An Experimental Study on the Development of Coherent Structure in a Plane Jet: 3rd Report, Multi-point Simultaneous Measurement of Main Streamwise Velocity and the Reconstruction of Velocity Field by the KL Expansion

In order to clarify the dynamics of the coherent structure in a turbulent plane jet, the simultaneous measurement of the main streamwise velocity at 21 points in the self-preserving region of a turbulent plane jet has been performed by an array of I-type hot wire probes. Then the KL (Karhunen Loeve) expansion was applied to extract the coherent structure in the jet. From the investigation of the random coefficients and the eigenfunctions (modes), it is found that the low-order modes correspond to the large scale (coherent) structure, the middle-order modes correspond to the small scale random structure and the higher-order modes correspond to the intermittent structure in the outer edge region. From the spatio-temporal velocity field reconstructed by the first KL mode, it is found that there exist a pair of fluid lumps with the positive and negative streamwise velocity fluctuation on the opposite sides of the jet centerline, and the signs of velocity fluctuation for those fluid lumps change alternately as the time advances. These charactereisitics are consistent with the so called “jet flapping” phenomenon.