Ahmed Body Challenge
Aim: To study the aerodynamic effects of flow over the Ahmed body and conduct a suitable grid independence test.
Introduction: As the burning of fossil fuels becomes a more pressing issue, manufacturers are introducing more fuel-efficient cars to the market. One main contributor to fuel burn is the car’s aerodynamic drag. Complexly shaped, cars are very challenging to model and it’s difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model while maintaining car-like geometry features.
Theory:
Source:https://www.cfd-online.com/Wiki/File:Ahmed.gif
The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, a height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees(20 degrees in our case).
Modeling Approach:
1.Computational Domain:
The airflow over the Ahmed body is simulated using enclosures. Two enclosures were created, one for capturing the entire computational domain which will be having coarse mesh and the other was a region of refinement with finer mesh to capture the physics close to the Ahmed body accurately.
The dimension of the bigger enclosure and smaller enclosure are:
Bigger Enclosure:
Length = 8m
Height = 1.338m
Width = 1.389m
Smaller Enclosure:
Length = 2.5m
Height = 0.838m
Width = 0.909m
Also using Split command the entire computational domain was split onto 2 parts. This was possible because the Ahmed body is perfectly symmetric, and this technique will still give us accurate results at lower computational cost and time.
The below figure shows the computational domain which was modeled in SpaceClaim:
Pre-Processing:
2.Meshing:
Meshing was done in 2 stages.
First, we generated a baseline automatically generated mesh and observed the simulation result.
Second, we employed meshing methods and conducted grid independency tests to observe the simulation result.
The named selections were as follows:
Stage 1:
Baseline Mesh:
The baseline mesh had an element size of 409.2 mm with no inflation layers.
Type of mesh elements created: Tetrahedral
Mesh Quality:
1. Element Quality
2. Aspect Ratio
3. Skewness
4. Orthogonal Quality:
Stage 2:
Refined mesh with methods and inflation layers.
Three trials of mesh independence tests were carried out.
2.a.
Mesh Details:
Global Element size:100 mm
Methods employed:
Multizone method applied at big enclosure to create a structured hexahedral mesh.
Body sizing applied at small enclosure(50mm)
Face sizing applied at Ahmed body(10mm) and cylindrical legs(2mm)
Inflation layer was applied at the Ahmed body and road with the following properties:
Mesh Quality:
1. Element Quality
2. Aspect Ratio
3. Skewness
4. Orthogonal Quality:
2.b.
Mesh Details:
Global Element size:80 mm
Methods employed:
Multizone method applied at big enclosure to create a structured hexahedral mesh.
Body sizing applied at small enclosure(40mm)
Face sizing applied at Ahmed body(10mm) and cylindrical legs(2mm)
Inflation layer was applied at the Ahmed body and road with the following properties:
Mesh Quality:
1. Element Quality
2. Aspect Ratio
3. Skewness
4. Orthogonal Quality:
2.c.
Mesh Details:
Global Element size:60 mm
Methods employed:
Multizone method applied at big enclosure to create a structured hexahedral mesh.
Body sizing applied at small enclosure(30mm)
Face sizing applied at Ahmed body(10mm) and cylindrical legs(2mm)
Inflation layer was applied at the Ahmed body and road with the following properties:
Mesh Quality:
1. Element Quality
2. Aspect Ratio
3. Skewness
4. Orthogonal Quality:
3. Fluent SETUP:
Under the Physics Tab, in General properties:
Solver-Pressure Based(Since the Inlet velocity is only 25 m/s, which is approximately 0.08 Mach number.)
Velocity Formulation-Absolute
Time-Steady State
Reference values were calculated according to geometry and BC:
Viscous Model: K-ε model with standard wall functions.
Material properties: Air
Boundary Conditions:
Velocity Inlet = 25m/s
Pressure Outlet = 1 atm(Gauge Pressure = 0Pa)
Ahmed body-wall(No slip BC)
symmetry BC to all the walls of the enclosures.
road - wall(No slip BC)
Solver: Pressure-Velocity Coupling- SIMPLE
Number of iterations-500
Stage 1 simulation results:
1.a. Residuals Plot
1.b. Coefficient of drag, Cd
Coefficient of drag value is 0.46775
1.c.Coefficient of lift, Cl
Coefficient of lift value is 0.29251
Post-Processing
1.d. Velocity Contour
1.e. Pressure Contour
1.f. Velocity Vector Plot
Stage 2.a. simulation results:
2.a.i Residuals Plot
2.a.ii. Coefficient of drag, Cd
Coefficient of drag value is 0.33155
2.a.iii. Coefficient of lift, Cl
Coefficient of lift value is 0.27820
Post-Processing
2.a.iv. Velocity Contour
2.a.v Pressure Contour
2.a.vi.Velocity Vector Plot
Stage 2.b. simulation results:
2.b.i Residuals Plot
2.b.ii. Coefficient of drag, Cd
Coefficient of drag value is 0.33172
2.b.iii. Coefficient of lift, Cl
Coefficient of lift value is 0.28168
Post-Processing
2.b.iv. Velocity Contour
2.b.v Pressure Contour
2.b.vi.Velocity Vector Plot
Stage 2.c. simulation results:
2.c.i Residuals Plot
2.c.ii. Coefficient of drag, Cd
Coefficient of drag value is 0.33209
2.c.iii. Coefficient of lift, Cl
Coefficient of lift value is 0.27368
Post-Processing
2.c.iv. Velocity Contour
2.c.v Pressure Contour
2.c.vi.Velocity Vector Plot
Grid Independence Test:
Hence we can observe that in the refined mesh models the change in Cd and Cl values is less than 0.5 percent. Therefore we can utilize the refined mesh with the least elements to get acceptable result accuracy while saving computational cost.
Result and Conclusion:
The baseline mesh gave a poor result, it wasn't able to capture the flow around the Ahmed body and the wake structure, due to coarse mesh.
The refined mesh gave better results. The plots show the wake formation accurately and the flow around the Ahmed body was also captured pretty well.
There is a variation of pressure across the Ahmed body. The pressure in front of the body is positive because that's the location where the flow first comes in contact with the body and comes to rest. At the corner, the flow accelerates, thereby decreasing the pressure.
The reason behind negative pressure in the wake region is due to flow separation, recirculating flow. It's a phenomenon that generally happens as the air moving past the body isn't able to pass through the immediate backside of the body creating a region, where there is no movement of air and thereby creating a pressure gradient.
As the wake flow is the main contributor to the drag force(pressure drag), the point of separation is an important aspect of the flow. The more attached the flow is to the body the recirculation zone will be smaller and hence reduce the adverse effect of pressure drag, improving the fuel efficiency of the body.
Generally, as the slant angle increases, the flow separation occurs early and vice-versa.
Animation Link:https://www.youtube.com/watch?v=tVA1vewwhMo