Passive Control Of A Planar Offset Attaching Jet Using A Perforated Plate

Flow separations and attachments are the common phenomenon in industrial production. With the development of the separated flow, the study has important significance to reduce the fluctuating wall pressure and achieve passive control of the reattachment position in order to increase the service life of the aircraft.The effect of porous wall on the dynamic motions and flow structures of an offset attaching planar jet were studied using experimental methods. The offset distance was equal to the height of the jet exit, Reynolds numbers was from7600to19000, the development of the jets was characterized using the measurements of the flow velocities and fluctuating wall pressure. The development of the large scale flow structures was also characterized using the two-time and two-point measurements of the wall pressure and velocity fluctuations.The offset jets develop along perforated walls with different open area ratios from0%(solid wall) to approximately44%. The fluctuating pressure on the bottom wall was measured using an array of microphones embedded in the wall along the centerline of the facility. The flow fields were also visualized using the smoke wire technique and the Particle Image Velocimetry, and obtained the distribution characteristics of the time averaged velocity, fluctuating velocity and Reynolds shear stress under the different open area ratios.It was found that the low frequency flapping motion disappeared after using the wall with opening holes, the root-mean-square values of the fluctuating wall pressure decreased when the wall porosity increased, while the averaged length of the recirculation region increased at the same time. The interaction between the vortex in the inner shear layer and the wall reduced while the dynamic characteristics of the jet gradually transformed to the free jet. The offset attaching jet behaved like a two-dimensional free jet when the open area ratio of the wall was about44%. The spectra of the wall pressure also showed that the large scale vortices in the inner shear layer of the attaching jet propagated a longer distance in the flow with a larger porosity suggesting the low-frequency flapping motion in the jets with small open area ratio hindered the development of higher-frequency shear layer vortices.