Week 3 - External flow simulation over an Ahmed body.

Aim:

To run external flow simulation over Ahmed body.

Objective:

1.Run the simulation for the velocity of 25 m/sec with the default air properties in fluent

2.To Perform the grid independency test and provide the values of drag and lift with each case. 

 

Introduction:

The Ahmed body was at first put forward by Ahmed et al. (1984). is a general car model which is used by the automotive industries to examine the wake forces and dynamics which is experienced in a verity of configurations. The Ahmed body is designed to have a smooth-edged front end with a flat roof and a flat bottom section and an angled back slant which basically acts as the rear window of a car and ending with a vertical base. The back-slant angle which is commonly designated as ϕ is very critical to the flow patterns which are fashioned at the near wake region and subsequently has an impact on the aerodynamic forces which act on the body. Car companies makes numerous attempts to develop modified designs to effectively reduce the aerodynamic drag force which occurs at the rear end without putting any constraints in the stability, comfort and safety of the passengers. The aerodynamic drag of road automobiles is firmly connected to the vehicle’s wake downstream flow. The separation zone size and the drag force FD directly rest mainly on the position of flow separation over the Ahmed body. Subsequently, comprehensive facts regarding the wake flow characteristics and its connection with the geometry of body is essential for a successful design of upcoming future cars.

 

Case Setup:

Geometry:

 Geometry with following dimensions is cerated using Sspaceclaim.

       

      

As we are simulating external flow over body,To create wind tunnel virtually create outer enclosure surrounding ahmed body using enclosure option in prepare tab.

       

Simmillarly create inner enclosure.

     

To ensure outer enclosure is not interferring in inner enclosure check interferrence.

    

As the body is perfectly symmetric, we can run the simulation by considering the only half body. This is the best practice where you can save on the number of cells and get the results faster as well.

To split the body go to design > split body

      

Meshing:

Open the meshing console and divide the different part using named selection as follows:

Inlet:

         

 

Outlet:

       

 

Symmetry:

       

 

Car-Wall:

       

 

Now click on generate mesh to cerate mesh.

      

To create the high quality mesh go to mesh > methods > multizone.

It will cretae the hexahedral mesh wherever possible.

    

To refine the inner enclosure, go to mesh > sizing > select the inner enclosure and put the sizing value.

Simmilarly, to refine ahmed body go to mesh > sizing > select ahmed body using named selction and put element size value as 10 mm.

     

 

To retain the size of car wall, create inflation layers.

Total thickness of inflation layers is calculated by:

     Total Thickness = Y*1.2(No.of layers -1)

    

 

Solver:

Boundary Conditions:

Inlet :   Velocity = 25 m/s

Outlet:    Pressure = 0 Pa (guage pressure)

Car-wall:  No slip condition

 

Solver : Density based steady state solver.

viscous model : K-epsilon

 

Setup the refference values:

      

 

Results:

To capture better values of drag and lift coeffiecient, Y+ should lie between 30 to 300.

Case 1:

Outer enclosure = 100 mm

Inner Enclosure = 50 mm

Y+ = 100

Density = 1.225 kg/m3

Viscosity = 1.7894e-5 Pa-s.

Velocity = 25 m/s

Length = 1.044 m

 

Calculations :

    

Mesh :

   

 

To retain the shape of ahmed body we have provided  6 inflation layers.

         

 

 

Residue:

Simulation is run for 1000 iterations.

   

 

Coefficient of drag:

   

 

Coefficient of lift:

   

 

Velocity vector:

  

 

Pressure vector:

      

 

Case 2:

Outer enclosure = 90 mm

Inner Enclosure =45 mm

Y+ = 150

Density = 1.225 kg/m3

Viscosity = 1.7894e-5 Pa-s.

Velocity = 25 m/s

Length = 1.044 m

 

Calculations:

  

Mesh:

  

   

 

Residue:

Simulation is run for 1200 iterations.

   

 

Coefficient of drag:

   

 

Coefficient of lift:

   

 

Velocity vector Plot:

  

 

Pressure vector plot:

      

 

Case 3:

Outer enclosure = 80 mm

Inner Enclosure =40 mm

Y+ = 200

Density = 1.225 kg/m3

Viscosity = 1.7894e-5 Pa-s.

Velocity = 25 m/s

Length = 1.044 m

 

Calculations:

  

Mesh:

  

  

 

Residue:

   

 

Coefficient of drag:

   

 

Coefficient of lift:

   

 

Velocity vector Plot:

    

 

Pressure vector plot:

      

 

Result Discussion:

Values of drag and lift coefficient for variouus mesh size is tabulated below:

   

Reason for the negative pressure in the wake region.

Ans:

From the pressure vector plot it is seen that static pressure increases in the direction of flow.This increases Potential energy of fluid and reduces kinetic energy and deccelerates the fluid flow.This makes the velocity of fluid inside the inner boundary to zero and thus the flow separation occures.And thus wake region is formed.These wake regions are nothing but small votices that causes Negative pressure.

 

Point of separation.

Ans:

      

 Flowing against an incresing pressure is known as flowing in an adverse pressure gradient.As shown in the figure, the velocity near the wall reduces and the boundary layer thickens. A continuous retardation of flow brings the wall shear stress at the point S on the wall to zero. From this point onwards the shear stress becomes negative and the flow reverses and a region of recirculating flow develops. We see that the flow no longer follows the contour of the body. We say that the flow has separated. The point S where the shear stress is zero is called the Point of Separation.

 

Conclusion:

It is not the grid size but the Y+ value that affects the values of drag and lift coefficients.