Tin-Kan Hung

Computational and experimental study of a captive annular eddy

Results of calculations and experiments on the flow of a viscous liquid through an axisymmetric conduit expansion are reported. The streamlines and vorticity contours are presented as functions of the Reynolds number of the flow. The dynamic interaction between the main flow and the captive eddy between it and the walls is analysed, and it is concluded that, for laminar flow, the main role of the eddy is that of shaping the flow with a rather small energy exchange.

Solid-particle motion in two-dimensional peristaltic flows

Some insight into the mechanism of solid-particle transport by peristalsis is sought experimentally through a two-dimensional model study (§ 2). The peristaltic wave is characterized by a single bolus sweeping by the particle, resulting in oscillatory motion of the particle. Because of fluid-particle interaction and the significant curvature in the wall wave, the peristaltic flow is highly nonlinear and time dependent. For a neutrally buoyant particle propelled along the axis of the channel by a single bolus, the net particle displacement can be either positive or negative. The instantaneous force acting upon the particle and the resultant particle trajectory are sensitive to the Reynolds number of the flow (§ 3 and 4). The net forward movement of the particle increases slightly with the particle size but decreases rapidly as the gap width of the bolus increases. The combined dynamic effects of the gap width and Reynolds number on the particle displacement are studied (§ 5). Changes in both the amplitude and the form of the wave have significant effects on particle motion. A decrease in wave amplitude along with an increase in wave speed may lead to a net retrograde particle motion (§ 6). For a non-neutrally buoyant particle, the gravitational effects on particle transport are modelled according to the ratio of the Froude number to the Reynolds number. The interaction of the particle with the wall for this case is also explored (§ 7). When the centre of the particle is off the longitudinal axis, the particle will undergo rotation as well as translation. Lateral migration of the particle is found to occur in the curvilinear flow region of the bolus, leading to a reduction in the net longitudinal transport (§ 8). The interaction of the curvilinear flow field with the particle is further discussed through comparison of flow patterns around a particle with the corresponding cases without a particle (§ 9).

Positioning the right atrial catheter: a model for reappraisal.

A flexible Silastic± casting of the human right atrium was developed to correspond to some in vivo human right atrium hemodynamic characteristics including chamber pressures, pulsatility, fluid output, and flow velocity. Using an infusion pump, air was introduced (10 ml in 30 s) into the superior vena cava of the model and aspirated via a catheter from different positions within the model atrial chamber. The tests were carried out at atrial inclinations of 60°, 80°, and 90° from the horizontal and compared the aspiration efficiency of a single-orificed 16-gauge catheter to a 16-gauge multiorified (5 aperatures) catheter. Optimal air aspiration occurred with the multiorificed catheter tip positioned within the area 2.0 cm below the junction of the superior vena cava (SVC) and the atrial chamber at an inclination of 80°. As much as 80 per cent of the incoming air could be aspirated under these conditions. At its optimal position the single-orificed catheter gave a maximal yield of 45 to 50 per cent aspiration when the tip was positioned 3.0 cm above the SVC and atrial chamber junction. Aspiration of air from mid right atrium (4.5 cm below the SVC-atrial junction) was poor regardless of the type of catheter used or atrial inclination. These data suggest a need for reappraisal of catheter design and placement.

An In-Vivo Measurement and Analysis of Viscoelastic Properties of the Spinal Cord of Cats

An in-vivo experimental technique was employed to determine the linear and nonlinear characteristics of viscoelastic properties of the spinal cord of anesthetized cats. The stress relaxation and recovery curves were reproducible in a group of cat experiments. The data of linear viscoelastic properties were used to develop a power law model with Boltzmanns convolution integral. The model was capable of predicting a prolonged stress relaxation and recovery curve. For larger deformation, the results were quantified using a nonlinear analysis of viscoelastic response of the spinal cord under the uniaxial experiment.

Analysis and comparison of venous air embolism detection methods.

Changes in precordial ultrasonic Doppler patterns in dogs were compared with changes of end-tidal carbon dioxide (EtCO2) and pulmonary arterial pressure (PAP). Possible mechanisms of pulmonary arterial hypertension after air embolism in dogs are discussed. After the injection of air, an immediate change in the Doppler ultrasonic pattern was detected, including a shift of the base line. The duration of change of Doppler patterns varied depending upon the length of time that trapped air remained in the right heart. There was a delay of 15 to 30 seconds before the EtCO2 and PAP responded. In a group of dogs pretreated with 0.3 mg of propranolol per kg, there was a significant decrease in the PAP response due to induced air embolism compared to the control group (P < 0.05). We found that the Doppler ultrasonic device was the most sensitive for detecting venous air embolism at a level as low as 0.05 ml of air per kg compared to 0.25 ml/kg for the PAP and EtCO2 responses.

Stress-strain relationship and neurological sequelae of uniaxial elongation of the spinal cord of cats**

An in vivo experimental method was developed to measure the mechanical or rheological properties of the spinal cord of anesthetized cats. This novel approach resulted in measurements of the tensile force in the cord and the modulus of the spinal cord tissue under longitudinal elongation. Both sensory and motor function of the cats recovered within a week after a spinal cord segment had been stretched by a 50% elongation.

Fluid-dynamics modelling of the human left ventricle with dynamic mesh for normal and myocardial infarction: Preliminary study

Pulsating blood flow patterns in the left ventricular (LV) were computed for three normal subjects and three patients after myocardial infarction (MI). Cardiac magnetic resonance (MR) images were obtained, segmented and transformed into 25 frames of LV for a computational fluid dynamics (CFD) study. Multi-block structure meshes were generated for 25 frames and 75 intermediate grids. The complete LV cycle was modelled by using ANSYS-CFX 12. The flow patterns and pressure drops in the LV chamber of this study provided some useful information on intra-LV flow patterns with heart diseases.

An implicit finite-difference method for solving the Navier-Stokes equation using orthogonal curvilinear coordinates

Abstract Orthogonal curvilinear mesh networks are generated numerically between the wavy walls of two-dimensional peristaltic channels. A dual iterative procedure is developed to facilitate the conformal mapping, as well as to adjust mesh dimensions when necessary to fit the boundaries of the flow region. An implicit finite-difference technique is employed to obtain transient solutions of the Navier-Stokes equations. The effects of initial conditions on the flow establishment are discussed, along with considerations of numerical accuracy. The effects of certain nonconservative difference forms of the governing equations are explored. A calculated velocity field for a two-dimensional nonlinear peristaltic flow is supported by laboratory flow observation. The present method is applicable for laminar flow in a nonuniform channel with or without wall peristalsis.

Stress-strain measurement of the spinal cord of puppies and their neurological evaluation.

An in-vivo experiment was developed to identify the biorheological properties of the spinal cord of puppies under uriaxial deformation. For strain less than 4.5%, the ratio between stress and strain was about 2.65 x 106 dyne/limbs of the puppies were recoverable after the spinal cord segment at L1 was stretched once by 10-50%.

Hydrodynamic forces induced by transient sloshing in a 3D rectangular tank due to oblique horizontal excitation

Abstract A time-independent finite difference method is developed to simulate fluid sloshing in a three-dimensional tank. The developed numerical scheme is verified by the rigorous benchmark tests. The experiment measurement of liquid sloshing in a 3D tank was also carried out in this study to further validate the accuracy of the present numerical results. Transient waves can change their types naturally in the time domain, especially for a tank excited by resonant frequencies. In this study, if the excitation frequencies are far from the fundamental natural frequency and there are four types of stable sloshing waves discovered due to the oblique horizontal excitation: “diagonal”, “single-directional”, “square-like”, and “irregular” waves. Besides, the swirling waves can only be generated for a partially-filled tank excited at near resonant frequency with oblique horizontal excitation. The evolution of forces induced by different sloshing waves acting on the tank walls is calculated and discussed in this work. In addition, the dynamics of sloshing force induced by swirling waves are explored in detail. The force of the single-directional waves acting on the tank bottom is time-invariant but the other types of sloshing waves show a beating phenomenon which is attributed to the momentum flux across the free surface and the vertical inertia of sloshing fluid. The effect of various oblique excitation directions of the tank on liquid sloshing is discussed as well. The horizontal hydrodynamic force of sloshing waves acting on the mid-section of the left wall of the tank is dominated by the added mass effect if the external excitation frequency is larger than 4 times the lowest natural frequency ( ω 1 ) of the tank with partially-filled fluid. On the other hand, the wave elevation of sloshing waves plays a key effect on the horizontal sloshing-induced force when the excitation frequency of the tank is less than 4 ω 1 . A novel mechanism is presented to describe the phenomenon of alternate switch directions of the swirling waves. The relationship between the external force and the sloshing hydrodynamic force is the major factor to trigger the switch direction of the swirling waves. The influence of different base ratios of a rectangular tank on kinematic and dynamic responses of sloshing fluid is also explored in this work.