluid flow in the bulk liquid and stress-strain evolution in the already solidified regions. With this aim in view, a new specific partitioned solver has been developed to model the liquid flow, essentially induced by the inlet jet distributed by the submerged nozzle, as well as the thermal deformation of the solid shell. The solver procedure allows simulating the transient regime, up to convergence to the steady-state regime. For this purpose, the computational finite element mesh moves and grows continuously. Within this evolving mesh, three different zones are defined: the solid shell as a pure Lagrangian zone, the liquid nozzle region as a pure Eulerian zone, and an intermediate Eulerian–Lagrangian zone. Conservation equations (energy, mass, and momentum) are solved in a general arbitrary Lagrangian–Eulerian framework, with a level-set formulation to track the free surface evolution at the meniscus. The article is composed of two parts. In the first part, the model is detailed with the resolution steps involved in the coupled resolution approach. In the second part, a simple verification test case is firstly proposed, followed by a more relevant and practical application to model an industrial pilot continuous casting process.

Optimization of the co-axial dispensing nozzle of a 3D bioprinter for the fabrication of tubular structures with micro-channel encapsulation

The 3D bio-printing has been developed as an effective approach to artificially create tubular tissue structures, which have been frequently found in body and organ systems. China Agricultural University (CAU) has developed a laboratory 3D bio-printer that can create tubular structures with the encapsulation of microfluidic channel. In order to create a tubular structure with more effective micro-channel encapsulation for better nutrient delivery and chemical stimulation, this work presents a design optimization of co-axial dispensing nozzle of this 3D bio-printer. In this study, an experimentally validated two-phase flow computational fluid dynamics modeling tool based on ANSYS CFX has been developed to analyze the microfluidic domain of fluid channels inside the nozzle. The simulations on the extrusion and encapsulation process of bio-inks have been conducted. Based on the response surface method, the simulation work has established an equation to predict the volume fraction of the encapsulating layer against a variety of influencing factors including the size of extrusion nozzle, pneumatic pressure condition and the dynamic viscosity of the bio-ink. This equation has been used to recommend optimal solutions of printing parameters for the CAU bio-printer, which is expected to improve the quality of bio-printed tubular structure with an encapsulation.

Mixing performance and continuous production of nanomaterials in an advanced-flow reactor

The Corning Advanced-FlowTM Reactor (AFR) whose productivity is ready for scale-up with limited loss of transport properties was implemented in the continuous synthesis of Ag nanoparticles (NPs). To demonstrate the effect of mixing on Ag NPs, the flow pattern and mixing characteristic (mainly on the meso- and macroscale) in AFR were studied by flow visualisation and computational fluid dynamics simulation first. It was found that increasing the total flow rate from 1 to 9 mL·min−1 could enhance mixing by the synergetic combination of secondary flows created by different mechanisms. The flow rate ratio in the range of 0.25–8 affected the degree of mixing by changing the initial concentration distribution profile across the combined stream. The highest degree of mixing was obtained at the flow rate ratios of 0.25 and 8. Subsequently, the micromixing performance was quantified by Villermaux-Dushman method. The micromixing time decreased from 17 to 4 ms as the total flow rate increased from 1 to 9 mL·min−1, while the micromixing time was nearly independent of the flow rate ratio. At last, the link between the mixing characteristics and average size and particle size distribution (PSD) of Ag NPs was established. As the total flow rate increased, the average particle size decreased, and PSD became narrower due to better micromixing efficiency. When the flow rate ratio varied, the PSD of Ag NPs was dependent on the width of the Ag precursor stream rather than the degree of mixing, because an excessive amount of NaBH4 was used.

3D printed Ti6Al4V bone scaffolds with different pore structure effects on bone ingrowth

The microstructure of porous scaffolds plays a vital role in bone regeneration, but its optimal shape is still unclear. In this study, four kinds of porous titanium alloy scaffolds with similar porosities (65%) and pore sizes (650 μm) and different structures were prepared by selective laser melting. Four scaffolds were implanted into the distal femur of rabbits to evaluate bone tissue growth in vivo. Micro-CT and hard tissue section analyses were performed 6 and 12 weeks after the operation to reveal the bone growth of the porous scaffold. The results show that diamond lattice unit (DIA) bone growth is the best of the four topological scaffolds. Through computational fluid dynamics (CFD) analysis, the permeability, velocity and flow trajectory inside the scaffold structure were calculated. The internal fluid velocity difference of the DIA structure is the smallest, and the trajectory of fluid flow inside the scaffold is the longest, which is beneficial for blood vessel growth, nutrient transport and bone formation. In this study, the mechanism of bone growth in different structures was revealed by in vivo experiments combined with CFD, providing a new theoretical basis for the design of bone scaffolds in the future.

The Galerkin least squares MLPG method for convection-dominated problems

The meshless method has been widely applied in the computational mechanics, materials science, computational heat transfer and fluid flow. However, the development of a high efficient method for convection term in the computational fluid dynamics is difficult. In this paper, a truly meshless method of the meshless local Petrov-Galerkin (MLPG) method was applied to solve convection-diffusion problem. Meanwhile, the Galerkin least squares (GLS) approximation method was proposed to overcome the influence of convection term. The accuracy and efficiency of the present method were validated by some cases with benchmark solutions. The computational results showed that the GLS method could be used in dealing with convection-diffusion problems with high computational precision.

The Galerkin least squares MLPG method for convection-dominated problems

The meshless method has been widely applied in the computational mechanics, materials science, computational heat transfer and fluid flow. However, the development of a high efficient method for convection term in the computational fluid dynamics is difficult. In this paper, a truly meshless method of the meshless local Petrov-Galerkin (MLPG) method was applied to solve convection-diffusion problem. Meanwhile, the Galerkin least squares (GLS) approximation method was proposed to overcome the influence of convection term. The accuracy and efficiency of the present method were validated by some cases with benchmark solutions. The computational results showed that the GLS method could be used in dealing with convection-diffusion problems with high computational precision.

Investigation of fluid flow velocity within the intervertebral disc

Enhancing fluid flow velocity within the intervertebral disc may allow to increase solute transportation rates and improve disc nutrition as the sufficient supply of the nutrients to cells of intervertebral disc is a key factor in order to avoid or delay processes of disc degeneration. Poroelastic finite element model of lumbar intervertebral disc is used to calculate fluid flow velocity with the intervertebral disc due to flexion, extension, lateral bending and axial rotation. When comparing the average of flow velocity, the highest values are calculated when flexion and extension moments were applied to the disc, but lateral bending induces the highest value of velocity component in Y direction in nucleus pulposus, yet the assumption that lateral bending could be more beneficial in order to improve nutrients supply to intervertebral disc cannot be substantiated only by this data, as calculated values of velocity component Y are significantly lower that total velocity values.

Simulation of Copper Electrodeposition in Millimeter Size Through-Silicon Vias

Computational predictions of copper deposition in millimeter size through-silicon vias (mm-TSV) are presented based on localized breakdown of a co-adsorbed polyether-chloride suppressor layer. The model builds upon previous work on localized Cu deposition in microscale TSV and through-holes by incorporating 3D fluid flow calculations to more effectively evaluate chemical transport of cupric ion and additives, both of which are critical to adlayer formation and disruption within the via. Simulations using potentiostatic and potentiodynamic waveforms are compared to previously reported filling experiments. Alternatively, the utility of galvanostatic control and variations in fluid flow are explored computationally. For appropriate applied potential(s) or current, deposition is localized to the via bottom, with subsequent growth proceeding in a bottom-up fashion. Selection of inappropriate current or potential waveforms, or forced convection conditions that supply insufficient cupric ion to the bottom of the via, results in prediction of voids. Simulations of deposition in via arrays (4 × 1) predict non-uniform growth across the arrays, with the passivation of individual vias associated with minor variations in convective flow and/or numerical perturbations in the simulation, that reflects the critical nature of the bifurcation process.

Modal decomposition methods for distributed excitation force field on tube bundle in cross flow

To rapidly and accurately identify the characteristics of the distributed force and reconstruct the force field on tube bundles in a cross flow, modal decomposition methods for a three-dimensional turbulent flow are studied. This is done with proper orthogonal decomposition (POD), dynamic mode decomposition (DMD), and the regularization method for DMD. A 3-D turbulent cross flow computational fluid dynamics model is simulated to obtain the temporal–spatial force field acting on the tubes. Applying direct Fourier transformation to the data yields the frequency contents of the field as a reference for other analyses. POD provides high-accuracy reconstruction of the force field with 43 modes, and each mode contains multiple frequency contents of the Fourier transformation. In comparison, DMD successfully demonstrates the relation between single frequencies and the spatial distribution on the tube surface. To solve the ill-posedness of singular value decomposition (SVD), the key mathematical processing of both decomposition methods, the Tikhonov regularization method is introduced into field reconstruction. The regularization operator and parameter determined by solving a functional extremum problem yield a criterion that can be applied to the SVD of the DMD, which effectively reduces the number of DMD modes required to reconstruct the field to 16 with the same level of norm error.