John A. Reizes

The Development of a Bubble Rising in a Viscous Liquid

The rise and deformation of a gas bubble in an otherwise stationary liquid contained in a closed, right vertical cylinder is investigated using a modified volume-of-fluid (VOF) method incorporating surface tension stresses. Starting from a perfectly spherical bubble which is initially at rest, the upward motion of the bubble in a gravitational field is studied by tracking the liquid–gas interface. The gas in the bubble can be treated as incompressible. The problem is simulated using primitive variables in a control-volume formulation in conjunction with a pressure–velocity coupling based on the SIMPLE algorithm. The modified VOF method used in this study is able to identify and physically treat features such as bubble deformation, cusp formation, breakup and joining. Results in a two-dimensional as well as a three-dimensional coordinate framework are presented. The bubble deformation and its motion are characterized by the Reynolds number, the Bond number, the density ratio, and the viscosity ratio. The effects of these parameters on the bubble rise are demonstrated. Physical mechanisms are discussed for the computational results obtained, especially the formation of a toroidal bubble. The results agree with experiments reported in the literature.

An experimental and numerical study of synthetic jet flow

The flow generated by a synthetic jet actuator with a circular orifice is investigated experimentally and computationally. The synthetic jet establishes itself much more rapidly than the steady jet, primarily because of turbulent dissipation. The oscillatory nature of synthetic jet flow also gives rise to a much greater entrainment of ambient fluid compared with the case of a steady jet. Finally, self-similarity seems to be established when the oscillations introduced by the actuator are reduced to negligible levels

Parameter-Optimized Model of Cardiovascular–Rotary Blood Pump Interactions

A lumped parameter model of human cardiovascular-implantable rotary blood pump (iRBP) interaction has been developed based on experimental data recorded in two healthy pigs with the iRBP in situ. The model includes descriptions of the left and right heart, direct ventricular interaction through the septum and pericardium, the systemic and pulmonary circulations, as well as the iRBP. A subset of parameters was optimized in a least squares sense to faithfully reproduce the experimental measurements (pressures, flows and pump variables). Our fitted model compares favorably with our experimental measurements at a range of pump operating points. Furthermore, we have also suggested the importance of various model features, such as the curvilinearity of the end systolic pressure-volume relationship, the Starling resistance, the suction resistance, the effect of respiration, as well as the influence of the pump inflow and outflow cannulae. Alterations of model parameters were done to investigate the circulatory response to rotary blood pump assistance under heart failure conditions. The present model provides a valuable tool for experiment designs, as well as a platform to aid in the development and evaluation of robust physiological pump control algorithms.

Mixed convection in vertical, cylindrical annuli

Abstract The laminar flow patterns and heat transfer for air contained in the enclosure formed between two vertical, concentric cylinders and two horizontal planes have been studied numerically. The inner cylinder and one of the horizontal planes are heated and rotated about the vertical axis; the other horizontal plane and outer cylinder are cooled and kept stationary. This geometry simulates the gaps at the ends of the rotor of a small, air-cooled, vertically mounted electric motor. The results facilitate the thermal design of such a motor. The influences of geometry (described by the radius ratio R and aspect ratio A), Ra and Re on temperature and velocity distributions have been investigated. Solutions have been obtained for 0.25 ⩽ A ⩽ 4.0, 1.2 ⩽ R ⩽ 8.0, 10 ⩽ Re ⩽ 300 and 103 ⩽ Ra ⩽ 105. It has been found that for low values of R and high values of Re the flow is dominated by centrifugal forces, whereas for high A and Ra buoyancy effects determine the flow patterns and, therefore, the heat transfer. Monocellular flow patterns have been found for the cases where one of these forces is dominant; otherwise two- or three-cell structures have been obtained.

An evaluation of synthetic jets for heat transfer enhancement in air cooled micro‐channels

Purpose – The development of novel cooling techniques is needed in order to be able to substantially increase the performance of integrated electronic circuits whose operations are limited by the maximum allowable temperature. Air cooled micro‐channels etched in the silicon substrate have the potential to remove heat directly from the chip. For reasonable pressure drops, the flow in micro‐channels is inherently laminar, so that the heat transfer is not very large. A synthetic jet may be used to improve mixing, thereby considerably increasing heat transfer. This paper seeks to address this issue.Design/methodology/approach – CFD has been used to study the flow and thermal fields in forced convection in a two‐dimensional micro‐channel with an inbuilt synthetic jet actuator. The unsteady Navier‐Stokes and energy equations are solved. The effects of variation of the frequency of the jet at a fixed pressure difference between the ends of the channel and with a fixed jet Reynolds number, have been studied with ...

Motion of Interacting Gas Bubbles in a Viscous Liquid Including Wall-Effects and Evaporation

Abstract The motion of single and multiple gas bubbles in an otherwise stationary liquid contained in a closed right vertical cylinder is investigated using a modified volume-of-fluid (VOF) method incorporating surface tension stresses. An isolated bubble was considered in a separate paper [4], where the initial bubble radius was small in comparison with that of the cylinder and watt effects were negligible. In this work the focus is on the interference effects during the motion of two initially spherical bubbles in a gravitational field, as well as the influence of the container wall on the bubble motion: the initial bubble diameter in the present study is more than half the cylinder diameter. The bubble size is also much larger than that required to satisfy the condition in which the gas can be treated as incompressible. In addition, the effect on bubble motion of the inclusion of evaporation at the gas-liquid interface as well as the bursting of a bubble through a five surface are considered. The modif...

Large-Eddy Simulation of Turbulent Natural Convection in Vertical Parallel-Plate Channels

A numerical investigation examining natural convection in a vertical parallel-plate channel with the simultaneous presence of laminar, transitional, and turbulent regimes is conducted using large-eddy simulation. The compressible three-dimensional Favre-filtered mass, momentum, and energy conservation equations are closed using the Smagorinsky and the Vreman subgrid-scale models. A two-stage predictor-corrector numerical methodology for low-Mach-number compressible flows is adopted. Time-averaged wall temperature and field profiles are well captured by the Vreman model, while the Smagorinsky model underpredicts wall behavior considerably. It is demonstrated that the present code is capable of capturing the flow development which is unachievable by conventional Reynolds-averaged Navier-Stokes approaches.

Noninvasive Average Flow and Differential Pressure Estimation for an Implantable Rotary Blood Pump Using Dimensional Analysis

Accurate noninvasive average flow and differential pressure estimation of implantable rotary blood pumps (IRBPs) is an important practical element for their physiological control. While most attempts at developing flow and differential pressure estimate models have involved purely empirical techniques, dimensional analysis utilizes theoretical principles of fluid mechanics that provides valuable insights into parameter relationships. Based on data obtained from a steady flow mock loop under a wide range of pump operating points and fluid viscosities, flow and differential pressure estimate models were thus obtained using dimensional analysis. The algorithm was then validated using data from two other VentrAssist IRBPs. Linear correlations between estimated and measured pump flow over a flow range of 0.5 to 8.0 L/min resulted in a slope of 0.98 (R 2 = 0.9848). The average flow error was 0.20 plusmn 0.14 L/min (mean plusmn standard deviation) and the average percentage error was 5.79%. Similarly, linear correlations between estimated and measured pump differential pressure resulted in a slope of 1.027 (R 2 = 0.997) over a pressure range of 60 to 180 mmHg. The average differential pressure error was 1.84 plusmn 1.54 mmHg and the average percentage error was 1.51%.

A Dynamic Lumped Parameter Model of the Left Ventricular Assisted Circulation

A lumped parameter model of the cardiovascular system (CVS) and its interaction with an implantable rotary blood pump (iRBP) is presented. The CVS model consists of the heart, the systemic and the pulmonary circulations. The pump model is made up of three differential equations, i.e. the motor equation, the torque equation and the hydraulic equation. Qualitative comparison with data from ex vivo porcine experiments suggests that the model is able to simulate different physiologically significant pumping states with varying pump speed set points. The combined CVS- iRBP model is suitable for use as a tool for investigating changes in the circulatory system parameters in the presence of the pump, and for testing control algorithms.

A Criterion for the Formation of Micro Synthetic Jets

A synthetic jet actuator is a zero net mass flow device, which under appropriate conditions generates a continuous jet always directed away from the orifice. Because of limited experimental and computational data on micro-sized jets, there is a need for a criterion to determine the onset of the sustained jet regime. A numerical study of axisymmetric micro synthetic jets for a frequency range from 250 to 50,000 Hz, orifice diameters range from 20 to 200 μm, and Reynolds numbers from 6.5 to 35 has been performed in order to identify a general jet formation criterion. The parametric study has allowed us to develop a new criterion for the onset of micro synthetic jets with Stokes numbers less than 7.Copyright