Week 3 - External flow simulation over an Ahmed body.

• Introduction :
• The Ahmed body represents a simplified, ground vehicle geometry of a bluff body type. Its shape is simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies.
• This model describes how to calculate the turbulent flow field around a simple car-like geometry using the Turbulent Flow.
• The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools.

Geometry:

Fig 1 (a) - Geometrical dimension of an ahmed body

Fig 1 (b) - CAD Model of the ahmed body

Mesh:

A baseline mesh was generated using a global element size of 100mm with local refinements around the body to capture the pressure distribution and wake region behind the ahmed body. The mesh generated consisted of 186,935 elements. A baseline mesh also helps us in quickly validating the physics of the problem to be solved.

Fig 2 - Section cut view of the baseline mesh

Numerical set up:

Fig 3 - Computation domain with boundary conditions

The flow inlet velocity of 50 m/s was defined at the inlet along with a guage pressure outlet condition. All the other faces were modelled as no-slip walls. The Reynolds number (Re) for our problem is in the range of 5∗106${\displaystyle 5\ast {10}^6}$ which means our flow is turbulent. So a k−ω${\displaystyle k-\omega }$SST turbulence model was used to solve for the turbulent flow characteristics within our flow domain. The model was used in this case The mach number (Ma) for our flow is 0.15 and density based solver is used to account for the compressibility of the fluid.

Results:

1. Case 1 - Baseline mesh, Global element size - 100mm

Residual monitor:

The residuals appear to have reached a converged state from 650 iterations onwards.

Velocity contour:

From the velocity contours we can see a low velocity and high pressure zone behind the wake region of the body. This effect causes a drag called as form drag. A recirculating region is also formed as the flow seperates at the top of the vertical back surface of the model. A line probe was placed at 2m from the body to monitor the velocity profile.

Pressure contour on the ahmed body:

A high pressure area is also seen which contributes to the pressure drag due to the blunt shape of the nose. The observed high pressure and low pressure zones are validatated by experimental data done by S.R Ahmed. A grid dependence study is performed by refining the baseline mesh to get more accurate results. The coefficient of drag Cd calculated for the baseline mesh by the formula

Cd = F${\displaystyle F}$ / 0.5∗A∗ρ∗V2${\displaystyle 0.5\ast A\ast \rho \ast V^2}$

where F=Drag force (N)

A = Area of body (m2${\displaystyle m^2}$)

ρ${\displaystyle \rho }$ = density of air (kg/m3${\displaystyle m^3}$)

V = Free stream velocity (ms−1${\displaystyle m{s}^{-1}}$)

The calculated Cd was 0.02.

Grid independence study:

• Two respective meshes were generated by changing the global element of the baseline mesh from 100mm to 80mm & 50mm respectively.

Mesh type	Global element size	No of elements	Coefficient of drag Cd
Baseline mesh	100mm	186935	0.025
Grid_2	80mm	215106	0.026
Grid_3	50mm	303195	0.026

• As we obtained a constant value of Cd for all the 3 meshes, we can conclude that the obtained results are mesh independent.