| Dual-throat fluidic thrust vectoring nozzle(DTN)is a hots topic in the researches of fluidic vectoring control due to its extensive advantages such as low thrust loss and high thrust vectoring efficiency.From the perspective of flow characteristics,the DTN has typical cavity flow characteristics.In order to explore the influence of the unsteady characteristics of nozzle cavity flow on the vibration of nozzle thin-walled structure and the stability of nozzle thrust,based on the computational fluid dynamics software FLUENT,the delayed detached-eddy simulation method based on SST k- turbulence model is used to numerically simulate the three-dimensional DTN.In addition,unsteady data processing methods such as dynamic mode decomposition(DMD),proper orthogonal decomposition(POD)and fast Fourier transform(FFT)are applied to analyze the frequency spectrum characteristics of unsteady flow structure in cavity.This thesis aims to reveal the coupling mechanism between nozzle mainstream and cavity flow and the internal relationship between unsteady flow structure in cavity and nozzle unsteady thrust.The accuracy of DMD and POD calculation programs are verified based on customized flow field and M219 cavity flow field.To verify the accuracy of the numerical method and grid distribution used in this work,the steady and unsteady results of DTN with two different secondary injection structures,one-row circular pass and slot pass,are compared with NASA experimental results.DTN with different secondary injection structures,different ratios of cavity length to depth,and different cavity lengths are calculated under six working conditions,where the nozzle mainstream pressure ratio NPR = 4 and the secondary flow mass flow accounts for 3%.Comparing the mean distribution of wall pressure coefficient within 0.5s under these six working conditions,it is found that the intensity of λ type shock is increased because the incident depth of the injection with slot pass is deeper than that with one-row circular pass,and thus the upper wall pressure coefficient deflects downward in the section of 0.01≤ x ≤ 0.02.In addition,the impact between the mainstream and the downside convergence section is significant when the cavity convergence angle is 20°,and the effect of this impact decreases with the increasing convergence angle.The pulsation curves of nozzle performance parameters including secondary mass flow coefficient,thrust coefficient and outlet mass flow coefficient within 0.06 s are systematically analyzed and compared.Results show that the secondary injections with one-row circular pass can effectively reduce the pulsation amplitude of nozzle performance parameters.While the secondary injections with slot pass can also effectively reduce the pulsation amplitude of nozzle performance parameters,achieved by reducing the ratio of cavity length to depth and the cavity length.Study on aeroacoustic characteristics of the DTN cavity under six working conditions suggests that the sound pressure level curves exhibit great difference at different positions in the DTN cavity,and the peak frequency of sound pressure level exists multiple values.The maximum value of the total sound pressure level under the 6 working conditions is about 203 d B.DMD and POD mode decompositions on the axisymmetric section of the DTN cavity is carried out,and the results are compared with the FFT analysis on the unsteady thrust.Results show that the low-frequency oscillation of the thrust is caused by the λtype shock wave in the cavity and the shear layer without instability between the main flow and the separation zone.The high-frequency oscillation of the thrust is caused by the impact of the downside convergence section and the vortex structure formed by the instability of the shear layer between the mainstream and the separation zone. |
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