Ahmed Body Challenge

AHMED BODY CHALLENGE

Objective:

1. What is Ahmed Body and its significance.
2. Simulate the flow over Ahmed Body and Evaluate the lift and drag effects and perform mesh dependence test.
3. State the reason for negative pressure in wake region
4. Understand the flow separation around the bluff body and its significance

What is an Ahmed Body?

Burning of fossil fuels is the more intense issue faced by automotive industry nowadays. One main factor to fuel burn is the aerodynamic drag, so the manufacturers are trying to achieve better fuel efficiency by specifically designed car bodies. It is difficult to calculate the aerodynamic drag over the complex shaped cars, which are very challenging to model computationally.

The Ahmed body is a benchmark model of simplified car body, which is used in automotive industry for validating simulation tools. It has simple shape to model and also maintains car like features. This body was first introduced by S.R. Ahmed in his research “Dome Salient features of the Time-Averaged Ground vehicle wake” in 1984.

It has simple shape with dimensions given below:

Length = 1.044 m

Height = 0.288 m

Width = 0.389 m

It also has 0.5 m cylindrical legs at the bottom.

Rear surface has the slant that falls off at 40 degrees.

• Significance of Ahmed Body:

The main goal of automotive industry is to reduce drag, keep noise to minimum level, prevent unwanted lift of vehicle and avoid instabilities that can occur at high speed. Hence to achieve above said goals the vehicle design and aerodynamics are the most important parameters. A lot of energy is wasted in overcoming the drag. The drag on vehicle is experienced due to pressure difference between the frontal and rear end of car.

To conduct aerodynamic analysis to get idea about turbulence and pressure a simplified model of car body called Ahmed body is used. The body is simplified bluff body which maintains the degree of accuracy when modelling the flow around the vehicle while maintaining the great features. The flow separation behind the Ahmed body is mainly due to drag forces.

• Case 1: Default mesh, Slant Angle =20⁰

• ^Geometry

As the body is perfectly symmetric, we can run the simulation by considering the only half portion of the body. So, we can get results faster as the number of cells will get reduced. The symmetric model is prepared from the given model using “split body” tool in Spaceclaim.

• Meshing:

Solver Settings:

• Reference values:

Projected area in the direction of is taken from fluent.

length - 1.044 m

velocity - 25 m/s

Residuals:

Drag Coefficient Plot:

Lift Coefficient Plot:

Velocity Contours on mid-plane

Pressure Contour on Car-wall:

Velocity variation along vertical lines at 2,3 and 4 m.

Vector Plot:

Drag and Lift Coefficient Value on Car-wall:

Case 2:

For refined mesh with

Element size at outer enclosure: 100 mm

Element size at inner enclosure: 50 mm

Inflation at car_wall with total thickness of 12.5 mm

Number of layer: 5

Growth rate: 1.2

Number of elements: 95955

Mesh:

Residuals:

Drag Coefficient Plot:

Lift Coefficient Plot:

Velocity Contours on mid-plane:

Pressure Contour on Car-wall:

Vector plot:

Velocity variation along vertical lines at 2,3 and 4 m.

Drag and Lift Coefficient Value on Car-wall:

Case 3:

• Element size at outer enclosure: 100 mm
• Element size at inner enclosure: 40 mm
• Inflation at car_wall with total thickness of 10 mm

Number of layer: 5

Growth rate: 1.2

• Number of elements:

Mesh:

Residuals:

Drag Coefficient Plot:

Lift Coefficient Plot:

Velocity Contours on mid-plane:

Pressure Contour on Car-wall:

Vector plot:

Velocity variation along vertical lines at 2,3 and 4 m.

Drag and Lift Coefficient Value on Car-wall:

Case 4:

• Element size at outer enclosure: 100 mm
• Element size at inner enclosure: 30 mm
• Inflation at car_wall with total thickness of 10 mm

Number of layer: 5

Growth rate: 1.2

• Number of elements:

Mesh:

Residuals:

Drag Coefficient Plot:

Lift Coefficient Plot:

Velocity Contours on mid-plane:

Pressure Contour on Car-wall:

Vector plot:

Velocity Streamline:

Velocity variation along vertical lines at 2,3 and 4 m.

Drag and Lift Coefficient Value on Car-wall:

Mesh independency test:

Case	Mesh elements	Outer Enclosure Size (mm)	inner Enclosure Size (mm)	Drag Coefficient	Lift coefficient
1	Default Mesh	-	-	0.3759	0.2316
2	95955	100	50	0.3539	0.2848
3	146953	100	40	0.3362	0.2888
4	284504	100	30	0.3111	0.2904

Why is the negative pressure in the wake region?

As the air flow strikes the vehicle it results in the stagnant condition in front of vehicle and it exerts the very high pressure on front side of vehicle. High pressure experienced at the front accelerates the flow and it travels over the Ahmed body causing further pressure drop. The flow reversal takes place in wake region i.e. the pressure force and opposes each other and the net pressure in wake region is observed to be negative.

Flow Separation and its Significance:

As we know that when there is relative motion between fluid and solid round body, boundary layer will exist near the wall of body. The boundary layer may be laminar or turbulent. The separation of flow occurs when flow is slowing down, with increasing pressure, after passing thickest part of streamlined body. The flow reversal takes place when there is negative pressure gradient also called as adverse pressure gradient. When the flow is detached from the surface, it takes the form of eddies and vortices. Separated flow results in reduced lift and increased pressure drag, as there exist the pressure differential between the front and rear end of object.

The separation takes place due to excessive momentum loss near the wall in the boundary layer trying to move downstream against increasing pressure i.e. dp/dx > 0, which is called adverse pressure gradient.

Till the point of separation pressure force and force due to stream wise acceleration i.e. inertia force are acting in same direction and after separation the acceleration force and pressure forces oppose each other. At separation point the velocity gradient is zero and thus wall shear stress is also zero.

The separated flow increases the pressure differential between the front and rear end of vehicle which contributes to the pressure drag on vehicle.

Conclusions:

1. As the mesh in flow domain is refined the values of the drag coefficient looks better. (Cd - 0.33)
2. Drag and lift on body shouid be constantly maintained so as to maintain the stability of vehicle.
3. Mesh sizing and Y+ greatly affects the accuracy of the model.
4. As the model symmetric the splitting of domain helps us to keep number of elements low and improves the calculation speed.