large-eddy simulation of airfoil flow using openfoam

50 Pages·2011·10.97 MB·English

LARGE-EDDY SIMULATION OF AIRFOIL FLOW USING
OPENFOAM

by

József RIDEG
/RY8ILC/

Submitted to the
Department of Fluid Mechanics of the
Budapest University of Technology and Economics
in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering Modelling

May, 2011

Project Report in Major Project /BMEGEÁTMWD1/

Supervisor:
László NAGY, assistant lecturer

Evaluation Team Members, advisors:
László NAGY, assistant lecturer
Dr. Gergely KRISTÓF, associate professor
Dr. Máté Márton LOHÁSZ, Assistant professor

Department of Fluid Mechanics
Faculty of Mechanical Engineering
Budapest University of Technology and Economics
ASSIGNMENT
MSc MAJOR PROJECT (BMEGEÁTMWD1)
Title: Large-Eddy Simulation of airfoil flow using OpenFOAM
Author’s name (code): József RIDEG (RY8ILC)
Curriculum : MSc in Mechanical Engineering Modelling / Fluid Mechanics
Supervisor’s name, title: László NAGY, assistant lecturer
Affiliation: Department of Fluid Mechanics / BME
Assistant supervisor’s name, title:
Affiliation:
Description / tasks of the project: 1. Draw conclusions from a BSc thesis discussing the same topic in 2D
2. Investigate the appropriate numerical setting further in 2D
3. Compare the difference scheme in OpenFOAM and in ANSYS Fluent.
4. Prepare LES (3D) in OpenFOAM using an existing mesh (without final
investigation)
Handed out / Deadline: 7th of February 2011. / 13th of May 2011.
Budapest, 7th of February 2011.
……………………………….
Head of Department
Received by: The undersigned declares that all prerequisite subjects of the Major Project have been fully
accomplished. Otherwise, the present assignment for the Final Project is to be considered
invalid. Signed in Budapest, on the 7th of February 2011.
……………………………….
Student
Large-Eddy Simulation of airfoil flow using OpenFOAM

DECLARATION

Full Name (as in ID): József RIDEG
Neptun Code: RY8ILC
University: Budapest University of Technology and Economics
Faculty: Faculty of Mechanical Engineering
Department: Department of Fluid Mechanics
Major/Minor: MSc in Mechanical Engineering Modelling
Spec. in Fluid Mechanics (major) and Solid
Mechanics (minor)
Project Report Title: Large-Eddy Simulation of airfoil using OpenFOAM
Academic year of submission: 2010 / 2011 - II.

I, the undersigned, hereby declare that the Project Report submitted for assessment and
defence, exclusively contains the results of my own work assisted by my supervisor.
Further to it, it is also stated that all other results taken from the technical literature or other
sources are clearly identified and referred to according to copyright (footnotes/references
are chapter and verse, and placed appropriately).
I accept that the scientific results presented in my Project Report can be utilised by the
Department of the supervisor for further research or teaching purposes.

Budapest, 13 May, 2011

__________________________________
(Signature)

FOR YOUR INFORMATION

The submitted Project Report in written and in electronic format can be found in the
Library of the Department of Fluid Mechanics at the Budapest University of Technology
and Economics. Address: H-1111 Budapest, Bertalan L. 4-6. „Ae” building of the BME.

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Large-Eddy Simulation of airfoil flow using OpenFOAM

ABSTRACT

The goal of this thesis was simulating turbulent flow around airfoil RAF-6E. At the
beginning, two dimensional simulations were performed by both OpenFOAM and
FLUENT. Then their results were compared to each other.

Then several numerical schemes were selected in OpenFOAM, then they were compared
to FLUENT and measurement.

Finally, a Large-Eddy Simulation of the airfoil in OpenFOAM was performed. Because of
the huge computation cost, the detailed LES will be the topic of the next semester.

KIVONAT

A dolgozat célja a RAF-6E típusú szárny körüli turbulens áramlás vizsgálata. Kezdetben
két dimenziós szimulációkat futtattunk OpenFOAM-ban és FLUENT-ben, majd az
eredményeket összehasonlítottuk egymással.

Később számos különböző numerikus sémát választottunk ki OpenFOAM-ban, melyek
eredményeit összevetettük a FLUENT-es szimuláció, illetve tanszéki mérések
eredményeivel.

Végül nagy örvény szimulációt végeztünk OpenFOAM-mal. A LES hatalmas számítási
igénye miatt, annak részletes vizsgálata a következő félév tárgya lesz.

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Large-Eddy Simulation of airfoil flow using OpenFOAM
CONTENTS

DECLARATION i
ACKNOLEDGEMENT ii
ABSTRACT iii
CONTENTS iv
NOMENCLATURE v
1. INTRODUCTION ............................................................................................. 1

1.1. Brief description of the case .............................................................................................................. 1

1.2. History of the project ......................................................................................................................... 1

1.3. Goals of this Major Project ............................................................................................................... 2

1.3.1. Repetition of earlier OpenFOAM simulation (2D) ......................................................................... 2

1.3.2. Comparison of different schemes in OpenFOAM (2D) .................................................................. 3

1.3.3. Setting LES case in OpenFOAM (3D) ............................................................................................ 3
2. DESCRIPTION OF THE MESH ...................................................................... 4

2.1. 2D mesh .............................................................................................................................................. 4

2.2. 3D mesh .............................................................................................................................................. 6

2.3. Mesh conversion to OpenFOAM ...................................................................................................... 7

2.3.1. 2D mesh conversion ........................................................................................................................ 7

2.3.2. 3D mesh conversion ........................................................................................................................ 7
3. COMPARISON OF FLUENT AND OPENFOAM .......................................... 8

3.1. Boundary conditions .......................................................................................................................... 8

3.1.1. Comparing boundary conditions of FLUENT and OpenFOAM ..................................................... 8

3.1.2. Defining velocity profile for OpenFOAM ...................................................................................... 8

3.2. Numeric schemes ............................................................................................................................... 9

3.2.1. Numeric schemes in FLUENT ........................................................................................................ 9

3.2.2. Numeric schemes in OpenFOAM ................................................................................................... 9

Table 3.2.2: List of numeric schemes used in OpenFOAM ........................................................................ 10

3.3. Solvers ............................................................................................................................................... 10

3.3.1. Solvers in FLUENT ...................................................................................................................... 10

3.3.2. Solvers in OpenFOAM ................................................................................................................. 10

Table 3.2.2: Parameters for solver in OpenFOAM ..................................................................................... 11

3.4. Results ............................................................................................................................................... 12

3.4.1. Wall Shear Stress .......................................................................................................................... 12

3.4.2. Contour plots of P ......................................................................................................................... 12

3.4.3. Contour plots of P .................................................................................................................... 13
mean

3.4.4. Contour plots of P .................................................................................................................... 13
RMS

3.4.5. Contour plots of u [m/s] ................................................................................................................ 14

3.4.6. Contour plots of v ......................................................................................................................... 15

3.4.7. Contour plots of u .................................................................................................................... 16
mean

3.4.8. Contour plots of v .................................................................................................................... 16
mean
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Large-Eddy Simulation of airfoil flow using OpenFOAM

3.4.9. Contour plots of u ................................................................................................................... 17
RMS

3.4.10. Contour plots of v ............................................................................................................... 18
RMS

3.4.11. Velocity and pressure profiles in wake .................................................................................... 19

3.4.12. Velocity and vorticity over horizontal line .............................................................................. 19
4. COMPARISON NUMERIC SCHEMES IN OPENFOAM .............................. 21

4.1. Numeric schemes ............................................................................................................................. 21

4.2. Results .............................................................................................................................................. 21

4.2.1. Speedtest....................................................................................................................................... 21

4.2.2. X component of instantaneous wall shear stress at suction side ................................................... 21

4.2.3. X component of in mean wall shear stress at suction side............................................................ 23

4.2.4. Mean of streamwise velocity (u ) in wake ............................................................................... 24
mean

4.2.5. RMS of streamwise velocity (u ) in wake ................................................................................ 25
RMS

4.2.6. Mean static pressure (P ) in wake ............................................................................................ 25
mean

4.2.7. RMS of static pressure (P ) in wake ......................................................................................... 26
RMS
5. LES ............................................................................................................... 27

5.1. LES properties ................................................................................................................................. 27

5.2. fvSchemes ......................................................................................................................................... 28

5.3. Boundary conditions ....................................................................................................................... 29

5.4. Results .............................................................................................................................................. 30

5.4.1. Lift and drag coefficients ............................................................................................................. 30

5.4.2. Coherent structure: Q iso-surface ................................................................................................. 30
6. SUMMARY .................................................................................................. 31
7. REFERENCES ............................................................................................... 32
8. APPENDIX ................................................................................................... 33

8.1. Boundary conditions of OpenFOAM in 2D .................................................................................. 33

8.1.1. u .................................................................................................................................................... 33

8.1.2. p .................................................................................................................................................... 35

8.1.3. nut ................................................................................................................................................. 36

8.1.4. nuTilda ......................................................................................................................................... 38

8.2. Numeric schemes of OpenFOAM in 2D ........................................................................................ 39

8.3. Solvers .............................................................................................................................................. 40

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Large-Eddy Simulation of airfoil flow using OpenFOAM
NOMENCLATURE

Mark Name Unit
c chord length m
C drag coefficient 1
D
C lift coefficient 1
L
FTN flow through number 1
N cells in streamwise dicetion 1
x
cells in wall-normal
N 1
y direction
N cells in spanwise dicetion 1
z
P static pressure Pa
Pa
P mean of static pressure
mean

root mean square of static
P Pa
RMS pressure
mean velocity in tangential
u m/s
mean direction
root mean square of velocity
u m/s
RMS in tangential direction
U reference velocity m/s
ref
instantaneous velocity in
v m/s
wall-normal direction
mean velocity in wall-
v m/s
mean normal direction
root mean square of velocity
v m/s
RMS in wall-normal direction
instantaneous velocity
w m/s
spanwise direction

GREEK INDEX
Mark Name Unit
α angle ot attack °
∆ time step s
t
∆ step size of the grid in x m
x direction
∆ step size of the grid in y m
y direction
m
∆ step size of the grid in z
z direction
τ x component of wall shear Pa
w,x stress
τ mean of x component of Pa
w,x,mean wall shear stress
2
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Large-Eddy Simulation of airfoil flow using OpenFOAM
1. INTRODUCTION

In this chapter a brief overview of this Major Project will be introduced. How is it
related to earlier studies, what and why will be investigated, and where will be the
conclusions used in future.
1.1. Brief description of the case

Fig.1.1: Profile of RAF6 E airfoil
Former studies have investigated the flow phenomana around RAF6 E airfoil. Both
RANS and LES computations were done for simulation the phenomena. The main
parameters of the flow and the geometry can be seen in this chapter.

Mark Value Name Unit
c 100 chord length [mm]
U 20 reference velocity [m/s]
ref
α 5 angle of attack [°]
ρ 1,225 density of air [kg/m3]
air
µ 1,7894·10-5 dynamic viscousity [kg/ms]
ν 1,4607·10-5 kinematic viscousity [m/s2]
Re 136917 Reynolds number [-]
c
Table 1.1: List of basic parameters

As a non-dimensional time parameter, flow through number (FTN) is defined in the
following way:
(referencevelocity)⋅(time) U ⋅t
FTN = = ref [−]
domain sizein streamwisedirection 3c

1.2. History of the project
CFD investigation of RAF6-E airfoil in 2D was performed by József Rideg [1] in 2009,
with using the commercial software of FLUENT. That project focused on mesh generation
with ICEM CFD software, and 2D simulation of the airfoil. Second order Upwind was set
as scheme. A brief LES simulation in 3D was also done.
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Large-Eddy Simulation of airfoil flow using OpenFOAM
In 2010 Dávid Virág [8] continued dealing with this problem still in 2D, with using the
freeware software of OpenFOAM. His goal was to compare the results obtained from
OpenFOAM and FLUENT. The effects of different schemes in OpenFOAM were also
investigated. In OpenFOAM linearUpwind, linear, upwind, MUSCL, QUICK were set as
schemes for different cases. The motivation for this topic is strongly related to the licence
differences between these two softwares. In case of OpenFOAM there are no limitations
for the number of cores, used for the parallel running of the case. But on the other hand,
definitely FLUENT is more user-friendly. So your choice depends on your preferences and
opportunities.

Fig.1.2: Overview of the projects

In the next semester, one can focus on OpenFOAM and LES, based on the gained
experiences and conclusions derived from this essay. Finally, the results can be useful later
as well for PHD thesis of László Nagy, since the earlier simulations were done with
FLUENT.
1.3. Goals of this Major Project
The goals of this thesis can be divided into three main parts: repetition and refined
postprocessing of former performed simulations in OpenFOAM and FLUENT. The second
part will be investigation of different numeric schemes in OpenFOAM and parallelization
efficiency. The third, final part will deal with defining a running LES case for the airfoil.
This third part can be handled as a preparation for the Final Project, which will focus on
LES.

1.3.1. Repetition of earlier OpenFOAM simulation (2D)
As a first step, getting familiar with OpenFOAM is a trivial, but important one. As it has
not got GUI (Graphical User Interface), one should set the cases in OpenFOAM by editing
text files, which are available in a strict, well-structured folder-tree.
On the other hand, some problem occurred during the BSc final thesis of Virág [2],
related to the part of postprocessing. Root mean square values should be corrected in the
generated contour plots. For better comparability of the results, I defined three profiles in
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Large-Eddy Simulation of airfoil flow using OpenFOAM
the wake of the airfoil, over which mean values and fluctuations of velocity and pressure
were plotted.
1.3.2. Comparison of different schemes in OpenFOAM (2D)
The selected numeric schemes can have great effects on the speed of the computation,
and on the credibility of the obtained results. With a proper decision, one can describe the
phenomena with feasible computational efforts. That is why we are interested in speed of
computation. For large domains, with huge amount of cells and computation time,
parallelization is an important issue. Therefore the effect of number of the cores used for
the cases will be examined as well.
1.3.3. Setting LES case in OpenFOAM (3D)
As earlier was mentioned, there are further opportunities with running LES in
OpenFOAM, because one can divide it into as many subdomains for calculation as many
he or she wishes, without licence limitations.
The first step will be mesh conversion from a mesh file with *.msh extension, which
was generated with GAMBIT. This mesh was used in several articles, which were written
at Department of Fluid Mechanics of the Budapest University of Technology and
Economics [1].
3