Effects of air injection on the characteristics of unsteady sheet/cloud cavitation shedding in the convergent-divergent channel

The objective of this paper is to investigate the effects of air injection on the characteristics of two different unsteady sheet/cloud cavitation shedding mechanisms, namely re-entrant flow mechanism and bubbly shock propagation mechanism, in the convergent-divergent channel. Experiments were conducted in the 10° divergent section with a ventilation slit near the throat, using a simultaneous sampling technique to synchronize the transient cavity behaviors with wall-pressure signals. Results are presented at a Reynolds number 9.33 × 105 and Froude number 10.24 for (1) under re-entrant flow mechanism (σ = 0.81) characterized by the intermittent shedding of small-scale cavities, and (2) under bubbly shock propagation mechanism (σ = 0.70) characterized by periodic large-scale cavity cloud shedding, with three non-dimensional air injection volumetric flow rates, namely 0, 1.135 × 10−3 and 2.270 × 10−3. The results show that air injection from the throat into the shear layer can significantly suppress cavitation induced unsteady pressure fluctuations. With the increasing of air flow rate, the size of the attached sheet cavity and the shedding cavity clusters grows larger, and the distance between the new attached sheet cavity and the shedding cavity cloud decreases, resulting in the cavitation region misty. The cavitation evolution tends to be stable, indicating the increase in the cavitation cycle, and the FFT analysis of the unsteady pressure fluctuations shows the decrease in dominant frequency. For the re-entrant flow mechanism condition, air injection rate at 1.135 × 10−3 will cause the transition from small-scale cavity clusters shedding to large-scale cavity cloud shedding, accompanying with pressure fluctuations increasing. With the air injection rate further increase, the pressure fluctuations decrease. Air injection can significantly reduce the unsteady pressure fluctuations spectral content in the frequency range of 1 kHz to 10 kHz. For the bubbly shock propagation mechanism condition, air injection can avoid the formation and propagation of the bubbly shock and suppress both the pressure peaks and pressure fluctuations. With the increase in the air injection rate, the unsteady pressure fluctuations are further suppressed. The unsteady pressure spectral content between 0.2 kHz and 10 kHz is effectively reduced.