One of the most important problems in the automotive industry is the general multiphysics simulation of coupled phenomena, where multiple — and sometimes conflicting — conditions need to be accounted for, all at the same time. One common application is the resistive heating of a car side mirror.

Designing the mechanism for keeping the mirror defrosted must also take into account the structural response of the mirror as the external environmental conditions, such as air pressure and cold temperature, cause physical stress and thermal deformation. The task is a base requirement of the automotive industry and requires a full multiphysics approach, which is still a challenge for common finite element method (FEM) simulation. In this post, we’ll show you how our engineers at SVS FEM used ANSYS AIM to model a side mirror and multiphysics analysis to solve some of its difficult design problems.

**Car Side Mirror Design**

Most of the things around us are more complicated than we imagine, and a car side mirror is good example. Air pressure at high speed, dynamic properties of the structure, and the various materials used to create the mirror have a synergistic impact. You also need to factor in structural strength and usability, electric conduction, mirror heating during various conditions and thermal transfer between the mirror and blowing air.

Virtual prototyping can save much of the time and expense of physical testing, but many simulation tools can only test one area of physics at a time. We chose AIM for its modeling and multiphysics testing capabilities, anticipating that this would save time and produce more accurate results.

**Fluid Flow Analysis**

AIM enabled us to solve steady-state air flow around the side mirror. We used the symmetry of air space around the modeled car and used dynamic air pressures as loads for the structural analysis.

**Static and Modal Analysis**

We applied the fluid pressure field from fluid analysis as a boundary condition for structural analysis, and set in advance the automatic force mapping from fluid analysis mesh onto a structural analysis mesh. Solved stresses and deformations in steel and plastic bearing parts corresponded to the simulated speed of the car.

To reduce the impact of vibrations on the mirror, we analyzed the dynamic behavior of the same structural model in modal analysis, simulating induced air blowing and car movement. We also obtained and analyzed several mode shapes with corresponding natural frequencies.

**Fluid Flow and Thermal Analysis**

We solved the steady-state air flow simulation a second time, simulating only the thermal flow effect, assuming a one degree Kelvin difference between the mirror glass and blowing air. The resulting heat fluxes on the mirror surface corresponded to the convective heat transfer effect.

**Geometry Preparation**

The ANSYS SpaceClaim geometry modeler enables fast and simple work with figures. We found its method for aligning to a plane and changing its scale to be very intuitive and useful. It is also easy to place curves on the face of the mirror glass geometry. We used an inserted heat flux figure as the basis for determining how to divide the mirror surface so we could more accurately apply boundary conditions for the heat transfer.

**Electric Conduction and Thermal Analysis**

We solved the steady-state electric potential field in the conductive paint together with thermal analysis using the contour-based convection boundary condition previously obtained from analysis. We used the resistivity of conductive printed circuit and the corresponding power input as the main result values for the analysis.

**Parametric Study**

The surface heat flux field is directly dependent on air flow caused by the speed of the car. We performed multiple calculations of fluid flow with thermal effect using the Design Point input table, which shows the heat flux field for each speed and calculates the average value.

**Transient Thermal Analysis**

We performed a thermal transient analysis of the mirror glass for chosen design points (DP1, DP2, DP3) of the previously calculated fluid flow. We then applied different input speeds as different fields of heat fluxes via the convection boundary condition.

**Conclusion**

Using ANSYS AIM, we were able to simulate the coupled phenomena of both the structure of the car side mirror and the effects of external forces upon it. The information we gained from our multiphysics simulation enabled us to optimize our design before creating a physical model. From this demonstration, we’ve concluded that AIM is a very useful tool suitable not only for design engineers but also for deep simulation of complex multiphysics responses.

To learn more about this simulation, please take a look at our video of this demonstration. You might also be interested in other simulation demonstration videos that we have posted on our YouTube channel, and other information about our company at our website.

Dear Dr. Cada,

Thank you for this very interesting post. I would like to discuss with you a perspective of a transient flow simulation with the resolved turbulence using either an LES or a hybrid RANS/LES approach. Please let me know your e-mail address.

With kind regards,

Yuri Egorov

yuri.egorov@ansys.com