Nahmkeon Hur

A COUPLED LEVEL SET AND VOLUME-OF-FLUID METHOD FOR THE BUOYANCY-DRIVEN MOTION OF FLUID PARTICLES

A level set method is combined with the volume-of-fluid method so that the coupled method not only can calculate an interfacial curvature accurately but also can achieve mass conservation well. The coupled level set and volume-of-fluid (CLSVOF) method is applied to the buoyancy-driven motion of fluid particles. For its easy and efficient implementation, we develop a complete and efficient interface reconstruction algorithm which is based on the explicit relationship between the interface configuration and the fluid volume function. Also, a cubic-interpolated propagation (CIP) scheme is combined with the CLSVOF method to calculate the advection terms of the momentum equation accurately. The improved CLSVOF method is applied for numerical simulation of bubbles and drops rising or falling in a quiescent fluid. The numerical results are found to preserve mass conservation and to be in good agreement with the data reported in the literature.

Design Analysis and Experimental Evaluation of an MR Fluid Clutch

An MRC (Magneto-Rheological Clutch), a device to transmit torque by shear stress of MR fluids, has the property that its power transmissibility changes quickly in response to control signal. In this study, we consider methods to predict performance of an MRC. First, we anticipate the performance of an MRC with a simplified mathematical model and second, we predict the performance in consideration of the applied magnetic field and viscosity distribution of fluids caused by the field. Between the two methods, compared with experimental results, it is shown that the numerical method is closer to reality than the simplified one.

Laminar secondary flows in curved rectangular ducts

The occurrence of secondary flow in curved ducts due to the centrifugal forces can often significantly influence the flow rate. In the present work, the secondary flow of an incompressible viscous fluid in a curved duct is studied by using a finite-volume method. It is shown that as the Dean number is increased the secondary flow structure evolves into a double vortex pair for low-aspect-ratio ducts and roll cells for ducts of high aspect ratio. A stability diagram is obtained in the domain of curvature ratio and Reynolds number. It is found that for ducts of high curvature the onset of transition from single vortex pair to double vortex pair or roll cells depends on the Dean number and the curvature ratio, while for ducts of small curvature the onset can be characterized by the Dean number alone. A comparison with the available theoretical and experimental results indicates good agreement. A correlation for the friction factor as a function of the Dean number and aspect ratio is developed and is found to be in good agreement with the available experimental and computational results for a wide range of parameters.

A highly-resolved numerical study of turbulent separated flow past a backward-facing step

Abstract A numerical study of fully-developed turbulent flow past a backward-facing step is performed to analyze the effect of mesh refinement on the computed results. The time averaged equations of conservation of mass and momentum are solved by a finite-volume method using two versions of the K-e model of turbulence. The computations are performed with both the standard and the non-linear K-e turbulence models for 166 × 73 and 332 × 146 mesh points . The comparison of the results with available experimental findings indicate that nonlinear terms must be incorporated into the K-e model to account for normal stress differences, and that even with very fine resolution, the standard K-e model fails to provide accurate predictions of the flow field.

Competitive entry of sodium and potassium into nanoscale pores

We have studied the competitive entry of potassium and sodium into carbon nanotubes using molecular dynamics simulations. Our results demonstrate how a combination of strong sodium hydration coupled with strong potassium-chlorine interaction leads to enhanced potassium selectivity at certain diameters. We detail the reasons behind this, and show how variation of nanotube diameter can cause a switch to sodium selectivity, or even cause a decrease in overall ion entry despite an increase in diameter. These results demonstrate the importance of considering inter-ion dependence in the theoretical study of pore selectivity and show that, with careful design, the practical separation of sodium and potassium is possible using diameter variation alone.

Coupled Turbulent Flow, Heat, and Solute Transport in Continuous Casting Processes with an Electromagnetic Brake

ABSTRACT This study describes the numerical modeling of coupled turbulent fluid flow, heat, and solute transport in a continuous slab caster with an electromagnetic brake (EMBr). Transport equations of total mass, momentum, energy, and species for a binary iron–carbon alloy system are solved using a continuum model. The turbulent effects are taken into account using the standard k–ϵ equations, where coefficients are appropriately modified for phase change. The electromagnetic field is described by Maxwell equations. A finite-volume method is employed to solve the conservation equations associated with appropriate boundary conditions. The process variables considered are the casting speed, magnetic flux density, and carbon segregation. The effects of these process variables on the velocity, temperature, and solute distributions are reported and discussed.

A Level Set Formulation for Incompressible Two-Phase Flows on Nonorthogonal Grids

A level set (LS) formulation is presented for computing two-phase flows on nonorthogonal grids. Compared with the volume-of-fluid (VOF) method based on a nonsmooth volume-fraction function, the LS method can calculate an interface curvature more accurately by using a smooth distance function. It is also more straightforward to implement for two- and three-dimensional irregular meshes than the VOF method, which requires much more complicated geometric calculations. The LS formulation on nonorthogonal grids is verified through computations of free-surface motions in two and three dimensions. Also, an efficient formulation for contact angle modeling is incorporated into the LS method to simulate droplet motion on an inclined wall.

A numerical study of a torque converter with various methods for the accuracy improvement of performance prediction

A comparative study was carried out on numerical methods for simulating a flow inside a torque converter. To investigate the effect of different methods for handling the relative motion of the parts, three methods were considered – the frozen rotor, sliding mesh and mixing plane methods. To improve the accuracy of performance prediction, the influence of viscosity variation with the temperature was studied by a thermo-fluid analysis. From parametric studies on the numerical scheme and the mesh resolution, it is observed that the results with the frozen rotor and sliding mesh methods agree well with the experimental data, whereas the mixing plane method induces a larger difference. The effect of viscosity variation on the accuracy of simulation is also investigated.

Blood flow velocities of cerebral arteries in lacunar infarction and other ischemic strokes

Blood flow velocity is an important determinant of vascular hemodynamics. The aim of the present study was to determine the mean flow velocities (MFVs) of cerebral arteries in patients with ischemic stroke, comparing lacunar and nonlacunar infarctions. 388 consecutive patients were examined for lacunar infarction, other subtypes of ischemic stroke, and the presence of underlying internal carotid artery steno-occlusion (ICS). MFVs were measured using transcranial Doppler along the full segments of each cerebral artery including both right and left middle cerebral arteries, basilar artery, and both of the vertebral arteries. The patients were categorized into two major groups: lacunar infarction, and nonlacunar infarction with or without underlying ICS. The characteristics of patients with lacunar infarction (n=83, 21.4%) were significantly different from those with nonlacunar infarctions: younger age, lower prevalence of type 2 diabetes, and lower concentration of plasma total homocysteine. The patients with lacunar infarction had lower MFVs in cerebral arteries than the patients with nonlacunar infarctions, especially in the posterior circulation vessels such as the basilar artery and both vertebral arteries. Different hemodynamics might be pathophysiologically associated with the lacunar infarction, compared with the other subtypes of ischemic stroke.

Numerical Study on Effects of Design Factors on Flow Characteristics of a Vane Pump

In the present study, the effects of the design factors and operating conditions on flow characteristics of a vane pump for the automotive power steering system has been analyzed numerically. An unsteady moving mesh technique with cell expansion/contraction method is used to simulate the rotation of vanes with respect to stationary inlet and outlet. As a result, the flow characteristics of the flow rate and pressure rise across the vane pump were obtained. The numerical analyses for the various design factors such as number of vanes and thickness between the rotor and camring and for various operating conditions such as rotational speed and pressure difference between inlet and outlet were extensively performed. And the results were discussed in the paper.