Shenq-Yuh Jaw

Numerical Simulation of Flow in a Screw-Type Blood Pump

AstractThis study presents a numerical investigation of the flow field in a screw pump designed to circulate biological fluid such as blood. A simplified channel flow model is used to allow analysis using a Cartesian set of coordinates. Finite analytic (FA) numerical simulation of the flow field inside the channel was performed to study the influence of Reynolds number and pressure gradient on velocity distribution and shear stresses across the channel cross-section. Simulation results were used to predict flow rates, circulatory flow and the shear stresses, which are known to be related to the level of red blood cell damage (hemolysis) caused by the pump. The study shows that high shear levels are confined to small regions within the channel cross-section, but the circulatory nature of the flow causes an increased percentage of blood elements to pass through the high shear regions, and increases the likelihood of cell damage.

Measurement of Pressure Distribution from PIV Experiments

In this study, a non-staggered grid SIMPLER pressure solution algorithm, which is able to produce correct pressure distribution directly if correct velocities are given, is proposed to solve the pressure distribution for PIV experiments. The cell face pseudo velocity required in the pressure equation is approximated by a simple linear average of the adjacent nodal pseudo velocities so that the velocity and pressure are collocated without causing the checkerboard pressure distribution problem. In addition, the proposed pressure solution algorithm has the features that upwind effects of the convective terms are considered, boundary conditions are not required, and the pressure distribution obtained can be used to correct the velocity field so that the continuity equation is satisfied. These features make the present algorithm a superior method to calculate the pressure distribution for PIV experiments. The pressure field solved is realistic and accurate. The proposed pressure equation solver is first calibrated with a two-dimensional cavity flow. It is found that the results are almost identical to the exact solution of the test flow. The algorithm is then applied to analyze a uniform flow past two side-by-side circular cylinders in a soap film channel. With the velocity and pressure distributions successfully measured, the structures of the complex shedding flow patterns are clearly manifested.

A two‐scale low‐Reynolds number turbulence model

A two-scale low-Reynolds number turbulence model is proposed. The Kolmogorov turbulence time scale, based on fluid kinematic viscosity and the dissipation rate of turbulent kinetic energy (ν, e), is adopted to address the viscous effects and the rapid increasing of dissipation rate in the near-wall region. As a wall is approached, the turbulence time scale transits smoothly from a turbulent kinetic energy based (κ, e) scale to a (ν, e) scale. The damping functions of the low-Reynolds number models can thus be simplified and the near-wall turbulence characteristics, such as the e distribution, are correctly reproduced. The proposed two-scale low-Reynolds number turbulence model is first examined in detail by predicting a two-dimensional channel flow, and then it is applied to predict a backward-facing step flow. Numerical results are compared with the direct numerical simulation (DNS) budgets, experimental data and the model results of Chien, and Lam and Bremhorst respectively. It is proved that the proposed two-scale model indeed improves the predictions of the turbulent flows considered

Development and Application of LED Illumination Color PIV

In this study, red, green, and blue light-emitting diodes (LED) are adopted as the light source to illuminate sequentially a two-dimensional soap film channel flow. Triple-exposure particle image is recorded on the same image frame by a 3-ccd color camera. Since the particles illuminated by the R, G, B LED will only be recorded on the R, G, B ccd-chip of the digital camera, three sequential exposure, R, G, B particle images can be obtained from separating the triple-exposure particle image. Two sequential velocity fields can be determined from the correlation analysis of the R-G and G-B sequential particle images. Time derivative of the velocity fields, and hence the evolution of the unsteady flow or the characteristics of turbulent flows can be analyzed from the two velocity fields determined. The color PIV method incorporated with the LED light has proven to be a cheap, safe, and powerful tool for the full-field flow measurements. Results of the flow past circular cylinder in the confined soap film channel flow are presented.Copyright

Development and Application of an Alternating-Color Micro-PIV System

In this study, light emitted from red, green, and blue LEDs is adopted as the light source of an alternating-color micro-PIV system for micro flows measurement. The strobe frequency of the LED can easily reach 20,000Hz, which is high enough to provide the time resolution required for most micro flows measurement. A cardioid annular condenser is adopted in the micro-PIV system so that the incident light from the LED is redirected and only the light emitted from fluorescent particles reaches the object lens of the microscope. The image quality is significantly improved. Clear dark background particle images of micro flows are recorded from the proposed LED micro-PIV system. In addition, the diffraction limit of the microscope is improved from half of the wavelength to one-fifth of the wavelength. For alternating-color multiple-exposure image application, a triple-exposure alternating-color image — sequentially illuminated by red, green, and blue LED light — is recorded on a single frame by a color CCD camera. Three unique color images — a blue, a green, and a red image respectively — are obtained from separating the triple-exposure image. With three sequential images available, the velocity, acceleration distributions of micro flow, and different phases of the multiphase flow can be measured from these unique color sequential images. The alternating-color micro-PIV system is then applied to measure micro flow past a cylinder, circulation in a micro-droplet, hydrodynamic focusing sheath flow, and two-phase flow in micro channels. Satisfactory results are obtained for all the flows measured.Copyright

Experimental Study on Generation of Single Cavitation Bubble Collapse Behavior by a High Speed Camera Record

It has been known that the collapse of the cavitation bubbles could cause serious destruction of pressure pipes, hydraulic machineries and turbine structures. After the cavitation bubble is generated, the variation of its surrounding velocity and pressure field could result in its collapse. If the process of the collapse of a cavitation bubble appears near the solid boundary, its impact to the boundary could generate an immense water-hammer pressure effect (Plesset and Chapman, 1971). The shock wave generated in this process of bubble collapse could possibly impact or even destroy the solid boundary of structure. The bubble collapse studies include the understanding of the shock wave, the characteristics of the resultant luminescence, and the jet related fields. If the cavitation bubble is located near the solid boundary at certain suitable distance, it is more possible for the production of counterjet in the process of bubble collapse. There has not been a firm conclusion for the exact characteristics which causes the destruction of the interface on the solid boundary.

PIV Measurements of Cavitation Bubble Collapse Flow Induced by Pressure Wave

In this study, a single cavitation bubble is generated by rotating a U-tube filled with water. A series of bubble collapse flows induced by pressure waves of different strengths are investigated by positioning the cavitation bubble at different stand-off distances to a solid boundary. Particle images of bubble collapse flow recorded by high speed CCD camera are analyzed by multi-grid, iterative particle image distortion method. Detail velocity variations of the transient bubble collapse flow are obtained. It is found that a Kelvin–Helmholtz vortex is formed when a liquid jet penetrates the bubble surface. If the bubble center to the solid boundary is within one to three times of the bubble radius, the liquid jet is able to impinge the solid boundary to form a stagnation ring. The fluid inside the stagnation ring will be squeezed toward the center of the ring to form a counter jet. At certain critical position, the bubble collapse flow will produce a Kelvin–Helmholtz vortex, the Richtmyer-Meshkov instability, or the generation of a counter jet flow, depending on the strengths of the pressure waves. If the bubble surface is in contact with the solid boundary, the liquid jet can only splash inside-out without producing the stagnation ring and the counter jet. The complex phenomenon of cavitation bubble collapse flows is clearly manifested in this study.Copyright

Numerical analysis of secondary instabilities of the incompressible boundary layer flow with suction

Based on the Euler-Maclaurin formula, a compact finite difference scheme is employed to solve a two-point boundary value problem for studying the secondary instabilities of the boundary layer flow. The parametric resonance of unstable waves is explored using the Floquet method. For both subharmonic and fundamental modes, two additional Fourier terms are added in the analysis, and the spatial growth rates are determined. The effect of suction mechanism on the secondary instability waves is also investigated