Spectral analysis and coherency of dendritic side branches: Video data from the Isothermal Dendritic Growth Experiment.

摘要

Evaluation and characterization of microgravity grown dendrites were performed to determine the steady-state nature of dendritic growth and to uncover any inherent or characteristic periodic aspects associated with the dendritic growth process. The experimental evaluation involved the use of photomicrographs as well as video images which captured the dendritic growth process over the entire growth cycle. The video images were captured during the microgravity phase of the flight of Columbia on STS-87, as part of the primary payload of the United States Microgravity Payload (USMP-4). The payload experiment, called the Isothermal Dendritic Growth Experiment (IDLE) permitted, for the first time, video recording at a rate of approximately 30 frames per second and provided ample amounts of data to analyze.; From the evaluation of the video data, it was found that the dendrites grown in microgravity achieved only an approximation to steady-state growth velocity during the solidification process. The space-grown dendrites appear to approach a more constant velocity towards the end of each growth cycle. In the growth cycles captured for this thesis, however, the dendrites were not able to reach a constant growth velocity. The video data were further processed and analyzed for the presence of any so-called characteristic or “eigenfrequencies” that might be associated with the mechanism of dendritic growth, especially regarding the formation of sidebranches. A comparison of the spectral analysis results obtained from the video data with optical measurements of sidebranch spacing obtained directly from the photomicrographs reveal the presence of a narrow band of frequencies during the dendritic growth cycle that could be associated with the formation of sidebranches. The photographic images showed that a correlation exists between the sidebranches produced behind the dendrite tip, and the characteristic frequencies extracted from the video data.