Ahmed Body Challenge
AIM: To describe the importance of Ahmed body and to explain the significance of point of separation.
To obtain the Pressure and Velocity contours and to plot the velocity profile which show the wake region.
To perform the grid independency test and provide the values of drag and lift with each case.
Theoritical background :
Ahmed body:
It is a benchmark model widely used in the automotive industry for validating simulation tools. It's shape is simple enough to model, while maintaining car-like geometry features. The Ahmed body is a generic car body (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. Ahmed body is considered as the base problem for the analysis of flow dynamics for all the four-wheel vehicles using computational fluid dynamics (CFD) both experimentally and numerically. The aerodynamic drag force is an important parameter to analyze the performance of the vehicle, and the automotive industry always tries to reduce the drag force to improve fuel efficiency. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.
The Ahmed body is made up of a round front part, a moveable slant plane placed at back part of the body to study the coefficient of drag and separation phenomena at different angles, and a rectangular box, whichconnects the front part and the rear slant plane. As the wake separation flow behind the Ahmed body is the main contributor to the drag force, accurate prediction of the separation process and the wake flow are the key to thesuccessful modelling of this case.
The external surface of the car is very important in aerodynamics aspects of vehicles such as stability, comfort and fuel consumption at high speeds. The total weight and modification of the external surfaces of the car are the most important to improve the aerodynamics characteristics of the car. These characteristics are helpful to improve the cruising conditions, greater stability of navigation, and lower fuel consumption.
Ahmed body Dimensions
Computational Domain for Fluid flow simulation
We want the flow to be simulated outside the Ahmed body, that is where the mesh needs to be created
General Case :
Meshing:
Residuals:
Drag plot:
Lift plot:
Results of Drag and Lift plot
Velocity Contour:
Pressure Contour:
Y plus plot:
Velocity contour with Vector:
Velocity plot:
Pressure plot
Grid Dependency test
A CFD Solution can never be trusted unless we check whether the result depends on the grid or not. or else we have any experimental data. The output result of a coarser mesh and finer mesh can neither be the same. So up to what extent, we need to vary the mesh to get the accepted level of tolerance, can be found out from Grid Independence test.
This is done by varying the mesh size from coarse to fine and checking the output result for each mesh. When varying the mesh does not affect the result much then we can stop and select that minimum mesh size for our final solution output.
In this case we are comparing the Geometry with 3 different mesh sizes
Case 1:
The above figure shows Car wall with inflation layers
The above figure shows the hatch lines of big and small enclosure
MESHING
The above figure shows the sectional view of the mesh
Residual plot:
Drag plot: Lift plot
Results of drag and lift plot
Velocity Contour: Pressure Contour:
Velocity contour with Vector:
Velocity plot:
Pressure plot:
Y + plot
Case 2:
Residuals plot:
Drag plot: Lift plot:
Results of Drag and Lift plot
Velocity Contour: Pressure Contour:
Velocity contour with Vector:
Velocity plot:
Pressure plot:
Y + plot
Case 3:
Residuals plot:
Drag plot: Lift plot:
Results of Drag and Lift plot
Velocity Contour: Pressure Contour:
Velocity contour with Vector:
Velocity plot:
Pressure plot:
Y + plot
The results of above 3 cases are tabulated below
TABULATION:
Note: the element size is expressed in terms of meters
the element size of 0.09m is considered to be an optimum one since it results in low drag value.
Three dimensional flow pattern and pressure difference between the regions in front of and behind the model develop some compressive drag force on the model. This necessitates obtaining the pattern of pressure variations across two end surfaces of the model to investigate the effect of flow across the region.
Total drag force applied to the model is made up of compressive and frictional drag forces applied to different regions of the model. Based on the pressure distribution
pattern on the tailing surfaces of the model negative pressure gradient behind the model results in the formation of pressure difference between the leading and tailing zones, ending up exerting compressive drag force onto the model. The frictional force is a result of the friction and direct collision of the air flow with lateral, lower, and upper surfaces of the model.
Point of Separation:
The point on the body at which the boundary layer is on the verge of separation from the surface is called point of separation. Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. The boundary layer separates when it has travelled far enough in an adverse pressure gradient that the speed of the boundary layer relative to the surface has stopped and reversed direction.