Week 3 - External flow simulation over an Ahmed body.
AIM:-
TO RUN THE SIMUATION OF VELOCITY 25m/s WITH THE DEFAULT AIR PROPERTIES IN THE FLUENT OVER THE AHMED BODY.
OBJECTIVES: -
- To Describe Ahmed's body and its importance.
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To Explain the reason for the negative pressure in the wake region.
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To Explain the significance of the point of separation.
Ahmed's body and its importance:-
The aim of the "Models for Vehicle Aerodynamics" (MOVA) Project is to develop, refine,
and validate the latest generation of turbulence models for selected examples encountered in
vehicle aerodynamics. The validation of turbulence models requires the availability of
detailed experimental data. These quantitative data should cover the most critical flow
regions around a bluff car-shaped body and they should give physical quantities that can
directly be correlated to the results of numerical simulations. Ahmed body is a simplified car body and was first created by S.R Ahmed. This shape provides a model to study geometrical effects on the wakes of ground vehicles. This is the kind of baseline geometry for automobile bodies.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.
Reason for the negative pressure in the wake region: -
The reason is at some point, the velocity gradient becomes zero, which is the point where the flow separates or detaches. Beyond this point, the velocity profile is zero both at the wall, because of the no-slip condition, and at the inflection point, which is where the velocity crosses from negative into positive. In the negative velocity region, between the surface and the inflection point, the air flows in a direction opposite to the main flow – this is called a recirculation. The separated flow will now surf on top of this recirculation flow and the boundary layer and wake will continue to grow, which can have a negative effect on drag.
The significance of the point of separation:-
The boundary layer seperation happens due to the flow which doesn't follows the solid body(ahmed body) regarding the shape of the solid body which where the seperation or detachment of flow starts ,in that region vortices and eddy forces will form, we can also see wake region and negative pressure in that region.
GEOMENTRY SET-UP:-
The ahmed body is a 3D model for a basic car simulation. For this project we're going to create enclosures for the given model and cut the model along z axis for the reduction of cells in mesh.
In the above image we've created two enclosures for the ahmed body, inner & outer enclosures.
OUTER ENCLOSURES FOR THE MODEL:-
The outer enclosure is 5m towards rear end of the ahmed body
Front side is 2
The base of the ahmed body is 0m
The top of the ahmed body is 1m
The width will be 0.5m for both sides
INNER ENCLOSURES FOR THE MODEL:-
The outer enclosure is 1m towards rear end of the ahmed body
Front side is 0.5m
The base of the ahmed body is 0m
The top of the ahmed body is 0.5m
The width will be 0.5m for both sides
MESH:-
Case1: -
The outer enclosure part is meshed using multizone mesh method to get accurate results based on the shape of the enclosure and it fits tetrahedral mesh and normal mesh wherever it needed in the model. The inner enclosure is meshed with default mesh.
For outer enclosure=90mm (multizone)
Inner enclosure = 42mm (default mesh)
No of inflation Layers=6, with first layer thickness using Y+ of 50.
Case2: -
The outer enclosure part is meshed using multizone mesh method to get accurate results based on the shape of the enclosure and it fits tetrahedral mesh and normal mesh wherever it needed in the model. The inner enclosure is meshed with default mesh.
For outer enclosure=90mm (multizone)
Inner enclosure = 40mm (default mesh)
No of inflation Layers=7, with first layer thickness using Y+ of 50.
Case3: -
The outer enclosure part is meshed using multizone mesh method to get accurate results based on the shape of the enclosure and it fits tetrahedral mesh and normal mesh wherever it needed in the model. The inner enclosure is meshed with default mesh.
For outer enclosure=90mm (multizone)
Inner enclosure = 50mm (default mesh)
No of inflation Layers=9, with first layer thickness using Y+ of 50.
While the bottom of the ahmed body the base circle which has to refine and the sizing is changed into 5m to get a perfect circle.
Name selections:-
- inlet
- outlet
- symmetry wall
- car wall
The mesh also carried by using named selection option in stead of geometry.
SET UP: -
The solver is pressure based type and the velocity are absolute
| SOLVER TYPE | PRESSURE BASED |
| VELOCITY FORMULATION | ABSOLUTE |
| TIME | STEADY STATE |
| VISCOUS MODEL | K-EPSILON |
| AREA | 0.065741m^2 |
| LENGTH | 1.044m |
| DENSITY | 1.225kg/m^3 |
| DYNAMIC VISCOSITY | 1.7894e-05kg/ms |
| PRESSURE | 0pascal |
| METHOD | HYBRID SIMULATION |
| INLET VELOCITY | 25m/s |
| MATERIAL | FLUID-AIR |
The simulation run upto 500 iteration for each and every case setup.
RESULTS:-
CASE1: -
RESIDUAL PLOT: -
The solution converged in 107th iteration.
Drag plot: -
Cd= 0.33735813
Lift plot: -
Cl= 0.22870985
Velocity contour:-
Pressure contour: -
Vector plot: -
CASE2: -
Residual plot: -
The solution is converged in 78th iteraton.
Drag report: -
Cd= 0.33283967
Lift report: -
Cl= 0.23655259
Velocity contour: -
Pressure contour plot: -
Vector plot: -
CASE3: -
Residual plot: -
The solution is converged in 83rd iteration
Drag report: -
Cd= 0.33946406
Lift report: -
Cl= 0.25458912
Velocity contour plot: -
Pressure contour plot:-
Vector plot: -
Grid independency test result comparision: -
| OUTER ENCLOSURE ELEMENT SIZE | INNER ENCLOSURE ELEMENT SIZE | INFLATION LAYERS WITH FIRST LAYER THICKNESS OF Y+ 50 | DRAG VALUE | LIFT VALUE |
| 90mm | 42mm | 6 | Cd=0.33735813 | Cl=0.22870985 |
| 90mm | 40mm | 7 | Cd=0.33283967 | Cl=0.23655259 |
| 90mm | 50mm | 9 | Cd=0.33946406 | Cl=0.25458912 |
Conclusion: -
Hence the smaller the element size the mesh will be more refined and drag,lift value will be near to the accurate value, possibilities of error will be less. As we discussed above, the flow detached from the ahmed body produces vortices and creates negative pressure in the wake region.That place is the point of seperation of the flow.