Section

B

Simulation

Steps

1.

Geometry

Figure 2.1 Design of the conical diffuser for

the axisymmetric wind tunnel

Figure

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

symmetry.

Figure

2.2 Geometry and Dimensions used for the simulation.

Remarks:

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.

Figure

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.

.

Meshing

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

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.

Remarks:

1.

K-epsilon is a

turbulence model where is has two transport equation method model. Both

equation determine different variables which are ;

a)

turbulent kinetic

energy, k

b)

turbulent

dissipation, ?

(Zohair

Uz Zaman, 2015)

2.

From the convergence absolute criteria set, the results will only

converge when the residuals is less than 0.001.

Table

2.3

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

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.

Analysis:

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.

Table

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.

Analysis:

ü There

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

Total

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

mm.

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

stated.

Table

2.8 ?P Grid Independent

Besides,

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.

Analysis:

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.

Analysis:

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.

Conclusion

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°.

Recommendations

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.