Week 3 - External flow simulation over an Ahmed body.

Aim:
Study of Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model)

objective:

Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.

For this challenge, you will have to provide answers to the following questions:

Q1. Describe Ahmed's body and its importance.

Q2. Explain the reason for the negative pressure in the wake region.

Q3. Explain the significance of the point of separation.

Q1. Describe Ahmed's body and its importance.

The Ahmed body is simplified car body used in 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 automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed body is made up of 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 like 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 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 in 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. Later, either experimentally or computationally it can be recreated all the same but with a desired geometry, new to come up to the market or a research.

The Ahmed body was described originally by S. R. Ahmed in 1984 [1]. 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.

Q2. 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.

Q3. Explain the significance of the point of separation.

The point of separation defined by the Ahmed body and the flow, the alpha angle depending on which value is at it causes change in 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 alpha and the Reynolds number.

Procedure:

Case 1

Geometry:

Endosure 1

from this we can see the overlapping is there we can clear or remove it by the interfernce tool in prepare

from this we can clearly see the hatch surface is deleted

now we can delete the one half of whole body because our ahmed body is symmetry so, taking the one half we can get the result easily because of computational time and cell count of the mesh by using th split body tool we can delete the body

Then important thing is we should do is share option because while mesh the inner body and outer body is need to be comunicate so, enable the share option spaceclaim itself.

Meshing part

insert the method and turn to automatic to multizone because we need 1st box has quad that mesh hexa mesh type

Now we need to insert the inflation layer because we need to know the solution near the car wall body

first layer thickness is 5mm and maximum layer is 5 growth rate is 1.2 as default

from the above fig the shape of inflation is not perfect so we can go to total layer thickness so the inflation layer will perfect in shaped

setup part

so, till this same for the all the cases we need the increase the cell count so, that we can get the better results

case 1

mesh - element size = 50 mm

body sizing = 40 mm

cell count = 1,96,758

Velocity Contour Pressure Contour

Drag plot Lift plot

Residual scaled plot

Velocity result Contour Pressure result Contour

Velocity Animation Pressure Animation

case 2

mesh - element size = 45 mm

body sizing = 33 mm

cell count = 3,56,953

In this case the mesh and body sizing as been reduced to get more refined mesh element

In this case 2 car wall faces have been improve by the face sizing option element sizing is 15mm

Velocity Contour Pressure Contour

Drag plot Lift plot

Residual scaled plot

Velocity result Contour Pressure result Contour

Velocity Animation Pressure Animation

Vector Contour Plot

Vertex Contour

case 3

mesh - element size = 43 mm

body sizing-element size = 28 mm

cell count = 4,90,025

In this case the mesh and body sizing as been reduced to get more refined mesh element

Velocity Contour Pressure Contour

Drag plot Lift plot

Residual scaled plot

Velocity result Contour Pressure result Contour

Velocity Animation Pressure Animation

Vector Contour Plot

Vertex Contour

This vertex plot for the 4 tem at the bottom of the body

Comparsion Table for drag and lift for all cases

CASE	CELL COUNT	DRAG	LIFT
CASE 1	1,98,758	0.32968931	0.22863514
CASE 2	3,56,953	0.31432759	0.23267479
CASE 3	4,90,025	0.31122161	0.2323107

Result:

From the above table we can say the when the mesh cell count less the drag increased and wake regoin also can't able to see in 1st case and in 2nd case the mesh cell count is increase more the 1st case but we get the drag coefficent is decrease campared to the 1st case and also the wake region also can able see at the end the body

In the 3rd case the mesh more fine the cell count also is increased. in result the drag coefficent also reduced and the wake region also can able seen more than the case 1 and 2

so, have we increase the cell count and maintened the y+ number so that we can get more fine result

CONCLUSION

• The baseline mesh gave a poor result, it wasn't able to capture the flow around the Ahmed body and the wake structure, due to coarse mesh.
• The refined mesh gave better results. The plots show the wake formation accurately and the flow around the Ahmed body was also captured pretty well.
• 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.
• 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.
• 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.

References

1. G. T. Yates. “How Microorganisms Move through Water,” American Scientist, 74, pp. 358–365, July–August, 1986.
2. R. J. Heinsohn and J. M. Cimbala. Indoor Air Quality Engineering. New York: Marcel-Dekker, 2003.
3. P. K. Kundu. Fluid Mechanics. San Diego, CA: Academic Press, 1990.
4. M. Van Dyke. An Album of Fluid Motion. Stanford, CA: The Parabolic Press, 1982.
5. F. M. White. Viscous Fluid Flow, 2nd ed. New York: McGraw-Hill, 1991.
6. R. L. Panton. Incompressible Flow, 2nd ed. New York: Wiley, 1996.
7. https://www.researchgate.net/publication/266883948_Flow_and_Turbulence_Structures_in_the_Wake_of_a_Simplified_Car_Model_Ahmed_Model
8. https://www.researchgate.net/publication/330383775_Experiments_and_numerical_simulations_on_the_aerodynamics_of_the_Ahmed_body