Abstract
A low-noise acoustic wind tunnels are usually used for the sake of clarification and reduction of aerodynamic noise. However, some cases of aerodynamic noise which are difficult to analyze enough by the experiment with only using acoustic wind tunnels have still remained without the best noise control. Three cases of self excited aerodynamic tone with acoustic resonance are introduced and consider reasons why it was difficult to measure and to analyze. Three cases are "air flow around a semi-cylindrical cavity", "air flow passing a inlet portion of compact compressor with ring-chamber", and "water jet from a nozzle connected to pipe wall in the water". The dominant reasons might be that unsteady flow pattern did not make clear by an acoustic wind tunnel and that acoustic resonance mode was more complexity to understand by only simple experiment.
