Investigation of the instabilities of supersonic impinging jets using unsteady pressure sensitive paint

At given nozzle to plate spacings, the flow field of high speed impinging jets is known to be characterized by a resonance phenomenon. Large coherent structures that convect downstream and impinge on the surface create strong acoustic waves that interact with the inherently unstable shear layer at the nozzle exit. This feedback mechanism, driven by the coherent structures in the jet shear layer, can either be axisymmetric or helical in nature. Fast response pressure sensitive paint is applied to the impingement surface to map the unsteady pressure distribution associated with these resonant modes. Phase-conditioned results at several kHz are obtained using a flush mounted unsteady pressure transducer on the impingement plate as a reference signal. Tests are conducted at nozzle to plate spacings of x/D_j = 4 and 4.5. The resulting phase-conditioned pressure distribution reveals dramatically different flow fields at the corresponding impingement heights. The existence of a purely axisymmetric mode is identified at x/D_j = 4.5 characterized by concentric rings of higher/lower pressure that propagate radially with increasing phase. Two simultaneous modes are observed at x/D_j = 4. One being a dominant symmetric mode and the second a sub-dominant helical mode exhibiting a unique 'yin-yang' pressure distribution. Phaseconditioned Schlieren images are also given to visualize the flow structures associated with each mode. Results at other impingement heights not shown here are also discussed in connection with the existence of axisymmetric and helical modes.