Emmanuelle Andrès

Experimental Analysis of a Bathtub Vortex in a Simplified Cowl Box of Automotive Vehicles

On automotive vehicles, the cowl box is a volume located at the bottom of the windshield, under the cowl top grille. It provides external fresh air to the HVAC (Heating, Ventilating and Air Conditioning) unit and it is used to collect water coming from the windshield under rain conditions. This box is designed as a tranquillisation chamber to segregate water from air and avoid the ingress of rainwater into the HVAC unit. However, as the area is awkward to access with measuring devices, our knowledge about the physics of flow in the cowl box is limited.The present work aims to advance our knowledge through experimental work on the air/water flow in a simplified cowl box in order to optimize the box size and improve numerical models. This paper will focus on the analysis of the bathtub vortex, which is potentially responsible for insufficient draining of the water collected in the cowl box.The experimental set-up consists of a Plexiglas parallelepiped representing a simplified cowl box with top cowl grille, HVAC inlet and drain. A blower generates airflow through the HVAC inlet. A water sheet, with controlled flow rate, is created on an inclined plane representing the windshield. Velocity measurements of all components are obtained by PIV (Particle Image Velocimetry) in the liquid phase and the surface level is recorded by a capacitance probe near the drain. Moreover, contour detection of the vortex core is achieved using a high-speed camera. Results show a relationship between the pressure loss generated by the airflow in the cowl box, the water level and the vortex structure. The modification of the vortex structure as well as the modification of velocity components near the air core are visible only in transient stages. These experimental results give us today some insight to understand the physical phenomena occurring in the cowl box.Copyright

Controlling the Intensity of a Bathtub Vortex by Acting on the Upstream Flow

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.Copyright