Week 3 - External flow simulation over an Ahmed body.
AHMED BODY
- The Ahmed body is the generic car body(a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around the automobile and was first defined and characterized in the experimental work of Ahmed. This model is used to describe the turbulent flow around the car like geometry and once the numerical model is validated it is used to design the new design of the car.
- The Ahmed body is the benchmark model used for the various automobile industries to calculate the drag coefficient for various complex shape bodies because one of the major contributors of fuel-burning for cars is the drag force acting on it.
- The lower drag coefficient of the car means that the car is more efficient.
- As well as affecting the car top speed of the car but we can not reduce the drag force drastically because it will adversely affect the handling of car, lower drag coefficient means lower downforce on the car and that will lead to a loss in road traction.
Figure: Geometry of Ahmed body
Figure: A model of Ahmed body for SpaceClaim
Figure: Computational domain for the Ahmed body
Figure: boundary layer separation
when there is any obsta
Case 1:Baseline Mesh
element size=0.4157m
Fig: Baseline mesh(default Ansys mesh)
Fig: Velocity contour
Fig:Pressure contour
coefficient of drag:0.34296
Case 2:Mesh details
element size=0.2m
maximum size=0.5
element size of inner enclosure=0.05m
total number of cells=209100
Fig:velocity contour for case 2
Fig: Pressure contour case 2
Fig: velocity plot at lines
Coefficient of drag for case 2:-0.37115
Case 3:refined mesh with inflation layers
Element size=0.1m
Maximum size=0.1m
Total number of elements:- 328579
Inflation layer
Fig: inflation layers
Fig: Refined Mesh
Fig:Velocity contour
Fig:Pressure contour
Fig:Coefficient of drag plot
Coefficient of drag=0.31790
Wake region
- Here at the backside of the Ahmed body, our flow is like diffuser flow if the angle of divergence is small friction and separation is less but our angle is quite large around 25 degrees so the flow separation takes place and some fluid will move in the opposite direction of the bulk motion of the fluid which is known as the wake region.
- At the back side of the ahmed body the flow separation takes place.The flow separation takes place because of the adverse pressure gradient.The separation of the flow is major cause of increased drag of the ahmed body.