Sung-Min Kim

Numerical Modeling of Laminar Pulsating Flow in Porous Media

The laminar pulsating flow through porous media was numerically studied. Two-dimensional flows in systems composed of a number of unit cells of generic porous structures were simulated using a computational fluid mechanics tool, with sinusoidal variations in flow with time as the boundary condition. The porous media were periodic arrays of square cylinders. Detailed numerical data for the porosity ranging from 0.64 to 0.84, with flow pulsation frequencies of 20-64 Hz were obtained. Based on these numerical data, the instantaneous as well as the cycle-average permeability and Forchheimer coefficients, to be used in the standard unsteady volume-averaged momentum conservation equation for flow in porous media, were derived. It was found that the cycle-average permeability coefficients were nearly the same as those for steady flow, but the cycle-average Forchheimer coefficients were significantly larger than those for steady flow and were sensitive to the flow oscillation frequency. Significant phase lags were observed between the volume-averaged velocity and the pressure waves. The phase difference between pressure and velocity waves, which is important for pulse tube cryocooling, depended strongly on porosity and the mean-flow Reynolds number.

Assessing radiocephalic wrist arteriovenous fistulas of obtuse anastomosis using computational fluid dynamics and clinical application

Introduction A radiocephalic arteriovenous fistula (AVF) is the best choice for achieving vascular access (VA) for hemodialysis, but this AVF has high rates of early failure due to juxta-anastomotic stenosis, making it impossible to use for dialysis. Low hemodynamic shear stress contributes to the pathophysiology of VA failure due to secondary thrombosis, stenosis, and re-occlusion after percutaneous intervention. Methods We used a computational fluid dynamics (CFDs) approach to evaluate the shear stress distribution and minimize its effects under various conditions including changes in the anastomosis angle. A three-dimensional computational domain was designed for arteriovenous end-to-side anastomosis based on anastomosis angles of 45°, 90° and including 135° angle of an obtuse anastomosis using three-dimensional design software. COMSOL Multiphysics® simulation software was used to identify the hemodynamic factors influencing wall shear stress at the anastomosis site using a low Reynolds number k-ε turbulence model that included non-Newtonian blood flow characteristics, the complete cardiac pulse cycle, and distention of blood vessels. In preliminary clinical study, all 201 patients who received a radiocephalic wrist AVF from January 2009 to February 2014 were divided into classic and obtuse angle groups. Results The CFD results showed that the largest anastomosis angle (135°) resulted in lower shear stress, which would help reduce AVF failures. This obtuse angle was preferred, as it minimized the development of anastomotic stenosis and tended to favor primary and primary-assisted patency in clinical study. Conclusions An obtuse radiocephalic wrist AVF shows more favorable patency compared to a classic radiocephalic AVF. Surgeons establishing a radiocephalic wrist AVF would be better to consider an AVF with an obtuse anastomosis.

Effect of Various Injection Hole Shapes on Film Cooling of Turbine Blade

Dispersion of coolant jets in a film cooling flow field is the result of a highly complex interaction between the film cooling jets and the mainstream. In order to investigate the effects of injection hole shapes and injection angle on the film cooling of turbine blade, four models having cylindrical and laterally-diffused holes were used. Three-dimensional Navier-Stokes code with k – e model was used to compute the film cooling coefficient on the turbine blade. A multi-block grid system was generated that was nearly orthogonal to the various surfaces. Mainstream Reynolds number based on the cylinder diameter was 7.1 × 104 . The turbulence intensity kept at 5.0% for all inlets. The effect of coolant flow rates was studied for blowing ratios of 0.9, 1.3 and 1.6, respectively. The temperature distribution of the cylindrical body surface is visualized by infrared thermography (IRT) and compared with computational results. Results show that the effects of injection hole shape and injection angle increase as the blowing ratio increases. As lateral injection angle increases, the adiabatic film cooling effectiveness is more broadly distributed and the area protected by coolant increases. The mass flow rate of the coolant through the first-row holes is less than that through the second-row holes due to the pressure distribution around the cylinder surface.© 2003 ASME