Week 3 - External flow simulation over an Ahmed body.
Aim: Import Ahmed body model to the Ansys fluent work bench perform drag co-efficient simulation for different mesh independent cases and plot the drag co-efficient graph and also take out the results of drag coefficient for three mesh independent cases
Theory:
- Describe Ahmed's body and its importance
Answer:- Ahmed body is a generic car body(a simplified car model) i.e the flow of air around the ahmed body captures the essential flow features around an automobile.
2. Explain the reason for the negative pressure in the wake region.
Answer:- Because in the wake region there is no air flow velocity, Due to this vaccum pressure forms in the wake region this we called negative pressure, There is no air flow velocity in the wake region because due to high velocity air flow over the body, Body wall reacts high viscous in nature due to this back side of body wake region formation takes place
3.Explain the significance of the point of separation.
Answer:-The separation point is the point where the air stops "sticking" to an object that is moving through the air, By this the drag for the car reduces
Procedure to simulate the ahmed body:
- First we importing the ahmed body in space claim geometry by drag the file and pull into the sketch
- After in above tool bar under prepare section enclosure option selecting and unenabling the symmetry option giving the dimension we creating the enclosure to the ahmed body with two enclosure and we supress the ahmed body
- two enclosure because to reduce the number of mesh element, This two enclosure is attached below in image1
- After the creating enclosure we split the body by using split option which option is there under prepare section by selecting plane like which make equal half we split the body, This spliting body is attached in image1
- After spliting the body we do the interference, we do interference because if we donot do the interference the air doesnot flow inside the enclosure of the inner enclosure
- After doing the interference we do the topo share this we do because to match the outer enclosure mesh to the inner enclosure mesh
- Then doing this all geometry cleanup we open the mesh window and performing mesh by giving some size based on problem statement, For to reduce mesh size of inner enclosure we use sizing option which is there when right click on the 'mesh' sizing option we can see by selecting that we give mesh to the inner enclosure but before that we hiding the outer enclosure by using face option we selecting outer enclosure and click the F8 button, Similarly for ahmed body foot using sizing option by selecting foot face we giving the mesh size over there Then last we generating the mesh
- Then we give the naming for inlet, outlet, symmetry and carwall this we do by selecting face and clicking shortcut key 'N' option
- Then next to the name selection we giving the inflation layer by calculating Yplus and giving this Yplus value as firstlayer thickness in the option and selcting car wall we create the inflation to the car wall
- So like this we meshing with different sizing we studing the three cases of simulation, This mesh and mesh quality is shown in the image2
- After the mesh to run the simulation we open the setup window, In set up window under viscous option we selecting the k-epsilon model and enhance wall treatment option
- And we take out the drag and lift graph sheet from under the report definition option selecting
- Next we giving the boundary condition to the inlet and outlet, Then in reference value we given where flow to compute from and in reference value we giving the frontal area value of the ahmed body and the length value
- next we initializing the problem and run the simulation for the particular iteration where the solution is get to converge
- Lastly we taking out the results from solution and from post solution, The final result is shown in the image3, image4, image5
- Note: To take yplus value in report option we open the surface integral task page in that report type we select average weightage area option, In field variable we select the turbulence option and the wall yplus option and last we selecting the body of what body yplus we want
Result in the images:
Image1: enclosure model and final split body model
image2: mesh of the three cases
1case:- 250mm mesh of outer enclosure, 100mm mesh of Inner enclosure, 2mm mesh of ahmed body foot and with 0.4mm first layer thickness inflation layers
2case:- 100mm mesh of outer enclosure, 50mm mesh of Inner enclosure, 2mm mesh of ahmed body foot and with 0.4mm first layer thickness inflation layers
3case:- 70mm mesh of outer enclosure, 40mm mesh of Inner enclosure, 2mm mesh of ahmed body foot and with 0.4mm first layer thickness inflation layers
image3: scaled residual plot, drag coefficient plot, lift coefficient for three cases
1case:-
scaled residual drag coefficient plot
lift coefficient plot
2case:-
scaled residual drag coefficient plot
lift coefficient plot
3case:-
scaled residual drag coefficient plot
lift coefficient plot
image4: results on console of three case
1case:-
2case:-
3case:-
image5: contour plots for the three cases
1case:-
velocity contour pressure contour
2case:-
velocity contour pressure contour
3case:-
velocity contour pressure contour
Validation :
- By looking above image of three case study third case study gives more accurate result than first two case
- By this we conclude finer the mesh more the accurate result
Conclusion: So by following above procedure the simulation run successfully