Steady State Fluid Flow Analysis over an Ahmed Body to Simulate Wake Formation using ANSYS FLUENT

Aim:-The aim of this challenge is to do steady state fluid analysis over an ahmed body.

Theory:-Ahmed Body was one such generic bluff body proposed to study the external aerodynamics of the vehicles. The Ahmed body is a simplified car-shaped made-up of a parallelepiped with rounded edges at the front and a slanted face at the rear. The slant at the rear side of the vehicle causes the separation of flow and also a significant factor for drag forces of Ahmed body. This standard automobile geometry shape was proposed by Ahmed and Ramm in 1984 who then brings out the experimental work related to the aerodynamics of the body for various slant angles.

In their study, Ahmed and Ramm had noticed that the most of the drag from the body is due to pressure drag, which was generated near the rear side of thevehicle.They also observed that the strength of the flow separation in the vehicle is regulated by the slant angle.

Importance of Ahmed Body

• The experiment have been conducted on actual ahmed body and hence a large amount of rich data is been available through their publication.So,the engineers around the world can use this experimental data to validate their results to formulate new CFD codes.

Now,in order to perform a grid dependency test on the ahmed body we will use three different case which as follows:-

CASE 1

Step1:-Geometry

Dimension:-

Length=1044mm

Width=389mm

Height=288mm

Wind tunnel:-Now,in order to simulate flow over an ahmed body we have to create an wind tunnel in spaceclaim.The Spaceclaim provides and very good option for this wind tunnel which is enclosure option.Since,the flow across over an ahmed body is importent so for that we have to create refine mesh around it for that we create second small enclosure which can been below

Dimension for bigger enclosure:-

Lenghth=8.04m

Width=1.38m

Height=1.33m

Dimension for smaller enclosure:-

Lenghth=2.54m

Width=0.909m

Height=0.83m

Interference:-Initially there was a huge interference between big and small enclosure and due to this there was a overlapping mesh was generated which is not desirable so,to avoid this we made share topology on and erase the interference region in spaceclaim after this we get a good intersection between big and small enclosure as shown in the below image

Step2:-Meshing

Before starting the meshing the first step is to assign the named selection for inlet,outlet and enclosure walls.We will use different approach for meshing to make it more effective which is explain below.

Method:-In this step,we will choose meshing method as multizone.This will create high quality Hexa mesh in the bigger enclosure,since this enclosure is quit simple in its geometry.The element size for meshing is 100mm.

Body sizing:-For smaller enclosure we will use body sizing method to make it more refine,since its very across to ahmed body.The element size for meshing is 50mm.

Face sizing:-The cylindrical leg of an ahmed body appear to be like polygon rather then to be circle,so for that we will do face sizing.The element size for meshing is 5mm.

Inflation layer:-In order to simulate the flow properly across an ahmed body we will add an inflation layer across it,which will make area more refine across ahmed body.Total 5 inflation layer is added by using total thickness approach whcih is 12.5mm.

Following are the images which we get after applying above methods which as follows:-

Element statistics:-

Step3:-Setup

• Steady-state analysis
• Solver:Density based to acquire more compressibility affect.
• Viscous model: K-epsilon,since the flow is turbulent
• Fulid flow:Air
• After above physics setup now we have define boundary condition:
• Inlet velocity=50m/s & Temperature=300k
• In order to view the flow over an ahmed body we will use use cut plane option which will be perpendicular to z-axis.
• Now,we will initialize the solution using Hybrid scheme,after this we will define contour plots for velocity and create solution animation to view how flow is taking palce across ahmed body.

Plots:-

Resdiual

Velocity

Pressure

Velocity Vector

• The above image is for velocity vector plotted on velocity contour.We can see that because of coarse mesh wake formation is not proper at rear end of ahmed body.

Chart

• The above chart is for velocity data plotted at 2m & 3m from the origin at the rear end of ahmed body.

CASE 2

Step1:-Geometry

Dimension:-

Length=1044mm

Width=389mm

Height=288mm

Wind tunnel:-Now,in order to simulate flow over an ahmed body we have to create an wind tunnel in spaceclaim.The Spaceclaim provides and very good option for this wind tunnel which is enclosure option.Since,the flow across over an ahmed body is importent so for that we have to create refine mesh around it for that we create second small enclosure which can been below

Dimension for bigger enclosure:-

Lenghth=8.04m

Width=1.38m

Height=1.33m

Dimension for smaller enclosure:-

Lenghth=2.54m

Width=0.909m

Height=0.83m

Interference:-Initially there was a huge interference between big and small enclosure and due to this there was a overlapping mesh was generated which is not desirable so,to avoid this we made share topology on and erase the interference region in spaceclaim after this we get a good intersection between big and small enclosure as shown in the below image

Step2:-Meshing

Before starting the meshing the first step is to assign the named selection for inlet,outlet and enclosure walls.We will use different approach for meshing to make it more effective which is explain below.

Method:-In this step,we will choose meshing method as multizone.This will create high quality Hexa mesh in the bigger enclosure,since this enclosure is quit simple in its geometry.The element size for meshing is 100mm.

Body sizing:-For smaller enclosure we will use body sizing method to make it more refine,since its very across to ahmed body.The element size for meshing is 40mm.

Face sizing:-The cylindrical leg of an ahmed body appear to be like polygon rather then to be circle,so for that we will do face sizing.The element size for meshing is 5mm.

Inflation layer:-In order to simulate the flow properly across an ahmed body we will add an inflation layer across it,which will make area more refine across ahmed body.Total 5 inflation layer is added by using total thickness approach whcih is 12.5mm.

Following are the images which we get after applying above methods which as follows:-

Element statistics:-

Step3:-Setup

• Steady-state analysis
• Solver:Density based to acquire more compressibility affect.
• Viscous model: K-epsilon,since the flow is turbulent
• Fulid flow:Air
• After above physics setup now we have define boundary condition:
• Inlet velocity=50m/s & Temperature=300k
• In order to view the flow over an ahmed body we will use use cut plane option which will be perpendicular to z-axis.
• Now,we will initialize the solution using Hybrid scheme,after this we will define contour plots for velocity and create solution animation to view how flow is taking palce across ahmed body.

Plots:-

Resdiual

Velocity

Pressure

Velocity Vector

• The above image is for velocity vector plotted on velocity contour.After refinning the mesh we can see the wake formation at the rear end but to captured it properly we need to refine it further.

Chart

• The above chart is for velocity data plotted at 2m & 3m from the origin at the rear end of ahmed body.

CASE 3

Step1:-Geometry

Dimension:-

Length=1044mm

Width=389mm

Height=288mm

Wind tunnel:-Now,in order to simulate flow over an ahmed body we have to create an wind tunnel in spaceclaim.The Spaceclaim provides and very good option for this wind tunnel which is enclosure option.Since,the flow across over an ahmed body is importent so for that we have to create refine mesh around it for that we create second small enclosure which can been below

Dimension for bigger enclosure:-

Lenghth=8.04m

Width=1.38m

Height=1.33m

Dimension for smaller enclosure:-

Lenghth=2.54m

Width=0.909m

Height=0.83m

Interference:-Initially there was a huge interference between big and small enclosure and due to this there was a overlapping mesh was generated which is not desirable so,to avoid this we made share topology on and erase the interference region in spaceclaim after this we get a good intersection between big and small enclosure as shown in the below image

Step2:-Meshing

Before starting the meshing the first step is to assign the named selection for inlet,outlet and enclosure walls.We will use different approach for meshing to make it more effective which is explain below.

Method:-In this step,we will choose meshing method as multizone.This will create high quality Hexa mesh in the bigger enclosure,since this enclosure is quit simple in its geometry.The element size for meshing is 100mm.

Body sizing:-For smaller enclosure we will use body sizing method to make it more refine,since its very across to ahmed body.The element size for meshing is 30mm.

Face sizing:-The cylindrical leg of an ahmed body appear to be like polygon rather then to be circle,so for that we will do face sizing.The element size for meshing is 5mm.

Inflation layer:-In order to simulate the flow properly across an ahmed body we will add an inflation layer across it,which will make area more refine across ahmed body.Total 5 inflation layer is added by using total thickness approach whcih is 12.5mm.

Following are the images which we get after applying above methods which as follows:-

Element statistics:-

Step3:-Setup

• Steady-state analysis
• Solver:Density based to acquire more compressibility affect.
• Viscous model: K-epsilon,since the flow is turbulent
• Fulid flow:Air
• After above physics setup now we have define boundary condition:
• Inlet velocity=50m/s & Temperature=300k
• In order to view the flow over an ahmed body we will use use cut plane option which will be perpendicular to z-axis.
• Now,we will initialize the solution using Hybrid scheme,after this we will define contour plots for velocity and create solution animation to view how flow is taking palce across ahmed body.

Plots:-

Resdiual

Velocity

Pressure

Velocity Vector

• The above image is for velocity vector plotted on velocity contour.After refinning the mesh more as compare to previous two cases here in this case we can see the wake formation at the rear end captured properly & also we can see recirculation of air at rear end properly.

Chart

• The above chart is for velocity data plotted at 2m & 3m from the origin at the rear end of ahmed body.

Conclusion:

• As we refined the mesh from case 1 to case 3 we can clearly see the wake formation properly at the rear end.

Why pressure goes Negative?

• The moment at which the air hits the front face of ahmed body,it become stationary & this point is called as stagnation point.This sudden obstruction in fluid flow causes very high pressure at the stagnation point.This high pressure area is colored in red & can been seen on the pressure contour.
• This high pressure air passes along the curved surafce of ahmed body and gained very high velocity.This gained in velocity causes pressure drop & hence pressure become negative.
• The pressure difference between top & the bottom end of the ahmed body form recirculation of air & this recirculation of air form wake formation at their rear end.