Ahmed Body Challenge
AHAMED BODY CHALLENGE
AIM:-
To run the Ahamed Body simulation for the velocity of 25 m/sec with the default air properties in fluent, and also to provide the importance of Ahamed Body and the reason for the negative pressure in the wake region and refine the simulation properly in accordance with the baseline simulation.
INTRODUCTION:-
The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model in validated,it is used to design new models of the car.The Shape of the Ahamed body has a length of 1.004meters ,height of 0.288 meters and a width of 0.389 meters.it has a 0.5 meter cylindrical legs attached to the bottom and the rear surface has a slant of 40 degrees.
THEORY:
ANSYS CFD solvers are based on the finite volume method,
- Domain is discretised into a finite set of control volume.
- General conservation equations for mass, momentum, energy, species, etc. are solved on this set of control volumes.
- Partial differential equations are discretised into a system of algebraic equations.
- All algebraic equations are then solved numerically to render the solution field.
Initially the process involves three major Categories,
Pre-Processing
- Geometry
- Mesh
- Physics
- Solver Settings
Solve
- Compute solution.
Post Processing
- Examine results.
Importance of Ahamed Body:-
- 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.
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The slant angle in the back of Ahamed body has a strong influence on the aerodynamic drag and lift at the back of the body. Both the drag and lift coefficients change abruptly at the angle ϕ=30°.
- Some of the important Characteristics that can be obtained by simulating the Ahamed body are Velocity mangnitude, Pressure magnitude, wake region, drag force, Lift force.
Baseline simulation:-
For this simulation since Ahamed body is Symmetrical we can split the body in order to reduce the element nodes and still pertain the same results.Intially for the baseline simulation we are not gonna use any refinement process to improve the qulaity of mesh but rather using the default mesh size and qulaity.
This is the Basic design of Ahamed body, Its dimensions are length of 1.004meters ,height of 0.288 meters and a width of 0.389 meters.it has a 0.5 meter cylindrical legs attached to the bottom and the rear surface has a slant of 40 degrees. To simulate the shape we need a air tunnel of dimensions 8 meters in length and height of 2.5 meters with width at 1.5 meters.We suppressed the Base of Ahamed Body for Physics as it is intervenes with the result.
This is the Enclosed area for the Ahamed body simulation.For Baseline Simulation we did not use any Refinement Design and also use the default mesh quality and size i.e. 0.4092meters.
Baseline Results:-
The results for the Baseline Simulations of Ahamed body Are as follows;
Scaled residules
Coefficient of Lift Coefficient of Drag
Pressure exerted on Ahamed Body
Velocity Contour Pressure contour
Velocity Vector Velocity diffrence between two lines
Refined Ahamned Body Simulation:-
It is simialr to the Baseline simulation but we are gonna add an additional geometry for a refinement measurement known as refinment region within the wind tunnel of the simulation just enclosing the Ahamed body barely with dimensions 3.5x1.5x1 meter in length height and thickness.
The above diagram shows the region of refinement within the wind tunnel.
Regarding the Meshing part of the refinement occurs mostly here, there are slight modifications we did to improve the mesh quality.
- Element structure is set at multiuse so that we could get the most hexa structural mesh.
- Element size and maximum element size is set at 0.1 meter.
- The region of refinement is set at 0.05 meter.
- Inflation layer is added to get a Body Fitting mesh.
- Further more the body of Ahamed body is set at 0.001 mesh size incresing further more element quality.
Inflation layers and region of fine refinement
Reults for refined Ahamed body:-
The following results provided from the above mentioned meshing size and refinement.
Scaled residuals for refined Ahamed Body
Coeffiecient of Drag Coefficient of Lift
Velocity Contour Pressure Contour
wake vortex of Velocity Velocity Vectors
Velocity Chart between two lines
Simulation of Velocity:-
Baseline Simulation of Velocity Refined Version of Velocity
Reason for the formation of negative pressure:-
Negative value for the static pressure indiactes that pressure value are lower thann that of the gauge pressure which is set at zero. Negative pressure is intially formed due to the formation of wake region.A wake is the region of re-circulating flow immediately a moving or a staionarysolid body due to the folw of surrounding fluid aroun the body. The local disturbances in pattern flow of the behind the vehicles causes a momentum loss thus causing drag whic extends far behind the body of the vehicle.The Kinetic energy of these turbulent air spirals caused by the wake acts in the direction that is negative realative to the direction of travel intended for.Vortices are released during the flow seperation and trail downstream to form structured or unstrtuctred wake patterns, Causing the negative pressure in the Simulation.
Results:-
Baseline Simulation Refined Simulation
| Line 1 | Line 1 | Line 2 | Line 2 | Line 1 | Line 1 | Line 2 | Line 2 | |
| Y [ m ] | Velocity u [ m s^-1 ] | Y [ m ] | Velocity u [ m s^-1 ] | Y [ m ] | Velocity u [ m s^-1 ] | Y [ m ] | Velocity u [ m s^-1 ] | |
| -5.00E-02 | 2.26E+01 | -5.00E-02 | 2.04E+01 | -5.00E-02 | 2.30E+01 | -5.00E-02 | 2.14E+01 | |
| 3.56E-01 | 2.46E+01 | 4.06E-03 | 2.11E+01 | 1.37E-01 | 2.42E+01 | 1.10E-01 | 2.41E+01 | |
| 4.25E-01 | 2.49E+01 | 2.49E-01 | 2.38E+01 | 3.24E-01 | 2.52E+01 | 2.75E-01 | 2.48E+01 | |
| 4.56E-01 | 2.50E+01 | 5.19E-01 | 2.51E+01 | 6.97E-01 | 2.52E+01 | 4.30E-01 | 2.50E+01 | |
| 8.99E-01 | 2.54E+01 | 5.68E-01 | 2.54E+01 | 6.97E-01 | 2.52E+01 | 6.82E-01 | 2.52E+01 | |
| 9.37E-01 | 2.54E+01 | 6.12E-01 | 2.54E+01 | 8.88E-01 | 2.53E+01 | 1.09E+00 | 2.55E+01 | |
| 9.42E-01 | 2.54E+01 | 1.27E+00 | 2.55E+01 | 1.09E+00 | 2.53E+01 | 1.19E+00 | 2.55E+01 | |
| 1.29E+00 | 2.54E+01 | 1.29E+00 | 2.55E+01 | 1.29E+00 | 2.53E+01 | 1.29E+00 | 2.55E+01 |
- Point of Seperation improves the Significance of the wake region and clearly gives the idea about the drag coefficient and lift of the body also it gives the accurate velocity at the points.
Conclusion:-
Comparing both the results of Baseline simulation and Refined simulation There is a significant diffrence in detail in respective of velocity drag and lift also the refined simulation give a smooth and reliable values than the baseline simulation.Hence it is advisable to refined the Ahamed body to get the accurate results.