An Experimental Study on the Development of Coherent Structure in a Plane Jet (4th Report, Multi-point Simultaneous Measurement of Two Velocity Components and the Large-scale Structure Model by the KL Expansion)

Abstract

In order to clarify the large-scale coherent structure in a turbulent plane jet, the simultaneous measurement of the main streamwise and the cross-streamwise velocity at 9 points in the self-preserving region of a turbulent plane jet has been preformed by an array of X-type hot wire probes. From the time variation of the main streamwise fluctuating velocity field, it is found that there exist a pair of fluid lumps with the positive and negative fluctuating velocity on opposite sides of the jet centerline. On the other hand, the instantaneous cross-streamwise fluctuating velocity shows the same sign over the cross-section, i.e., the vertically-striped pattern is formed. As the result of the KL expansion, it is found hat the first u and v modes present a character of the “flapping”, and the second and third modes show the one of the “puffing”. From the combination of the “flapping” and the “puffing”, a new coherent structure model in the self-preserving region of a turbulent plane jet has been developed.