Develpment of Karman Vortex Velocimeter by Use of Laser Diode

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A velocimeter which is applicable to gas and liquid flow was developed. The frequency of the Karman vortex downstream of a small diameter wire (0.3mm, 0.5mm and 1mm) set perpendicular to the flow is measured. The parallel light (diameter : 1mm) from a laser diode is refracted by the vortex and it is detected with a photo diode. The wire is heated by applying a low-voltage electrical charge in order to strengthen the refraction of light. The frequency is measured with FFT analyzer.The frequency of the Karman vortex and the velocity are almost linear. It was confirmed experimentally that the velocity range between 3m/s and 200m/s could be measured, and it is supposed that higher velocity is also avairable.Two methods were carried out to measure the velocity profile. One is a method which measures the frequency by scanning the laser light along the wire (vortex generator). The another is a method which measures the frequency by moving the wire.Almost same results between this method and Pitot tube were obtained as for the wire of a small diameter such as 0.3mm and 0.5mm. As the diameter became larger, the difference between the two measurements became higher. Beside, the wire of small diameter showed favorable results in a smoll disturbance of the flow, a fine wave form and a low electrical power consumption.A flowmeter for water flow in a small diameter pipe (diameter : 5mm) was also developed in a same way to the velocimeter. The wire of small diameter from 0.1mm to 0.5mm were used as the vortex generator. The relation between the flow rate and the frequency which is almost linear. F/V converter was also used successfully to measure the frequency. The converter was available to measure a sinusioidal flow rate up to 40Hz.