Robert R. Hwang

Optimization of particle image distortion for PIV measurements

In this study, a jump matching correlation scheme is proposed. For the multi-grid, iterative particle image distortion analysis, the coarse grid solutions are not obtained from coarse pixel image constructed by binning small pixels to large pixel, but by jumping the interrogation window the number of pixels specified during the particle image matching process. With the jump matching scheme applied and particle image interpolated, the central difference particle image pattern matching and image distortion analysis are easily linked together to resolve flow fields from coarsest grid to super-resolution grid. Optimization of particle image distortion is achieved by averaging a forward and a backward image distortion correlation for every iteration process. The proposed method is first verified by standard particle images of impinging jet flow that has exact solution available, and then applied to analyze uniform flow past two side-by-side circular cylinders. Satisfactory results are obtained from the jump matching scheme proposed.

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

Effect of Plateau Length on the Transformation of Internal Solitary Waves

Abstract—The propagation and dissipation of internal waves over continental shelf bathymetry are complex phenomenon. The waveform would be re-generated while transmitting a submerged deep-shallow-deep topography. To study the effect of the marine topography on the evolution of an internal wave, numerical simulation is utilized to perform the flow evolution and waveform inversion of a large depression internal wave over a trapezoidal obstacle with different plateau. A finite volume based Cartesian grid method is adopted to solve the Reynolds averaged Navier-Stokes equations using a k-ε model for the turbulence closure. Numerical results reveal that the re-generated waveform does not occur due to baroclinic wave. The shorter plateau length would induce strong vortex in back of the obstacle. Moreover, the wave amplitude, vorticity and turbulent kinetic energy are dissipated significantly. However, the level of the phenomenon decreases as the plateau length is larger than the wavelength.

Multiple‐port buoyant jets in density stratified cross flow

Abstract This study investigates theoretically and experimentally the mixing characteristics of a flow with initial momentum and density differences ejected from a multiple‐port diffuser into a uniform cross flow of a density stratified fluid. With the integral approach, the governing equations are established by taking the cross‐section integral over the jet and closed the problem with the specification of the entrainment function. The model predicts the trajectory and dilution of the jet as a function of horizontal position. In the laboratory experiment, a salt‐water filling process was used to create a linearly density‐stratified environment. The experimental results were obtained by towing a multiple‐port buoyant jet discharged at constant velocity through a stagnant, stratified fluid. Results show good agreement between the model prediction and the laboratory experiments.