Ahmed body Mesh independence study
Ahmed body simulation: Grid dependence study
Importance of Ahmed body:
Ahmed body captures the flow features around the body which was first characterized and experimented by Ahmed. The Ahmed body is used to validate the simulation becuase it is simpler to design and it roughly represents a car model.
Ahmed body is a simple geometrical representation of a car which is used to validate the simulation tool. Since the experimental data is available it can be used to validate the code or software that is used for simulation. Once the numerical code is validated a new model is used for simulation.
Steady state simulation:
The geomtry was prepared by setting up enclosure which will act as a wind tunnel in the simulation. Then a second enclosure was provided to further refine the mesh locally. To check the grid independency of the mesh 3 cases were tested in which the mesh was refined for local and global scales.
The preparation of geometry is shown below:
FLUENT solver was used to simulate the ahmed body models. Density based solver is used to simulate the Ahmed body. since the velocity of air inside the wind tunnel is 55m/s and mach number is 0.16, therefore there can be changes in density when the wind hits the ahmed body. Then the velocity contour and pressure contour were used to the determine the characteristics of the flow.
For post-processing the results the vector plot for pressure and velocity was plotted. For further study of characteristics at specfic portions of the domain vertical and horizontal lines were drawn. Then, the effects of velocity and pressure were studied and plotted for these specific portions in the domain.
The picture given show the vertical and horizontal lines drawn for characteristics study:
Meshing:
In meshing as stated two enclosures were created. In the outer enclosure, Multizoned meshing technique were used in which the mesh in the outer zone is converted into Hexa mesh which is the most accurate mesh. For inner enclosure tetrahedral mesh is used. Then inflation layers are setup near the ahmed body to capture the boundary layer effects accurately and an uniform mesh is created around ahmed body. Inner enclosure was further refined to capture the flow more accurately in that region.
FLUENT solver conditions:
Boundary conditions:
Inlet velocity: 55 m/s
Tempreature: 300 K
Results:
The simulation was done for different sizes of mesh to check mesh dependency.
Case-1:
Domain size : 100 mm
Cell count: 213676
Mesh:
Velocity Contour:
Pressure Contour:
Velocity graph w.r.t change in magnitude in X-direction ( X= 0 to 2.5 m, Y=0.3 m):
Velocity graph w.r.t change in magnitude in Y-direction for different locations of X ( Y= 0 to 0.5 m, X1=0, X2= 0.5m, X3=1m, X4=1.5m):
Pressure graph w.r.t change in magnitude in X-direction ( X= 0 to 2.5 m, Y=0.3 m):
Pressure graph w.r.t change in magnitude in Y-direction for different locations of X ( Y= 0 to 0.5 m, X1=0, X2= 0.5m, X3=1m, X4=1.5m):
Velocity vector Plot:
Case 2:
Domain size : 40mm
Cell count: 410192
Velocity Contour:
Pressure Contour:
Velocity graph w.r.t change in magnitude in X-direction ( X= 0 to 2.5 m, Y=0.3 m):
Velocity graph w.r.t change in magnitude in Y-direction for different locations of X ( Y= 0 to 0.5 m, X1=0, X2= 0.5m, X3=1m, X4=1.5m):
Pressure graph w.r.t change in magnitude in X-direction ( X= 0 to 2.5 m, Y=0.3 m):
Pressure graph w.r.t change in magnitude in Y-direction for different locations of X ( Y= 0 to 0.5 m, X1=0, X2= 0.5m, X3=1m, X4=1.5m):
Velocity vector Plot:
Case 3:
Domain size : 37mm
Cell count: 520973
Velocity Contour:
Pressure Contour:
Velocity graph w.r.t change in magnitude in X-direction ( X= 0 to 2.5 m, Y=0.3 m):
Velocity graph w.r.t change in magnitude in Y-direction for different locations of X ( Y= 0 to 0.5 m, X1=0, X2= 0.5m, X3=1m, X4=1.5m):
Pressure graph w.r.t change in magnitude in X-direction ( X= 0 to 2.5 m, Y=0.3 m):
Pressure graph w.r.t change in magnitude in Y-direction for different locations of X ( Y= 0 to 0.5 m, X1=0, X2= 0.5m, X3=1m, X4=1.5m):
Velocity vector Plot:
Results and discussion:
It is observed that as we refine or increase the no of cells in the mesh the vector velocity plot becomes finer and finer. Also, a refined mesh also makes the wake region smoother. In the 3 cases we can see the vector and contour plots have become smoother with little deviation in results.
Explain the variation in pressure as to why it goes negative:
It is seen in the pressure graph in y- direction for series 1 (x=0 y=0 to 0.5m) the pressure suddenly rises, it is becuase the air is getting compressed when it comes into contact with the ahmed body. Then as we move towards the right side of ahmed body the pressure gets released sudden due to which a vaccum region is formed and the velocity of the gets increased. This increase in velocity phenomena is observed in Velocity graph in x- direction( x=0 to 2.5m, y=0.3m).
Due to this sudden compression and expansion of waves a weak shock wave is generated due to which the pressure goes negative or a vaccum is created.