Ahmed body
AHMED BODY SIMULATION CHALLENGE
# Ahmed body description and its importance.
The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Ahmed body is the benchmark model for aerodynamics simulation tools.
The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.
Ahmed body represents the simplified ground vehicle geometry of a bluff body type. Its shape is simple enough for accurate flow simulation and retains some important practical features relevant to automobiles body. This model helps us to calculate turbulent flow field around the simple car using the k-epsilon model.
Some of the key characteristics which can be obtained from simulating Ahmed's Body are Velocity Magnitude, Pressure Magnitude, Wake region, Drag Force, Lift Force.
Now using the enclosure option , we will enclose this ahmed body (with inner enclosure and outer enclosure) in order to create the mesh outside the ahmed body and to simulate the flow outside of the ahmed body.
Now grid dependency test will be performed on this enclosures by taking the cell count close to the academic license limit (512000)
Meshing
In meshing, a multizone method was used to distinguish between two domains(outer and inner) and concentrate on the inner domain where the air is going to flow over Ahmed's body. For this purpose 'Multizone Method ' was used with Hexa mapped mesh type.
The inner domain was set up using the 'sizing' feature where we performed the grid dependency test with 3 different element size from Coarser to finer.
Steady Density-based solver was been used with Implicit Solution Method
Boundary Conditions
At inlet,
Velocity of air(fluid): 50 m/s
The temperature of air: 300k
Case 1:
Inner domain element size : 50mm
Cell count: 188471
Mesh
# Inflation layer around the Body
# Residuals plot:
# Coefficient of drag(Cd) plot:
(NOTE: ALL THE VALUES OF COEFFICIENT OF DRAG IS MULTPLIED BY 10E-3)
# Velocity distribution on plane:
# Pressure on the ahmed body:
# Vector plot of velocity:
Case 2:
Inner domain element size : 32mm
Cell count: 481395
# Mesh:
# Residuals plot:
# Coefficient of drag(Cd) plot:
(NOTE: ALL THE VALUES OF COEFFICIENT OF DRAG IS MULTPLIED BY 10E-3)
# Velocity distribution on plane:
# Pressure on the ahmed body:
# Vector plot of velocity:
Case 3 :
Inner domain element size : 31.4mm
Cell count: 503048
# Mesh
# Residuals plot:
# Coefficient of drag(Cd) plot:
(NOTE: ALL THE VALUES OF COEFFICIENT OF DRAG IS MULTPLIED BY 10E-3)
# Velocity distribution on plane:
# Pressure on the ahmed body:
# Vector plot of velocity:
Comments on the results:
The grid dependency test allows us to see the smoother velocity profile , As we make the mesh more finer i.e when we increase the cell counts (closer to the limit), we can see from the vector plots(from all the three cases) of the velocity, that it becomes more and more smoother and we get more close to the accurate results.
Other thing to notice is that , making the mesh finer allows us to see the formation of the wake region more clearly.
Therefore clear Take away is that as we refine the mesh we can get accurate results.
3. WHY THE PRESSURE GOES NEGATIVE:
As from the above three cases , from the pressure plot on the car wall , we can see there are some areas where pressure is negative, areas like the cylindrical stands of the car.
There can be many reasons for such negative pressure.
1. Velocity increases when it flow overs the ahmed body, so in order to maintain the flow pressure becomes negative. (Simple case of law of conservation of energy)
2. Flow seperation takes place when the fluid hits the ahmed body,which leads to the formation of the air bubbles which are at very low pressure.
3. Pressure could also become negative in order to over come the shear stress.
4. Other reason could be due to flow recirculation.