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2008 International

ANSYS Conference

Strongly Coupled FSI Simulation Moving Compressible Pressure Pulse through a Tube

Daniel L. Cler,

US Army RDECOM/ ARDEC/

WSEC/ Benet Labs

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Overview

• Background

• Problem Description

• Objectives and Assumptions

• Workflow

• Results

• Future Work

• Conclusion

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A Collage of Real Life Multiphysics

Applications

• Examples of multiphysics

are plentiful

• Realistic FSI analysis is

needed to address design

and performance issues.

– Wing flutter, engine noise,

VIV, oil exploration, offshore

structures, air crafts and

components, defense

equipment, pumps, valves,

arteries, bones, ...

Courtesy of Pluere

Total deformation contours

on a pump impeller in a

coupled structural and flow

analysis

Coupled FSI of bio-med valve

Typical torsional

blade modes:

impact of gas on

swept surfaces

ANSYS Adv.v1,n3,2007,

FSI of distributor header

Courtesy: CADFEM

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Background

Multiphysics Solutions: State-of-the-Art

• Much has been achieved

– In-depth single discipline solutions

– Ability to make these solutions communicate

– Efforts to facilitate communication at data level• MPCCI, dedicated solvers, and communicators

• Multi-disciplinary applications need real world collaboration of discipline specialists

• Analysis tools need framework to share multiple data fields that represent the true physics

• Technology maturity is providing new opportunities• Need for improvement stays ahead of the progresses made

• Hardware and software capabilities entice practitioners with increased demand of complex real life analysis towards Simulation Driven Product Development

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Current RealityIntegrated Process in Workbench

Geometry Model

CHT Solid Mesh

CFD with CHT

Thermal Stress Setup

Thermal Loads from CFX

Thermal Stress

Solution

Base level coupling

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A Sample Engineering Case

• Consider a problem with flow physics involving– High speed compressible flow in a tube

• From pavement / concrete digger to oil well drills

• From BB guns to more serious defense equipment

• From musical instruments to pneumatic control equipment...

– Very high cyclic pressure and thermal loads over long time

• Simple principle– Use potential energy of compressed fluid through systematic release

in kinetic form

– Focus on optimized delivery of an object or a force on a target

– The release segment of these cycles involve a reaction force

• Engineering challenge– Minimize the reaction impact without any degradation on the forward

motion, direction and force fields

• Important to analyze the response of the tube material– Accuracy of the target motion, direction and force fields

– Long term fatigue behavior of the equipment (tube)

Time

P / T

Sp

ikes

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Engineering Solution

• Consider a lightweight structure attached at the end of the tube to reduce the recoil action

– Loads on the new attachment device• Pressure pulse from inlet end of the tube

• Thermal loads from the source of the pressure pulse

• Design objective

– Maximize braking by smart design to minimize forces and moments (torques) on the tube• Smartness defined by minimum space and

material use without loss of strength or life

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Engineering Solution

• The case studied here is a muzzle block

– The attachment geometry is for exemplification only and is not for any real equipment or any design

– The total engineering of such system require analysis of multiple fields of physics • Propulsion system, material science, aero-acoustics,

turbulence, fatigue life estimation, stress-concentration and micro-cracks, solidification, heat treatment, machining, surface finish, etc.

• This study focuses only on analysis of coupled FSI

problem to demonstrate maturity of the analysis tool

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Objectives

• Explore the state of the art in simulation for two-way fluid structure

interaction to predict the pressure, thermal loads, on the fluid side and

deformations of the structures

– Multiple loading cycles in a single analysis

– Objective is NOT to do all the detailed simulation of 1 cycle

• Requirements:

– Two way coupled, unsteady, FEA & CFD analysis

– Robust, easy to use, flexible

– Automated with minimal user intervention

– Optimization tools

• Develop the initial “Proof-of-Concept” !

– Feasibility study

– Focus on correctness of physics by establishing proper coupling

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Assumptions

• Fluid simulation

– Blow down simulation

• No moving solid-load in the tube

• Typical chamber pressure is compensated

– Half geometry, vertical symmetry

– Fluid material properties

• NASATM 4647 ; NASA/TP-2002-211556

• FEA simulation

– Flexible multi body dynamics

– Tube inlet is fixed in space

– Solid material properties

• Alloy Steel

• http://www.efunda.com/materials/alloys/alloy_home/steels_properties.cfm

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Workflow and Data Transfer

• Pre-processing– ANSYS Simulation: Solid mesh, mechanical simulation setup

– ICEM CFD: Fluid mesh

– ANSYS CFX-Pre: CFD, FSI simulation setup

• Solver and Execution– ANSYS Multiphysics: A single integrated, fully coupled environment

• FEA Solver: ANSYS Simulation

• CFD solver: CFX-Solver

• Data Transfer– CFD to FEA: Wall heat flux and total force

– FEA to CFD: Wall temperature and displacement

– Data transfer between ANSYS and CFX solvers is fully automated

• Post-processing– ANSYS CFX Post

• Solid and Fluid field variables can be post processed together

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Fluid Domain and Mesh

• Mesh Generated using ICEM CFD

– Blocking concept, hexahedral mesh

• Initial “proof of concept” mesh

• Number of Nodes – 49322

Outer domain: External flow

Chamber:High pressure, high temperature

Barrel

Ground

Muzzle block

FLUID Domain

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Solid Domain and Mesh

• Mesh Generated using ANSYS Simulation

– Easy to use, highly automated and fast!

– Hex mesh in the barrel

– Tet mesh in the muzzle block

Pulse Source

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Workflow SnapshotsMechanical Simulation Setup

Boundary Conditions : Solid

• Material: Steel Alloy

• Analysis Type : Flexible Dynamics

• Coupled Field Element • Solve for thermal and structural stresses

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Workflow SnapshotsCFD Simulation Setup

• Material properties– Density: Ideal Gas mixture

– Temperature dependent properties

• NASATM 4647;

• NASA/TP-2002-211556

• Physical Models– SST K- turbulence model

– Natural convection

• Initial condition– 1atm and 300K, zero velocity

• Source term approach for initial high P and T– Mass and energy sources

corresponding to 820 atm and 1120 K at pulse source

– Applied at the first time step of each cycle

• Adaptive Time stepping– Time step size ramp up from

7e-6 s (min) to 0.25 s (max)

• B.C. types• Green : Opening

• Red : FSI interface

• Cyan : Ground

• Open face: Symmetry

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Workflow SnapshotsFSI simulation set-up in ANSYS CFX Pre

FSI Simulation Setup

External coupling

Interface load transfer

CFD solver controls

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CFX Solver input ANSYS Solver input

Workflow SnapshotsFSI simulation start-up using ANSYS CFX-Solver

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MFX Multi-Field Solver

• Supports Simulation between Multiple Executables

– FEA CFD

• Third Party Coupling Scheme Not Required

– Uses Client/Server architecture

– TCP/IP

– Heterogeneous Networks (LAN/WAN/Internet)

• Supports Large Models

– Solution on Two Machines

– CFX Solution can use Parallel Processing

• Supports Nonconformal Meshes

• Transfers Surface Loads

• Automatically Morphs CFD Mesh

ANSYS Multi-Field Solver

Sequential load transfer coupling

MFS

Coupling within a

single executable

MFX

Coupling between

multiple executables

ANSYS/CFXFSI

Other combinations in

future releases!

Coupling of structural,

thermal, electric and

electromagnetic fields

in ANSYS Multiphysics.

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ResultsSolvers output – CFX & ANSYS

Cooling period

b/w the pressure

pulses ~10sec

CFX Output

ANSYS Output

Total force (N) Fx, Fy, Fz

on FSI interface

Maximum mesh

displacement (m) in the

Fluid Domain

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Workflow Snapshots Post Processing

• Coupled simulation post-processing in ANSYS CFX Post

– Common Graphical User Interface

– Can analyze intermediate time step data

– Easy to create/save animations

Geometry definition

Post surfacesAnimation controls

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Results – CFD

Mach no. @ Symmetry Plane Temperature @ Symmetry Plane

16 - Cycles

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Results – CFD

Temperature iso-surface, 500K Pressure iso-surface, 0.1 atm(g)

5 - cycles

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Results – Mechanical

Temperature on FSI interface Temperature on the Muzzle block

7- cycles

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Results – Mechanical

Structural deformation x25000

5 - cycles

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Future Work

• Appropriately finer mesh

• Inclusion of additional physics– Radiation model

– Real gas effects

– Including moving solid „loads‟

• Moving solid considered „rigid‟

– Approximated by a moving interior plane in “layering”

• Fully coupled 1-DOF solid‟s motion

– Significant time saving

• Source terms to model energy impulse of the cartridge detonation

• Improve run time performance– Effects of tightness of the convergence criteria on solution accuracy

• Ensure parallel performance

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Conclusion

• Two way coupled FEA & CFD

– Demonstrate seamless two way fluid/thermal and structural coupling for high speed compressible flow simulation

• Easy to Use

– Single, intuitive environment for the entire FSI simulation setup

• Robust

– Minimal user intervention for the FSI run

– Robust FEA and CFD solvers, even with larger time step size

• Automation with minimal user intervention

– Full automation through scripts possible

• Flexible– Ability to add advanced models to include more physics

• Optimization– Design Explorer in Workbench

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Questions and Answers

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Multi-field Solver Field Interface

Loads Transferred Across Field Interface

Physics Fields SEND RECEIVE

Structural Displacement Force, Temperature

ThermalTemperature, Heat

Generation, Heat Flux

Displacement, Heat

Generation

Electric Field Force, Heat Generation Displacement, Temperature

Magnetic Force, Heat Generation Displacement, Temperature

FluidSurface Force, Surface

Temperature

Displacement, Surface

Temperature

HF Electromagnetic Heat Generation Temperature

Please refer to ANSYS coupled field analysis guide for thorough details.

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MFX Multi-field Solver

ANSYS Master CFX Slave

Time Loop

End Time

Loop

Stagger Loop

End Stagger

Loop

Time Loop

End Time

Loop

Stagger Controls (ANSYS to CFX)

Load Transfers

Stagger Controls (Bidirectional)

Stagger Loop

End Stagger

Loop

CFX

SolverANSYS

Solver

Time Controls

Time Controls

Do Mapping