Mechanisms Of Corner Separation Control In Compressor Cascades With Synthetic Jets

Corner separation is one of the common complex flow phenomena in the endwall region of highly-loaded compressors.It not only causes an rapid increase of flow loss,but also influences the working stability of compressors.On the basis of deeply understanding the mechanism of corner separation,it is of great significance to put forward advanced flow control methods for the design of highly-loaded compressors.Synthetic jet is one of the active flow control technique based on the motion of vortex.With unique characteristics of zero net mass-flux and non-zero mean momentum flux,the synthetic jet is not only compacted in structure but also simple in configuration,and does not need complex system of pumps and pipes in comparison with conventional injection-based methods.As a result,it has currently become one of the most promising techniques in the field of active flow control in compressors.In the thesis,numerical investigation of corner separation control on both the low-speed and high-speed compressor cascades is performed with synthetic jet.The synthetic jet actuators are mounted on the blade suction surface and the hub endwall,respectively,of which the endwall one includes slot and tube structures.The effect of the flow control technique on the aerodynamic performance of the cascade is systematically investigated.The underlying physics of corner separation control are discussed in detail and the adaptability of the flow control method under off-design conditions is verified as well.In addition,a comparative analysis is performed on the influence of jet parameters on the time-averaged aerodynamic performance of both the low-speed and high-speed cascades.Based on the theory of vortex dynamic and topological principle,both the topology of surface flow pattern and the vortex model are discussed.In addition,the relationships between the vortex model and the loss are revealed.Firstly,the present study systematically investigates the corner separation control on both the low-speed and high-speed cascades with the suction surface synthetic jet.The results indicate that the flow control technique can significantly improve the time-averaged aerodynamic performance of the cascade and it has good adaptability to off-design conditions.Besides,the flow control technique can effectively delay the flow separation downstream of the actuator.It should be noted that the improvements of the low-speed cascade are superior to that of the high-speed one.Streamwise momentum addition is the main mechanism of the suction surface synthetic jet,and the flow mixing also plays an important role.The actuation frequency determines the dynamic response characteristic of the flow fields to the synthetic jet and affects the induced flow mixing.This is the main reason that it influences the flow control effect.The effect of jet momentum and jet angle on both the compressor cascades are consistent.With the increase of jet momentum and the decrease of jet angle,the flow control effect is gradually enhanced.However,the influence of jet location on the low-speed and high-speed cascade are inconsistent.It has moderate effect on the performance of the low-speed cascade,while the performance of the high-speed one is improved with the jet location moving towards the downstream.In addition,the improvements obtained by the partly-span jet structure are roughly the same as the full-span one.As a result,the flow control efficiency of the partly-span one is obviously higher.Secondly,the present paper systematically investigates the corner separation control on both the low-speed and high-speed cascades with the endwall synthetic jet.The results indicate that the flow control method can also significantly improve the time-averaged aerodynamic performance of the compressor cascade and it has good adaptiability to off-design conditions.However,only small changes in the flow patterns on the blade surface and endwall can be observed.In other words,the flow control technique just slightly delays flow separation in the corner between the suction surface and the endwall.Streamwise momentum addition and flow mixing are the key factors in the mechanism of the endwal synthetic jet.The former plays a dominant role in the slot structure,while the above two play an important role in the hole structure.The influence of jet momentum,jet angle and jet location on both compressor cascades are consistent.The flow control effect is improved gradually with the increase of jet momentum and the decreases of jet angle.The prerequisite of achieving effective separation control is that the jet momentum must exceed its threshold,and the threshold of the high-speed cascade is significantly higher than that of the low-speed one.The endwall synthetic jet located near the root of separation line on the blade suction surface can remarkably amplify the instability of T-S wave in the endwall boundary layer,which helps to strenghthen the flow mixing.However,the influence of jet momentum on the endwall synthetic jet with hole structure is different from that with slot structure.The jet momentum has an optimum value and its threshold is obviously lower than that of slot structure,both of which are associated with the streamwise jet vortex and the induced mixing loss.This is also the reason why the mechanism of corner separation control is different from the slot structure.At last,the topological patterns on the blade surface and the vortex model in the blade passage are also discussed.Compared with the endwall synthetic jet,the influence of the suction surface synthetic jet on the topological patterns is more prominent.With the suction surface synthetic jet,jet momentum,jet angle and jet location significantly affect the topological patterns of the low-speed cascade,while the influence of actuation frequency and jet location on the topological patterns of the high-speed cascade is more obvious.The synthetic jet improves the flow characteristic of the compressor cascade by controlling the migration of the boundary layer.Then it further affects the topological patterns on the blade surface and the evolution of passage vortex and concentrated shedding vortex.When selecting a proper set of jet parameters,the compliexity of topological patters and the numer of singularities are greatly reduced.In this case,the flow control effect is the most prominent.