Week 3 - External flow simulation over an Ahmed body.

Aim:

Our aim is to perform fluent flow analysis on Ahemd body and explain reseaon for negative pressure in wake region also the significance of the point of separation. 

Objective:

As for the objective its said to provide:

1. Velocity and pressure contours. 

2. The drag coefficient plot for a refined case. ( For velocity of 25m/sec, the drag coefficient should be around 0.33).

3. The vector plot clearly showing the wake region. 

4. Perform the grid independency test and provide the values of drag and lift with each case. 

Body of content:

1. Ahemed body:

Ahemed body is nothing but a simulation tool use study airflow over an automobile, it was made by SR ahemed. By studying analysis on ahemed body one can also learn the flow over any simulation on automobile.

2. Negative pressure in wake region:

before going with negative pressure in wake region lets first understand what is wake region, wake region is the region of disturbuted flow of turbulent downstream of a solid body moving through fluid which is also caused by fluid around body, negative pressure wake region is nothing but the pressure drag generated by wake region.

3. Significance of point of separation:

Point of seperation is nothing but the point of the body at which the boundary layer is on the verge of sepration from the surface.

 Preprocessing:

Here first we load the geometry in spaceclaim and start with geometry clean up, here we also provide the geometry with encloser and also use the split comand at the plane from Y, here is a step by step explation in below figure.

Than we move to next step that is meshing and named selection of our ahmed body, as for the base line simulation we go with normal mesh and we give the named as inlet, outlet, symmetry-wall and car-wall. as shown in below figure.

Postprocesing:

Now for post procesing we open the module in setup and assign the physics for that, then we give the reference value and the material property which is air we select the viscous model as k-epsilon for the analysis and for intialsation we go with hybrid intialization, as shown in below figure.

Now in postprocsing we also created cut plane at z to see the veloctiy and pressure change along ahemd body , in report definiton we go with lift coefficient and drag coefficient in our analysis. As shown in below figure.

Result:

Now after that we perform a base line setup to analyse the flow over ahemd body, the below is the result and plot of ahemed body base line simulation.

Residual plot:

Coefficient of drag:

Coefficient of lift:

Velocity contour:

Pressure contour:

Vector plot near the wake region:

Below table shows about the drag and lift coefficiet.

Drag coefficient	112.24981
lift coefficient	66.593598

As we perform only baseline simulation the result is not acurate from what we expected so in order to achieve preciese result we are going to simulate our ahemed body with more dense meshing so that it can capture more geometry and give us a acurate result.

1. Performing meshing of 100mm on ahemed body.

Here we perform multizone mesh. we have give the 100mm mesh to our ahemd body and for internal enclousre body we have given the 80mm body sizing to our mesh, we have given the 5 inflation layer of 5mm thickness over ahemed body so that we can capture acturate drag over the body. below shown figure also contain the statistics of meshing nodes and element.

The result generat from the above meshing creteria is shown in below.

Residual plot:

Coefficient of drag:

Coefficient of lift:

Velocity contour:

Pressure contour:

Vector plot near the wake region:

Below table shows about the drag and lift coefficiet.

Drag coefficient	0.34157707
lift coefficient	0.24906017

2. Performing meshing of 90mm on ahemed body.

 Here we perform multizone mesh. we have give the 90mm mesh to our ahemd body and for internal enclousre body we have given the 50mm body sizing to our mesh, we have given the 5 inflation layer of 4 mm thickness over ahemed body so that we can capture acturate drag over the body. below shown figure also contain the statistics of meshing nodes and element.

The result generat from the above meshing creteria is shown in below.

Residual plot:

Coefficient of drag:

Coefficient of lift:

Velocity contour:

Pressure contour:

Vector plot near the wake region:

Below table shows about the drag and lift coefficiet.

Drag coefficient	0.33029258
lift coefficient	0.25308573

3. Performing meshing of 80mm on ahemed body.

 Here we perform multizone mesh. we have give the 80mm mesh to our ahemd body and for internal enclousre body we have given the 40mm body sizing to our mesh, we have given the 5 inflation layer of 2 mm thickness over ahemed body so that we can capture acturate drag over the body. below shown figure also contain the statistics of meshing nodes and element.

The result generat from the above meshing creteria is shown in below.

Residual plot:

Coefficient of drag:

Coefficient of lift:

Velocity contour:

Pressure contour:

Vector plot near the wake region:

Below table shows about the drag and lift coefficiet.

Drag coefficient	0.29608445
lift coefficient	0.23881753

Now if we compare all the meshing cretria with the drag and lifit cofficient and perform grid independence test we found the following result which is shown in below table.

Grid independence test	Elements	Coefficient of lift	Coefficient of drag
Baseline setup	84443	112.24981	66.593598
100 mm mesh	73267	0.24906017	0.34157707
90 mm mesh	155719	0.25308573	0.33029258
80 mm mesh	338432	0.23881753	0.29608445

Conclusion:

As we can observe from the grid independence study, that the more we refine our mesh the colser we get to the acurate result. the maine conclusion of these analysis is to see how in general the drag and lift coefficient can be calculated on a automobile, also if the body is symmetry to reduce the computional load we can perform the analysis on the half body to get the desire result.