Ahmed Body Challenge

AIM: To Perform CFD simulation of flow around Ahmed Body.

INTRODUCTION: Automotive aerodynamics comprises of the study of aerodynamics of road vehicles. Its main goals are reducing drag, minimizing noise emission, improving fuel economy, preventing undesired lift forces and minimising other causes of aerodynamic instability at high speeds. Also, in order to maintain better control for steering and braking, we look into design and aerodynamics of a vehicle. Drag is caused due to the pressure difference between the frontal and the rear end of the vehicle. It can be reduced by modification of the design of the vehicle or the modification of the air flow around the vehicle. 50% of the mechanical energy of the vehicle is wasted to overcome drag at highway speed of nearly 88.5 to 96.5 kph.

It is necessary, at times, to generate down force - to improve traction and thus cornering abilities.Lift can be dangerous for an automobile, especially at high speeds. So, in order to maintain control for steering and braking, cars are designed so that the automobile exerts a downward force as their speed increases. However, increasing this downward force increases drag, which in turn, limits the top speed and increases fuel consumption. Hence, these two forces must be carefully balanced.

Air has a tendency to curl downwards around the ends of a car, travelling upwards from the highpressure region under the car to the low-pressure region on top, at the rear end of the automobile and subsequently collides with moving low-pressure air. A wake is the region of re-circulating flow immediately behind a moving or stationary solid body, caused by the flow of surrounding fluid around the body.

The generic Ahmed Body reference model has been chosen as the benchmark for carrying out computations for studying of the aerodynamic parameters. The body was first proposed by Ahmed (1984).
The Ahmed body is a very simple bluff body which has its shape simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies. It has a slant on its rear end, whose angle can be manipulated and the corresponding drag and lift coefficients calculated. This is done to exhibit the air flow over the different geometry sections of an automobile and in its vicinity, at different slant angles. This model portrays how to calculate the turbulent flow field around a simple car-like geometry using the turbulent flow, k-epsilon interface.

PROCEDURE:

1. Geomerty setup:

• To begin the simulation the model has to be imported in spaceclaim.

• The Ahmed Body model has to be enclosed for which we use enclose option available in PREPARE->ENCLOSURE. Enclosure acts as a wind tunnel used for aerodynamic analysis in which air is blown on to the body to get the results. The dimensions of the wind tunnel is as below.

• Now to get still more accurate results, the ahmed body is covered with another enclosure.

2. Mesh:

• Open the Mesh module and faces are allotted their respective names which are Inlet,Outlet,Symmetry walls,Car walls.
• Now mesh the model with default settings.
• Under mesh, a multi zone method is inserted by selecting the outer enclosure area.

• Similarly body sizing method is inserted by selecting the inner enclosure.

• The mesh size for the outer enclosure is taken as 100mm with mesh type as hexahedral and for the body sizing the mesh length is 50mm with mesh type as tetrahedral.

• The circular holes in the bottom have been distorted and can be rectified by meshig it with different mesh size.

• To rectify this face sizing method has to be inserted and the four faces have to be selected and mesh size of 5mm is given.

• The rectified circular region is as follows:

• In the boundary of the ahmed body to have a good mesh flow, inflation layers are applied with 5 layers , total thickness and maximum thickness of 12.5mm.

Grid Independency Test:

I. Case-1: No.of Elements = 178124

Mesh size of outer enclosure = 100mm

Mesh size of inner enclosure = 50mm

• The mesh is as follows:

• Once the mesh is obtained, launch Fluent and set up the values as shown below and select k-epsilon as the turbulence model.

• Area = 389mm*288mm = 0.112032m
• Length = 1044mm = 1.044m
• Here residual values,drag coefficient and lift coefficient values are calculated and velocity and pressure contour are also plotted.

Residuals Plot:

Drag coefficient:

Lift Coefficient:

Drag and Lift Coefficient:

Velocity Contour:

Pressure Contour:

Vector Distribution:

II. Case-2: No.of Elements = 270722

Mesh size of outer enclosure = 100mm

Mesh size of inner enclosure = 40mm

• The mesh is as follows:

Residuals Plot:

Drag coefficient:

Lift Coefficient:

Drag and Lift Coefficient Values:

Velocity Contour:

Pressure Contour:

Vector Distribution:

III. Case-3: No.of Elements = 474952

Mesh size of outer enclosure = 100mm

Mesh size of inner enclosure = 32mm

• The mesh is as follows:

Residuals Plot:

Drag coefficient:

Lift Coefficient:

Drag and Lift Coefficient Values:

Velocity Contour:

Pressure Contour:

Vector Distribution:

Summary of Grid Indepency Test:

Why Negative pressure in wake region?

The air becomes almost stagnant as it strikes the vehicle which results in air exerting very high pressure on front engine grill of the vehicle represented by the red area. The airflow then gets divided between the upper and lower surface of the vehicle. The higher pressure air on front surface accelerates as it travels over the curved nose surface of Ahmed body, causing the pressure to drop. This lower pressure creates lifts over the roof surface as the air passes over it. As the air continues to flow and make its way to the rear, a notch is created by the rear slant owing to flow separation, leaving a vacuum or low pressure space which the air is not able to fill properly.The resulting lower pressure creates lift that then acts upon the surface area of the rear slant.

What is y+ value?

y+ is a non-dimensional distance. It is often used to describe how coarse or fine a mesh is for a particular flow pattern. It is important in turbulence modeling to determine the proper size of the cells near domain walls. The turbulence model wall laws have restrictions on the y+ value at the wall. For instance, the standard K-epsilon model requires a wall y+ value between 30 - 300. A faster flow near the wall will produce higher values of y+, so the grid size near the wall must be reduced.

As shown in the above image, most of the y+ value recorded in the plot of the current model is also in the range of 30-300. Nevertheless some are falling below 30 value but those can be neglected as major of them are above 30.

Point of Separation:

• Point of separation is related to boundary layer separation occuring in the ahmed body. This is the point where the velocity and pressure tends to zero or negative and this can be observed in the beginning of the slant area. Here the velocity is zero and pressure is negative(-544 Pa). Also if observed in the vector distribution, the point where the vector starts recirculating indicates the point of separation.

• The point of separation is also seen w.r.t the slant angle.The minimum drag is obtained for the rear slant angle φ= 7.5 degree for the attached flow range of angles which exists up to 30 degree and thereafter, the flow becomes unattached at rear slant and correspondingly the values of drag and lift tend to be randomly dispersed.The velocity contours clearly indicate that the wake region, and consequently the form drag,
  continually decreases as the rear slant angle is increased from 0-30 degree and thereafter, boundary layer separation occurs on the rear slant.

Conclusion:

From the above simulation of the Ahmed Body and its results from the grid independency test, we can infer that:

1. In the 3rd case of 474952 elements we could see that the drag coefficient was around 0.35 which is very close to the prescribed value 0.33 and lift was observed to be 0.26. It is always advised to have low value of drag and lift coeffcient in order to have a smooth movement of the vehicle.
2. In the vector distribution of case-3, we see that there is a lot of recirculation happening in the wake region(negative pressure wake region) and this helps in obtaining low drag coefficient. So as mesh size increases recirculation increases and drag coefficient decreases. This is applicable only when the recirculation increases in the negative pressure wake region.
3. The point of separation which is in the beginning of the slant area was located in the velocity and pressure contour.