Ahmed Body Challenge
Ahmed Body Challenge
SOLUTION1:
Ahmed body:-
The Ahmed body was originally described By S.R. Ahmed in 1984. Ahmed body is a simplified car body. The shape provides a model to study geometric effects on the wakes of the ground vehicles.
- They are a kind of baseline geometry for automobile bodies.
- They have some important practicle features relevent to automobile bodies.
- The flow of air around the ahmed body captures the essential flow features arround an automobile.
- Its shape is simple enough to model, While maintaining the car like geometry features.
- The model describes how to calculate turbulent flow field around a simple car like geometry using the turbulent flow, k-epsilon interface.
The basic dimentions of Ahmed body:-
The A recirculation region that is form as the flow seperates at the top of the verticle back surface of the model.
The B recirculation region is formed due to the seperation at the base of the model.
The c-piller vortices that form as the velocity in the side boundary layers roll up over the slant edges.
The wake was shown to be highly dependent on the slant angle. For the slant angles less than 12 degrees the flow remains attached over the slant. The flow is essentially two dimentional and has low drag. Between 12 and 30 degrees, the flow becomes much more three dimentional as the c-pillar vortices form. These reach maximum strength at 30 degree. The drag increases significantly as the low pressure cores acts on the rear surfaces. Past 30 degree the flow the flow seperates fully off the slant. This results in a sudden decrease in drag and weaker c-piller vortices.
Geometry after preparation:-
Parameters and values:-
Length=1044mm
height =288mm
width =389mm
slant angle=35 degree.
People working in the 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 is validated it is used to design the new models of the car.
They are used for the faster calculation and for the comparison of analytical results.
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Solution2:
With respect to boundary conditions velocity inlet is 50m/s and outlet is zero atmospheric pressure is defined. since velocity is 50m/s (1/6 of mach no.) density based solver with the SIMPLE second-order upwind method is used for solving the physics of the problem.
Case1:-
number of nodes=16529
Number of elements=83759
Geometry after preperation:-
As the first case, meshing is done with default parameters without any inflation layers. Meshing for this first case:-
Mesh:-
Mesh quality:-
Scaled residuals:-
Velocity contours:-
Pressure contours:-
From the results obtained the wake region flow, in this case, is not much evident. So mesh refinement has been done using multizone hexahedral elements throughout surrounding. The new enclosure has been created which surrounds the ahmed body well and different mesh sizes are tried for new enclosure. As well inflation layers are too used which captures the results better around the Ahmed body.
Case 2(Mesh Refined):-
Second Enclosure
number of nodes=49831
Number of elements=165059
Geometry after preperation:-
Mesh:-
Mesh quality:-
Scaled residuals:-
Velocity contours:-
Pressure contours:-
Velocity vector:-
Case 3:-
number of nodes=106940
Number of elements=475502
Geometry after preperation:-
Mesh:-
Mesh quality:-
Scaled residuals:-
Velocity contours:-
Pressure contours:-
Velocity vector:-
Thus from the above plots, All velocity and pressure contours seems similar but in the last case the plots are more smooth and more discriptive. These kind of refinement in mesh consumes lot of computation time but provide more accurate and closer values to the true values.
Grid Dependency test:-
- There are notable changes in the mesh in all three cases.
- The mesh was more refined from case 1 to case 3 to chech grid dependency.
- A smoother mesh provides a smoother and more accurate values.
- From the three cases we can conclude the the flow is nore defined at Higher element size.
- The finer the mesh gets, the easier it gets in visualizing the contours.
- Mesh refinement increases the closeness of the actual values to the true values.
SOLUTION3:
Why the pressure goes negative:-
The pressure goes negative in some regions of Ahmed body. The reasons are:-
- The pressure goes low(negative) to maintain the flow when there is an increase in the velocity of the flow.
- It can be the result of flow seperation at the curved surface interface of the body.
- Seperation bubbles are produced due to flow seperation which results in negative pressure values.
- The pressure also become negative due to flow recirculation.
- The pressure turns negative to counter balance the effects of shear stress
- The flow gets accelerated in those regions which results in negative pressure.