Week 3 - External flow simulation over an Ahmed body.

AHMED 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 aerodynamic 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 characterised by high turbulent and three dimensional flow seperation as well as three is a growing need for more insight into the physical feature of these dynamical flows. thr ahmed body is a simplifier car used in the automobile 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 movable z slat plane in the rear of the body to study the detachable phenomena at different angle and a rectangular box which link the front part and the rear slant plane. the principle objective to study such a simplified car body is to tackle the flow process involved in drag production. through perceving the mechanicm involved in creating drag one can be able to design a car to minimize drag and therefore reducing fuel consumption and maximixe vehicle performance.

an ahmed body represent a simplified vehicle volume which helps understand the fundamental physics defining by the engineer the lift and drag forces and coefficient at certain characteristically velocities , pressure differences and seeing and analyzing the vorties, turbulance, and reyolds number obtained .ANSYS is a general purpose FEA and CFD simulation software it gives a numericaly prediction of the behavior of a system.

these numericaly prediction need to be validate by experimental data. people working in vehicle aerodynamic 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 invalidation it is used to desighn new model of cars.

its shape is simple enough to allow for accurate flow simulation but retain some importance praticaly feature to automobilebodies. this model describes how to calculate the turbulent flow field around a simple car like geometry using the turbulent flow k eplision and k omega interfance. later either experimentally or computationally it can be recreated all the same but with the described geometry new to come up to the market or research.

in ahmed body the main three features were seen in the wake :

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

2 the B recirculation region that is formed due to seperation 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 degree the flow remain attached over the slant . the flow is essentially two dimensional and has low drag between  12 degree and 30 degree the flow become much more three dimensional as the c pillar vortices form. these reach maximum strength at 30 degree the drag increases significantly as the low pressure cores act on the rear surfaces. past 30degree the flow seperates fully off the slant. the results in a sudden decreases in drag and weaker c pillar votices.

* Reason for negative pressure in the wake region:

The path line showing the exact circular motion or vortices generation just at the rear end of the car. the seperation takes two region 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 posses 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 approch from the higher pressure region to the lower pressure region thus creating a circulating exactly in the wake region created.

the body penetrating the fluid creayes a region of emptiness just beside its end due to the nature of the geometry of the body here which can be least in case of a perfectly streamlined shaped body. the emptiness created is a low pressure towards which the fluid column exactly the wake region which of atmosperic pressure tries to fill the space thus draging the fluid column with it thus creating the region of vorticity. this also result in the huge pressure gradient in the region exactly rear of the body creating the negative pressure at the end . also this is the laymans reason why there is negative gauge pressure in the wake region.

* significance of the point of seperation:

the fluid seperation point can be defined as a point where the dv/dy 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 seperation starts occuring or the negative pressure gradient starts forming after that point. the point of seperation cannot be understood just by single countour in hard and fast way. it can be supportively studied with the help of the dp/dx plots or velocity plotsthe 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 expercing ina particular region of the fluid and body interation.

GEOMETRY OF AHMED BODY:

as the body is perfectly symmetric we can run the simulation by considering only half the body this is the best way where we can save on the number of cell and get the result faster as well. we need to use 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 set it to meters units

2 go to prepare and click on the encloser and select the ahmed body . enter the encloser dimesion as shown below

3 once the encloser is done we can create another encloser closely around ahmed body so that we can refine the mesh in the encloser and still keep the total cell countless.

ahmed body with both encloser

4 after the encloser we can check interfence and avoid it overlaping of meshing in the 2nd encloser by applying interference and delecting the overlap.

5 we also need to go to properties and select share topology.

MESH

CASE 1

Outer encloser=100mm

inner encloser=50mm

face sizing for ahmed body=5mm

inflation for ahmed body=5 layer total thickness 12.4mm

setup for fluent

-undate the mesh 

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

-consider pressure density solver

-take 25 m/s as velocity inlet boundary

-initilize solution

- need to create cut plane z and create pressure and velocity contour

-create drag and lift coefficient

- run the simulation for 500 iteration.

RESULTS:

1 resiudal plot

2 velocity and pressure contour

3 drag and lift coefficient

 vector plot

chart

CASE 2

outer encloser=90mm

inner encloser=40mm

face sizing for ahmed body=5mm

inflation for ahmed body=5 layer total thickness 12.4mm

mesh:

result

1 residual plot

2 pressure and velocity contour

3 lift and drag coefficient

4 vector plot and chart

CASE 3

outer encloser=85mm

inner encloser=35mm

face sizing for ahmed body=5mm

inflation for ahmed body=5 layer total thickness 12.4mm

MESH:

RESULT

1 residual plot

2 velocity and pressure countour

3 drag and lift coefficient

4vector plot and chart

conclusion:

- high pressure is formed at the frontal area of the ahmed body due to loss of kinematic energy of the air.

-the adverse pressure gradient is created near the wake region because of the flow seperation which is occuring because of the geometry of the model. as a result flow seperation alters the pressure gradient which in turn affects the drag and lift coefficient.

- 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 result approch closer to experimental results.