Author(s): Sebastian Vecchi Created using ANSYS AIM 18.1 ANSYS AIM Tutorial Flow over an Ahmed Body Problem Specification Start Up Geometry Import Geometry Enclose Suppress Mesh Set Mesh Controls Generate Mesh Physics Set-Up Boundary Conditions / Forces Solution/Results Validation References
Problem Specification The Ahmed Body is a standard wind tunnel model used to represent ground vehicles. It was first defined and characterized by S.R. Ahmed in 1984 and has been used since to study the effects of drag and aspect ratio. Below is the blueprint definition of an Ahmed body. In this tutorial, we will subject an Ahmed body to air at 40 m/s in order to find the velocity vectors, pressures on the body, and streamlines around the body.
Start Up A few words on the formatting on the following instructions: 1) Notes that require you to perform an action are colored in blue 2) General information is colored in black, but does not require any action 3) Words that are bolded are labels for items found in ANSYS AIM 4) Most important notes are colored in red We are ready begin simulating in ANSYS AIM. Open ANSYS AIM by going to Start > All Apps > ANSYS 18.1 > ANSYS AIM 18.1. Once you are at the starting page of AIM, select the Fluid Flow template as shown below. You will be prompted by the Fluid Flow template to either Define new geometry, Import geometry file, or Connect to active CAD session. Select Import geometry file and press Next.
Geometry Import Geometry For this problem, we are going to import the geometry into ANSYS from a CAD package. Download the part file NewAhmed.scdoc from here. Select and open the file. Once successfully imported, press Finish. Enclose A volume needs to be created to around the Ahmed body using the enclosure feature, to represent the fluid flow area. Press Geometry in the Workflow tab and select Edit Geometry in the Geometry panel. The Enclosure tool can be found in the Analysis section of the toolbar
under the Prepare tab. Select the Ahmed body and a box will appear around the geometry. Input 500% into the Default cushion box. Uncheck the Symmetric dimensions box. Change the distance from the body to the bottom of the enclosure and distance from the midplane to the enclosure to 0m, then the distance from the front of the body to the enclosure to 2.61m. Use the picture below for guidance. Press the green checkmark and the enclosure will be generated. Suppress Now that the geometry of the flow volume has been created, we can suppress the Ahmed body from the physics calculation. Right click the Solid in the geometry tree and select Suppress for Physics.
Mesh Once you have exited the modeling window, initiate the meshing process by clicking on Mesh in the Workflow. Set Mesh Controls Under Global Sizing, change the Size function method to Curvature and proximity. The curvature option will automatically refine the mesh near the curved surfaces of the body. The proximity option will automatically refine the mesh between the bottom of the body and the ground. In the Boundary Layer Settings, under Collision avoidance, use the Layer compression setting. This will ensure continuous boundary layers around the body, which can improve accuracy for external flows.
AIM will prompt you to fix the boundary layer before generating the mesh. Click on Boundary Layer under Mesh Controls. Select the faces of the volume in contact with the Ahmed body, as shown below. The simplest way to do this is to drag a box around the Ahmed body, from the upper left to the lower right. AIM will select all faces that are completely enclosed by the box. Generate Mesh Return to the Mesh panel, then click Generate Mesh under Output or at the top of the screen by the status window for Mesh. AIM will detect that you are ready to generate the mesh and highlight the buttons in blue.
Physics Set-Up Boundary Conditions / Forces First, the inlet must be defined using the Fluid Flow Conditions. In the Add drop down menu by Fluid Flow Conditions, select Inlet. Then, using the face selection tool, define an inlet at the face upstream of the Ahmed body. The front of the body is the taller end with the rounded edges and corners. Make sure to input the Velocity magnitude as 40 m/s. Once the inlet is defined, the outlet is next. In the same Add menu, use the Outlet condition to define an outlet downstream of the body. Assign a Gauge static pressure of 0 psi.
Create openings for the sides of the flow volume by selecting Opening in the Add drop down menu. Select the top face of the flow volume, and the side face away from the Ahmed body, then input 0 Pa for the Gauge entrainment pressure.
Add a Symmetry condition from the Add drop down menu to the face of the flow volume which passes through the Ahmed body.
A Wall condition needs to be added for the remaining faces of the flow volume. In the Add menu, select Wall. AIM will automatically select all faces not already assigned.
Press Solve Physics in the Physics panel to run the calculations, then move on to the next step.
Solution/Results Press the Results button in the Workflow to extract information from the simulation. In order to find information that can be readily used, first press Evaluate Results. Once the evaluation is complete, AIM will automatically output a Velocity Vector in the Results section under Objects. Select the Velocity Vector to edit the settings with which the vectors are defined. Change Symbol distribution to Based on mesh and change At every Nth item to 4. If desired, change the Symbol sizing in the Appearance section to alter how big the arrows are. Press the Play button in the model window to see how these velocity vectors develop over time. Flow along the midplane can be visualized by displaying the velocity vectors on the symmetry plane. Change the Location to Symmetry 1, change At every Nth item to 1, and press Evaluate. Press the Play button in the model window to see how these velocity vectors develop over time.
Pressure on the Ahmed body can be plotted by adding a Contour in the Add drop down menu of the Results panel. Use Total Pressure as the Variable and then select the faces of the flow volume touching the faces of the Ahmed body.
Streamlines can also be computed, by picking the Streamline option in the Add drop down menu near the Results category. Select the faces of the Ahmed body as the Seed location. Then, change Distribution to Based on mesh and At every Nth item to 10.
Press Evaluate. If desired, change the Wire thickness in the Appearance section to alter how big the streamlines are. Press the Play button in the model window to see how these streamlines develop over time.
Validation An excellent way of validating simulations is by comparing them to research papers which are relevant. Since the Ahmed body is so widely studied and used as validation, it is not difficult to find supporting evidence. For this tutorial, the information gathered from the simulation will be compared to Embedded Large Eddy Simulation of Flow around the Ahmed Body by Domenico Caridi done in ANSYS FLUENT. Below is a contour of the pressure on the rear surface of the Ahmed body. This can be compared to our model by creating a similar contour in our simulation. Add a Contour from the Add drop down menu and select the top, side and rear faces as the Location. After changing the Variable to Pressure, the following contour plot will be created.
While qualitatively similar, the numerical results are significantly different than those found in the validation. By refining the mesh, the results can be calculated to a more accurate degree. Go back to the Mesh task in the workflow and increase the Mesh resolution all the way up. Return to the Results and press Evaluate Results, then display the Pressure contour again.
These results compare more favorably to the FLUENT results, though they are still significantly different. However, the mesh in the FLUENT study was much more refined than the one used here, especially in the critical wake region immediately behind the Ahmed body. Additional refinement of the AIM mesh would further improve the results and is recommended practice for external flow around vehicles.