CFX-FSI 14.5 Lect-09 Immersed Solids

8/13/2019 CFX-FSI 14.5 Lect-09 Immersed Solids

1/26

2011 ANSYS, Inc. July 26, 20131 Release 14.5

14. 5 Release

Solving FSI Applications Using

ANSYS Mechanical and ANSYS CFX

Lecture 9

Immersed Solids in CFX


2/26


Overview

A way to represent a moving solid

without deforming the mesh

Fluid mesh is created that covers

both the fluid and solid regions

Overlapping solid mesh is created

that represent the initial position ofthe solid

The solver locates the fluid nodes

that overlap with the solid mesh

Fluid velocity at overlapping nodes

set equal to solid body velocity,

capturing the influence of the solid Immersed Solid Domain

Mesh to represent theimmersed solid

Fluid domain

Cartesian mesh shown in the example, but

can be any mesh in general

Fluid nodes covered by immersed solid(IMS) are forced to satisfy V = Vsolid


3/26


4/26


Overview

The solid body mesh is only used to find overlapping fluidnodes

No solution on the solid mesh

No direct influence of the solid on the fluid

Solid boundary mesh should accurately represent solidbody, but solid mesh quality doesnt matter

Principle advantage no need for remeshing or mesh

morphing

Arbitrary motion easily handled

Robust and simple to set up


5/26


Limitations

Viscous forces on rigid body are not well resolved, especially forturbulent flows

Mesh usually doesnt resolve the boundary layer

No Wall Functions

Immersed solid approach is applicable when forces on immersed body are

pressure-dominated

Does not work with variable density flows for transient runs

Transient runs should be incompressible, single phase, no cavitation

But variable density can occur away from the immersed solid

Physics Limitations

No interaction with particles

No combustion, radiation, heat transfer, additional variables, CHT

No automatic replication of immersed solids across periodic boundaries


6/26


When can it be used?

Include Blockage Without Meshing

Internal Flows

Electronics enclosure

Automotive Underhood

Engine Nacelle

HVAC

Debris in ducts

Occupants, furniture or other

obstructions in a room

External flows

Tree placement near a building

Body passing across the path of a car

or aircraft

Permit Large Relative Motion of Bodies

Positive displacement pumps and

blowers

Gear pump

Roots blower

Vane pump Ge-rotor pump

Valve shutting and opening


7/26 2011 ANSYS, Inc. July 26, 20137 Release 14.5

When should it 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

Positive displacement compressors

Compressible flow

Screw and scroll compressors

Compressible flow

Piston compressors (use mesh morphing instead)

Compressible flow

But can use immersed solid to allow valve to close



Basic Solver Implementation

At the beginning of each time step, the solver:

Updates the position of the immersed solid

Cannot update IMS position each Coefficient Loop

Determines which fluid nodes lie inside the immersed solids

The velocity in the fluid region that overlaps the immersed solid

is enforced through a body force in the momentum equations:

S= -C(VVIMS

)

C come from the coefficients in the momentum equation

is the Momentum Source Scaling Factor

Pressure field inside the immersed body may look unphysical dueto the forcing momentum source terms

Unimportant; can be manually set using inside( ) function if desired



Immersed Solid Domains

The fluid and immersed solid domainshave independent overlapping meshes

No domain interface between immersed

solid domain and regular domains



Domain Basic Settings

Insert a domain as usual, useDomain Type = Immersed Solid

Local Coordinate Frame Local frame can be created to facilitate

the representation of the domainmotion

Domain Motion types Stationary

Speed and Direction

Specified Displacement

Rotating

General Motion

Rigid Body Solution


11/26


Domain Motion Types General Motion

General Motion

Chasles Theorem: The general displacement

of a rigid body is a linear motion of a originpoint plus a rotation around the origin point

Origin Motion

Describes the three translatory degrees of

freedom for the linear motion of the origin

point

Body Rotation

Describes the three rotating degrees offreedom for the angular motion of the rigid

body around the origin point


12/26


Domain Motion Types Rigid BodyMotion

Rigid Body Motion

Immersed Solid motion is determined

by 6-DOF Rigid Body Solver

Immersed Solid position is updated

when the Rigid Body solution isupdated

See Lecture 6 for details


13/26


Domain Solver Control

Fluid velocity wont be exactly equal

to the solid body velocity Will be closer for larger values of

Momentum Source Scaling Factor

Can set the Momentum Source Scaling

Factorfor each immersed soliddomain

Also set on the usual Solver Control panel

Numerical system can be stiff as forcing term becomes large

More likely for Solver to fail

For Momentum Source Scaling Factors > 10, may need to activate the

expert parameter: smooth inside ims = t


14/26


Domain Solver Control

A scaling factor of 0 turns off theinfluence of the immersed solid

Can be combined with step( ) functions toturn on and off immersed solid

Results observed for a gear pump:

Flow Rate Error vs

Experiment

Momentum Source Scaling Factor

55% 10

13% 50

9 % 100


15/26


In the global Solver Control settings aBoundary Modeloption is available:

None default treatment

Modified Forcing

The Immersed Solid boundary is modeled

using a modified forcing term based on a

constant shear assumption for laminar

flow and wall functions for turbulent flow

The modified force is applied at the fluid

nodes near the immersed solid boundary

these need to be located:

Boundary Face Extrusion: a virtual volume is formed by extruding the immersed solid

boundary faces by the specified distance, then expanding the volume by the tolerance.

Fluid nodes are projected accurately to the immersed solid faces.

Search Through Elements: no inputs required, but a less accurate association of the near-

immersed-boundary fluid nodes with the immersed solid elements

Solver Control Boundary Model


16/26


Initialization

There is no physics defined inimmersed solid domains, so

theres no variables to initialize

In the fluid domain can use theinside( ) function to initialize fluidvelocity to a different value at

nodes covered by the immersed

solid


17/26


Mesh Considerations

Need relatively fine background fluid mesh along

the swept path of the solid boundary

If the swept path is known, design mesh accordingly

Solid mesh need only be fine at surface in order to

resolve geometry

Roots Blower Geometry

Immersed Solid MeshFluid Mesh


18/26


Examples: Forward-Facing Step

Comparison of a forward-facing step using standardmesh-resolved approach and

immersed solid approach

No motion here, theimmersed solid is just used to

block-off the step region

Compare boundary layer and

flow pattern

Note recirculation region after

the step

Mesh Resolved Approach

Immersed Solid Approach


19/26



Recirculation in front of thestep is properly captured

Reattachment downstream

of the step is not captured(no Boundary Model usedhere)


Immersed Solid Approach


20/26


With the Boundary Model set toModified Forcing significant

improvements in the near-boundary

flow are observed

Better separation prediction over a

forward facing step shown

In other cases, particularly at

transitional Reynolds numbers,

results have not shown

improvements and can be less robust

when using the Boundary Model



Immersed Solid (default)

Immersed Solid with Boundary Model


21/26


Examples: Gear Pump Tutorial

Ge-rotor tutorial from CFX

documentation

Inner immersed solid rotates at 7 rev/s

Rotating fluid domain at 6 rev/s

Fine fluid mesh to resolve the flow

in the gap between the rotors


22/26


Examples: Gear Pump Tutorial

Flow rate shown for two different

scaling factors

Scaling factor of 100 shows a

larger amplitude of the mass flow

rate but a lower mean mass flowrate

Consistent with some leakage

occurring through the immersed

solid at the lower scaling factor

Momentum Source Scaling Factor = 10

Momentum Source Scaling Factor = 100


23/26


Examples: Roots Blower

Positive displacement blowerFine mesh to resolve theleakage path and the solid

boundary path

Accurate leakage prediction

unlikely with IMS approach


24/26


Examples: Free Surface Initialization

A novel application ofimmersed solids

Create IMS to represent

desired liquid region

Set Momentum Source

Scaling Factor = 0

Set liquid volume

fraction = inside()@IMS

Note that in general multiphase simulation do not interact

with immersed solids in the current release, but here we

are just initializing a variable in a multiphase simulation


25/26


In moving mesh cases that would normally result in the meshpinching off (e.g. valve closing/opening), and immersed solid canprovide a sink region for the mesh to move into

Can be combined with FSI, so that contact occurs on the structural side(assuming one side of the contact is rigid)

Examples: Avoid Mesh Pinching

Flexible flap: Initially the flap is in contact with a wall. As the upstream

pressure increases it deforms and allows flow to pass to the downstream side.

The black region is an immersed solid. Meshes are shown on the next slide.


26/26

2011 ANSYS Inc July 26 201326 Release 14 5

Examples: Avoid Mesh Pinching

Immersed solid region

Fluid domain mesh get pulled downand allows the flap to move away

from the wall

Date post:	04-Jun-2018
Category:	Documents
Upload:	shaheen-s-ratnani
View:	733 times
Download:	64 times

CFX-FSI 14.5 Lect-09 Immersed Solids

Documents