Steady State simulation of Flow over Ahmed Body

Aim :

• To perform Steady state simulation on Ahmed Body and to perform a Grid Dependency test.
• To plot velocity, pressure contours and a vector plot, to show the wake region.
• To find the Coeffecient of Drag for various grid sizes

Ahmed body :

The Ahmed body was an experimental work by S.R. Ahmed in 1984, in order to simulate the flow over a vehicle.The Ahmed body was primarily used to validate the codes of flow simulation and to compare them with the origianal data which is available. The ahmed body helps us the identify the wake region, where the recirculation is formed when the flow seperates from the vertical back surface.The ahmed body is mainly used for determining the drag resitance ,fuel effeciency and lift factors of the vehicle.

Procedure:

Geometry Creation:

• The Geometry of Ahmed body is taken and enclosure command is used to add a enclosure for the body, which will act as the fluid layer
• A refinement layer is added around the Ahmed body, by using the enclosure command again, the interference is checked and removed, inorder to avoid the overlap of the two enclosure.
• The body being symmetric about its center, the split body command is used ,to split into two equal halves. This is done to reduce the total number of cel
• The share topology is selected, inorder to make the nodes share information between the two layers and ahmed body.
• The Ahmed body is suppressed for physics.

Primary Enclosure

Refinement enclosure

After removing Interference

Meshing :

• In the meshing , the sections are named using named section option or using the hot key N
• The car walls are set as wall condition, the two enclosure's bottom are given wall to represent the road,rest of the enclosure are given symmetry boundary condition
• The inflation layer is given along the car walls to capture the turbulent nature along it.

Solution:

Model Used: K-omega SST model

Solver: Simple Solver

Initialization: Hybrid

Velocity at inlet : 25m\s

Air Density : 1.255 kg\m^3

Case 1 :

Outer enclosure : 225mm

Inner enlosure : 30mm

Inflation- First layer thickness : 0.00142mm

Total elements: 4419264

Scaled Residuals

Velocity contour

Coeffecient of Drag = 0.27

Pressure Contour

Velocity Vector Plot

Case 2 :

Outer enclosure : 200mm

Inner enlosure : 30mm

Inflation- First layer thickness : 0.00112mm

Total elements: 420343

Scaled Residuals

Velocity Contour

Coeffecient of Drag= 0.32

Pressure Contour

Velocity Vector Plot

Case 3:

Outer enclosure : 250mm

Inner enlosure : 50mm

Inflation- First layer thickness : 0.009mm

Total elements: 331981

Scaled Residuals

Velocity Contour

Coeffecient of Drag = 0.29

Pressure Contour

Velocity Vector Plot

Inference :

Reason for Negative pressure

When the air strikes the vehicle, it exerts a high pressure at the front. As it travels further, it gets seperated to upward and downward portion of the vehicle.When the air passes the curved front surface, the velocity increases, which inturn reduces the pressure.As the air reaches the tail, boundary layer seperation takes place causing a recirculating flow which inturn produces a negative pressure region.

Point of seperation

The point of seperation is the point at which the boundary layer leaves the body, the point of seperation occurs when flow continues in regions of adverse pressure gradient. If the Point of seperation is earlier, it results in loss and lift.

Result:

The Grid dependency test was done

From the table , we can see that , when the first layer height of the inflation layer is low(i.e. assuming a y+ value lesser than 0.5) we can achieve the actual calue of coeffecient of drag.

This is because, when we use k-Omega model, the y+ value must be in the range of 0 to 5 inorder to accurately determine the changes near the wall.