Research On The Flow Control Mechanism Of Unsteady Blowing On Axial Compressor Cascade Corner Separation

Higher compressor pressure ratio is requested with the higher demands for aeroengine thrust-weight ratio.Corner separations,which not only cause aerodynamic loss,but also lead to serious blockage,and even affects the stability of the engine,are widely observed under the effect of strong adverse pressure gradient and end wall secondary flows in compressor cascade.Large-scale flow separations also show obvious unsteady behaviors especially in off design conditions,which is more likely to cause compressor rotating stall or surge.Therefore,it is of great significance to control the corner separations.Compared with conventional steady flow control methods,unsteady flow control methods could reduce the demands for extra excitation energy and improve the flow anti-disturbunce ability.The current dissertation utilized pulsed jets and sweeping jets to carry out the research of corner separation control based on unsteady blowing methods.Firstly,the experimental study in a low-speed cascade wind tunnel demonstrated the effectiveness of pulsed jets and sweeping jets in controlling the corner separation of in linear compressor cascade.The flow control mechanism of unsteady jets were primarily analyzed.Steady jets achieved a 5.72% total pressure loss reduction with a cost of0.85% excitation jet momentum coefficient,while pulsed jets achieved 7.85% reduction with a cost of half the excitation jet momentum coefficient of steady jets.Based on the flow visualization results,the steady jets and induced horse-shoe vortex restricted the accumulation of endwall secondary flows to the corner region.Meanwhile the steady jets had acceleration effects on the endwall boundary layer.Pulsed jets reserved these effects and decreased the intensity of the horse-shoe vortex,narrowed the endwall separation region apparently and restricted the suction side spiral node scale compared with steady jets.The energy distribution of characteristic frequency in the flow field could be obtained by deconstructing the frequency spectrum of densely distributed measuring points.The distribution of pulsed jets governing frequency corresponded to the concentrated shedding vortex region,which indicated an obvious frequency lock effect of pulsed jets to the vortex shedding.Besides,the effectiveness of arranging sweeping jet actuator on blade suction surface to control corner separation was validated by experiment method.Sweeping jets achieved 12.7% total pressure loss reduction without consideration of excitation jet energy.With the help of flow field visualization and measurement of governing frequency locations,it was found that sweeping jets accelarated suction surface boundary layer in a relatively larger region and restricted accumulation of low momentum fluids.No spiral node was observed in the suction surface visualization.Secondly,numerical method based on unsteady Reynolds Averaged Navier Stokes equations were used to carry out the schemes which were difficult to fulfill and the flow field data which were difficult to obtain in the experimental method.Detailed parametric studies were also carried out.The investigation on pulsed blowing frequency effects revealed that the excitation frequency showed an apparent frequency lock effect to the concentrated shedding vortex.Shedding vortex shedded without a thoroughly development process at high excitation frequency and shedded with a thoroughly development process with a larger scale and higher strength at low excitation frequency.The investigation of sweeping jets were divided into two parts: using sweeping jet actuator(SJA)directly and using moduled sweeping jets on the blade suction surface.It is found that the flow control performance was not sensitive to SJA mounting angle,exit position and excitation jet pressure ratio,but sensitive to SJA axial and spanwise locations in the current study.With the arrangement of dual SJAs,the flow control performance was even more sensitive to SJA locations.Moduled sweeping jet investigations showed that flow control performance was increased with the increase of sweeping jet frequency and became stable as the non-dimensional frequency reached 1.The corresponding excitation cycle related to the fluids flowed from the blade leading edge to the trailing edge.At designed working condition,moduled sweeping jet reduced total pressure loss by 6.1% and the same case reduced total pressure loss by 10.19% at 9°attack angle.Finally,Large Eddy Simulation and mode analysis methods were used to capture fluctuation information and extract governing flow structures in order to analyze the influence mechanism of unsteady jets on corner separation.It was observed shedding vortex features occupied the flow field at relevant positions with frequency spectra analysis,Proper Orthogonal Decomposition(POD)and Direct Mode Decomposition(DMD)analysis under original case.The two unsteady jet schemes made the energy distribution in the flow field reconstructed by POD more dispersed than that in the uncontrolled flow field.The pulsed jets and induced small scale structures occupied lots of high energy modes.Pulsed jets restricted endwall secondary flow developments with the help of these small scale structures.The large scale axial vortex dominated the sweeping jets scheme at location off the excitation plane,indicating that the sweeping jets accelerated the low momentum fluids in the separation region by the jets themselves directly in a larger region.The DMD reconstructed mode with most energy occupied corresponded to excitation frequency under all unsteady blowing cases.