AC 6-07 CFD Simulations

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Description

Test Data

CFD Simulations

Evaluation

Best Practice Advice




Draft tube

Application Challenge 6-07               © copyright ERCOFTAC 2004


CFD Simulations

Overview of CFD Simulations

Two international workshops, focused on this test case, have been organised: Turbine-99 - Workshop on Draft Tube Flow, Porjus, Sweden, 20-23 June 1999 (for Proceedings see Gebart et al., 2000a, hereafter in the text called Proc.T99W1), and Turbine-99 - Workshop 2 on Draft Tube Flows, Älvkarleby, Sweden, June 18-20, 2001 (for Proceedings see Engström et al. 2003, http://epubl.luth.se/1402-1536/2000/11/index-en.html, hereafter in the text called Proc.T99W2)). Brief summaries of the workshops have also been presented at the 20th and 21st IAHR Symposia on Hydraulic Machinery and Systems in Charlotte, N.C. 2000 (Gebart et al., 2000b), and in Lausanne, respectively, (Engström et al., 2002).

The first workshop was organised as a blind test and attracted 18 participants who contributed with comparison data and papers to the proceedings. The geometry and the axial and tangential velocity components at the inlet section were specified, while most of the remaining options were left to the participants to decide upon, e.g. grid and radial velocity component at the inlet. The focus was from an engineering point of view, to get an understanding of the difficulties of CFD-simulations and possible variations between different CFD-simulations of the test case. The first workshop showed that the case was very sensitive to different settings and the resulting Cp varied with up to ± 50 %. The reasons were as stated above: the choice of inlet boundary condition for the radial velocity had a large effect on the pressure recovery, many solutions were obtained on very coarse grid, various turbulence models were used, etc. To summarize: most of the computations were not performed according to the ERCOFTAC Best Practice Guidelines for Quality and Trust in Industrial CFD, which formally came out in the beginning of 2000 (Casey and Wintergerste, editors, 2000). Detailed results from the first workshop are available from Luleå University of Technology, Div. of Fluid Mechanics, see Proc.T99W1, Gebart et al (2000a). Since these results to a large extent are superseded by the results from the second workshop, we will not further discuss the results from the first workshop.

The second workshop focused more on quality and it was recommended to use the ERCOFTAC Best Practice Guidelines (Casey and Wintergerste, 2000) as a guide when performing computations. For this workshop the grid, the radial velocity component, turbulence model etc were specified for one test case, (case T), to get a reference solution from every participant in order to reduce the spread between the calculations. And for the 12 participants the variation in Cpr was reduced to ± 20 %. Engström et al. 2001, Engström et al., 2003. The difference was still very big based on Cpr wall, considering that all affecting variables should be the same. If one instead looks at Cprmean the difference is only ± 7 %.

In this document a brief comparison between the CFD-simulations will be given to demonstrate the agreement and differences between CFD results and experiments, with focus on the results from the second workshop. Results regarding engineering quantities (of which many are exactly the design or assessment parameters, DOAPs, defined for the test case) will be given for computations by all participants. A “typical” simulation of the standard test case T is then chosen to enable more detailed comparisons with the experiments.

The participating groups and their acronyms are given in Table 3.1

Table 3.1. List of acronyms

Acronym

Authors

Affiliation

CFD code

AEAT

Holger Grotjans

AEA Technology GmbH

CFX-TASCflow

Germany

CKD

Ales Skotak

CKD Blansko Engineering, a.s.

FLUENT

Czech Republic

HQ

Maryse Page

IREQ – Institute de recherche

CFX-TASCflow

Anne-Marie Giroux

d’Hydro-Quebec

FIDAP

Canada

FINE/Turbo

HTC

Katsumasa Shimmei

Hitachi, Ltd.

Takanori Ishii

Kazuo Niikura

NUT

Sadao Kurosawa

Toshiba Corporation, Japan

Tomotatsu Nagafuji

Nagoya University, Japan

Debasish Biswas

Toshiba Corporation, Japan

TEV

Per Egil Skåre

Sintef Energy Research, Norway

FLUENT

Ole Gunnar Dahlhaug

Norwegian University of

Science and Technology, Norway

VUAB

Staffan Jonzén

Vattenfall Utveckling AB

FLUENT

Bengt Hemström

Urban Andersson

Iowa

Yong. G. Lai

Iowa Institute of Hydraulic

U2RANS

V.C. Patel

Research, U.S.A.

LTU

Michel Cervantes

Luleå University of Technology

CFX-4.3

Fredrik Engström

Sweden

EXA

Alain Bélanger

Exa Corporation, U.S.A.

PowerFlow

Experiment

Urban Andersson

Vattenfall Utveckling AB

Table 3.2 contains a summary description of all test cases.

Table 3.3 is a description of all available data files which the computors were asked to provide from their computations. The data from VUAB:s computation are chosen to illustrate typical computational results for the case T(r ).

NAME

GNDPs
PDPs (problem definition parameters)
MPs (measured parameters)
N11
(√m/60s)
Q11
(√m/s)
ReH
(10-6)
Head (m)
Flow rate (m3/s)
Runner speed (rpm)
detailed data
DOAPs

CFD 1a

(T(r)-case)

140.4
0.98
4.1
4.5
0.516
595
Cp, U, V, k
Cpr wall, Cpr average ζ, α, β, S

CFD 2a

(R(r)-case)

140.4
1.02 / 1.05
4.1
4.5
0.542 / 0.554
595
Cp, U,V, k
Cpr wall, Cpr average ζ, &alpha, β, S

Table 3.2. Summary description of all test cases

Section Ia
Section Ib
Section II
Section III
U V W Cp
k ε
U V W Cp
U V W Cp
U V W Cp
k ε

CFD 1a

Tick.gif TrIa_VelCp_CFD.dat
Tick.gif TrIa_keps_CFD.dat
Tick.gif TrIb_CFD.dat
Tick.gif TrII_CFD.dat
Tick.gif TrIII_CFD.dat
Tick.gif TrIII_keps_CFD.dat

CFD 2a

Tick.gif RrIa_VelCp_CFD.dat
Tick.gif RrIa_keps_CFD.dat
Tick.gif RrIb_CFD.dat
Tick.gif RrII_CFD.dat
Tick.gif RrIII_CFD.dat
Tick.gif RrIII_keps_CFD.dat
Section Ia W
Section IVa
Section IVb
Upper centre line
Lower centre line
DOAPs, or other miscellaneous data
Cp
U V W Cp
U V W Cp
Cp Cf
Cp Cf

CFD 1a

Tick.gif TrIaW_CFD.dat
Tick.gif TrIVa_CFD.dat
Tick.gif TrIVb_CFD.dat
Wall pressure: TrIVbW_CFD.dat
Tick.gif TrUcl_Cp_CFD.dat
TrUcl_Cf_CFD.dat
Tick.gif TrLcl_Cp_CFD.dat
TrLcl_Cf_CFD.dat
Tick.gif TrDOAP_CFD.dat

CFD 2a

Tick.gif RrIaW_CFD.dat
Tick.gif RrIVa_CFD.dat
Tick.gif RrIVb_CFD.dat
Wall pressure: TrIVbW_CFD.dat
Tick.gif RrUcl_Cp_CFD.dat
RrUcl_Cf_CFD.dat
Tick.gif RrLcl_Cp_CFD.dat
RrLcl_Cf_CFD.dat
Tick.gif RrDOAP_CFD.dat

Table 3.3. Description of all available data files, and simulated parameters.

Simulation Case

Solution strategy

The different workshop participants used different codes, both commercial and in-house codes, based on either FVM and FEM, as shown in Table 3.1 where some basic information is available. It was also planned that each group should fill in a Table like Table 3.4. This was however not possible. As an example, however, we show Table 3.4 filled in by ourselves (VUAB), since VUAB:s results for test case T has been chosen as representing typical results.

All relevant settings for the group can be seen in Table 3.4.

Table 3.4. Solution strategy chosen by VUAB, test case T.

Solution acronym

VUAB

Test case

T

Operating condition

CFD code (version)

Fluent 6.0.20

Authors

Jonzén S., Hemström B. & Andersson U.

Affiliations

Vattenfall Utveckling AB.

Report date

June 2001, later revised

References

Jonzén S., Hemström B. & Andersson U: “Turbine 99 – Accuracy in CFD Simulations on Draft Tube Flow”,paper in Proc.T99W2.



Computational topics

Description

Parameter

Discretization method

Governing equations

Continuity, momentum, turbulent kinetic energy, turbulent dissipation.

Turbulence modelling

Standard k-epsilon

Near-wall treatment

Standard wall functions

K=0.42, E=9.793

Discretization scheme

QUICK. Second order upwind for pressure

Mesh

Modification of first grid by Lai, 707760 cells

Fluid properties

ρ=1000 kg/m3, μ=1006*10-3
Ns/m2

Inlet boundary conditions

Velocities, k and ε are specified

Outlet boundary conditions

Static pressure=0

Convergence criteria

Stable residuals and monitor value of velocity.

Although not provided in the same format here or in ProcT99W2, the corresponding information for the other computations according to the table of acronyms can be extracted from the separate papers included in Proc.T99W2.

Computational Domain

To reduce the variation in results from the first workshop, a common grid was supplied to the workshop participants. The grid is described in http://epubl.luth.se/1402-1536/2000/11/index-en.html.

Due to geometrical problems at the first part of the grid and a rather bad grid quality at the elbow, one conclusion from the second workshop was that an improved grid was needed. Some authors modified in various ways the given grid or used their own grid.

Boundary Conditions

A full summary of experimental data needed to set up a CFD-problem can be found in http://epubl.luth.se/1402-1536/2000/11/index-en.html . Here will be found information on the two main velocity components at the inlet, information on the wall roughness, wall pressures at the outlet and suggested assumptions for the third (radial) velocity component at the inlet that can be used to verify CFD-calculations against other simulations.

Both workshops used experimental data from (r) data set and for verification of the quality of a code it is recommended that these data is used with the specifications of Case 1 (T) of the second workshop since this offers the best opportunity for comparisons with other computations. However, for the future validation of the codes the calculations should be focused on the (n) data set since this offers the most consistent data set with a ‘complete’ set of velocity measurements from section I to section III. In the next chapter some additional requirements to obtain simulations with a good agreement with the experiments will be discussed.

Application of Physical Models

Most groups used wall functions.

For the second workshop a constant turbulent length scale at the inlet was suggested. Several workshop participants pointed out that this was not the best possible estimate. A better estimate would be a smaller length than recommended by the organisers.

Numerical Accuracy

Bergström (2000) made a thorough study of the numerical accuracy. The grid error was < 7 % and the iterative error was less than 0.8 %, however, a new grid caused an increase in e.g. Cp of 20%. This shows that the grid topology is very important and that ordinary refinement strategies might not be enough to reveal the total grid error.

The variation in the results between the participants serves as a measurement of the total accuracy comparing different CFD-codes and discretisation schemes. For Case T(r) most participants used the recommended grid, which facilitates comparisons between different computed results.

Comparing different computations revealed that the evaluation of the DOAPs was extremely sensitive to the evaluation algorithm. Therefore, these parameters were recomputed by the organisers to reduce the error and to give a constant numerical error. Details are given in Engström et al (2003).

CFD Results

Grid:

Workshop 2-grid

New grid

Derived data for comparison:

ASCII-files with DOAPs for the different cases submitted to the workshop for comparisons.

DOAPs calculated at available cross sections for case T(r). Both workshop 1 and 2.

TrDOAP_CFD.dat

DOAPs calculated at available cross sections for case R(r). Workshop 2.

RrDOAP_CFD.dat

Case T

Velocity and pressure values:

Section Ia

Computed velocity components and normalised pressure (Cp)
at cross section Ia, for Case T(r). (See Figure 1.5).

TrIa_VelCp_CFD.dat

Computed turbulent kinetic energy and turbulent dissipation rate
at cross section Ia, for Case T(r). (See Figure 1.5).

TrIa_keps_CFD.dat

Computed normalised wall pressure (Cp) at cross section Ia, for Case T(r).
(See Figure 1.5).

TrIaW_CFD.dat

Section Ib

Computed velocity components and pressure normalised pressure (Cp) at cross section Ib,
for Case T(r). (See Figure 1.5).

TrIb_CFD.dat

Section II

Computed velocity components and normalised pressure (Cp) at cross section II
for Case T(r). (See Figure 1.4).

TrII_CFD.dat

Section III

Computed velocity components and normalised pressure (Cp) at cross section III,
for Case T(r). (See Figure 1.4).

TrIII_CFD.dat

Computed turbulent kinetic energy and turbulent dissipation rate at cross section III,
for Case T(r). (See Figure 1.5).

TrIII_keps_CFD.dat

Section IVa

Computed velocity components and normalised pressure (Cp) at cross section IVa,
for Case T(r). (See Figure 1.4).

TrIVa_CFD.dat

Section IVb

Computed velocity components and normalised pressure (Cp) at cross section IVb,
for Case T(r). (See Figure 1.4).

TrIVb_CFD.dat

Computed normalised wall pressure (Cp) at cross section IVb, for Case T(r).
(See Figure 1.4).

TrIVbW_CFD.dat

Centrelines

Computed normalised wall pressure along the upper centre line, for Case T(r).
(See Figure 1.7).

TrUcl_Cp_CFD.dat

Computed normalised wall pressure along the lower centre line, for Case T(r).
(See Figure 1.7).

TrLcl_Cp_CFD.dat

Computed normalised wall shear stress (Cf) along the upper centre line,
for Case T(r). (See Figure 1.7).

TrUcl_Cf_CFD.dat

Computed normalised wall shear stress (Cf) along the lower centre line,
for Case T(r). (See Figure 1.7).

TrLcl_Cf_CFD.dat

Case R

Velocity and pressure values:

Section Ia

Computed velocity components and normalised pressure (Cp)
at cross section Ia for Case R(r). (See Figure 1.5).

RrIa_VelCp_CFD.dat

Computed turbulent kinetic energy and turbulent dissipation rate
at cross section Ia, for Case R(r). (See Figure 1.5).

RrIa_keps_CFD.dat

Computed normalised wall pressure (Cp) at cross section Ia, for Case R(r).
(See Figure 1.5).

RrIaW_CFD.dat

Section Ib

Computed velocity components and pressure normalised pressure (Cp)
at cross section Ib, for Case R(r). (See Figure 1.5).

RrIb_CFD.dat

Section II

Computed velocity components and normalised pressure (Cp)
at cross section II for Case R(r). (See Figure 1.4).

RrII_CFD.dat

Section III

Computed velocity components and normalised pressure (Cp)
at cross section III, for Case R(r). (See Figure 1.4).

RrIII_CFD.dat

Computed turbulent kinetic energy and turbulent dissipation rate
at cross section III, for Case R(r). (See Figure 1.5).

RrIII_keps_CFD.dat

Section IVa

Computed velocity components and normalised pressure (Cp)
at cross section IVa, for Case R(r). (See Figure 1.4).

RrIVa_CFD.dat

Section IVb

Computed velocity components and normalised pressure (Cp)
at cross section IVb, for Case R(r). (See Figure 1.4).

RrIVb_CFD.dat

Computed normalised wall pressure (Cp) at cross section IVb, for Case R(r).
(See Figure 1.4).

RrIVbW_CFD.dat

Centrelines

Computed normalised wall pressure along the upper centre line, for Case R(r).
(See Figure 1.7).

RrUcl_Cp_CFD.dat

Computed normalised wall pressure along the lower centre line, for Case R(r).
(See Figure 1.7).

RrLcl_Cp_CFD.dat

Computed normalised wall shear stress (Cf) along the upper centre line, for Case R(r).
(See Figure 1.7).

RrUcl_Cf_CFD.dat

Computed normalised wall shear stress (Cf) along the lower centre line,
for Case R(r). (See Figure 1.7).

RrLcl_Cf_CFD.dat

References

Bergström, J. (2000). “Modeling and Numerical Simulation of Hydro Power Flows”. Doctoral Thesis 2000-06, Luleå University of Technology, Department of Mechanical Engineering, Division of Fluid Mechanics.

Casey M. & Wintergerste T. (editors). (2000) ERCOFTAC Best Practice Guidelines. ERCOFTAC Special Interest Group on “Quality and Trust in Industrial CFD”.

Gebart B.R., Gustavsson L.H. & Karlsson R.I. (editors) (2000a) "Turbine 99 – Workshop on Draft Tube Flow”. Technical Report 2000-11, Luleå University of Technology, Department of Mechanical Engineering, Division of Fluid Mechanics.

Gebart B.R., Gustavsson L.H. & Karlsson R.I. (2000b) "Report from Turbine 99 – Workshop on Draft Tube Flow in Porjus, Sweden, 20-23 June 1999”. Paper presented at the 20th IAHR Symposium Hydraulic Machinery and Systems. Aug. 6-9, 2000, Charlotte, North Carolina, U.S.A.

Engström T.F., Gustavsson L.H. & Karlsson R.I. (2002) "Report from Turbine 99 – Workshop 2 on Draft Tube Flow”. The second ERCOFTAC Workshop on Draft Tube Flow, held in Älvkarleby, Sweden, June 18-20, 2001. Paper presented at the 21st IAHR Symposium Hydraulic Machinery and Systems, Lausanne, Switzerland, Sept. 2002

Engström, T.F., Gustavsson, L.H., & Karlsson, R.I. (2003), Proceedings of Turbine-99 - Workshop 2. The second ERCOFTAC Workshop on Draft Tube Flow. Älvkarleby, Sweden, June 18-20 2001. http://epubl.luth.se/1402-1536/2000/11/index-en.html

In text called Proc.W2.

© copyright ERCOFTAC 2004



Contributors: Rolf Karlsson - Vattenfall Utveckling AB


Front Page

Description

Test Data

CFD Simulations

Evaluation

Best Practice Advice