Introduction:

When the older cars were designed first, the aerodynamic factors weren’t taken much into consideration. The frontal panel was primarily designed to hold the engine. However, as time passed by, engineers realized that the design of the car was impacting the airflow around the car. the older models were structured in such a way that the aerodynamic drag was quite high. In the presence of such drag forces, the vehicle will require an extra power to overcome the drag force and this extra power came from the engine. In other words, the shape of the car was causing the engine to use up more fuel to power the car.

In order that the air might flow streamlined, the frontal panel was redesigned later, like the ones we have today. Although the change in the shape of cars from boxy to curvy resulted in reduced drag, engineers encountered a minor problem – the side view mirrors were interfering with the streamlined flow of air. They were found to induce vortex streets which merge with the main flow of air.

It is also not possible to remove the side view mirrors as they are needed for better visibility of the driver. Hence, the best possible solution is to check the design of the side view mirrors and make sure that they do not interfere with the airflow.

Many investigations have been carried out to investigate the drag contribution of the side view mirrors. Stephanie Lei [1] performed analysis at different speeds ranging from 80-120 km/h and found that at low subsonic speeds, the coefficient of drag was almost constant and would produce lesser drag by varying the curvature of housing, implementing shorter mounts for the mirrors and by decreasing the overall size of the mirror. Olsson [2] analysed the effect of side mirrors on a car and then implementing different design cases to arrive at a solution. Both CFD and wind tunnel simulations were performed in this case. P.Murukesavan [3] investigated 3 different designs of rearview mirrors at two different velocities and arrived at a design which was more efficient.

Objective:

In this project, we will design a side view mirror and simulate airflow over it to understand how much drag is generated by the side view mirrors.

Steps to be done:

  • Design a side view mirror
  • Meshing
  • Simulate flow over the design
  • Study the results

Design a side view mirror:

 Software used: SolidWorks

The side view mirror geometry is a generic model created using SolidWorks 2018. The end design is shown here:

You can find the dimensions of the design here:

Mirror front view:

Mirror top view:

Meshing:

Software used: ANSYS SpaceClaim

The model was imported onto ANSYS SpaceClaim and an enclosure was created around the model using the tool provided in SpaceClaim. The enclosure is the region within which we will simulate and study the flow pattern.

The enclosure around the side view mirror

The enclosure was created such that the inlet is placed 500 mm away from the front of the mirror and the outlet is placed 1000mm away from the back.  This distance is placed so that the flow can develop accurately. Another enclosure was created inside the existing enclosure so that more refinement can be provided in the newly created enclosure. The model is then meshed using ANSYS Mesher. When meshing, the entire geometry is broken down into individual fragments called Finite Volume Cells. Inside these volumes, the governing equations are solved. The larger enclosure was given a hexahedral dominant mesh while the inner enclosure was given tetrahedrons with a much finer sizing so as to get a reasonably good mesh within the limits of the academic license of ANSYS. It is vital that we capture the boundary layers when the flow is happening or the flow will be diffused and the results will be inaccurate. Hence, inflation layers employed on the mirror geometry to capture the boundary layers.

The cut view of the mesh

 In the image here, you can see the mesh settings given:

The mesh statistics specifying information about the mesh we have set:

With the current meshing scheme, a reasonably good mesh has been obtained with the majority of the mesh being of very good quality. The bad elements were found to be present near the frontal curved area of the mirrors. Further refinement is needed to be studied but due to the limitations of the academic version of the software, the present setup is deemed acceptable.

The model was studied under three different velocities of 80, 100 and 120 km/h. The inputs were given in m/s to FLUENT as the inlet condition was set to velocity inlet. The outlet was set to pressure outlet at 0 Pa. The sides of the enclosure were set to symmetry so that the sides will be considered as open.

After calculating Reynold’s number for the flow, the flow was found to be in the turbulent region. In order to analyse the drag over external flows, the k-ω SST model is more accurate and hence it was used in this simulation. The simulation was performed and the results are discussed below.

Results:

The results for the study are as follows:

Velocity( km/h) Drag Coefficient
80 0.7102
100 0.7145
120 0.7168

In order to visualize the flow in 2D, the Velocity contours are used. The velocity contours are clearly captured in the refined enclosure region while it gets more diffused in the enclosure region with the coarser mesh.As expected, the side views are producing vortex streets behind them leading to complicated flow regimes. The vortex shedding phenomenon will induce vibrations on the mirror and this will lead to an increased drag on the body, as studied by Ramnarayan Gopalakrishnan [4]. The value of the drag coefficient obtained for these velocities are found to increase slightly as the velocity increases.

Visualizing the flow using Velocity contours

A volume rendering done on CFD Post in ANSYS FLUENT helps visualize the velocity in 3D:

The pressure acting on the mirror body clearly shows regions of high pressure and low pressure and this explains the behaviour of the flow around the body. On plotting the pressure, the following result can be visualized:

You can view the animation by clicking on this video:

Conclusion:

The results for this particular case falls in line with the predictions made by Stephanie Lei Shen[1], that the mirrors don’t contribute much to drag under low-velocity conditions. Further analysis will have to be undertaken to study the drag forces on the body at higher velocities and another avenue of investigation for this particular simulation could be to find out the overall drag while considering the car body too. In this study, only the mirrors were accounted for. But in real life conditions, the flow around the car body will also interact with the mirror and this will further complicate the flow. Studies can also be carried out on different shapes of the mirror.

References:
[1] Stephanie Lei Shen Ai, et al. “Effect of Size and Shape of Side Mirrors on the Drag of a Personal Vehicle” Eureca 2016 Conference Paper, Number 2ME26.
[2] Martin Olsson, “Designing and Optimizing Side-View mirrors” , Chalmers University of Technology, Goteborg, Sweden 2011.
[3] P Murukesan, “Computational Analysis of an effect of an aerodynamic pressure on the side view mirror geometry” ICMER 2013, 012039.
[4] Ramnarayan GopalKrishnan, “Vortex induced forces on oscillating bluff cylinders”,Masachusetts Institute of Technology, Cambridge, 1993- MIT/WHOI-92-38.

Project submitted by,

Aditya Ram


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