Abstract
To measure non-stationary flow in fluidic elements, a new type laser Doppler velocimeter was developed with variable frequency shift and by the use of a frequency tracker. The frequency tracker provides a real time demodulation of the FM signal and yields an analogue signal continually which is proportional to the component of the local fluid velocity to which the optical system is sensitive. However, the measurable variation of the Doppler signal frequency is limited by the dynamic response of the frequency tracker. To give more allowance to the limitation of the frequency tracker, the mean frequency of the Doppler signal was shifted up by two acousto-optic modulators. So the width and the frequency of the signal frequency variation were kept within the measurable range of the frequency tracker.
Using this laser Doppler velocimeter, firstly when a step flow appeared in the main nozzle with and without a control flow, the transient responses of flow velocity at the points near the jet attached wall were measured. Without a control flow the velocity fluctuates but later becomes constant. With a control flow the velocity becomes constants in about 1/20 of the time experienced without a control flow. Secondly the switching characteristics of the attached jet with a control flow were studied in the fluidic elements with and without a cylinder in the main nozzle. The switching speed in the element with the cylinder becomes faster than that in the element without the cylinder. Lastly, to investigate the mechanism of the faster switching, the turbulent power spectra in their elements were obtained from the Fourier transform of the autocorrelation functions of the velocity fluctuations. In the element with a cylinder, the turbulent scale becomes larger than that in the element without a cylinder. As the large-scaled turbulence diffuses the attached jet, the jet becomes wider and draws near to the wall of the fluidic element. This makes the switching speed faster.
