Ahmed Body simulation using Ansys Fluent

Objective: To perform a grid independency test on the Ahmed body.

Expected Results:

1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case.
3. The vector plot clearly showing the wake region. 
4. Perform the grid independence test and provide the values of drag and lift with each case. 

Given:

 The geometry of the Ahmed body along with all dimensions is as below:

Ahmed Body in Spaceclaim is as:

Theory:

`tt"Ahmed Body"`

The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed. Although it has a very simple shape, the Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.

This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model is validated, it is used to design new models of the car.

The geometrical shape of the Ahmed body investigated is depicted in the above Figure. Despite the considerable deviation of its geometry from usual vehicles, the body represents the basic aerodynamic properties of a vehicle, especially in the rear part. The slant angle ϕ has a strong influence on the aerodynamic drag and lift at the back. Both the drag and lift coefficients change abruptly at the angle of ϕ=30°. Therefore, this angle is called the critical slant angle. For slant
angles higher than this value, the adverse pressure gradient in between the slant and the roof is so intense that the flow fully detaches over the slant.
Below these critical slant angles (30 degrees), the flow still separates but the pressure difference between the slant region and the side walls is still sturdy enough to generate substantial stream-wise vortices at the lateral slant edges. These prompt a downward motion over the slant, mainly in the downstream part. As a result, the flow separates at the upstream end of the slant can couple further to the downstream. At a slant angle of 30°, i.e., directly before the transition to the full squareback flow, this pair of vortices is developed most strongly, and the flow separates at the lower edge of the slant. This produces strong underpressures at that position so that the aerodynamic drag reaches its maximum in this state.

Importance of Ahmed body:

The research of three-dimensional flow around a vehicle has become a subject of significant importance in the automotive industry. One apparent technique of improving the fuel efficiency of vehicles is to reduce aerodynamic drag force by optimizing the body shape. Execution of fine aerodynamic design under stylistic constraints requires an immense understanding of the flow pattern phenomena and, especially, how the aerodynamics are impacted by changes in body shape. 
Ahmed Body was one such generic bluff body proposed to study the external aerodynamics of the vehicles. The flow around the Ahmed body has several flow separations from the front to the rear of the vehicle. The flow recirculation caused by these flow detachment contributes the vehicle’s drag. The location point at which the flow separates determines the size of the separation zone, and accordingly the drag force, thus an exact simulation of the wake flow and of the separation process is essential for the accurate result of drag predictions

The wake separation flow behind the Ahmed body is the main contributor to the drag force, accurate prediction of the separation process and the wake flow are the key to the successful modelling of this case.

`tt"Negative pressure in the wake region and Point of Separation"` 

Wake Region:
Consider a fluid particle flows withing the boundary layer around the circular cylinder. From pressure distribution measured, the pressure is a maximum at the stagnation point and gradually decreases along the front half of the cylinder. The flow stays attached in this favorable pressure region as expected. However, the pressure starts to increase in the rear half of the cylinder and the particle now experiences as adverse pressure gradient. Consequently, the flow separates from the surface and creating a highly turbulent region behind the cylinder called 'WAKE'. The pressure inside the wake region remaining low as the flow separates and a net pressure force (pressure drag) is produced.

Flow Separation: 
The presence of the fluid viscosity slows down the fluid particles very close to the solid surface and forms a thin slow-moving fluid layer called a boundary layer.  The flow velocity is zero at the surface to satisfy the no-slip boundary condition.  Inside the boundary layer, flow momentum is quite low since it experiences a strong viscous flow resistance. Therefore, the boundary layer flow is sensitive to the external pressure gradient (as the form of a pressure force acting upon fluid particles). If the pressure decreases in the direction of the flow, the pressure gradient is said to be favorable. In this case, the pressure force can assist the fluid movement and there is no flow retardation. However, if the pressure is increasing in the direction of the flow, an adverse pressure gradient condition as so it is called exist. In addition to the presence of a strong viscous force, the fluid particles now have to move against the increasing pressure force. Therefore, the fluid particles could be stopped or reversed, causing the neighboring particles to move away from the surface. This phenomenon is called the boundary layer separation.

The pressure gradient is an is one of the factors that influence a flow immensely. As discussed above, A negative pressure gradient is termed a Favourable pressure gradient. Such a gradient enables the flow. A positive pressure gradient has the opposite effect and is termed the Adverse Pressure Gradient. Fluid might find it difficult to negotiate an adverse pressure gradient. Sometimes, we say the fluid has to climb the pressure hill. One of the severe effects of an adverse pressure gradient is to separate the flow.
Consider flow past a curved surface as shown in Fig. The geometry of the surface is such that we have a favorable gradient in pressure to start with and up to a point P. The negative pressure gradient will counteract the retarding effect of the shear stress (which is due to viscosity) in the boundary layer. For geometry considered we have an adverse pressure gradient downstream of P.
Now the adverse pressure gradient begins to retard. This effect is felt more strongly in the regions close to the wall where the momentum is lower than in the regions near the free stream. As indicated in the figure, the velocity near the wall reduces and the boundary layer thickens. Continuous retardation of flow brings the wall shear stress at the point S on the wall to zero. From this point onwards the shear stress becomes negative and the flow reverses and a region of recirculating flow develops. We see that the flow no longer follows the contour of the body. We say that the flow has separated. The point S where the shear stress is zero is called the Point of Separation.

`tt"CASE SETUP"` 

The aim of this challenge is to perform a grid dependence test in simulating external flow i.e. flow over Ahmed body and Compute values of aerodynamic forces such as Drag and Lift.

The Case studies we are going to do is described as below:

In order to perform all the above grid dependence test. The following steps we need to employ. The enclosure option of Spaceclaim allows us to define regions and to create appropriately refined meshes to accurately capture the complex physical phenomenon. High quality and high-density meshes are required to accurately capture the complex physical phenomena but it is computationally expensive. Therefore mesh was locally refined in regions that are important and coarser mesh was used at less relevant places to reduce the computational expense with a sufficient number of grid sizes needed to be solving the physics accurately. Thus, we have restricted our study to above 4 cases further refinement in mesh sizes will not be possible since the academic license of Ansys limited to 512000 nos. of elements.

STEP-1:

Import Ahmed body geometry into SpaceClaim.

STEP-2:

Form a bigger computational domain using the Enclosure option in Spaceclaim as per the below dimension.

The bigger enclosure is shown below:

STEP-3:

Form a smaller computational domain using the Enclosure option in Spaceclaim as per the below dimension.

The Smaller enclosure is shown below:

STEP-4:
Our geometry of Ahmed body is perfectly symmetric so using Split Body option in Spaceclaim allows us to reduce nos. of cells and brings out an accurate result using lesser nos. of cells at a lower computational time. Here, we are simulating only half of complete domain.

STEP-5:
We must look for interference errors since we have enclosed Ahmed body into two bigger enclosures.

STEP-6:
Name selection done is as below:

`tt"FLUENT SETUP"` 

Before processing for case setup we must ensure perform 'Check Mesh operation' to ensure mesh generated is free from errors.

Reference Values:

Viscous Model- 
We are here working on velocity i.e. 25m/s. Reynold's number also will be higher hence adding the Turbulence model is our necessity.

Checked Energy equation.

Model - K-epsilon (2 equations)
K-epsilon model- Standard
Near-wall Treatment- Standard wall functions

Material- Air with default properties.

Boundary Conditions-

Inlet- Type= Velocity inlet 
        Velocity Magnitude= 25 m/s
        Temperature= 300 K

Outlet- Type= Pressure outlet
           Gauge Pressure= 0 Pa

Symmetry- Type= Symmetry

Walls- Type= Wall
          Wall Motion= Stationary wall
          Shear conditon= No slip
          Roughness model= Standard

`tt"RESULTS"` 

CASE-1:

In this case, we are proceeding with baseline autogenerated mesh by Ansys Fluent.

Baseline Mesh: 0.4092m

Nodes: 8624
Elements: 42790

Mesh:

Coefficient of Drag And Lift plot:

Velocity and Pressure contour Plots:

As we can see observe from the above contour plot to the coarse grid size we are not able to capture data correctly. There is no distinct form of the wake region behind the body.

Here, we can observe that pressure is high in front of the body due to the stagnation of fluid.

Vector Plot:

CASE- 2:

In this case, the body has enclosed inside enclosures to refine the grid wherever we expect accuracy in the results.

MESH (Big enclosure): Element size- 0.1m
                                 Max. Size- 0.1m

Inflation (Car-wall): Nos. of layers- 5
              Growth Rate- 1.2
              Inflation option- Total Thickness
              Max. Thickness- 9.5e-3m

Body Sizing (Small enclosure): Element size- 5e-2m

Face Sizing (Bottom legs): Element size- 5e-3m

Node: 24641
Elements: 75462

Mesh: Coefficient of Drag And Lift plot:
Velocity and Pressure contour Plots:

We can observe from the above contour plot is that refinement in grid size gives us quite good results. we can clearly distinguish between high and low-velocity region.

Vector Plot:

CASE- 3:

MESH (Big enclosure): Element size- 0.07m
                                 Max. Size- 0.07m

Inflation (Car-wall): Nos. of layers- 15
              Growth Rate- 1.2
              Inflation option- Total Thickness
              Max. Thickness- 1.075e-2m

Body Sizing (Small enclosure): Element size- 3e-2m

Face Sizing (Bottom legs): Element size- 5e-3m

Node: 110347
Elements: 378600

Mesh:

Coefficient of Drag And Lift plot:

Velocity and Pressure contour Plots:

Vector Plot:

CASE- 4:

MESH (Big enclosure): Element size- 0.06m
                                 Max. Size- 0.06m

Inflation (Car-wall): Nos. of layers- 15
              Growth Rate- 1.2
              Inflation option- Total Thickness
              Max. Thickness- 1.2e-2m

Body Sizing (Small enclosure): Element size- 2.8e-2m

Face Sizing (Bottom legs): Element size- 5e-3m

Node: 134730
Elements: 457073

Mesh:

Coefficient of Drag And Lift plot:

Velocity and Pressure contour Plots:

Vector Plot:

Residual Plots:

`tt"CONCLUSION"`

In this way, we have successfully investigated/simulated the case studies of the effect of different grid sizes on flow over the Ahmed body and compute values for aerodynamic properties like DRAG and LIFT.

`***`We can observe that the value for the Coefficient of Drag is reducing and the Coefficient of lift is increasing as we are refining the grid this is happening because we are refining near walls of the car-body based on Y+ values.
`***` In the wake region we can see that there is a formation of recirculation zone due to boundary layer separation at the back of the body.
`***` All the case studies performed by considering the velocity of 25m/s. It is expected to see that an increase in velocity will cause boundary layer separation near the beginning of the slant surface and more vortex formation at the back of Ahmed body(wake region).
`***`Study of static pressure contours help in the understanding sudden deceleration of flow and red color indicates that static pressure raises the front of the body.