Vortex shedding characteristics in air-assisted sprays: A phenomenological and quantitative study

Vortex shedding at the edge of twin-fluid spray was systematically identified and analyzed. High-speed photography was used to capture air-assisted spray under varying fuel injection durations (T_f) and gas injection pressures (P_a), revealing vortex shedding at the spray edge. Image processing techniques enabled quantitative analysis of the spray edge near the outlet of the air-assisted injector. The dimensionless timing of vortex shedding demonstrated a linear relationship with the ratio of dimensionless fuel injection durations (T^*) to dimensionless gas injection pressures (P^*). The shedding location shifted further from the injector as P_a increased, regardless of T_f. Power spectral density analysis combined with sliding-window sampling techniques enabled detailed examination of spray edge evolution and provided both full time and time-resolved characteristic frequencies of vortex shedding under various conditions. The inherent instability of air-assisted sprays resulted in multiple characteristic peaks in the full time vortex shedding frequency. The time-resolved characteristic frequency initially increased and then stabilized. Across all conditions, the vortex shedding frequency decreased with longer T_f and increased with higher P_a. The dimensionless maximum vortex shedding characteristics frequency for each condition demonstrated a linear relationship with T^* · 1 − l n P^*. The autocorrelation function was employed to calculate the wave propagation delay, thereby determining wave propagation velocity and, subsequently, the minimum wavelength. Wave velocity increased exclusively with rising P_a, highlighting the dominant influence of the gas phase in wave propagation. A nearly linear relationship was observed between wave velocity and maximum vortex shedding frequency. The minimum wavelength increases exponentially, primarily with T_f.