Fluid Structure Interaction Simulations with COMSOL

Daniel Ahlman, Ph.D.

COMSOL

2012 COMSOL. COMSOL and COMSOL Multiphysics are registered trademarks of COMSOL AB. Capture the Concept, COMSOL Desktop, and LiveLink are trademarks of COMSOL AB. Other product or brand names are trademarks or registered trademarks of their respective holders.

Nagi Elabbasi, Ph.D.

Veryst Engineering

Traditional approach to modeling

Chemical Reactions

Acoustics

Electromagnetic Fields

Heat Transfer

Structural Mechanics

Fluid Flow

The COMSOL Multiphysics approach

Chemical Reactions

Acoustics

Electromagnetic Fields

Heat Transfer

Structural Mechanics

User Defined Equations

Fluid Flow

The COMSOL Multiphysics approach

Chemical Reactions

Acoustics

Electromagnetic Fields

Heat Transfer

Structural Mechanics

User Defined Equations

Fluid Flow

FSI

Introduction to Fluid-Structure Interaction (FSI) simulations

POLL QUESTION 1

Which simulation tool do you currently use to model FSI?

COMSOL Multiphysics

Other commercial software

Non-commercial software (in-house code, freeware, open

source, )

None

Different levels of FSI

Small elastic deformation

Fluid flow Solid stress

Large rigid displacement

Rigid structure displacement Fluid flow

Large elastic deformation

Structural deformation Fluid flow

1. Solve for fluid flow.

2. Calculate total stresses.

3. Apply fluid stresses on solid boundaries.

4. Solve for the displacements in solids.

1

2

3

4

Fluid/solid coupling; Small elastic deformations

All steps automated in COMSOL! Fluid-Solid interface on shared mesh

boundary!

1. Solve for displacements of the rigid structure to compute a fluid flow mesh.

2. Apply the velocity of the solid boundaries on the fluid walls.

3. Solve for fluid flow

4. Repeat for next time step.

1

2

3

Fluid/solid coupling;

Large rigid rotations

All steps automated in COMSOL!

1. Solve for fluid flow.

2. Calculate total stresses.

3. Apply fluid stresses on solid boundaries.

4. Solve for the displacements in solids.

5. Use the solid displacements to;

Update the fluid flow mesh

Apply the solid boundary velocity to the fluid

6. Repeat until solution converges.

7. If Transient; Compute next time step

1

2

3

4

5

Large elastic deformations

All steps automated in COMSOL!

POLL QUESTION 2

Which type of solid deformation, or displacement, best defines your FSI application?

Small elastic deformations (Fluid Solid )

Large rigid displacements (Solid Fluid)

Large elastic deformations (Fluid Solid)

How to handle large elastic deformations?

Account for large deformation in solids

Solve for computational mesh for fluid flow

Large Deformation; Moving mesh

Reference mesh

Structural mechanics

Moving mesh

Fluid flow

Obstacle in a flow channel

Demo Exercise

Modeling overview

Inlet: u_fluid = f(t,y) Outlet: p = 0

Fluid domain

Solid domain

Lower boundary of solid is fixed. Top and bottom boundaries of fluid domains are walls, i.e. zero fluid

velocity.

Modeling in COMSOL

INSERT DEMO

FSI Analysis of a Peristaltic Pump

Veryst Engineering , LLC Engineering Through the Fundamentals

nelabbasi@veryst.com www.veryst.com

Nagi Elabbasi, Ph.D.

Peristaltic Pumps

Valuable for pumping abrasive fluids, corrosive fluids and delicate fluids

Rugged pump design requiring minimal maintenance

Used in pharmaceutical, petrochemical, biomedical and food processing industries

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Pump Modeling

Nonlinear, coupled fluid-structure interaction

Structural nonlinearities include:

Tube material behavior

Contact

Large deformations

Important to know tube stresses and strains for lifetime prediction

Optimal design depends on fluid properties and flow rate

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Rotary Peristaltic Pump Model

Two metallic rollers and an elastomeric tube pumping a viscous Newtonian fluid at a speed of 60 Hz

Model setup parametrically using native COMSOL geometry

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Tube Modeling

Hyperelastic tube

Tube material one of main limiting factors affecting pump performance

Typically made of elastomers like silicone or thermoplastic elastomers such as Norprene or Tygon

Used Mooney-Rivlin hyperelastic material model

Fixed along outermost edge for simplicity

Fixed boundary

Fixed boundary

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Fluid Modeling

Open fluid boundaries

Incompressible Newtonian fluid

Laminar Navier-Stokes equations

Peristaltic pumps commonly used for non-Newtonian fluids

Moving fluid-mesh follows solid deformation

FSI interface automatically handled by COMSOL

Fluid-structure interface:

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Contact Modeling

Contact Pair 1 Contact Pair 2

Source surfaces

Rollers are contact source, tube is contact destination due to higher relative stiffness of rollers

No friction, rollers not allowed to rotate

Contact Lagrange multipliers solved for as Lumped Step in segregated solver

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Step 1: Move Rollers in Place

Damped transient analysis to get rollers in correct starting configuration

Initial model configuration Initial pump configuration

Roller displacement

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Step 2: Rotate Rollers

Transient analysis with prescribed roller rotation Wait until transient dies out

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Solution Approaches for FSI Problem

Solve both solid and fluid fields simultaneously

Two-way coupled solution

More accurate (for some pump configurations)

More computational resources (memory and time)

Solve solid field followed by fluid field

Coupling only from solid to fluid

Less accurate

Less computational resources

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Mises Stresses and Fluid Velocity

Solid contours: Mises stress (MPa) Fluid arrows: Velocity field

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Flow Rate (and Fluctuations)

Significant flow fluctuations including flow reversal at point of roller separation from tube (vertical roller position)

In practice fluctuations reduced by roller design

For more details on peristaltic pump modeling visit www.veryst.com/peristalticpump.html

Veryst Engineering

Benefits of using COMSOL for FSI

Automatic calculation of total fluid forces.

Automatic identification of solid-fluid interface.

Automatic application of correct boundary conditions at the solid-fluid interface.

Ability to move computational mesh based on deformation of solid.

Accurate calculation of fluid flow stresses on moving mesh.

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