Project 1 : Flow over Ahmed Body

AIM: To perform an external aerodynamic simulation over Ahmed Body with two different turbulence models which are K-Epsilon and K-Omega SST and varying slant angles 25 and 35 degrees.

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:

CAD modelling:

• The ahmed body model is drawn using the 3D CAD modeller of STAR CCM.
• The dimension of the body are:

                                    

• Now using the above dimensions the model is drawn in the CAD modeller.

                                  

 

• After the model is drawn, it extruded using the extrude option and now the bottom parts of the model is drawn using the below dimensions and these parts also extruded. 

 

                                  

 

• Then the final obtained geometry is:

                               

 

• Now the ahmed body is placed in a wind tunnel of certain dimensions and conditions as shown below:

 

                                 

 

  

                               Solid view                                                                                                  Transparent view

 

• Since this simulation is an external aerodynamic, hence boolean subtract has to be done to eliminate the inner fluid volume of the ahmed body. 
• The model is checked for surface errors if any. This model is free from major errors.

                                                 

 

Assigning regions:

1. Inlet - Velocity inlet = 40 m/s
2. Outlet - Pressure outlet = 0 Pa
3. Sides - Symmetry
4. Floor 1st part - wall (slip)
5. Floor 2nd part - wall (no slip)
6. Ahmed body - wall

Meshing:

• Meshing criteria:

1. Surface remesher
2. Automatic surface repair
3. Trimmed cell mesher
4. Prism layer mesher

• Mesh controls:

1. Base size = 0.1m
2. No.of prism layers = 6
3. Prism layer near wall thickness = 0.474mm (Prism layer near wall thickness is calculated based on the Y+ value. Assumed Y+ value is 50 as turbulence is included and y+ should be in the range of 30-500.)
4. Prism layer total thickness = 0.05m
5. Prism transition ratio = 2
6. Volume growth rate = medium
7. Maximum cell size = 0.5m

• Y+ calculation: here the assumed Y+ value is 50. hence below are the Y+ calculations.

            

                                 

 

• Custom control for wake and the tunnel are also given.

 

• Meshed model:

                                  

 

• Sectional view:

                                

 

Physics continua: (same for both k-epsilon and omega sst models)

1. Space - 3D
2. Time - steady
3. material - gas
4. flow - coupled 
5. equation of state - constant density
6. viscous regime - turbulent
7. wall treatment - all y+

 

Results:

I. 25 degrees and K - Epsilon model

Residuals:

                        

 

velocity:

                         

II. 25 degrees and K - Omega SST model:

Residual:

               

Velocity:

                 

 

III. 35 degrees and K - Epsilon model:

Residual:

                          

Velocity:

                      

 

IV. 35 degrees and K - Omega SST model:

Residual:

                         

 

Velocity:

                            

 

Drag coefficient:

 I. 25 degrees and K - Epsilon model:

                                   

 

II. 25 degrees and K - Omega SST model:

                                    

 

III. 35 degrees and K - Epsilon model:

                           

 

IV. 35 degrees and K - Omega SST model:

                            

 

Drag coefficient table:

                                

• From the above table we see that as the slant angle increases the drag coefficient decreases which is an advantage. 

Lift coefficient table:

                                     

 

 

Velocity contours:

I. 25 degrees and K - Epsilon model:

                                       

 

 

II. 25 degrees and K - Omega SST model:

                                     

 

 

III. 35 degrees and K - Epsilon model:

                                        

 

 

IV. 35 degrees and K - Omega SST model:

                                          

 

 

 Conclusion:

•  The wake region for 25 degree K-epsilon model was not captured well and the same was well captured in the 25 degree K-omega model as the flow separation can be clearly seen.
• As the wake angle is increased to 35 degrees both the turbulence models were successful in capturing the wake region and flow separation and k-omega SST proved to be the best among the two models.