This study reveals the blockage effects on vortex shedding, the lock-in mechanism of a forced oscillating cylinder, and the coupling effects of blockage and oscillation. The wind tunnel experiments and large eddy simulations were conducted at a Reynolds number of 2000, encompassing a range of blockage ratios from 0.15 to 0.5. The cylinder is subjected to forced harmonic oscillations in a direction perpendicular to the incoming flow to investigate the frequency lock-in mechanism. The research findings demonstrate that both blockage and forced vibration significantly influence the dynamics of vortex shedding by altering the development of the shear layer instability (SLI). For a fixed cylinder, high blockage effectively suppresses the growth of transverse disturbances, promoting a stable maintenance of the shear layer (SL). In high blockage, the flow deceleration induces the separation of the boundary layer from the sidewalls and the subsequent constriction of the mainstream toward the channel centerline, consequently leading to an increase in the Strouhal number. For the case of a fixed blockage ratio, forced vibration alters the flow supplement in the near-SL region. During the unlocked phase, a significant occurrence of backflow is observed near the SL, which promotes the development of SLI. However, proper vibration induces the transverse flow supplement in the near-SL region, which balance the amount carried away by mainstream, thereby suppressing the backflow. The coupling effects of blockage and vibration are evident in the shift of the lock-in region and the triggering of hysteresis, both of which are explained by the proposed mechanisms of blockage and lock-in.

The performance and stability of the transonic fan stage with a nonuniform inflow is a key issue in aero engine operation, and the coupling of inlet distortion and low Reynolds numbers at high altitude creates more severe challenges to aero engines. This work used computational methods to explore the performance and stability of a transonic fan stage with inlet distortion and a low Reynolds number. To investigate this issue, the transonic fan stage NASA stage 67 with a 180° total pressure distortion at a low Reynolds number was selected as the test case. Full-annulus 3D unsteady simulations were conducted to reveal the details of the flow field under coupling conditions. The computational results under different conditions were analyzed and showed that the coupling effect was not linearly stacked with single factors. The low Reynolds number in the coupling case thickened the boundary layer on the blade surface, and, on the one hand, the profile loss in the hub region increased. On the other hand, the structure of shock wave was converted in the tip region, which increased the shock loss significantly. In addition, the variation in the shock wave structure reconstructed the pressure distribution on the blade surface, allowing the fluid to migrate to the tip region, which affected the tip flow structure and ultimately delayed the stability boundary.

Active flow control methods are commonly used in expanding the operating range of compressors. Indeed, unsteady active control methods are the main focus of researchers due to their effectiveness. For constructing an unsteady active control system, reliable actuators are significant. To compare with conventional actuators such as synthetic jet actuators and rotating valves, fluidic oscillators have structurally robust characteristics and can generate self-excited and self-sustained oscillating jets, which leads to its higher applicability in compressors under severe working conditions. Thus, to explore the feasibility of unsteady active control systems by the usage of fluidic oscillators, a low-frequency and low-speed oscillator is first designed and experimentally studied for improving the stability of a low-speed axial flow compressor. During the experiments, a special casing is designed to install 15 uniformly distributed oscillators in the tip region of compressor. Based on the unsteady micro injections of the rotor tip with rotor rotation frequency, the results indicate that the frequency/period of oscillators are flexible, in which the values are decoupled with the variation of inlet pressure. When the inlet-to-outlet pressure ratio of the oscillator is in the range of 1.1~2.0, the maximum velocity ranges from 30 m/s to 80 m/s. Moreover, the mass flow rate of the single oscillator only varies from 0.017‰ to 0.059‰ from the designed compressor mass flow rate. For the improvement of the compressor stall margin, the value is 3.45% when the total mass flow of oscillators is 0.08% of the designed compressor mass flow.