Passive control of an offset attaching j et using a perforated plate

The effect of porous wall on the dynamic motions of an offset attaching planar j et was studied using experimental methods.The offset jets developed along perforated walls with different open area ratios (OAR)from 0% (solid wall)to approximately 40%.The fluctuating pressure on the bottom wall was measured using an array of microphones embedded in the wall a-long the centerline of the facility.The flow fields were also visualized using the smoke wire tech-nique.It was found that the root-mean-square values of the fluctuating wall pressure decreased when the wall porosity increased,while the average length of the recirculation region increased at the same time.The cross-spectra of the fluctuating wall pressure measured at different down-stream locations showed that the low frequency flapping motion disappeared when the open area ratio was more than 20% of the total area of the wall.The attaching jet behaved like a two-di-mensional free j et when the open area ratio of the wall was more than 40%.The spectra of the wall pressure also showed that the large scale vortices in the inner shear layer of the attaching j et propagated a longer distance in the flow with a larger porosity,which indicated the low-frequency flapping motions in the j ets with small open area ratio hindered the development of higher-fre-quency shear layer vortices.