INIVOL and Fluid Structure Interaction (Drop Container)

Input Files

Model Files (1.6 MB)

Introduction

The aim of this example is to introduce /INIVOL for initial volume fractions of different materials in multi-material ALE elements, /SURF/PLANE for infinite plane, and fluid structure interaction (FSI) with a Lagrange container.

Options and Keywords Used

Keyword documentation may be found in the reference guide available from

OpenRadioss User Documentation

  • Solid element (/BRICK)

  • Material law (/MAT/LAW51 (MULTIMAT) and /ALE/MAT)

  • ALE grid velocity (/ALE/GRID/DONEA)

  • MUSCL scheme for ALE LAW51 (/ALE/MUSCL)

  • Property (/PROP/TYPE14 (SOLID))

  • Load (/INIVEL)

  • Define initial volume (/INIVOL)

  • Infinite surface plane (/SURF/PLANE)

  • Fluid structure contact (/INTER/TYPE18)

Model Description

A hex mesh is created that fully encloses the structural container. The mesh size of the hex mesh should be ½ the size of the structural mesh. Ideally the hex mesh should also be ¼ of the structural mesh size in the direction of impact. To simplify this example, the hex mesh in this model does not adhere to the ¼ mesh size guideline.

A container partially filled with water is simulated being dropped from a height of 1 meter. The container is partially filled with water with the remainder filled with air.

Figure1. Problem Description

Boundary Conditions

Each outer side of the hex mesh is constrained to prevent displacement in the direction normal to the side. For example, the top and bottom of the hex mesh is constrained in the z translation DOF (Figure 2). The same is done for the other four sides. The velocity at impact of a drop from 1 meter would be 4429 mm/s. Since the simulation is started right before impact, an initial velocity of 4429 mm/s is applied to the container and the fluid hex mesh (Figure 2).

Figure 2. Boundary Condition of Container in z-direction

Units: mm, s, Mg, N, MPa

In one /MAT/LAW51 card, three different phases can be defined. The two phases are: Air and Water

Air is defined with the following characteristics:

Water is defined with the following characteristics:

Coupled Euler_Lagrange (CEL) Interface

To define the contact between the fluid and the structure a visco-elastic penalty formulation /INTER/TYPE18 interface is defined as:

  • Main is the Lagrange structure

  • Secondary is the ALE fluid nodes

Gap is the Interface gap. The recommended value is 1.5 times fluid element size along the normal direction to contact.

Where:

 

For this example:

Simulation Iterations and Modeling

Fill Container with /INIVOL.

With /INIVOL, the water line can be defined in this part.

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

/INIVOL/part_ID/inivol_ID

inivol_title

surf_ID

ALE_PHASE

FILL_OPT

ICUMU

FILL_RATIO

 

 

 

 

surf_ID

ALE_PHASE

FILL_OPT

ICUMU

FILL_RATIO

 

 

 

 

etc

etc

etc

etc

etc

 

 

 

 

surf_IDn

ALE_PHASE

FILL_OPT

ICUMU

FILL_RATIO

 

 

 

 

/INIVOL uses successive filling actions of the initial background multi-material ALE mesh, to get the final configuration of the initial volume fractions (three containers and three ALE phases). Initially the volume is filled by the first material defined in the /MAT/LAW51 field. In this case, the first material is air, so the entire hex mesh is first filled with air. Next, a surface is defined from the container part ID.

/SURF/PART/998 Vessel_Surf_Part 85

Since the surface normal of container part point outside, use FILL_OPT = 1 to fill the water (phase 3) inside the container (filling the side which against surface normal direction).

/INIVOL/86/10003507 INIVOL # Surf_ID ALE_PHASE FILL_OPT ICUMU FILL_RATIO 998 3 1 0 1.0

Now, ALE mesh is filled with ALE material 1 (air) from /MAT/LAW51 on the outside of the container and material 3 (water) inside the container. Lastly, define a surface plane, /SURF/PLANE to define the fill height. The normal of this plane points upward, use FILL_OPT = 0 to fill the air (phase 2) above the plane (filling the side along normal direction).

# Surf_ID ALE_PHASE FILL_OPT ICUMU FILL_RATIO 9999 2 0 0 1.0

 

To check the initial fill, the following animation options can be used in the Engine file.

  • /ANIM/ELEM/DENSITY

  • /ANIM/ELEM/VFRAC

You can contour the model and use section cut to see inside, or use iso-surface, as shown in Figure 4.

/ALE/MUSCL - Anti-diffusive Technique

/ALE/MUSCL allowing for a better localization of the interface between fluids, and much less numerical diffusion. In this example, use default for Beta.

/ALE/GRID/DONEA - ALE Grid Velocity

This activates the J. Donea Grid Formulation, where the velocity of a given grid node depends on velocity and displacements of neighboring grid nodes.

Engine Control

It is recommended to use time step scale factor 0.5 for ALE in /DT/BRICK in order to keep computation stable, and use fac=1.0 in /UPWM/SUPG. This option provides better velocity field in Cartesian grids when ALE material velocity is not normal to brick faces.

Results

To see the movement of the water in the container, and iso-surface plot of results type "density” can be done. If the simple averaging method is used in HyperView, the results will look smoother.

Also notice that water is starting to splash up the sides of the container at the end of the simulation.