Week 3 - External flow simulation over an Ahmed body.
AIM: To study the aerodynamics of Ahmed's body and Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.
OBJECTIVE:
- Describe Ahmed's body and its importance.
- Explain the reason for the negative pressure in the wake region.
- Explain the significance of the point of separation.
Ahmed's Body and its importance
- It represents a simplified ground vehicle geometry of a bluff body type. Its shape is simple enough to allow for accurate flow simulation but retains some important practical features similar to automobile bodies.
- The airflow around the body captures the essential flow features of an automobile and was first defined and characterized in the work of Ahmed.
- Though it has a simple shape,it allows to capture essential features that are relevant to the bodies in the automobile industry.
- This model describes how to calculate the turbulent flow field around a simple car geometry.
- People working in vehicle aerodynamics use Ahmed's body to validate their model because the experimental data from wind tunnel testing is available for Ahmed's body
- Once the numerical model is validated, it is used to design new models of the car
The Ahmed body was described originally by S. R. Ahmed in 1984. Three main features were seen in the wake:
1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model
2. The B recirculation region that is formed due to the separation at the base of the model.
3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.
The wake was shown to be highly dependent on slant angle. For slant angles less than 12°, the flow remains attached over the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30° the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.
Geometry of Ahmed's Body
Reason for the negative pressure in the wake region.
The negative pressure is due to the velocity difference applied at a certain Reynold number and the rear angle, which at a low Reynolds number when this difference in pressure happens the wake obtained increases the drag due to the pressure drop behind the Ahmed body, which at a high Reynolds number due to the vortices generated it produces small amounts of pressure thrusting the body.
The air becomes almost stagnant as it strikes Ahmed's body which results in air exerting very high pressure on the front face of Ahmed's body. The airflow then gets divided between the upper and lower surface of Ahmed's body. The higher-pressure air on the front surface accelerates as it travels over the curved nose surface of Ahmed's body, causing the pressure to drop. This lower pressure creates lifts over the roof surface as the air passes over it. As the air continues to flow and makes its way to the rear, a notch is created by the rear slant owing to flow separation, leaving a vacuum or low-pressure space which the air is not able to fill properly. The resulting lower pressure creates lift and drag that then acts upon the surface area of the rear slant.
The significance of the point of separation.
Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. Separation occurs in flow that is slowing down, with pressure increasing, after passing the thickest part of a streamlined body or passing through a widening passage.
The point of separation defined by the Ahmed body and the flow, the alpha angle depending on which value is at it causes a change in the flow direction, forces required, lift on the body, induced drag due to “tip vortex”, at a critical value flow in slant stalls and reduced lift-reducing as well drag, all depending on the value of alfa and the Reynolds number.
Procedure
- Setting up the geometry
Isometric view
Side view
Rear View
Front view
Creation of Enclosures around the body for baseline mesh generation
Outer enclosure
- The outer enclosure is at a distance of 2.5m from the Front side of Ahmed's body on the X-axis.
- The outer enclosure is at a distance of 5m from the Rear side of the Ahmed body on the X-axis.
- From the top, it is at a distance of 1.5m in the Y-axis.
- At the bottom, it is merged means it is at zero distance.
- From the left and right side of Ahmed's body, it is 0.5m along Z-axis.
Inner enclosure
- The inner enclosure is at a distance of 0.5m from the Front side of Ahmed's body on the X-axis.
- The inner enclosure is at a distance of 1m from the Rear side of the Ahmed body on the X-axis.
- From the top, it is at a distance of 0.5m on the Y-axis.
- At the bottom, it is merged means it is at zero distance.
- From the left and right sides of Ahmed's body, it is 0.28m along Z-axis.
Interface Region
Meshing
The mesh generation on the enclosures i.e. the outer has hexahedral grids to capture the flow structure and the inner has tetrahedral grids and inflation layers created along the walls to capture the point of separation.
Setup
Conditions
- Steady-state, density-based solver
- viscous model,k-epsilon
- air as flow material
- inlet velocity- 25m/s
- Uncheck all the convergence criteria.
- Solution method- hybrid-initialize
The inlet velocity will be 25 m/s, the density of the fluid will be 1.225 kg/m^3, dynamic viscosity will be 1.7894x10^-5 kg/m-s. - boundary conditions
In the case of the solution, we have the residual plots, drag, lift, and other reports.
Results
- CASE 1.
- Y-plus = 50
- Viscous model: K-epsilon
- 1st box domain =100 mm size of hexahedral mesh
- 2nd box domain = 50 mm size of tetrahedral mesh
- Inflation layer thickness = 0.7 mm
- Elements = 77,055
- Nodes = 25,415
CASE 2.
- Y-plus = 125
- Viscous model: K-epsilon
- 1st box domain =75 mm size of hexahedral mesh
- 2nd box domain = 35 mm size of tetrahedral mesh
- Inflation layer thickness = 1.79 mm
- Elements = 1,95,799
- Nodes = 56,779
Velocity contour plane
Pressure contour plane
Residuals plot
Drag coefficient
Vector plot
CASE 3.
In the third case, the simulation was done for the following cell count.
- Y-plus = 150
- Viscous model: K-epsilon
- 1st box domain =75 mm size of hexahedral mesh
- 2nd box domain = 35 mm size of tetrahedral mesh
- Inflation layer thickness = 1.43 mm
- Elements = 1,96,895
- Nodes = 56,975
Velocity contour plane
Pressure contour
CASE 4.
In the third case, the simulation was done for the following cell count.
- Y-plus = 5
- Viscous model: K-Omega
- 1st box domain =75 mm size of hexahedral mesh
- 2nd box domain = 35 mm size of tetrahedral mesh
- Inflation layer thickness = 0.0716 mm
- Elements = 2,01,422
- Nodes = 57,752
Velocity contour plane
Pressure contour plane
Residual plot
Drag coefficient
Velocity contour
Comparison Table
| K-epsilon | ELEMENTS | Cd | Cl |
| CASE 1 | 77052 | 0.3622 | 0.25149 |
| CASE 2 | 1,95,799 | 0.3349 | 0.2780 |
| CASE 3 | 1,96,895 | 0.3327 | 0.2831 |
| K-Omega | |||
| CASE 4 | 201422 | 0.3006 | 0.2029 |
Conclusion
There is a variation of pressure across the body. The pressure near the front is positive as that is the one that comes in contact and comes to rest. It decreases at the corner as the flow accelerates.
The reason behind negative pressure in the wake region is due to flow separation, and 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.