Wall-sheared thermal convection: heat transfer enhancement and turbulence relaminarization

We studied the flow organization and heat transfer properties in two-dimensional and three-dimensional Rayleigh-B\'enard cells that are imposed with different types of wall shear. The external wall shear is added with the motivation of manipulating flow mode to control heat transfer efficiency. We imposed three types of wall shear that may facilitate the single-roll, the horizontally stacked double-roll, and the vertically stacked double-roll flow modes, respectively. Direct numerical simulations are performed for fixed Rayleigh number $Ra = 10^{8}$ and fixed Prandtl number $Pr = 5.3$, while the wall-shear Reynolds number ($Re_{w}$) is in the range $60 \le Re_{w} \le 6000$. Generally, we found enhanced heat transfer efficiency and global flow strength with the increase of $Re_{w}$. However, even with the same magnitude of global flow strength, the heat transfer efficiency varies significantly when the cells are under different types of wall shear. An interesting finding is that by increasing the wall-shear strength, the thermal turbulence is relaminarized, and more surprisingly, the heat transfer efficiency in the laminar state is higher than that in the turbulent state. We found that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that the origin of thermal turbulence laminarization is the reduced amount of thermal plumes. Because plumes are mainly responsible for turbulent kinetic energy production, when the detached plumes are swept away by the wall shear, the reduced number of plumes leads to weaker turbulent kinetic energy production. We also quantify the efficiency of facilitating heat transport via external shearing, and find that for larger $Re_{w}$, the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.Comment: 27 pages, 16 figure

Pore-scale statistics of temperature and thermal energy dissipation rate in turbulent porous convection

We report pore-scale statistical properties of temperature and thermal energy dissipation rate in a two-dimensional porous Rayleigh-B\'enard (RB) cell. High-resolution direct numerical simulations were carried out for the fixed Rayleigh number ($Ra$) of $10^{9}$ and the Prandtl numbers ($Pr$) of 5.3 and 0.7. We consider sparse porous media where the solid porous matrix is impermeable to both fluid and heat flux. The porosity ($\phi$) range $0.86 \leq \phi \le 0.98$, the corresponding Darcy number ($Da$) range $10^{-4}<Da<10^{-2}$ and the porous Rayleigh number ($Ra^{*}=Ra\cdot Da$) range $10^{5} < Ra^{*} < 10^{7}$. Our results indicate that the plume dynamics in porous RB convection are less coherent when the solid porous matrix is impermeable to heat flux, as compared to the case where it is permeable. The averaged vertical temperature profiles remain almost a constant value in the bulk, whilst the mean square fluctuations of temperature increases with decreasing porosity. Furthermore, the absolute values of skewness and flatness of the temperature are much smaller in the porous RB cell than in the canonical RB cell. We found that intense thermal energy dissipation occurs near the top and bottom walls, as well as in the bulk region of the porous RB cell. In comparison with the canonical RB cell, the small-scale thermal energy dissipation field is more intermittent in the porous cell, although both cells exhibit a non-log-normal distribution of thermal energy dissipation rate. This work highlights the impact of impermeable solid porous matrices on the statistical properties of temperature and thermal energy dissipation rate, and the findings may have practical applications in geophysics, energy and environmental engineering, as well as other fields that involve the transport of heat through porous media.Comment: 30 pages, 16 figure

Finite element modeling and fatigue life prediction of helicopter composite tail structure under multipoint coordinated loading spectrum

This paper presents a numerical study on fatigue life prediction of helicopter composite tail structure under multipoint coordinated loading spectrum. The FE model of a full-scale helicopter composite tail structure was established and then validated with the experimental results of strain and displacement distributions. Good agreement has been achieved between simulation and experiments under two multipoint coordinated static loading conditions representing left yawing and two-point horizontal landing. A progressive damage analysis was performed on the tail structure model under multipoint coordinated spectrum loading. The predicted fatigue life of the helicopter tail structure is 64 repeated applications of the load spectrum. No element failure was predicted on the tail structure after 48 repeated applications of the load spectrum. The impact damage introduced on the left-side wall thereafter propagated after another 6 repeated applications of the load spectrum, which agrees well with experimental observations. The progressive damage analysis has been shown to be a practical engineering tool for life prediction of helicopter composite structure

Effects of sodium arsenite on liver fibrosis and expression of epithelial-mesenchymal transformation-related proteins in SD rats

BackgroundLong-term exposure to sodium arsenite leads to its accumulation in the liver and liver injury as a result. Previous studies showed that mesenchymal cells play an important role in hepatic fibrosis, and epithelial-mesenchymal transformation (EMT) is considered to be a main source of mesenchymal cells.ObjectiveTo investigate the effects of sodium arsenite at different doses on liver fibrosis and EMT-related protein expressions in SD rats.MethodsTwenty-four healthy weaned SD rats, half male and half female, were randomly divided into four groups according to body weight, with 6 rats in each group. The four groups were control group (gavage with 10.0 mL·kg−1 physiological saline), 2.5 mg·kg−1 sodium arsenite group, 5.0 mg·kg−1 sodium arsenite group, and 10.0 mg·kg−1 sodium arsenite group. All rats were gavaged 6 d per week for 36 weeks and weighed once a week, the serum and liver tissues of rats were collected and weighed, then the organ coefficient was calculated. Hematoxylin-eosin staining and Masson's trichrome staining were used to determine the pathological changes of hepatic fibrosis in rats. The serum secretion levels of hyaluronic acid (HA), laminin (LN), procollagen Ⅲ N-terminal propeptide (PⅢNP), and collagen Ⅳ (COL-Ⅳ) in rats were detected by enzyme-linked immunosorbent assay (ELISA). The protein expressions of HSCs activation-related proteins, such as α-smooth muscle actin (α-SMA) and transforming growth factor-β1 (TGF-β1), as well as EMT-related markers, such as E-cadherin, N-cadherin, Vimentin, and Snail, were detected by Western blotting.ResultsCompared with the control group, the 10.0 mg·kg−1 sodium arsenite group showed decreased body weight (P<0.05) and increased liver coefficient (P<0.05) of female and male rats. The pathological staining showed that, compared with the control group, a large number of inflammatory cells were observed in liver tissue of rats exposed to sodium arsenite, liver parenchymal cells were also liquefied, necrotic, and denatured, and the collagen positive staining area of liver tissue showed an upward trend along with the increase of arsenic exposure dose (P<0.05). The results of ELISA and Western blotting showed that the serum secretion levels of HA, LN, PⅢNP, and COL-Ⅳ in the 5.0 and 10.0 mg·kg−1 sodium arsenite groups were higher than those in the control group and the 2.5 mg·kg−1 sodium arsenite group (P<0.05). Compared with the control group, the expressions of α-SMA and TGF-β1 proteins in liver tissue were increased in each sodium arsenite exposure group (P<0.05), the expression levels of E-cadherin protein were decreased (P<0.05), and the expression levels of N-cadherin, Vimentin, and Snail were increased (P<0.05).ConclusionSodium arsenite exposure can induce HSCs activation and liver fibrosis injury in SD rats, resulting in increased extracellular matrix secretion levels, accompanied by EMT in liver tissue, suggesting that EMT is closely related to the process of liver fibrosis caused by arsenic