Week 3 - External flow simulation over an Ahmed body.
AIM :- To perform grid independency test on the Ahmad body and compare the results.
DISCUSSION :- As the body is perfectly symmetric, we can run the simulation by considering the only half body. This is the best practice where we can save on the number of cells and get the results faster as well.
The geometry which is provided with the challenge needs a modification which we should be able to do at this stage of a course. We need to use the \'split body\' command in SpaceClaim to perform the operation and then we can use the symmetry boundary condition in fluent to perform the simulation.
Run the simulation for the velocity of 25 m/s with the default air proparties in fluent.
For this challenge, we will have to provide answers to the following questions-
Q1. Describe Ahmad body and its importance.
Q2. Explain the reason for the negative pressure in the wake region.
Q3. Explain the significance of the point of separation.
Expected Results :-
1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case. (for velocity of 25m/s, the drag coefficient should be around 0.33).
3. The vector plot clearly showing the wake region.
4. Perform the grid indipendency test and provide the values of drag and lift with each case.
QUE.1. Describe Ahmad body and its importance.
ANS.:- The evaluation of car since from last few decades has been extremely fast because of increase compitition between the car companies. The computational Fluid Dynamics is a powerfull tool used to find the flow parameters in the automobile industry today. This analysis is used to understand the stability , fuel consumption , passenger comfort and emmission level before and after of the vehicle. In automobile design the complexity involved due to the great amount of accessories and devices are used in it, makes the experimental validation unaffordable, and the flow around vehicle under normal operating condition is principally turbulent. It is highly charecterised by large separation, recircuilation region, a complex wake flow and interaction of boundary layer flow in this work the drag force and drag coefficient is calculated by using ANSYS CFD Workbench 2020 R1. For this study a simplified car modelis used to investigate the flow analysis and find the wake flow around the body. Ahmad body as ground vehicle is commonly used as test case in industry today. The present study is attempted to impliment a computational study of the flow over a two-dimensional car model Reynold\'s Average Navier-Stokes (RANS) with a combination of most popular k-epsilon turbulence model produces sufficiently accurate results. Working fluid is \'Air\'.
Q.2. Explain the reason for the negative pressure in the wake region.
ANS.:- Pressure gradient is an one of the factors that influences a flow immensely. It is easy to see that the shear stress caused by viscosity has a retarding effect upon the flow. This effect can however be overcaome if there is a negative pressure gradient is termed a favourable pressure gradient. Such a gradient enables the flow. A positive pressure gradient has the opposite effect and is termed the Adverse pressure gradient. Fluid might find it difficult to negotiate an adverse pressure gradient. Sometimes, we say the fluid has to climb the pressure hill.
One of the vevere effects of an adverse pressure gradient is to separate the flow. consider flow past a curved surface as shown in above figure. The geometry of the surface is such that we have a favourable gradient in pressure to start with and upto a point \'p\'. The negative pressure gradient will counteract the retarding effect of the shear stress (which is due to viscosity) in the boundary layer. For the geometry considered we have an adverse pressure gradient downstream of \'p\'.
Now, the adverse pressure gradient begins to retard. This effect is felt more strongly in the region close to the wall where the momentum is lower than in the region near the free stream. As indicated in the figure, the velocity near the wall reduces and the boundary layer thickness. A contineous retardation of flow brings the wall shear stress become negative and the flow reverses and a region of recircuilating flow developes. We see that the flow no longer follows the contour of the body. We see that the flow no longer follows the contour of the body. We say that the flow has separated. The point \'s\' where the shear stress is zero is called the point of separation.
Depending on the flow conditions the recircuilating flow terminate and the flow may become reattached to a body. A separation bubble is formed. There are a veriety of factors that could influence this reattachment. The pressure gradient may be now initially laminar may undergo transition within the bubble and may become turbulent. A turbulent flow has more energy and momentum than a laminar flow. This can kill separation and flow may reattached. A short bubble may not be of much consequence.
On aerofoils sometimes the separation occurs near the leading edge and gives rise to a short bubble. What can be dengerous is the separation occuring more towards the trailing edge and the flow not reattaching. In this situation the seperated region merges with the wake and may result in stall of the aerofoils (loss or lift).
Q.3. Explain the significance of the point of seperation.
ANS.:- There are three main reasons why we need to point of seperation. They are-
1) To remove unwanted particle :-
- A bhel is a mixture of various ingradients like onion, potato, chillies..etc. Someone who does not like spicy food will remove the chillies to prevent mouth burn. Therefore, seperation is necessary to remove the chillies from food.
- Our mother always removes stone from rice before the cooks. Because stones are not wanted. Seperation is necessary to remove the stones.
- Also, our mother removes the tea leaves from the using a stainer. We can not drink tea with tea leaves. Hence, they are seperated.
- Air also contained dirt and dust. We have hair in our nostrils which seperate the dust and dirt particles from the air.
2) To obtain important substances :-
Some substances are more important than other in a mixture. Therefore, these need tobe obtained by seperation of substances.
- We all love butter, we spread it on a bread and eat it. It is used it for cooking. Butter is prepared from milk. The milk is churned at a high speed which creates a froth up. This froth is actually butter. The oil droplets are collected. Thus butter is seperated from milk.
- Crude oil is raw oil. It is mixture of many oils. It has petrol, diesel, kerosene, parrafin wax. But it is also cantains impurities. We all know petrol and diesel are used in vehicles. Vehicle can not run without them. Thus, they have tobe seperated from the crude oil.
3) To obtain pure substances :-
- A mixture is actually made up of pure substances only. But they get mixed with each other in a mixture. Sometimes the individual pure substances are more important than the mixture. e.g. Salt water is not very important to us. But salt and water individually are very important to us. Hence, we need to seperate both from each other. Thus, seperation of substances is necessary to obtain pure subsatnce for domestic purposes, industrial purposes and research work.
- Even the example used earlier of gasses present in air is also an example of obtaining pure substances. We can obtain oxygen in uts pure form for many reasons.
So, as we read, point of seperation is very important in our day to day life.
Seperation point is the position at which the boundary layer leaves the surface of a solid body. It may occure with the laminar flow (laminar seperation) or when the boundary layer is turbulent (turbulent seperation). Seperation is sometimes followed by reattachments of the flow.
# Split body command in spaceclaim are as follows-
For this I use plane along z-axis in front face and use the split body command and escape command on keyboard in design menu are as follows-
RESULTS :-
In my case \'Multizone method\' I shouldn\'t use due to some critical error. So, I just use face and body sizing commands.
Inside the domain there have multiple body, so I use just inner Ahmad body and outer boundary i.e. inlet, outlet, symmetry and bottom wall. By selecting both inner and outer boundary I put different elements size for grid dependency test.
For next sizing command I use face body selection tool for selecting four pins from bottom wall because that was in hexahedral shape but I want circuilar shape so i select that and give the size of 5mm.
In Mesh I use maximum 5 layers and total sizing 12.5mm by including 1.2 times growth rate.
In this way, I use 3 different element size for performing grid indipendency test and provide the values of drag and lift with each case. And for this drag coefficient I maintain y+ appropriate range.
Also, I plot velocity and pressure conours, drag and lift coefficient plots, vector plots which clearly showing the wake region and more respected plots as well for all three cases and also I plot some extra plots for given condition which are also describe in below side.
Steady Density based solver was been used with Implicit solution method because drag coefficient is more than 0.3 (given).
Commen geometry for all cases are as follows-
Boundary conditions :-
At inlet,
velocity of air (fluid) = 25 m/s ......(given)
Temperature of air = 300k ......(room temperature)
CASE-1 :-
Inner domain element size = 200mm
Cell count at each elements = 83491
Cell count at each nodes = 24197
1. Mesh :-
2. Drag Coefficient :-
3. Lift coefficient :-
4. Vector plane :-
5. Scaled residuals :-
6. Pressure Car wall :-
7. Velocity conours at cut plane z :- At points are (0, 0, 0) and (0, 0, 1).
8. Velocity plane :-
9. Line-1 :- At points (2, 0, 0) and (2, 2, 0).
10. Line-2 :- At ponts (3, 0, 0) and (3, 2, 0).
11. Velocity chart :-
12. Post data for CFD :-
[Name]
Series 1 at /LINE:Line 1
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
6.38E-03 7.15E+00
2.65E-02 9.95E+00
9.58E-02 1.98E+01
1.73E-01 2.19E+01
2.24E-01 2.36E+01
2.52E-01 2.42E+01
3.21E-01 2.48E+01
3.71E-01 2.53E+01
4.18E-01 2.54E+01
4.26E-01 2.54E+01
4.57E-01 2.55E+01
5.23E-01 2.56E+01
6.54E-01 2.56E+01
6.84E-01 2.56E+01
7.16E-01 2.56E+01
7.17E-01 2.56E+01
7.17E-01 2.56E+01
8.14E-01 2.57E+01
8.99E-01 2.57E+01
9.27E-01 2.57E+01
1.04E+00 2.57E+01
1.06E+00 2.57E+01
1.09E+00 2.57E+01
1.14E+00 2.57E+01
1.29E+00 2.57E+01
[Name]
Series 2 at /LINE:Line 2
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
-2.86E-02 3.23E+00
6.75E-02 1.78E+01
9.40E-02 2.15E+01
2.28E-01 2.44E+01
2.33E-01 2.45E+01
2.37E-01 2.46E+01
2.61E-01 2.49E+01
2.68E-01 2.49E+01
3.30E-01 2.52E+01
4.27E-01 2.56E+01
5.06E-01 2.56E+01
5.36E-01 2.56E+01
5.73E-01 2.56E+01
5.75E-01 2.56E+01
6.19E-01 2.56E+01
6.59E-01 2.56E+01
7.37E-01 2.56E+01
7.88E-01 2.56E+01
8.24E-01 2.56E+01
8.89E-01 2.56E+01
9.38E-01 2.56E+01
9.89E-01 2.56E+01
1.08E+00 2.56E+01
1.09E+00 2.56E+01
1.10E+00 2.56E+01
1.14E+00 2.56E+01
1.15E+00 2.56E+01
1.22E+00 2.56E+01
1.29E+00 2.56E+01
13. Case-1 report :-
CASE-2 :-
Inner domain element size = 100mm
Cell count at each element = 96962
Cell count at each Nodes = 19091
1. Mesh :-
2. Drag coefficient :-
3. Lift coefficient :-
4. Vector plane :-
5. Scaled residuals :-
6. Pressure car wall :-
7. Velocity contours at cut plane z :- At points are (0, 0, 0) and (0, 0, 1).
8. Velocity Plane :-
9. Line-1 :- At points (2, 0, 0) and (2, 2, 0).
10. Line-2 :- At points (3, 0, 0) and (3, 2, 0).
11. Velocity chart :-
12. Post data for CFD :-
[Name]
Series 1 at /LINE:Line 1
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
2.84E-02 1.81E+01
3.04E-02 1.85E+01
3.58E-02 1.99E+01
3.92E-02 1.99E+01
4.11E-02 2.00E+01
1.58E-01 2.35E+01
1.64E-01 2.37E+01
1.70E-01 2.39E+01
1.75E-01 2.40E+01
1.97E-01 2.43E+01
2.63E-01 2.52E+01
2.94E-01 2.54E+01
2.96E-01 2.55E+01
3.63E-01 2.58E+01
3.86E-01 2.59E+01
4.10E-01 2.60E+01
4.64E-01 2.61E+01
4.78E-01 2.61E+01
4.84E-01 2.61E+01
5.11E-01 2.61E+01
5.59E-01 2.61E+01
5.64E-01 2.61E+01
5.65E-01 2.61E+01
6.14E-01 2.61E+01
6.45E-01 2.61E+01
6.88E-01 2.61E+01
6.89E-01 2.61E+01
6.92E-01 2.61E+01
7.21E-01 2.62E+01
7.53E-01 2.62E+01
7.92E-01 2.62E+01
8.01E-01 2.62E+01
8.24E-01 2.62E+01
8.32E-01 2.62E+01
8.61E-01 2.62E+01
8.88E-01 2.62E+01
9.21E-01 2.62E+01
9.25E-01 2.62E+01
9.68E-01 2.62E+01
1.01E+00 2.62E+01
1.05E+00 2.62E+01
1.06E+00 2.62E+01
1.06E+00 2.62E+01
1.10E+00 2.63E+01
1.12E+00 2.63E+01
1.14E+00 2.63E+01
1.18E+00 2.63E+01
1.21E+00 2.63E+01
1.21E+00 2.63E+01
1.21E+00 2.63E+01
1.24E+00 2.63E+01
1.26E+00 2.63E+01
1.29E+00 2.63E+01
[Name]
Series 2 at /LINE:Line 2
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
-3.34E-02 5.16E+00
-2.76E-02 6.94E+00
-1.91E-02 9.48E+00
2.70E-02 2.29E+01
6.27E-02 2.34E+01
9.07E-02 2.37E+01
1.04E-01 2.38E+01
1.11E-01 2.39E+01
1.17E-01 2.40E+01
1.26E-01 2.41E+01
2.01E-01 2.51E+01
2.10E-01 2.53E+01
2.14E-01 2.54E+01
2.28E-01 2.55E+01
2.73E-01 2.56E+01
2.90E-01 2.56E+01
3.37E-01 2.60E+01
3.44E-01 2.60E+01
3.80E-01 2.61E+01
3.92E-01 2.61E+01
4.27E-01 2.62E+01
4.42E-01 2.62E+01
4.72E-01 2.62E+01
5.03E-01 2.62E+01
5.19E-01 2.62E+01
5.43E-01 2.62E+01
5.45E-01 2.62E+01
5.89E-01 2.61E+01
6.22E-01 2.61E+01
6.41E-01 2.61E+01
6.74E-01 2.61E+01
6.77E-01 2.61E+01
7.05E-01 2.61E+01
7.55E-01 2.61E+01
7.90E-01 2.61E+01
8.13E-01 2.61E+01
8.34E-01 2.61E+01
8.62E-01 2.61E+01
8.93E-01 2.61E+01
9.14E-01 2.61E+01
9.35E-01 2.61E+01
9.51E-01 2.61E+01
9.70E-01 2.61E+01
9.94E-01 2.61E+01
1.05E+00 2.61E+01
1.06E+00 2.61E+01
1.09E+00 2.61E+01
1.10E+00 2.61E+01
1.14E+00 2.61E+01
1.16E+00 2.61E+01
1.21E+00 2.61E+01
1.22E+00 2.61E+01
1.22E+00 2.61E+01
1.25E+00 2.61E+01
1.28E+00 2.61E+01
1.29E+00 2.61E+01
13. Case-2 report :-
a)
b)
CASE-3 :-
Inner domain element size = 300mm
cell count at each elements = 16258
Cell count at each nodes = 3736
1. Mesh :-
2. Drag coefficient :-
3. Lift coefficient :-
4. Vector plane :-
5. Scaled residuals :-
6. Pressure car wall :-
7. Velocity contours at cut plane z :- At points (0, 0, 0) and (0, 0, 1).
8. Velocity plane :-
9. Line-1 :- At points (2, 0, 0) and (2, 2, 0).
10. Line-2 :- At point (3, 0, 0) and (3, 2, 0).
11. Velocity chart :-
12. Post data for CFD :-
[Name]
Series 1 at /LINE:Line 1
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
4.33E-02 3.41E+01
1.29E-01 6.53E+01
1.58E-01 8.00E+01
1.65E-01 8.21E+01
1.98E-01 8.43E+01
2.01E-01 8.44E+01
3.89E-01 1.02E+02
4.75E-01 1.16E+02
5.88E-01 1.47E+02
6.33E-01 1.61E+02
6.76E-01 1.71E+02
7.69E-01 1.92E+02
8.18E-01 2.03E+02
8.43E-01 2.09E+02
1.02E+00 2.49E+02
1.11E+00 2.60E+02
1.12E+00 2.61E+02
1.29E+00 2.73E+02
[Name]
Series 2 at /LINE:Line 2
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
1.19E-01 1.21E+02
1.30E-01 1.29E+02
1.90E-01 1.73E+02
2.12E-01 1.73E+02
4.31E-01 1.71E+02
4.52E-01 1.71E+02
5.47E-01 1.78E+02
5.57E-01 1.79E+02
7.43E-01 1.97E+02
7.67E-01 1.98E+02
8.34E-01 1.96E+02
9.87E-01 2.04E+02
1.04E+00 2.08E+02
1.04E+00 2.08E+02
1.05E+00 2.08E+02
1.10E+00 2.10E+02
1.10E+00 2.11E+02
1.29E+00 2.26E+02
1
13. Case-3 report :-
a)
b)
Some other refined cases are as follows-
1. Mesh with elements size :-
2. Mesh with face size :-
3. Mesh without multizones :-
Also, some commen results for the Ahmad body are as follows-
1. Velocity counters at all boundary :-
2. Velocity vectors by static pressure :-
3. Pressure vectors by static pressure :-
4. Pathline velocity :-
5. Cummulative force vs distance at point (1,0,0) :-
6. Drag coefficient :-
7. Lift coefficient :-
8. scaled residuals :-
9. Histogral velocity magnitude :-
10. Static pressure vs position :-
11. Velocity chart :- At points, line-1 :-a) (2, 0, 0) and (2, 2, 0) b) (3, 0, 0) and (3, 2, 0).
12. Converged solution :-
13. Post data for CFD at each point :-
[Name]
Series 1 at /LINE:Line 1
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
1.37E-01 1.57E+01
2.09E-01 2.26E+01
2.23E-01 2.36E+01
2.65E-01 2.41E+01
2.75E-01 2.41E+01
4.85E-01 2.57E+01
5.72E-01 2.57E+01
5.78E-01 2.57E+01
7.23E-01 2.58E+01
8.76E-01 2.59E+01
9.28E-01 2.59E+01
1.01E+00 2.59E+01
1.11E+00 2.59E+01
1.29E+00 2.60E+01
[Name]
Series 2 at /LINE:Line 2
[Data]
Y [ m ] Velocity u [ m s^-1 ]
-5.00E-02 0.00E+00
-4.98E-02 2.38E-02
8.40E-02 8.69E+00
1.85E-01 1.34E+01
3.59E-01 2.48E+01
4.77E-01 2.53E+01
5.00E-01 2.54E+01
6.28E-01 2.55E+01
7.72E-01 2.57E+01
7.87E-01 2.57E+01
9.61E-01 2.57E+01
1.17E+00 2.57E+01
1.29E+00 2.57E+01
COMMENTS ON THE RESULTS :-
The grid independency test allows us to see the smoother velocity profile, as we make the mesh more finer i.e. when we increase the cell counts (closer to the limit), we can see from the vector plots (from all the three cases) of the velocity, that it becomes more and more smoother and we get more close to the accurate results.
Other things noticed that, making the fresh finer allows us to see the formation of the wake region more clearly.
Therefore clear take away is that as we refine the mesh we can get accurate results.
# More details on, why the pressure goes negative ?
As from the above three cases, from the pressure plot on the car wall, we can see there are some areas where pressure is negative, areas like the cylindrical stands of the car.
There can be many reasons for such negative pressure.
1. Velocity increases when it flow overs the ahmad body, so in order to maintain the flow pressure becomes negative. (simple case of law of conservation of energy).
2. Flow seperation takes place when the fluid hits the ahmad body, which leads to the formation of the air bubbles which are at very low pressure.
3. Pressure could also becomes negative in order to over come the shear stress.
4. Other reasons could be due to flow recirculation.
CONCLUSION :-
From the above grid independency test and other results we can conclude that as number of body elements increases we get coarse mesh and rear vector plot, but as number of body elements decreases we get finer mesh and vice versa. For this conclusion I proove everythings in this project. Ahmad body is the very basic and useful body of finding analysis about any automobile and aerofoil industry.
In this way I successfully completed this whole project and anlyse everythings from the plots and using different cases.