Ahmed Body Challenge
Abstract:To objective of this Project is to study flow simulation over Ahmed body in Ansys Fluent.By doing so we will understand flow over simplified car body and the phenomenon that occur around it during flow.
Importance of Study:Study will help in understanding aerodynamic design of Ahmed body,factor that lead to increase in Drag coefficient(which is very important parameter for any vehicle to be economical),fuel consumption,locations where is Point of flow separation,Wake formation,Vortex formation.
Rest will be discussed below.
Practical Applications:For every Automobile Manufacturer there are only two important prime factors,Person comfort and Fuel economy.
So Fuel economy for a vehicle is handled with these studies ,to make vehicle more aerodynamically efficient.Vehicle can be designed in any way to make it look beautiful but for every change in design,lot of factors add in which directly affect the performance of vehicle.
This Ahmed body ,very simplified body gives lot of experimental results for change in its slant angle,corner radius.So everything at the end matter for vehicle performance.
Solving And Modelling approach:
Geometry will be prepared forming enclosure around Ahmed body and only half body will be used for simulation as it is symmetric.
In Meshing ,both enclosure will be Meshed.Outer encloure will be meshed somewhat coarse and inner enclosure around Ahmed body will be meshed fine.
Inflation layers will also be added near faces of Ahmed Body according to Y plus value.
Pressure based steady state solver will be used using Kwsst model.
Results that will be obtained then post processed to study velocity and pressure around body,wake region etc.
Pre-processing and Solver setup:
Geometry:
Ahmed body already prepared file imported in Space claim.
With the use of Enclosure command,two enclosure was made.
One was designed for creating space for model (coarse and bigger one) like small wind tunnel and
another was created (small and refined one ),keeping in mind area where wake formation takes
place.Meaning for capturing wake phenomenon and point of separation more accurately. Small enclosure
surround Ahmed body Model and Bigger enclosure surround both Small enclosure and Ahmed Body.
As the Ahmed body is symmetry,so for getting results faster with great accuracy also,we split the body
into two halves.So only one half will be simulated.This will save time and we can refine mesh more as
number of cells that can be used are limited to some extent.Split body command used to split the body
in Spaceclaim.
#Ahmed Body
#Small or Inner Enclosure
#Big or Outer Enclosure:
#Splitted Body from Centre:
#Section view:
Meshing:
The outer enclosure is Meshed with Hexahedrally somewhat coarse with element size 0.1m
Inner enclosure is mesh Tetrahedrally refined with
Three different cases:
a).Element size-0.0255 m
b).Element size-0.03 m
c).Element size-0.035 m
We will discuss for case (a).
Inflation layers added on faces of Ahmed body to capture results near its walls more accurately.
First layer thickness=0.00003 m
No of Inflation Layers-6
#First layer thickness is decided based on Y plus value(which is kept less than 1)
Face sizing -5 mm
##Setup:
Solver used-Pressure based and Steady state.
Model used -`komegasst`
Material used- Air
Reference Values:
Area-0.056016 `m^2`
Density-1.225 `(kg)/m^3`
Length- 0.288 m
Velocity - 25 `m/sec`
Dynamic Viscocity-1.7894e-05`(kg)/(m-s)`
Velocity Inlet-25 `m/sec`
Symmetry boundary condition is used for side walls and for lower walls ,Wall Boundary condition is used.
Pressure at Outlet-0 Pa (Gauge Pressure).
Drag and Lift coefficient report generated at Ahmed body.
Results:
a).Velocity Contour Plot:
b).Pressure Contour Plot:
c).Velocity Vector Plot indicating Wake region:
d).Velocity simulation plot:
##For case (b) :
Outer enclosure is same with element size 0.1m
Inner enclosure -element size-0.03 m
Face sizing -5 mm
First layer thickness-0.00003 m
No of Inflation Layers-6
##For case (c):
Outer enclosure is same with element size 0.1m
Inner enclosure -element size-0.035 m
Face sizing -5 mm
First layer thickness-0.00006 m
No of Inflation Layers-6
##Grid independency test and the values of drag and lift coefficients with each case.
| S.No | No. of Elements | Drag Coefficient | Lift Coefficient |
| 1 | 497393 | 0.3305 | 0.2858 |
| 2 | 309712 | 0.3638 | 0.3057 |
| 3 | 236554 | 0.4102 | 0.3613 |
Ahmed's body:
A real-life automobile is very complex shape to model or to study experimentally. However, the simplified
vehicle shape employed by Ahmed (1984), generates fully three-dimensional regions of separated flow
which may enable a better understanding of such flows. Ahmed's body is 1044mm long 288mm high and
389mm width. The slant part is 222mm long, whatever the angle. The bottom surface of the Ahmed body
is located at 50mm above the ground.The flow around this body is strongly influenced by the angle of the
rear slant surface, which indicates that the large portion of aerodynamic drag is generated by the
development of three-dimensional vortex separation from the rear slant surface. The Ahmed body was first
defined and its characteristics described in the experimental work of (Ahmed, 1984). Two configurations
with slant angles of 25°and 35°are considered as a test case. In the experiment perform by the (Ahmed,
1984), flow velocity was taken 40 m/s, Reynolds number was 4.29 million based on model length.
Importance of Ahmed body:
The flow around road vehicle (car, buses, trucks) under normal operating condition is principally
turbulent. It is typically characterized by large-scale separation and recirculation regions, a complex
wake flow, long trailing vortices and interaction of boundary layer flow on vehicle and ground. In
developing new road vehicle it is essential for the designer to understand thoroughly the structure of
flow around the vehicle. This will have influence on such principal feature as the shape of vehicle,
aerodynamic drag, fuel consumption, noise production and road handling.
Significance of Point of Separation(FLOW SEPARATION):
The flow near the surface may actually be reversed by the action of the pressure. The point where the
flow stops is known as the separation point.At this point, the main stream is no longer attached to the
body but able to break free and continue in more or less straight line. It tries to entrain air from the
region behind the body, the pressure in this re on drop below the ambient. Vortices form and the flow
is very irregular in this region.The phenomenon of the separation prevents the flow from simply
proceeding down the back side of the car. The pressure in the seperation region is below that imposed
on the front of the vehicle and the difference in these overall pressure force is responsible for drag form.
# Negative Pressure in Wake Region:
a).Plot 1
b).Plot 2
c).Plot 3
As we can see in above plots,
Plot no 1 showing wake region is behind Ahmed body trailing end.(Blue region forming circles shapes is wake)
Plot no 2 and 3 showing ,at 1.14 m in plot ,Velocity is maximum.As mass flow rate is constant,therefore Pressure is minimum.
Conclusion:
1.Simulation of flow over the Ahmed body done successfully,the results we obtained are similar to the figure shown below.
We captured Wake on trailing end of body or recirculation region,lower vortex,foresbody and backlight separation bubble and little C-pillar vortex.
2.We have performed simulation for only `25^o` of slant angle.But according to study done by other researchers,it is found that -The drag coefficient was found to increase with increase in the slant angles due to the development of recircular region at the downstream side of the Ahmed body.
3.It has been found from the grid independence test,with the increase in number of cells,the drag coefficient reduces.So in our most refined case setup ,we achieved 0.3305.
4.Because of limitations in the use of number of cells,we could not achieve clearly point of flow separation.It could be only achieved at the cost of less accurate Drag coefficient.
5.As far as Gaps are concerned in challenge,
i think,this challenge could be made better by including different rear slant angle.