Joon Hock Yeo

Velocity measurements downstream of a lobed-forced mixer with different trailing-edge configurations

Velocity and turbulence characteristics have been measured downstream of a lobed-forced mixer with three different trailing-edge configurations, using a two-component laser-Doppler anemometer at a Reynolds number of 2.27 x 10 4 (based on the bulk mean velocity of the two streams at 10 m/s, and a nominal wavelength of the lobe at 33 mm), and with velocity ratios of 1:1,1:2, and 1:3 across the lobe. The three trailing-edge configurations under investigation have the shape of a square wave, a semicircular wave, and a triangular wave. The results for the velocity ratio 1:1 indicate that the mixing is not only affected by the strength of the secondary flow shed by a lobe, but also by the boundary-layer thickness grown along the side walls of the lobe penetration and the subsequent shedding of the boundary layer to the wake region. No region of high turbulence was found within six wavelengths downstream of the trailing edge. The results for the higher velocity ratios, 1:2 and 1:3, reveal a very different flow development from those of an equal velocity ratio. A high-turbulence region appeared at around two to three wavelengths downstream of the trailing edge, and was followed by a gradual decay in magnitude. Analyzing the production terms in the Reynolds stresses equations at the corresponding location suggested that positive production of turbulent kinetic energy existed and was a consequence of large mean axial velocity gradients that coincided with shear stresses of the opposite signs. Thus, the present investigations suggest that the high-turbulence region responsible for rapid mixing may not be due to vortex breakdown (at six wavelengths downstream), but may be due to the positive production of turbulent kinetic energy at a location (at about two to three wavelengths downstream) earlier than previously suggested.

Design, testing, and simulation of NiTi shape-memory-alloy thin-film-based microgrippers

We present a way to fabricate microgrippers that can meet the industrys needs well, i.e., low cost and large tip deflection, etc. The microgripper is fabricated by bonding two identical micro NiTi-Si cantile- ver beams together with a silicon spacer in between. It can be actuated by electrical current directly. We have tested the behavior of micro NiTi-Si cantilever beams of three different sizes, and compared that with our simulation results. According to our simulation, the maximum strain and the maximum stress in NiTi should enable the grippers to survive after 10 6 cycles. Due to the simple fabrication process, this design is very suitable for batch production at low cost, which is a significant ad- vantage in both medical and manufacturing industries.

Blood cell counting and classification by nonflowing laser light scattering method

We present a nonflowing laser light scattering method for automatically counting and classifying blood cells. A linear charge-coupled device (CCD) and a silicon photoelectric cell (which is placed behind a pinhole plate on the CCD) form a double-detector structure: the CCD is used to detect the scattered light intensity distribution of the blood cells and the silicon photoelectric cell to complete the focusing process. An isotropic sphere, with relative refractivity near 1, is used to model the blood cell. Mie theory is used to describe the scattering of white blood cells and platelets, and anomalous diffraction, red blood cells. To obtain the size distribution of blood cells from their scattered light intensity distribution, the nonnegative constraint least-squares (NNLS) method combined with the Powell method and the precision punishment method are used. Both numerical simulation and experimental results are presented. This method can be used not only to measure the mean and the distribution of red blood cell size, but also to divide the white blood cells into three classes: lymphocytes, middle-sized cells, and neutrocytes. The experimental results show a linear relationship between the blood cell (both white and red blood cells) concentration and the scattered light intensity, and therefore, the number of blood cells in a unit volume can be determined from this relationship.

Experimental study of hemodynamics in the circle of willis

BackgroundThe Circle of Willis (CoW) is an important collateral pathway of the cerebral blood flow. An experimental study of the cerebral blood flow (CBF) distribution in different anatomical variations may help to a better understanding of the collateral mechanism of the CoW.MethodsAn in-vitro test rig was developed to simulate the physiological cerebral blood flow in the CoW. Ten anatomical variations were considered in this study, include a set of different degrees of stenosis in L-ICA and L-ICA occlusion coexist with common anatomical variations. Volume flow rates of efferent arteries and pressure signals at the end of communicating arteries of each case were recorded. Physiological pressure waveforms were applied as inlet boundary condition.ResultsIn the development of L-ICA stenosis, the total CBF decreases with the increase of stenosis degree. The blood supply of ipsilateral middle cerebral artery (MCA) was affected most by the stenosis of L-ICA. Anterior communicating artery (ACoA) and ipsilateral posterior communicating artery (PCoA) function as important collateral pathways of cerebral collateral circulation when unilateral stenosis occurred. The blood supply of anterior cerebral circulation was compensated by the posterior cerebral circulation through ipsilateral PCoA when L-ICA stenosis degree is greater than 40% and the affected side was compensated immediately by the unaffected side through ACoA. Blood flow of the anterior circulation and the total CBF reached the minimum among all cases studied when L-ICA occlusion coexist with the absence of PCoA.ConclusionThe results demonstrated the flow distribution patterns of the CoW under anatomical variations and clarified the collateral mechanism of the CoW. The flow ACoA is the most sensitive indexes to the morphology change of ipsilateral ICA. The relative independence of the circulation in anterior and posterior sections of the CoW is not broken and the function of ipsilateral PCoA is not activated until a severe stenosis of unilateral ICA occurs. PCoA is the most important collateral pathway of the collateral circulation and the missing of PCoA has the highest risk of stroke when the ipsilateral ICA has severe stenosis. These findings may provide the basis for future therapeutic and diagnosis applications.

Truly stentless molded autologous pericardial aortic valve prosthesis with single point attached commissures in a sheep model.

OBJECTIVE Aortic valve cusp extension and free-hand aortic valve replacement with autologous pericardium has been described. The long-term results were shown to be comparable with commercially available aortic bioprostheses. Nevertheless the relatively demanding surgical technique could not find wide acceptance. We developed a new design of a molded aortic valve, fashioned from autologous pericardium, treated briefly with glutaraldehyde, and simplified the implantation technique using single point attached commissures (SPAC). METHODS Molded autologous valve prostheses were implanted in the subcoronary aortic position in 10 sheep with the commissures connected to the aortic wall at three single commissural points (SPAC). The prosthesis mean size was 21.6+/-1.3 mm and the construction time (excluding 10 min glutaraldehyde treatment) was 6.2+/-1.2 min. Cardiopulmonary bypass and cross-clamp time was 111.1+/-12.4 min and 75.0+/-16.3 min, respectively. Six sheep were euthanized after 201.2+/-10.3 days (6 months) and four sheep were euthanized after 330.8+/-6.5 days (11 months) postoperatively. RESULTS In all sheep, the valve was immediately competent. At sacrifice, SPAC has proven to be well anchored to the aortic wall and the pericardial valve to be pliable in all cases. The maximum transvalvular gradient after cardiopulmonary bypass and at sacrifice was 3.7+/-2.2 mmHg and 10.6+/-5.2 mmHg, respectively. CONCLUSIONS This new truly stentless molded autologous aortic valve with simplified implantation technique (SPAC) makes a reliable implantation in a standard timeframe possible. The simplicity of construction, low cost and absent need for anticoagulation of this molded autologous aortic bioprosthesis offers an attractive alternative and not only for patients in the developing world.

Extension of unsymmetric finite elements US‐QUAD8 and US‐HEXA20 for geometric nonlinear analyses

Purpose – This paper aims to present an extension of two recently published elements (which are based on Petrov‐Galerkin formulation) to geometric nonlinear (GNL) problems.Design/methodology/approach – Two different sets of shape functions, namely isoparametric and metric, suitably chosen to satisfy the necessary compatibility and completeness conditions, are used as test and trial functions, respectively. Total Lagrangian formulation is used for the implementation of the element.Findings – In implementing the unsymmetric formulation for nonlinear problems, the deformation gradient tensor can be evaluated invariably using either isoparametric or metric shape functions. The developed elements are found to exhibit improved performance in the presence of mesh distortions.Research limitations/implications – The numerical problems in this paper involve linear elastic materials.Originality/value – Extension of US‐QUAD8 and US‐HEXA20 for GNL problems is new.

Shear stress investigation across mechanical heart valve.

The particle image velocimetry technique was used to study the shear field across a transparent mechanical heart valve model in one cardiac cycle. Shear stress was continuously increased until peak systole and high turbulent stress was observed at the orifice of the central channel and also around the occluder trailing tips. The peak Reynolds shear stress was up to 500 N/m2 at peak systole, which was higher than the normal threshold for hemolysis.

In vitro measurements of velocity and wall shear stress in a novel sequential anastomotic graft design model under pulsatile flow conditions

This study documents the superior hemodynamics of a novel coupled sequential anastomoses (SQA) graft design in comparison with the routine conventional end-to-side (ETS) anastomoses in coronary artery bypass grafts (CABG). The flow fields inside three polydimethylsiloxane (PDMS) models of coronary artery bypass grafts, including the coupled SQA graft design, a conventional ETS anastomosis, and a parallel side-to-side (STS) anastomosis, are investigated under pulsatile flow conditions using particle image velocimetry (PIV). The velocity field and distributions of wall shear stress (WSS) in the models are studied and compared with each other. The measurement results and WSS distributions, computed from the near wall velocity gradients reveal that the novel coupled SQA design provides: (i) a uniform and smooth flow at its ETS anastomosis, without any stagnation point on the artery bed and vortex formation in the heel region of the ETS anastomosis within the coronary artery; (ii) more favorable WSS distribution; and (iii) a spare route for the blood flow to the coronary artery, to avoid re-operation in case of re-stenosis in either of the anastomoses. This in vitro investigation complements the previous computational studies of blood flow in this coupled SQA design, and is another necessary step taken toward the clinical application of this novel design. At this point and prior to the clinical adoption of this novel design, in vivo animal trials are warranted, in order to investigate the biological effects and overall performance of this anastomotic configuration in vivo.

Noninvasive blood glucose measurement using multiple laser diodes

In the event of diabetes clinicians have advocated that frequent monitoring of a diabetics blood glucose level is the key to avoid future complications (kidney failure, blindness, amputations, premature death, etc.,) associated with the disease. While the test-strip glucose meters available in current consumer markets allow for frequent monitoring, a more convenient technique that is accurate, painless and sample-free is preferable in a diabetics daily routine. This paper presents a non-invasive blood glucose measurement technique using diffuse reflectance near infrared (NIR) signals. This technique uses a set of laser diodes, each operating at fixed wavelengths in the first overtone region. The NIR signals from the laser diodes are channeled to the measurement site viz., the nail-bed by means of optical fibers. A series of in vivo experiments have been performed on eight normal human subjects using a standard Oral Glucose Tolerance Test (OGTT) protocol. The reflected NIR signals are inputs to a Partial Least Squares (PLS) algorithm for calibration and future predictions. The calibration models used are developed using in vivo datasets and are unique to a particular individual. The 1218 paired points collected from the eight test subjects plotted on the Clarke Error Grid, revealed that 87.3% of these points fall within the A zone while the remainder, within the B zone, both of which, are clinically accepted. The standard error of prediction was ±13.14mg/dL for the best calibration model. A Bland-Altman analysis of the 1218 paired points yields a 76.3% confidence level for a measurement accuracy of ±20mg/dL. These results demonstrate the initial potential of the technique for non-invasive blood glucose measurements in vivo.

Some measurements downstream of a lobed forced mixer with different trailing edge configurations

Abstract Velocity and turbulence characteristics have been measured downstream of a lobed forced mixer with three different trailing edge configurations, using a two-component laser-Doppler anemometer at a Reynolds number of 2.27 × 10 4 (based on a reference velocity of 10m/s, and a nominal wavelength of the lobe of 33 mm). The three trailing edge configurations under investigation have the shape of a square wave, a semi-circular wave and a triangular wave. Measurements for a velocity ratio of 1:1 indicate that the strength of the secondary flow shed by a lobe is not the only parameter which determines the effectiveness of mixing, as previously suggested. The boundary layer thickness which grows along the side-walls of the lobe penetration and the subsequent shedding of the boundary layer to the wake region are of equal importance. High turbulence regions do not appear downstream of the mixers within six wavelengths downstream of the trailing edge. The normal and shear stresses in two cases (the semi-circular and triangular wave lobes) are shown to have a similar trend of decaying in magnitude.