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A new method called simplified and highly stable thermal lattice Boltzmann method, which is based on the lattice Boltzmann framework, was used to simulate the hybrid nanofluid natural convection and heat transfer in a square enclosure with a heating obstacle at high Rayleigh numbers. Four fins protrude from the heating obstacle to affect the flow pattern and heat transfer performance. The effects of the Rayleigh number (106 ≤ Ra ≤ 109), nanoparticle volume fraction (0 ≤ ϕ ≤ 0.05), and length of the fin (0.1 ≤ h ≤ 0.3) on the flow pattern, temperature distribution, and heat transfer characteristics were illustrated and analyzed. To validate the present method, the benchmark simulation results were performed. Three kinds of flow patterns (steady symmetry, unsteady symmetry, and unsteady asymmetry) can be identified at various Rayleigh numbers. At different lengths of fins, the critical Rayleigh number of flow pattern transition (from steady symmetry to steady asymmetry and from steady asymmetry to unsteady asymmetry) is different. ϕ and h also significantly affect the flow pattern. At higher ϕ, the flow inside the enclosure is steadier and the effect of h on the flow pattern varies at different Ra. [ABSTRACT FROM AUTHOR]

In the present study, the impact process of droplets onto a rotational surface was experimentally studied. The influences of the rotational speed, liquid surface tension, and viscosity on the impact phenomena were carefully investigated. The phenomena of asymmetric spreading, splashing with and without liquid filament, finger formation, and secondary droplets were observed. The results showed that, at the same rotational speed, decrease in the surface tension could result in increase in the wetting length in angular direction and the wetting area. In addition, the splashing threshold was discussed based on the analysis of the lift force. In addition, increase in the linear velocity of the surface could promote the splashing behavior and generate more fingers and secondary droplets. Moreover, droplets with higher viscosity could produce more secondary droplets and less fingers. Meanwhile, droplets with higher surface tension could produce more fingers and less secondary droplets. In addition, new correlations for the spreading ratio, the number of fingers formed at the angle of 150° after impact, and the number of secondary droplets were proposed. [ABSTRACT FROM AUTHOR]

Water droplet freezing on an ice surface is a common phenomenon and poses hazards to a lot of applications, including wind turbines, aircraft, and power transmission lines. Since the water droplet temperature is critical, many studies have been carried out to understand the influence of the water droplet temperature on both the impact and freezing processes of droplets on different surfaces. However, the past research studies mainly focused on supercooled water droplets, not on hot water droplets. For applications such as hot-water ice-drilling, the understanding of freezing of hot water droplets on an ice surface is necessary. In the present study, we report the detailed dynamic motions of a hot water droplet impacting on an ice surface. The impact and freezing processes of the hot water droplet on the ice surface are recorded by two cameras. The effects of the water droplet temperature and the ice surface temperature on the impact and freezing processes of the water droplet were experimentally investigated. The results showed that, at the same ice surface temperature, the increase of the water droplet temperature resulted in the increase of the maximum spreading factor, the reduction of the height of the ice bead, and the slight increase of the freezing time. In addition, during the droplet spreading process, the experimental results of the normalized contact diameter fitted well with the exponential model and the water droplet temperature was found to have an apparent influence on the lamella thickness. [ABSTRACT FROM AUTHOR]

A droplet impinging on a liquid film finds numerous applications in diverse scientific fields. Although studies have primarily focused on spherical droplets, a droplet can present both spherical shape and non-spherical shape at the moment of impact. In the present study, the impact dynamics of a non-spherical droplet on a thin liquid film is investigated experimentally. The results show that, under the same film thickness and Weber number conditions, the splashing and deposition regimes are different for different droplet shapes such as prolate, sphere, and oblate. It is found that the splashing threshold (Ksp) is changed according to the shape of the droplet. Under the same condition when a spherical droplet is in the deposition mode, an oblate droplet is more likely to splash and form secondary droplets, while a prolate droplet is more splash-resistant even at higher Weber numbers. A modified theoretical model for crown radius evolution is developed. The predictions of theoretical model are found to agree well with the experimental results for spherical and non-spherical droplets. Moreover, at the same Weber number, a prolate droplet has a relatively higher maximum crown height than those of an oblate droplet and a spherical droplet. [ABSTRACT FROM AUTHOR]

In the present study, the impact of a water droplet onto a rotational surface was experimentally studied. The effects of the impact velocity, the rotational speed, and the impact radial position on the impact phenomena were carefully investigated. The phenomena of asymmetric spreading, finger formation, and secondary droplets were observed. Moreover, experiments of a water droplet impacting onto a stationary surface with different impact velocities were also conducted for comparisons. The results showed that, at the same impact velocity, the increase in the rotational speed or the impact radial position could result in the increase in the wetting length in the angular direction and the wetting area. In addition, at the same impact velocity, the dimensionless wetting length in the radial direction on the rotational surface was generally lower than the spreading factor on the stationary surface. In addition, new correlations for the spreading ratio, the dimensionless excess spread area, the dimensionless maximum wetting area, and the number of fingers appearing at the angle of 50° after impact were proposed. The increase in the impact velocity and the surface linear velocity could both promote the formation of the fingers at the angle of 50° after impact. In addition, the dimensionless horizontal displacement of the secondary droplet was found to be mainly influenced by the impact velocity, while the surface linear velocity only had a minor influence. [ABSTRACT FROM AUTHOR]

In the present study, we carried out an experimental investigation of the impinging and freezing processes of a supercooled large water droplet on an ice surface. One high speed camera was used to measure the dynamic motions of the water droplet while two charge coupled device (CCD) cameras were adopted to obtain the images of the freezing process and the freezing morphologies, respectively. The effects of the water droplet temperature and the ice surface temperature on the impact and freezing processes of the water droplet were carefully evaluated. The results showed that the subcooling degree of the water droplet had an apparent influence not only on the spreading process but on the freezing morphology as well. When the subcooling temperature of the water droplet was high (e.g., Tw = –3.0 °C), a triangle cross-section profile formed. However, once the subcooling temperature of the water droplet was relatively low (e.g., Tw = –9.0 °C), the final ice morphology consisted of two parts: one part was a cone-like bead in the center while the other part was an ice ring at the periphery. Moreover, at the same water droplet temperature, raising the ice surface temperature led to an increase in the maximum spreading factor. [ABSTRACT FROM AUTHOR]

We propose a novel reduced-order model and examine its applicability to the complex three-dimensional turbulent wake of a generic square-backed bluff body called the Ahmed body at the Reynolds number ReH = U∞H/ν = 9.2 × 104 (where U∞ is free-stream velocity, H the height of the body, and ν viscosity). Training datasets are obtained by large eddy simulation. The model reduction method consists of two components—a Visual Geometry Group (VGG)-based hierarchical autoencoder (H-VGG-AE) and a temporal convolutional neural network (TCN). The first step is to map the high-dimensional flow attributes into low-dimensional features, namely latent modes, which are employed as the input for the second step. The TCN is then trained to predict the low-dimensional features in a time series. We compare this method with a TCN based on proper orthogonal decomposition (POD), which utilizes time coefficients as the input in the second part. It turns out that the H-VGG-AE has a lower reconstruction error than POD when the number of latent modes is relatively small in the first part. As the number of latent modes increases, POD exceeds in the performance of model reduction. However, the H-VGG-AE-based TCN is still more effective in terms of spatiotemporal predictions because it has a lower prediction error and costs much less time. [ABSTRACT FROM AUTHOR]

In this work, the impact of the isentropic and Kantrowitz limits on the aerodynamic behavior of evacuated tube transportation (ETT) was numerically explored. Two tube train systems with different blockage ratios (β), that is, β = 0.09 and β = 0.25, were employed for the comparative study of aerodynamic drag and flow structure. The results revealed three distinct aerodynamic behaviors, corresponding to the three speed regions separated by the two critical Mach numbers. Furthermore, the influence of head and tail lengths on drag reduction was investigated in these three speed ranges. An increase in head length appeared to be more sensitive to drag reduction at a speed of 600 m/s, while a long tail was found to induce a pronounced drag reduction at 200 m/s. In addition, the combined effect of the head and tail lengths on drag reduction was close to the superposition of their individual optimization effect. Based on the results, this study concludes that the individual designs of the head and tail of ETT systems may be rather demanding to achieve the desired optimization when considering distinct cruising speeds. [ABSTRACT FROM AUTHOR]

The flow around a seal-vibrissa-shaped cylinder (SVSC) is numerically investigated using the large eddy simulation framework at a Reynolds number of 20 000. Compared with a circular cylinder (CC), the wake of the SVSC presents more stable three-dimensional separation, a longer vortex formation length, and a weaker vortex strength. The mean drag and fluctuation of the lift coefficient are 59.5% and 87.7% lower than those of the CC, respectively. Three-dimensional spectral proper orthogonal decomposition (SPOD) is used to investigate the turbulent flow around these two types of cylinders in terms of the spatial modes, mode energy, mode coefficients, and reconstructed flow by a reduced-order modeling. Four typical vortex shedding patterns are first extracted by SPOD for the SVSC, producing crescent-, twist-, branch-, and knot-shaped vortices. A concept model is proposed for the wake dynamics of the SVSC, allowing the formation and transformation of these modes to be elucidated. Detailed analysis of the impact of the flow pattern on the associated forces indicates that the dominant out-phase vortex shedding at the upper and lower saddle planes makes a significant contribution to the reduction in lift fluctuations. [ABSTRACT FROM AUTHOR]

The wake dynamics of a rectangular flat-backed bluff body is studied using both the wind tunnel experiment and Improved Delayed Detached Eddy Simulation (IDDES) at Re = 9.2 × 104. Both approaches are systematically investigated in order to provide a quantitative comparison. The wake barycenter deficit and base pressure gradient dynamics are first investigated to characterize the wake states. Second, the known global dynamics such as long-term bi-stability, vortex shedding, and wake pumping are analyzed using proper orthogonal decomposition. It is found that the wake dynamics is globally well captured by the IDDES, but with a more intense activity due to the absence of the fore-body separations observed in the experiment. The coupling of these global dynamics is explored by utilizing low-order modeling in cross planes and elaborating the evolution of the three-dimensional (3D) instantaneous wake flow from IDDES. The shedding of a large-scale hairpin vortex from the horizontal shear layer is closely associated with the pumping motion during wake switchings or switching attempts. A concept model is proposed for 3D bi-stable wake topology, which attempts to elucidate both asymmetric and symmetric wake configurations. [ABSTRACT FROM AUTHOR]

In this paper, the laminar forced convection heat transfer of nanofluid through a bent channel was numerically investigated. The lattice Boltzmann method was used for solving the governing equations in the domain. The effect of different parameters such as Reynolds number (50 ≤ Re ≤ 150), vertical passage ratio (2.0 ≤ M ≤ 4.0), and nanoparticle solid volume fractions (Φ = 0, 0.01, 0.03, 0.05) are analyzed in terms of streamlines, isotherms, and local Nusselt numbers. It was concluded from this study that the local and average Nusselt number increased with increasing nanoparticle volume fraction regardless of Re and M. Moreover, the effect of the nanofluid concentration on the increment of heat transfer was more remarkable at higher values of the Reynolds number. Simulations show that by increasing the Reynolds number or decreasing the vertical passage ratio, the local and average Nusselt number increases. [ABSTRACT FROM AUTHOR]