Week 3 - External flow simulation over an Ahmed body.

WEEK 3: EXTERNAL FLOW OVER AN AHMED BODY.

AIM: TO SIMULATE THE EXTERNAL FLOW AND EXAMINE THE FLOW BEHAVIOUR AROUND THE AHMED BODY.

OBJECTIVES:

1. Describe Ahmed body and its importance.

2. Explain the reason of the negative pressure in the wake region behind the Ahmed body.

3. Explain the importance of the Point of Separation.

4. Obtain the Drag Coefficient and examine the results for the pressure and velocity contours as well as the vector plots.

5. Perform Grid Dependency Test.

1. Describe Ahmed body and its importance.

Ahmed body is a simple geometry which is used to represent a vehicle geometry of a bluff body type. In spite of being a simple geometry, the Ahmed body does retain some practical features of the vehicle to allow the simulations produce accurate flow.

The external flow simulation around the Ahmed body captures essential flow features that are seen around any typical automobile. This body is also used in describing the flow around cars.

Typically, Ahmed body is used in validating the numerical model flow data with the experimental data. The Ahmed body provides significant data about the different wake regions, recirculation regions formed behind and the base of the Ahmed body.

2. The Separation Point.

When air or some fluid flows over a flat surface there is very thin layer of the fluid close to the surface. In this layer the viscous effects are predominant and the laminar flow behaviour is exhibited by the fluid.

However, if the same flat surface bulges out from central region and acquires convex shape then the fluid flow near surface boundary changes. The changes are likewise:

As the fluid moves upstream, the projected flow cross-section area reduces till the point its reaches minimum value. On moving further, the projected flow area starts to increase.

As the area reduces due to the continuity equation the flow velocity increases consequently, the pressure reduces and reaches to minimum value. If, the pressure gradient along the flow length during this is still negative then the fluid will not leave the surface.

But if a condition reaches, wherein, the pressure gradient value becomes zero then that can result in fluid leaving the surface. This phenomenon of the fluid leaving the surface is known as the Flow Separation and the point at which the pressure gradient changed direction is known as the Separation Point.

3. Reason for the negative pressure in the wake region.

Consider the same surface as discussed in above section, if the flow occurs beyond the point P the projected area of flow is increasing. This results in decrease in velocity and increase in the pressure in direction opposite to the flow. This leads in flow regime having cyclic pressure patterns and as the flow moves further this cyclic flow pattern gets strong and results in recirculation region.

This Recirculation region is the reason of the negative pressure observed in the wake region.

PREPARING THE CFD MODEL:

The model for this analysis is prepared using ANSYS tool, geometry editing is done in SpaceClaim and meshing in ANSYS Workbench. The model setup in ANSYS Fluent and post processing in CFD-Post. The steps are as follows:

1. GEOMETRY CLEANUP:

Import the Ahmed Body.

Create enclosures around Ahmed body.

Suppress Ahmed body, split the cad and remove interference error.

Finally, set the Topology to Share.

The final cad will look like this:

MESHING:

The meshing will be done is shown in the figures below.

First, creating the named selections of INLET, OUTLET, CAR WALL & SYMMETRY faces.

Outer Enclosure:

Multi-zone meshing with average and maximum size equal to 100 mm.

Inner Enclosure:

Meshed using the sizing method and the size used is 50 mm.

Ahmed Body Cavity:

The pillars are meshed using the Face sizing method with 5 mm as the average element size.

The Ahmed body cavity faces are provided with the Inflation Layers to capture the boundary layer around it. The layers are incorporated using First Layer Thickness Method, wherein, the first layer thickness is equal to 0.42 mm and there are 10 layers with a growth rate of 1.2.

After incorporating all the above-mentioned steps, the mesh generated is:

The mesh generated will be refined in sub-sequent process, in order to perform the Grid Dependency test using the Drag Coefficient result values to decide.

Analysis Setup:

The model setup is prepared in Ansys Fluent; the steps are as follows:

For this analysis the Density Based Solver is used with the process being considered in steady state.

The turbulence model used is K-epsilon model.

The inlet velocity is equal to 25 m/s and the fluid used is Air. The rest boundary conditions are all default.

The projected area of the Ahmed body front part is computed and substituted in the reference values of the Inlet boundary.

The analysis is initialized using Hybrid initialization to assign starting values of the velocity. The analysis is required to give outputs for the Pressure & Velocity and Drag & Lift Coefficient variations near the Ahmed body and their respective animation and contours.

The analysis runs for 750 iterations.

RESULTS:

The analysis after completion will give the following results:

DRAG COEFFICIENT:

The above figure shows the value is coming out to be as 0.3313. This value is in accordance with the desired value of the drag coefficient.

LIFT COEFFICIENT:

The lift coefficient is coming out to be as 0.2488.

VELOCITY:

The velocity variation around the Ahmed Body is seen in the figure shown below:

Above figure shows the recirculation regions forming behind the Ahmed Body. These regions are responsible for the negative pressure values in the system.

PRESSURE:

Above figure shows the variation of the pressure around Ahmed body. It is seen that pressure is negative behind the Ahmed body.

The velocity vector lines can be seen in the figure below:

Figure shows the scarcity in the back region signifying the low pressure and recirculation region.

GRID DEPENDENCY TEST:

To perform this test, the mesh is made finer and finer and the same analysis is performed again to obtain the converged values of the Drag and Lift coefficients. The converged results will point out that the mesh size is at or near the optimum mesh size.

The mesh refinement will be carried out by preparing three models in which the different regions will have mesh sizes as follows:

CASE 1: Base Run Model.

Outer Enclosure = 100 mm.

Inner Enclosure = 50 mm.

Pillar Size = 5 mm.

CASE 2: Refined Model 1.

Outer Enclosure = 75 mm.

Inner Enclosure = 37.5 mm.

Pillar Size = 5 mm.

CASE 3: Refined Model 2.

Outer Enclosure = 50 mm.

Inner Enclosure = 25 mm.

Pillar Size = 5 mm.

The inflation Layers setting is same in all three models for simplicity. The results obtained from the remaining test runs are as follows:

Refined Model 1:

The above figure shows the value is coming out to be as 0.3124.

The above figure shows the value is near 0.24.

Refined Model 2:

As seen in above figures, the curves are not showing flat lined values for either of the desired quantity, but the drag coefficient value can be assumed to be averaged to 0.32. similarly, for lift coefficient value can be assumed to be equal to 0.25.

The recirculation region is seen very clearly for this model.

CONCLUSION:

The results from the above three models can be summarised into the following table:

S. No	MODEL	CELL COUNT	DRAG COEFFICIENT	LIFT COEFFICIENT
1	BASE RUN MODEL	110507	0.3313	0.2488
2	REFINED MODEL 1	196366	0.3124	0.2395
3	REFINED MODEL 2	569839	0.32	0.25

From the results of the analysis, it is seen that:

The base run model results are converged for the given number of iterations, other models are approaching the converged results but not yet achieved it.

Other models will also give the converged results if the number of iterations is further increased, but that will result in more computational resources utilization. This is not justified due to the fact that base run is providing the accurate results with less time consumption.

So, the Basse run model mesh sizing can be accepted for meshing this geometry.