Week 3 - External flow simulation over an Ahmed body.

AIM: To study the aerodynamics of the Ahmed's body and Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.

OBJECTIVE:

 1. To Describe Ahmed's body and its importance.

 2. To Explain the reason for the negative pressure in the wake region. 

 3. To Explain the significance of the point of separation.

1.Ahmed's body and its importance:

The Ahmed body 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 relevant to automobile bodies. This model describes how to calculate the turbulent flow field around a simple car-like geometry.

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, the Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.

ANSYS is a general-purpose FEA and CFD simulation software. It gives a numerical prediction of the behaviour of a system. These numerical predictions need to be validated by experimental data.

This model is also used to describe the turbulent flow field around a car-like geometry. 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 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.

The Geometry of Ahmed's Body is as shown in the below figure.

 

 

2. Explain the 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 body which results in air exerting very high pressure on the front face of Ahmed body. The airflow then gets divided between the upper and lower surface of the Ahmed body. The higher pressure air on the front surface accelerates as it travels over the curved nose surface of Ahmed 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 make 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 the drag that then acts upon the surface area of the rear slant.

The following figure shows the pressure thrusting.

 

 

3. 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 streamline 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.

 

Workflow: Geometry  ---> Meshing ---> Solving ---> Results

Geometry:

First, we need to set up the geometry of Ahmed's Body for our simulation.

 

 

 

                 

                                                Isometric view of the three-dimensional model of Ahmed body.

 

 

                                          

                           

                                                                       Front view of the Ahmed body.

 

 

                            

                                                                           Side view of Ahmed body.

 

                  

                                                                     Rear view of Ahmed body.

we can also calculate the frontal area and length for reference in solver

Area = 0.115032 m^2

Length = 1.044 m

 

Creating the enclosures around the Ahmed Body for baseline mesh generation.

                  

Outer enclosure

• The outer enclosure is at a distance of 2m from the Front side of the Ahmed body in the X-axis.
• The outer enclosure is at a distance of 5m from the Rear side of the Ahmed body in the X-axis.
• From the top, it is at a distance of 1m in the Y-axis.
• At the bottom, it is merged means it is at zero distance.
• From the left and right side of the Ahmed 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 the Ahmed body in the X-axis.
• The inner enclosure is at a distance of 1m from the Rear side of the Ahmed body in the X-axis.
• From the top, it is at a distance of 0.5m in the Y-axis.
• At the bottom, it is merged means it is at zero distance.
• From the left and right side of the Ahmed body, it is 0.26m along Z-axis.

 

Interference region.

 

Initially, there was a huge interference between the Outer and Inner encloser so that the mesh gets overlapped which gets not acceptable so to avoid this we made share topology and get deleted the Interference region that by using the interference command after this we get a good interaction between the Inner and Outer enclosers so that we can get the mesh properly.

We have made 3 casese of mesh for same geometry

Case 1 : Baseline mesh

• Y-plus = not used
• Viscous model: K-omega
• 1st box domain =not specified
• 2nd box domain = not specified
• Inflation layer thickness = none
• Elements = 84,798
• Nodes = 17,168

MESH

• It was a base line mesh

 

SET UP

• in the setup>physics>steady state and density-based solver.
• setup>physics>viscous model>k-omega>ok.
• setup>physics>materials, select air as a flow material>ok
• setup>physics>boundaries>inlet  -velocity as 25 m/s, outlet gauge pressure as 0, other conditions will be the default values we are considering>click on ok
• setup>solution>report>definitions>new>force report>drag>drag coefficients>car-wall>select report file and report plot >save/display
• setup>solution>method>Hybrid>initialize
• Result>create>plane>point and normal>ok
• setup>solution>run calculation

DO NOT FORGET TO GIVE REFERENCE VALUES

SOLUTION

1. Residuals

2. Drag co-efficient

3. Lift co-efficient

4. Velocity contour

5. Pressure contour

RESULTS:

1. Vector on xy plane

 zoomed in for better understanding of vectors

2. Vectors on yz plane 200mm after the ahmed body

 3. Vectors on yz plane 400mm after the ahmed body

 

 

 

Case 2 : Slightly Refined mesh

• Y-plus = 100
• Viscous model: K-omega
• 1st box domain =100 mm size of hexahedral mesh
• 2nd box domain = 50 mm size of tetrahedral mesh
• Inflation layer thickness = 6 layers, 3.63mm total thickness
• Elements = 212,687
• Nodes = 65,209

MESH

 

Quality of mesh

 

SET UP

• same for every case

SOLUTION

1. Residuals

2. Drag co-efficient

3. Lift co-efficient

4. Velocity contour

5. Pressure contour

RESULTS:

Vector on xy plane

 zoomed in for better understanding of vectors

Vectors on yz plane 

 

 

• 200mm after the ahmed body

 

 

• 400mm after the ahmed body

   

• 600mm after the ahmed body

 

• 800mm after the ahmed body

 

 

 

 

Case 3 : More Refined mesh

• Y-plus = 100
• 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 = 6 layers, 3.63mm total thickness
• Elements = 506,492
• Nodes = 142,253

MESH

     

Quality of mesh

 

SET UP

• same for every case

SOLUTION

1. Residuals

2. Drag co-efficient

3. Lift co-efficient

4. Velocity contour

5. Pressure contour

RESULTS:

Vector on xy plane

zoomed in for better understanding of vectors

 

Vectors on yz plane 

 

 

• 200mm after the ahmed body

 

• 400mm after the ahmed body

   

• 600mm after the ahmed body

 

• 800mm after the ahmed body

 

 STREAMLINE VIEW :

 

Negative pressure gradient

When fluid velocity increases, the pressure decrease. The wake region is formed due to adverse pressure in the rear end causes the boundary layer to separate out from the body. The wall shear stress and velocity becomes zero and wake formation starts. These wakes are nothing but small vortices that create negative pressure. These create circulation of velocity and induces drag forces.

Point of seperation

Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculating flow and the flow through the central region  is called the dividing streamline. The point where the dividing streamline attaches to the wall again is called the reattachment point. As the flow goes farther downstream it eventually achieves an equilibrium state and has no reverse flow.When the boundary layer separates, its remnants form a shear layer and the presence of a separated flow region between the shear layer and surface modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag.

 

Comparision.

	ELEMENTS	NODES	Cd	Cl
CASE 1	84,798	17,168	0.451	0.232
CASE 2	2,12,687	65,209	0.381	0.233
CASE 3	5,06,492	1,42,253	0.339	0.240

Conclusion

1. From running a baseline simulation, the post CFD results gave inaccurate results at the region of mesh overlapping.
2. The grid independency test shows best results close to the experimental data. With refining the mesh size with y+ factor, the error is significantly reduced.
3. The y+ is chosen in between a stable range to give a correct prediction of velocity and pressure contour plots.
4. The flow seperation and appearence of vortices gets more prominent as y+ is increased. As seen in baseline meshing no wake formation is captured properly, on refining the mesh, the flow is captured properly. (Y+ plus values were changed and recorded but its not in report)

5. 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.

6. 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.

7. 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.