Performing External Aerodynamic Simulation Over An Ahmed Body By Creating A Virtual Wind Tunnel Using SpaceClaim & Ansys Fluent.

Abstract:- The experiment is focused on investigating the flow characteristics over an AHMED body inside a virtually designed low subsonic wind tunnel created using the geometry editing tools available in SpaceClaim, apply multiple mesh refinement strategies i.e. perform grid scaling or grid independence test, assign proper boundary conditions to the inlet & outlet, investigate improved aerodynamic forces ( Lift & Drag co-efficient plots) with each mesh refinement strategy & also visualize the pressure & velocity contour plots for each case. The simulation will be run for 4 distinct cases using a pressure-based steady-state solver & the turbulence model used is the standard k – ε model. The design of the model used in this paper is done through the CATIA V5 and CFD simulation carried out in FLUENT (ANSYS 20). 

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                                                                            Keywords: Ahmed Body,  Wake Region,  k – ε turbulence model, `C_L` &  `C_D` Co-efficient 

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          1. Theory

               The flow around road vehicles (cars, buses, trucks) under normal operating conditions is principally turbulent. It is typically characterized by large-scale separation and recirculation regions, a complex wake flow, long trailing vortices, and interaction of boundary layer flow on vehicle and ground. In developing a new road vehicle it is essential for the designer to understand thoroughly the structure of flow around the vehicle. This will have an influence on such principal features as the shape of the vehicle, aerodynamic drag, fuel consumption, noise production, and road handling. Traditionally, vehicle designer has gained their understanding of the airflow around a vehicle through extensive wind tunnel testing. Automotive aerodynamics comprises the study of aerodynamics of road vehicles. Its main goals are reducing drag, minimizing noise emission, improving fuel economy, preventing undesired lift forces, and minimizing other causes of aerodynamic instability at high speeds.

Air has a tendency to curl downwards around the ends of a car, traveling 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 region is a recirculation region generated behind a moving vehicle, this is where flow separation takes place, caused by the flow of surrounding fluid around the body. The local disturbances in the flow pattern behind the vehicle cause a momentum loss thus causing form drag which extends far behind the bodyworks of a vehicle. The local disturbances in the flow pattern behind the vehicle cause a momentum loss thus causing form drag which extends far behind the bodyworks of a vehicle. 

The Ahmed body is a simplified car model created by Ahmed et in the 1980s to investigate the behavior of newly developed turbulence models for complex geometry cases. The Ahmed body is made up of a curved front part, a movable slant plane placed in the rear of the body to study the separation phenomena at different angles, and a rectangular box, which connects the front and rear slant plane as shown in Fig 1.1. Ahmed body is a simplified model of a car body, but it demonstrates all the flow features involved in an actual case of a moving car.

                                                                                                                          Fig 1.1:-  Ahmed Body Geometry

          2. Geometry Importing & Editing

We start by importing the pre-modeled Ahmed body geometry to Spaceclaim, where we will be editing our geometry according to our simulation requirements. For any external aerodynamic simulation, the first step of the editing process involves designing a virtual wind tunnel. The dimensions of the wind tunnel vary according to the geometry of the object that is to be simulated. For our case, our Ahmed body geometry is of length 1.04m & our wind tunnel will have dimensions of 2m upstream of the Ahmed body & 5m downstream with a height of 1m from the origin (bottom of the Ahmed body). The designing of the wind tunnel geometry is carried out using the "Enclosure" feature available in Spaceclaim software. The "Enclosure" feature allows users to create an enclosure around the geometry, the enclosure can be a box, cylinder, or sphere. For our purpose, we used a "Box Enclosure" as shown in Fig 2.1.

                                                                                                      Fig 2.1:- Ahmed Body geometry inside a virtual wind tunnel (Case 1)

This above setup will be used for case 1, but for cases 2,3 & 4 we will be adding a second "Box Enclosure" around the Ahmed body geometry for local refinement & the dimensions for the box are shown below in Fig 2.2.

                                                                                                       Fig 2.2:- Ahmed Body geometry inside a virtual wind tunnel

Creating an additional box volume inside the wind tunnel creates an overlapping region as shown in Fig 2.3, this may lead to uneven mesh distribution along the entire surface. To fix this we use the "Interference" feature to mark the area & the feature solves the problem automatically. The next & final step in the geometry editing module is to "suppress for Physics" for the Ahmed body & turn on "Shared Topology" for the mesh generation process to work uniformly. 

                                                                                                       Fig 2.3:- Over Lapping Region fixed using the "Interference" feature.

          3.  Setting up the Mesh & Specifying Boundary Conditions

The process of discretizing our domain of interest i.e. the geometry into smaller pieces is called mesh generation. Ansys Fluent uses a technique called the "bounding box" method to generate a body-fitted mesh. This is done by creating the smallest size hypothetical box possible that can be used to fit the entire geometry into it. We will be running a total of 4 case setup & each of the cases will have varying mesh refinement, therefore each case will have varying "total number of cells" & "total number of nodes". This is done to perform a Grid independence test & compare the simulation results.

For Case 1, we will be using a tetrahedral mesh with a size of 100mm, which generates a total cell count of 140550 & no additional layers of mesh will be added. This will be used as our base mesh & the mesh will be evenly spread around the Ahmed body wall & the wind tunnel section as shown in Fig 3.1.

                                                                                                                     Fig 3.1:- Cut plane view of the Base Mesh for Case 1

For Case 2, we will be adding a volume of "Box Enclosure" near the Ahmed body geometry with dimensions of 0.5m upstream, 1m downstream & 0.5m height. This region of enclosure is added to provide a more refined mesh near the Ahmed body where the wake region will be more prominent. We will be making use of the "multi-zone" feature available in ANSYS, the MultiZone mesh method provides a combination of pure hexahedral mesh wherever possible & tetrahedral mesh near the geometry region. For our case, the hexahedral mesh will dominate the entire "outer box" or the wind tunnel region with a mesh element size of 100mm i.e. will have a coarse mesh. The inner volume or the "smaller box" will have a mesh element size of 60mm i.e. has a finer mesh as shown in Fig 3.2, the total number of cells generated is 81519.

                                                                                                                       Fig 3.2:- Cut plane view of the mesh for Case 2

For Case 3, we will be adding a volume of "Box Enclosure" near the Ahmed body geometry with dimensions of 0.5m upstream, 1m downstream & 0.5m height as shown in Fig 2.2. This region of enclosure is added to provide a more refined mesh near the Ahmed body where the wake region will be more prominent. We will be making use of the "multi-zone" feature available in ANSYS, the MultiZone mesh method provides a combination of pure hexahedral mesh wherever possible & tetrahedral mesh near the geometry region. For our case, the hexahedral mesh will dominate the entire "outer box" or the wind tunnel region with a mesh element size of 100mm i.e. will have a coarse mesh. The inner volume or the "smaller box" will have a mesh element size of 60mm i.e. has a finer mesh as shown in Fig 3.3, in addition, we will also be adding 4 inflation layers near the Ahmed body & the total number of cells generated is 85011.

                                                                                                                      Fig 3.3:- Cut plane view of the mesh for Case 3

For Case 4, we will be adding a volume of "Box Enclosure" near the Ahmed body geometry with dimensions of 0.5m upstream, 1m downstream & 0.5m height as shown in Fig 2.2. This region of enclosure is added to provide a more refined mesh near the Ahmed body where the wake region will be more prominent. We will be making use of the "multi-zone" feature available in ANSYS, the MultiZone mesh method provides a combination of pure hexahedral mesh wherever possible & tetrahedral mesh near the geometry region. For our case, the hexahedral mesh will dominate the entire "outer box" or the wind tunnel region with a mesh element size of 100mm i.e. will have a coarse mesh. The inner volume or the "smaller box" will have a mesh element size of 35mm i.e. has a finer mesh as shown in Fig 3.4, in addition, we will also be adding 8 inflation layers near the Ahmed body & the total number of cells generated is 342024. Case setup 4 has the finest mesh amongst all the cases & thus also has the maximum number of cell count while case 2 has the least number of cell count. 

The final step in the mesh module is to assign the right boundary names. For all 4 cases we have 5 boundaries, inlet, outlet, Wall (Ahmed body), bottom wall, symmetry boundaries.  

          4. Setting up the Flow Physics for the Computational Model

In Ansys Fluent after creating the mesh & assigning the boundaries, the next stage of the process is to set up the simulation parameters. This step involves specifying certain properties to capture the physics of the problem perfectly, such as materials used, simulation time parameters, solver parameters, initial & boundary conditions to solve the Navier Stokes equation which are PDEs, defining body forces, type of fluid flow & species involved.

We start by setting up the Domain, here we will be using a pressure-based steady-state solver with absolute velocity formation. We can justify using a density-based solver since the velocity or Mach number at the inlet is lower than M=2 (V= 25 m/s). Along with the standard PDEs, we will also be solving the Energy equation. 

The next parameter that we need to set up is the Flow Physics, here we will be navigating to the viscous feature & use an upgraded version of the original K-epsilon (k-ε) turbulence model i.e. the Standard k-epsilon (k-ε) turbulence with standard wall function in which the magnitudes of two turbulence quantities, the turbulence kinetic energy ‘k’ and its dissipation rate ‘ϵ’, is calculated from transport equations solved simultaneously with those governing equations for capturing better flow physics. In the flow physics parameters itself, we will be defining the materials, our choice of fluid for the experiment is "air". 

We then navigate to the boundary zone, these are user-defined parameters where we assign the inlet velocity & temperature for the inlet (25m/s) & we define the pressure near the outlet area. The walls of the Ahmed body & the Bottom wall are stationary walls with a "no-slip" condition. The final step includes performing a hybrid initialization & running the simulation for 800-1000 iterations until the solutions converge.

          5. Results & Observations

                    A. Velocity Contours

                    B. Pressure Contours

                    C. Lift Coefficient Plots

                    D. Drag Coefficient Plots

          6. Conclusion

For case 1, the mesh generated was coarse as it had no enclosure & refinements but the total cells generated were more than that of case 2 & case 3 hence the results observed are slightly better than cases 2 & 3. Even though cases 2 & 3 had an additional enclosure with refined mesh near the Ahmed body the total cell count decreased drastically. Thus, we can conclude adding an enclosure & mesh refinement doesn't necessarily improve the final results of the simulations as we can observe from the above table. The `C_L  &  C_D`results obtained for cases 2 & 3 are not reliable & should not be used to make any engineering decision. Case 3 had by far the maximum number of cells generated with 0.49 million cells (Student license has a maximum limitation of 0.5 million cells). The results obtained from the final case are close to the values obtained from the actual experiment but the results can further be improved by adding a few more cells to the geometry, for which high computational power is required. From the velocity contour plots, we observe a flow separation region which occurs due to boundary layer separation (wake region) downstream the Ahmed body.