New algorithms for particle-tracking velocimetry are proposed and tested with typical particle images showing two-dimensional fluid flows. There are new ideas not only in the algorithm of the particle tracking itself but also in that of the individual-particle detection. The performance of the particle tracking is much improved by the new relaxation method and that of the individual-particle detection by the use of the dynamic threshold-binarization method. The special concern of the authors about the new algorithms is the applicability of particle-tracking velocimetry to high-density particle images, contrary to what is usually believed regarding this type of particle-imaging velocimetry. These new algorithms are first tested in synthetic images with a variety of particle parameters and then in other types of experimental visualizations showing jet and wake flows.

The starting flows past a two-dimensional oscillating and translating airfoil are investigated by visualization experiments and numerical calculations. The airfoil, elliptic in cross-section, is set in motion impulsively and subjected simultaneously to a steady translation and a harmonic oscillation in pitch. The incidence of the airfoil is variable between 0° and 45° and the Reynolds number based on the chord length is between 1500 and 10000. The main object of the present study is to reveal some marked characteristics of the unsteady vortices produced from the oscillating airfoil set at large incidences in excess of the static stall angle. Another purpose is to examine, in some detail, the respective and combined effects of the major experimental parameters on the vortex wake development. It is shown that, in general, the dominant parameter of the flow is the reduced frequency not only when the airfoil oscillates at incidences close to the static stall angle but also at larger incidences. It is also demonstrated that, as the pitching frequency is increased, the patterns of the vortex wake are dependent on the product of the reduced frequency and the amplitude rather than on the frequency itself. It is noted that the combined effect of a high reduced frequency and a large amplitude can give rise to cyclic superposition of leading-edge vortices from which a gradually expanding standing vortex is developed on the upper surface.

A new system has been developed for estimating experimentally some of the principal physical variables of fluid flows, through flow-visualization and image-processing techniques. Distributions of stream function, vorticity and pressure are calculated by this system with reasonable accuracy for two examples of two-dimensional flow: namely unsteady twin-vortex flow behind a circular cylinder accelerated impulsively to constant speed, and Kármán vortices behind a circular cylinder moving at constant speed. A detailed explanation of the image-processing technique and the numerical calculation process is given first, and then some consideration is given to calculated results in these two types of flow. Comparison shows that some results of the unsteady twin-vortex experiment coincide well with those of previously published experimental investigations and theoretical calculations. Errors introduced at each stage of this system are estimated in some detail.

The starting flows past a two-dimensional NACA 0012 airfoil translating and oscillating at large incidences are investigated by visualization experiments and numerical calculations. The airfoil model is set in motion impulsively and subjected simultaneously to a constant translation and harmonic oscillation in pitch. The evolution of the vortex wake is followed in a sequence of streamline visualizations and the wake pattern generated is analysed. The parameters varied in the visualization experiment are the Reynolds number ranging from 1500 to 10000, the reduced frequency from 0.1 to 1.0, the mean incidence 30° or 15° and the angular amplitude 15° or 7°. There are also two additional parameters of special interest: the airfoil cross-section and the pitching axis. The effects of these parameters are discussed in relation to the resultant wake patterns. Some comparison is made with the results of earlier experiments.

A new concept genetic algorithm (GA) has been implemented and tested for the use in the 2-D and 3-D Particle Tracking Velocimetry (PTV). The algorithm is applicable to particle images with larger (greater than 2000) number of particles without losing the excellent accuracy in the particle matching results. This is mainly due to a new fitness function as well as unique genetic operations devised especially for the purpose of particle matching problem. The new fitness function is based on the relaxation of movement of a group of particles and is particularly suited for an increased density of particle images. The unique genetic operations give rise to the concentration of more fit genes in the forward part of the gene strings where the crossover and mutation processes are suppressed. The new algorithm also profits from the new genetic encoding scheme which can deal with the loss-of-pair particles (i.e., those particles which exist in one frame but do not have their matching pair in the other frame), a typical problem in the real image particle tracking velocimetry. In the present study, the new method is tested with 2-D and 3-D synthetic as well as real particle images with a large number of particles.