criteria for cfd applications in nuclear engineering - esss€¦ · criteria for cfd applications...

Criteria for CFD applications

in Nuclear Engineering

(Criterios para la aplicación de la CFD en la Ingeniería Nuclear)

Juan Carlos Ferreri

Scientific Advisor, ESSS Argentina

Emeritus Advisor, Autoridad Regulatoria Nuclear, Argentina

Foreword 2

This presentation is not a recipes book on how to

implement CFD solutions to thermal-hydraulic (TH)

problems in Nuclear Engineering.

It presents a hopefully thorough (anyway time limited)

discussion on criteria and sources of detailed information

on the subject and considers some trends regarding TH.

Contents 3

• Background

• V&V procedures, BPGs & Accuracy metrics

• Trends

• Conclusions

Background from my mind chest of remembered things…

4

Reference: Internal report DF-01-78, Depto. de Termodinámica, INTI, 1978

5

• Natural circulation in a uniformly heated container partially filled with a

solid and water (1982) – 1331 fluid cells – 3D, SOLA like algorithm

Background

6

Background

• Same example – revisited with FLUENT 6.3 (2009)

No more physics than before.

It´s just colored.

With thanks to Fabio Moretti (UPisa)

7

Annals of Nuclear Energy, 34, 339-395, 2007

Natural Circulation in a simple TH loop.

Background

Experiments show

that the flow is

unstable

8

Background

• Use of wall laws allows

coarse grids near the wall

or coarse grids force use

of wall laws?

• RNG k-ε turbulence

CODE: FLUENT 6.2

CELL No.: about 60000

CPU time to span 1000 s:

~ 3 days on a standard PC

Using RELAP5 or TRANLOOP

CPU time to span 1000 s:

some minutes on the same PC

9

Onset of unstable flow: captured due to stratification allowed by this somewhat detailed CFD approximation

Background

PS: recall, these are physical

oscillations

The paper avalanche… 10

0

500

1000

1500

2000

Previous 2012 2013 2014

Nu

mb

er o

f o

ccu

ren

ces

Keyword “CFD” & “Computational Fluid Dynamics” in title+keywords+abstract

G-Scholar

NED

ANE

Google search: CFD or “computational fluid dynamics” and nuclear 223000

Google search: CFD or “computational fluid dynamics” and PWR 35800

G-Scholar search: "computational fluid dynamics" and (fluent or cfx) 756

Some definitions on V&V 11

REFERENCE: “Assessing the Reliability of Complex Models: Mathematical and Statistical Foundations of Verification, Validation, and Uncertainty Quantification”, Committee on Mathematical Foundations of Verification, Validation, and Uncertainty Quantification, NAS, 2012

12

CONTEXT: V&V

“Validation: the process of determining the

degree to which a model {and its associated

data} is an accurate representation of the real

world from the perspective of the intended uses

of the model”

“No experimental data no validation”

“We can describe validation (legitimate, minimal

validation) as the comparison of model results

and their associated uncertainties with

experimental results and their associated

uncertainties”

Reference: Patrick J. Roache, “Fundamentals of Verification and Validation”, Hermosa Publishers, 2009

PS: a model is more

than a code…

ON THE PRESENT (~ 2013) STATUS OF CFD IN RELATION TO

NUCLEAR SAFETY

13

1. computational model (Synonym: computer model) Computer code that (approximately) solves the equations of the mathematical model.

2. mathematical model (Synonym: conceptual model) A model that uses mathematical language (sets of equations, inequalities, etc.) to describe the behavior of a system

3. Verification The process of determining whether a computer program (“code”) correctly solves the mathematical-model equations. This includes code verification (determining whether the code correctly implements the intended algorithms) and solution verification (determining the accuracy with which the algorithms solve the mathematical model equations for specified quantities of interest).

Some definitions on V&V

14

4. validation The process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model.

5. Extra[intra]polative prediction The use of a model to make statements about quantities of interest in settings (initial conditions, physical regimes, parameter values, etc.) that are outside [inside] the conditions for which the model validation effort occurred.

End of cite…

15

Validation Domain

ITFs + SETs Prediction Domain

Safe predictions, no code needed

(Scaling distortions assessed) Desirable condition for predictions

(Scaling distortions assessed)

Usual, undesirable condition for predictions

Uncertainty determination mandatory

Extra[intra]polative prediction


16

REFERENCE: W.L. Oberkampf and T.G. Trucano, Verification and Validation in CFD,

Progress in Aerospace Sciences, vol. 38, pp. 209-272, 2002,

A valid approach to CFD predictions

17


Cautionary comment to the previous slide:

It is somewhat difficult to accept that the jump

from comparison to sound extrapolate

predictions can be performed more precisely

than by induction, as one possible way of

inference,

e.g. →

INFER

1: to derive as a conclusion from facts or premises

INDUCE

3: to determine by induction; specifically : to infer from particulars

DEDUCE

1: to determine by deduction; specifically: to infer from a general

principle Source Merriam-Webster Dictionary online

18

• Evaluation of CFD methods for reactor safety analysis - ECORA - 12 partner institutions, started ~2005 (see M. Scheuerer el al., NED, vol. 235, pp. 359-368)

• Established to a) show to end-users, utilities and regulators

to which extent CFD can enhance the accuracy of safety analysis, b) that it implies “addressing the lack of certainty on CFD results and c) defining related Best Practice Guidelines (BPGs from hereon) to evaluate these results”

• Present efforts to define accuracy metrics for CFD in limited Nuclear Safety contexts

Some collaborative efforts to elucidate the needs on CFD in relation to Nuclear Safety evaluations


NUCLEAR SAFETY

19

Reference: M. Scheuerer el al., NED, vol. 235, pp. 359-368, 2005

Objectives reached, according to authors:

• The establishment of BPGs for ensuring high-quality results and for the formalized judgment of CFD calculations and experimental data.

• The assessment of the potential, and of current limitations of CFD methods for flows in the primary system and in LWR containments, with special emphasis on PTS.

• The definition of experimental requirements for the verification and validation of CFD software for flows in the primary system and in LWR containments.

• The identification of improvements and extensions to the current CFD packages that are necessary for primary loop and containment flow analysis.

• The implementation and validation of improved turbulence and two-phase flow models for the simulation of PTS phenomena in PWR primary systems.


NUCLEAR SAFETY

20

The BPGs are documented in Menter (2002). They contain detailed information on:

• The formalized judgment of results obtained with different CFD software packages. This includes the definition and quantification of round-off, iteration, and discretization errors, and the assessment of modelling errors.

• The consistent use of CFD methods for reactor safety problems. These guidelines relate to geometry and grid generation, boundary and initial condition specification, selection of suitable physical models, and handling of solution algorithms.

• The judgment of experiments regarding their use for verification and validation of CFD methods.

The guidelines include criteria for checking global mass, momentum, and energy balances, consistency checks for field data, and plausibility checks. Experiments are

grouped in a hierarchy ranging from laboratory studies to industrial field tests. The BPGs report is intended as a living document.

REFERENCE: M. Scheuerer el al., NED, 235, pp. 359-368, 2005


NUCLEAR SAFETY

21

In 2007 the BPGs have been updated and given a more general scope in the OECD document NEA/CSNI/R(2007)5, under the authorship of 17 specialists leaded by John Mahaffy (PSU) and an additional group of experts, whose opinions have been considered, either verbally or by e-mail.

The guidelines include (in 164 pages)

• A historical introduction, based on institutional and country contributions (most activities are reported starting in the 80’s)

• An account of all the aspects to be considered in setting up and applying criteria CFD methods to Nuclear Reactor Safety, including problem definitions, ranking of phenomena, gridding, verification and validation of codes. Opinions are stated in many cases and practical advises are given.

• Appropriate consideration to the fact that “computer simulation is much more than generating an input and observing results”

• A “check list for a calculation” that is included (curious about this? )


NUCLEAR SAFETY

BPGsCheckList.pdf

22

Comments on BPGs

• BPGs are a most valuable contribution for any CFD practitioner. • Some are incorporated as advise in code manuals and in the problem check options

in codes like FLUENT • Interested readers may find that many of the aspects considered in this presentation

are coherent with the OECD NEA/CSNI/R(2007)5 document. Careful consideration of this document is mandatory

• Evaluation of results validity beyond the known experimental data base range has always been one objective and an obvious difficulty of safety evaluations. Perhaps, whenever possible, designing SETs to interpolate should be a major goal to achieve

• I believe that written BPGs are not enough. Perhaps emphasizing on the need of a discipline of analysis in depth and to promote pursuing on well founded Engineering Judgment at the Academia should be done.

• BPGs may be a way to consolidate Engineering Judgment traditions… • It must be recalled that the interaction of complex codes may increase the

complexity of the analysis


NUCLEAR SAFETY

23

To be honest…

No one is free from formulating BPGs

in his/her life, it is just a matter of elapsed time

and tools…

1989 “On Expert System Assisted, Finite-Difference Schemes Selection in Computational Fluid Dynamics”, J.C. Ferreri and G.M. Grandi Proc. of Sixth Int. Conf. on Numerical Methods in Laminar & Turbulent Flows, C. Taylor, P.M. Gresho, J. Thompson, R.L. Sani and J. Hauser (Eds.), Swansea, July 1989, Pineridge Press, 2, pp. 1663-1673.


NUCLEAR SAFETY

24

Verification and validation (V&V) of CFD codes is perhaps the major present challenge for their application in NRS – Task is now (since 2012) leaded by ASME

A special number appeared in NED (vol. 238, 2008) showing outcomes from “Benchmarking of CFD Codes for Application to Nuclear Safety (CFD4NRS)”, 5–7 September 2006, Garching, Munich, Germany

Conclusions emerging from the meeting were:

• Best Practice Guidelines should be followed as far as practical to ensure that CFD simulation results are free of numerical errors, and that the physical models employed are well validated against data appropriate to the flow regimes and physical phenomena being investigated.

• Experimental data used for code validation should include estimates of measurement uncertainties, and should include detailed information concerning initial and boundary conditions.

• Experimenters involved in producing data for validating CFD models and/or applications should collaborate actively with CFD practitioners in advance of setting up their instrumentation. This interface is vital in ensuring that the information needed to set up the CFD simulation will actually be available, the selection of “target variables” (i.e. the most significant measurements against which to compare code predictions) is optimal, and the frequency of data acquisition is appropriate to the time-scale(s) of significant fluid-dynamic/heat-transfer/phase-exchange events.


NUCLEAR SAFETY

25

• It would be worth giving consideration to “Verification and validation benchmarks”, W.L. Oberkampf and T.G. Trucano (pp. 716-743), because of their precise definitions and overview.

• It was also emphasized that subsequent meetings would be worth. • Then, a second Meeting on the subject was held, “XCFD4NRS,

Experiments and CFD Code Applications to Nuclear Reactor Safety”, OECD/NEA & IAEA Workshop CEA, Grenoble, France, 10 – 12 September, 2008, on single-phase and two-phase CFD simulations.

• Emphasis was on validation for boiling flows, free-surface flows, direct contact condensation and turbulent mixing in relation to NRS- relevant issues. Discussion of validation of the CFD tool, use of systematic error quantification and Best Practice Guidelines (BPGs) was encouraged.

• Regarding BPGs, J. Mahaffy stated that they must be supplemented on application to specific cases and for two-phase flow. As a living document, a Wiki like site could be developed at NEA

• Full satisfaction of BPGs is rarely accomplished…


NUCLEAR SAFETY

ASME Standard on V&V 20 (2009)

26 Example of procedural specification

for V&V and UQ determination

Non-mandatory:

Method of manufactured solutions

(or MMS)

Procedural approaches to V&VUQ 27

REFERENCES:

“V&V METHODOLOGY COMPARISONS: AIAA G-077(1998), ASME V&V 20 (2009), ASTM E1355-05a(2005),

NEA/CSNI/R(2007), and NRC CSAU(1988)”

S. Peters, L. Tschaepe, B. Zhan and A. Ruggles, The 14th International Topical Meeting on Nuclear Reactor

Thermalhydraulics, NURETH-14, Paper 586, Toronto, Ontario, Canada, September 25-30, 2011

NRC CSAU (1988, NucEng specific, TH systems codes)

AIAA G-077(1998)

ASTM E1355-05a (2005, NUREG-1824, for fire simulation in NPPs)

NEA/CSNI/R (2007, NucEng specific)

NASA TECHNICAL STANDARD, NASA-STD-7009, 2008

ASME V&V 30 (NucEng specific, compliant with ASME NQA-1, to be

issued)

ON ASME V&V 30 Standard 28

REFERENCE: An Assessment of the Relationship of ASME NQA-1 and ASME V&V 20 Standards to the draft V&V 30 Standard

for Verification and Validation of Software for Nuclear Applications

Edwin A. Harvego (INL), Richard G. Hills (Sandia NL), Richard R. Schultz (INL), Ryan Crane (ASME)

ASME 2012 Verification & Validation Symposium Las Vegas, NV, May 2-4, 2012

29

More recent information that is worth considering comes in the context of: • The NEA 2010 Meeting on “Experimental Validation and Application of CFD

and CMFD Codes to Nuclear Reactor Safety Issues” (CFD4NRS-3) held in September 14-16, 2010, Bethesda, USA.

• The CFD4NRS-4 Workshop on CFD for Nuclear Reactor Safety Applications September 10 – 12, 2012 Daejeon, Korea

The original extent of the meeting has been widened, with emphasis on V&V, systematic application of assessed BPGs, experimental data analysis and related aspects

Many valuable contributions, may be found at the proceedings of NURETH-14 (Canada, 2011) and NURETH-15 (Pisa, May 2013)


NUCLEAR SAFETY

SUGGESTIONS (for users) 30

Start studying your code manual and check what is “behind the screen” in terms of physical models, numerics and correlations. Decide which sub-model is more appropriate for your problem in close relation with your computational resources

Get a clear view of truncation and discretization errors; evaluate numerical diffusion relative to physical and turbulent viscosity coefficients; decide on the type of boundary conditions as related to gridding scales.

When forced to make a choice between parameters or models consider the one giving conservative results. Otherwise perform UQ analysis

Use a tiered approach, from simple cases to more complex ones. Search for a validation example similar to your problem and perform the simulation. Blind benchmark specifications are sometimes preferable to check your approximation against other authors results. Follow procedural standards (e.g. ASME)

Follow expert advices embedded in codes but it would be better to verify this a-posteriori of your choices (i.e. rely on observation)

By the way… 31

• The existence of spurious oscillations (reflected in numerically stable computations and non physical) in the solution of conservation equations may be the consequence of non appropriate resolution of the boundary layer behavior of the solution.

• Then, suppressing the oscillations may be non conservative or, equivalently, allow

computing a solution not showing all the important aspects of the physics. Work done in the early 80´s served to clarify these aspects.

METRICS 32

Metrics is used to evaluate the accuracy of the computed solution

Metrics is problem (or context) specific. Select an appropriate metric

Depending on your (a priori) knowledge of the expected solution, metrics may be

more relaxed

Most procedural standards specify basic metrics that must be considered.

METRICS 33

REFERENCE: Accuracy quantification metrics for CFD simulation of in-vessel flows, F. Moretti and F. D’Auria, NED, in press, 2014

TRENDS 34

• Presently, the collaborative inter-institutional efforts continue. In the USA

are leaded by the Consortium for Advanced Simulation of Light water

reactors (CASL) that includes academia, DOE National Laboratories and

industry

Industry participation comes from software

products and provision, problems and a link to

affordable computer clusters, instead of

“frontline” systems like Titan at ORNL

PS: Walt Schwarz is the ANSYS representative to the “Industry Council”

REFERENCE: report CASL-U-2014-0006-000

TRENDS 35

Virtual Environment for Reactor Analysis (VERA)

Codes integration through VERA 36

Codes integration through VERA 37

CONCLUSIONS 38

Maturity of CFD is to be reached regarding TH applications for NPP

This is particularly true when dealing with Nuclear Safety and licensing

Reasons for the above two assertions come from the need of unified, more general approaches for physical models (e.g. two-phase flows, turbulence models, etc.) and the need of huge efforts and resources dedicated to V&V and UQ

It is verifiable that even in the leading projects like CASL, systems TH codes (1D) will be used to model out-of-core TH and heat transfer, providing the linking between the whole system TH behavior and the in-core advanced simulations. As a consequence, developments like TRACE, maintenance of RELAP and other non-CFD codes is worth considering

There is plenty of literature on CFD applied to nuclear installations and due to the different methodologies and originating schools it is somewhat difficult to discern on their validity

CONCLUSIONS 39

The so-called “user effects” are even more important now because code dials are still present explicitly or implicitly (interaction of grids and sub-models, code enhancements by users, codes validity extrapolations, etc.)

Reproducibility of results is almost impossible without detailed access to input data. Different codes produce different results. Colored graphs only are not helpful to evaluate reports

CFD provides access to dependent variables that are otherwise impossible to check In these cases coloring becomes essential

Perhaps the way of “working isolated” is old-fashioned to face the near future

Collaborative efforts are widespread, aimed at accomplishing high tech goals It is suggested to create a “downearth” forum for this, despite of skepticism and institutional selfishness

THE END 40

Thanks for attending and for your attention

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