Rainfall on windshield: coupling airflow with rain and wipers movement

During heavy rain, the rain falling on a car windshield may alter driver’s visibility, especially when it overflows onto the side windows as wipers clear water from the windshield and create new flows.

Your application

The design of a car which ensures appropriate evacuation of rainwater is anything but trivial. Indeed, depending on the driving speed, the relative wind influence grows, and the water flow is driven by a complex and changeable coupling between aero and hydrodynamics. The unsteady wipers motion also influences the flow and increases its complexity.

Nowadays, to ensure appropriate designs, the automotive industry needs to perform complex experiments in climatic wind tunnels. Cars are spread with a fluorescent dye to visualize the flow under ultraviolet light. Car prototypes and climatic wind tunnel facilities provide high-quality results but their very high cost precludes systematic use.

CFD allows numerous geometries to be tested and enables individual physical phenomena to be isolated more easily and at lower cost.

Our solution

The study of the rain falling on a car in driving conditions at 80 km/h is split into 2 kinds of studies: aero and hydrodynamics, respectively the airflow and the rain. This choice is driven by the fact that SPH-flow software based on a Lagrangian approach, is more efficient than an Eulerian approach for hydrodynamics computation, whereas it is the contrary for aerodynamics studies.

      The aerodynamics field around the car was previously simulated using CS’ Lattice Boltzmann Method (LBM) solver ProLB. The resulting mean airflow field is used as input data for the rain simulation carried out using SPH-flow. A coupling method was developed in SPH-flow to impose a drag force on the water droplets. Hence, two configurations are simulated:

      • A 5mm thick water film is initialized on the windshield blown by the airflow
      • Tropical heavy rain condition (800 mm/h) modelized by 42 injectors of 6 mm of diameter with a constant water flow rate, distributed over a 1.5 m² surface and positioned at a height of 2.5 m. The wiper kinematics is also simulated in SPH-flow.


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