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THE STATISTICAL ANALYSIS OF FLAPPING PHENOMENON IN A TWO-DIMENSIONAL TURBULENT JET

Osamu Terashima
Department of Mechanical Science and Engineering, Nagoya University, Nagoya, 464-8603, Japan

Yasuhiko Sakai
Department of Mechanical Science and Engineering, Nagoya University, Nagoya, 464-8603, Japan

Kouji Nagata
Department of Mechanical Science and Engineering, Nagoya University, Nagoya, 464-8603, Japan

Аннотация

The characteristics of a two-dimensional turbulent jet in flapping motion are investigated by simultaneous measurement of the velocity and the pressure. The simultaneous measurements are performed by using a combined probe, which was developed by our laboratory (Terashima et al., 2010), that consists of an X-type hot-wire probe and a static pressure probe. The measured data are analyzed by using conditional-sampling technique and ensemble-averaging technique, on the basis of an intermittency function to determine whether the jet is flapping or not. The intermittency function is obtained by applying a continuous wavelet transform with Gabor mother wavelet to the measured signals of hot-wire anemometers set in the self-preserving region of the jet.
The measured results obtained by the present new method show that the characteristics of the flapping motion clearly, and the ensemble-averaged velocity field during the flapping motion shows a good agreement with the results obtained by Sakai et al. (2006) through the multi-point simultaneous measurement of the velocity. Further, the ensemble-averaged static pressure field in the flapping motion indicates the existence of a coherent vortex structure, which is interpreted as a combination of flapping and puffing motion in the self-preserving region. The statistical analysis of the flapping motion shows that the flapping motion arises at random, and the average interval time of the arising of the flapping motion is about 6.1 times the flapping periods (0.24 sec), and the average duration time of the flapping motion is about 1.7 times the flapping period (0.07 sec) and the one that continues over 3.0 times the flapping period is very few. Moreover, the two-points spatial velocity correlation, which becomes negative value, substantially depends on whether the flapping motion arises or not. These results indicate that the flapping motion in a two-dimensional turbulent jet has a very important role to the properties of the jet, although the flapping motion is a rare phenomenon.