Study On Numerical Simulation Of Impact Loads Of Airplane Ditching

Airplane may not be able to land on the right site due to the failure of engine or landing gear.It is relatively safe to make an emergency landing on the nearby water.The ditching of aircraft is a strong non-linear coupled hydrodynamic problem.The landing speed of aircraft generally exceeds 240 km/h.The overload of fuselage caused by the huge hydrodynamic impact load will lead to the damage of the fuselage structure and even the breakage of the fuselage.In this paper,the commercial CFD software STAR-CCM+is used to simulate airplane ditching in quiescent water and wave environment.The effects of initial motion conditions,wave factors on the motion law and impact load of ditching are studied.Overset mesh method and 6-DOF fluid-solid coupling model are used to simulate airplane motion,and VOF model is used to capture free surface.As the airplane ditching is a high Reynolds number problem,the Realiable k-? turbulence model is used to simulate the unsteady flow.Firstly,the free fall water entry of wedge and cone is investigated.It is found that the current numerical results are in good agreement with the existing experimental data,which verifies the reliability of the numerical model for the water entry of simple structures.The vertical and oblique water entry of wedges are simulated as further validation.Comparing with the results of boundary element method(BEM),it is found that the pressure on the leeward side of wedge is quite different from that of BEM,while the pressure at other positions is in good agreement.In this paper,the influence of oblique velocity and deadrise angle of water entry of wedge on pressure disribution is analyzed.A noticeable vortex flow and low pressure area on the leeward side of the wedge have been observed and a low pressure zone will be formed in the vortex region.The location of the vortex center corresponds to the peak of the low pressure.The peak value of the low pressure varies with the increase of water entry depth.Therefore,the sharp structure at the bottom of the fuselage should be avoided in the process of ditching.In this paper,NACA TN2929 airplane model is used to study airplane ditching on initial quiescent water and waves.The effects of initial horizontal velocity,vertical velocity,pitch angle,wavelength and wave height on ditching process and impact load are studied.The appropriate size of mesh and time step are determined by convergence analysis.The convergence results are compared with the existing experimental results.The results show that the load and pitch angle of the airplane will reach a huge peak in a very short time after contacting water,and then decrease rapidly.The initial horizontal velocity has a direct impact on hydrodynamic load.The larger the initial horizontal velocity is,the larger the peak load is.The influence of initial vertical velocity and pitch angle is relatively small.The peak pressure at the bottom of the fuselage appears at the interaction of the free surface and the fuselage.Larger vertical velocity would result quick progress of ditching.The increase of pitch angle and decrease of vertical velocity would ease the violent behavior during ditching.The numerical results suggest that the aeroplane shall minimize its ditching velocity and adopt a proper initial pitch angle.The aerodynamic on the wing has less effect,and the load on the tail increases obviously when it contacts water.Fifth-order Stokes waves are employed to study the effects of waves on airplane ditching An experiment is designed to study the effect of wave factors on fuselage load when the fuselage is flying at a constant speed in the wave.Study the coupling motion of the aircraft ditching in the crest and trough,and the changes of load and motion are compared.It is found that when the airplane ditching in the trough and the wavelength is twice the length of the fuselage,the peak load is smaller and the motion is relatively gentle.