Week 3 - External flow simulation over an Ahmed body.

A1. Describe Ahmed's body and its importance.

The Ahmed body is a simplified car body used in the automotive field to study the impact of the flow pattern on the 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 separations as well as

there is a growing need for more insight 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 which link the front part and the rear slant plane. The principal objective to study

such a simplified car body is to tackle the flow processes 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

maximize vehicle performance.

 

Important of Ahmed body:

 

An Ahmed body represents a simplified vehicle volume which helps understand the fundamental physics defining by the

engineers the lift and drag forces and coefficients at certain characteristically velocities, pressure differences 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 behavior of a system.

These numerical predictions need 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 important 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 [1]. Three main features were seen in the wake:

The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model

The B recirculation region that is formed due to the separation at the base of the model.

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.

 

A2. Reason for 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 regions 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 the higher pressure beside the wake

region try to approach from the higher pressure region to the lower pressure region thus creating a circularity 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 least in 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.

 

A3. 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 separations 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 supportively studied with the help of the dp/dx plots or velocity plots 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 in eraction.

 

Geometry setup:

The geometry of the Ahmed body is  shown below.

                                                       3D view of  Ahmed body 

 

                                              Ahmed body front view

 

                                            Ahmed body side view

 

 

                                                        Ahmed body rear view

 

 

 Geometry setup in SpaceClaim:

As the body is perfectly symmetric, we can run the simulation by considering only half the body. This is the best practice

where you can save on the number of cells and get the results faster as well. The geometry which is provided with the

challenge needs a modification which you should be able to do at this stage of a course. You need to use the 'split body'

command in SpaceClaim to perform the operation and then you can use the symmetry boundary condition in fluent to

perform the simulation. 

1. Load the Ahmed body model into SpaceClaim and convert the units from millimeters to meters.

 

 

2. Go to prepare and click on the enclosure and select the Ahmed body. Enter the enclosure dimensions as shown in the

below snap.And uncheck symmetric option before giving the value

 

3. once the enclosure is done, we can create another enclosure closely around Ahmed body so that we can refine the mesh in

that enclosure and still keep the total cell countless. 

Ahmed body with inner and outer enclosure:

After both the enclosure is created, we can check interference and avoid overlapping of meshing in the 2nd enclosure by

applying check interference and deleting the overlap. This is automatically done by Spaceclaim when the interference option

is selected inside prepare.

 

5. After clearing the interference, we can now split the body in half using the split body command in design, but before that,

we have to create a XY plane at the origin to generate 2 equal halves. Select a design and choose planes and bring the cursor

near the origin and left-click. we get 3 planes, hide the unnecessary planes and use the XY plane.   choose split body and

choose the enclosure needed to split, select the plane and choose the part needed to be removed. Similarly, we can achieve

split on both the enclosures and the body.

6. After achieving the split, hide the Ahmed body, suppress the physics and go to properties, and select Share topology. Now

the mesh from both the enclosures can share the information and have a symmetry.

 

Mesh:

Case 1:

Outer enclosure: 100mm(elemental size)

inner enclosure:50mm(elemental size)

Face sizing for Ahmed body :5mm(elemental size)

inflation for ahmed body  :5layer with total thickness 12.4mm

Total number of elements=90433

Fluent Set up procedure:

Update the mesh before opening the fluent setup and mesh checking should be done

Take the k-epsilon turbulence model because we are taking the inlet velocity 25m/s which is smaller

Consider the pressure density solver 

Take 25m/s as a velocity for the inlet boundary

Then initialize the solution

After initialize we should go to the post process to create a cut plane to see the velocity profile on it

And create a contour plot for both pressure and velocity 

Also, create a plot for drag and lift using the definition tool

And give the reference values for the Ahmed body 

Run the simulation for 500 iterations.

                        

 

Results:

We should insert  the plane along the z axis to get the velocity and pressure variation

1.Residual plot

2.velocity and pressure contour plot

3.drag and lift the coefficient plot 

4.vector plot

 

5.Velocity variation:

Case 2:

Outer enclosure: 90mm(elemental size)

inner enclosure:40mm(elemental size)

Face sizing for Ahmed body:5mm(elemental size)

inflation for Ahmed body:5layer with a total thickness of 12.4mm

Mesh

Cell counts:144529

Results:

1.Residual plot

2.velocity and pressure contour plot

3.drag and lift the coefficient plot 

4.vector plot

 5.Velocity variation:

Case 3:

Outer enclosure: 80mm(elemental size)

inner enclosure:30mm(elemental size)

Face sizing for Ahmed body:5mm(elemental size)

inflation for Ahmed body:5layer with a total thickness of 12.4mm

Mesh

Results:

1.Residual plot

 

2.velocity and pressure contour plot

 

3.drag and lift the coefficient plot 

 

4.vector plot

 

Grid independence test

number of cells

$C_d$

$C_L$

case 1	90433	0.3592	0.235
case 2	144529	0.338	0.238
case 3	293485	0.3185	0.2384

 

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 coefficients.

Conclusion:

High pressure is formed at the frontal area of the Ahmed body due to the loss of kinetic energy of the air.

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

coefficients.

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. 

With the increase in mesh size, the numerical results approach closer to experimental results