Week 3 - External flow simulation over an Ahmed body.

1.Objective:

To do external flow over the Ahmed body.

Q1.1 Ahmed Body

• The Ahmed body was described originally by S. R. Ahmed in 1984
• The Ahmed body is a simplified vehicle model. The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed.
• The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features.

Fig: Ahmed Body Dimension

• The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters.

Q.1.2 Importance of Ahmed body

• The Ahmed body is easy to model, it can be effortlessly utilized for researching various properties like turbulence, drag coefficient, wake region, lift forces, velocity magnitude at various regions of the car, magnitude of pressure around the car which helps in determining what will be resistance, fuel.
• It has become a benchmark of the CFD simulation of the famous external aerodynamic test case.

Q2.The reason for the negative pressure in the wake region.

• The flow around a road vehicle is a rather complex phenomena that includes massive separation and development of a wake. The boundary layer on the surface of the body eventually separates as it reaches the rear edge and the separated flow experiences high shear and forms a wake. The aerodynamic resistive force that a road vehicle experiences is mostly due to pressure which means that there is a large pressure difference at the front and the rear of the body. The lowpressure region at the rear is due to the existence of a wake which is made up with multiple length and time scales and is inherently unsteady and three dimensional. These flow features arise due to the bluffness of the geometry and this bluffness is associated with comfort and functionality which cannot be negated. illustrates the common flow features associated with road vehicle aerodynamics.

Fig: Negative pressure at wake region

Q3.The significance of the point of separation.

8. Inflation sizing:

This option is used to accurately capture the boundary layers on the surface of the body.

First layer thickness=0.005m

Incremental ratio = 1.2 times of previous layer (or) 20% of previous layer

Total layer and Total thickness in this problem = 5 layers and 0.0005m

Fig: Inflation sizing

Fig: Inflation layers

Mesh

Fig:Final mesh

Fig: Diamond meshing of Inner enclosure

2.3 Setup and solution

Fig: Mesh

9.Exit mesh and open setup. First, Check mesh, it is throwing no error then only we can proceed further. We are going to do Density based steady state simulation, So choose these options on task page.

Fig: Task page

10. Go to physics menu and select viscous model. It will open a menu, choose realizable K-omega model and close it.

Reynolds Number:

Fig: Viscous model parameters

11.Click on material and choose air as working fluid. check working fluid by clicking on cell zone conditions option, if it is air then proceed.

Fig: Assigning Mateials

12.Now go to boundaries option to set boundary conditions. Select inlet x and click on option edit. It will open a menu, Where it is thermal radiation, momentum etc. Put the value of velocity as 25m/s and go to thermal parameter to set temperature as 300k

13.We want to generate temperature report of outer layer of car body velocity

14.Now click on intialization to calculate approximate values of velocity and pressure.

Fig: velocity inlet

15.Change the number of iteration 200 and click on calculate.

2.4 Results

In this Ahmed body is helps to know the outside of the car body velocity, negative pressure at wake region, drag and lift. We used two different turbalent solvers for it first was realized k-epsilon(initial mesh) and second was k-omega(Finner mesh) SST.

Residual

Drag co-efficient of car body

Lift co-efficient of car body

Pressure contour

Velocity contour

vector contour

zoom the wake zone (vector contour)

futhur zoom the wake zone (vector contour)

Mesh with velocity contour

Mesh with velocity contour

velocity contour

velocity contour with mesh of car outer body

zoom the wake zone(velocity contour)

Pressure contour

zoom the wake zone(pressure contour)

Chart of line 1 & 2 velocity

4.Grid Dependancy:

A Grid Dependance Test is used to as certain whether the final values of the solution are dependent on the mesh or not. This is an important test to ensure the simulation results are correct.

	Mesh quality(m)	Body sizing(m)	Edge sizing(m)	Number of nodes	Number of elements
1st Refindment	0.08	0.045	0.005	74165	223039
2nd Refindment	0.075	0.045	0.005	76961	225343
3rd Refindment	0.08	0.04	0.005	87758	295141
4th Refindment	0.08	0.04	0.004	89097	299319
5th Refindment	0.075	0.040	0.004	91908	301331
6th Refindment	0.070	0.040	0.004	100285	308994
7th Refindment	0.070	0.035	0.0035	122651	429386

5.Conclusion:

• The velocity decreases at the front of the Ahmed body because of the impact of the fluid with the body due to which the pressure increases.
• Due to the impact, there is an initial acceleration of the flow on the top, followed by which the velocity decreases gradually.The pressure decreases in the regions where velocity increases.
• The flow once again speeds up because of the sudden change in geometry at the end of the top surface, which results in the separation of flow and the formation of the wake.
• This wake formation leads to a significant reduction in pressure at the region of the Ahmed body as compared to the front the region, where pressure is higher due to the impact of flow on the body, and this results in the generation of a drag force.