Week 3 - External flow simulation over an Ahmed body.

Aim:

To Simulate the external flow over an Ahmed Body. And we can run the simulation by considering the only half body. To run the simulation for the velocity of 25 m/sec with the default air properties in fluent.

Objective:

In this challenge we have to do simulation of the flow over a circular cylinder and then we have to get some pressure, velocity, vectors and streamline plots and contours from the simulation. Below are the objectives of this challenge we have to follow these steps.

Q1. Describe Ahmed's body and its importance.

Q2. Explain the reason for the negative pressure in the wake region.

Q3. Explain the significance of the point of separation.

1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case. (For velocity of 25m/sec, the drag coefficient should be around 0.33).
3. The vector plot clearly showing the wake region.
4. Perform the grid independency test and provide the values of drag and lift with each case.

Ahmed body:

Ahmed body is a simplified car model geometry. Ahmed body is used to capture the basic flow characteristics of an automobile. CFD is used to study the aerodynamic behaviour of Ahmed body.

The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features.

Ahmed body was developed by S.R. Ahmed & presented in his research paper “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. This body is used as a generic car body because it captures all the essential flow features around an automobile and is easy to model.

The simple model makes meshing easier, reduces the computational power required and the time taken to run the simulations. Hence the body is widely used by researchers & engineers to validate their numerical model or CFD codes and sometimes without even wind tunnel testing as vast experimental data is already available (for different scale models of Ahmed body) from validation studies over the years.

Negative pressure in the wake region:

• Negative pressure in the wake region the fluid as it flows Ahmed body there would be stagnation of air particles at the front of the Ahmed body and pressure would be more which can be seen from the pressure contour, then there would be gradual increase in the acceleration of the air particles till the start of the slant angle
• Reason for negative pressure in the wake region: The path lines showing the exact circular motion or vorticity generation just at the rear end of the car. The separation takes two regions of distribution. One at the upper part of the rear and the other at the lower part of the rear end.
• The flow around the vehicle is characterized by high turbulent and three-dimensional flow separations as well as there is a growing need for more insight into the physical features of these dynamical flows. The Ahmed Body is a simplified car, used in the automotive field to investigate the flow analysis and find the wake flow around the body.

Point of separation:

The rear slant angle of the Ahmed body has a considerable effect on the flow separation at the rear end and the 25-degree configuration, particularly, is the one of the critical cases where flow experiences both flow separation and reattachment over the rear slant surface.

The flow separation happens at different points for a body based on the type of flow, basically, Laminar or Turbulent. For the laminar flow, the flow separation happens quickly resulting in minimal shear drag and a large pressure drag which forms a wake region (low pressure) behind the body, while for the turbulent flow, the flow separation is delayed causing minimal pressure drag but a slight increase in shear drag since the velocity profile is thicker for turbulent flow in comparison with laminar flow.

Challenge approach:

In this challenge we have to use ANSYS Fluent solver to create the geometry and perform unstructured tetra CFD mesh. We have to use five major steps in this challenge, there are Geometry, mesh, set-up, solution, result. We have to follow these steps. So, in this challenge we have to create

Case Element Outer Enclosure Inner enclosure Car wall

size in (mm)

Case 1 100 50 10

Case 2 120 90 9

Case 3 140 70 7

with boundary definitions, setup the physics of fluid flow and run the iterations to get solutions, using post processing to interpret the data in form of plots, contours, vectors, streamlines etc., With velocity of 25 m/s and k-epsilon model we shall study grid independence for Ahmed Body with RANS. When we say grid independence study, we shall study for 3 cases as follows.

Conclusion:

Finally, we conclude from the results of all 3 cases, and we got the drag coefficient and lift coefficient values.

Case 1 Case 2 Case 3

Outer enclosure 100 120 140

Inner enclosure 50 90 75

Car wall 10 9 7

Drag coefficient 0.300 0.352 0.295

Lift coefficient 0.210 0.321 0.223

Inferences:

From this challenge the expected drag coefficient of 0.33 at the case 3. As the drag coefficient decreases the lift coefficient increases. Where the case 2 and 3 in finer mesh the iterations are converged around 120 iterations itself. So, mesh size impacts the convergence as well. The distance from the tail of car Vs Velocity plot shows that as the distance from tail increases the velocity is more