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“Best” Practice Guidelines for the use of CFD

in Nuclear Reactor Safety Applications

OECD NEA workgroup on CFD in reactor studies

www.nea.fr/html/nsd/csni/cfd.htm

http://www.nea.fr/html/nsd/docs/2007/csni-r2007-5.pdf

following the « KNOO »

CFD Workshop on Test Cases, Databases

& BPG for Nuclear Power Plants Applications. University of Manchester, 16 July 2008

http://cfd.mace.manchester.ac.uk/Main/KnooWorkshop

By D. Laurence

Turbulence modelling in OECD BPG :

- incompetence ? or disrespect for academic publication “best practice” ?
- “In the ECORA and FLOMIX-R projects… it was found that
- Reynolds stress models provided better results for buoyancy driven mixing”
- Merits of Second Moment Closures re-discovered 30 years later !
- Out of 265 references, none of the following turbulence modeling experts
- are mentionned: B. Launder(1), W. Rodi, S. Pope, T Gatski, C Speziale.
- At least these should be mentionned:
- - Launder, B. E., Reece, G. J. and Rodi, W. (1975), "Progress in the Development of a Reynolds-Stress Turbulent Closure.", Journal of Fluid Mechanics, Vol. 68(3), pp. 537-566.
- It scores 1,380 references under Google Scholars and it was actually sponsored by the CEGB, (Central Electricity Generating Board)
- Rodi, W. (1993). Turbulence Models and their Application in Hydraulics.
- IAHR Monograph, Balkema, The Netherlands, 116 pages [ISBN 90-5410-150-4
- this book is most relevant to thermal-hydraulics and describes algebraic models correctly
- - K. Hanjalic and B. E. Launder A Reynolds stress model of turbulence and its application to thin shear flows Journal of Fluid Mechanics Digital Archive, Volume 52, Issue 04, April 1972, pp 609-638 doi: 10.1017/S002211207200268X,

[1]however Brian Launder appears once in the text: “In 1979 with assistance from Brian Launder, a modified k-e two-equation turbulence model (with addition of buoyancy terms) was incorporated into the code.” sic !

Obsolete and biased toward certain commercial codes?

Code Name N of occurrences

CFX 64

FLUENT 51

TRIO or TRIO-U 13

STAR CD or STAR-CD 9 + 6 (isn’t Framatome/AREVA

in the Nuc. En. and using this code?)

ESTET and N3S (obsolete EDF codes) 2

EDF’s current software Code_Saturne 0

- “CFD codes, mainly devoted to Nuclear applications, have been developed inFrance, among them : TRIO-U and CAST3M (CASTEM Fluid, TONUS) by the CEA, and ESTET and N3S by EDF. Industrial codes such as CFX, Fluent and STAR CD are also used by the different organizations.”
- “Primary Flowrate (Hot Leg Heterogeneity): the comparison study was based on Banquise
- tests and used CFX [19] and TRIO-U calculation in order to assess EDF’s demonstration
- based on STAR CD”
- EDF does not use STAR CD for it studies and has moved on from the OLD FD and FE codes N3S and ESTET to the unstructured-grid finite-volume solver “Code_Saturne” 10 years ago.

A publicity coup for the premature SST SAS model ?

- There seems to be a lot of excitement around CFX’s SST-SAS, but is this not a bit too preliminary to be included in Best Practice Guidelines? Has there been any applications to Nuclear Reactors yet?

- “Menter F. R. and Egorov, Y., Re-visiting the turbulent scale equation,î Proc. IUTAM Symposium; One hundred years of boundary layer research, Gottingen, 2004.”

- “Menter F. R. and Egorov, Y., ìA Scale-Adaptive Simulation Model using Two-Equation Modelsî, AIAA

paper 2005-1095, Reno/NV, 2005.”

- “Menter F.R. and Egorov, Y., Turbulence Models based on the Length-Scale Equationî, Fourth

International Symposium on Turbulent Shear Flow Phenomena, Williamsburg, 2004 ñ Paper TSFP4-268,

2005.”

CFX-team is not the only one attempting Transient RANS. The refs below would seem to be more relevant to the NEA BPG than the Aerodynamics SAS applications it has included!

Kenjeres, S. and Hanjalic, K. (2006),“LES, T-RANS and Hybrid simulations for thermal convection at high Ra numbers”, International Journal of Heat and Fluid Flow , Vol.27, pp.800-810.

Kenjeres S., Hanjalic K. and Gunarjo S. B. (2002),"A T-RANS/VLES Approach to Indoor Climate Simulations", ASME paper, FEDSM2002-31400, pp.1—9.

Kenjeres, S. and Hanjalic, K. (2002),"Combined effects of terrain orography and thermal stratification on pollutant dispersion in a town valley: a T-RANS simulation", Journal of Turbulence , Vol.3 (26), pp.1-25.

Hanjalic, K. and Kenjeres, S. (2001),"VLES of flows driven by thermal buoyancy and magnetic field", ERCOFTAC Bulletin No.48, pp.40-43

Hanjalic, K. and Kenjeres, S. (2001),"T-RANS Simulation of Deterministic Eddy Structure in Flows Driven by Thermal Buoyancy and Lorentz Force", Flow, Turbulence and Combustion , Vol. 66, pp.427-451.

Kenjeres, S. and Hanjalic (2000),"Convective rolls and heat transfer in finite-length Rayleigh-Benard convection: a two-dimensional numerical study ", Physical Review E , Vol.62, No.6, pp.1-12. 7. Rayleigh-Benard convection: A T-RANS study", Journal of Turbulence , Vol.1 (8),pp.1-22.

Kenjeres, S. and Hanjalic, K. (1999),"Identification and Visualization of Coherent Structures in Rayleigh-Benard Convection with a Time-Dependent RANS", Journal of Visualization , Vol.2 (2), pp.169-176. 4. Kenjeres, S. and Hanjalic, K. (1999),"Transient analysis of Rayleigh-B

A publicity coup for the premature SST SAS model ?

- - Development of this model was funded by the EU AEROSPACE project DESIDER 2004-2007.
- Very premature, has no relation with Réactor thermal hydraulics!
- Below are the Only 2 CFD illustrations between p 1. and P. 111 !
- With little space devoted to turbulence modelling was this SAS the most relevant ??

Premature publicity for SST SAS model ?

- An example of proper evaluation of DES, SAS and RANS
- Flow ventilation of room with release of passive scalar

- From Dr S. Gant, Health and Safety Laboratory.'Challenges in validation of unsteady modeling approaches (DES/SAS/URANS)', presented at: http://cfd.mace.manchester.ac.uk/Main/KnooWorkshop
- To appear on http://www.hse.gov.uk/

From Dr S. Gant, 'Challenges in validation of unsteady modeling approaches (DES/SAS/URANS)',

- http://cfd.mace.manchester.ac.uk/Main/KnooWorkshop

Turbulence modelling section: incompetence ? or disrespect for academic publication “best practice” ?

- Clearly incompetence ! Errors in nearly every turbulence related equation,
- (index errors, symbols are not defined, numerous errors, inconsistent notations
- (use S or repeated index summation, or full expansion, but don’t mix !)
- OECD could focus on BPG on typing Mathematical expressions ?
- “Experts” named in introduction clearly did not or were not asked to proof read the document.

- - First term on r.h.s. should be d./dx_k instead of d./dx_j
- Explain why l.h.s. has ‘primes’ (u’ ro’) but not the r.h.s.
- Perhaps something should be said about Favre’s
- density weighted averaging ?

Why expand suddenly ? why not :

- What does the “i ” subscript on the “ro” subscript stand for ? (it is missing on the r.h.s ?)
- do not use subscripts-on-subscripts

“Conclusions and Recommendations”

“The need for Best Practice Guidelines … became very clear during the assembly of this report.” Indeed the authors need help !

“With the exception of individuals working within the ECORA project, most CFD practitioners worked from personal experience, advice from co-workers, and at times code manuals. For those with experience in development and use of classic thermalhydraulic safety codes this should come as no surprise.”

- What does it mean ? “with the exception of ECORA people, all CFD users are idiots?”

- Good CFD practice does indeed require a lot of experience (own and co-workers), moreover, Code manuals and Tutorial quality tend to be very high, because they need to be value for customers money, in opposition to this taxpayer funded piece of

-“it should have significant value as a repository of expertise for training inexperienced CFD users.”

-“verification of the final discretization will also be needed before accepting results.” “Frequently, available time and computer resources restrict the rigor in estimation of discretization error. not an excuse to abandon verification.”

“examples are boron dilution and pressurized thermal shock. These scenarios

have been analysed for many years by a number of organizations,”

Very poor examples and lessons learnt for so much effort !

The third example explores the use of Fluent for simulation of dry cask storage of spent fuel.

Then in Conclusions of this example

“The application of Best Practice Guidelines is currently restricted due to the prohibitive numerical effort to carry out mesh sensitivity studies and comprehensive investigations on different turbulence models.” sic !!

But Authors wrote in general recommendations:

-“verification of the final discretization will also be needed before accepting results.” “Frequently, available time and computer resources restrict the rigor in estimation of discretization error. not an excuse to abandon verification.”

Yet the examples chosen do not even follow the recommendations !

« Discretization schemes

« The calculations were done with the CFX « High Resolution » option for spatial discretization, which adjusts local discretization to provide something close to second order spatial accuracy. «

- « Since modelling the UPTF geometry is computationally very demanding, it is

impossible to strictly follow the BPG, which, e.g., state that a 2x2x2 refinement should be tested, instead, a 1st order solution (case B) has been compared with a 2nd order solution (case D). «

When only 3 applications are shown, is a test case

Where a laminar simuation is nearly as good

as the low Re solution the best choice ? ?

Laminar

k-esp

k-omega

Verification and Validation of Selected Fire Models for Nuclear Power Plant Applications

NUREG-1788

CFD Analysis of Full-Scale Steam Generator Inlet Plenum Mixing During a PWR Severe Accident

NUREG-1781

CFD Analysis of 1/7th Scale Steam Generator Inlet Plenum Mixing During a PWR Severe Accident

Prepared by C.F. Boyd, K. Hardesty

”As more experience is gained through OECD sponsored benchmarks and other activities, we recommend that this document be used as a template for application specific best practice guidelines.

For example experience with ISP 47 could be used to generate detailed guidelines for modelling hydrogen mixing and combustion in a containment.”

Please don’t bother OECD NEA, excellent V&V work can be found online here:

UNITED STATES NUCLEAR REGULATORY COMMISSION

(www.nrc.gov/reading-rm/doc-collections/nuregs/staff/).

- "Verification and Validation of Selected Fire Models for Nuclear Power Plant Applications,"3 (NUREG1824). Is a very thorough comparison of 5 codes on several dozen tests.
- “CFD Analysis of 1/7th Scale Steam” is an excellent CFD V&V study including sensitivity of input parameters and models

Why compare k-epsilon and SST on a LAMINAR

Natural/mixed convection test case in the BPG?

The « vertical heated pipe » flow has been a reference for Nu Reactor studies for years!

But the SST fails to capture the relaminarization.

Buoyancy aided heated pipe flow

Gr/Re**2 = 0.087 (relaminarization)

Buoyancy aided heated pipe flow

Gr/Re**2 = 0.087 (relaminarization)

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