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 dry air to the passenger compartment through the ventilation system. When rainwater accumulates in the box, a bathtub vortex forms above the drain pipe. This vortex sucks up air into the pipe and creates an air core that decreases the water drain rate in the pipe. This increases the water level in the box and can cause a water overflow into the ventilation system. The behavior of this bathtub vortex was experimentally studied using a simplified geometry representative of a real cowl box. The inlet water flow rate was controlled and a capacitive probe measured the water level in the box. The flow was studied using Particle Image Velocimetry to measure the 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 also shifts the vortex axis away 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 was tested in order to reduce the vortex intensity and consequently the water level. Resulting velocity fields showed a better distribution of the upstream flow. The vortex intensity decreased by up to 55% and the water level by up to 53%. While there was still a vortex with an air core, the water level was therefore significantly reduced. These results are particularly interesting for cowl box design: as this device can guarantee a lower water level, the cowl box depth can be reduced and space can be saved.