# Influence of Active Flow Control of Synthetic Jet at
## 6 Influence and Analysis of Coupled Flow Control on Compressor Performance
### Mechanism of Coupled Flow Control to Improve Compressor Performance  
Figure 19 presents the internal flow situation of the compressor under different flow control methods. It is clear to compare the differences between the tip leakage flow, the radial velocity of flow at the channel outlet, and the radial velocity at the bottom of the slot in three different scenarios. During the prototype compressor, due to the high back pressure, the tip leakage flow could not pass out smoothly at the top of the blade, resulting in extensive blockage. After the casing treatment, due to the pressure difference between the suction surface and the pressure surface at the blade top, when the casing treatment passes through the blade top, under the action of the pressure difference, the low-speed flow at the pressure surface blade top is sucked into the slot casing treatment and then sprayed out on one side of the suction surface. This effectively decreased the blade top load, reduced the angle between the leakage flow at the blade top and the suction surface, and significantly increased the speed of tip leakage flow. Therefore, the stability margin of the compressor with casing treatment is significantly increased. When coupled with flow control, the casing treatment still effectively plays a stabilizing role. From the radial velocity distribution of the flow on the outlet surface of the blade channel, the radial velocity (time-averaged value) after the action of the synthetic jet is greater than that of the prototype compressor, which is consistent with the influence of the synthetic jet on the separation and separation loss of the suction surface mentioned earlier. The driving effect of the synthetic jet on radial flow will reduce the degree of separation at the trailing edge of the suction surface, effectively reduce separation losses, and improve the aerodynamic efficiency of the compressor.  
Figure 18: Effect of coupled control on the total performance of compressor.  
Figures 20 and 21, respectively, present the changes in total loss and internal loss sources of the compressor. From the change in the total loss, the compressor loss is reduced after the casing treatment, but from the overall performance chart, the introduction of the slot leads to a decrease in the compressor efficiency, which indicates that the power capacity of the compressor is reduced due to the introduction of the casing treatment. After the coupled flow control, the loss of the compressor is further reduced due to the excitation of the synthetic jet. Therefore, the compressor efficiency is improved compared with the prototype and the simple casing treatment. From the change in each loss source, the import loss and hub loss have little change. The channel loss is slightly reduced, indicating that the flow in the compressor channel is more uniform than that in the prototype compressor. The tip leakage loss is reduced most significantly, which corresponds to the suppression of tip leakage flow and the great improvement of tip flow after the above-mentioned slot casing treatment. The casing treatment loss is reduced, and the outlet loss is also significantly reduced. This is because after the slot casing treatment, the wake loss of the tip leakage flow is reduced, and the outlet flow is more uniform. Especially after the coupled flow control, the suction surface separation is also restrained to a certain extent, the outlet flow is more uniform than the pure slot casing treatment, and the outlet loss is further reduced.  
Figure 19: Comparison of internal flow in compressors under different flow control methods.  
Figure 20: Changes of total loss.