Investigation of impinging jet resonant modes using unsteady pressure-sensitive paint measurements

Abstract

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 (PSP) is applied to the impingement surface to map the unsteady pressure distribution associated with these resonant modes. Phase-averaged results acquired at several kHz are obtained using a flush mounted unsteady pressure transducer on the impingement plate as a reference signal. Tests are conducted on a Mach 1.5 jet at nozzle to plate spacings of \(h/D_{j} = 4\, \text{ and}\, 4.5\). The resulting phase-averaged distribution reveals dramatically different flow fields at the corresponding impingement heights. The existence of a purely axisymmetric mode with a frequency of 6.3 kHz is identified at \(h/D_{j} = 4.5\) and is characterized by concentric rings of higher/lower pressure that propagate radially with increasing phase. Two simultaneous modes are observed at \(h/D_{j} = 4\) with one being a dominant symmetric mode at 7.1 kHz and the second a sub-dominant helical mode at 4.3 kHz. Complimentary phase-conditioned Schlieren images are also obtained visualizing the flow structures associated with each mode and are consistent with the PSP results.