Week 3 - External flow simulation over an Ahmed body.

External flow simulation over an Ahmed body

AIM: To study external aerodynamic simulation over an Ahmed body for a velocity of 25 m/s and to perform mesh independence test with 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.

Ahmed body:

The Ahmed body is a generic car body (a simplified vehicle model). It is used in the automotive field to study the impact of the flow pattern on the drag. The external 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. 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.

An Ahmed body represents a simplified vehicle volume which helps understand the fundamental physics, lift and drag forces and coefficients at certain velocities, pressure differences and analyzing the vortices, turbulence, and Reynolds number. Ahmed body cfd simulations can be used to validate numerical models with the available experimental data. Once the numerical model is validated, 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.

Significance of Ahmed Body:
1. Ahmed Body plays a vital role in testing the aerodynamic properties of vehicle.
2. It is also used to describe the turbulent flow field around a car-like geometry and later to design new car models with validated results.

Wake Region:The region of recirculating flow or disturbed flow behind a moving or stationary blunt body, caused by viscosity, which may be accompanied by flow separation and turbulence. 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.

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 where atmospheric pressure tries to fill the space, thus dragging the fluid column with it. which creates the region of vorticity. This also results in the huge pressure gradient in the region exactly rear of the body creating the negative pressure at the end. This is the layman’s reason why there is negative gauge pressure in the wake region.

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 interaction.

Geometry :

Double click on geometry, we can edit the geometry in spaceclaim to desired dimensions.

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.

4. 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.

Meshing:

Open the meshing console and divide the different part using named selection as follows

Inlet

Outlet

Symmetry

Car-wall(Ahmed body)

Case 1:

Outer enclosure = 110 mm

Inner Enclosure = 55 mm

Ahmed body size=10mm

Y+=100

Density = 1.225 kg/m3

Viscosity = 1.7894e-5 Pa-s.

Velocity = 25 m/s

Length = 1.044 m

first layer thickness is calculated

https://www.pointwise.com/yplus/index.html

To capture better values of drag and lift coeffiecient, Y+ should lie between 30 to 300.

MESH

Residuals

Co-efficient of drag

Co-efficient of lift

Velocity Contour

Pressure Contour

Vector plot

Case 2

Outer enclosure = 90 mm

Inner Enclosure = 50 mm

Ahmed body size=10mm

Y+=200

Density = 1.225 kg/m3

Viscosity = 1.7894e-5 Pa-s.

Velocity = 25 m/s

Length = 1.044 m

Mesh

Residuals

Co-efficient of drag

Co-efficient of lift

Velocity contour

Pressure contour

Vector plot

Case 3

Outer enclosure = 80 mm

Inner Enclosure = 45 mm

Ahmed body size=10mm

Y+=200

Density = 1.225 kg/m3

Viscosity = 1.7894e-5 Pa-s.

Velocity = 25 m/s

Length = 1.044 m

Mesh

Residuals

Co-efficient of drag

Co-efficient of lift

Velocity Contour

Pressure Contour

Vector plot

	Inner body(mm)	Outer body(mm)	Ahmed body(mm)	Y+	Cd	Cl
Case 1	110	55	10	100	0.362	0.288
Case 2	90	50	10	150	0.33	0.248
Case 3	80	45	10	100	0.32	0.25

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