Ahmed Body Challenge

Ahmed Body Challenge

Problem Description: -To perform a grid dependency test on the ahmed body and compare the results. 

Objective: -

  • To describe ahmed body and its importance
  • To perform grid dependency test on geometry by varying the mesh around geometry.
  • To explain why pressure goes to negative.

Objective 1: -

  • Air has a tendency to curl downwards around the ends of a car, travelling upwards from the high-pressure 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. The kinetic energy of these turbulent air spirals acts in a direction that is negative relative to the direction of travel intended. Thus, the car engine must compensate for the losses created by this drag.so in order to simulate these characteristics this model is proposed and evaluated the characteristics properties in wind tunnel.
  • Ahmed body is simple bluff body to simulate the flow over automobile body. The body has slant at rear end in order to calculate effect of this angle on drag coefficient and velocity profile and pressure distribution.

 

                                                                                                   

People working in vehicle aerodynamics use Ahmed body to validate their numerical model because the experimental data from wind tunnel testing is available for Ahmed body. Once the numerical model in validated, it is used to design new models of the car.

 

Objective 2: -

Ahmed body Baseline meshing

Geometry: -

Ahmed body geometry file is available with us in STEP format, same has been imported in ANSYS designmodeler to create enclosure for CFD analysis. Basic of enclosure dimension will be given as follows.

 

Meshing:-

Meshing done with element size of 100 mm and 5 laysers of inflation with max thicknes  of 12.5 mm.

Node:-40828

Elements:-156941

 

Boundary condition:-

 

Velocity @inlet:- 50m/sec

Solution: -

As the velocity at start is nearby to Mach no 0.3 we will be using density based solver with K ₼ model of viscosity by keeping energy equation on. We have run the analysis for 500 interation.

Velocity @ Cut plane: -

 

Vector Plot: -

Chart of velocity @ various location: -

 

Grid Dependency Test: -

For getting more accurate result near car body we have added one more enclosure to efficiently mesh fine near car body region we will call it as inner enclosure.

 

CASE 1: -

For case one outer enclosure has been meshed using multizone meshing with hexagonal element of size 100 mm.for getting accurate result near car wall we have used inflation layered meshing with max thickness will be of 12.5 mm.for getting fine meshing we have used body sizing in inner enclosure with element size of 50 mm.

No of Elements: -87902

No of Nodes: -34015

Velocity Contour: -

Vector Plot: -

Pressure Plot: -

Chart for Velocity @ 3 Different location: -

 

CASE 2: -

For case one outer enclosure has been meshed using multizone meshing with hexagonal element of size 100 mm.for getting accurate result near car wall we have used inflation layered meshing with max thickness will be of 12.5 mm.for getting fine meshing we have used body sizing in inner enclosure with element size of 35 mm.

No of Elements: -217168

No of Nodes: -59344

Velocity Contour: -

Vector Plot: -

Chart for Velocity @ 3 Different location: -

 

Case 3:-

For case one outer enclosure has been meshed using multizone meshing with hexagonal element of size 100 mm.for getting accurate result near car wall we have used inflation layered meshing with max thickness will be of 12.5 mm.for getting fine meshing we have used body sizing in inner enclosure with element size of 28 mm.

No of Elements: -450175

No of Nodes: -104608

Velocity Contour: -

Vector Plot: -

Pressure Contour: -

Chart for Velocity @ 3 Different location: -

 

Objective 3: -

Velocity at front side of ahmed body is large but when it impacts on front end velocity decrease’s which cause high pressure at front side but when flow passes over radial edge velocity again increases when it flows over slant edge recirculation of flow happens and also due to vortex shedding pressure goes to negative. 

Conclusion: -

As we reduce the element size from 50 mm to 28 mm flow characteristics around ahmed body is easily captured.

 

 


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