Fluid Structure Interaction Analysis Of A Supersonic Intake

Along with the development of supersonic aircraft, higher requirements are made for the intake design. To improve engine thrust weight ratio and aircraft performance,intake is tended to be designed with flexible wall, which introduce outstanding intake fluid structure interaction problem. However, traditional rigid wall design method could not describe this problem. And fluid structure interaction method must be applied to analysis the intake fluid structure interaction problem. Less reports are found about fluid structure interaction problem of intake. Most of them focus on the coupling problem which tend to static status, while shock oscillation induced fluid structure interaction problem is rarely reported.Based on these background, fluid structure interaction method was applied to study the coupled problem between shock oscillation induced unsteady flow in supersonic intake and local flexible structure. In view of the complexity of multi-physics analysis, the thesis was organized from simple to complex. The fluid structure interaction problem was firstly reduced to three independently problems, e.g. flow excitation problem, nonlinear vibration problem and coupling scheme problem, which were studied with three simple objects respectively. Then, a ramjet intake was chose to study the fluid structure interaction problem between shock-boundary layer interaction induced unsteady flow and local flexible wall. Finally, a wind-tunnel test was performed to study the unsteady flow in a ramjet intake, and to validate the numerical model and simulation process. Mainly research contents are listed as follow.Firstly, transonic flow model was generated to describe shock-boundary layer interaction in a simple intake, where effects of turbulence model, inflow turbulence parameters and outflow perturbation on steady state flow and unsteady flow were analyzed, and two methods were proposed to improve solving precision of shock-boundary layer interaction and solving speed of self-sustained oscillation of shock-boundary layer interaction. It is found that the standard k- ω turbulence model with modified inflow turbulence parameters accurately describes shock-boundary layer interaction and the unsteady flow induced by self-sustained oscillation. Outflow perturbation affects intake flow through nonlinear‘frequency acquisition’. And complicated excitation is formed on the intake wall due to shock oscillation, including traveling wave excitation in shock oscillation region and its downstream quasi harmonic excitation.Then, finite element model was generated to describe local flexible intake wall nonlinear vibration based on Von K`arm`an geometric nonlinearity, and flexible intake wall’s nonlinear vibration response was determined under harmonic excitation, traveling wave excitation and real oscillated shock excitation. It is found that nonlinear ‘jump’ phenomena in wall vibration response is caused by geometric nonlinearity. It leads to dual vibration amplitude in certain excitation frequency interval, and that actual vibration status depends on excitation history. Beside, nonlinear ‘jump’ phenomena may cause large vibration amplitude increase under a small excitation frequency variation, which adverses to flexible wall vibration control. However, nonlinear ‘jump’ phenomena could be weaken through decreasing excitation amplitude, and even be suppressed through adding flexible wall damping.Furthermore, a coupling scheme was generated for fluid structure interaction between intake unsteady flow and local flexible wall, and a method was proposed with adaptive coupling time step, and three adaptive criterion’s effects were also studied. It is concluded that this method improves fluid structure interaction calculation speed with the adaptive criterion of coupling interface vibration velocity, and maintains a accurate solution. However results with the other two adaptive criterion are not accurate.In addition, unsteady flow caused by shock-boundary interaction in a ramjet intake was studied, together with the fluid structure interaction problems. It is found that the fluid structure interaction status induced by self-sustained shock oscillation is different from that caused by back pressure perturbation. With a constant back pressure, selfsustained flow oscillation in the intake influences its fluid structure interaction status.When self-sustained shock-boundary interaction oscillation does not occur, the interaction obtains a quasi-static status. When self-sustained oscillation occurs, the interaction is also affected by flexible wall damping, which obtains flutter status with a small damping or without damping, and quasi-static status with large damping. Fluid structure interaction induced by back perturbation only keeps disturbed status, However, flexible wall vibration amplitude and shock oscillation amplitude could be limited through adding flexible wall damping and reducing perturbation amplitude.Finally, a wind-tunnel pressure fluctuation test was performed for a ramjet two dimensional intake, which partially validated the flow model and simulation process in this thesis, and effects of inflow Mach number, attack angle and back pressure on intake flow was also determined. It is concluded that steady state numerical flow solution in critical conditions agrees well with test data, while pressure fluctuation solution in super critical conditions agrees partially with test data, including basic tallies in fluctuation frequency results and some differences in fluctuation amplitude results.Above all, fluid structure interaction problem between unsteady flow in ramjet intake and local flexible wall was studied in this thesis, based on numerical and experimental methods. Intake flow model, nonlinear vibration model and fluid structure coupling scheme were generated. Variable parameters’ effects on unsteady flow and fluid structure interaction were also analyzed. This thesis could provide technical reference for flexible design of modern supersonic intake.