Causality correlations of jet noise using a shadowgraph technique for source detection

The “source” of “causal” signal is produced by a low‐power laser beam incident on a photodiode. The beam passes through a turbulent flow, measuring light‐intensity fluctuations proportional to the two‐dimensional Laplacian of density (principle of shadowgraph). This gives an approximation to the source term in the dilatation model of jet noise. The diode signal is cross‐correlated with the farfield sound, producing a causality correlation function. The method gives only a linewise resolution of the source region since the beam integrates the conditions along its path. This technique is potentially superior to the insertion of probes measuring velocity or pressure, since the flow is not disturbed and no extra noise due to turbulence‐probe interaction is generated. The source region (about the first 10 diameters) of a 20‐mm‐diameter model jet was surveyed by moving the laser beam radially and axially. The resulting distribution of causality data are presented for different exit Mach numbers and farfield mic...