Development of a High Speed Three-dimensional Flow Visualization Technique

Abstract

A high-speed three-dimensional flow visualization technique has been developed. The technique involves the generation of laser pulses at a high repetition rate, which are subsequently converted to sheets of light and incrementally scanned across the flow field. The interaction of the laser sheet with particles contained in the flow results in scattering of the laser beam. This scattering is then imaged with a high-speed camera and the images are transferred to a computer where they are post-processed to produce three-dimensional images and movies of the flow. The development of the technique is described in terms of the development and characterization of two of its chief components: a pulse burst laser system and a high-speed scanning mirror. The 3rd generation pulse burst laser is capable of generating pulses at a repetition rate between DC to 10MHz with pulse widths as short as 20ns. This flexibility in repetition rate and pulse width was achieved by the use of an Acousto-Optic Modulator (AOM) for pulse slicing. The contrast ratio of the pulses was 20000:1.The pulses were subsequently amplified by double passing them through a chain of 3 amplifiers. The gain of the 1st and 2nd amplifier was observed to be 16 and 142 respectively versus an anticipated gain of 200. The pulses could be uniformly amplified within duration of 600 microseconds in the gain curve. Various laser scanning options are discussed. Out of the various options the galvanometric scanning mirror was found to be most suitable for the development of the current technique. The maximum angular velocity of the mirror was measured to be in excess of 100,000 degrees/second over a 150 microsecond period with the mirror capable of accessing over 18 resolvable spots every 1 µsec. The mirror was found to be highly repeatable in its position at a given time past the electronic trigger. Also given the high deflection efficiency of the mirror (>98%), it proves to be the best option for high-speed laser scanning. Low speed 3-D visualization experiments were conducted to test the effectiveness of various components of the high-speed system and gauge the potential of the laser scanning concept. The experiments were conducted using a 15mW He-Ne laser in place of the pulse burst laser system. A low-speed flow field was created using the natural convection of smoke where the flow velocity was less than 1m/s. A DRS Hadland Ultra 68 camera was used to capture sequence of 2-D images. The 2-D sequence of images obtained was reconstructed into a 3-D image using Matlab. The camera resolution was 220*220 pixels with the overall volume of the imaging region being 3.8” x 3.8” x 3.1”.The 3-D results revealed several interesting features of the flow field. These results clearly demonstrate exciting possibilities arising from 3-D flow visualization.