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Figure 2.1 Design of the conical diffuser for
the axisymmetric wind tunnel

2.1 shows the 2D axisymmetric wind tunnel model with default angle of 90°.
From the diagram, it clearly shown that the global x-axis is the axis of

2.2 Geometry and Dimensions used for the simulation.


By using half of the geometry can reduce the
simulation time. As simulation time reduced, a more accurate result with finer
meshes can be generated as finer meshes need longer simulation time. The flow
field above and below the global x-axis is assumed to be the same so it will
not affecting results generated.   

2.3 name selection of each boundary

1.      Inlet
and Outlet is where the flow enters and leave the tunnel respectively.

2.      1-wall,
2-wall and 3- wall is the wall of the wind tunnel that are named separately

3.      Symmetry
is the axis of symmetry of the wind tunnel.

The wall of the wind tunnel is named separately so
that the data obtained can be more accurate as the flow field of the respective
wall can be observed or analysed separately.


The accuracy of the
results obtained from the simulation is increasing when the mesh of the model becomes
finer. However, the finer the mesh, the higher the number of elements or cells
which will cause the simulation time to increase. There are several setting
which can be set to change the mesh size.

Table 2.1 is setting that do not change for all the
simulation. To change the mesh size of the geometry, only the following values
are changed;

Min Size: the minimum element edge size that the mesher
will create.

Max Face Size: maximum size that the
surface mesher will allow.

Max Size: maximum size that the volume mesher will allow.


Figure 2.4 shows the inflation layer of 20 that is
used on the 1-wall, 2-wall and 3-wall which are marked red. As shown in the
figure, by using inflation layer it generate a more detailed elements which can
provide more data after the simulation.


K-epsilon is a
turbulence model where is has two transport equation method model. Both
equation determine different variables which are ;

turbulent kinetic
energy, k

dissipation, ?

Uz Zaman, 2015)

From the convergence absolute criteria set, the results will only
converge when the residuals is less than 0.001.


In order to get the grid independent for the
resistance force in the diffuser and the change in pressure, ?P, different
number of cells were used as shown in table 2.3 by using different values of
min size, max face size and max size.

From table 2.2, it shows the resistance forces in the
1-wall, 2-wall and 3-wall section. The total resistance forces for each wall is
calculated by the summation of pressure drag and viscous drag on each wall. The
highlighted region is where the stable reading of resistance forces on the
three wall.


Graph 2.1 shows the relationship between the
resistance force in the diffuser section and number of cells that are used for
the simulation. From both table 2.1 and graph 2.1, they show a stable reading
of resistance forces in the diffuser section (2-wall) start from 91527 number
of cells. Thus, 91527 number of cells with min size of 7mm, max face size of
8mm and max size of 8mm.



ü  From
table 2.3, table 2.4 and graph 2.1, the values and dot that are marked red is
the grid independent for the resistance force in the diffuser section.

2.5 Resistance Force Grid Independent

The grid independent is obtained by
using the greatest mesh or grid which has the stable reading for the resistance
force (between 15.3N and 15.8N as shown in table 2.4). Besides, by observing
the graph 2.1, the graph after number of cells of 91527 has less fluctuation
which indicates a stable result.

From table 2.6, the vector at x=1510mm have shown the
flow field at small number of mesh has high velocity when near to the boundary
of symmetry and the flow field at high number of mesh has lower velocity when
near to the boundary of symmetry. This can be deduced that the different mesh
number causes the flow field in the wind tunnel to differ. For the velocity
vector diagrams, the smaller mesh number has more empty space as compared with
the larger mesh number. From these statements, it can be concluded that the
smaller mesh number will has a more simple flow field and it might miss out
important areas which are necessary in order to obtain correct flow field of
the tunnel. The larger mesh number will has more complex flow field and it
shows more detail flow field.


ü  There
are two type of pressure which are static pressure and dynamic pressure, where,

pressure = static pressure + dynamic pressure.

ü  In
this study, static pressure is used. For static pressure, the outlet pressure
is zero as it is set as shown in table 2.2. For the real static pressure is
summation of the gauge pressure and operating pressure. The inlet pressure
obtained in table 2.7 in negative is gauge pressure. It shows negative values
because Ansys is comparing the gauge pressure to the operating pressure and it
is less than the operating pressure.


ü  Furthermore,
the pressure inlet, Pin is obtained from average of pressure from
coordinate 0 ? y ? 250 at x = -1500 mm and the pressure outlet, Pout
is obtained from average of pressure from coordinate 0 ? y ? 500 at x = 8000

ü  From
table 2.3, table 2.7 and graph 2.2, the values and dot that are marked red is
the grid independent for the change in pressure, ?P = Pin – Pout.
This is deduced by analyse the data in table 2.7 where it show stable value of
?P (between -52.000Pa and -52.200Pa). The mesh number of 91527 is the grid
independent as it is the greatest mesh size which has ?P in the stable range

2.8 ?P Grid Independent

by observing the graph 2.2, the data after number of cells of 91527 has the
least fluctuation which indicates stable readings.

ü  Part
I and Part II have the same grid independent of mesh number = 91527. Thus, the
following setting is then fixed for the simulation of Part III and Part IV.

ü  Velocity
of x-direction is plotted for angle of 5°, 8°, 12°, 17°, 23°, 30°, 38°, 47°,
57°, 68° and 80°. This is to determine whether there are any backflow of velocity
since the air is flow at x-direction.

ü  Red
colour is the maximum velocity of flow field and blue colour is the minimum
velocity of flow field.


ü  From
figure 2.5 until figure 2.15, it shows different angles that are used for the
diffuser and its velocity diagram. For flow separation to occur, there must be
turbulence flow where there will be backflow of air in the wind tunnel. Thus,
if the minimum velocity of that particular angle is negative, there will be
separation occur on the diffuser wall.


ü  By
observations, velocity on diffuser wall is the lowest in all the cases from
figure 2.5 until figure 2.15. The velocity drop dramatically when reached the
diffuser wall.


ü  From
table 2.10 and figure 2.5 to figure 2.15, only angle of diffuser of 5°, 8° and
12° do not have back flow and no flow separation on diffuser wall. This is
because only angle of diffuser of 5°, 8° and 12° do not have negative velocity
of flow field as shown in table 2.10. Lastly, 12° is the maximum angle to be
used for the angle of diffuser to avoid separation.


ü  From
table 2.11, it shows that the 5° has the greatest pressure difference among all
the angle which is ideal because the largest the pressure difference indicate
the effectiveness of the diffuser is the highest among the other angles. The
diffuser with 5° recover more pressure.

ü  However,
8°, 12° and 17° still in the acceptable range of pressure difference.


The objective of this study is to
achieve a reasonable of pressure recovery through the diffuser while keeping
the length of diffuser as small as possible. From the CFD simulations and
analysis, angle of diffuser of 5°, 8° and 12° is in the acceptable range where
the pressure recovery is in the acceptable range. They have the highest
efficiency of diffuser among the angles. Besides, three of these angles also
show no backflow on the diffuser wall.

Hence, angle of diffuser of 12° is
the ideal angle to achieve the stated objective because it has the smallest
length of diffuser (maximum value of ?)
among the diffuser of 5°, 8° and 12°.


Ø  In our CFD simulation, we always assume that temperature is
constant and the friction does not cause any heat. Due to this reason, the CFD
simulation may not present the real situation. Therefore, thermodynamics
knowledge should also be considered and included in the calculations of the CFD
simulation to obtain a more accurate result.  


Ø  Besides, the mesh density at the inlet and the outlet should be
finer compare to the one at the body. This is because the result at the inlet
and outlet is being study. Through this a more accurate result can be obtained.

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