Narrow Results

Natural rivers are one of the main sources of water and energy for humans. The design and management of exploitation of natural rivers systems will require a complete understanding of flow and sediment transport mechanics. Meanders are one of the most common types of rivers in nature, which have a very complex flow. The three-dimensional (3D) and complex characteristics of flow in meanderings bring up the necessity of investigating the flow pattern in this channel with 3D numerical models. This study aimed to investigate the flow pattern and predict the erosion and sedimentation site in the meandering. Also, the effect of the Froude number (Fr), the ratio of width-to-depth ($B$/$h)$ and meander angle parameters on the flow field are investigated. In this study, the turbulent flow pattern in a sine-generated channel and the angle of 50^∘ with a rectangular section with width = 40 cm and the rigid bed is studied using the 3D mathematical model SSIIM1.1. The primary and secondary flow patterns were studied using the turbulence models of $k$-$\epsilon$ and SST $k$-$\omega$. Comparison of numerical models and experimental results showed that the flow pattern is predicted well by both turbulence models of turbulence. But in some cases, the SST $k$-$\omega$ model has provided closer results to the experimental results. The results show that the shear stress pattern depends on the changes in the Fr. But, the secondary flow pattern does not change much by changing the Fr. Also, changes in the $B$/$h$ ratio affect the secondary flow pattern and shear stress pattern, so that for $B$/$h<8$, a small rotary cell is made in addition to the main rotary cell near the outer coast. The results also show that the flow pattern and shear stress depend on the channel angle.

This work is based on a numerical simulation to study the flow pattern and heat transfer characteristics inside a cavity in the form of an Islamic knot. The effect of height and length of the obstacle are investigated. Nine different obstacles with different lengths (a) and heights (b) are considered. It can be found that the variation in length and height of the hot obstacle leads to the obvious change in the flow field and temperature field. In addition, the small gap inside the cavity has a limited effect on the flow motion. Besides, the small gap between the cold and hot surfaces leads to higher conduction heat transport. For high Ra, the flow and heat transfer characteristics can be described in two situations. The first is included in the cases of $(a,b)=(10,1)$, (10, 2) and (10, 3), where the primary vortices are located on the top and bottom of the cavity. The second one consists of cases $(a,b)=(3,3)$, (4, 3), (5, 3), (6, 3), (7, 3) and (8, 3), where the primary flow circulations are established on the left and right sides. For all the cases, the average Nu increases by rising the Ra. By increasing the Ra, the average Nu in the second situation increases more significantly than that in the first one. The cases in the second situation always have better heat transfer performance than those in the first one, especially at higher Ra.

A heated plane water jet impinging vertically onto free water surface has been numerically studied based on large eddy simulation method coupled with the volume of fluid approach. The Boussinesq approximation is adopted to simulate the effect of buoyancy. Results showed that there exist two flow patterns for the plane thermal buoyant jet, which are the stable impinging flow pattern and the flapping impinging flow pattern. Distinct temperature stratification can be found in the stable impinging flow pattern, while it disappears in the flapping impinging flow pattern.

Plenty of well-established medical research works have shown that many vascular diseases such as stenosis and atherosclerosis are prone to appear in curved arteries. In this paper, we investigated the influence of wall compliance on flow pattern in curved arteries exposed to dynamic physiological environments in order to understand the hemodynamic mechanism and provide a basis for clinical research in related areas. Representative curved arteries with elastic and rigid walls are constructed by computers. The fluid-structure interaction (FSI) effect is considered in our calculations. Physiological velocity profile is assigned as the inlet boundary condition. No-slip boundary condition is applied at the blood-wall interface. Our results show that the maximum axial velocity in the rigid wall model is larger than that in the elastic wall model. Wall compliance also has a remarkable effect on backflow patterns. Significant differences in pressure distribution are found between the elastic and rigid wall models. Blood strain rate distribution patterns in the two models were also compared. It was interesting to discover that in the straight part of the artery, the flexible wall made the counter-rotating vortices induced by the curvature gradually disappear along a downstream direction. However, for the flow feature in the rigid wall model, strong vortices existed throughout the entire straight part of the artery. It revealed that the increment of wall rigidity results in a reduction in wall movement capacity, thus affecting the physiological function of the arterial wall, making it incapable of effectively regulating the flow pattern inside the artery. The current work indicates that the influence of wall compliance on flow pattern in curved artery is significant.

The thrombus is the inappropriate activation of hemostasis in vascular system. In this paper, biomechanical factors affecting the behaviors of artery with intraluminal thrombus were studies. Results indicated that heart rate and blood viscosity had strong impact on the compliance of the stenosis artery and flow pattern. The alteration in blood viscosity had stronger influence than cardiac cycle on the volume change of the fluid region surrounded by thrombus. von Mises stress measured at the thinnest region of the plaque had the largest time-averaged value. The alteration of these parameters could potentially lead to stress redistribution at intraluminal thrombus.

The spectral element method, a direct numerical technique, is used to study the behavior of flow past two cylinders in tandem array. A control cylinder is employed in front of the main cylinder to study the drag reduction performance and flow patterns. Three major flow patterns are found, including single cylinder vortex shedding, two cylinders vortex shedding and suppression. The flow patterns are affected by the distance between two cylinders, Reynolds number and the diameter ratios of cylinders. In a bistable regime, when there is a critical distance between cylinders, drag is reduced dramatically.

This paper presents a comprehensive review of oil retention and performance of oil separator in a heat pump system. Liquid volume fraction model, viscous film model and analytical model from continuity and momentum equations for predicting the oil retention amount in a heat pump system are reviewed. In addition, weighing a test section, using an oil injection and extraction device and optical analysis for measuring the oil retention amount are introduced and experimental results are analyzed. Numerical studies which used commercial tools for computational fluid dynamics or suggested a new model by simplifying the flow in cyclone separator are reviewed. Effects of droplet size and angular velocity of gas phase flow in cyclone separator on the performance are discussed. Experimental methods for measuring the efficiency of oil separator are introduced, and theoretical analysis on the efficiency and pressure drop of cyclone separator is suggested.

This study experimentally investigated the flow pattern, void fraction, and evaporation heat transfer characteristic of R134a upward flow in a vertical narrow rectangular channel having a hydraulic diameter of 0.99mm, resembling a plate heat exchanger. Experiments were performed with mass velocities and vapor quality ranging between 30–200kgm${}^{-2}$s${}^{-1}$ and 0.05–0.9, respectively, at a saturation temperature of 15${}^{\circ }$C. Flow patterns were classified into bubble, slug, churn, and annular flows. The void fraction increased with increasing quality, and decreased with decreasing mass velocity in the low-quality region. Further, the influence of flow inlet/outlet positions remarkably appeared when the superficial gas velocity was low. The observed flow patterns and the measured void fraction were compared to that in previous studies. The effects of mass velocity and heat flux on the evaporation heat transfer coefficient were small, and the heat transfer through the thin liquid film was dominant.

Recently, percutaneous valve replacement has emerged as an alternative treatment for stenosis of mitral, aortic, and pulmonary valves, replacing the surgical approach and providing a new perspective on transcatheter placement of cardiac valves. The conventional open heart surgery does not suit most of the cardiothoracic patient population for various reasons. Percutaneous valve replacement has started becoming the first choice for surgical replacement of the cardiac valves. Under such a scenario where good potent porcine valves made from the pericardium of pigs are being made available, the real question is designing and developing cost effective stents to bear these valves. The ones that are imported are highly expensive which cannot be afforded by some of the Indian population. This also has substantial benefits from the standpoints of health, safety, and cost. The manufacturing of a stented aortic valve is an absolutely critical job, which requires proper designs, finite element analysis, and flow dynamics studies. This paper forms the base for an eventual manufacture of stented aortic valves, giving in-depth details pertaining to design and implantation of the bioprosthesis in the aorta with flow pattern analysis postimplantation and its hemodynamic efficacy analysis. Blood flow analysis and associated hemodynamic analysis help to understand the leakage resistance of the stented valve. A valve of this kind will enable minimal invasive cardiac surgeons to perform percutaneous aortic valve replacement with ease. This would also be an economic procedure allaying the high costs that are typically involved in conventional open heart surgery. We believe that this model has great potential for helping to set up a protocol for the growing of a tissue engineered heart valve construct.