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ANSYS CFD Tutorial: Immersed Solids

ANSYS CFD tutorial: The immersed solids method is an additional FSI option in ANSYS CFX that allows simulation of unlimited motion of solid objects through fluid regions, as it avoids any mesh deformation or re-meshing. This article may help you get started!

Introduction

The Immersed Solid capability stems from a simple idea involving simulations where rigid solid objects moves inside the fluid domain. The immersed solid method can be used in cases where the geometry is too hard to mesh accurately, allow the fluid mesh to overlap the volume the solid occupies. In other words, mesh the space as though the solid body were not there (see figure 1 below). The solver then searches for nodes that are “inside” the solid body and assigns to these nodes the velocity of the solid body, thereby including its effect on the fluid.

This tutorial shows how to set up a simple 2D case involving rotating gear.

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ANSYS CFD Tutorial: Immersed Solid Method

Create the fluid domain which will be affected by the solid, and create the immersed solid part in the same coordinate system. Be sure that the control volumes in the fluid part are smaller than the elements of the immersed solid parts. Fluid and solid domain can overlap.

Figure 1. Fluid domain. The hole in the middle will always be covered by the solid part and it’s therefore not necessary to include the area in the calculation.

Figure 2. Rotating part.

After you have loaded the mesh files you define your fluid domain and solid domain separately.

Figure 3. This case has three domains in total. The solid domains are contra rotating.

Define the solid domain as immersed solid and define its motion. The fluid nodes covered by the immersed solid will be forced to have the same velocity as the moving part, V = Vsolid.

Figure 4. The domain tab.

When to use the Immersed Solid approach

- When mesh deformation is not practical.

- When solid motion can be specified or calculated by the 6 DOF solver. In cases where the motion is decided by gravity and center of mass, Immersed Solid motion can be determined by 6-DOF Rigid Body Solver.

- Include Blockage in internal and external flows without meshing:

  • Electronics enclosure
  • Automotive underhood
  • Engine nacelle
  • Debris in ducts
  • Occupants, furniture or other obstructions in a room.
  • Tree placement near a building
  • Body passing across the path of a car or aircraft

- Positive displacement pumps and blowers

  • Gear pump
  • Roots blower
  • Vane pump
  • Ge-rotor pump

Valve shutting and opening.

Limitations

This model cannot be used as a two-way-coupled analysis. The immersed solid will change the fluid behavior, but not opposite, which means that forces as drag and lift on the immersed solid will not be predicted accurately.

Mesh usually doesn’t resolve the boundary layer and viscous forces on rigid body will not be well resolved, especially would turbulent flows be hard to predict since wall functions cannot be applied to the boundary of the solid body. The immersed solid approach is applicable when forces on immersed body are pressure-dominated.

General multiphase simulations do not interact with immersed solids in the current release. The immersed solid model does not interact correctly with variable density flows for transient runs, i.e. transient runs should be incompressible and single phase.

Cases where Immersed solid method should not be used:

  • Accurate prediction of drag around bodies: Requires accurate boundary layer prediction
  • Vessels or objects floating in water: Variable density not correctly handled (use Rigid Body solver and sliding mesh instead).
  • Screw and scroll compressors: Compressible flow
  • Piston compressors (use mesh morphing instead): Compressible flow, but can use immersed solid to allow valve to close
  • Particle tracking: Particles will not interact with the wall of an immersed solid.
  • Physics as combustion, radiation, heat transfer, additional variables, and CHT cannot be modelled.

Want to learn more?

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