Week 3 - External flow simulation over an Ahmed body.
Describe Ahmed's body and its importance?
The Ahmed body is a simplified car body used in the automobile field to study the impact of the flow pattern on drag. The external aerodynamics of the car defines many major traits of an automobile like stability, comfort, and fuel consumption at high speeds. The flow around the vehicle is characterized by high turbulent and three-dimensional flow separation as well as there is a growing need for more insights into the physical features of these dynamical flows. The Ahmed body is a simplified car, used in the automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed body is made up of a round front part, a moveable z slant plane in the rear of the body to study the detachment phenomena at different angles, and a rectangular box that links the front part and rear slant plane. The principal objective to study such a simplified car body is to tackle the flow process involved in drag production. Through perceiving the mechanisms involved in creating drag one can be able to design a car to minimize drag and therefore reducing fuel consumption and maximizing vehicle performance.
An Ahmed body represents a simplified vehicle volume which helps understands the fundamental physics defining by the engineers the lift and drag forces and coefficients at certain characteristically velocities, pressure difference and seeing and analyzing the vortices, turbulence, and Reynolds number obtained. ANSYS is a general purpose FEA and CFD simulation software. It gives a numerical prediction of the behaviour of a system.
These numerical predictions needs to be validated by experimental data. 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 invalidated, it is used to design new models of the car.
Its shape is simple enough to allow for accurate flow simulation but retains some importance practical features relevant to automobile bodies. This model describes how to calculate the turbulent flow field around a simple car like geometry using the Turbulent Flow, K-epsilon and K-omega interface. Later, either experimentally or computationally it can be recreated all the same but with the desired geometry, new to come up to the market or research.
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.
Explain the reason for the negative pressure in the wake region?
The path lines showing the exact circular motion or vorticity generation just at the rear end of the car. The separation takes two region of distribution. One at the upper part of the rear and the other at the lower part of the rear end.
The path lines show about the nature of the fluid interaction with the rear end of the Ahmed body which here shows that the fluid just after leaving the contact from the slope still possess the momentum in it and the fluid particles near to the surface almost show the no-slip region which means the very least possible velocity of the fluid particles is in contact with the slope and base of the rear part. Thus the fluid particles away from the rear region having high pressure beside the wake region try to approach from the higher pressure region to the lower pressure region thus creating exactly in the wake region created.
The body penetrating the fluid creates a region of emptiness just beside its end due to the nature of the geometry of the body here which can be seen last in the case of a perfectly streamlined shaped body. This emptiness created is a low-pressure region towards which the fluid column exactly behind the wake region which of atmospheric pressure tries to fill the space thus dragging the fluid column with it thus creating the region of vorticity. This also results in the huge pressure gradient in the region exactly rear of the body creating the-ve pressure at the end. Also, this is the layman's reason why there is negative gauge pressure in the wake region.
Explain the significance of the point of separation?
The fluid separation point can be defined as a point where the dv/dy i.e. gradient of velocity goes zero along with the dp/dx = 0, for the specific point. In more simplified term we can say that the point where the velocity gradient goes zero and the pressure gradient along the x-direction goes zero the point of separation starts occurring or the negative pressure gradient starts forming after that point. The point of separation cannot be understood just by single contour in hard and fast way. It can be supportive studied with the help of the dp/dx plots or velocity w.r.t the Y direction and other such plots in which we can see the nature of such physical properties like dp/dx=0, and dv/dy=0 experiencing in a particular region of fluid and body interaction.
Geometry Setup
Isometric View of Ahmed Body
Front View of Ahmed Body
Side View of Ahmed Body
Back View of Ahmed Body
Adding an Enclosure to the Ahmed body from the "Prepare" tab, and it is defined with the above dimensions.
Dimension of the Enclosure from the front view.
Adding another Enclosure inside the above-made enclosure, it is made to get a more refined answer around the Ahmed body
The isometric view of Ahmed body with both the Enclosure-s
After the Enclosure is made, the next step is to check the interference to avoid the overlapping of mesh between the inner and outer enclosure. This is automatically done by the interference option in the SpaceClaim.
After the interference option is done we can split the whole body into two from the x-y plane. Here the whole body is split in a half, which is done using the split body command.
On completing the split body command, the next step is to suppress for physics the Ahmed body from the geometry. This is done because we don't need meshing to be done inside Ahmed body. Now select Share topology. This shares information from both the mesh and have an exact symmetry.
MESH
CASE 1
Outer Enclosure - Element size = 100mm
Inner Inclosure - Element Size = 50mm
Support stand for Ahmed Body = 5mm
Inflation Layer - 5 layer with total thickness = 12.4mm
Total Number of elements = 89200
Fluent
Viscous Model - K-epsilon (Because the inlet velocity given is 25m/s which is smaller)
Take the Pressure-Density Solver
Inlet velocity - 25m/s
The area in the Reference value is the projected frontal area and the length is the characteristic length of the body.
Now the Solution is Run for 600 Iterations
Results
1.Residual Plot
2. Velocity Plot
3.Pressure plot
4. Drag plot
5. Lift Plot
6. Vector plot of Velocity
CASE 2
MESH
Outer Enclosure - Element size = 90mm
Inner Inclosure - Element Size = 40mm
Support stand for Ahmed Body = 5mm
Inflation Layer - 5 layers with total thickness = 12.4mm
Total number of elements = 140474
Results
1. Velocity Plot
2. Pressure Plot
3. Drag plot
4. Lift Plot
5. Residual Plot
A Total Number of 600 iterations is done
6. Velocity Vector Plot
Case 3
Outer Enclosure - Element size = 80mm
Inner Inclosure - Element Size = 30mm
Support stand for Ahmed Body = 5mm
Inflation Layer - 5 layers with total thickness = 12.4mm
Total Number of Elements = 283603
Results
1. Velocity Plot
2. Pressure plot
3.Drag plot
4. Lift Plot
5. Residual Plot
6. Velocity Vector
Grid Independence Test
From the above table with the refinement of the mesh drag coefficient approaches closer to the analytical values that are because more cells are assigned at the boundary layer which results in more accurate results of drag coefficient.
Conclusion
1. High pressure is formed at the frontal area of the Ahmed body due to the loss of Kinetic energy of the air
2. The adverse pressure gradient is created near the wake regions because of the flow separation which is occurring because of the geometry of the model. As a result flow separation alters the pressure gradient which in turn affects the drag and lift coefficient.
3. In order to reduce the size of the wake region, detachment of the fluid should be farther away from the body which can be achieved by an increase in the slant angle
4. With the decrease in the mesh size, the numerical results approach closer to the experimental results.