Week 3 - External flow simulation over an Ahmed body.
The Ahmed body was first created by SR Ahmed in his research "some salient feature 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, a 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 boady and the rear surface has a slant has falls off at 40 degrees.
Ahmed's body represents the simplified ground vehicle geometry of bluff body type .its shape is simple enough for accurate flow simulation and retains some important practical features relevant to automobiles body. this helps us to calculate the turbulent flow field around the simple car using the k-epsilon model'
some of the key characteristics which can be obtained from the simulation of Ahmed body are velocity magnitude, pressure magnitude, wake region, drag force, lift force
#encloser
#meshing
'Multizone method ' with Hexa mapped mesh
element size 100 mm
maximum size of element 100mm
inner domain 'sizing'
#Grid independence test
inflation layer around the body with 3 different first layer thicknesses using Y+ value
1st Y+ value 100 where thickness is 1.43 mm
1st Y+ value 200 where thickness is 2.8 mm
1st Y+ value 250 where thickness is 3.58 mm
| y+ | nodes | elemants |
| 100 | 163002 | 940121 |
| 200 | 162844 | 875464 |
| 250 | 1627881 | 874914 |
case 1
Residuls
cd graph
vector
pressure countor
velocity countour
case 2
Residuals
Cd graph
vector graph
pressure graph
velocity graph
pressure graph
case 3
Residuals graph
Cd graph
vector graph
pressure graph
pressure distribution around the body
velocity graph
Q2 negative pressure wake region because
1)velocity increases when it flows over the body, so in order to maintain the flow pressure becomes negative
2) due to flow separation taking place over the body, which leads to low pressure.
Q3
1)the separation point is the point where the air stops "sticking to a body that is moving through the air
2)the point on the body where the boundary layer lifts off the surface
conclusion
when grid independency test was made we observed that y+ value increase cd valve dresses the and number nodes and elements come down