AC 607 CFD Simulations
Contents
Draft tube
Application Challenge 607 © copyright ERCOFTAC 2004
CFD Simulations
Overview of CFD Simulations
Two international workshops, focused on this test case, have been organised: Turbine99  Workshop on Draft Tube Flow, Porjus, Sweden, 2023 June 1999 (for Proceedings see Gebart et al., 2000a, hereafter in the text called Proc.T99W1), and Turbine99  Workshop 2 on Draft Tube Flows, Älvkarleby, Sweden, June 1820, 2001 (for Proceedings see Engström et al. 2003, http://epubl.luth.se/14021536/2000/11/indexen.html, hereafter in the text called Proc.T99W2)). Brief summaries of the workshops have also been presented at the 20^{th} and 21^{st} 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 CFDsimulations and possible variations between different CFDsimulations of the test case. The first workshop showed that the case was very sensitive to different settings and the resulting C_{p} 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 Since these results to a large extent are superseded by the results from the second workshop, we will not further discuss the results from the first workshop.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 Cp_{r} was reduced to ± 20 %. Engström et al. 2001, Engström et al., 2003. The difference was still very big based on Cp_{r} wall, considering that all affecting variables should be the same. If one instead looks at Cp_{r}mean the difference is only ± 7 %.
In this document a brief comparison between the CFDsimulations 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 
CFXTASCflow 
Germany 

CKD 
Ales Skotak 
CKD Blansko Engineering, a.s. 
FLUENT 
Czech Republic 

HQ 
Maryse Page 
IREQ – Institute de recherche 
CFXTASCflow 
AnneMarie Giroux 
d’HydroQuebec 
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 
CFX4.3 
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 


 







 
CFD 1a (T(r)case) 








CFD 2a (R(r)case) 








Table 3.2. Summary description of all test cases



 





 
CFD 1a 






CFD 2a 











 





 
CFD 1a 






CFD 2a 






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 inhouse 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 kepsilon 

Nearwall 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*103 

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/14021536/2000/11/indexen.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 CFDproblem can be found in http://epubl.luth.se/14021536/2000/11/indexen.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 CFDcalculations 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 CFDcodes 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 2grid
New grid
Derived data for comparison:
ASCIIfiles 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.
DOAPs calculated at available cross sections for case R(r). Workshop 2.
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).
Computed turbulent kinetic energy and turbulent dissipation rate
at cross section Ia, for Case T(r). (See Figure 1.5).
Computed normalised wall pressure (Cp) at cross section Ia, for Case T(r).
(See Figure 1.5).
Section Ib
Computed velocity components and pressure normalised pressure (Cp) at cross section Ib,
for Case T(r). (See Figure 1.5).
Section II
Computed velocity components and normalised pressure (Cp) at cross section II
for Case T(r). (See Figure 1.4).
Section III
Computed velocity components and normalised pressure (Cp) at cross section III,
for Case T(r). (See Figure 1.4).
Computed turbulent kinetic energy and turbulent dissipation rate at cross section III,
for Case T(r). (See Figure 1.5).
Section IVa
Computed velocity components and normalised pressure (Cp) at cross section IVa,
for Case T(r). (See Figure 1.4).
Section IVb
Computed velocity components and normalised pressure (Cp) at cross section IVb,
for Case T(r). (See Figure 1.4).
Computed normalised wall pressure (Cp) at cross section IVb, for Case T(r).
(See Figure 1.4).
Centrelines
Computed normalised wall pressure along the upper centre line, for Case T(r).
(See Figure 1.7).
Computed normalised wall pressure along the lower centre line, for Case T(r).
(See Figure 1.7).
Computed normalised wall shear stress (Cf) along the upper centre line,
for Case T(r). (See Figure 1.7).
Computed normalised wall shear stress (Cf) along the lower centre line,
for Case T(r). (See Figure 1.7).
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).
Computed turbulent kinetic energy and turbulent dissipation rate
at cross section Ia, for Case R(r). (See Figure 1.5).
Computed normalised wall pressure (Cp) at cross section Ia, for Case R(r).
(See Figure 1.5).
Section Ib
Computed velocity components and pressure normalised pressure (Cp)
at cross section Ib, for Case R(r). (See Figure 1.5).
Section II
Computed velocity components and normalised pressure (Cp)
at cross section II for Case R(r). (See Figure 1.4).
Section III
Computed velocity components and normalised pressure (Cp)
at cross section III, for Case R(r). (See Figure 1.4).
Computed turbulent kinetic energy and turbulent dissipation rate
at cross section III, for Case R(r). (See Figure 1.5).
Section IVa
Computed velocity components and normalised pressure (Cp)
at cross section IVa, for Case R(r). (See Figure 1.4).
Section IVb
Computed velocity components and normalised pressure (Cp)
at cross section IVb, for Case R(r). (See Figure 1.4).
Computed normalised wall pressure (Cp) at cross section IVb, for Case R(r).
(See Figure 1.4).
Centrelines
Computed normalised wall pressure along the upper centre line, for Case R(r).
(See Figure 1.7).
Computed normalised wall pressure along the lower centre line, for Case R(r).
(See Figure 1.7).
Computed normalised wall shear stress (Cf) along the upper centre line, for Case R(r).
(See Figure 1.7).
Computed normalised wall shear stress (Cf) along the lower centre line,
for Case R(r). (See Figure 1.7).
References
Bergström, J. (2000). “Modeling and Numerical Simulation of Hydro Power Flows”. Doctoral Thesis 200006, 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 200011, 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, 2023 June 1999”. Paper presented at the 20^{th} IAHR Symposium Hydraulic Machinery and Systems. Aug. 69, 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 1820, 2001. Paper presented at the 21^{st} IAHR Symposium Hydraulic Machinery and Systems, Lausanne, Switzerland, Sept. 2002
Engström, T.F., Gustavsson, L.H., & Karlsson, R.I. (2003), Proceedings of Turbine99  Workshop 2. The second ERCOFTAC Workshop on Draft Tube Flow. Älvkarleby, Sweden, June 1820 2001. http://epubl.luth.se/14021536/2000/11/indexen.html
In text called Proc.W2.
© copyright ERCOFTAC 2004
Contributors: Rolf Karlsson  Vattenfall Utveckling AB