Three-dimensional Flow Phenomena Research Articles

On the Darcy–Forchheimer flow of carbon nanotubes nanofluid across a stretching surface for the impact of heat source/sink and Ohmic heating

This research leads to carrying out the productivity and the efficiency of the carbon nanotubes (CNTs) that have extensive applications in solar collectors. Due to the superior thermal as well as electrical properties, the use of CNTs has an important contribution to the nanotechnology revolution. Therefore, owing to the aforementioned vital points, this investigation intended to put forth the thermophysical properties of both single and multi-walled CNT nanofluids past a stretching surface. Additionally, an electrically conducting nanofluid flow phenomenon enriches due to the inclusion of dissipation (Ohmic heating) and external heat source/sink. The dimensional form of the three-dimensional fluid flow phenomena is transformed to a non-dimensional form with the use of similarity transformation and further numerical procedure is implemented to solve the nonlinear governing equations. The substantial significance of the characterizing parameters is presented briefly via figures and the comparative analysis with the earlier investigation is deployed through the table. However, the main findings of this study are as follows: A significant attenuation in the shear rate is marked for the enhanced inertial drag but it augments for the augmented values of the magnetization; further, particle concentrations of both the CNTs favor accelerating the fluid momentum as well as temperature distribution.

A graphics-accelerated deep neural network approach for turbomachinery flows based on large eddy simulation

In turbomachinery, strongly unsteady rotor–stator interaction triggers complex three-dimensional turbulent flow phenomena such as flow separation and vortex dynamics. Large eddy simulation (LES) is an advanced numerical method that has recently been used to resolve large-scale turbulent motions and model subgrid-scale turbulence in turbomachinery. To largely reduce the computing cost of LES for turbomachinery flow, a graphics processing unit (GPU)-accelerated deep neural network-based flow field prediction approach is explored, which combines convolutional neural network autoencoder (CNN-AE) with long short-term memory (LSTM). CNN-AE extracts spatial features of turbomachinery flow by mapping high-dimensional flow fields into low-dimensional space, while LSTM is used to predict the temporal evolution of fluid dynamics. Automatic mixed precision (AMP) is employed to achieve rapid neural network training using Nvidia GTX 1080 Ti GPU, which shows a significant speedup compared with that without AMP. We evaluated the proposed CNN-AE-LSTM (CAL) method against gated recurrent units (GRU) and simple recurrent network (SRN) on two types of turbomachinery, i.e., centrifugal and axial flow pumps. The results show that the proposed CAL shows better capability of capturing the vortex structure details of turbomachinery. When predicting the temporal vorticity field, the mean square error of CAL results is 0.105%–0.124% for centrifugal pumps and 0.071%–0.072% for axial flow pumps. Meanwhile, the structural similarity index measure of the CAL results is 92.51%–92.77% for centrifugal pumps and 93.81%–94.61% for axial flow pumps. The proposed CAL is noticeably better than GRU and SRN in terms of both mean square error and structural similarity index measure.

Magnetohydrodynamic squeezing micropolar nanofluid flow confined in parallel disks with implication of Maxwell-Cattaneo law

Time-dependent electrically conducting squeezing flow of micropolar nanofluid is analyzed between two parallel disks. The bottom porous disk is fixed while the upper disk moved orthogonally along axial direction. Nanofluid features such as thermophoresis and the Brownian motion are considered using the Buongiorno theory of nanoparticles. Cattaneo-Christov model (CCM) is executed in the expression of energy equation. Uniform suction/injection is considered at bottom disk. Inclined magnetic field is accounted at an acute angle along axial direction. The constitutive model is normalized by utilizing the similarity functions. The numerical solution of flow model is developed by MATLAB built-in bvp4c function. The physical parameters are explored through graphical depiction, two-dimensional contours, three-dimensional flow phenomenon and streamlines. Since, the micropolar parameters tend to move the fluid particles in opposite way. Therefore, micro-rotational field has an increasing tendency in the left half region and decreasing behavior in the right half plane of the central region. It is obtained that the temperature field is boost up against increased squeezing Reynolds number, parameter of thermal relaxation stress and Prandtl number. Brownian movement and thermophoretic factors have a reverse phenomenon on concentration curves.

Preliminary Design and Numerical Investigation of a Centrifugal Compressor for Supercritical Carbon Dioxide Operating in the Vicinity of Its Critical Thermodynamic State

Waste heat recovery is one of the main practices used to reduce the carbon footprint of the industrial sector regarding environmental concern. The supercritical carbon dioxide (s-CO2) cycle is one of the most attractive heat-to-power technologies; due to the abrupt variation in CO2 properties in the vicinity of its critical point, small compression work is required and finally a high cycle efficiency is achieved. In the literature, among the various proposed layouts, the recompression s-CO2 Brayton cycle is considered to be the most efficient one. The most critical component of such a cycle is definitely the main compressor, as the related usual design procedures have been developed in the past for ideal gas as a working fluid. This study presents a methodology for the preliminary design of a centrifugal compressor with a vaned diffuser, suitable for fulfilling the desired operating requirements of a particular supercritical CO2 recompression Brayton cycle. Furthermore, it demonstrates the numerical investigation of the three-dimensional (3D) flow phenomena occurring in it, focusing on the investigation of possible condensation. To this end, a one-dimensional flow model was developed to provide information regarding the geometry of the compressor and predict its prospective performance. Commercial computational fluid dynamics (CFD) software was then employed to examine the three-dimensional flow. The effect of accuracy in the evaluation of real gas properties approaching the critical point was examined, showing that a look-up table with more points around the critical point can reduce the numerical relative error by up to 0.3% for the value of specific heat capacity. In addition, the possibility of condensation occurrence was investigated at the impeller’s inlet, where the flow is accelerated. The supersaturation pressure ratio was defined and implemented in order to identify regions where static pressure is lower than saturation pressure, possibly leading to local two-phase flow.

Swirling flow analysis of Eyring–Powell fluid between coaxial disks with variable property

Abstract The main objective of this study is to examine an unsteady swirling flow of a non-Newtonian Eyring–Powell fluid between two coaxial disks. The lower and upper disks are considered to rotate with different angular velocities. The three-dimensional axisymmetric flow phenomenon and heat transfer mechanism are observed with the consequences of the magnetic field and variable thermal conductivity of the fluid. The variable thermal conductivity is taken to be dependent on the fluid temperature. The implementation of the Von Karman similarity transformations on the constituting equations of the flow phenomenon yields the dimensionless system of the non-linear equations. An optimal homotopy analysis technique is adopted to obtain analytical solutions for highly non-linear equations. In view of the same and opposite directions of disks rotation, the various aspects of the flow system corresponding to the pertinent parameters are discussed with physical significance. The obtained results indicate that both radial and axial fields are the escalating functions of the Eyring–Powell fluid parameter. Moreover, the heat transfer rate enhances with the improving variable thermal conductivity parameter.

Numerical analysis of the three-dimensional flow phenomena in a 19-pin wire-wrapped tight lattice bundle

The wire-wrapped tight lattice assembly has the advantages of compact arrangement, small volume, and high power density, and it has been widely used in the design of advanced nuclear reactors. A wire-wrapped tight lattice assembly has been proposed at Shanghai Jiao Tong University, and the mixing characteristics of this assembly have been experimentally investigated in a 19-pin wire-wrapped bundle in the previous work. In the present study, the numerical simulation for this bundle based on the Reynolds-Averaged-Navier-Stokes SST k-ω model has been carried out, and the numerical method was verified by the mixing experiment data. Then the three-dimensional flow phenomena have been discussed for a wide range of Reynolds numbers from 5843 to 100,000. The axial flow split parameters, vortex behaviors in cross-section, and the flow sweeping behaviors in subchannel gaps were analyzed in a typical condition, and the effect of the Reynolds number also has been discussed, which shows little effect on these parameters. This study is a continuation of our previous work, and they both aim to provide a reference for the design of wire-wrapped tight lattice assembly.

Comparison of one-dimensional and three-dimensional glottal flow models in left-right asymmetric vocal fold conditions

While the glottal flow is often simplified as one-dimensional (1D) in computational models of phonation to reduce computational costs, the 1D flow model has not been validated in left-right asymmetric vocal fold conditions, as often occur in both normal and pathological voice production. It is unclear to what extent the 1D model approximates the effect of three-dimensional (3D) flow phenomena and fluid-structure interaction. In this study, we performed 1D and 3D flow simulations coupled with the two-mass vocal fold model and compared vocal fold vibration patterns at different degrees of left-right stiffness asymmetry. The flow and acoustic fields in 3D were predicted by solving the compressible Navier-Stokes equations using the volume penalization method and considering the moving vocal fold wall as an immersed boundary. The results showed that vocal fold vibration amplitudes and left-right phase differences in the 3D flow were predicted by the 1D flow model under conditions of small left-right asymmetry, while vocal fold vibration amplitudes were underestimated at conditions of large left-right asymmetry. This indicates that 1D flow models may be sufficient in modeling phonation under left-right asymmetric conditions, although the performance can be further improved by more accurately predicting air pressure on vocal fold surface.

Experimental evaluation of longitudinal and transverse vortex generators on the endwall of a serpentine passage

A 180-deg turning channel can be seen in many heat exchange systems, and a strong three-dimensional flow phenomenon will occur at the turning point. In order to strengthen the heat transfer inside the turn and considering the lack of relevant enhanced endwall heat transfer approach, the present study innovatively demonstrates the measures of enhanced internal endwall heat transfer using arrays of delta-winglet and conical vortex generators. The studied parameters of the vortex generators include aspect ratio (AR), spacing, and angle of attack, etc. The experimental results indicate that the longitudinal vortex generators have more advantages in increasing the heat transfer than the transverse conical vortex generators in certain configurations with the same order of magnitude of the increased area. Besides, transverse conical vortex generators are more helpful to improve the endwall heat transfer uniformity than longitudinal vortex generators. The array of longitudinal vortex generators with 45-deg angle of attack, 0.02 m pitch, and 2 aspect ratio provides the highest thermal performance for all studied cases, indicating that the longitudinal delta-winglet vortex generators can induce near-wall flow turbulence as well as convective heat transfer of the endwall with a modest pressure loss penalty. For this optimally designed endwall, the Nusselt number is augmented by up to 35%. From the perspective of energy saving, the present studied vortex generators provide a superior thermal performance at identical pumping power, which can be useful in the future design of endwalls in sharp-turn channels.

Reynolds stress tensor and pressure-related turbulence transport terms measured by time-resolved tomographic-PIV

Turbulence is inherently a three-dimensional and time dependent flow phenomenon (Pope, 2001). Because of the ubiquitous existence of turbulent flows in nature, accurate characterization of turbulent flows, either through experimental measurements or through direct numerical simulations, is of paramount importance for modeling turbulence (Liu and Katz, 2018). Since its inception in 1984 (Adrian, 1984), Particle Image Velocimetry (PIV), among several other conventional techniques used for turbulence measurements, has been a valuable tool for providing reliable experimental data for turbulence research. Several advancements in hardware such as high-speed cameras, together with innovative algorithms and procedures, have extended the scope of PIV to a variety of applications. Westerweel et al. (2013) point out in a recent review article that one of the main advantages of the PIV measurement is its unique ability in measuring quantitatively spatial derivatives of the flow field. With the development of Tomographic PIV introduced by Elsinga et al. (2006), it is now possible to measure simultaneously the distributions of three velocity components in a three-dimensional flow field, thus enabling us to measure all the velocity derivatives of a turbulent flow. However, for a thorough characterization of a turbulent flow, in addition to the velocity gradients, the instantaneous pressure distribution in the 3D flow field also needs to be measured.

Characterization of temperature-dependent fluid properties in compressible viscous fluid flow induced by oscillation of disk

Abstract Time-dependent compressible flow of viscous fluid with temperature dependent density, thermal conductivity and viscosity is considered. The motion in fluid is generated due to periodic oscillations of rotating disk. The normalized system of partial differential equations is achieved by adopting similarity transformations. The obtained system is then solved numerically using Successive over Relaxation (SOR) method and finite difference schemes. The local similar solutions are presented. One-dimensional graphs, two-dimensional contours and three-dimensional flow phenomenon are drawn graphically. Tabular description of various physical parameters is also presented.

Finite difference approach and successive over relaxation (SOR) method for MHD micropolar fluid with Maxwell–Cattaneo law and porous medium

This analysis considered the flow of micropolar fluid to describe the micro-rotational influence of fluid particles. The oscillatory motion of stretchable disk is responsible for the fluid flow. The impacts of magnetic field and porous medium are incorporated in a momentum equation. The Maxwell–Cattaneo law of material-invariant version having damped-hyperbolic equation is proposed in order to overcome the limitations of Fourier’s law. The model of three-dimensional flow problem is then normalized by adopting suitable similarity variables. The resulting normalized system of partial differential equations is solved in a numerical way using first and second order central and backward finite difference approximations by converting the semi-infinite domain into the finite one. The resulting expressions are solved iteratively using the successive over relaxation parameter method. The impacts of various dimensionless parameters are elaborated through graphs and tables. Higher values of porosity and magnetic parameters reduced the radial velocity curves. Micropolar parameters, namely vortex and spin gradient viscosity parameters, enhanced the microrotational tangential velocity profiles. Radial shear stresses and frictional torque are decreased due to larger values of the vortex viscosity parameter. Couple stresses along radial and tangential directions are enhanced for increasing values of porosity parameter. The three-dimensional and two-dimensional flow phenomenon is also sketched. The numerical results for the limiting scenario are compared for various physical parameters to validate the numerical scheme.

Crosswise Wind Shear Represented as a Ramped Velocity Profile Impacting a Forward-Moving Aircraft

Abrupt changes in wind velocities over small distances in a lateral or vertical direction can produce wind shear which is known to have serious effects upon the performance of an aircraft. Brought about by large-scale changes in the atmospheric conditions, it is a three-dimensional flow phenomenon imposing severe velocity gradients on an aircraft from all possible directions. While it would be difficult to model an instantaneous velocity gradient in a lateral plane, a vortical flow impinging from the sides which represents a wind shear in a vertical direction is imposed on a forward-moving aircraft to investigate the effect on the aerodynamic performance. The maximum shear wind speed from the side was fixed at 0.3 times the forward velocity. After due validations under no-wind shear conditions on simpler half-reflection plane models, a BGK airfoil-based full 3D wing and the ONERA M6 3D wing model were selected for preliminary studies. The investigation was concluded using the ARA M100 wing-fuselage model.

Utilization of Maxwell-Cattaneo law for MHD swirling flow through oscillatory disk subject to porous medium

The present study aims to investigate the salient features of incompressible, hydromagnetic, three-dimensional flow of viscous fluid subject to the oscillatory motion of a disk. The rotating disk is contained in a porous medium. Furthermore, a time-invariant version of the Maxwell-Cattaneo law is implemented in the energy equation. The flow problem is normalized by obtaining similarity variables. The resulting nonlinear system is solved numerically using the successive over-relaxation method. The main results are discussed through graphical representations and tables. It is perceived that the thermal relaxation time parameter decreases the temperature curves and increases the heat transfer rate. The oscillatory curves for the velocity field demonstrate a decreasing tendency with the increasing porosity parameter values. Two- and three-dimensional flow phenomena are also shown through graphical results.

Water surface topology of supercritical junction flow

Abstract The complexity of flow conditions at junctions amplifies significantly with supercritical flow. It is a pronounced three-dimensional two-phased flow phenomenon, where standing waves with non-stationary water surface are formed. To analyse the hydrodynamic conditions at an asymmetric right-angled junction with incoming supercritical flows at Froude numbers between 2 and 12, an experimental approach was used. For a phenomenological determination of the relations between the integral parameters of incoming flows and the characteristics of standing waves at the junction area, water surface topographies for 168 scenarios at the junction were measured using non-intrusive measurement techniques. The new, phenomenologically derived equations allow for determination of location, height and extent of the main standing waves at the junction. Research results give important information on the processes and their magnitude for engineering applications.

Numerical Analysis of an Instrumented Turbine Blade Cascade

The measurement accuracy of the temperature/pressure probe mounted at the leading edge of a turbine/compressor blade is crucial for estimating the fuel consumption of a turbo-fan engine. Apart from the measurement error itself, the probe also introduces extra losses. This again would compromise the measurement accuracy of the overall engine efficiency. This paper utilizes high-fidelity numerical analysis to understand the complex flow around the probe and quantify the loss sources due to the interaction between the blade and its instrumentation. With the inclusion of leading-edge probes, three-dimensional flow phenomena develop, with some flow features acting in a similar manner to a jet in cross flow. The separated flow formed at the leading edge of the probe blocks a large area of the probe bleedhole, which is one of the reasons why the probe accuracy can be sensitive to Mach and Reynolds numbers. The addition of 4% free-stream turbulence is shown to have a marginal impact on the jet trajectory originated from the probe bleedhole. However, a slight reduction is observed in the size of the separation bubble formed at the leading edge of the probe, preceding the two bleedhole exits. The free-stream turbulence also has a significant impact on the size of the separation bubble near the trailing edge of the blade. With the addition of the free-stream turbulence, the loss observed within the trailing edge wake is reduced. More than 50% of the losses at the cascade exit are generated by the leading-edge probe. A breakdown of the dissipation terms from the mean flow kinetic energy equation demonstrates that the Reynolds stresses are the key terms in dissipating the counter-rotating vortex pairs with the viscous stresses responsible for the boundary layer.

Experimental Investigation of Local Film Thickness and Velocity Distribution Inside Falling Liquid Films on Corrugated Structured Packings

Understanding the complex fluid dynamics of the liquid phase in structured packing is crucial for the prediction of the separation performance of packed tower and for further optimization of the packing geometry. Detailed numerical investigations are useful to study a wide range of parameters, but require high computational effort and validation is required. Therefore, in this study, experimental investigations on the gravity driven liquid film flow phenomena over corrugated structured packing geometries are presented and evaluated. Due to the fragile nature of thin liquid films, nonintrusive optical measurements where performed through the backside of transparent moldings of packing geometries, thereby reducing optical distortion. To enable the investigation of three-dimensional flow phenomena a µ-Stereoscopic Particle Image Velocimetry (µSPIV) was adapted for thin liquid films. Measurements are performed on MONTZ-Pak B1.250 macro-structure with a smooth surface or a micro-structured surface at different locations. Values of film thicknesses and velocity distributions inside the falling liquid film are discussed in detail.

Microfluidic effects on the heat transfer enhancement and optimal design of microchannels heat sinks

Due to the rapid progress in miniaturization and utilization of low-dimensional structures, the optimal design of microchannel heat sinks has received a great deal of attention. However, the concept of ‘micro’ has been solely used as a word and therefore, microchannels are normally modeled just like the (macro-scale) channels. The aim of this article is to provide an optimal geometric structure of a micro-scale channel heat sink, by assuming slip boundary condition, instead of no-slip boundary condition which breaks down in the rigid walls of a low-dimensional microchannel or a channel. The optimization objective is to reduce the maximum temperature of the system (or to minimize the thermal resistance) so that the pressure drop in the channel is kept to a desirable level. Three-dimensional fluid flow and heat transfer phenomena inside the microchannel heat sink are investigated numerically. To reveal the effects of velocity slip on the optimal design of a microchannel heat sink, the optimization results (optimal geometric parameters and the minimal thermal resistance) assuming slip boundary condition are compared with the optimization results assuming no-slip condition. Compared with the case of no-slip boundary condition, it is indicated that the slippage reduces the minimum thermal resistance drastically and changes the geometric design points such as the optimal number and the optimal aspect ratio of the microchannels. For example, it is shown that the aspect ratio of the microchannel is enlarged and the microchannel tends to be narrow when the slip parameter increases.

CFD investigation on thermal-hydraulic behaviors of a wire-wrapped fuel subassembly for sodium-cooled fast reactor

Different from the square-arrayed fuel rod bundles in pressurized water reactors, the fuel subassembly for sodium-cooled fast reactors features with wire spacers helically wrapped on the fuel rod. This intricate geometry consequently imposes a significant challenge in numerical simulation. In the present study, a new marked and separated meshing method is developed to reduce the required mesh and modeling time. Instead of directly generating the complex geometry and mesh, the hexahedral wire wrapper mesh was dealt with locus equations. First, the numerical modeling method was validated by combining the experimental data of a 19-pin wire-wrapped rod bundle and the simplified Cheng and Todreas correlation. Furthermore, a comparative study of different meshing methods was conducted. Then, an investigation on the three-dimensional flow phenomenon and heat transfer characteristics of a 61-pin wire-wrapped fuel subassembly for the China Experimental Fast Reactor was carried out. Finally, complex axial and transverse flow behaviors and temperature field in different types of subchannels were quantitatively analyzed. Results indicate that the axial velocity exhibits periodic oscillation with a maximum normalized value of 1.54. The flow oscillation in the interior subchannels presents a smaller amplitude and higher frequency than that in the corner and edge subchannels. Local hot spots occur in the gaps beneath the spacer wires. Sweeping and cross flow patterns exist in the peripheral and interior subchannels, respectively. In addition, the secondary transverse flow induced by the spacer wires is stronger in the corner subchannel. The cross flow strength reaches a peak value equal to 30% of the average axial velocity, which promotes the thermal mixing.

CFD investigation of three-dimensional flow phenomena in a JAEA 127-pin wire-wrapped fuel assembly

This paper presents a RANS (Reynolds Averaged Navier-Stokes simulation) based CFD (Computational Fluid Dynamics) investigation of the three-dimensional flow phenomena in a JAEA (Japan Atomic Energy Agency) 127-pin wire-wrapped fuel assembly. Complicated and vortical flow phenomena in the wire-wrapped fuel bundles have been elucidated by a CFD analysis with a high resolution scheme and a shear stress transport (SST) turbulence model, and by a vortex structure identification technique based on the critical point theory. The CFD results show a good agreement with the JAEA experiment results with the 127-pin wire-wrapped fuel assembly, which was implemented using water for validating the pressure drop formulas in the JAEA sub-channel analysis code, ASFRE. The generation and demise of the vortex structures in the edge, corner, and interior sub-channels were periodically synchronized with the angular positions of the wire spacers. The axial and tangential flows in the edge and corner sub-channels are much stronger than those in the interior sub-channels. The vortex structures in the edge sub-channels have higher axial and tangential velocities than those in the corner sub-channels, interior sub-channels, and the wake region generated by wire spacers. The vortex structures in the edge sub-channels are a type of longitudinal vortex, and have a larger scale than the other sub-channels. The vortex structures in the corner sub-channels have lower axial and tangential velocities than those in the edge and interior sub-channels. The turbulence intensity in the sub-channels is remarkably dominated by the magnitude of the axial velocity. The vortex structures in the edge, corner, and interior sub-channels have a higher turbulence intensity and lower vorticity than the small-scale wakes near the wire spacers. The driving forces on the wire in the X-, Y-, and Z-directions are not only dependent on the axial velocity, but are also significantly dependent on the relative angular position between the wire spacer and rod, and the relative position between the wire spacer and duct wall.

Analysis of vortical flow field related to aero-acoustic sound in an air conditioning system by wall pressure measurement and CFD (2nd Report, blade passing frequency in a blower)

This paper presents an elucidation of three-dimensional flow phenomena that induce an aero-acoustic sound of Blade passing frequency (BPF). Three-dimensional separated and vortical flow fields in an air conditioning system have been investigated by Experimental fluid dynamics (EFD) analysis using an unsteady wall pressure measurement system and by Computational fluid dynamics (CFD) analysis using a Reynolds-averaged Navier-Stokes (RANS) simulation as described in the first report of the present study. Generally, the aero-acoustic sound of BPF is one of the main aero-acoustic sounds in air conditioning systems. Based on the EFD analysis, the wall pressure fluctuation of BPF is found to be not only located near the scroll tongue region but also widely located around the scroll casing wall. It was found that there is no correlation between the sensor positions in the circumferential direction and the pressure fluctuation level of BPF. Based on the CFD analysis results, it was found that an attachment line on the center of the scroll casing wall develops by interacting with secondary flow from the multi-blade fan. A pair of secondary vortex structures in the scroll casing longitudinally develops in the circumferential direction. As the pair of secondary flow structures with high vorticity develops in the circumferential direction and interacts with the scroll casing, wall pressure fluctuation of BPF is generated even though it is located far away from the scroll tongue.