FEM Analysis: Simula Technologies, Inc.)

Figure 3(a) shows ripples in the form of contours on the surface of the water, which represents the splash formation. Figure 3(b), Illustrates the results from the FEM analysis and the experiments conducted by Kindervater, wherein the experimental graphs were passed through 600 Hz SAE [7] filter, and FEM was passed through a 1000 Hz filter. Acceleration values for the FEM analysis and the experimental data was 91G and 69G respectively. Figure 3(c) shows the later phase in which a half ,non-rigid fuselage model of 90 cm long , having a diameter of 30cm, with an impact velocity around 10 m/sec and a pitch angle of 1.3o. The FEM analysis consisted of 2,192 shell elements for the half fuselage model using results from PAM-CRASH as input for DRI-KRASH.
Army conducted experiment in which a helicopter was dropped from a height of 9 feet at velocity around 24 ft/sec to test the impact of the fuselage (weighing around 1450 pounds) on the surface of water using crane to suspend the fuselage as shown in figure 4. The impact pulses were recorded using transducers and an accelerometer. Accelerometer data as shown in Figure 5, was obtained using a 10,000 Hz VXI-based data acquisition system and digitally filtered at 300 Hz. The minimum and maximum peak was 27.9 g and 69g respectively, as shown in Figure 5 (b). From Figure 6, the damage occurred on the bottom of the fuselage can be noticed; a cracked small Plexiglas bubble and a bulged panel was found. Using the ALE method along with LS-DYNA by Randhawa, MAT_ELASTIC_FULID was used for the material properties in the analysis and water was validated using the above conducted experiment. Using the solid elements, the geometry for water pool was created using dimensions, 960x960x240 cm and mesh seeding was 20x20x15 (Tri-mesh for helicopter, quad mesh for the water and air
Tay, P.S. Bhonge & H.M. Lankarani[9], conducted experiments using SPH Technique as it has ability to undergo large deformation without mesh distortion. The water was treated as a collection of particles and the mass of each particle was calculated by dividing the total mass of the fluid with the number of particles. To perform the vertical impact of the fuselage section onto a body of water, the correlated fuselage and water numerical models were coupled. Trial simulations with number of SPH particles was performed, it is found that difference in outcomes is insignificant above 1:1*106 SPH particles. To provide proper constraint to the body of water, Planar walls were defined at five corners of the water block. An initial vertical velocity of 9.14 m/s is applied to the fuselage model to replicate a 4.3 m (14 ft) vertical drop. LS-DYNA contact codes were applied to the numerical model, where the contact interaction between the fuselage skin and the water particles was based on the soft contact-penalty algorithm. Numerical model for the vertical impact of the fuselage section onto a body of water is as shown in the figure 7

Figure 7: Numerical model for the vertical impact of the fuselage section onto a body of water [9]

At velocity of 9.14 m/s, the behaviour of the fuselage section impacting a body of water is shown in figure 8. It is seen the deformation of the fuselage section is found to be asymmetric. After 0.08 s, it can be seen the fuselage section has penetrated