coefficient of drag analysis on Ahmed body using Ansys

Ahmed body

• A ground vehicle can be termed as bluff bodies moving in close vicinity of the road surface. The shape and size of the body have a significant effect on fuel economy. Ahmed body is a lab generated model by S.S Ahmed and experimental data of pressure, velocity and force data is gathered by many researchers as well as S.S Ahmed; due to its slant blunt-body feature at rear of body and also frontal area as curved.
• As a velocity strike on the body pressure at that point is very high and velocity tends to zero.

Separation of flow

below image shows a separation of flow

• Start of separation of flow has occurred at point (s point in below image) when pressure gradient in x-direction becomes zero, as well as velocity gradient in the y-direction (s point in below image), should be zero.
• When pressure gradient in x-direction is negative as well as velocity gradient in y-direction should be positive then the boundary layer stick to body and vice versa is for starting of boundary layer separation.
• Once the shear stress becomes zero (s point in below image) to negative on a wall at that is when pressure gradient in the x-direction is starting from zero to positive and velocity gradient becomes in the y-direction from zero to negative the reverse recirculating flow has becomes started and it causes the separation of the boundary layer.
• For flow over the square plate; due to higher pressure drag and higher values of negative pressure in wake generation the pressure drag is higher on a plate and less lift
• For flow over cylinder ; due to less pressure drag compare to square plate there is higher values of coefficient of lift.

Steps in analysis

1. Geometry preparation
2. Meshing
3. Setup
4. Solution
5. Post-processing
6. Geometry

Above image shows side view while view plan as YZ plane and below image shows back view of Ahmed body (slant angle 20 degrees)

Meshing

• First enclosure

• Second enclosure

Mesh set

M1

• The first enclosure as hex mesh element size as 100 mm
• A second enclosure as a tetrahedral mesh with element size as 50 mm
• car support or car stand on-road as element size of 10 mm
• inflation layer around car body that is Ahmed body as Total thickness method and maximum thickness as 12.5mm with 6 inflation layer and growth rate of layer as 1.2
• Number of Elements: 214008

M2

• The first enclosure as hex mesh element size as 100 mm
• A second enclosure as a tetrahedral mesh with element size as 20 mm
• car support or car stand on-road as element size of 10 mm
• inflation layer around car body that is Ahmed body as Total thickness method and maximum thickness as 12.5mm with 6 inflation layer and growth rate of layer as 1.1
• Number of Elements: 286977

2 .setup

Boundary conditions as shown in below image

Velocity inlet as 25 m/s

Outlet as Neumann boundary condition

Symmetry wall as symmetry

Car wall as wall that is also Neumann wall

And all other remaining wall as that is also Neumann wall

Solver used

1. Steady state, density based, energy enabled viscous based K-epsilon realizable model for M1 set of mesh.
2. Steady state, density based, energy enabled viscous based K-epsilon model for M2 set of mesh.

And all other parameter as software defaults

Air has property of density as 1.225 kg/meter cube

And viscosity of 1.7894e-05 kg/meter. Sec

Post-processed results

1. velocity plot mesh set M1 and M2

Gauge Pressure contour for M1 and M2

Velocity contour on xy plane and velocity vector for M1 and M2 mesh set

From above images it\'s clearly shows how recirculation of velocity at rear end of Ahmed body

And also below image shows boundary layer on rear end of body for M2 set of mesh

Static Pressure on car wall and symmetry line interaction that is polyline curve length vs pressure for M1 and M2 mesh set

M1

At front of Ahmed body where velocity is zero and pressure is 395.44 pa

M2

At front of Ahmed body where velocity is zero and pressure is 416.405 pa

Coefficient of drag on M1 and M2 mesh set

Coefficient of lift on M1 and M2 mesh set

Static Pressure on car wall and XY plane as velocity contour for m1 and m2 set

Table of analysis

Mesh set	M1	M2
Viscous model	K-E Realizable	K-E simple
Coefficient _ drag CD	0.3765	0.3396
Coefficient _ lift CL	0.3270	0.2797
Max Pressure at front	395.44 pa	416.405 pa
Reynolds number height based density as 1.225 kg/meter cube Viscosity of 1.7894e-05 kg/meter. Sec V=25 m/s Height=0.29 m	5.2e5	5.2e5

Ahmed body and wake region

• At back side of Ahmed body when negative pressure gradient in x direction becomes positive and velocity gradient in y direction goes negative and thus tends formation of turbulence in flow and vortices are form as fluid completely separate from body.
• Due to recirculation fluid in wake region pressure goes on downside i.e. behind the car; hear note that the negative pressure in wake is because while calculating pressure in Ansys software it will give pressure as gauge pressure, and yes our outlet condition for gauge pressure as zero that is one atmospheric.
• In context of total pressure that is summation of static and dynamic pressure. So, there is just low pressure/negative pressure zone in separated region that is in wake region. Also absolute pressure that is difference between atmospheric pressure and vacuum pressure.
• So, Due to outlet condition as one atmospheric pressure the gauge/vacuum pressure in flow region goes negative to full fill Bernoulli’s principle that is valid for incompressible, steady and in-viscid flow, hear air is compressible but Mach number is very low and that is why Bernoulli’s principle is applicable.
• As pressure head goes down; velocity head increases to compensate to flow of fluid in particular direction; so, that conservation of energy principle gets satisfied. This situation in car body happen in between road and bottom surface of car and vice versa is also true.
• As fluid travels from above Ahmed body the pressure above body and in font of body is high and ones its detached or separated from blunt rear end of body turbulence takes place and due to recirculation and shape of body large pressure difference is generated and that is why in the case of blunt Ahmed body has large pressure dag in compare to frictional drag
• This low-pressure region trying to pull body backward thus to contract this drag more traction force needed thus it will increase fuel requirement.
• Large the length of wake it will increase drag coefficient also.
• To reduce wake region vortices trapper are used and also wake is greatly influenced by shape of body so streamline body is a good option but also streamline body has less coefficient drag but high skin friction coefficient and that will decrease by smooth surface finish of car body.
• Due to separated flow in wake region vortices are formed and if the frequency of vortex shredding gets equal to the resonance frequency of car that will do serious destruction on cars structure and that is why!! Also, blunt body and wake region study are important.

Conclusion

The analysis is done on two sets of mesh and results for a coefficient of drag is obtained and compared its observed that as mesh gets finer the flow of physics can be captured may be equal to experimental values.

Scope

To obtain good results of coefficient of drag the total thickness layer should be 1.2e-5m*1.1^(5) for five layers and that for the desired value of Y plus as 1 for this Reynolds number flow.