Andrew Ooi

Bio: Andrew Ooi is an academic researcher from University of Melbourne. The author has contributed to research in topics: Reynolds number & Turbulence. The author has an hindex of 37, co-authored 239 publications receiving 4696 citations. Previous affiliations of Andrew Ooi include Monash University & Bushfire and Natural Hazards CRC.

Reynolds averaged simulation of unsteady separated flow

Abstract: The accuracy of Reynolds averaged Navier–Stokes (RANS) turbulence models in predicting complex flows with separation is examined The unsteady flow around square cylinder and over a wall-mounted cube are simulated and compared with experimental data For the cube case, none of the previously published numerical predictions obtained by steady-state RANS produced a good match with experimental data However, evidence exists that coherent vortex shedding occurs in this flow Its presence demands unsteady RANS computation because the flow is not statistically stationary The present study demonstrates that unsteady RANS does indeed predict periodic shedding, and leads to much better concurrence with available experimental data than has been achieved with steady computation

Cavitation microstreaming and stress fields created by microbubbles.

Abstract: Cavitation microstreaming plays a role in the therapeutic action of microbubbles driven by ultrasound, such as the sonoporative and sonothrombolytic phenomena. Microscopic particle-image velocimetry experiments are presented. Results show that many different microstreaming patterns are possible around a microbubble when it is on a surface, albeit for microbubbles much larger than used in clinical practice. Each pattern is associated with a particular oscillation mode of the bubble, and changing between patterns is achieved by changing the sound frequency. Each microstreaming pattern also generates different shear stress and stretch/compression distributions in the vicinity of a bubble on a wall. Analysis of the micro-PIV results also shows that ultrasound-driven microstreaming flows around bubbles are feasible mechanisms for mixing therapeutic agents into the surrounding blood, as well as assisting sonoporative delivery of molecules across cell membranes. Patterns show significant variations around the bubble, suggesting sonoporation may be either enhanced or inhibited in different zones across a cellular surface. Thus, alternating the patterns may result in improved sonoporation and sonothrombolysis. The clear and reproducible delineation of microstreaming patterns based on driving frequency makes frequency-based pattern alternation a feasible alternative to the clinically less desirable practice of increasing sound pressure for equivalent sonoporative or sonothrombolytic effect. Surface divergence is proposed as a measure relevant to sonoporation.

A Study of the Evolution and Characteristics of the Invariants of the Velocity Gradient Tensor in Isotropic Turbulence

Abstract: Since the availability of data from direct numerical simulation (DNS) of turbulence, researchers have utilized the joint PDFs of invariants of the velocity gradient tensor to study the geometry of small-scale motions of turbulence. However, the joint PDFs only give an instantaneous static representation of the properties of fluid particles and dynamical Lagrangian information cannot be extracted. In this paper, the Lagrangian evolution of the invariants of the velocity gradient tensor is studied using conditional mean trajectories (CMT). These CMT are derived using the concept of the conditional mean time rate of change of invariants calculated from a numerical simulation of isotropic turbulence. The study of the CMT in the invariant space (RA, QA) of the velocity-gradient tensor, invariant space (RS, QS) of the rate-of-strain tensor, and invariant space (RW, QW) of the rate-of-rotation tensor show that the mean evolution in the (Σ, QW) phase plane, where Σ is the vortex stretching, is cyclic with a characteristic period similar to that found by Martin et al. (1998) in the cyclic mean evolution of the CMT in the (RA, QA) phase plane. Conditional mean trajectories in the (Σ, QW) phase plane suggest that the initial reduction of QW in regions of high QW is due to viscous diffusion and that vorticity contraction only plays a secondary role subsequent to this initial decay. It is also found that in regions of the flow with small values of QW, the local values of QW do not begin to increase, even in the presence of self-stretching, until a certain self-stretching rate threshold is reached, i.e. when Σ≈0.25 〈QW〉1/2. This study also shows that in regions where the kinematic vorticity number (as defined by Truesdell 1954) is low, the local value of dissipation tends to increase in the mean as observed from a Lagrangian frame of reference. However, in regions where the kinematic vorticity number is high, the local value of enstrophy tends to decrease. From the CMT in the (−QS, RS phase plane, it is also deduced that for large values of dissipation, there is a tendency for fluid particles to evolve towards having a positive local value of the intermediate principal rate of strain.

CFD analysis of ejector in a combined ejector cooling system

Abstract: One-dimensional ejector analyses often use coefficients derived from experimental data for a set of operating conditions with limited functionality. In this study, several ejector designs were modelled using finite volume CFD techniques to resolve the flow dynamics in the ejectors. The CFD results were validated with available experimental data. Flow field analyses and predictions of ejector performance outside the experimental range were also carried out. During validation, data from CFD predicted the entrainment ratios with greater accuracy on definite area ratios, although no shock was recorded in the ejector. Predictions outside the experimental range—at operating conditions in a combined ejector–vapour compression system—and flow conditions resulting from ejector geometry variations are discussed. It is found that the maximum entrainment ratio happens in the ejector just before a shock occurs and that the position of the nozzle is an important ejector design parameter.

Evaporation and dispersion of respiratory droplets from coughing.

Abstract: Understanding how respiratory droplets become droplet nuclei and their dispersion is essential for understanding the mechanisms and control of disease transmission via droplet-borne and airborne routes. A theoretical model was developed to estimate the size of droplet nuclei and their dispersion as a function of the ambient humidity and droplet composition. The model-predicted dried droplet nuclei size was 32% of the original diameter, which agrees with the maximum residue size in the classic study by Duguid, 1946, Edinburg Med. J., 52, 335 and the validation experiment in this study, but is smaller than the 50% size predicted by Nicas et al., 2005, J. Occup. Environ. Hyg., 2, 143. The droplet nuclei size at a relative humidity of 90% (25°C) could be 30% larger than the size of the same droplet at a relative humidity of less than 67.3% (25°C). The trajectories of respiratory droplets in a cough jet are significantly affected by turbulence, which promotes the wide dispersion of droplets. We found that medium-sized droplets (e.g., 60 μm) are more influenced by humidity than are smaller and larger droplets, while large droplets (≥100 μm), whose travel is less influenced by humidity, quickly settle out of the jet.

社会研究方法基础 = The Basics of Social Research

Abstract: Part I: AN INTRODUCTION TO INQUIRY. 1. Human Inquiry and Science. 2. Paradigms, Theory, and Research. 3. The Ethics and Politics of Social Research. Part II: THE STRUCTURING OF INQUIRY: QUANTITATIVE AND QUALITATIVE. 4. Research Design. 5. Conceptualization, Operationalization, and Measurement. 6. Indexes, Scales, and Typologies. 7. The Logic of Sampling. Part III: MODES OF OBSERVATION: QUANTITATIVE AND QUALITATIVE. 8. Experiments. 9. Survey Research. 10. Qualitative Field Research. 11. Unobtrusive Research. 12. Evaluation Research. Part IV: ANALYSIS OF DATA:QUANTITATIVE AND QUALITATIVE . 13. Qualitative Data Analysis. 14. Quantitative Data Analysis. 15. Reading and Writing Social Research. Appendix A. Using the Library. Appendix B. Random Numbers. Appendix C. Distribution of Chi Square. Appendix D. Normal Curve Areas. Appendix E. Estimated Sampling Error.

PIVlab – Towards User-friendly, Affordable and Accurate Digital Particle Image Velocimetry in MATLAB

Abstract: Digital particle image velocimetry (DPIV) is a non-intrusive analysis technique that is very popular for mapping flows quantitatively. To get accurate results, in particular in complex flow fields, a number of challenges have to be faced and solved: The quality of the flow measurements is affected by computational details such as image pre-conditioning, sub-pixel peak estimators, data validation procedures, interpolation algorithms and smoothing methods. The accuracy of several algorithms was determined and the best performing methods were implemented in a user-friendly open-source tool for performing DPIV flow analysis in Matlab.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Abstract: To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

The Structure of Turbulent Shear Flow

Abstract: The Structure of Turbulent Shear Flow By Dr. A. A. Townsend. Pp. xii + 315. 8¾ in. × 5½ in. (Cambridge: At the University Press.) 40s.