Aerodynamics : Flow around the Ahmed Body using ANSYS FLUENT

I. Introduction : 

Automotive aerodynamics comprises of the study of aerodynamics of road vehicles. Its main goals are reducing drag, minimizing noise emission, improving fuel economy, preventing undesired lift forces and minimising other causes of aerodynamic instability at high speeds. Also, in order to maintain better control for steering and braking, we look into design and aerodynamics of a vehicle. Drag is caused due to the pressure difference between the frontal and the rear end of the vehicle. It can be reduced by modification of the design of the vehicle or the modification of the air flow around the vehicle. 50% of the mechanical energy of the vehicle is wasted to overcome drag at highway speed of nearly 88.5 to 96.5 kph.

Air has a tendency to curl downwards around the ends of a car, travelling upwards from the high-pressure region under the car to the low-pressure region on top, at the rear end of the automobile and subsequently collides with moving low-pressure air. The kinetic energy of these turbulent air spirals acts in a direction that is negative relative to the direction of travel intended. Thus, the car engine must compensate for the losses created by this drag. Vortices are released during flow separation and trail downstream to form structured or unstructured wake patterns. A wake is the region of re-circulating flow mmediately behind a moving or stationary solid body, caused by the flow of surrounding fluid around the body. The local disturbances in the flow pattern behind the vehicle causes a momentum loss thus causing form drag which extends far behind the bodyworks of a vehicle.

The Ahmed body has the form of a highly simplified car, consisting of a blunt nose with rounded edges fixed onto a box-like middle section and a rear end that has an upper slanted surface, the angle of which can be varied. The model is supported on circular-sectioned legs or stilts, rather than wheels. Despite neglecting a number of features of a real car (rotating wheels, rough underside, surface projections etc.) the Ahmed body generates the essential features of flow around a car, namely: flow impingement and displacement around the nose, relatively uniform flow around the middle and flow separation and wake generation at the rear. It was first invented or used in experiments in 1984 by Ahmed. It helps demonstrate how the drag of a body is mainly the effect of pressure drag generated at the rear portion of the body.

                             

In the original experiments undertaken the angle of the rear slant was varied from β = 0 ◦ to 40 ◦. Visualization techniques were employed to examine the structure of the wake and time-averaged velocity measurements were made on the centreline plane and at transverse planes in the wake. Measurements of the total drag were made at 5 ◦ intervals for slant angles from β = 0 ◦ to 40 ◦. The total drag was observed to fall from β = 0 ◦ to 15 ◦ and then rise to a maximum at 30 ◦ , followed by a sudden decrease, thereafter remaining almost constant between 30 ◦ and 40 ◦ slant.

 

II. Study Objective : 

1. Simulate a turbulent flow around Ahmed body inside a wind tunnel

2. Do local refinement of mesh & perform Grid dependency test 

3. Post Process Velocity & Pressure

 

III. Geometry Creation : 

The Wind Tunnel is developed in Ansys spaceclaim and Appropriate measurements are given to capture the flow, a Second enclosure is created near the Ahmed body for Local Mesh Refinement.

A check for interference is done and its eliminated and Topology is set to share. 

The slant in the ahmed body is 20 deg . 

 

IV. Meshing : 

After naming the boundaries, We begin by creating a basic mesh with case A. The inner enclosure is the local refinement zone where the mesh is of finer size than the outer enclosure - wind tunnel. This helps in capturing the area near the ahmed body with more accuracy so we can validate our data and or post process it. The inner facing wall body of ahmed body is further given a finer mesh size and an inflation layer is created around it to capture the border data accurately with clean shape of mesh. Face sizing is given to the legs for same reason. As we move further with the cases the refinement is increased in the end to be able to perform grid dependency test.

1. Case A. 

Details of the Mesh : 

i. Main enclosure – 200 mm

ii. Small enclosure – 100 mm

iii. Face sizing of ahmed body – 20 mm

iv. Inflation : First Layer thickness - 3 layers, 5 mm each

v. 20 % Growth

vi. Elements : 160897 

vii. Nodes : 41924

 

 

2. Case B.

For the same geometry we further refine the mesh. 

Details of the Mesh : 

i. Main enclosure – 100 mm - Multizone (hexa)

ii. Small enclosure – 50 mm

iii. Face sizing of ahmed body – 25 mm

iv. Face sizing of legs of ahmed body : 5 mm

v. Inflation : Total thickness : 5 layers - 12.442 mm

vi. 20 % Growth

vii. Elements : 231470 

viii. Nodes : 62978

 

Cut section mesh display -

Inflation Layers : 

Legs : 

 

3. Case C. 

Details of the Mesh : 

i. Main enclosure – 90 mm - Multizone (hexa)

ii. Small enclosure – 35 mm

iii. Face sizing of ahmed body – 20 mm

iv. Face sizing of legs of ahmed body : 5 mm

v. Inflation : Total thickness : 8 layers - 15 mm

vi. 20 % Growth

vii. Elements : 468930

viii. Nodes : 132088

 

V. Case Setup : 

1. Type - Density based.

2. Velocity Formulation - Absolute

3. Time - Steady 

4. Energy; Viscous Model : k - epsilon (2 eqn) 

5. Inlet velocity - 50 m/s

6. outlet : Gauge pressure : 0 pascal : 1 atm

 

7. Hybrid Initialization

8. Material : Fluid : Air

9. Density : 1.225 kg/m3 (constant)

10. Viscosity : 1.7894e-05 kg/m-s (constant)

11. Lower wall - wall boundary condition

12. Side walls - Symmetry conditon

13 Other B.c conditions such as inlet, Outlet

14. Plane creation to capture velocity & capture animation

 

VI. Post Processing the Results : 

1. Case A. 

Residual Plot : 

The Solution converges at around 1800 iterations. it was ran for 2000 iterations & execution time was 2868 seconds. 

    

 

Velocity Plot : 

A cut plane is created in the Post Processing in the center of wind tunnel over the ahmed body, This gives the velocity of air along different geometry sections of the Ahmed body. The green region displays the velocity of 50 m/s while the lower wall captures the boundary layer starting from nearly 0 velocity inceasing and as it enters the second enclosure it is further refined. we can observe the wake region , its sharper as the slant angle is 20deg. in Experimental results a little wider wake regions have been observed with lesser angles such as 12.5 deg. The wake region indicates low velocity region. 

 

Pressure Plot : 

The air becomes almost stagnant as it strikes the vehicle which results in air exerting very high pressure on front engine grill of the vehicle represented by the red area. The airflow then gets divided between the upper and lower surface of the vehicle.

The higher pressure air on front surface accelerates as it travels over the curved nose surface of Ahmed body to the top, sides and lower region to escape, this causes pressure to drop which can be validated from Bernoulli's fluid laws. This lower pressure creates lifts over the roof surface as the air passes over it. As the air continues to flow and make its way to the rear, a notch is created by the rear slant owing to flow separation, leaving a vacuum or low pressure space which the air is not able to fill properly. The resulting lower pressure creates lift that then acts upon the surface area of the rear slant.

 

                       

 

 

2. Case B. 

Residual Plot : 

The Solution converges at around 2000 iterations. it was ran for 2676 iterations & execution time was 3624.7 seconds , around 1 hr. 

    

 

Velocity Plot : 

We can observe the maximum velocity for case B is higher than Case A.  The green area corresponds to 50 m/s. the blue area show low velocity and high pressure region and red area on the surface of ahmed body gives high velocity & Low pressure region. We can see the wake region more clearly in this refined plot. 

 The boundary layer seperation occurs on the rear slant with increase in lift due to larger pressure difference generated in the slant. The sudden vaccumish zone created on the wake region generate eddies

                

 

Pressure Plot : For case B. 

 

3. Case C. 

Residual Plot : 

The Solution converges at around 2600 iterations. it was ran for 2706 iterations & execution time was 8648.376 seconds , around 2.4 hrs. 

 

Velocity Plot : 

A more defined contour is observed with fine color observations in the wake and front region of the air velocity.

StreamLines Velocity :

The wake region boundary layer seperation, the lift at the slant and eddies created can be clearly observed in this plot. The stream lines traces the velocity data. 

 

Pressure Plot : 

A more accurate result is obtained as the mesh is finer. 

 

VII. Velocity Distribution at Probes & Grid dependency test : 

For all the cases, probes are created at fixed locations in the wake region and  near ahmed body & velocity across Y distance is plotted. This observation and study gives data of how the velocity behaves in a graphical manner between two distant probes, their values and can be compared across different Cases. It allows for flexibility to export the data and use it for further calculations as well. 

 

      

The black vertical lines are the probes. 

Case A. 

For case A, an extra probe at the front of ahmed body was created at location -0.04 m, the other locations are followed serially in increasing order as per legend . 

We can observe that the velocity at front of ahmed body increases from the ground up then decreases near the impact stagnation zone increasing again on its way above. 

Similarly with other 3 in the wake region, but depending on their location, the closer the probe is to the center of wake region : 1.11 m the lower the velocity has dropped and at later probe locations they slighly increase and the very last one does not falls into negative velocity meaning no reverse vortices are formed. 

      

 

Case B. 

Similar velocity profiles can be observed with a much fine mesh than case A with some slight variation in velocity towards the probes in the end of wake region. 

       

 

Case C. 

As the mesh was increased to more refinement, it is observed that the wake region expanded a little as we can observe the middle and last probe shows nearly same results in this case which wasnt with the above two . This we can conclude as some margin of error allowed. 

      

 

VIII. Grid Dependency Conclusion : 

From above observations, we conclude that Grid is independent but there is some margin of error which can be caused by a number of other simulation parameters, for eg the turbulence model chosen for this simulation isnt the best one for it. 

Academic license & computational limit further limits the possibility of better results. But for all practical purposes of this study the Grid is independent and all our cases points to it with a certain margin of acceptable error. 

 

IX. Conclusion : 

1. Ahmed body has become a basis for external flow study of car body with so much experimental data available.

2. Local refinement of the enclosure is a great tool for performing analysis and it improved the results without which wouldnt have been so accurate and cleared grid dependency in ansys fluent.

3. This study for all practical purposes gives a great real world scenario of aerodynamic impact on a car body, which when improved can help overcome many challenges faced by automobile. 

 

X. References : 

1. http://www.up22.com/Aerodynamics.htm

2. https://www.simscale.com/docs/content/validation/AhmedBody/AhmedBody.html#id13

3. https://www.comsol.com/blogs/studying-the-airflow-over-a-car-using-an-ahmed-body/

4. IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) e-ISSN: 2278-1684,p-ISSN: 2320-        334X, Volume 12, Issue 4 Ver. III (Jul. - Aug. 2015), PP 87-94

5. Experiments and numerical simulations on the aerodynamics of the Ahmed body - cfd Letters

 

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keywords - AERODYNAMICS, CFD, ANSYS-FLUENT, CAE.