RD-E: 2202 Ditching using ALE (Mono-Domain)

Impact of a simple object on water simulated by ALE approach.

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

Options and Keywords Used

Input Files

The input files used in this example include:


Model Description

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:
Material Properties
Initial density
7.8 x 10-6 kg.mm-3
Young's modulus
206.9 GPa
Poisson ratio
The material law for water is BIPHAS law (/MAT/LAW37) with the following characteristics:

Figure 1. Problem Data

Model Method

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 main node.

The water is modeled using 15x15x15 mm mesh with a total of 166023 elements.

The air is modeled using a BIPHAS material with the following characteristics:
#            Psh
#          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:
  • Z displacement component fixed for lower and upper faces
  • Y displacement component fixed for lateral faces normal to Y
  • X displacement component fixed for lateral faces normal to X

An interface TYPE18 is defined to manage the contact between the solid in Lagrangian mesh (Prism) and the fluid (pool). The diedra is defined as main and the nodes in the pool (air and water) as secondary.

The interface TYPE18 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.


The interface TYPE18 forces are computed by Penalty method. The forces are proportional to the stiffness factor Stfac which should be calibrated. Figure 2 shows the dependence of results (acceleration at the accelerometer) to the mesh and Stfac.

Figure 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 (Figure 3).

Figure 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 Figure 4.

Figure 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.
  • The computation time is similar for both SPH and ALE approaches.
  • Using a fine enough mesh, both Radioss methods SPH and ALE provide satisfactory results, compared to experience and analytical solution.
  • The ALE approach is more robust and stable when the SPH signal is highly vibrated.
  • The use of a coarse mesh in the ALE approach requires the interfaced stiffness calibration.