Abstract
In the previous papers we reported that the wall-attachment fluidic element (Amp. 2) with parallel walls and diverging area in the main nozzle has shorter switching time than that of the element (Amp. 1) with parallel walls.
To make the effect of turbulence, especially coherent structure, on the switching time in those elements clear, their space correlation coefficients are measured by the use of the new developed LDV. The LDV makes it possible to measure the space-time correlation of flow velocities at two points easily, and to reduce the ambiguity noise by correlating signals from two frequency trackers at two points close to each other having the same velocity for measurement of space correlation coefficient. The space iso-correlation contours are obtained from the measured space correlation coefficients. The results are as follows.
At the jet unattaching side in the down stream near by the exit of the main nozzle, the shapes of the eddies in Amps. 1 and 2 become ellipsoid. The eddy in Amp. 2 becomes more flat and extended longer to the jet center than that in Amp. 1. And the major axis of the ellipsoid in Amp. 2 is inclined at larger angle with the flow direction than that in Amp. 1. At the jet attaching side in that down stream and at the wall side in the space correlation abscissa the eddy is constrained by the wall. Consequently the sizes of the eddies in both elements become almost same. At the jet side of the space correlation abscissa the eddy is not constrained by the wall, so that the eddy in Amp. 2 becomes extended longer to the jet center than that in Amp. 1. The larger coherent structure of Amp. 2, compared with that of Amp. 1. makes the entrainment of the surrounding fluid larger and the diffusion of the jet larger. This plays an important role in shortening the switching time
