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3 pages, 175 KiB

Open AccessEditorial

Symmetry in Fluid Flow

by Toshio Tagawa

Symmetry 2023, 15(3), 653; https://doi.org/10.3390/sym15030653 - 05 Mar 2023

Cited by 1 | Viewed by 802

Abstract

Fluid flows sometimes exhibit symmetricity under certain conditions [...] Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

Research

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19 pages, 5595 KiB

Open AccessFeature PaperArticle

Symmetry and Asymmetry in the Thermo-Magnetic Convection of Silver Nanofluid

by Elzbieta Fornalik-Wajs, Aleksandra Roszko and Janusz Donizak

Symmetry 2020, 12(11), 1891; https://doi.org/10.3390/sym12111891 - 18 Nov 2020

Cited by 4 | Viewed by 1724

Abstract

Application of nanofluids is aimed at enhancing the heat transfer performance the same as the utilization of a strong magnetic field. The potential of the combined effect of these passive and active methods was analyzed numerically. The silver nanofluid thermo-magnetic convection in a [...] Read more.

Application of nanofluids is aimed at enhancing the heat transfer performance the same as the utilization of a strong magnetic field. The potential of the combined effect of these passive and active methods was analyzed numerically. The silver nanofluid thermo-magnetic convection in a cubical enclosure placed in the Rayleigh–Benard configuration was investigated for various concentrations of nanoparticles and various values of magnetic induction at constant temperature difference. The nanofluid flow was considered as a two-phase flow and studied with the Euler–Euler approach. The main outcome was related to the heat transfer performance, but also a lot of attention was paid to the flow structure, which is very difficult to obtain by experimental methods. The results exhibited a flow structure with diagonal axis of symmetry in all analyzed cases and stabilizing effect of magnetic field. The heat transfer performance is indicated by the Nusselt number, which increases with an increasing value of magnetic induction but decreases with an increasing concentration of nanoparticles. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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19 pages, 6422 KiB

Open AccessArticle

Numerical Analysis of Turbulent Heat Transfer in the Case of Minijets Array

by Sebastian Gurgul, Tomasz Kura and Elzbieta Fornalik-Wajs

Symmetry 2020, 12(11), 1785; https://doi.org/10.3390/sym12111785 - 28 Oct 2020

Cited by 2 | Viewed by 1687

Abstract

The presented numerical investigations show an analysis of the turbulent single-phase array of ten minijets impinging a heated surface, which lead to the intensification of heat transfer between the fluid and the surface. Attention was devoted to the comparison between phenomena occurring for [...] Read more.

The presented numerical investigations show an analysis of the turbulent single-phase array of ten minijets impinging a heated surface, which lead to the intensification of heat transfer between the fluid and the surface. Attention was devoted to the comparison between phenomena occurring for the heated flat and concave surface geometry. The selection of the shapes was based on the impinging jets applications. From the numerical point of view, the focus was placed on a comparison of the Reynolds Averaged Navier–Stokes (RANS) turbulence model implementations in ANSYS Fluent software, and their impact on the modeling precision of the thermal and hydrodynamic boundary layers phenomena. The 3D numerical model was based on the continuity, momentum, and energy transport equations, together with three various RANS turbulence models: k-ω SST Kato-Launder, k-ε RNG Kato-Launder, and Intermittency Transition. The water submerged minijets, characterized by three various values of Reynolds number, were considered. Average surface Nusselt number values for all analyzed cases were compared with the experimental correlations and exhibited the same tendency but differed in detail. Numerically obtained average Nusselt number values agreed with the results of two from three correlations in the range of 10–20%. The flat surface was characterized by higher heat transfer than the concave one and an influence of the cross flow, changing the symmetrical distribution of the Nusselt number, was more visible for it. A cross flow impact was found in fuzzy hexagonal or tetragonal symmetry of this distribution. Additionally, the areas of high temperature gradient values were identified in the region of the strongest jets’ interactions, which can be important for mechanical strength analysis. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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16 pages, 10253 KiB

Open AccessFeature PaperArticle

Effect of the Direction of Uniform Horizontal Magnetic Field on the Linear Stability of Natural Convection in a Long Vertical Rectangular Enclosure

by Toshio Tagawa

Symmetry 2020, 12(10), 1689; https://doi.org/10.3390/sym12101689 - 15 Oct 2020

Cited by 4 | Viewed by 2522

Abstract

The effect of the direction of external horizontal magnetic fields on the linear stability of natural convection of liquid metal in an infinitely long vertical rectangular enclosure is numerically studied. A vertical side wall is heated and the opposing vertical wall is cooled [...] Read more.

The effect of the direction of external horizontal magnetic fields on the linear stability of natural convection of liquid metal in an infinitely long vertical rectangular enclosure is numerically studied. A vertical side wall is heated and the opposing vertical wall is cooled both isothermally, whereas the other two vertical walls are adiabatic. A uniform horizontal magnetic field is applied either in the direction parallel or perpendicular to the temperature gradient. In this study, the height of the enclosure is so long as to neglect the top and bottom effects where returning flow takes place, and thus the basic flow is assumed to be a parallel flow and the temperature field is in heat conduction state. The Prandtl number is limited to the value of 0.025 and horizontal cross-section is square. The natural convection is monotonously stabilized as increase in the Hartmann number when the applied magnetic field is parallel to the temperature gradient. However, when the applied magnetic field is perpendicular to the temperature gradient, it is once destabilized at a certain low Hartmann number, but it is stabilized at high Hartmann numbers. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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19 pages, 386 KiB

Open AccessArticle

Conformal Invariance of Characteristic Lines in a Class of Hydrodynamic Models

by Marta Wacławczyk, Vladimir N. Grebenev and Martin Oberlack

Symmetry 2020, 12(9), 1482; https://doi.org/10.3390/sym12091482 - 09 Sep 2020

Cited by 12 | Viewed by 1934

Abstract

This paper addresses the problem of the existence of conformal invariance in a class of hydrodynamic models. For this we analyse an underlying transport equation for the one-point probability density function, subject to zero-scalar constraint. We account for the presence of non-zero viscosity [...] Read more.

This paper addresses the problem of the existence of conformal invariance in a class of hydrodynamic models. For this we analyse an underlying transport equation for the one-point probability density function, subject to zero-scalar constraint. We account for the presence of non-zero viscosity and large-scale friction. It is shown analytically, that zero-scalar characteristics of this equation are invariant under conformal transformations in the presence of large-scale friction. However, the non-zero molecular diffusivity breaks the conformal group (CG). This connects our study with previous observations where CG invariance of zero-vorticity isolines of the 2D Navier–Stokes equation was analysed numerically and confirmed only for large scales in the inverse energy cascade. In this paper, an example of CG is analysed and possible interpretations of the analytical results are discussed. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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15 pages, 5403 KiB

Open AccessArticle

Heat Transfer Enhancement in a Novel Annular Tube with Outer Straight and Inner Twisted Oval Tubes

by Chao Luo, KeWei Song, Toshio Tagawa and TengFei Liu

Symmetry 2020, 12(8), 1213; https://doi.org/10.3390/sym12081213 - 23 Jul 2020

Cited by 11 | Viewed by 3301

Abstract

The thermal-hydraulic performance in a novel annular tube formed by outer straight and inner twisted oval tubes is numerically investigated. An annular tube formed by two straight oval tubes is also studied for comparison. Inner twisted oval tubes with different aspect ratios and [...] Read more.

The thermal-hydraulic performance in a novel annular tube formed by outer straight and inner twisted oval tubes is numerically investigated. An annular tube formed by two straight oval tubes is also studied for comparison. Inner twisted oval tubes with different aspect ratios and twist ratios are studied. The heat transfer is well improved by the symmetrical secondary flow in the annulus. The Nusselt number generally increases when the inner oval tube becomes flatter and the twists stronger in the studied range of geometrical parameters. The largest Nusselt number Nu of the inner twisted tube increases by 116% while the friction factor f increases by only 46% compared with that of the inner straight tube, and the largest value of the thermal performance factor (JF) can be up to 1.9. Correlations of the Nusselt number and friction factor are proposed for laminar and turbulent flows, and the deviations of the correlations are within ±5% and ±4% for Nu and f, respectively. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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13 pages, 4023 KiB

Open AccessArticle

Asymmetrical Velocity Distribution in the Drag-Reducing Channel Flow of Surfactant Solution Caused by an Injected Ultrathin Water Layer

by Zaiguo Fu, Xiaotian Liang and Kang Zhang

Symmetry 2020, 12(5), 846; https://doi.org/10.3390/sym12050846 - 21 May 2020

Cited by 3 | Viewed by 2507

Abstract

Although the turbulent intensity is suppressed in the drag-reducing channel flow by viscoelastic additives, the mean velocity distribution in the channel flow is symmetrical and tends to be similar to the laminar flow. In the study of near-wall modulation of the drag-reducing flow [...] Read more.

Although the turbulent intensity is suppressed in the drag-reducing channel flow by viscoelastic additives, the mean velocity distribution in the channel flow is symmetrical and tends to be similar to the laminar flow. In the study of near-wall modulation of the drag-reducing flow with an injected ultrathin water layer, an asymmetrical mean velocity distribution was found. To further investigate this phenomenon and the underlying cause, an experiment was carried out with the water injected from a porous channel wall at a small velocity (~10⁻⁴ m/s) into the drag-reducing flow of surfactant solution. The instantaneous concentration and flow fields were measured by using planar laser-induced fluorescence (PLIF) and particle imaging velocimetry (PIV) techniques, respectively. Moreover, analyses on turbulent statistical characteristics and spatial distribution of viscoelastic structures were carried out on the basis of comparison among various flow cases. The results showed that the injected ultrathin water layer under present experimental conditions affected the anisotropy of the drag-reducing flow. The characteristics, such as turbulence intensity, showed the zonal feature in the wall-normal direction. The Reynolds shear stress was enhanced in the near-wall region, and the viscoelastic structure was modified severely due to the redistributed stress. These results may provide experimental supports for the near-wall modulation of turbulence and the exploration of the drag-reducing mechanism by viscoelastic additives. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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11 pages, 3944 KiB

Open AccessArticle

Rayleigh-Bénard Convection of Paramagnetic Liquid under a Magnetic Field from Permanent Magnets

by Kengo Wada, Masayuki Kaneda and Kazuhiko Suga

Symmetry 2020, 12(3), 341; https://doi.org/10.3390/sym12030341 - 28 Feb 2020

Cited by 6 | Viewed by 3480

Abstract

The convection control is important in terms of the heat transfer enhancement and improvement of the applied devices and resultant products. In this study, the convection control by a magnetic field from block permanent magnets is numerically investigated on the Rayleigh-Bénard convection of [...] Read more.

The convection control is important in terms of the heat transfer enhancement and improvement of the applied devices and resultant products. In this study, the convection control by a magnetic field from block permanent magnets is numerically investigated on the Rayleigh-Bénard convection of paramagnetic fluid. To enhance the magnetic force from the available permanent magnets, pairs of alternating-pole magnets are employed and aligned near the bottom heated wall. The lattice Boltzmann method is employed for the computation of the heat and fluid flow with the consideration of buoyancy and magnetothermal force on the working fluid. It is found that, since the magnetic force at the junction of pair magnets becomes strong remarkably and in the same direction as the gravity, descending convection flow is locally enhanced and the pair of symmetrical roll cells near the magnet junction becomes longitudinal. The local heat transfer corresponds to the affected roll cell pattern; locally enhanced at the magnet junctions and low heat transfer area is shifted aside the magnet outer edge. The averaged Nusselt number on the hot wall also increases proportionally to the magnetic induction but it is saturated at high magnetic induction. This suggests the roll cell pattern is no more largely affected at extremely-high magnetic induction. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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14 pages, 3417 KiB

Open AccessArticle

Flow Symmetry and Heat Transfer Characteristics of Winglet Vortex Generators Arranged in Common Flow up Configuration

by Kewei Song, Lu Wang, Yajun Hu and Qi Liu

Symmetry 2020, 12(2), 247; https://doi.org/10.3390/sym12020247 - 05 Feb 2020

Cited by 11 | Viewed by 2267

Abstract

The generation of longitudinal vortices is an effective method for promoting thermal performance with a relative low-pressure penalty in heat exchangers. The winglet pair can generate symmetrical longitudinal vortices on the cross-section of the channel. The heat transfer and pressure-loss characteristics of a [...] Read more.

The generation of longitudinal vortices is an effective method for promoting thermal performance with a relative low-pressure penalty in heat exchangers. The winglet pair can generate symmetrical longitudinal vortices on the cross-section of the channel. The heat transfer and pressure-loss characteristics of a pair of winglet vortex generators with different transverse pitches are numerically studied in this paper. The winglet pair arranged in a common flow up configuration generates a pair of symmetrical longitudinal main vortices with counter-rotating directions. The symmetrical flow structure induces fluid to flow from the bottom towards the top of the channel in the common flow region between the longitudinal vortices. The flow symmetry of the longitudinal vortices and the heat transfer performance are strongly affected by the transverse pitch of the winglet pair owing to the interaction between the longitudinal vortices. The optimal transverse pitch of the studied winglet pair with the best thermal performance is reported. The increments in the vortex intensity and the Nusselt number for the optimal pitch are increased by up to 21.4% and 29.2%, respectively. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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12 pages, 4879 KiB

Open AccessArticle

Characteristics of Flow Symmetry and Heat Transfer of Winglet Pair in Common Flow Down Configuration

by KeWei Song, WeiNa Shi, Xiang Wu and LiangBi Wang

Symmetry 2020, 12(2), 209; https://doi.org/10.3390/sym12020209 - 02 Feb 2020

Cited by 6 | Viewed by 2300

Abstract

The effect of transverse pitch between a pair of delta-winglet vortex generators arranged in a common flow down configuration on the symmetrical flow structure and heat-transfer performance was numerically investigated. The results showed that symmetrical longitudinal vortices form a common flow down region [...] Read more.

The effect of transverse pitch between a pair of delta-winglet vortex generators arranged in a common flow down configuration on the symmetrical flow structure and heat-transfer performance was numerically investigated. The results showed that symmetrical longitudinal vortices form a common flow down region between the vortices. The fluid is induced to flow from the top towards the bottom of the channel in the common flow region, which is advantageous to the heat transfer of the bottom fin. The vortex interaction increases and the vortex intensity decreases along with the decrease in transverse pitch of vortex generators. Vortex interaction has a slight influence on pressure penalty. The Nusselt number decreases with increasing vortex interaction. The vortices gradually attenuate and depart from each other during the process of flowing downward. A reasonable transverse pitch of delta-winglet vortex generators in a common-flow-down configuration is recommended for high thermal performance. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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10 pages, 3618 KiB

Open AccessArticle

Shock Waves Asymmetry in a Symmetric Nozzle

by Janusz Telega, Ryszard Szwaba and Piotr Doerffer

Symmetry 2019, 11(12), 1477; https://doi.org/10.3390/sym11121477 - 04 Dec 2019

Cited by 5 | Viewed by 2961

Abstract

The results of the experimental research on the symmetry of supersonic flow in a symmetric convergent-divergent nozzle are presented. The investigations were focused on the fact that for some flow conditions the flow in a precisely symmetric nozzle becomes asymmetric. Starting from a [...] Read more.

The results of the experimental research on the symmetry of supersonic flow in a symmetric convergent-divergent nozzle are presented. The investigations were focused on the fact that for some flow conditions the flow in a precisely symmetric nozzle becomes asymmetric. Starting from a specific value of Mach number, the flow becomes asymmetric in terms of shock wave λ-foot geometry on both sides of a symmetric nozzle. The evolution of the abovementioned asymmetry has been analysed for Mach number value ranging from M = 1.26 to M = 1.59 with the nozzle opening angle of up to 6.5° on each side. The presented results indicate that for the same flow parameters as Mach number and Reynolds number, and for the same geometry of the nozzle, different λ-foot size is formed at each wall. This unexpected behaviour is responsible for the flow asymmetry. Numerical simulations carried out earlier confirm the appearance of shock wave asymmetry. The side in which the asymmetry takes place is accidental, as the full symmetry of simulation mesh and experiment setup was secured. In numerical simulation the asymmetry follows always the same direction. In experiments the direction of asymmetry happens alternatively without any apparent reason. The explanation of the phenomena is provided in this paper. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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18 pages, 4161 KiB

Open AccessArticle

Quasi-Periodic Oscillating Flows in a Channel with a Suddenly Expanded Section

by Takuya Masuda and Toshio Tagawa

Symmetry 2019, 11(11), 1403; https://doi.org/10.3390/sym11111403 - 13 Nov 2019

Cited by 4 | Viewed by 3325

Abstract

In this study, two-dimensional numerical simulation was carried out for an oscillatory flow between parallel flat plates having a suddenly expanded section. Governing equations were discretized with the second-order accuracy by a finite volume method on an unequal interval mesh system resolving finer [...] Read more.

In this study, two-dimensional numerical simulation was carried out for an oscillatory flow between parallel flat plates having a suddenly expanded section. Governing equations were discretized with the second-order accuracy by a finite volume method on an unequal interval mesh system resolving finer near walls and corners to obtain the characteristics of the oscillatory flow accurately. Amplitude spectrums of a velocity component were obtained to investigate the periodic characteristics of the oscillatory flow. At low Reynolds numbers, the flow is periodic because the spectrum mostly consists of harmonic components, and then at high Reynolds numbers, it transits to a quasi-periodic flow mixed with non-harmonic components. In conjunction with the periodic oscillation of a main flow, separation vortices that are not uniform in size are generated from the corner of a sudden contraction part and pass through a downstream region coming into contact with the wall. The number of the vortices decreases rapidly after they are generated, but the vortices are generated again in the downstream region. In order to specify where aperiodicity is generated, the turbulent kinetic energy is introduced, and it is decomposed into the harmonic and non-harmonic components. The peaks of the non-harmonic component are generated in the region of the expanded section. Full article

(This article belongs to the Special Issue Symmetry in Fluid Flow)

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