Applying Fluid Structure Interaction to Automotive Aerodynamics

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Aerodynamic studies on vehicles have become a mandate for manufacturers. Whether using wind tunnel tests or using CFD simulations, studying the impact of air flow over the exterior body of the vehicle is crucial, in order to reduce the drag forces and improve fuel efficiency.

A typical aerodynamic analysis involves determining the forces that affect the vehicle motion and formation of wake regions, allowing designers to identify better outer body designs. The study of air flow is even more important in case of high-speed racing cars, as even a slight increase in drag leads to drop in speeds and ultimately loose races.

Fluid Structure Interaction to Automotive Aerodynamics
Fluid Structure Interaction to Automotive Aerodynamics

However, aerodynamic analysis considers the behavior of air flow over the vehicle geometry. For high speed applications, it is important to realize the effect of these forces on the structure as well. In aerodynamic analysis, the vehicle body is treated as a rigid body and does not consider any structural interaction of the forces acting on it.

However, there are many areas in a vehicle consisting of weak structures, which can deform under the effect of aerodynamic forces. These areas include the front grille and underbody panels which are designed to be lightweight but cover a significant area of the vehicle. The purpose of underbody panels is usually to avoid the formation of turbulence underneath the vehicle, which would otherwise increase the drag and affect the efficiency.

As modern vehicles are required to be low in weight, the panels and grilles are made lightweight, increasing the chances of getting deformed under the impact of high pressure forces from the air flow. As such, incorporating fluid structure interaction (FSI) along with aerodynamic analysis helps in evaluating these lightweight components more effectively.

The use of FSI helps in determining the impact of fluid forces on the structure by coupling the fluid and solid solvers. The region before the solid boundary is solved using usual governing equations of fluid, while the solid region is solved using solid mechanics equations. However, at the interface between fluid and solid, coupling equations are solved which transfers the data from fluid to solid using a mapping algorithm that maps the data from each fluid element to solid elements.

A FSI problem could be solved using one-way or two-way coupling; wherein, one-way coupling the transfer quantities are sent from one domain to another, but not in reverse direction. This coupling is suitable for cases where deformations are not large. A two-way coupling on the other hand transfers the quantities bi-directionally.

Incorporating FSI with aerodynamic analysis will help in determining the deformation regions on the panels and grilles, which can be further evaluated to improve strength or modify design to reduce the impact of pressure forces.

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About the Author: Mehul Patel CFD specialist at Hi-Tech, is an expert at ANSYS FLUENT, OpenFOAM and many more. With more than 8 years of experience, Mehul has successfully planned, coordinated and executed CFD Projects for Aerodynamics, Combustion, Turbomachinery, Multi-phase flow & HVAC analysis. Mehul adept at co-ordination and QA/QC, handles a team of CFD engineers contributing to CFD projects for aviation, automotive, building design & construction, plant design and heavy engineering industries.
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