Investigations On The Characteristics Of Novel Dual Synthetic Jet And Its Applications On Supersonic Boundary Layer Flow Control

The flow state of supersonic boundary layer is not only very crucial for aerodynamic design and thermal protection of supersonic/hypersonic vehicles,but also an important part of fundamental research on the flow mechanism of compressible boundary layer.The transition of supersonic boundary layer from laminar flow to turbulent flow will bring about great difference between wall friction and heat flux.The separation of supersonic boundary layer flow will not only bring about the same problem,but also the low-frequency oscillation in separation zone will bring potential damage to the structural safety and maneuverability of aircraft.Therefore,it is urgent to control the flow state of supersonic boundary layer.Based on the high performance dual synthetic jet(DSJ)actuator invented by our research team,the working characteristics of dual synthetic cold/hot jet,the flow stability control of supersonic flat-plate boundary layer and the separation control of supersonic forward-facing step are studied by numerical simulations and wind tunnel experiments.Firstly,the current research status of supersonic boundary layer flow control technologies and separation control of supersonic forward/backward-facing step flow is summarized,and the characteristics and shortcomings of single velocity-based control or temperature-based control of current flow control technologies are summarized.On this basis,combining the advantages of velocity-based control and temperature-based control,an innovative control method of supersonic boundary layer coupling both velocity-based and temperature-based control based on dual synthetic cold/hot jet technology is proposed to further improve the control ability of synthetic jet to supersonic boundary layer.Based on large-eddy simulation(LES)and particle image velocimetry(PIV),numerical simulations and experimental studies were carried out to study the flow field characteristics of dual synthetic cold/hot jets.The effects of jet temperature on the mean flow fields and instantaneous flow fields were obtained.For the dual synthetic cold jets,two strands of jets will converge near the orifice of actuator and form a potential-core with its height ranging from 1cm to 3cm.In the case of dual synthetic hot jet,the two strands of jets emerged with each other until the far field region.Mathematical derivation of two mode decomposition tools,proper orthogonal decomposition(POD)and dynamic mode decomposition(DMD),are introduced in detail.They are used to reduce the order of massive and complex turbulent flow fields.The flow field characteristics and frequency characteristics of decomposed flow field under different energy structures are analyzed.The symmetrical and anti-symmetrical distributions of POD modes of the experimental data are discovered.The flow fields are reconstructed based on a small number of low-order modes,and the effectiveness of the reduced-order processing is verified.Aiming at the mechanism of supersonic boundary layer controlled by dual synthetic cold/hot jet,numerical simulations are conducted to study the propagation and evolvement of unstable waves in transition process in near wall region of Ma4.5 supersonic flat-plate boundary layer.The flow field and control effects are analyzed using the temporal mode in linear stability theory(LST).Results show that the synthetic jet can effectively suppress the first mode whereas aggravate the instability of the second mode.The control law of the parameters such as wall blowing and suction,jet temperature,perturbation frequency and perturbation amplitude on the growth rate of the perturbation mode in the boundary layer is analyzed and the control mechanism is revealed.Aiming at the flow separation of supersonic forward-facing step,twodimensional numerical simulations of flow separation control of Ma3 supersonic forward-facing step using dual synthetic cold/hot jet were carried out.After the actuator worked,it was found that the separation area was reduced by about 5 mm,and the vorticities were mainly concentrated around the boundary layer,separation area and shock.With the help of modal decomposition tool,the flow field characteristics under different modes were obtained.The fine flow structures of interaction between self-sustaining dual synthetic jets and Ma2.95 supersonic laminar boundary layer were obtained by means of supersonic quiet wind tunnel,high-speed schlieren and nanoparticle-based planar laser scattering(NPLS).The typical flow field structures,such as separation shock,“W” bow shock,barrel shock,horseshoe vortex,Kelvin-Helmholtz(K-H)vortex and counter-rotating vortex pair,were clearly identified by streamwise and spanwise flow field images.The curves of jet penetration depth were fitted,and corresponding momentum energy ratio between synthetic jet and supersonic main flow is about 0.55.Wind tunnel experiments were carried out to study the separation control of two-dimensional forward-facing step flow and three-dimensional forward-facing step with self-sustaining dual synthetic jet under Ma2.95 supersonic laminar flow.The influence of orifice positions on separation control was analyzed in detail.It was found that large-scale streamwise vortex structures generated by synthetic jet could effectively reduce the length of separation region,compel the oblique shock induced by the step to distort and reduce its intensity and finally,weaken or even eliminate the separation shock.