Journal articles on the topic 'Reynolds Ranges'
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1
Huang, Xiao Qing, Xu Zhang, and Chun Guang Li. "Experimental Research on Resistance and Heat Transfer Properties of Corrugated Plate Air-Cooled Heat Exchanger." Advanced Materials Research 354-355 (October 2011): 153–58. http://dx.doi.org/10.4028/www.scientific.net/amr.354-355.153.
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Experimental research on resistance and heat transfer properties of corrugated plate air-cooled heat exchanger under the condition of variable air and hot water flow rates has been conducted. The pressure drop and convection heat transfer coefficient correlation expressions both the air side and hot water side are acquired, where the Reynolds number for air side ranges from 401 to 6602 and the Reynolds number for water side ranges from 2536 to 19301 are adaptable.
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Elhadi Kh. Abugnah, Wan Saiful-Islam Wan Salim, Abdulhafid M. Elfaghi, and Zamani Ngali. "Comparison of 2D and 3D Modelling Applied to Single Phase Flow of Nanofluid through Corrugated Channels." CFD Letters 14, no. 1 (January 11, 2022): 128–39. http://dx.doi.org/10.37934/cfdl.14.1.128139.
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Nanofluid flow through non-corrugated and corrugated channels is studied using a two-dimensional (2D) and three dimensions (3D) numerical simplification. Due to the high computational costs of a full 3D grid model, the 2D approach offer a more practical advantage. However, little information about its validity is available. The aim of this study is to explore to which extent 2D simulations can describe the flow within a 3D geometry, and to investigate how effective the commonly used 2D numerical simplification is in nanofluid flow through non-corrugated and corrugated channels. A case study has implemented with 2D and 3D mesh models to compare their results taking into consideration the analysis of heat transfer and pressure drop. A simulation has been carried out using Ansys fluent software to compare the results for different Reynolds Numbers ranges from 10000 to 30000 and different geometries non-corrugated, semicircle and rectangular channels. The results show that for non-corrugated channel there is a slight difference between 2D and 3D results for all Reynolds number ranges, while for both semicircle and rectangular corrugated channels, the difference becomes larger for high Reynold’s Number.
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Raza, Wasim, Shakhawat Hossain, and Kwang-Yong Kim. "A Review of Passive Micromixers with a Comparative Analysis." Micromachines 11, no. 5 (April 27, 2020): 455. http://dx.doi.org/10.3390/mi11050455.
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A wide range of existing passive micromixers are reviewed, and quantitative analyses of ten typical passive micromixers were performed to compare their mixing indices, pressure drops, and mixing costs under the same axial length and flow conditions across a wide Reynolds number range of 0.01–120. The tested micromixers were selected from five types of micromixer designs. The analyses of flow and mixing were performed using continuity, Navier-Stokes and convection-diffusion equations. The results of the comparative analysis were presented for three different Reynolds number ranges: low-Re (Re ≤ 1), intermediate-Re (1 < Re ≤ 40), and high-Re (Re > 40) ranges, where the mixing mechanisms are different. The results show a two-dimensional micromixer of Tesla structure is recommended in the intermediate- and high-Re ranges, while two three-dimensional micromixers with two layers are recommended in the low-Re range due to their excellent mixing performance.
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FARAZMAND, M. M., N. K. R. KEVLAHAN, and B. PROTAS. "Controlling the dual cascade of two-dimensional turbulence." Journal of Fluid Mechanics 668 (November 30, 2010): 202–22. http://dx.doi.org/10.1017/s0022112010004635.
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The Kraichnan–Leith–Batchelor (KLB) theory of statistically stationary forced homogeneous isotropic two-dimensional turbulence predicts the existence of two inertial ranges: an energy inertial range with an energy spectrum scaling of k−5/3, and an enstrophy inertial range with an energy spectrum scaling of k−3. However, unlike the analogous Kolmogorov theory for three-dimensional turbulence, the scaling of the enstrophy range in the two-dimensional turbulence seems to be Reynolds-number-dependent: numerical simulations have shown that as Reynolds number tends to infinity, the enstrophy range of the energy spectrum converges to the KLB prediction, i.e. E ~ k−3. The present paper uses a novel optimal control approach to find a forcing that does produce the KLB scaling of the energy spectrum in a moderate Reynolds number flow. We show that the time–space structure of the forcing can significantly alter the scaling of the energy spectrum over inertial ranges. A careful analysis of the optimal forcing suggests that it is unlikely to be realized in nature, or by a simple numerical model.
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Iwata, Naoyuki, Hiroki Suzuki, and Shinsuke Mochizuki. "Numerical simulation of viscosity/implicit large-eddy steady turbulence with the Reynolds number dependency." Journal of Physics: Conference Series 2047, no. 1 (October 1, 2021): 012007. http://dx.doi.org/10.1088/1742-6596/2047/1/012007.
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Abstract This study presents a numerical analysis that models small scale turbulence using numerical viscosity or implicit large-eddy simulation (LES). The motivation for focusing on these models is that the sub-grid scale components of LES are assumed to have a sufficiently high Reynolds number turbulence. The Reynolds number dependence of steady isotropic turbulence is used to validate the present analysis. Here, this dependency ranges from low to high Reynolds numbers. The results of this analysis are validated by comparing them with those of direct numerical simulation. The donor cell method and quick method are used as schemes of the numerical viscosity. Analysis based on the numerical viscosity can give accurate turbulent kinetic energy values at high Reynolds numbers and implicit LES at low Reynolds numbers. However, these models did not accurately predict static pressure fluctuations. These results were discussed by visualizing the large-scale turbulent structures.
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Oo, Zaw Zaw, Muhammad Younis Yamin, Hua Zhang, Muhammad Zaka, and Bo Hu. "Study of Laminar Horseshoe Vortex Using Particle Image Velocimetry." Applied Mechanics and Materials 110-116 (October 2011): 3249–54. http://dx.doi.org/10.4028/www.scientific.net/amm.110-116.3249.
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—This study investigates the upstream of the juncture flows generated by the circular cross section cylindrical body mounted on a flat plate using PIV (Particle Image Velocimetry) technique. The flow structure of laminar horseshoe vortex and a topological insight into the flow pattern of the vortex system were observed. Vortex structures for ReD(Diameter Reynolds number) 1600, 2000, 2400 and 3500 are predicted and discussed in detail. Experiments were conducted to investigate the structure of steady and periodic horseshoe vortex, the effect of Diameter Reynolds number, location of horseshoe vortex core and its variation with the change in Diameter Reynolds number and the location and nature of the saddle point located most upstream of the leading edge of the cylinder. The results revealed that (a) two different flow regimes were observed corresponding to four Reynolds number ranges; (b) the upstream vortex systems approach closer to the cylinder whereas the distance of saddle point located upstream of the leading edge of the cylinder moves away from the wall when the Reynolds number increases.
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Chen, Yi, Udaya Kahangamage, Quan Zhou, and Chun Wah Leung. "Can hydrogen enriched biogas be used as domestic fuel? - Part I – Thermal Characteristics of Blended Biogas/H2 Impinging Flames." HKIE Transactions 28, no. 2 (June 30, 2021): 60–67. http://dx.doi.org/10.33430/v28n2thie-2020-0040.
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Biogas is a renewable energy source widely produced by breakdowns of organic matters in natural environment and industry. However, it is not yet an ideal replacement of fossil fuels because its high CO2 content would deteriorate its thermal performance. To upgrade biogas for possible domestic application, hydrogen enrichment is proposed by adding high-grade hydrogen (H2) to biogas in order to improve its flammability and heating value, and reduce pollutant emission. However, most previous studies on blended Biogas/H2 focus on analysing the effects of H2 fraction and nozzle-to-plate distance on the heat flux profile and flame temperature. No comprehensive study has ever demonstrated the influence of the Reynolds number and equivalence ratio under a wide operating range. In this study, a test rig was built to investigate the effects of the Reynolds number and equivalence ratio on heat flux and thermal efficiency of blended biogas/H2 impinging flame. The blended biogas/H2 consisted of 80% biogas and 20% H2 addition in volume. Biogas was artificially made by 60% CH4 and 40% CO2 (BG60). The Reynolds number ranges from 300 to 1500 and equivalence ratio ranges from 1 to 3. A comparative study was also conducted between pure biogas (BG60) and biogas with 20% H2 enrichment.
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GOTOH, TOSHIYUKI, and ROBERT S. ROGALLO. "Intermittency and scaling of pressure at small scales in forced isotropic turbulence." Journal of Fluid Mechanics 396 (October 10, 1999): 257–85. http://dx.doi.org/10.1017/s0022112099005972.
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The intermittency of pressure and pressure gradient in stationary isotropic turbulence at low to moderate Reynolds numbers is studied by direct numerical simulation (DNS) and theoretically. The energy spectra scale in Kolmogorov units as required by the universal-equilibrium hypothesis, but the pressure spectra do not. It is found that the variances of the pressure and pressure gradient are larger than those computed using the Gaussian approximation for the fourth-order moments of velocity, and that the variance of the pressure gradient, normalized by Kolmogorov units, increases roughly as [Rscr ]1/2λ, where [Rscr ]λ is the Taylor microscale Reynolds number. A theoretical explanation of the Reynolds number dependence is presented which assumes that the small-scale pressure field is driven by coherent small-scale vorticity–strain domains. The variance of the pressure gradient given by the model is the product of the variance of ui,juj,i, the source term of the Poisson equation for pressure, and the square of an effective length of the small-scale coherent vorticity–strain structures. This length can be expressed in terms of the Taylor and Kolmogorov microscales, and the ratio between them gives the observed Reynolds number dependence. Formal asymptotic matching of the spectral scaling observed at small scales in the DNS with the classical scaling at large scales suggests that at high Reynolds numbers the pressure spectrum in these forced flows consists of three scaling ranges which are joined by two inertial ranges, the classical k−7/3 range and a k−5/3 range at smaller scale. It is not possible, within the classical Kolmogorov theory, to determine the length scale at which the inertial range transition occurs because information beyond the energy dissipation rate is required.
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DONZIS, D. A., and K. R. SREENIVASAN. "The bottleneck effect and the Kolmogorov constant in isotropic turbulence." Journal of Fluid Mechanics 657 (June 10, 2010): 171–88. http://dx.doi.org/10.1017/s0022112010001400.
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A large database from direct numerical simulations of isotropic turbulence, including recent simulations for box sizes up to 40963 and the Taylor–Reynolds number Rλ ≈ 1000, is used to investigate the bottleneck effect in the three-dimensional energy spectrum and second-order structure functions, and to determine the Kolmogorov constant, CK. The difficulties in estimating CK at any finite Reynolds number, introduced by intermittency and the bottleneck, are assessed. The data conclusively show that the bottleneck effect decreases with the Reynolds number. On this basis, an alternative to the usual procedure for determining CK is suggested; this proposal does not depend on the particular choices of fitting ranges or power-law behaviour in the inertial range. Within the resolution of the numerical data, CK thus determined is a Reynolds-number-independent constant of ≈1.58 in the three-dimensional spectrum. A simple model including non-local transfer is proposed to reproduce the observed scaling features of the bottleneck.
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Luan, Yi Gang, and Hai Ou Sun. "Simplification Model for Prediction of Pressure Drop in Wire Mesh Mist Eliminator by CFD." Applied Mechanics and Materials 26-28 (June 2010): 297–302. http://dx.doi.org/10.4028/www.scientific.net/amm.26-28.297.
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In this article, computational fluid dynamics(CFD) method is used to predict the pressure drop of a wire mesh mist eliminator. A simplification method is used during the simulation process to solve the difficulty of model building during the simulation of the mist pad. A two-dimensional model is employed to acquire the resistance of mesh pad with different layer spacing. The flow field is calculated using 2D Reynolds-averaged Navier-Stokes equations. turbulence model is used to simulate the Reynold stress. And pressure drop of wire mesh mist eliminator is expressed as a function of broad ranges of inlet velocity. After CFD simulation, model experiment study is carried on using a small scale wind-tunnel. The pressure drop is gained to testify the numerical simulation result.
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Jahnke, Craig C., and Daniel T. Valentine. "Recirculation Zones in a Cylindrical Container." Journal of Fluids Engineering 120, no. 4 (December 1, 1998): 680–84. http://dx.doi.org/10.1115/1.2820723.
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The flow field induced inside a cylindrical container by the rotation of the two end walls is described. It is shown that stagnation points leading to separation bubbles occur on the axis of rotation and/or the bottom end wall for certain ranges of the characteristic parameters; the Reynolds number, the aspect ratio of the container, and the ratio of the rotation rates of the end walls. Flow fields in a container of aspect ratio 2.0 are examined for Reynolds numbers from 100 to 3000 and ratios of the rotation rates of the top and bottom end walls from −0.10 to 1.0. For a range of ratios of the rotation rates of the top and bottom end walls and Reynolds numbers it is shown that ring vortices surrounding a columnar vortex core exist.
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DONG, S. "Direct numerical simulation of turbulent Taylor–Couette flow." Journal of Fluid Mechanics 587 (August 31, 2007): 373–93. http://dx.doi.org/10.1017/s0022112007007367.
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We investigate the dynamical and statistical features of turbulent Taylor–Couette flow (for a radius ratio 0.5) through three-dimensional direct numerical simulations (DNS) at Reynolds numbers ranging from 1000 to 8000. We show that in three-dimensional space the Görtler vortices are randomly distributed in banded regions on the wall, concentrating at the outflow boundaries of Taylor vortex cells, which spread over the entirecylinder surface with increasing Reynolds number. Görtler vortices cause streaky structures that form herringbone-like patterns near the wall. For the Reynolds numbers studied here, the average axial spacing of the streaks is approximately 100 viscous wall units, and the average tilting angle ranges from 16° to 20°. Simulationresults have been compared to the experimental data in the literature, and the flow dynamics and statistics are discussed in detail.
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Zhou, Xun, Bo Dong, and Weizhong Li. "Phase-Field-Based LBM Analysis of KHI and RTI in Wide Ranges of Density Ratio, Viscosity Ratio, and Reynolds Number." International Journal of Aerospace Engineering 2020 (June 19, 2020): 1–14. http://dx.doi.org/10.1155/2020/8885226.
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Numerous studies have elaborated the dominated roles of Kelvin-Helmholtz instability (KHI) and Rayleigh-Taylor instability (RTI) in the liquid sheet breakup and primary atomization. As for applications in aeronautics, the liquid-gas mixing generally occurs at the challenging conditions of a large density ratio and high Reynolds number. Hence, the evaluation of KHI and RTI under such challenging conditions will have great significance in better understanding the destabilizing mechanism of the liquid layer. To this end, a lattice Boltzmann multiple-relaxation-time (MRT) two-phase model, based on the conservative Allen-Cahn equation, is reconstructed for the present study. Preliminarily, the numerical stability and accuracy of this MRT model are tested by Laplace’s law under a large density ratio and high Reynolds number, including the sensitivity study to the values of mobility. Afterward, KHI and RTI are investigated in wide ranges of the Reynolds number, density ratio, and viscosity ratio. Numerical results indicate that the enhanced viscous force of light fluid with an increasing viscosity ratio notably suppresses the roll-ups of heavy fluid in KHI and RTI. As for the density ratio, it generally shows negative impacts on fluid-mixing in KHI and spike-spiraling in RTI. However, when the density ratio and the Reynolds number both arrive at high levels, the Kelvin-Helmholtz wavelets aroused by a dominated inertia force of heavy fluid trigger severe interface disintegration. The above results once more demonstrate the excellent ability of the present model in dealing with challenging conditions. Besides, the morphological characteristics of KHI and RTI at a high Reynolds number and large density ratio also greatly support the typical interface breakup mechanism observed in primary atomization.
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Ahmed I. ElShafei, Amr Guaily, and Mohammed A. Boraey. "Turbulent Axisymmetric Non-Isothermal Flow of The Hitec Molten Salt with Temperature Dependent Properties: A Numerical Investigation." Journal of Advanced Research in Fluid Mechanics and Thermal Sciences 86, no. 2 (August 22, 2021): 1–14. http://dx.doi.org/10.37934/arfmts.86.2.114.
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This study aims to investigate the Hitec molten salt’s thermal-hydraulic behavior in a smooth round pipe under broad ranges of surface heat flux and Reynolds number (q = 104 – 105 W/m2, Re = 104 – 105). Mesh independent study was performed to ensure the robustness of the model to achieve accurate solutions. Presentation of temperature, pressure and thermophysical properties for multiple cases are presented and discussed. Temperature gradient decreases at high Reynolds number leading to small change in thermo-physical properties. While pressure seems not to be affected by the change in the applied surface heat flux, it increasess linearly across the pipe with the increase in Reynolds number. This analysis aims to provide better understanding of the thermal-hydraulic behavior for fluids with temperature dependent properties for a wide range of Re and surface heat flux.
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Hirata, Katsuya, Ryo Nozawa, Shogo Kondo, Kazuki Onishi, and Hirochika Tanigawa. "On High-Performance Airfoil at Very Low Reynolds Number." Journal of Robotics and Mechatronics 28, no. 3 (June 17, 2016): 273–85. http://dx.doi.org/10.20965/jrm.2016.p0273.
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[abstFig src='/00280003/02.jpg' width=""300"" text='Iso-Q surfaces of very-slow flow past an iNACA0015' ] The airfoil is often used as the elemental device for flying/swimming robots, determining its basic performances. However, most of the aerodynamic characteristics of the airfoil have been investigated at Reynolds numbers Re’s more than 106. On the other hand, our knowledge is not enough in low Reynolds-number ranges, in spite of the recent miniaturisation of robots. In the present study, referring to our previous findings (Hirata et al., 2011), we numerically examine three kinds of high-performance airfoils proposed for very-low Reynolds numbers; namely, an iNACA0015 (the NACA0015 placed back to front), an FPBi (a flat plate blended with iNACA0015 as its upper half) and an FPBN (a flat plate blended with the NACA0015 as its upper half), in comparison with such basic airfoils as a NACA0015 and an FP (a flat plate), at a Reynolds number Re = 1.0 × 102 using two- and three-dimensional computations. As a result, the FPBi shows the best performance among the five kinds of airfoils.
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Elfmark, Ola, Robert Reid, and Lars Morten Bardal. "Blockage Correction and Reynolds Number Dependency of an Alpine Skier: A Comparison Between Two Closed-Section Wind Tunnels." Proceedings 49, no. 1 (June 15, 2020): 19. http://dx.doi.org/10.3390/proceedings2020049019.
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The purpose of this study was to investigate the impact of blockage effect and Reynolds Number dependency by comparing measurements of an alpine skier in standardized positions between two wind tunnels with varying blockage ratios and speed ranges. The results indicated significant blockage effects which need to be corrected for accurate comparison between tunnels, or for generalization to performance in the field. Using an optimized blockage constant, Maskell’s blockage correction method improved the mean absolute error between the two wind tunnels from 7.7% to 2.2%. At lower Reynolds Numbers (<8 × 105, or approximately 25 m/s in this case), skier drag changed significantly with Reynolds Number, indicating the importance of testing at competition specific wind speeds. However, at Reynolds Numbers above 8 × 105, skier drag remained relatively constant for the tested positions. This may be advantageous when testing athletes from high speed sports since testing at slightly lower speeds may not only be safer, but may also allow the athlete to reliably maintain difficult positions during measurements.
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Budi, Utomo Kukuh W., Kamal Samsul, Suhanan, and I. Made Suardjaja. "Heat Transfer Effectiveness and Coefficient of Pressure Drop on the Shell Side of a Staggered Elliptical Tubes Bank." Applied Mechanics and Materials 493 (January 2014): 134–39. http://dx.doi.org/10.4028/www.scientific.net/amm.493.134.
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The effectiveness of heat transfer and the pressure drop coefficient of staggered elliptical tube banks are studied experimentally. The bank consists of 11 elliptical tubes of 0.75 equivalent diameter in an arrangement of 4-3-4. The major and the minor sub-axis of each tube are 24.70 mm and 12.35 mm respectively, and therefore the aspect ratio (AR) of the tube is 2.0. The geometric parameters of the bank are ST = 24.70 mm, SL = 37.00 mm and minimum frontal area B = 12.35 mm. Seven mid-tubes are internally heated by electrical heater of 69.6 Watt each. Experiment is conducted in a sub sonic wind tunnel and run with the wind velocities of 1 m/s 12.6 m/s which correspond with Reynolds number of = 346-6904. The results show that the effectiveness (ε) varied from 2144.44 to 15.26. It decreases exponentially at low Reynolds numbers and tended asymptotically at higher Reynolds number. The coefficient of pressure drop (CΔp) ranges from 7.21 to 4.41 decreases continuously at low Reynolds number and asymptotic at higher one.
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BUNNER, BERNARD, and GRÉTAR TRYGGVASON. "Dynamics of homogeneous bubbly flows Part 2. Velocity fluctuations." Journal of Fluid Mechanics 466 (September 10, 2002): 53–84. http://dx.doi.org/10.1017/s0022112002001180.
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Direct numerical simulations of the motion of up to 216 three-dimensional buoyant bubbles in periodic domains are presented. The bubbles are nearly spherical and have a rise Reynolds number of about 20. The void fraction ranges from 2% to 24%. Part 1 analysed the rise velocity and the microstructure of the bubbles. This paper examines the fluctuation velocities and the dispersion of the bubbles and the ‘pseudo-turbulence’ of the liquid phase induced by the motion of the bubbles. It is found that the turbulent kinetic energy increases with void fraction and scales with the void fraction multiplied by the square of the average rise velocity of the bubbles. The vertical Reynolds stress is greater than the horizontal Reynolds stress, but the anisotropy decreases when the void fraction increases. The kinetic energy spectrum follows a power law with a slope of approximately −3.6 at high wavenumbers.
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DONG, S. "Turbulent flow between counter-rotating concentric cylinders: a direct numerical simulation study." Journal of Fluid Mechanics 615 (November 25, 2008): 371–99. http://dx.doi.org/10.1017/s0022112008003716.
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We report three-dimensional direct numerical simulations of the turbulent flow between counter-rotating concentric cylinders with a radius ratio 0.5. The inner- and outer-cylinder Reynolds numbers have the same magnitude, which ranges from 500 to 4000 in the simulations. We show that with the increase of Reynolds number, the prevailing structures in the flow are azimuthal vortices with scales much smaller than the cylinder gap. At high Reynolds numbers, while the instantaneous small-scale vortices permeate the entire domain, the large-scale Taylor vortex motions manifested by the time-averaged field do not penetrate a layer of fluid near the outer cylinder. Comparisons between the standard Taylor–Couette system (rotating inner cylinder, fixed outer cylinder) and the counter-rotating system demonstrate the profound effects of the Coriolis force on the mean flow and other statistical quantities. The dynamical and statistical features of the flow have been investigated in detail.
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Wang, Xuegeng, and Charles Dalton. "Oscillating Flow Past a Rigid Circular Cylinder: A Finite-Difference Calculation." Journal of Fluids Engineering 113, no. 3 (September 1, 1991): 377–83. http://dx.doi.org/10.1115/1.2909507.
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A finite-difference study of the sinusoidally oscillating flow past a fixed circular cylinder is made using vorticity and stream function as the dependent variables. Calculations are performed for conditions which lead to both a symmetric wake and an unsymmetric wake. The Reynolds number ranges from 100 to 3000 and the Keulegan-Carpenter number ranges from 1 to 12. A hybrid differencing scheme is introduced to provide a stable for large values of the parameters. Good comparison to flow visualization results and calculated force coefficients is found. The results are given a physical interpretation for the various vortex patterns observed.
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Balatka, K., and S. Mochizuki. "Numerical Analysis of the Flow in an Annular-Conical Passage." Journal of Fluids Engineering 120, no. 3 (September 1, 1998): 513–19. http://dx.doi.org/10.1115/1.2820692.
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The purpose of this paper is to bring new insights into the flow phenomena in an annular-conical passage, which—as previously disclosed experimentally—forms a toroidal-vortex street. The two-dimensional, time dependent Navier-Stokes equations are solved with an explicit finite-difference scheme based on the Marker and Cell method. Solutions are obtained for four different cone apex angles (β = 60, 90, 120, and 180 deg). For each apex angle three cases of different passage spacing are computed. The Reynolds number ranges from Re = 100 ˜ 5000. The stress is put on the initiation and subsequent development of the toroidal-vortex street. Critical Reynolds number for several parameter settings is determined and compared with experimental results.
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Lei, U., and Arthur C. Y. Yang. "Convective Heat Transfer of the Flow through a Rotating Circular Straight Pipe." Journal of Mechanics 17, no. 2 (June 2001): 79–91. http://dx.doi.org/10.1017/s1727719100003154.
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ABSTRACTLaminar heat transfer for large ranges of Reynolds numbers, rotational Reynolds numbers, and Prandtl numbers are studied numerically for incompressible fully developed flow in a circular straight pipe, which is rotating constantly about an axis perpendicular to its own axis under the constant wall temperature gradient condition. There exist four types of local Nusselt number distributions associated with the four different flow regimes for different parameters depending on the relative importance of different forces. Correlations of the averaged Nusselt number are also provided. When the Prandtl number is sufficiently large, the temperature distribution in the core is determined essentially by the secondary flow. Scaling analyses are provided for understanding the essential physics of the problem.
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Erguvan, Mustafa, and David MacPhee. "Energy and Exergy Analyses of Tube Banks in Waste Heat Recovery Applications." Energies 11, no. 8 (August 12, 2018): 2094. http://dx.doi.org/10.3390/en11082094.
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In this study, energy and exergy analyses have been investigated numerically for unsteady cross-flow over heated circular cylinders. Numerous simulations were conducted varying the number of inline tubes, inlet velocity, dimensionless pitch ratios and Reynolds number. Heat leakage into the domain is modeled as a source term. Numerical results compare favorably to published data in terms of Nusselt number and pressure drop. It was found that the energy efficiency varies between 72% and 98% for all cases, and viscous dissipation has a very low effect on the energy efficiency for low Reynolds number cases. The exergy efficiency ranges from 40–64%, and the entropy generation due to heat transfer was found to have a significant effect on exergy efficiency. The results suggest that exergy efficiency can be maximized by choosing specific pitch ratios for various Reynolds numbers. The results could be useful in designing more efficient heat recovery systems, especially for low temperature applications.
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Ambreen, Tehmina, Arslan Saleem, and Cheol Woo Park. "Homogeneous and Multiphase Analysis of Nanofluids Containing Nonspherical MWCNT and GNP Nanoparticles Considering the Influence of Interfacial Layering." Nanomaterials 11, no. 2 (January 21, 2021): 277. http://dx.doi.org/10.3390/nano11020277.
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The practical implication of nanofluids is essentially dependent on their accurate modelling, particularly in comparison with the high cost of experimental investigations, yet the accuracy of different computational approaches to simulate nanofluids remains controversial to this day. Therefore, the present study is aimed at analysing the homogenous, multiphase Eulerian–Eulerian (volume of fluid, mixture, Eulerian) and Lagrangian–Eulerian approximation of nanofluids containing nonspherical nanoparticles. The heat transfer and pressure drop characteristics of the multiwalled carbon nanotubes (MWCNT)-based and multiwalled carbon nanotubes/graphene nanoplatelets (MWCNT/GNP)-based nanofluids are computed by incorporating the influence of several physical mechanisms, including interfacial nanolayering. The accuracy of tested computational approaches is evaluated by considering particle concentration and Reynolds number ranges of 0.075–0.25 wt% and 200–470, respectively. The results demonstrate that for all nanofluid combinations and operational conditions, the Lagrangian–Eulerian approximation provides the most accurate convective heat transfer coefficient values with a maximum deviation of 5.34% for 0.25 wt% of MWCNT–water nanofluid at the largest Reynolds number, while single-phase and Eulerian–Eulerian multiphase models accurately estimate the thermal fields of the diluted nanofluids at low Reynolds numbers, but overestimate the results for denser nanofluids at high Reynolds numbers.
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Sunil, Arjun Kozhikkatil, and Rakesh Kumar. "LBM Analysis of Micro-Convection in MHD Nanofluid Flow." Strojniški vestnik - Journal of Mechanical Engineering 63, no. 7-8 (July 17, 2017): 426. http://dx.doi.org/10.5545/sv-jme.2016.4248.
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The lattice Boltzmann-Bhatnagar-Gross-Krook method was used to simulate Al2O3-water nanofluid to find the effects of Reynolds, Rayleigh and Hartmann numbers, slip coefficient, nanoparticle volume fraction and axial distance on forced convection heat transfer in MATLAB. The ranges of studied Reynolds number, Rayleigh number, magnetic field strength, nanoparticle volume concentration and slip coefficient include 200 ≤ Re ≤ 4000; 103 ≤ Ra ≤ 106; 0 ≤ Ha 90; 0 ≤ φ ≤ 2%; 0.005 ≤ B ≤ 0.02, respectively. The results show that increasing Reynolds number and nanoparticle volume fractions improve heat transfer in the 2D microtube under laminar, turbulent, slip and temperature jump boundary conditions. Decreasing the values of slip coefficient decreases the temperature jump and enhances the Nusselt number. A critical value for the Rayleigh number (105) and magnetic field strength (Ha 10) exists, at which the impacts of the solid volume fraction and slip coefficient effects are the most pronounced. The pressure drop shows a similar type of enhancement in magnitude, as observed in the case of the Nusselt number. However, application of nanofluids for low Reynolds numbers is more beneficial, and the effect of volume fractions are more pronounced in comparison to slip coefficient, though the effects are marginal.
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Hong, Chungpyo, Yutaka Asako, Stephen E. Turner, and Mohammad Faghri. "Friction Factor Correlations for Gas Flow in Slip Flow Regime." Journal of Fluids Engineering 129, no. 10 (April 11, 2007): 1268–76. http://dx.doi.org/10.1115/1.2776966.
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Poiseuille number, the product of friction factor and Reynolds number (fRe) for quasi-fully-developed gas microchannel flow in the slip flow regime, was obtained numerically based on the arbitrary-Lagrangian-Eulerian method. Two-dimensional compressible momentum and energy equations were solved for a wide range of Reynolds and Mach numbers for constant wall temperatures that are lower or higher than the inlet temperature. The channel height ranges from 2 μm to 10 μm and the channel aspect ratio is 200. The stagnation pressure pstg is chosen such that the exit Mach number ranges from 0.1 to 1.0. The outlet pressure is fixed at atmospheric conditon. Mach and Knudsen numbers are systematically varied to determine their effects on fRe. The correlation for fRe for the slip flow is obtained from that of fRe of no-slip flow and incompressible theory as a function of Mach and Knudsen numbers. The results are in excellent agreement with the available experimental measurements. It was found that fRe is a function of Mach and Knudsen numbers and is different from the values by 96/(1+12Kn) obtained from the incompressible flow theory.
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TECHET, A. H., F. S. HOVER, and M. S. TRIANTAFYLLOU. "Vortical patterns behind a tapered cylinder oscillating transversely to a uniform flow." Journal of Fluid Mechanics 363 (May 25, 1998): 79–96. http://dx.doi.org/10.1017/s0022112098001104.
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Visualization studies of the flow behind an oscillating tapered cylinder are performed at Reynolds numbers from 400 to 1500. The cylinder has taper ratio 40[ratio ]1 and is moving at constant forward speed U while being forced to oscillate harmonically in the transverse direction. It is shown that within the lock-in region and above a threshold amplitude, no cells form and, instead, a single frequency of response dominates the entire span. Within certain frequency ranges a single mode dominates in the wake, consisting of shedding along the entire span of either two vortices per cycle (‘2S’ mode), or four vortices per cycle (‘2P’ mode); but within specific parametric ranges a hybrid mode is observed, consisting of a ‘2S’ pattern along the part of the span with the larger diameter and a ‘2P’ pattern along the part of the span with the smaller diameter. A distinct vortex split connects the two patterns which are phase-locked and have the same frequency. The hybrid mode is periodic, unlike vortex dislocations, and the location of the vortex split remains stable and repeatable, within one to two diameters, depending on the amplitude and frequency of oscillation and the Reynolds number.
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Pastore, Douglas M., Richard N. Peterson, Diane B. Fribance, Richard Viso, and Erin E. Hackett. "Hydrodynamic Drivers of Dissolved Oxygen Variability within a Tidal Creek in Myrtle Beach, South Carolina." Water 11, no. 8 (August 19, 2019): 1723. http://dx.doi.org/10.3390/w11081723.
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Beach erosion and water quality degradation have been observed in Singleton Swash, a tidal creek that traverses the beach-face connecting land and ocean in Myrtle Beach, SC. The objective of this study in Singleton Swash is to explore relationships between water quality and hydrodynamics, where the latter are influenced by beach face morphology. We measure water velocities, water levels, and dissolved oxygen concentrations (DO) (a proxy for water quality) and apply correlation analysis to examine the relationships between physical processes and dissolved oxygen variations. Results show that larger tidal ranges are associated with higher mean levels of DO in the tidal creek. The larger tidal ranges are linked to larger magnitude currents, which increase both the DO transport via larger fluxes of oxygenated oceanic water into the swash and the magnitude of Reynolds shear stresses; due to tidal asymmetries, flood currents are stronger than ebb currents in this system. Based on these results, it is concluded that the combined transport of oxygenated waters into the tidal creek from the ocean on large flood tides and subsequent mixing due to large Reynolds shear stresses result in the observed net DO concentration increases in the creek over tidal cycles.
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Badano, Nicolás D., and Angel N. Menéndez. "Accuracy of boundary layer treatments at different Reynolds scales." Open Engineering 10, no. 1 (April 8, 2020): 295–310. http://dx.doi.org/10.1515/eng-2020-0033.
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AbstractResistive forces associated to boundary layers (‘friction’) are usually out of scale in physical models of hydraulic structures, especially in the case of hydraulically smooth walls, generating distortions in the model results known as scale effects, that can be problematic in some relevant engineering problems. These scale effects can be quantified and corrected using suitable numerical models. In this paper the accuracy of using numerical simulation through the Reynolds Averaged Navier-Stokes (RANS) approximation in order to represent the head losses introduced by friction in hydraulically smooth walls is evaluated for a wide range of Reynolds scales. This is performed by comparing the numerical results for fully developed flow on circular pipes and between parallel plates against experimental results, using the most popular wall treatments. The associated numerical errors, mesh requirements and ranges of application are established for each treatment. It is shown that, when properly applied, RANS models are able to simulate the head losses produced by smooth wall friction accurately enough as to quantify the scale effects present in physical models. A methodology for upscaling physical model results to prototype scale, free of scale effects, is proposed.
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Karami, K., and Z. Ebrahimi. "Phase-Mixing and Dissipation of Standing Shear Alfvén Waves." Publications of the Astronomical Society of Australia 26, no. 4 (2009): 448–53. http://dx.doi.org/10.1071/as09019.
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AbstractWe study the phase mixing and dissipation of a packet of standing shear Alfvén waves localized in a region with non-uniform Alfvén background velocity. We investigate the validity of the exponential damping law in time, exp (–At3), presented by Heyvaerts & Priest (1983) for different ranges of Lundquist, S, and Reynolds, R, numbers. Our numerical results shows that it is valid for (R,S) ≥ 107.
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Golijanek-Jędrzejczyk, Anna, Andrzej Mrowiec, Robert Hanus, Marcin Zych, and Dariusz Świsulski. "Determination of the uncertainty of mass flow measurement using the orifice for different values of the Reynolds number." EPJ Web of Conferences 213 (2019): 02022. http://dx.doi.org/10.1051/epjconf/201921302022.
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Standard orifice flowmeters are widely used in the chemical and energy industry. Therefore, it is essential to know how accurate the measurements made with these instruments are. The paper presents an estimation of measurement uncertainty of a liquid mass flow using the orifice plate. The authors will present the influence of ranges of the Reynolds number on the estimated uncertainty, obtained on the basis of simulation and laboratory experiments. The research was conducted for the central orifice in the Reynolds number 8,000 < Re < 21,000. The results of estimating the extended uncertainty of the measurement of water flow using simulation and experimental method, are convergent. The maximum difference in the extended uncertainty values of flow measurement for the simulation and experiment was 0.04.10-3 kg/s.
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Kouadri, A., Y. Lasbet, and M. Makhlouf. "High mixing performances of shear-thinning fluids in two-layer crossing channels micromixer at very low Reynolds numbers." Journal of Mechanical Engineering and Sciences 13, no. 4 (December 30, 2019): 5938–60. http://dx.doi.org/10.15282/jmes.13.4.2019.15.0471.
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In a recent study, the Two-Layer Crossing Channels Micromixer (TLCCM) exhibited good mixing capacities in the case of the Newtonian fluids (close to 100%) for all considered Reynolds number values. However, since the majority of the used fluids in the industrial sectors are non-Newtonians, this work details the mixing evolution of power-law fluids in the considered geometry. In this paper, the power-law index ranges from 0.73 to 1 and the generalized Reynolds number is bounded between 0.1 and 50. The conservation equations of momentum, mass and species transport are numerically solved using a CFD code, considering the species transport model. The flow structure at the cross-sectional planes of our micromixer was studied using the dynamic systems theory. The evolutions of the intensity, also the axial, radial and tangential velocity profiles were examined for different values of the Reynolds number and the power-law index. Besides, the pressure drop of the power-law fluids under different Reynolds number was calculated and represented. Furthermore, the mixing efficiency is evaluated by the computation of the mixing index (MI), based on the standard deviation of the mass fraction in different cross-sections. In such geometry, a perfect mixing is achieved with MI closed to 99.47 %, at very small Reynolds number (from the value 0.1) whatever the power-law index and generalized Reynolds numbers taken in this investigation. Consequently, the targeted channel presents a useful tool for pertinent mass transfer improvements, it is highly recommended to include it in various microfluidic systems.
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Rajamuni, Methma M., Mark C. Thompson, and Kerry Hourigan. "Transverse flow-induced vibrations of a sphere." Journal of Fluid Mechanics 837 (January 5, 2018): 931–66. http://dx.doi.org/10.1017/jfm.2017.881.
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Flow-induced vibration of an elastically mounted sphere was investigated computationally for the classic case where the sphere motion was constrained to move in a direction transverse to the free stream. This study, therefore, provides additional insight into, and comparison with, corresponding experimental studies of transverse motion, and distinction from numerical and experimental studies with specific constraints such as tethering (Williamson & Govardhan, J. Fluids Struct., vol. 11, 1997, pp. 293–305) or motion in all three directions (Behara et al., J. Fluid Mech., vol. 686, 2011, pp. 426–450). Two sets of simulations were conducted by fixing the Reynolds number at $Re=300$ or 800 over the reduced velocity ranges $3.5\leqslant U^{\ast }\leqslant 100$ and $3\leqslant U^{\ast }\leqslant 50$ respectively. The reduced mass of the sphere was kept constant at $m_{r}=1.5$ for both sets. The flow satisfied the incompressible Navier–Stokes equations, while the coupled sphere motion was modelled by a spring–mass–damper system, with damping set to zero. The sphere showed a highly periodic large-amplitude vortex-induced vibration response over a lower reduced velocity range at both Reynolds numbers considered. This response was designated as branch A, rather than the initial/upper or mode I/II branch, in order to allow it to be discussed independently from the observed experimental response at higher Reynolds numbers which shows both similarities and differences. At $Re=300$, it occurred over the range $5.5\leqslant U^{\ast }\leqslant 10$, with a maximum oscillation amplitude of ${\approx}0.4D$. On increasing the Reynolds number to 800, this branch widened to cover the range $4.5\leqslant U^{\ast }\leqslant 13$ and the oscillation amplitude increased (maximum amplitude ${\approx}0.6D$). In terms of wake dynamics, within this response branch, two streets of interlaced hairpin-type vortex loops were formed behind the sphere. The upper and lower sets of vortex loops were disconnected, as were their accompanying tails. The wake maintained symmetry relative to the plane defined by the streamwise and sphere motion directions. The topology of this wake structure was analogous to that seen experimentally at higher Reynolds numbers by Govardhan & Williamson (J. Fluid Mech., vol. 531, 2005, pp. 11–47). At even higher reduced velocities, the sphere showed distinct oscillatory behaviour at both Reynolds numbers examined. At $Re=300$, small but non-negligible oscillations were found to occur (amplitude of ${\approx}0.05D$) within the reduced velocity ranges $13\leqslant U^{\ast }\leqslant 16$ and $26\leqslant U^{\ast }\leqslant 100$, named branch B and branch C respectively. Moreover, within these reduced velocity ranges, the centre of motion of the sphere shifted from its static position. In contrast, at $Re=800$, the sphere showed an aperiodic intermittent mode IV vibration state immediately beyond branch A, for $U^{\ast }\geqslant 14$. This vibration state was designated as the intermittent branch. Interestingly, the dominant frequency of the sphere vibration was close to the natural frequency of the system, as observed by Jauvtis et al. (J. Fluids Struct., vol. 15(3), 2001, pp. 555–563) in higher-mass-ratio higher-Reynolds-number experiments. The oscillation amplitude increased as the reduced velocity increased and reached a value of ${\approx}0.9D$ at $U^{\ast }=50$. The wake was irregular, with multiple vortex shedding cycles during each cycle of sphere oscillation.
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Klein, Markus, Theresa Trummler, Noah Urban, and Nilanjan Chakraborty. "Multiscale Analysis of Anisotropy of Reynolds Stresses, Subgrid Stresses and Dissipation in Statistically Planar Turbulent Premixed Flames." Applied Sciences 12, no. 5 (February 22, 2022): 2275. http://dx.doi.org/10.3390/app12052275.
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The characterisation of small-scale turbulence has been an active area of research for decades and this includes, particularly, the analysis of small-scale isotropy, as postulated by Kolmogorov. In particular, the question if the dissipation tensor is isotropic or not, and how it is related to the anisotropy of the Reynolds stresses is of particular interest for modelling purposes. While this subject has been extensively studied in the context of isothermal flows, the situation is more complicated in turbulent reacting flows because of heat release. Furthermore, the landscape of Computational Fluid Dynamics is characterised by a multitude of methods ranging from Reynolds-averaged to Large Eddy Simulation techniques, and they address different ranges of scales of the turbulence kinetic energy spectrum. Therefore, a multiscale analysis of the anisotropies of Reynolds stress, dissipation and sub-grid scale tensor has been performed by using a DNS database of statistically planar turbulent premixed flames. Results show that the coupling between dissipation tensor and Reynolds stress tensor is weaker compared to isothermal turbulent boundary layer flows. In particular, for low and moderate turbulence intensities, heat release induces pronounced anisotropies which affect not only fluctuation strengths but also the characteristic size of structures associated with different velocity components.
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RIEDINGER, XAVIER, STÉPHANE LE DIZÈS, and PATRICE MEUNIER. "Viscous stability properties of a Lamb–Oseen vortex in a stratified fluid." Journal of Fluid Mechanics 645 (February 22, 2010): 255–78. http://dx.doi.org/10.1017/s002211200999262x.
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In this work, we analyse the linear stability of a frozen Lamb–Oseen vortex in a fluid linearly stratified along the vortex axis. The temporal stability properties of three-dimensional normal modes are obtained under the Boussinesq approximation with a Chebychev collocation spectral code for large ranges of Froude numbers and Reynolds numbers (the Schmidt number being fixed to 700). A specific integration technique in the complex plane is used in order to apply the condition of radiation at infinity. For large Reynolds numbers and small Froude numbers, we show that the vortex is unstable with respect to all non-axisymmetrical waves. The most unstable mode is however always a helical radiative mode (m = 1) which resembles either a displacement mode or a ring mode. The displacement mode is found to be unstable for all Reynolds numbers and for moderate Froude numbers (F ~ 1). The radiative ring mode is by contrast unstable only for large Reynolds numbers above 104 and is the most unstable mode for large Froude numbers (F > 2). The destabilization of this mode for large Froude numbers is shown to be associated with a resonance mechanism which is analysed in detail. Analyses of the scaling and of the spatial structure of the different unstable modes are also provided.
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Tso, C. P., G. P. Xu, and K. W. Tou. "An Experimental Study on Forced Convection Heat Transfer From Flush-Mounted Discrete Heat Sources." Journal of Heat Transfer 121, no. 2 (May 1, 1999): 326–32. http://dx.doi.org/10.1115/1.2825984.
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Experiments have been performed using water to determine the single-phase forced convection heat transfer from in-line four simulated electronic chips, which are flush-mounted to one wall of a vertical rectangular channel. The effects of the most influential geometric parameters on heat transfer including chip number, and channel height are tested. The channel height is varied over values of 0.5, 0.7, and 1.0 times the heat source length. The heat flux is set at the three values of 5 W/cm2, 10 W/cm2, and 20 W/cm2, and the Reynolds number based on the heat source length ranges from 6 × 102 to 8 × 104. Transition Reynolds numbers are deduced from the heat transfer data. The experimental results indicate that the heat transfer coefficient is affected strongly by the number of chips and the Reynolds number and weakly by the channel height. Finally, the present results from liquid-cooling are compared with other results from air-cooling, and Prandtl number scaling between air and water is investigated.
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Al-Najem, N. M., K. Y. Ezuddin, and M. A. Darwish. "Heat Transfer Analysis of Local Evaporative Turbulent Falling Liquid Films." Journal of Heat Transfer 114, no. 3 (August 1, 1992): 688–94. http://dx.doi.org/10.1115/1.2911335.
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A theoretical study has been conducted for evaporative heating of turbulent free-falling liquid films inside long vertical tubes. The methodology of the present work is based on splitting the energy equation into homogeneous and nonhomogeneous problems. Solving these simple problems yields a rapidly converging solution, which is convenient for computational purposes. The eigenvalues associated with the homogeneous problem can be computed efficiently, without missing any one of them, by the sign-count algorithm. A new correlation for the local evaporative heat transfer coefficient along the tube length is developed over wide ranges of Reynolds and Prandtl numbers. Furthermore, the average heat transfer coefficient is correlated as a function of Reynolds and Prandtl numbers as well as the interfacial shear stress. A correlation for the heat transfer coefficient in the fully developed region is also presented in terms of Reynolds and Prandtl numbers. Typical numerical results showed excellent agreement of the present approach with the available data in the literature. Moreover, a parametric study is made to illustrate the general effects of various variables on the velocity and temperature profiles.
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SHIELS, D., and A. LEONARD. "Investigation of a drag reduction on a circular cylinder in rotary oscillation." Journal of Fluid Mechanics 431 (March 25, 2001): 297–322. http://dx.doi.org/10.1017/s002211200000313x.
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Drag reduction in two-dimensional flow over a circular cylinder, achieved using rotary oscillation, was investigated with computational simulations. In the experiments of Tokumaru & Dimotakis (1991), this mechanism was observed to yield up to 80% drag reduction at Re = 15 000 for certain ranges of frequency and amplitude of sinusoidal rotary oscillation. Simulations with a high-resolution viscous vortex method were carried out over a range of Reynolds numbers (150–15 000) to explore the effects of oscillatory rotational forcing. Significant drag reduction was observed for a rotational forcing which had been very effective in the experiments. The impact of the forcing is strongly Reynolds number dependent. The cylinder oscillation appears to trigger a distinctive shedding pattern which is related to the Reynolds number dependence of the drag reduction. It appears that the source of this unusual shedding pattern and associated drag reduction is vortex dynamics in the boundary layer initiated by the oscillatory cylinder rotation. The practical efficiency of the drag reduction procedure is also discussed.
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Hwang, Jenn-Jiang, and Tong-Miin Liou. "Effect of Permeable Ribs on Heat Transfer and Friction in a Rectangular Channel." Journal of Turbomachinery 117, no. 2 (April 1, 1995): 265–71. http://dx.doi.org/10.1115/1.2835655.
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Heat transfer and friction characteristics in a rectangular channel with perforated ribs arranged in-line on two opposite walls are investigated experimentally. Five perforated rib open-area ratios (0, 10, 22, 38, and 44 percent) and three rib pitch-to-height ratios (10, 15, and 20) are examined. The Reynolds number ranges from 5000 to 50,000. The rib height-to-channel hydraulic diameter ratio and the channel aspect ratio are 0.081 and 4, respectively. Laser holographic interferometry is employed not only to measure the heat transfer coefficients of the ribbed wall but also to determine the rib apparent permeability. It is found that ribs with appropriately high open-area ratio and high Reynolds number are permeable, and the critical Reynolds number for evidence of flow permeability decreases with increasing rib open-area ratio. Results of local heat transfer coefficients further show that the permeable ribs have an advantage of obviating hot spots. Moreover, the duct with permeable ribs gives a higher thermal performance than that with solid ribs.
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Derksen, J. J. "Particle-resolved simulations of liquid fluidization of rigid and flexible fibers." Acta Mechanica 231, no. 12 (October 2, 2020): 5193–203. http://dx.doi.org/10.1007/s00707-020-02832-2.
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Abstract Particle-resolved, three-dimensional, time-dependent simulations of rigid and flexible cylinders fluidized by a liquid flow in fully periodic domains have been performed by means of the lattice-Boltzmann method supplemented with immersed boundaries. The solids volume fraction ranges from 0.10 to 0.48 and the length-over-diameter aspect ratio of the cylinders from 4 to 12. The bending stiffness of the cylinders is the third major input parameter. The resulting Reynolds numbers based on the average slip velocity of the cylinders and their equivalent diameter range from 6 to 70. It is shown that increasing the flexibility—that is, decreasing the bending stiffness—reduces the Reynolds number, an effect that is most pronounced for low solids volume fractions and long cylinders. As for rigid cylinders, the distribution of the orientation relative to the direction of gravity of the flexible cylinders is a pronounced function of the solids volume fraction and the aspect ratio. Flexibility tends to somewhat randomize the orientation distribution, which could explain the effect of flexibility on the slip velocity and thus the Reynolds number.
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WOSNIK, MARTIN, LUCIANO CASTILLO, and WILLIAM K. GEORGE. "A theory for turbulent pipe and channel flows." Journal of Fluid Mechanics 421 (October 25, 2000): 115–45. http://dx.doi.org/10.1017/s0022112000001385.
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A theory for fully developed turbulent pipe and channel flows is proposed which extends the classical analysis to include the effects of finite Reynolds number. The proper scaling for these flows at finite Reynolds number is developed from dimensional and physical considerations using the Reynolds-averaged Navier–Stokes equations. In the limit of infinite Reynolds number, these reduce to the familiar law of the wall and velocity deficit law respectively.The fact that both scaled profiles describe the entire flow for finite values of Reynolds number but reduce to inner and outer profiles is used to determine their functional forms in the ‘overlap’ region which both retain in the limit. This overlap region corresponds to the constant, Reynolds shear stress region (30 < y+ < 0.1R+ approximately, where R+ = u*R/v). The profiles in this overlap region are logarithmic, but in the variable y + a where a is an offset. Unlike the classical theory, the additive parameters, Bi, Bo, and log coefficient, 1/κ, depend on R+. They are asymptotically constant, however, and are linked by a constraint equation. The corresponding friction law is also logarithmic and entirely determined by the velocity profile parameters, or vice versa.It is also argued that there exists a mesolayer near the bottom of the overlap region approximately bounded by 30 < y+ < 300 where there is not the necessary scale separation between the energy and dissipation ranges for inertially dominated turbulence. As a consequence, the Reynolds stress and mean flow retain a Reynolds number dependence, even though the terms explicitly containing the viscosity are negligible in the single-point Reynolds-averaged equations. A simple turbulence model shows that the offset parameter a accounts for the mesolayer, and because of it a logarithmic behaviour in y applies only beyond y+ > 300, well outside where it has commonly been sought.The experimental data from the superpipe experiment and DNS of channel flow are carefully examined and shown to be in excellent agreement with the new theory over the entire range 1.8 × 102 < R+ < 5.3 × 105. The Reynolds number dependence of all the parameters and the friction law can be determined from the single empirical function, H = A/(ln R+)α for α > 0, just as for boundary layers. The Reynolds number dependence of the parameters diminishes very slowly with increasing Reynolds number, and the asymptotic behaviour is reached only when R+ [Gt ] 105.
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Benouis, Fatima Zohra, and Ould Amer Yacine. "Heat Transfer Enhancement of Heat Sources at its Optimum Position in a Square Enclosure with Ventilation Ports." Defect and Diffusion Forum 406 (January 2021): 12–24. http://dx.doi.org/10.4028/www.scientific.net/ddf.406.12.
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Forced convection in a ventilated enclosure with aspect ratio 2 is studied. Three heat sources simulating electronic component are placed in the bottom wall of the cavity, all walls are kept insulated. With varying the inlet and the outlet location of cold air firstly then swapping the location of the heat sources, the optimal cooling strategy was identified. Consideration was given to steady two-dimensional laminar flow and Reynolds number (Re) in the range 10–1500. The governing equations along with the boundary conditions are solved by using the control volume method. Calculations showed that enhancement in heat transfer occurred, and the results indicate that there exists an optimal location of ventilation ports and an optimal disposition of heat sources for which the heat transfer is maximized for all ranges of Reynolds numbers.
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Benouis, Fatima Zohra, and Ould Amer Yacine. "Heat Transfer Enhancement of Heat Sources at its Optimum Position in a Square Enclosure with Ventilation Ports." Defect and Diffusion Forum 406 (January 2021): 12–24. http://dx.doi.org/10.4028/www.scientific.net/ddf.406.12.
Full textAbstract:
Forced convection in a ventilated enclosure with aspect ratio 2 is studied. Three heat sources simulating electronic component are placed in the bottom wall of the cavity, all walls are kept insulated. With varying the inlet and the outlet location of cold air firstly then swapping the location of the heat sources, the optimal cooling strategy was identified. Consideration was given to steady two-dimensional laminar flow and Reynolds number (Re) in the range 10–1500. The governing equations along with the boundary conditions are solved by using the control volume method. Calculations showed that enhancement in heat transfer occurred, and the results indicate that there exists an optimal location of ventilation ports and an optimal disposition of heat sources for which the heat transfer is maximized for all ranges of Reynolds numbers.
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Kapat, J. S., J. Ratnathicam, and B. B. Mikic´. "Experimental Determination of Transition to Turbulence in a Rectangular Channel With Eddy Promoters." Journal of Fluids Engineering 116, no. 3 (September 1, 1994): 484–87. http://dx.doi.org/10.1115/1.2910302.
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We report on laminar-to-turbulent transition in a rectangular channel in the presence of periodically placed cylindrical eddy promoters. Transition is identified through the analysis of power spectral density (PSD) of velocity fluctuations. Placement of the eddy promoters in the channel, depending on the geometric configuration, can significantly reduce the value of Reynolds number at transition. The critical Reynolds number (based on the average velocity and the channel height) ranges from 1500 (for an unobstructed channel) to about 400 (for the most unstable configuration we have deployed). For all the configurations tested, demarcation of transition can be correlated with the expression: Reτ≡τ¯w,αv/ρH/2/ν=44˜51, where τw,αv is the spatially averaged value of mean wall shear stress and H is the channel height.
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Mu¨ller-Karger, Carmen M., and Andre´s L. Granados. "Derivation of Hydrodynamic Bearing Coefficients Using the Minimum Square Method." Journal of Tribology 119, no. 4 (October 1, 1997): 802–7. http://dx.doi.org/10.1115/1.2833888.
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A linear analysis of the parameters for the orbital transient response of journal-bearing systems is made with the purpose of computing the bearing dynamic coefficients using the minimum square method. The journal-bearing response is obtained from a nonlinear simulation that includes a transient solution of the Reynolds equation. The minimum square method permits the adjustment of coefficients with only one orbit and does not need prior linearization of the response. Therefore it was found to be advantageous compared with the more traditional experimental method of using a frequency domain method with two orbital responses. Three different Sommerfeld numbers were analyzed. Comparisons between the eight adjusted coefficients and the linear coefficients obtained from perturbations of the Reynolds equation about the equilibrium position permit the establishment of the ranges where the bearings behave linearly.
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Ahmad, Shafee, and Shams Ul-Islam. "Numerical Investigation of Fluid Flow past Four Cylinders at Low Reynolds Numbers." Mathematical Problems in Engineering 2021 (July 26, 2021): 1–24. http://dx.doi.org/10.1155/2021/1127324.
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A numerical investigation on the effects of separation ratios and Reynolds numbers on the flow around four square cylinders in diamond arrangement has been carried out using the lattice Boltzmann method. The separation ratios between the cylinders vary from g ∗ = 1 to 15. The Reynolds numbers based on the diameter of the square cylinder and the inlet uniform inflow velocity are selected from Re = 80 to 160. The computations show that a total of five different flow regimes are observed over the selected ranges: single bluff-body, quasi-unsteady, chaotic flow, in-phase synchronized vortex shedding, and antiphase synchronized vortex shedding flow regimes. It is found that the flow features significantly depend on both the separation ratio and Reynolds number, with the former’s influence being more than the latter’s. We found that the critical spacing for four square cylinders in diamond arrangement for selected Reynolds numbers (80 ≤ Re ≤ 160) is in the range of 2 ≤ g ∗ ≤ 5. The results reveal that the presence of secondary cylinder interaction frequencies indicates that, for chaotic flow regime, the wake pattern is not stable and there is a strong interaction of gap flows and continuous change in the direction of shed vortices behind the cylinders. The effects of the g ∗ and Re on fluid forces, vortex shedding frequency, and flow separation have been examined in detail.
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Sukhotskii, A. B., and G. S. Sidorik. "EXPERIMENTAL STUDY OF HEAT TRANSFER OF A SINGLE-ROW BUNDLE OF FINNED TUBES IN MIXED CONVECTION OF AIR." ENERGETIKA. Proceedings of CIS higher education institutions and power engineering associations 60, no. 4 (July 7, 2017): 352–66. http://dx.doi.org/10.21122/1029-7448-2017-60-4-352-366.
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The technique and results of experimental study of heat transfer of a single bundle consisting of bimetallic tubes with helically knurled edges, in natural and mixed convection of air are presented. Mixed convection, i.e. a heat transfer, when the contribution of free and forced convection is comparable, was created with the help of the exhaust shaft mounted above the heat exchanger bundle and forced air movement was created by the difference in density of the air in the shaft and the environment. The experimental dependence of the heat transfer of finned single row of bundles in the selected ranges of Grashof and Reynolds numbers has been determined. It is demonstrated that heat transfer in the mixed convection is 2.5−3 times higher than in free one and the growth rate of heat transfer with increasing Reynolds number is more than in the forced convection. Different forms of representation of results of experiments were analyzed and it was determined that the Nusselt number has a single power dependence on the Reynolds number at any height of the exhaust shafts. A linear dependence of the Reynolds number on the square root of the Grashof number was determined as well as the proportionality factors for different shaft heights. It is noted that the characteristics of the motion of air particles in the bundle in free convection is identical to the motion of particles in forced convection at small Reynolds numbers, i.e. a free convection flow smoothly flows into a forced convection one without the typical failures or surges if additional driving forces arise.
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Ferreira, Germán, Artur Sucena, Luís L. Ferrás, Fernando T. Pinho, and Alexandre M. Afonso. "Hydrodynamic Entrance Length for Laminar Flow in Microchannels with Rectangular Cross Section." Fluids 6, no. 7 (July 1, 2021): 240. http://dx.doi.org/10.3390/fluids6070240.
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This work presents a detailed numerical investigation on the required development length (L=L/B) in laminar Newtonian fluid flow in microchannels with rectangular cross section and different aspect ratios (AR). The advent of new microfluidic technologies shifted the practical Reynolds numbers (Re) to the range of unitary (and even lower) orders of magnitude, i.e., creeping flow conditions. Therefore, accurate estimations of L at Re≤O(1) are important for microsystem design. At such low Reynolds numbers, in which inertial forces are less dominant than viscous forces, flow characteristics become necessarily different from those at the macroscale where Re is typically much larger. A judicious choice of mesh refinement and adequate numerical methods allowed obtaining accurate results and a general correlation for estimating L, valid in the ranges 0≤Re≤2000 and 0.1≤AR≤1, thus covering applications in both macro and microfluidics.
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Mohamed, Hany A. "Effect of Rotation and Surface Roughness on Heat Transfer Rate to Flow through Vertical Cylinders in Steam Condensation Process." Journal of Heat Transfer 128, no. 3 (April 12, 2005): 318–23. http://dx.doi.org/10.1115/1.2098862.
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The enhancement in the rate of the heat transfer resulting from rotating smooth and rough vertical cylinders, of 1.28 and 21.75μm average roughness, respectively, are experimentally studied. Experiments were carried out for cooling fluid Reynolds numbers from 3300 to 7800 with varying the rotational speed up to 280rpm. Experimental runs at the stationary case showed an acceptable agreement with the theoretical values. The experimental Nusselt number values at various rotational speeds are correlated as functions of Reynolds, Weber, and Prandtl numbers for smooth and rough surfaces. The correlated equations were compared with the correlation obtained by another author. The results show that the enhancement of the heat transfer rate becomes more appreciable for low Reynolds numbers at high rotational speeds and for high Reynolds numbers at low rotational speeds. The rotation causes an enhancement in the overall heat transfer coefficient of ∼89% at Re=7800, We=1084, and Pr=1.48 for smooth surface and of ∼13.7% at Re=4700, We=4891, and Pr=1.696 for rough surface. Also, the enhancement in the heat transfer rates utilizing rotary surface becomes more pronounced for the smooth surface compared with the rough one, therefore the choice of the heat transfer surface is very important. The present work shows a reduction in the heat transfer rate below its peak value depending on the type of the heat transfer surface. It is shown that the enhancement in the heat transfer, i.e., enhancement in the Nusselt number, depends on the Weber number value and the surface type while the Nusselt number value mainly depends on the Reynolds and Prandtl numbers. Correlated equation have been developed to represent the Nusselt number values as functions of the Weber and Reynolds numbers within the stated ranges of the parameters.
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Siddiqui, Aysha, Waqas Arshad, Hafiz Ali, Muzaffar Ali, and Muhammad Nasir. "Evaluation of nanofluids performance for simulated microprocessor." Thermal Science 21, no. 5 (2017): 2227–36. http://dx.doi.org/10.2298/tsci150131159s.
Full textAbstract:
In this investigation, deionized water was used as base fluid. Two different types of nanoparticles, namely Al2O3 and Cu were used with 0.251% and 0.11% volumetric concentrations in the base fluid, respectively. Nanofluids cooling rate for flat heat sink used to cool a microprocessor was observed and compared with the cooling rate of pure water. An equivalent microprocessor heat generator i. e. a heated Cu cylinder was used for controlled experimentation. Two surface heaters, each of 130 W power, were responsible for heat generation. The experiment was performed at the flow rates of 0.45, 0.55, 0.65, 0.75, and 0.85 liter per minute. The main focus of this research was to minimize the base temperature and to increase the overall heat transfer coefficient. The lowest base temperature achieved was 79.45 oC by Al2O3 nanofluid at Reynolds number of 751. Although, Al2O3-water nanofluid showed superior performance in overall heat transfer coefficient enhancement and thermal resistance reduction as compared to other tested fluids. However, with the increase of Reynolds number, Cu-water nanofluid showed better trends of thermal enhancement than Al2O3-water nanofluid, particularly at high Reynolds number ranges.