Investigations On A Fluidic Control Method For Improving Self-starting Ability Of Hypersonic Inlets

In order to improve the self-starting ability of hypersonic inlets at low Mach number and expend the working range, a closed-type flow control concept of backflow duct was proposed in this paper. With a combined numerical and experimental method, the self-starting process, flowfield variation and aerodynamic performance of inlets were investigated.First, the geometric profile of a generic hypersonic inlet was determined, and flowfield characteristics under start and unstart states were analyzed through preliminary simulations. Results indicated: inlet exhibited completely different flowfield features under different operating statuses and the surface pressure distribution of the compression ramp near the inlet entrance altered significantly. Take advantage of the pressure difference generated by the separation induced shock under unstart condition, the fluidic control method of backflow duct was put forward.Second, with steady numerical simulations, the underlying control mechanism of backflow duct was revealed, and inlet’s flowfield characteristics and aerodynamic performances influenced by variable geometry parameters of the backflow duct were studied. Compared with performances of the inlet without backflow duct, results showed that due to the backflow duct, the self-starting Mach number of inlet(internal area contraction ratio: CR=1.6) was declined from Ma=4.7 to Ma=3.6, expending the operating Mach range remarkably. The location of the backflow duct’s entrance affected the self-starting performance significantly, while the location of backflow duct’s exit and the width of backflow duct’s section( b ?8mm 8mm) scarcely impacted the self-starting capability. At low Mach number, the unstarted flowfield of inlet was improved by the backflow duct considerably. But at high Mach number, the influence of backflow duct would be slight.Then, 3-D CFD calculations and wind tunnel experiments of inlets without/with backflow duct(CR=2.0) have been performed. Results demonstrated: the inlet without backflow duct was unable to start at Ma=5.0, ? ?0?, when a periodically varying separation bubble appears near the inlet entrance. However, the previous separation bubble and induced shock disappeared with the backflow duct introduced at Ma=5.0, ? ?0?, during which the backflow-duct inlet operated properly. Besides, the start flowfields were still established under states with large attack angles of ? ?4? and ? ?6? respectively, indicating that the self-starting Mach number of the backflow-duct inlet at ? ?0? was markedly lower than Ma5.0.Last, unsteady numerations have been conducted to simulate the fluid introducing process of backflow duct. Results indicated that at the moment when the backflow duct switched to open, the backflow rate just rose rapidly to the maximum value and then dropped gradually before reaching to final steadiness. However, the mass flow rate of inlet exerted a completely opposite variation trend through the whole procedure. In addition, the restart process of hypersonic inlet at Ma=6.0,? ?0? were researched through moving the plug backwardly, one can note that from the experimental phenomenon the restart process can be divided into three continuous stages: big buzz, shock train flow downstream in the inlet’s internal duct and stable high-frequency oscillations around the isolator exit.