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Thermal-structural FEM Simulations in an Aero-Structures

Design CourseAlan Zehnder

Course taught in collaboration with Tony Ingraffea

Cornell University and

Barry DavidsonSyracuse University

Presented at 2007 Swanson Engineering Simulation Program Advisory Committee Meeting, October 19, 2007

Supported by NASA Langley Research Center through Cooperative Agreement No. NCC-1-01004, the AT&T Foundation, the State of New York, Syracuse and Cornell Universities.

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Course and Project Overview

• From 2001 to 2006 Cornell and Syracuse offered “Collaborative Engineering Design” (CED) for seniors.

• Student teams (3 CU & 3 SU) perform preliminary thermal-structural design of “panels” for manned re-entry vehicle

• Emphasis on– Aerospace structural analysis and design– Thermal analysis and design– Team work– Presentation skills– How to work in a “virtual team”

• Project was designed as a pathfinder for NASA – a geographically dispersed organization

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IT Infrastructure

• Distance Learning Classrooms (DLC)– Teleconferencing system (Polycom) for video/audio– Presentations shared using Netmeeting– Multi/screen, camera, projector, microphones– Smartboard Sympodium

• Design Studios• Wireless networking and Tablet PC’s• Synchronous web-based collaboration

environment (IBM Lotus SameTime) • Asynchronous web-based collaboration

environment (IBM Lotus QuickPlace)

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Asynchronous Web-Based Collaboration Environment

• Based on IBM Lotus QuickPlace– Spaces for providing

• course assignments, reading, announcements• team space for document sharing, discussion,

calendar, tasks

– Individual and team drop boxes for posting assignments and evaluations

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Synchronous Web-Based Collaboration Environment: Used for Design Team Meetings and for Instruction

Example of Two Person

Collaboration; Shared

PowerPoint

– Based on IBM Lotus SameTime

– Multi-person communications with audio and video

– Shared applications and whiteboard

– Chat

– Polling

– Hand-raising

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Design Assignment for Fall 2006

• Preliminary thermal protection system (TPS) and structural system design for Crew Exploration Vehicle (CEV)

• 10 loading cases• Re-entry and launch

heating• Structural material (i.e. Al-

Li, Ti-6-4,…) assigned• Structural and TPS

configuration constrained to a given conceptual design

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Expectations

• Virtual teams collaborate using IP-based tools.• Each member contributes substantially • Deliver and present summary report containing:

– Design drawings– Show correspondence of closed-form and FEM

methods of analysis– Assess manufacturability, cost, risk and weight– Design must withstand given mechanical and thermal

loads within specified safety factors. • Material strength, structural stability, maximum allowable

material temperatures. Assess micrometeorite damage resistance.

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FEM in Course Preparation

• Tutorials on shell analysis in Ansys developed with Raj Bhaskaran. Emphasize parametric modeling and validation of FEM against analytical model.

• Extensive test runs performed and checked against analytical methods both to check FEM results and to select analytical approximations that we can then teach to the students. For example:– When must shear deformation be considered?– How should orthotropic stiffnesses be computed for a built-up structure?

• Supplementary notes developed to cover tricky points such as importing thermal loads

• Prototypes of all analyses performed and used to develop lecture notes– Stress and deflection of built up plate under pressure, in-plane and

thermal loads– Global and local buckling– Transient heat conduction

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Shell Analysis Tutorial

Create areas, then mesh, then copy mesh and areas to create complete model. Tutorial shows how to parametrize the model so that the number and dimensions of stiffeners can easily be changed

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FEM Instruction in Course - I

• ½ of students studied FEM, other ½ analytical methods• Two Lectures on FEM basics using truss and plane

stress analyses as examples– Sources of approximation?– What is a shape function? – Principle of virtual displacements – Assembling the equations– Solution and compare to analytical result– How to assess if solution is accurate– Use of consistent units

• Students asked to work truss and plane-stress tutorial problems.

• In plane stress problem consider convergence of solution as mesh refined and higher order elements used.

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FEM Instruction in Course - II

• Three FEM application lectures– Shell problems, buckling, introduction to parametric

modeling, use of symmetry– Thermal analysis in ANSYS– Building up shell model parametrically

• Each lecture ends with an assignment to build and solve model problems that are similar to those that teams will need to solve for design.

• How to apply given mechanical and thermal loads and how to compare simple models to analytical results and how to interpret FEM results are emphasized.

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Example Problem from Lectures 1D Transient Thermal Analysis

x

y 0y

T

0x

T 100,,0 tyT

h

500,, yxTWhat makes this a 1D problem?

h=30 mm

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Steps in the setup and solution of ANSYS Model

• Preferences – Thermal Analysis• Plane77, 2D thermal solid element, no real constants• Specify thermal properties (stick with SI units) If you use length in mm you need to

convert the thermal properties to base units in mm. – Density: 1000 kg/m3=1e-6 kg/mm3

– Specific heat: 1000 J/kg K = 1e+9 mm2/s2 K– Conductivity: 1 W/m k = 1000 kg mm/s

• Specify transient analysis– Give max. temperature, max. time step (see load step options and analysis options) Can select

program chosen for most options. Generally gives good results. – Save results at each substep (see solution controls)

• Specify initial and boundary conditions– Zero heat flux boundaries– Specified temperature boundary– Temperature at all nodes set to initial temperature

• Solve LS and then view results– Use results viewer to easily scroll through different time steps– Time history viewer will let you plot or export data for the time variation of a solution variable

(nodal temperature in this case) at a chosen node. Pick node using GUI. • Setup, run and post-process model during class time.

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Comparison of FEM and analytical solutions at t=1200 sec.

ANSYS solution

Analytical solution

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Student Use of FEM in Design

• Students ran Ansys using computers in Hollister Hall “design studio” or on their Tablet PC’s.

• Teams generally split up the work so that one member worked on thermal analysis, two on structural analysis, often considering two concepts.

• Teams were successful in the basic thermal analysis for design and in stress and deflection analysis under mechanical loadings.

• Most groups were not able to accurately perform the more difficult analysis, such as buckling and stress due to thermal gradients.

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Sample Student Work – Design Concept

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Sample Student Work – Parametric Study of Temperature Rise During Re-entry

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Sample Student Work – Deflection of Pressure Loaded Stiffened Skin Structure

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Sample student work – global buckling of compression loaded stiffened sheet

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Summary and Concerns - I

• Ansys FEM introduced. – Covered transient thermal analysis, shell structures,

parametric modeling, buckling and stress analysis

• Students used FEM and closed form analysis methods to complete preliminary design

• Student reviews very positive!

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Summary and Concerns - II• Tutorials are a big help• Students had trouble on problems for which we don’t

have tutorials– Difficult to get students up to speed with FEM in very short time.

• Following lecture via SameTime meeting problematic – refresh rate of screen is slow. Must work slowly in order for students to see every menu that is opened up in ANSYS GUI.

• Lectures were recorded, which helped with above, but time consuming to scroll through recorded lecture and find exactly what you are looking for.

• Similar courses or projects may want to consider alternate modes of instruction.

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• BACK UP SLIDES FOLLOW

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Analytical Solution

smc

ka

h

ne

ntxT

n

htan

/1075.1

2sin

200100,

27

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4/ 222

.sec5100

/1075.1

03.

caseour In

is re temperatuuniformreach

todimension ofobject for , time,sticCharacteri

:conductionin scalinglength and Time

27

2

2

sm

m

a

h

h

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Asynchronous AIDE Screen Shot

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Course Evaluation - I

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Course Evaluation - II

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Samples of Course Preparation Analyses

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MN

MX

X

Y

Z

thermal-example-2

312.575346.46

380.345414.231

448.116482.001

515.887549.772

583.657617.543

JUN 27 200614:01:06

NODAL SOLUTION

STEP=1SUB =17TIME=929.931TEMP (AVG)RSYS=0SMN =312.575SMX =617.543

•Stress and deflection•Global and local buckling•Transient heat conduction