When a car travels through air, it basically pushes the air aside, causing the air to flow along the body. This flow of air along the car body impacts the forces acting on the car. The earlier models of cars which were box-shaped generated an increased amount of drag force as the air flows along the body. Hence, the cars were later designed to be curvy so that the flow of air will be streamlined. However, it soon became apparent that in spite of the change in design, there was a slight drag force generated by the side view mirrors which merged with the main airflow.
Since the side view mirrors are important in imparting better visibility to the drivers, they cannot be removed altogether. While it has been debated temporarily that cameras can be used to replace the side view mirrors, the idea was later discarded. This was because an electrical system failure or even a malfunctioning in the camera can render the driver partially blind in terms of visibility. Hence, the best solution to ensure that side view mirrors are designed to cause as less drag force as possible. In order to do that, engineers carry a variety of simulations on various side view mirror designs to check the amount of drag force generated, forces acting on the mirror and deformation etc.
In this project, we will simulate airflow over a side mirror to map the effect of pressure acting on it and the deformation that occurs as a result of the pressure. To study the effect of such pressure due to airflow over the mirror, Fluid Structure Interaction (FSI) analyses are carried out. FSI is preferred because it is difficult to simulate the effect of fluid loads acting on a surface when the load is not being constant along the surface it acts upon. In other words, airflow over a side view mirror is not constant but varies as the mirror curves in design. It is extremely difficult to perform a typical structural analysis due to various complexities. So to study the effects of air as it flows over such a curved design, we perform FSI. The side view mirror designed for one of our previous projects “Analysis of drag for side mirrors of an automobile” will be used to carry out the FSI analysis.
Steps to be done:
- Import the previous side view mirror model
- Map the pressure acting on the side view mirror
- Study the areas where deformation occurs
Import the previous side view mirror model:
Software used: ANSYS
Fig 1 shows how the geometry and the solutions from the previous CFD setup using ANSYS FLUENT are being shared with the Static Structural module in ANSYS. The model shared from the CFD analysis will consist of the enclosure that was defined to analyse the fluid flow. For the static structural analysis, only the solid mirror geometry will be considered. This is done by suppressing the enclosures as shown in Fig 2.
Meshing was performed in ANSYS itself with an average element size of 3.5mm. This enabled the overall number of elements to be within the academic limit of ANSYS. The meshed geometry is as shown in Fig 3. The mesh metrics for element quality is as shown in Fig 4
One fixed support was given to the end face of the mirror mount which is shown in Fig 5.
Mapping the areas of pressure:
The fluid pressure acting on the mirror geometry in the wind tunnel needs to be imported from the FLUENT setup. This option is readily available within ANSYS Mechanical setup. After importing, the pressure is mapped onto the corresponding geometry. With a finer degree of meshing, better mapping of the pressure is obtained. While using a coarse mesh, the pressure was mapped as shown in Fig 6.
When using the aforementioned element size of 3.5mm (length of the element), the pressure mapping was found to be better, which is shown in Fig 7.
The material assigned to the geometry was ABS Plastic which is available within the ANSYS material library. The setup was solved to find out the total deformation that will happen on the geometry with the Wind tunnel analysis that was undertaken at a speed of 120kmph.
Studying the deformation on the side view mirror:
Deformation was found to occur at the unsupported end of the side mirror geometry. The value was found to be 0.012536 mm. The animation for a scale of 1.4e3 is also shown below.
The mirror expressed slight deformation towards the end (shown in red). In this snapshot here taken from the video, you can see the deformation at the edge of the mirror:
By identifying the pressure areas and the deformation on the mirror, we can update the design to withstand the forces acting on it. We can also perform the same study on different shapes of side view mirrors to deduce the which design produces less drag force while undergoing minimum deformation.
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