Flow Over Ahmed Body

Objective: The objective of this project is to determine the Aerodynamic forces on the Ahmed body and Also to compare the velocity profile with experimental and computational data.

What is an Ahmed Body?

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. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, the height of 0.381 meters, and a width of 0.389 meters. It also has 50-millimetre cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 35 degrees.

CASE SETUP:

The virtual wind tunnel is having a dimension of 7.5x1x0.8 metres. The virtual wind tunnel is shortened due to the computational restrictions, otherwise, the typical dimensions for wind tunnel are 10 times in X directions of the length of the car. Twice the length of a car for the width of the wind tunnel and a length of the car for the height of it. The fluid for simulation is air with 23% oxygen and 77% nitrogen. A transient case setup is used for this simulation with 0.375 sec as the end time. The one flow cycle computed was 0.18 sec. The Reynolds number calculated was:

`Re= (rho.V.L)/v`

Therefore Re= 2826298.343

V= inlet velocity m/s= 40

L= characteristic length= 1.044 m

`v` = kinematic viscosity of air= 1.81x10^-5 Pa

`rho`= density of air at 300K= 1.225 kg/m^3

The experimental data shows that the boundary layer thickness in front of the body at 400mm is equal to 30mm. So,

`del=(0.382x)/(Rex)^(1/5)`

where `del`= boundary layer thickness

`x`= distance from the ground

Rex= Reynolds number

Therefore, `x`= 1.53 metres.

therefore, the car is placed (1.53+0.4)= 1.93 ~ 2 metres away from Inlet.

Boundary Conditions:

• For inlet:

A Neumann Boundary condition for pressure was set up with 40 m/s in the X-direction.

• For outlet:

The static pressure of 101325 Pa was setup. Neumann Boundary condition for velocity is considered.

• Side faces and top face:

These faces are considered to be symmetry.

• Bottom face:

It is considered to be a wall and law of wall boundary was setup. It represents the road for Ahmed body.

• Ahmed Body:

It is considered as a wall and law of wall boundary is given to capture the boundary layer condition and forces around it.

• Turbulence Model:

As the case is of external aerodynamic simulations, K-`omega`SST model is used. It also has the advantage of automatic wall treatment which further helps to enhance the near-wall turbulence.

MESH:

Due to the lack of computation resource a coarse mesh is used. 0.08625x 0.0575x0.08625 metres is the mesh for this case. Furthermore, boundary layer embedding is given with a scale of 3 and the number of a layer as 4 and also box embedding is given with a scale of 2.

Results:

LIFT/Down FORCE:

The lift force computed from the case is -126 N. Therefore, it is observed that there is a downforce acting on the Ahmed Body. Therefore, the coefficient of lift is CL= 0.323

`CL= (2L)/(rhou^2S)`

where CL= coeff of lift

L= lift force= -126 N

`rho`= density of air= 1.225 kg/m^3

u= Flow velocity= 40 m/s

S= relevant surface area= 0.398 m^2

DRAG FORCE:

The drag force computed from the case is 23 N.

Therefore coefficient of drag is Cd= 0.2023

`Cd=(2Fd)/(rhou^2A)`

where Cd= coeff. of drag,

Fd= drag force,

`rho`= Density of air= 1.225 kg/m3

`u`= flow velocity of object= 40 m/s

A= frontal area= 0.116 m^2

Y+ :

The Y+ value fluctuates around 300.

CONTOURS:

Velocity:

The picture depicts the velocity contours along the Ahmed Body.

As seen in the frontal area, the velocity gets stagnated causing a zero velocity region at the nose. Then the velocity sweeps through the curve or fillet part of the nose causing a high-velocity area near the fillets reaching its maximum magnitude. As the flow sweeps through the fillets the detachment takes flow between surface and airflow causing a sudden decrease at the edge of the fillet. This air is then carried downstream to the back of the car. The picture below depicts the streamline of the air. the lines represent the velocity and the car and road represent the pressure acting on them.

As the flow reaches back of the car, the slant edge increases the velocity of the air and departs them to the downward direction. The sudden change of geometry results in boundary layer separation causing a turbulent wake region at the back of the car, resulting in the backflow of the air. The slant of the car results in changing the air direction and eventually giving the downforce.

The flow passing from the downside of the car meets the cylindrical extension depicting tyres/wheels of the car. We can observe the Von Karman Vortex effect on a greater extent. The flow is relatively average except the wheels.

Animation- https://youtu.be/uBRKruuVH_Y

PRESSURE:

The picture above depicts the pressure distribution around the AHMED body.

The front area of the car, as seen shows the high-pressure region due to stagnation of velocity. As the air flows through the fillets of the car, the velocity increases eventually decreasing the air pressure around the edges. As the flow stables over the car top surface, there is a sudden drop in the slant area at the back of the car. This eventually drops the pressure, which can be seen as a sharp line at the edge of the slant. Pressure drag is created when the shape of the surface changes abruptly, as at the point where the roof of an automobile ends. The drop from the roof increases the space through which the airstream flows. This slows down the flow and, by Bernoulli's principle, increases the static pressure. The air stream is unable to flow against this sudden increase in pressure and the boundary layer gets detached from the surface creating an area of low-pressure turbulent wake or flow. Since the pressure in the wake is much lower than the pressure in front of the car, a net backward drag or force is exerted on the car.

ANIMATION- https://youtu.be/vXa9PXv3oKg

Comparison between Simulation & Experimental Data:

Line probes were placed at various X-locations and the velocity along X was placed at various Y-locations. The simulation data and experimental data for the same location was compared with the help of line plots. It is observed that both the plots follow the same trend and simulation data mimics the experimental data. Nonetheless, the simulation data does not follow the 5% error range. This can be eliminated by using finer mesh and better computational resources. We can agree that through the following plots, the simulation data and experimental data have a strong agreement of similar trends.