In a motor vehicle, the cowl box is a volume located at the bottom of the windshield. It collects rainwater and drains it to provide clean and dry air to the passenger compartment through the ventilation system. When rainwater is accumulated into the box, a bathtub vortex appears above the drain pipe. This vortex sucks up air into the pipe and creates an air core responsible of the decreased water drain rate in the pipe. It leads to an increased water level in the box and can cause a water overflow into the ventilation system.

The behavior of this bathtub vortex has been experimentally studied using a simplified geometry representative of a real cowl box. The inlet water flow rate is controlled and a capacitive probe measures water level in the box. The flow has been studied using Particle Image Velocimetry to measure velocity field around the vortex.

The flow pattern is described using these data. Due to geometry and inlet conditions, the upstream flow forces the vortex counter-clockwise. It is also responsible for a shift of the vortex axis from the drain axis. In this configuration, the upstream flow is strongly asymmetric and feeds the vortex using less than half the width of the box.

Based on these observations, a device has been tested in order to reduce the vortex intensity and consequently the water level. Resulting velocity fields show a better distribution of the upstream flow. The vortex intensity is decreased up to 55% and the water level up to 53%. Despite there is still a vortex with an air core, the water level is therefore significantly reduced.

These results are particularly interesting for the cowl box design: if this device can guarantee a lower water level, the cowl box depth can be reduced and space can be saved.

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