Week 3 - External flow simulation over an Ahmed body.

AIM :

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.

EXPECTED RESULTS :

1. Velocity and pressure contours.

2. The drag coefficient plot for a refined case. ( For velocity of 25m/sec, the drag coefficient should be around 0.33).

3. The vector plot clearly showing the wake region.

4. Perform the grid independency test and provide the values of drag and lift with each case.

THEORY :

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 alfa and the Reynolds number.

SOLVING AND MODELLING APPROACH :

FIRST WE IMPORT THE AHMED BODY GEOMETRY IN SPACECLAIM .

CHANGE THE UNITS INTO METERS FROM SPACECLAIM OPTION.

THE GEOMETRY DESCRIPTION IS SHOWN AS BELOW.

GENERATE OUTER ENCLOSURE TO SIMULATE THE MESH .

1. Right face is located at a distance of 5m from the right side face of Ahmed body along the X axis.
2. Left face is located at a distance of 2m from the left side face of Ahmed body along the X axis.
3. Top face of the encloser is at a distance of 1 m from the top side face of the Ahmed body along the Y axis.
4. The bottom face of the encloser is attached to the bottom face of the Ahmed body along the YZ plane.
5. The front and back face of the encloser is at a distance of 0.5 m along the Z axis.

GENERATE INNER ENCLOSURE :

1. Right face is located at a distance of 1 m from the right side face of Ahmed body along the X axis.
2. Left face is located at a distance of 0.5 m from the left side face of Ahmed body along the X axis.
3. Top face of the encloser is at a distance of 0.5m from the top side face of the Ahmed body along the Y axis.
4. The bottom face of the encloser is attached to the bottom face of the Ahmed body along the YZ plane.
5. The front and back face of the encloser is at a distance of 0.26 m along the Z axis.

BOTH INNER AND OUTER ENCLOSURE LOOKS LIKE -

USE THE PLANE TOOL TO CREATE SECTIONS.

TO AVOID OVERLAPPING IN THE MESH WE USE THE TOOL INTERFERENCE .

NAME ALL THE SECTIONS FOR BOUNDARY CONDITION AS

INLET (VELOCITY = 25 M/S), OUTLET , SYMMETRY_WALL , CAR_WALL

MESH :

# CASE 1 - WITH OUTER ENCLOSURE

MESH IT WITH NO. OF NODES = 16596 AND ELEMENTS = 84073

THE NEXT STEP IN THE SIMULATION AFTER GENERATING THE MESHING IS TO SET-UP THE PHYSICAL PROPERTIES FOR THE CASE.

• TYPE OF SOLVER: DENSITY-BASED
• SIMULATION METHOD: STEADY-STATE SIMULATION
• TURBULENT MODEL: K-EPSILON STANDARD
• MATERIAL PROPERTIES OF FLUID (AIR)

DENSITY : 1.225 KG/M3

CP (SPECIFIC HEAT) : 1006.43 J/KG-K

THERMAL CONDUCTIVITY : 0.0242 W/M-K

VISCOSITY : 1.7894E-05 KG/M-S

• INLET VELOCITY: 25 M/S
• INLET TEMPERATURE: 300 K
• OUTLET: PRESSURE TYPE, 0-GAUGE PRESSURE.
• WALLS: WALL TYPE WITH NO-SLIP
• SET PLANE
• SET CONTOURS VELOCITY/PRESSURE
• SET ANIMATIONS VELOCITY/PRESSURE

RESULTS :

AFTER SETTING UP A CASE AND PROVIDING APPROPRIATE INITIAL AND BOUNDARY CONDITIONS, NUMBERS OF SIMULATIONS ARE PERFORMED ON AHMED BODY, MAINLY FOR THE COMPARISON OF RESULTS FROM DIFFERENT MESH SIZING. SET 500 NO. OF ITERATIONS AND CALCULATE

RESIDUALS -

VELOCITY CONTOUR PLOT -

PRESSURE CONTOUR PLOT -

USE VECTOR COMMAND TO SEE RESULT OF VELOCITY VECTOR

VELOCITY VECTOR PLOT -

LIFT COEFFICIENT PLOT -

DRAG COEFFICIENT PLOT -

# CASE 2 - WITH INNER AND OUTER ENCLOSURE AND MEDIUM SIZE MESH

RESULTS :

RESIDUALS -

VELOCITY CONTOUR -

PRESSURE CONTOUR -

VELOCITY VECTOR PLOT -

LIFT COEFFICIENT PLOT -

DRAG COEFFICIENT PLOT -

# CASE 3 - WITH INNER AND OUTER ENCLOSURE AND FINE SIZE MESH

MESH -

RESULTS :

RESIDUALS -

VELOCITY CONTOUR -

PRESSURE CONTOUR -

VELOCITY VECTOR -

LIFT COEFFICIENT PLOT -

DRAG COEFFICIENT PLOT-

GRID INDEPENDENCE TEST :

MESH	No. of elements	No. of Nodes	Lift Coefficient	Drag Coefficient
Coarse Mesh	16596	84073	0.27120537	0.40138095
Medium Mesh	179224	70680	0.28960068	0.39337236
Fine Mesh	276107	103477	0.29017916	0.3688416

CONCLUSION :

After performing numerous simulations on Ahmed body with different meshing parameters, we can draw a few conclusions from this study of CFD simulations.

1) As we refined the meshing, the results are more accurate and clearly showing the wake region behind the Ahmed Body.

2) The negative pressure in a wake region with an increase in velocity is because the Ahmed body is a bluff body. The boundary separation and recirculating of the flow and also the effect of shear stress. To overcome this stress, the pressure turns negative for the balancing effect.

3) Flow separation occurs when the fluid boundary layer encounters an adverse pressure gradient. The point on the body where the boundary layer lifts off the surface is called the separation point. When the airflow separates off the surface, the pressure drag usually increases. To have better performance of an Automobile or any vehicles this separation needs to be avoided or delayed, for smooth flow over the surface of the vehicle.

(FAIZ AMIR AZMI)