Investigating the Kaps whistle tone at low- and high- flow rates of nitrogen gas

Flow-driven sound generation in fipple-type instruments arises from complex coupling between a jet, a sharp edge, and an acoustic resonator. In this work, we experimentally investigate the flow-dependent acoustic behavior of a Kaps-style whistle driven by nitrogen gas over a wide range of volumetric flow rates. Spectral measurements reveal four distinct regimes. At low-flow rates (3–4.6 l/min), the system exhibits passive resonance excitation governed by the fundamental pipe mode. As the flow increases, the oscillation transitions to a Strouhal-controlled edge-tone regime characterized by St≈0.6. At intermediate flow rates (9–30 l/min), a pronounced resonance lock-in occurs, where the oscillation frequency becomes weakly dependent on the flow rate, while acoustic amplitude continues to grow, indicating strong jet–resonator coupling. Beyond a critical flow rate, an abrupt transition to second-harmonic dominance is observed, accompanied by a reduction of the effective Strouhal number to approximately 0.36. Two-dimensional numerical simulations are used to visualize jet deflection and recirculation patterns, providing a physical interpretation of the regime transitions. The results demonstrate that flow-dependent coupling governs frequency selection, as described by a flow-corrected resonance relation, resonance locking, and harmonic mode switching in fipple-type aeroacoustic systems, offering insight into nonlinear acoustic mode selection in wind instruments.