RD-E: 2202 Ditching using SPH

The ditching of an object into a pool of water is studied.

Figure 1.

The simulation results are compared to the experimental data and the analytical results. The ditching object is modeled as a rigid prism. The water is modeled using SPH elements with an outlet condition to allow for a non-reflecting boundary condition.

Options and Keywords Used

Input Files

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The model files used in this example are available in:


Model Description

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

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

The impact of a triangular prism object on water is simulated and the results are compared qualitatively 2, also using the experimental data obtained from the Politecnico di Milano. 1

Figure 2. Problem Data
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's ratio
The material law for water is a hydrodynamic viscous fluid law (/MAT/LAW6) with linear stiffened gas (/EOS/STIFF-GAS) with the following characteristics:the following characteristics:
Material Properties
Initial density
1 x 10-6 kg.mm-3
Kinematic viscosity
0 GPa
1e-4 GPa
0 GPa
0.36885 GPa

The impacting prism is modeled using shell elements with an average mesh size of 15 mm x 15 mm. To shorten the computation, it is made rigid with an accelerometer on the main node of the rigid body. Additional mass is added to the main node of the rigid body, so the total mass of the prism is 27 kg.

The water is modeled using SPH particles with a face-centered cubic mesh and smoothing length "ho" equal to 21 mm. Each SPH weighs m=2.61 g. The SPH particle mesh can be generated in HyperWorks using the SPH panel with a face-centered cubic pitch=15, and a material density of 1.0e-6 k g m m 3 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSGaaeaaca WGRbGaam4zaaqaaiaad2gacaWGTbWaaWbaaSqabeaacaaIZaaaaaaa aaa@3AB3@ . HyperWorks will then automatically create SPH property /PROP/SPH. This results in 222005 SPH elements. The size of the water is adapted to the shape of the object to reduce the model’s size and the simulation’s CPU time. For an SPH analysis, air is not typically modeled.

Boundary Setup

An initial velocity of 11 m/s in the Z direction is defined on the main node of the rigid body of the prism. Gravity is also applied to the prism.

An outlet non-reflective boundary /SPH/INOUT, Ityp=3 is defined using a surface at a distance equal to 2 x ho inside the SPH water elements. The surface is placed at a distance equal to 2 x ho inside the water. The surface is shown in green in Figure 3 and is oriented so that its normal vector points inside the domain.

Figure 3. SPH Outlet Surface Non-reflective Frontiers (NRF) (green)

The fluid-structure contact between the impacting prism and SPH water is modeled using a /INTER/TYPE7 sliding penalty contact interface. The impacting prism is the main surface and the SPH elements are the secondary nodes. The contact thickness gap is defined as Gapmin=3. For situations when the contact thickness gap is important such as when the SPH particles are in a tank, define Gapmin=1/2 * (shell thickness + SPH diameter) or for solid elements Gapmin=1/2 * (SPH diameter).


The SPH particles are supported as MASS elements in HyperView. Their display type and size can be set in HyperView by using Preferences > Visualization. In this example, the animation results are directly output to an h3d file using the /H3D option. The pressure in the water can be plotted.

Figure 4 shows the variation in the pressure when the wedge ditches into the water at an impact velocity of 11 m/s.

Figure 4. Pressure results from a section cut

Output Acceleration

To measure the acceleration, an accelerometer is defined at the main node of the prism rigid body. The acceleration values were converted to g’s and compared to both the experimental values 1 and the analytic solution proposed by Von Karman. 2 The simulation acceleration is filtered using the same CFC 60 (-3db) filter frequency as the test data was filtered.

Figure 5. Deceleration of the prism for an impact velocity of 11 m/s

The SPH results are very close to the experimental test results and also to the analytical solution. The simulations show that the SPH approach using the OUTLET option /SPH/INOUT correctly models a vertical ditching event without any numerical problems.

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