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					COMPUTATIONAL FLUID
DYNAMICS
GAMBIT PORTION


SUBMITTED TO: MR. GULRAIZ AHMED
SUBMITTED BY: ARSLAN SHAFIQ BE-05 (AERO)
Table of Contents
1 Introduction: ............................................................................................. 3
  1.1 Airfoil: ................................................................................................. 3
  1.2 GAMBIT Software:............................................................................... 3
2 Gambit: ..................................................................................................... 4
  2.1 Coordinate Format: ............................................................................. 4
  2.2 Creating Airfoil Geometry: .................................................................. 4
  2.3 Create Farfield Boundary: ................................................................... 5
  2.4 Create Faces: ..................................................................................... 6
3 Mesh Geometry in GAMBIT ........................................................................ 7
  3.1 Mesh Edges: ........................................................................................ 7
  3.2 Boundary Layer Approximation ........................................................... 8
    3.2.1 Split edge: .................................................................................... 8
    3.2.2 Meshing on airfoil edges: .............................................................. 8
    3.2.3 Y+ Approximation......................................................................... 9
4 Specify Boundary Types in GAMBIT ......................................................... 11
  4.1 Group Edges: .................................................................................... 11
5 Quality of the Structured Mesh ................................................................ 13
6 Unstructured Mesh: ................................................................................. 14
  6.1 Meshing the Edges ............................................................................ 14
  6.2 Meshing the Faces............................................................................. 14
  6.3 Quality of the Unstructured Mesh ..................................................... 16
7 References .............................................................................................. 17
1 Introduction:

In this report we first of select an airfoil and then generate the data points
of the airfoil. This file is imported in gambit and then we define a
computational domain in it. Boundary layer is then plotted around the airfoil.
Then the structured and unstructured meshing is done inside the domain. If
the meshing is not fine then it is refined.


1.1 Airfoil:

The airfoil which I used is NACA 2415. I generated it using the software
profilli. The number of points on the upper and lower surface are equal.
Points on each surface are 127.


1.2 GAMBIT Software:

Gambit is basically meshing software. In this we will define a computational
domain around our system under consideration e.g airfoil. In this
computational domain the meshing is generated. Meshing helps in the flow
analysis of the prototype model. The numerical techniques are used for
evaluation from one grid point to other. This is the first step in building and
analyzing a flow model.
2 Gambit:


2.1 Coordinate Format:

The format of the vertices of the airfoil are in .dat format. The points
generarted by the softwareprofilli are saved in it. The fiirst line represents
the number of points on each edge.



2.2 Creating Airfoil Geometry:

The .dat file of the vertices of the airfoil is imported in the gambit through
ICEM input.
2.3   Create Farfield Boundary:

Then the farfield boundary is created.
LABEL               X                  Y          Z
A                   1                  13.5       0
B                   22                 13.5       0
C                   22                 0          0
D                   22                 -13.5      0
E                   1                  -13.5      0
F                   -12.5              0          0
G                   1                  0          0




After generating the points the edges are made.
2.4 Create Faces:

There are basically four faces are drawn namely, rec1, rec2, airfol and circ1.
3 Mesh Geometry in GAMBIT


3.1 Mesh Edges:

After making the faces the next step is meshing. All the edges are meshed.
First we are doing the structured meshing.

Edges              Arrow direction    First length        Interval count
GA                 Upwards            .01                 70
BC                 Upwards            .01                 70
AB                 Left to right      .01                 85
CG                 Left to right      .01                 85
DE                 Left to right      .01                 85
EG                 Downwards          .01                 70
CD                 Downwards          .01                 70
3.2 Boundary Layer Approximation

3.2.1 Split edge:

Next, we will split the top and bottom edges of the airfoil into two edges so
that we have better control of the mesh point distribution. Figure of the
splitting edges is shown below.




We need to do this because a non-uniform grid spacing will be used for
x<0.3 and a uniform grid spacing for x>0.3. We will split the top edge into
HI and IG and the bottom edge into HJ and GJ.


3.2.2 Meshing on airfoil edges:

Before making the boundary layer, there should be meshing done on the
airfoil.

Edges               Arrow direction          First length/      Interval count
                                             Successive ratio
HI                  From H to I              .025               35
HJ                  From H to J              /1                 76
IG                  Left to right            .025               35
JG                  Left to right            /1                 76

For edges AF and EF, the number of divisions needs to be equal to the
number of divisions on the line opposite to it.

In order to calculate the boundary layer thickness we have the formula,
                              0.2
                     U x 
BL thickness  0.37 x             where U  =10m/s and  =1.46e-05 and x =1m
                       
So the boundary layer equation comes out to be,

                               BL thickness = 0.0252m

Now create the boundary layer 0.0252m thick. But before that approximate
y+ as below,
3.2.3   Y+ Approximation

y+ can be approximated by the formula given below,

                              y 
                                      y1u* 
                                                -------- (1)
                                       
Where u * can be approximated from the following equation,
                          2                        0.2
                      u*          U x 
                     
                      U   0.0296  
                                                         -------- (2)
                                      
Where for my case,
                               U  = 10m/s
                                 = 1.46e-05 m2/s
                                    And x = 1m
So, from eqn (2) we can get the       u * which is equal to,
                                 u * = 0.4488 m/s
Now using eqn (2) to get the value of y+, I have taken the value of y1 to be
equal to,
                                 y1 = 0.00055 m
Then plugging the values in eqn (1), we have,

                                     y  = 16.9068
The number of rows of the boundary layer is 15.
After the making of boundary layer all the faces are meshed.
4 Specify Boundary Types in GAMBIT


4.1 Group Edges:

We will create groups of edges and then create boundary entities from these
groups. First, we will group AF and EF together. Select Edges and enter
farfield1 for Label, which is the name of the group. In the transcript window,
you will see the message “Created group: farfield1 group”.




Similarly, create the other two far field groups. We should have created a
total of three groups:
          Group Name                              Edges in Group
             farfield1                                  AF,EF
             farfield2                                 AB,DE
             farfield3                                 BC,CD
               airfoil                              HI,IG,HJ,JG

4.2 Define Boundary Types:

We have grouped each of the edges into the desired groups; we can assign
appropriate boundary types to these groups. Select any edge belonging to
the airfoil surface and that will select the airfoil group. Next to name: enter
airfoil. Leave the Type as WALL.

Similarly, create boundary entities corresponding to farfield1, farfield2 and
farfield3 groups. Set the Type to Pressure Far field in each case.
5 Quality of the Structured Mesh

    The quality of the structured mesh is:




    The worst element is .370028 and lies in circ1.
6 Unstructured Mesh:


6.1 Meshing the Edges

All the edges of the same computational domain are meshed again for the
unstructured case.

Edges              Arrow direction    First length/Last   Interval count
                                      length/
                                      Successive ratio
GA                 Upwards            .03                 115
BC                 Upwards            /-/1                115
AB                 Left to right      .2                  100
CG                 Left to right      .05                 150
DE                 Left to right      .3                  100
EG                 Downwards          .03                 115
CD                 Downwards          /-/1                100
HI                 From H to I        /.025/              25
HJ                 From H to J        /-/1                40
IG                 Left to right      /.025/              25
JG                 Left to right      /-/1                40


6.2 Meshing the Faces

After meshing the edges, now comes the turn of meshing the faces. But
before that the boundary layer of same approximation as that of structured
is drawn. Then mesh the faces.
6.3 Quality of the Unstructured Mesh

    The quality of the structured mesh is:




    The worst element is .482127 and lies in rec1.
7 References

      http://courses.cit.cornell.edu/fluent/airfoil/index.htm

      http://www.pdas.com/

      Profilli software

     http://www.fluent.com/solutions/whatcfd.htm

     http://www.cfd-online.com/Wiki/Fluent_FAQ#What_is_Gambit.3F

				
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posted:10/1/2011
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Ruqia Sultana Ruqia Sultana Vortex Accademy www.mechinfolibrary.blogspot.com
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