On the velocity and pressure statistics during the flapping motion in a planar turbulent jet

•A “Flapping motion” observed in a self-preserving region of a plane jet is investigated.•We performed simultaneous measurement of two components of velocity and pressure in the jet.•Flapping motion enhances the production of the turbulent energy and its turbulent diffusion.•Flapping motion enhances the pressure diffusion of turbulent energy to the inner region of the jet.•Pressure field during the flapping motion showed the existence of a large-scale vortex structure.

The role of a large-scale coherent vortex structure in a planar jet, which is intermittently observed and called “flapping motion”, on the turbulent energy transport process is investigated experimentally. The experiment is performed by simultaneously measuring the two components of velocity and pressure in the self-preserving region of the jet. The probe for the simultaneous measurement of the velocity and pressure consists of an X-type hot-wire sensor and a static pressure probe. The measurement data are analyzed using a conditional sampling technique and the phase-averaging technique, on the basis of an intermittency function which shows whether the jet is now flapping or not. This intermittency function is obtained by the measurement results of the streamwise velocity fluctuation by means of the two I-type hot-wire sensors set in the self-preserving region of the jet with applying a continuous wavelet transform analysis to the data. The experimental results show that the phase-averaged velocity field during the flapping motion shows a good agreement with those obtained through the 23 points simultaneous measurement of the streamwise velocity in the previous studies. Further, the phase-averaged pressure field during the flapping motion indicates the existence of a large-scale coherent vortex structure, interpreted as a combination of the flapping and puffing motions in the self-preserving region of the jet. In addition, it is found that the production of the turbulent energy and its diffusion from the inner region to the outer region of the jet are enhanced by the flapping motion. In particular, this enhancement of the turbulent energy diffusion is caused by the combination of an increase of the turbulent diffusion to the outer region of the jet and a decrease of the pressure diffusion to the inner region of the jet.