Overview

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Physical Problem Description

The problem consists of a simple object falling into water simulating the ditching of a helicopter.

Units: mm, ms, KN, GPa, kg.

Impact of a triangular object on water is performed and the results are compared qualitatively [2], also using the experimental data obtained from the Politechnico di Milano [1].

The computation is performed using several impact velocities of 3.5 and 11 m/s.

The material used for the object follows a linear elastic law (/MAT/LAW1) with the following characteristics:

The material law for water is BIPHAS law (/MAT/LAW37) with the following characteristics:

Fig 1: Problem data.

Analysis, Assumptions and Modeling Description

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Modeling Methodology

The object is modeled using shell elements with an average mesh size of 15 x 15 mm2. In order to simplify the computation, it is put in a rigid body with an accelerometer on the master node.

The water is modeled using 15x15x15 mm mesh with a total of 166023 elements. The material law BIPHAS (/MAT/LAW37) is used.

The air is modeled using a BIPHAS material with the following characteristics:

#blank

 

#          RHO_L              CL       ALPHA_L         NU_L     LAMBDA_ON_RHO_L

           1e-6           2.089             0       .00089                   0

#          RHO_G           GAMMA            P0         NU_G     LAMBDA_ON_RHO_G

        1.22E-9             1.4         .1e-3      .014607                   0

 

The boundary conditions are applied to the pool as following:

An interface type 18 is defined to manage the contact between the solid in Lagrangian mesh (Prism) and the fluid (pool). The diedra is defined as master and the nodes in the pool (air and water) as slave.

The interface type 18 forces are computed by Penalty method. The interface stiffness is proportional to impact velocity. The results obtained by the ALE approach can be highly dependent to the interface stiffness factor Stfac, which should be adjusted in function of size of element and fluid properties.

Simulation Results and Conclusions

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The interface type 18 forces are computed by Penalty method. The forces are proportional to the stiffness factor Stfac which should be calibrated. The following graph shows the dependence of results (acceleration at the accelerometer) to the mesh and Stfac.

Fig 2: Higher peak force with coarse mesh and interface stiffness dependence

Higher peak forces are obtained with the coarse mesh. That can be partially corrected by filtering, as shown in Fig 3.

Fig 3: Filtering of results for coarse and fine meshes

Using a filter CFC 60, -3 dB, the simulation results of the ALE and SPH approaches are compared to Von Karman theoretical solution and experimental measures in Fig 4.

Fig 4: Comparison between simulation results, theoretical solution and experimental measures (acceleration)

SPH and ALE approaches respectively give the maximum acceleration of 83g and 84g. However, the Von Karman theory delivers 82g; with the maximum value by experience is between 83g and 73g.

On the other hand, the duration for acceleration beyond 40g is respectively 7.9ms and 8.2ms for SPH and ALE simulation methods, where the experience provides values between 7.5ms and 8.5ms and the Von Karman theory provides 8 ms.

Note that:

References

[1] CAST Deliverable 5.5.1 Generic Water Impact Tests performed at Politecnico di Milano (Polytechnic University of Milan).

[2] Olivier Pastore Study and modelization of rigid bodies impact during sea landing phase; Annex 1 Von Karman's Theoretical Models, T. Miloh et al. May.