Session 5 Van Eekelen

8/20/2019 Session 5 Van Eekelen

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2 7 th European Workshop on TPS and Hot Structures – 8-10 April, 2013

Contents

Introduction

Test-case series overview

Code types

Material definition

Summary of previous test-cases

Test-case series #3

Next steps


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Motivation: Why did we start this? -> pure curiosityHow do codes compare? – if same model.How do models compare? – if different physics implemented.

Goalpropose problems of increasing complexity until it is agreed that the most-elaboratedwell-defined problem is formulated

Method to design a test case1. census on problems of interest2. census on code capabilities3. draft a proposition of test case (necessarily a compromise)4. iterate with the community until the test-case definition is clear and complete

We try our best to propose SOFT test-casesSimple, Open, Focused, Trouble-free.

Introduction


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TACOT: Theoretical Ablative Composite for Open Testing created from literature data. It is alow-density carbon/phenolic.

1 st test-case (2011) : 15 participants / 25 codes in the open literatureMostly a simple heat transfer problem chosen for it’s simplicity

2nd test-case series (2012) – progress: convective boundary condition & recession

2.1 - bridge between 1 st and 2.2 (non-physical but useful for code developers)2.2 - 1D state-of-the art design level – low heat-flux2.3 - 1D state-of-the art design level – high heat-flux2.4 - Comparison of methods to compute recession rates (e.g. B’ tables)

3 rd test-case series (2013) – progress: 2D & 3D,3.0: high heat-flux, isotropic material, no recession (non-physical but useful for codedevelopers)3.1: high heat-flux, isotropic material, with recession3.2: high heat-flux, orthotropic-material, axis-symmetric model3.3: high heat-flux, orthotropic-material, full 3D model

Test-case overview


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Three types of material-response codes have been identified in the community

Type 1: CMA-type codes (heat transfer, pyrolysis decomposition, simplified transport of

the pyrolysis gases, equilibrium chemistry);Type 2: CMA model augmented with an averaged momentum equation for the transportof the pyrolysis gases;Type 3: Higher fidelity codes (possibly including finite-rate chemistry, multi-componentdiffusion, in-depth ablation/cocking, conduction-radiation coupling, etc).

Different test cases with two objectives has been defined

Inter-calibration of codes of the same type (focus: numerical methods and datainterpretation)

Comparison of codes of different types (focus: modeling approach).

Code types


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Elemental compositionReinforcement: ex-cellulose carbon fibers, heat treated at 2000 K, density 1600 kg/m 3,

length: 1mm, diameter: 10 microns.

Matrix: ex-novolac/formaldehyde polymer, virgin density 1200 kg/m 3

ArchitectureRandom fiber distribution and orientationFiber volume fraction: 10 %

Fiber-coating matrixMatrix volume fraction: 10 %Initial porosity: 80 %

Properties (given)Inspired from open literature data - when available for similar materials

• conductivity, heat capacity, pyrolysis gases (composition, decomposition, finite-rate chemistry)

Derived/computed - when not found in the literature• formation enthalpy of the solid, thermodynamic properties of the pyrolysis gases at

equilibrium, viscosity, permeability, tortuosity, B’ table for air.

Material definition

3D numerical construction of the architecture of TACOT


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Material properties are given

B’-table material TACOT (char is pure graphite); p= 1 atm; T=300-4000K; under air, with thefollowing constraints:

Air (in mol fractions): O 2=0.21, N 2=0.79Pyrolysis gas (in mol fractions): C=0.206 / H=0.679 / O=0.115Equal diffusion coefficients, frozen chemistry in the boundary layer, no erosion or failure,CEA database, equilibrium chemistry.Mixture (25 species): C; H; O; N; CH 4; CN; CO; CO 2; C 2; C 2H; C 2H2,acetylene; C 3; C 4;C 4H2,butadiyne; C 5; HCN; H 2; H 2O; N 2; CH 2OH; CNN; CNC; CNCOCN; C 6H6; HNC.

Material definition

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0 500 1000 1500 2000 2500 3000

C o n

d u c t

i v i t y

[ W / m

. K ]

Temperature [K]

virgin Char


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Objective : comparison of the “in-depth physics and chemistry”Simple : 1D, fixed surface temperature, no recession.FIAT baseline provided

Test-case series #1

time

1644 K

1 minute

Tsurface (K)

0.1 sBottom B.C.

Adiabatic, impermeable

Top B.C. Tsurface = f(time)p surface = 1 atm

h = 5cm

Initial conditions: T=298 K, p= 1 atm, initial gas composition left open (air, Ar, pyrolysis gas, …)


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Objectivesreach the state-of-the art design levelkeep as much as possible from test-case 1 – to optimize time investment.

NEW in 2 nd seriesconvective boundary condition (instead of fixed surface temperature)surface recession

Structure of 2 nd series : 4 cases2.1: low heating, no recession (non-physical intermediate test case found useful by code

developers)2.2: low heating, recession – should be in the finite-rate chemistry regime for model

comparison2.3: high heating, recession – should be in the equilibrium chemistry regime2.4: computation of the ablation rate of TACOT for a temperature range of 300K-4000K

and an air pressure of 101325 Pa (1 atm).

Test-case series #2


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References describing the convective boundary condition as implemented in CMA (and still usedin most of the design codes) are made available. This does not mean that the CMA model mustbe used.

Test-case series #2

Bottom B.C. Adiabatic, impermeable

Convective B.C.

h = 50 mm

h e= f(t)

rho eu eCH

time

60 s.h e= f(t)

0.1 s

Initial conditions: T=300 K, p= 101325 Pa (1 atm), air.

120 s.60.1 s


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Definition of the “iso-q” test specimenExample from the literature

• Geometry definition• Load and boundary conditions

Initial results presented at 5 th Ablation Workshop in Kentucky (2012)Problems identified

• Coupling between boundary conditions and ablation

Non-physical peak heat load at shoulder • Available pressure distribution not accurately known

Structure of 3 rd series : 4 cases3.0: high heating, isotropic material, no recession (non-physical intermediate test case

found useful by code developers)3.1: high heating, iso-tropic material, recession3.2: high heating, orthotropic-material oriented along the axis of axis-symmetry - axis-

symmetric model3.3: high heating, orthotropic-material oriented under and angle with the axis of axis-

symmetry – full 3D model

Test-case series #3


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Changing the test-case series #3Loads and boundary conditions

• Initial uniform temperature

• Initial uniform pressure• Adiabatic/impermeable bottom surface• Initial gas state (type 3 codes)

Re-radiation (uniform at outer surface)

Enthalpy type load (stagnation point)

Reduced the mass flow – avoid localizedmesh deformation

Test-case series #3

time

60 s.h e= f(t)

0.1 s

Initial conditions: T=300 K, p= 101325 Pa (1 atm), air.

120 s.60.1 s

44

wT T q

wggwccheewehee hh Bhh BC uhhC uq ''

1

2'0

0

2

'

0

Bh

he

BC C

5.0


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Changing the test-case series #3Geometry:

• Elliptic arc geometry

• Less pronounced heat load peak on shoulder

Loads and boundary conditions – non-equilibrium aero-thermodynamic hypersonic CFD

code (super catalytic wall at 255 K).• Heat flux distribution – scaling of stagnation point heat flux q w(0)

• Pressure distribution – scaling of stagnation point pressure p w(0) = 0.1 atm.

Test-case series #3

0

0

w

wheehee q

qC usC u


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Results need to be generated for all four test-cases:Temperature and density curves

• At stagnation point

• At all 10 thermo-couple positions

Isotropic material:3.0: no recession3.1: with recession

Orthotropic material3.2: Axis-symmetric model3.3: Full 3D model

Test-case series #3

isotropic IP

TTT

20

01

0

0


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Preliminary results are generated with SAMCEF AmaryllisResults are for test-case 3.3

A constant pressure (p w = 0.1 atm.) along the outer surface is used

Results at time t = 40 seconds:Temperature distributionPressure distributionDensity distribution

Test-case results


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Temperature evolution (Test 3.3)

Density evolution (Test 3.3)

Test-case results


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Why constant pressure around the outer surface?Pressure distribution will cause gas mass flow insidematerial.

Gas out-flow will take place at the shoulder

SAMCEF is a type 2 code:• No initial gas (Air) inside structure• But pyrolysis gas with enthalpy h g• Applied heat flux is:

Non-physical cool-down due to:• Initial gas hypothesis (solution type 3 code)• Equilibrium hypothesis

Test-case series #3

wgghee hh BC uq

'...

-1.00E+07

-5.00E+06

0.00E+00

5.00E+06

1.00E+07

1.50E+07

2.00E+07

2.50E+07

3.00E+07

3.50E+07

4.00E+07

0 500 1000 1500 2000 2500 3000 3500 4000

Temperature [K]

E n t

h a

l p y

[ J / k

g ]

Wall enthalpy Gas enthalpy


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Next steps

Pressure at the outer surface has changed

We must therefore update the following material data:

• Non-dimensional ablation speed Bc’

• Wall enthalpy table

Re-run the test-cases

Distribute the test-case definition

Session 5 Van Eekelen

Documents

Transcript of Session 5 Van Eekelen